@article{xue_palmese_sekely_little_kish_muth_wierer_2024, title={Growth and characterization of AlInN/GaN superlattices}, volume={630}, ISSN={["1873-5002"]}, url={https://doi.org/10.1016/j.jcrysgro.2024.127567}, DOI={10.1016/j.jcrysgro.2024.127567}, abstractNote={Data are presented on near-lattice-matched Al1-xInxN/GaN superlattices (SLs) with superior morphology to thick AlInN layers. The SLs are grown by metalorganic chemical vapor deposition and consist of ∼3 nm thick AlInN, ∼1 nm thick GaN layers, and x=0.153 to 0.203. The SLs are grown with either 20 or 100 periods on GaN-on-sapphire or free-standing GaN substrates. Growth conditions are explored, and the In-content of the AlInN layers within the SL increases with growth temperature and pressure, while the growth rate decreases with pressure. Thick AlInN layers grown on GaN-on-sapphire exhibit island growth with a root mean square (rms) roughness of ∼0.65 nm, while the AlInN/GaN SLs have steplike morphology and rms ∼0.3 nm. Also, 80 nm thick AlInN/GaN SLs grown on GaN substrates exhibit nearly perfect steplike morphology with a lower rms of ∼0.13 nm and extremely low pit densities. The refractive index of the SLs is the weighted average of AlInN and GaN, and they emit light from the quantum states within the thin GaN layers. These AlInN/GaN SLs are a potential replacement for AlInN layers in optoelectronic and electronic devices that require steplike morphology and controlled pitting.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Xue, Haotian and Palmese, Elia and Sekely, Ben J. and Little, Brian D. and Kish, Fred A. and Muth, John F. and Wierer, Jonathan J.}, year={2024}, month={Mar} } @article{wei_muyeed_xue_wierer_2023, title={Anisotropic Etching of InGaN Thin Films with Photoelectrochemical Etching to Form Quantum Dots}, volume={16}, url={http://dx.doi.org/10.3390/ma16051890}, DOI={10.3390/ma16051890}, abstractNote={Traditional methods for synthesizing InGaN quantum dots (QDs), such as the Stranski-Krastanov growth, often result in QD ensembles with low density and non-uniform size distribution. To overcome these challenges, forming QDs using photoelectrochemical (PEC) etching with coherent light has been developed. Anisotropic etching of InGaN thin films is demonstrated here with PEC etching. InGaN films are etched in dilute H2SO4 and exposed to a pulsed 445 nm laser with a 100 mW/cm2 average power density. Two potentials (0.4 V or 0.9 V) measured with respect to an AgCl|Ag reference electrode are applied during PEC etching, resulting in different QDs. Atomic force microscope images show that while the QD density and sizes are similar for both applied potentials, the heights are more uniform and match the initial InGaN thickness at the lower applied potential. Schrodinger-Poisson simulations show that polarization-induced fields in the thin InGaN layer prevent positively charged carriers (holes) from arriving at the c-plane surface. These fields are mitigated in the less polar planes resulting in high etch selectivity for the different planes. The higher applied potential overcomes the polarization fields and breaks the anisotropic etching.}, number={5}, journal={Materials}, publisher={MDPI AG}, author={Wei, Xiongliang and Muyeed, Syed Ahmed Al and Xue, Haotian and Wierer, Jonathan J.}, year={2023}, month={Feb}, pages={1890} } @article{shvilberg_mimura_xue_wierer jr_paisley_heinrich_ihlefeld_2023, title={Electrical Performance of Sputtered Epitaxial Magnesium Oxide on n-Type Gallium Nitride Metal-Oxide-Semiconductor Devices}, volume={5}, ISSN={["1557-9646"]}, url={https://doi.org/10.1109/TED.2023.3269406}, DOI={10.1109/TED.2023.3269406}, abstractNote={A challenge facing specific classes of wide bandgap semiconductor devices is the development of a robust dielectric insulating layer that is chemically and thermally compatible with the semiconductor. Requirements for gate dielectrics include sufficient band offsets, low leakage currents, and low interface state densities. Magnesium oxide holds promise for integration with gallium nitride (GaN), as it has been shown to meet many of these requirements in epitaxial films synthesized by molecular-beam epitaxy (MBE), pulsed laser deposition (PLD), and atomic layer deposition (ALD). Large area growth with low impurity content and smooth growth surfaces are the remaining obstacles. This study presents the results of the epitaxial growth of magnesium oxide by radio frequency (RF) magnetron sputtering on n-type GaN. An epitaxial intermixing layer between the GaN top layer and the oxide was found. Electrical characterization shows explicit depletion and accumulation of charge in metal–oxide–semiconductor devices up to 500 °C. The density of interface states at room temperature was determined using photo-assisted capacitance–voltage measurements to be as low as

$2.9\times 10^{{11}}$

$^{-{2}}$

$^{-{1}}$

, showing that the interfacial phase is not detrimental to the interfacial electronic properties of the metal–oxide–semiconductor capacitor (MOSCap) devices.}, journal={IEEE TRANSACTIONS ON ELECTRON DEVICES}, author={Shvilberg, Liron and Mimura, Takanori and Xue, Haotian and Wierer Jr, Jonathan J. and Paisley, Elizabeth A. and Heinrich, Helge and Ihlefeld, Jon F.}, year={2023}, month={May} } @article{palmese_xue_pavlidis_wierer_2023, title={Enhancement-Mode AlInN/GaN High-Electron-Mobility Transistors Enabled by Thermally Oxidized Gates}, volume={12}, ISSN={["1557-9646"]}, url={https://doi.org/10.1109/TED.2023.3343313}, DOI={10.1109/TED.2023.3343313}, abstractNote={Enhancement mode AlInN/gallium nitride (GaN) high-electron-mobility transistors (HEMTs) are fabricated by thermally oxidizing the barrier region under the gate. The oxidation is performed at 850 °C in

$\text{O}_{{2}}$

, and a SiNx mask is used to achieve selective oxidization of the AlInN layer. For comparison, a standard Schottky gate and atomic layer deposition (ALD) Al2O3 metal–insulator–semiconductor (MIS) HEMTs are fabricated from the same structure and show depletion mode behavior as expected. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) mappings are performed to characterize the gate of the oxidized HEMTs, showing complete oxidation of the AlInN barrier. All the devices are tested to determine their transfer and output characteristics. The results show that the thermally oxidized gate produces a positive shift in threshold voltage at ~4 V and low currents (

$\sim 2\times 10^{-{7}}$

mA/mm) at zero gate voltage. The oxidized HEMTs are also subjected to postmetallization annealing (PMA) at 400 °C and 500 °C for 10 min flowing 1000 sccm of

$\text{N}_{{2}}$

, retaining enhancement mode behavior and leading to a further positive shift in threshold voltage.}, journal={IEEE TRANSACTIONS ON ELECTRON DEVICES}, author={Palmese, Elia and Xue, Haotian and Pavlidis, Spyridon and Wierer, Jonathan J.}, year={2023}, month={Dec} } @article{xue_muyeed_palmese_rogers_song_tansu_wierer_2023, title={Recombination Rate Analysis of InGaN-Based Red-Emitting Light-Emitting Diodes}, volume={59}, url={http://dx.doi.org/10.1109/jqe.2023.3246981}, DOI={10.1109/jqe.2023.3246981}, abstractNote={The recombination rates are measured and analyzed for red-emitting InGaN light-emitting diodes (LEDs) to better understand the factors that limit their efficiency. InGaN/AlGaN/GaN multiple quantum well (MQWs) are grown with

$\text{x}\ge 0.28$

in the InxGa

$_{\mathrm {1-x}}\text{N}$

quantum well. The AlyGa

$_{\mathrm {1-y}}\text{N}$

interlayers (ILs) with high Al-content (

$\text{y}>$

0.8) are employed because they result in smoother surfaces with smaller V-pits and higher photoluminescence efficiency. The IL-MQWs are formed on GaN and InzGa

$_{\mathrm {1-z}}\text{N}$

/GaN superlattice (SL) underlayers (ULs) with

$z =0.015$

, 0.025, and 0.065. Differences in

$B$

coefficients (radiative recombination) within this set result from changes in wavefunction overlap caused by differences in layer thickness and composition in the IL-MQW. IL-MQWs grown on SL-ULs have

$A$

coefficients (Shockley-Reed-Hall recombination) that are lower than expected, indicating that the SL-ULs help reduce defect formation. Compared to shorter wavelength InGaN-based LEDs, the

$B$

coefficients are

$\sim $

100 times lower due to lower wavefunction overlap. A and

$C$

coefficients are higher because of a higher number of defects.}, number={2}, journal={IEEE Journal of Quantum Electronics}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Xue, Haotian and Muyeed, Syed Ahmed Al and Palmese, Elia and Rogers, Daniel and Song, Renbo and Tansu, Nelson and Wierer, Jonathan}, year={2023}, month={Apr}, pages={1–9} } @article{xue_palmese_song_chowdhury_strandwitz_wierer jr_2023, title={Structural and optical characterization of thin AlInN films on c-plane GaN substrates}, volume={134}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0136004}, DOI={10.1063/5.0136004}, abstractNote={The structure and optical characteristics of thin (∼30 nm) wurtzite AlInN films grown pseudomorphic on free-standing, c-plane GaN substrates are presented. The Al1−xInxN layers are grown by metalorganic chemical vapor deposition, resulting in films with varying In content from x = 0.142 to 0.225. They are measured using atomic force microscopy, x-ray diffraction, reciprocal space mapping, and spectroscopic ellipsometry (SE). The pseudomorphic AlInN layers provide a set where optical properties can be determined without additional variability caused by lattice relaxation, a crucial need for designing devices. They have smooth surfaces (rms < 0.29 nm) with minimum pit areas when the In content is near lattice-matched to GaN. As expected, SE shows that the refractive index increases and the bandgap energy decreases with increased In-content. Plots of bandgap energy vs In content are fitted with a single bowing parameter of 3.19 eV when using bandgap energies for AlN and InN pseudomorphic to GaN, which is lower than previous measurements and closer to theoretical predictions.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Xue, Haotian and Palmese, Elia and Song, Renbo and Chowdhury, Md Istiaque and Strandwitz, Nicholas C. and Wierer Jr, Jonathan J.}, year={2023}, month={Aug} } @article{palmese_xue_song_wierer_2023, title={Thermal oxidation of lattice mismatched Al1-xInxN films on GaN}, url={https://doi.org/10.1016/j.prime.2023.100208}, DOI={10.1016/j.prime.2023.100208}, abstractNote={Lattice-mismatched Al1-xInxN layers grown on GaN and with varying x are thermally oxidized to understand how alloy content affects the oxidation process and oxide films. The samples are oxidized in a horizontal tube furnace at 830 oC and 900 oC for 2 h under O2. The samples are characterized using atomic force microscopy to determine root mean square roughness before and after oxidation. The oxide thickness for each sample is determined by spectroscopic ellipsometry. The AlInN layers with less indium produce smoother oxide layers, and the oxidation rate of the samples increases with increasing indium content. Energy dispersive X-ray spectroscopy of scanning transmission electron microscopy images of the oxide layers show the In collects on or near the surface of the oxide layer. Overall, the results indicate that oxides formed from AlInN layers with less indium produce smoother oxide films and are more suitable for device applications.}, journal={e-Prime - Advances in Electrical Engineering, Electronics and Energy}, author={Palmese, Elia and Xue, Haotian and Song, Renbo and Wierer, Jonathan J., Jr.}, year={2023}, month={Sep} } @article{fragkos_sun_borovac_song_wierer_tansu_2022, title={Delta InN-InGaN Quantum Wells With AlGaN Interlayers for Long Wavelength Emission}, volume={58}, ISSN={["1558-1713"]}, url={http://dx.doi.org/10.1109/jqe.2022.3142270}, DOI={10.1109/JQE.2022.3142270}, abstractNote={An active region design based on InGaN / delta-InN quantum well (QW) with AlGaN interlayer (IL) and GaN barriers (delta-structure) is investigated for potential high-efficiency visible light emitters. Numerical simulations demonstrate a large wavelength redshift with a simultaneous increase of the electron-hole wavefunction overlap for the delta-structure as compared to the conventional InGaN QW with AlGaN IL and GaN barriers. Proof of concept experimental growths via the metalorganic chemical vapor deposition demonstrate the effect of the delta-InN insertion into the conventional InGaN-based QW.}, number={2}, journal={IEEE JOURNAL OF QUANTUM ELECTRONICS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Fragkos, Ioannis E. and Sun, Wei and Borovac, Damir and Song, Renbo and Wierer, Jonathan J., Jr. and Tansu, Nelson}, year={2022}, month={Apr} } @article{wei_muyeed_xue_palmese_song_tansu_wierer_2022, title={Near-infrared electroluminescence of AlGaN capped InGaN quantum dots formed by controlled growth on photoelectrochemical etched quantum dot templates}, volume={10}, url={https://doi.org/10.1364/PRJ.441122}, DOI={10.1364/PRJ.441122}, abstractNote={Near-infrared electroluminescence of InGaN quantum dots (QDs) formed by controlled growth on photoelectrochemical (PEC) etched QD templates is demonstrated. The QD template consists of PEC InGaN QDs with high density and controlled sizes, an AlGaN capping layer to protect the QDs, and a GaN barrier layer to planarize the surface. Scanning transmission electron microscopy (STEM) of Stranski–Krastanov (SK) growth on the QD template shows high-In-content InGaN QDs that align vertically to the PEC QDs due to localized strain. A high-Al-content Al 0.9 Ga 0.1 N capping layer prevents the collapse of the SK QDs due to intermixing or decomposition during higher temperature GaN growth as verified by STEM. Growth of low-temperature (830°C) p-type layers is used to complete the p-n junction and further ensure QD integrity. Finally, electroluminescence shows a significant wavelength shift (800 nm to 500 nm), caused by the SK QDs’ tall height, high In content, and strong polarization-induced electric fields.}, number={1}, journal={Photonics Research}, publisher={The Optical Society}, author={Wei, Xiongliang and Muyeed, Syed Ahmed Al and Xue, Haotian and Palmese, Elia and Song, Renbo and Tansu, Nelson and Wierer, Jonathan J.}, year={2022}, month={Jan}, pages={33} } @article{muyeed_borovac_xue_wei_song_tansu_wierer_2021, title={Recombination Rates of InxGa1−xN/AlyGa1−yN/GaN Multiple Quantum Wells Emitting From 640 to 565 nm}, volume={57}, url={http://dx.doi.org/10.1109/jqe.2021.3111402}, DOI={10.1109/jqe.2021.3111402}, abstractNote={The recombination rates in an In0.25Ga0.75N/Al0.48Ga0.52N/GaN multiple quantum well (MQW) structure are measured to identify the cause of low efficiencies in high In-content InGaN quantum wells. The MQWs emit from 640 to 565 nm and are grown using metal-organic chemical vapor deposition (MOCVD). The addition of AlGaN interlayers within the MQW provides strain balancing and suppresses defect formation in high In-content InGaN quantum wells. The rates are found by transforming the optically measured radiative efficiency, differential carrier lifetimes, and optical absorption. Both components of non-radiative recombination rates, Shockley-Read-Hall (SRH) and Auger recombination, are found to be similar to the values of blue-emitting InGaN MQWs. The low SRH recombination rate is attributed to the use of the AlGaN interlayer. The radiative recombination rate, however, is more than an order of magnitude lower compared to blue emitting (lower In-content) InGaN-based MQWs. While this large reduction in radiative rate can be attributed to differences in carrier overlap and transition energy, it may also include effects of variations in thickness and compositional inhomogeneities.}, number={6}, journal={IEEE Journal of Quantum Electronics}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Muyeed, Syed Ahmed Al and Borovac, Damir and Xue, Haotian and Wei, Xiongliang and Song, Renbo and Tansu, Nelson and Wierer, Jonathan J.}, year={2021}, month={Dec}, pages={1–7} } @article{palmese_peart_borovac_song_tansu_wierer_2021, title={Thermal oxidation rates and resulting optical constants of Al0.83In0.17N films grown on GaN}, volume={129}, url={http://dx.doi.org/10.1063/5.0035711}, DOI={10.1063/5.0035711}, abstractNote={The thermal oxidation rates of Al0.83In0.17N layers grown lattice-matched to GaN and the oxide's optical constants are studied. The ∼230 nm thick AlInN layers are placed into a horizontal furnace at elevated temperatures and exposed to either O2 (dry) or H2O vapor with an N2 carrier gas (wet). The samples are oxidized at different temperatures (830–870 °C) and at a constant time or at various times at a constant temperature of 830 °C. Spectroscopic ellipsometry is used to determine the oxide thicknesses, refractive index, and extinction coefficients. The oxidation rate for the wet conditions is faster than for the dry conditions, and both increase with temperature, as expected. However, the oxidation rate is also dynamic with time and can be fitted with the Deal–Grove model, suggesting reaction rate and diffusion-limited processes like other more mature semiconductors. Finally, the dry conditions produce oxides that expand more than the oxides produced under wet conditions. The ability to thermally oxidize Al0.83In0.17N layers lattice-matched to GaN is a promising process technique for producing new III-nitride-based electronic devices.}, number={12}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Palmese, Elia and Peart, Matthew R. and Borovac, Damir and Song, Renbo and Tansu, Nelson and Wierer, Jonathan J.}, year={2021}, month={Mar}, pages={125105} } @article{peart_borovac_sun_song_tansu_wierer_2020, title={AlInN/GaN diodes for power electronic devices}, url={https://doi.org/10.35848/1882-0786/abb180}, DOI={10.35848/1882-0786/abb180}, abstractNote={AlInN/GaN power diodes consisting of a p-type GaN and a 300 nm thick n-type AlInN drift layer are demonstrated. The p–n junction is grown using metalorganic chemical vapor deposition, and the AlxIn1−xN drift layer is lattice-matched to GaN (x ∼ 0.82) with an electron concentration of ∼8 × 1016 cm−3 after correcting for the 2-dimensional electron gas. The diodes exhibit ∼−60 V blocking capability. Under forward bias, the diode has a turn-on voltage of ∼4 V. If experimental challenges are overcome, the ultrawide bandgap and high mobility of an AlInN drift layer could increase the performance of GaN-based power devices.}, journal={Applied Physics Express}, author={Peart, Matthew R. and Borovac, Damir and Sun, Wei and Song, Renbo and Tansu, Nelson and Wierer, Jonathan J., Jr.}, year={2020}, month={Sep} } @article{muyeed_wei_borovac_song_tansu_wierer_2020, title={Controlled growth of InGaN quantum dots on photoelectrochemically etched InGaN quantum dot templates}, url={https://doi.org/10.1016/j.jcrysgro.2020.125652}, DOI={10.1016/j.jcrysgro.2020.125652}, abstractNote={• InGaN quantum dot (QD) growth templates made by photoelectrochemical (PEC) etching. • Self-assembled (SA) InGaN QD growth is controlled using PEC InGaN QD templates. • SA QDs are smaller sized and higher density on PEC QDs compared to planar GaN. • Atomic force microscopy suggests the SA QDs are aligning to the underlying PEC QDs. • Multiple layers of QDs formed on PEC QD templates exhibit better photoluminescence. Controlled growth of InGaN quantum dots (QDs) using photoelectrochemically (PEC) etched InGaN QD templates is demonstrated. The InGaN QDs are grown by a self-assembly (SA) method using metal-organic chemical vapor deposition on templates consisting of planar GaN and PEC etched InGaN QDs for comparison. The InGaN QD templates are formed using quantum-size-controlled PEC etching of planar InGaN layers on GaN, which produces controlled QD radiuses with a statistical mean (μ) of 17.3 nm and standard deviation (σ) of 6.2 nm, and densities of 1.2 × 10 10 cm −2 . The PEC etched QDs are capped with an AlGaN interlayer and GaN barrier layer to recover a planar surface morphology for subsequent SA growth of QDs. The PEC QD templates behave as seeds via localize strain near the PEC QDs which provide improved control of the SA QD growth. The SA grown QDs on PEC QD templates are smaller and have controlled radiuses with μ = 21.7 nm and σ = 11.7 nm compared to the SA QDs on planar GaN templates with radiuses of μ = 37.8 nm and σ = 17.8 nm. Additionally, the dot densities of the SA QDs on PEC QD templates are ~3 times higher and more closely match the underlying densities of the template (8.1 × 10 9 cm −2 ). Multiple quantum dots (MQDs) are also grown on both templates that consist of 4 periods of SA QDs and AlGaN/GaN interlayer/barrier layers. The MQDs grown on PEC QD templates better retain their planarized smooth surfaces after barrier layer growth, and exhibit ~3 times stronger PL intensity at room temperature compared to MQDs grown on planar GaN.}, journal={Journal of Crystal Growth}, author={Muyeed, Syed Ahmed Al and Wei, Xiongliang and Borovac, Damir and Song, Renbo and Tansu, Nelson and Wierer, Jonathan J., Jr.}, year={2020}, month={Jun} } @article{peart_wierer_2020, title={Edge Termination for III-Nitride Vertical Power Devices Using Polarization Engineering}, url={http://dx.doi.org/10.1109/ted.2019.2958485}, DOI={10.1109/ted.2019.2958485}, abstractNote={A method for edge termination utilizing polarization-induced charge for GaN vertical power devices is presented. The polarization edge termination is simulated on a GaN power diode and consists of a 5-nm-thick n-type AlGaN layer on top of a p-type GaN layer that is located at the periphery of the main p-n junction. The spontaneous and piezoelectric polarization present in III-nitrides result in fixed charges at the AlGaN/GaN heterointerface, and the p-GaN layer becomes depleted at this interface under reverse bias. Numerical simulations show that this AlGaN/GaN heterointerface can be engineered to control the depletion region under reverse bias to prevent localization of electric fields and premature avalanche breakdown. Nearly parallel-plate reverse breakdown performance can be achieved. In addition, a simple analytical model based on charge balancing predicts the performance of this edge termination method.}, journal={IEEE Transactions on Electron Devices}, author={Peart, Matthew R. and Wierer, Jonathan J.}, year={2020}, month={Feb} } @article{electrical properties of mgo/gan metal-oxide-semiconductor structures_2020, url={http://dx.doi.org/10.1016/j.sse.2020.107881}, DOI={10.1016/j.sse.2020.107881}, abstractNote={Electrical properties of metal-oxide semiconductor (MOS) capacitors were measured with MgO/Al2O3 gate dielectrics deposited by atomic layer deposition (ALD) on GaN. For an Al2O3 (1 nm)/MgO (20 nm) dielectric layer, a leakage current density of 0.25 mA/cm2 at 1 V was measured for the MOS capacitor. A peak capacitance of ~0.1 μF/cm2 was obtained from the C-V measurements with significant hysteresis observed. In addition, a 15-minute forming gas anneal at 450 °C resulted in an increased leakage current density of 1 A/cm2 at 1 V while also increasing the peak capacitance by approximately 30%. To improve the performance, an Al2O3 (20 nm)/MgO (20 nm) dielectric stack was deposited that exhibited a leakage current density of ~1 × 10−5 mA/cm2 at 1 V, which corresponds to ~4 orders of magnitude lower current density than that of the single layer dielectric. Additionally, a 3-layer Al2O3 (10 nm)/MgO (20 nm)/Al2O3 (10 nm) stack also shows a leakage current density reduction of ~4 orders of magnitude, and a reduced density of interface states while remaining a high-k dielectric. The density of interface states was estimated to be between 6.8 × 1011 eV−1 cm−2 and 1.5 × 1012 eV−1 cm−2 for the 3-layer stack using the photo-assisted C-V method.}, journal={Solid-State Electronics}, year={2020}, month={Oct} } @article{borovac_sun_peart_song_wierer_tansu_2020, title={Low background doping in AlInN grown on GaN via metalorganic vapor phase epitaxy}, url={https://doi.org/10.1016/j.jcrysgro.2020.125847}, DOI={10.1016/j.jcrysgro.2020.125847}, abstractNote={Nearly lattice-matched and unintentionally doped AlInN films with low background doping grown via metalorganic vapor phase epitaxy on GaN/sapphire are investigated. The lattice-matched condition is verified with x-ray diffraction (XRD), and the films exhibit typical morphological characteristics for AlInN. The optical constants (nr & k) and thicknesses of the AlInN films are determined via spectroscopic ellipsometry, finding an nr ~ 2.2 at 500 nm and a bandgap of ~4.366 eV. Temperature-dependent Hall measurements in the Van der Pauw configuration are performed for temperatures from 80 K up to 350 K, and a low background doping concentration (n ~ 3 × 1017 cm−3) and high electron mobility (μe ~ 320 cm2/V∙s) are found at room temperature. Simulations are performed to determine the influence of the 2-D electron gas (2DEG) caused by polarization fields from the GaN/AlInN interface and validate the Hall measurements. Thus, this work shows the potential of achieving high-quality AlInN films with low background doping densities for use in power electronic devices and deep-ultraviolet light-emitting diodes.}, journal={Journal of Crystal Growth}, author={Borovac, Damir and Sun, Wei and Peart, Matthew R. and Song, Renbo and Wierer, Jonathan J., Jr. and Tansu, Nelson}, year={2020}, month={Oct} } @article{borovac_sun_song_wierer_tansu_2020, title={On the thermal stability of nearly lattice-matched AlInN films grown on GaN via MOVPE}, url={http://dx.doi.org/10.1016/j.jcrysgro.2019.125469}, DOI={10.1016/j.jcrysgro.2019.125469}, abstractNote={The thermal stability of nearly lattice-matched AlInN films grown via metalorganic vapor phase epitaxy on GaN/sapphire is investigated. The structural and morphological changes of the AlInN layers, as determined by x-ray diffraction (XRD) and atomic force microscopy (AFM), are studied when systematically varying annealing times, temperatures, and ambients to gain a better understanding of the temperature limits of the AlInN films. The samples are annealed either in the growth chamber with the same conditions (gases, pressure, and flow rates) as the original growth conditions of the AlInN samples, or in the XRD under N2. In general, the surface of the AlInN changes at temperatures >850 °C under growth and N2 conditions mostly likely due to a loss of In as determined by AFM. However, the bulk crystal structure of the AlInN remains stable up to temperatures of 950–1050 °C (depending on ambient) as determined by XRD. These findings provide a helpful guide for future experiments involving high-temperatures (790–1050 °C) for subsequent or transition layers during epitaxial growth, and for fabricating device structures employing AlInN layers.}, journal={Journal of Crystal Growth}, author={Borovac, Damir and Sun, Wei and Song, Renbo and Wierer, Jonathan J., Jr. and Tansu, Nelson}, year={2020}, month={Mar} } @article{surface pretreatment and deposition temperature dependence of mgo epitaxy on gan by thermal atomic layer deposition_2020, url={http://dx.doi.org/10.1016/j.jcrysgro.2020.125568}, DOI={10.1016/j.jcrysgro.2020.125568}, abstractNote={We report on the structural properties of MgO films deposited on GaN templates on sapphire substrates via atomic layer deposition (ALD). Analysis of the crystal quality and structure as a function of surface treatment and growth temperature are presented. Our results indicate deposition temperatures greater than 250 °C are preferable for achieving a high quality MgO thin film. Rotational scans of the samples show a six-fold symmetry at all deposition temperatures, indicating the existence of two rotational symmetric MgO crystal domains on the GaN surface, which were confirmed using electron backscatter diffraction.}, journal={Journal of Crystal Growth}, year={2020}, month={Apr} } @article{wierer_tansu_2019, title={III‐Nitride Micro‐LEDs for Efficient Emissive Displays}, url={http://dx.doi.org/10.1002/lpor.201900141}, DOI={10.1002/lpor.201900141}, abstractNote={Abstract}, journal={Laser & Photonics Reviews}, author={Wierer, Jonathan J., Jr. and Tansu, Nelson}, year={2019}, month={Aug} } @article{muyeed_sun_peart_lentz_wei_borovac_song_tansu_wierer_2019, title={Recombination rates in green-yellow InGaN-based multiple quantum wells with AlGaN interlayers}, url={http://dx.doi.org/10.1063/1.5126965}, DOI={10.1063/1.5126965}, abstractNote={The recombination rates in InGaN/AlGaN/GaN multiple quantum wells (MQWs) emitting in the green-yellow and grown with different Al compositions in the AlGaN interlayer (IL) are shown. By transforming measurements on radiative efficiency, absorption, and differential carrier lifetime, the radiative and nonradiative rates are determined. The IL Al composition controls lattice relaxation of the MQWs, as determined by X-ray reciprocal space mapping, and, therefore, defect formation. For the most pseudomorphic MQWs, the Shockley-Read-Hall (SRH) A coefficient is minimized and is similar to reports at shorter (blue and green) wavelengths. It is an order of magnitude smaller than a conventional InGaN/GaN MQW and is the most significant factor behind the improvement in radiative efficiency using the IL. The radiative B coefficient is also reduced and a minimum for the most pseudomorphic MQWs due to a reduction in the electron-hole wavefunction overlap. However, the decrease in A is more significant and leads to an overall improvement in the radiative efficiency. These recombination rate measurements confirm that if the SRH recombination is controlled, then the severe reduction of radiative recombination with an increased emitting wavelength is one of the main challenges in realizing high efficiency, long-wavelength InGaN-based MQW emitters operating at low to moderate current densities.}, journal={Journal of Applied Physics}, author={Muyeed, Syed Ahmed Al and Sun, Wei and Peart, Matthew R. and Lentz, Rebecca M. and Wei, Xiongliang and Borovac, Damir and Song, Renbo and Tansu, Nelson and Wierer, Jonathan J., Jr.}, year={2019}, month={Dec} } @article{thermal oxidation of alinn for iii-nitride electronic and optoelectronic devices_2019, url={http://dx.doi.org/10.1021/acsaelm.9b00266}, DOI={10.1021/acsaelm.9b00266}, abstractNote={The oxidation of semiconductors is a fundamental building block of many modern electronic devices. The prime example is the oxidation of silicon into silicon dioxide, which is used as a gate dielectric, waveguides, masking layer, and a device isolation layer. The ability to form an analogous stable and insulating oxide in III-nitride semiconductors would enable a new generation of III-nitride-based electronic and optoelectronic devices. Here we present data on the conversion of thick (>100 nm) AlInN epitaxial layers into oxides with H2O vapor in an N2 carrier gas (wet oxidation) at elevated temperatures (900 °C). The AlxIn1-xN layers are grown on and latticed matched (x=0.82) to GaN layers. The oxide can be formed over its entirety or selectively by patterning the surface. The conversion to a oxide is confirmed and characterized by atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, spectroscopic ellipsometry, and electrical measurements. The oxide i...}, journal={ACS Applied Electronic Materials}, year={2019}, month={Aug} } @article{peart_tansu_wierer_2018, title={AlInN for Vertical Power Electronic Devices}, url={http://dx.doi.org/10.1109/ted.2018.2866980}, DOI={10.1109/ted.2018.2866980}, abstractNote={The known benefits and challenges of AlInN as a next-generation power electronic semiconductor are presented. Al_xIn1_−xN is lattice matched to GaN at

${x} = {0.82}$

and has the advantages of an available substrate, a wide bandgap (~4.4 eV), and high mobility (~450 cm²

$/\text {V} \cdot \text {s}$

). The power figure of merit (FOM), determined using empirical and theoretical values of mobility and estimated critical electric fields determined from reported bandgaps, spans from ~20% to 130% times greater than GaN. In order to realize and precisely determine these high AlInN FOM values, experimental challenges will need to be overcome such as polarization-induced electric fields and bandgap discontinuities at AlInN/GaN interfaces, and controlling carrier concentration levels.}, journal={IEEE Transactions on Electron Devices}, author={Peart, Matthew R. and Tansu, Nelson and Wierer, Jonathan J.}, year={2018}, month={Oct} } @article{sun_al muyeed_song_wierer_tansu_2018, title={Integrating AlInN interlayers into InGaN/GaN multiple quantum wells for enhanced green emission}, volume={112}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000432553900006&KeyUID=WOS:000432553900006}, DOI={10.1063/1.5028257}, abstractNote={Significant enhancement in green emission by integrating a thin AlInN barrier layer, or interlayer (IL), in an InGaN/GaN multiple quantum well (MQW) is demonstrated. The MQWs investigated here contains 5 periods of an InGaN QW, a 1 nm thick AlInN IL, and a 10 nm thick GaN barrier grown by metalorganic chemical vapor deposition. To accommodate the optimum low-pressure (20 Torr) growth of the AlInN layer a growth flow sequence with changing pressure is devised. The AlInN IL MQWs are compared to InGaN/AlGaN/GaN MQWs (AlGaN IL MQWs) and conventional InGaN/GaN MQWs. The AlInN IL MQWs provide benefits that are similar to AlGaN ILs, by aiding in the formation of abrupt heterointerfaces as indicated by X-ray diffraction omega-2theta (ω-2θ) scans, and also efficiency improvements due to high temperature annealing schedules during barrier growth. Room temperature photoluminescence of the MQW with AlInN ILs shows similar performance to MQWs with AlGaN ILs, and ∼4–7 times larger radiative efficiency (pump intensity dependent) at green wavelengths than conventional InGaN/GaN MQWs. This study shows the InGaN-based MQWs with AlInN ILs are capable of achieving superior performance to conventional InGaN MQWs emitting at green wavelengths.}, number={20}, journal={Applied Physics Letters}, author={Sun, W. and Al Muyeed, S. A. and Song, R. B. and Wierer, J. J. and Tansu, N.}, year={2018}, pages={5} } @article{room temperature luminescence of passivated ingan quantum dots formed by quantum-sized-controlled photoelectrochemical etching_2018, url={http://dx.doi.org/10.1063/1.5046857}, DOI={10.1063/1.5046857}, abstractNote={Room temperature luminescence of epitaxial InGaN quantum dots (QDs) formed by quantum sized-controlled photoelectrochemical (QSC-PEC) etching and passivation layer regrowth is demonstrated. QSC-PEC etching is performed on a 7.5 nm thick In0.20Ga0.80N layer emitting at ∼514–521 nm and with a laser diode emitting at 445 nm. Parameters such as etch bias (0.9 V and 1.5 V), laser average power (20 mW/cm2 and 100 mW/cm2), and laser operating conditions (pulsed and continuous wave) are explored. QSC-PEC etching of In0.20Ga0.80N requires a minimum bias (>0.9 V) and pulsed laser conditions in order to form QDs. After etching, the QDs do not exhibit photoluminescence due to defect recombination. Regrowth of passivation layers consisting of a 2 nm thick Al0.45Ga0.55N layer and a 11 nm thick GaN layer reduce the defect recombination, and room temperature photoluminescence is observed at room temperature at ∼435–445 nm with narrow full-width at half-maximum of ∼35 nm.}, journal={Applied Physics Letters}, year={2018}, month={Sep} } @article{sun_tan_wierer_tansu_2018, title={Ultra-Broadband Optical Gain in III-Nitride Digital Alloys}, volume={8}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000425190500013&KeyUID=WOS:000425190500013}, DOI={10.1038/s41598-018-21434-6}, abstractNote={Abstract}, journal={Scientific Reports}, author={Sun, W. and Tan, C. K. and Wierer, J. J. and Tansu, N.}, year={2018}, pages={7} } @article{tan_sun_wierer_tansu_2017, title={Effect of interface roughness on Auger recombination in semiconductor quantum wells}, volume={7}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000397862300056&KeyUID=WOS:000397862300056}, DOI={10.1063/1.4978777}, abstractNote={Auger recombination in a semiconductor is a three-carrier process, wherein the energy from the recombination of an electron and hole pair promotes a third carrier to a higher energy state. In semiconductor quantum wells with increased carrier densities, the Auger recombination becomes an appreciable fraction of the total recombination rate and degrades luminescence efficiency. Gaining insight into the variables that influence Auger recombination in semiconductor quantum wells could lead to further advances in optoelectronic and electronic devices. Here we demonstrate the important role that interface roughness has on Auger recombination within quantum wells. Our computational studies find that as the ratio of interface roughness to quantum well thickness is increased, Auger recombination is significantly enhanced. Specifically, when considering a realistic interface roughness for an InGaN quantum well, the enhancement in Auger recombination rate over a quantum well with perfect heterointerfaces can be approximately four orders of magnitude.}, number={3}, journal={Aip Advances}, author={Tan, C. K. and Sun, W. and Wierer, J. J. and Tansu, N.}, year={2017}, pages={8} } @article{feezell_wierer_chowdhury_shen_2017, title={Nitride Semiconductors Preface}, volume={214}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000407683800051&KeyUID=WOS:000407683800051}, DOI={10.1002/pssa.201770146}, number={8}, journal={Physica Status Solidi a-Applications and Materials Science}, author={Feezell, D. and Wierer, J. and Chowdhury, S. and Shen, S. C.}, year={2017}, pages={2} } @article{wierer_dickerson_allerman_armstrong_crawford_kaplar_2017, title={Simulations of Junction Termination Extensions in Vertical GaN Power Diodes}, volume={64}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000399935800059&KeyUID=WOS:000399935800059}, DOI={10.1109/ted.2017.2684093}, abstractNote={Simulations of reverse breakdown behavior of GaN power diodes with junction termination extensions (JTEs) are presented. The p-type JTE is located at the edge of the main p-n-junction, and under reverse bias, the charge in the JTE causes spreading and reduction of the peak electric fields to avoid premature avalanche breakdown. To determine the available charge in the JTE, it is shown that the electric field under reverse bias causes severe band bending within the JTE and full ionization of the Mg acceptor. Therefore, all the Mg dopants contribute charge and determine the performance of the JTE. The dependence of the breakdown voltage on the JTE’s acceptor concentration and thickness is shown. When the JTE is properly designed, the simulations show improved reverse breakdown behavior and breakdown efficiencies approaching 98% of the ideal limit for planar geometry. Finally, the challenges of creating JTEs within GaN power diodes are discussed.}, number={5}, journal={Ieee Transactions on Electron Devices}, author={Wierer, Jonathan J. and Dickerson, Jeramy R. and Allerman, Andrew A. and Armstrong, Andrew M. and Crawford, Mary H. and Kaplar, Robert J.}, year={2017}, pages={2291–2297} } @article{al muyeed_sun_wei_song_koleske_tansu_wierer_2017, title={Strain compensation in InGaN-based multiple quantum wells using AlGaN interlayers}, volume={7}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000414246100084&KeyUID=WOS:000414246100084}, DOI={10.1063/1.5000519}, abstractNote={Data are presented on strain compensation in InGaN-based multiple quantum wells (MQW) using AlGaN interlayers (ILs). The MQWs consist of five periods of InxGa1-xN/AlyGa1-yN/GaN emitting in the green (λ ∼ 535 nm ± 15 nm), and the AlyGa1-yN IL has an Al composition of y = 0.42. The IL is varied from 0 - 2.1 nm, and the relaxation of the MQW with respect to the GaN template layer varies with IL thickness as determined by reciprocal space mapping about the (202¯5) reflection. The minimum in the relaxation occurs at an interlayer thickness of 1 nm, and the MQW is nearly pseudomorphic to GaN. Both thinner and thicker ILs display increased relaxation. Photoluminescence data shows enhanced spectral intensity and narrower full width at half maximum for the MQW with 1 nm thick ILs, which is a product of pseudomorphic layers with lower defect density and non-radiative recombination.}, number={10}, journal={Aip Advances}, author={Al Muyeed, S. A. and Sun, W. and Wei, X. L. and Song, R. B. and Koleske, D. D. and Tansu, N. and Wierer, J. J.}, year={2017}, pages={7} } @inbook{ultra-efficient solid-state lighting: likely characteristics, economic benefits, technological approaches_2017, booktitle={Springer}, year={2017} } @article{allerman_armstrong_fischer_dickerson_crawford_king_moseley_wierer_kaplar_2016, title={Al0.3Ga0.7N PN diode with breakdown voltage > 1600 V}, volume={52}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000381001600024&KeyUID=WOS:000381001600024}, DOI={10.1049/el.2016.1280}, abstractNote={Demonstration of Al0.3Ga0.7N PN diodes grown with breakdown voltages in excess of 1600 V is reported. The total epilayer thickness is 9.1 μm and was grown by metal-organic vapour-phase epitaxy on 1.3-mm-thick sapphire in order to achieve crack-free structures. A junction termination edge structure was employed to control the lateral electric fields. A current density of 3.5 kA/cm2 was achieved under DC forward bias and a reverse leakage current <3 nA was measured for voltages <1200 V. The differential on-resistance of 16 mΩ cm2 is limited by the lateral conductivity of the n-type contact layer required by the front-surface contact geometry of the device. An effective critical electric field of 5.9 MV/cm was determined from the epilayer properties and the reverse current–voltage characteristics. To our knowledge, this is the first aluminium gallium nitride (AlGaN)-based PN diode exhibiting a breakdown voltage in excess of 1 kV. It is noted that a Baliga figure of merit (V br 2/R spec,on) of 150 MW/cm2 found is the highest reported for an AlGaN PN diode and illustrates the potential of larger-bandgap AlGaN alloys for high-voltage devices.}, number={15}, journal={Electronics Letters}, author={Allerman, A. A. and Armstrong, A. M. and Fischer, A. J. and Dickerson, J. R. and Crawford, M. H. and King, M. P. and Moseley, M. W. and Wierer, J. J. and Kaplar, R. J.}, year={2016}, pages={1319–1320} } @article{armstrong_allerman_fischer_king_heukelom_moseley_kaplar_wierer_crawford_dickerson_2016, title={High voltage and high current density vertical GaN power diodes}, volume={52}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000378886300044&KeyUID=WOS:000378886300044}, DOI={10.1049/el.2016.1156}, abstractNote={We report on the realization of a GaN high voltage vertical p-n diode operating at > 3.9 kV breakdown with a specific on-resistance 1.4 kA/cm2. An effective critical electric field of 3.9 MV/cm was estimated for the devices from analysis of the forward and reverse current-voltage characteristics. Furthermore this suggests that the fundamental limit to the GaN critical electric field is significantly greater than previously believed.}, number={13}, journal={Electronics Letters}, author={Armstrong, A. M. and Allerman, A. A. and Fischer, A. J. and King, M. P. and Heukelom, M. S. and Moseley, M. W. and Kaplar, R. J. and Wierer, J. J. and Crawford, M. H. and Dickerson, J. R.}, year={2016}, pages={1170–1171} } @article{wierer_tansu_fischer_tsao_2016, title={III-nitride quantum dots for ultra-efficient solid-state lighting}, volume={10}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000379958800006&KeyUID=WOS:000379958800006}, DOI={10.1002/lpor.201500332}, abstractNote={Abstract}, number={4}, journal={Laser & Photonics Reviews}, author={Wierer, J. J. and Tansu, N. and Fischer, A. J. and Tsao, J. Y.}, year={2016}, pages={612–622} } @article{dickerson_allerman_bryant_fischer_king_moseley_armstrong_kaplar_kizilyalli_aktas_et al._2016, title={Vertical GaN Power Diodes With a Bilayer Edge Termination}, volume={63}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000367259600056&KeyUID=WOS:000367259600056}, DOI={10.1109/ted.2015.2502186}, abstractNote={Vertical GaN power diodes with a bilayer edge termination (ET) are demonstrated. The GaN p-n junction is formed on a low threading dislocation defect density (104 - 105 cm-2) GaN substrate, and has a 15-μm-thick n-type drift layer with a free carrier concentration of 5 × 1015 cm-3. The ET structure is formed by N implantation into the p+-GaN epilayer just outside the p-type contact to create compensating defects. The implant defect profile may be approximated by a bilayer structure consisting of a fully compensated layer near the surface, followed by a 90% compensated (p) layer near the n-type drift region. These devices exhibit avalanche breakdown as high as 2.6 kV at room temperature. Simulations show that the ET created by implantation is an effective way to laterally distribute the electric field over a large area. This increases the voltage at which impact ionization occurs and leads to the observed higher breakdown voltages.}, number={1}, journal={Ieee Transactions on Electron Devices}, author={Dickerson, J. R. and Allerman, A. A. and Bryant, B. N. and Fischer, A. J. and King, M. P. and Moseley, M. W. and Armstrong, A. M. and Kaplar, R. J. and Kizilyalli, I. C. and Aktas, O. and et al.}, year={2016}, pages={419–425} } @article{wierer_tsao_2015, title={Advantages of III-nitride laser diodes in solid-state lighting}, volume={212}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000354405000014&KeyUID=WOS:000354405000014}, DOI={10.1002/pssa.201431700}, abstractNote={III‐nitride laser diodes (LDs) are an interesting light source for solid‐state lighting (SSL). Modelling of LDs is performed to reveal the potential advantages over traditionally used light‐emitting diodes (LEDs). The first, and most notable, advantage is LDs have higher efficiency at higher currents when compared to LEDs. This is because Auger recombination that causes efficiency droop can no longer grow after laser threshold. Second, the same phosphor‐converted methods used with LEDs can also be used with LDs to produce white light with similar color rendering and color temperature. Third, producing white light from color mixed emitters is equally challenging for both LEDs and LDs, with neither source having a direct advantage. Fourth, the LD emission is directional and can be more readily captured and focused, leading to the possibility of novel and more compact luminaires. Finally, the smaller area and higher current density operation of LDs provides them with a potential cost advantage over LEDs. These advantages make LDs a compelling source for future SSL.}, number={5}, journal={Physica Status Solidi a-Applications and Materials Science}, author={Wierer, Jonathan J., Jr. and Tsao, Jeffrey Y.}, year={2015}, pages={980–985} } @article{moseley_allerman_crawford_wierer_smith_biedermann_2015, title={Defect-enabled electrical current leakage in ultraviolet light-emitting diodes}, volume={212}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000352820100002&KeyUID=WOS:000352820100002}, DOI={10.1002/pssa.201400182}, abstractNote={Electrical current leakage paths in AlGaN‐based ultraviolet (UV) light‐emitting diodes (LEDs) are identified using conductive atomic force microscopy. Open‐core threading dislocations are found to conduct current through insulating Al0.7Ga0.3N layers. A defect‐sensitive H3PO4 etch reveals these open‐core threading dislocations as 1–2 µm wide hexagonal etch pits visible with optical microscopy. Additionally, closed‐core threading dislocations are decorated with smaller and more numerous nanometer‐scale pits, which are quantifiable by atomic‐force microscopy. The performances of UV‐LEDs fabricated on similar Si‐doped Al0.7Ga0.3N templates are found to have a strong correlation to the density of these electrically conductive open‐core dislocations, while the total threading dislocation densities of the UV‐LEDs remain relatively unchanged.}, number={4}, journal={Physica Status Solidi a-Applications and Materials Science}, author={Moseley, Michael W. and Allerman, Andrew A. and Crawford, Mary H. and Wierer, Jonathan J., Jr. and Smith, Michael L. and Biedermann, Laura B.}, year={2015}, pages={723–726} } @article{armstrong_bryant_crawford_koleske_lee_wierer_2015, title={Defect-reduction mechanism for improving radiative efficiency in InGaN/GaN light-emitting diodes using InGaN underlayers}, volume={117}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000352645100037&KeyUID=WOS:000352645100037}, DOI={10.1063/1.4916727}, abstractNote={The influence of a dilute InxGa1-xN (x ∼ 0.03) underlayer (UL) grown below a single In0.16Ga0.84N quantum well (SQW), within a light-emitting diode (LED), on the radiative efficiency and deep level defect properties was studied using differential carrier lifetime (DCL) measurements and deep level optical spectroscopy (DLOS). DCL measurements found that inclusion of the UL significantly improved LED radiative efficiency. At low current densities, the non-radiative recombination rate of the LED with an UL was found to be 3.9 times lower than the LED without an UL, while the radiative recombination rates were nearly identical. This suggests that the improved radiative efficiency resulted from reduced non-radiative defect concentration within the SQW. DLOS measurement found the same type of defects in the InGaN SQWs with and without ULs. However, lighted capacitance-voltage measurements of the LEDs revealed a 3.4 times reduction in a SQW-related near-mid-gap defect state for the LED with an UL. Quantitative agreement in the reduction of both the non-radiative recombination rate (3.9×) and deep level density (3.4×) upon insertion of an UL corroborates deep level defect reduction as the mechanism for improved LED efficiency.}, number={13}, journal={Journal of Applied Physics}, author={Armstrong, Andrew M. and Bryant, Benjamin N. and Crawford, Mary H. and Koleske, Daniel D. and Lee, Stephen R. and Wierer, Jonathan J., Jr.}, year={2015} } @article{moseley_allerman_crawford_wierer_smith_armstrong_2015, title={Detection and modeling of leakage current in AlGaN-based deep ultraviolet light-emitting diodes}, volume={117}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000351134400036&KeyUID=WOS:000351134400036}, DOI={10.1063/1.4908543}, abstractNote={Current-voltage (IV) characteristics of two AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) with differing densities of open-core threading dislocations (nanopipes) are analyzed. A three-diode circuit is simulated to emulate the forward-bias IV characteristics of the DUV-LEDs, but is only able to accurately model the lower leakage current, lower nanopipe density DUV-LED. It was found that current leakage through the nanopipes in these structures is rectifying, despite nanopipes being previously established as inherently n-type. Using defect-sensitive etching, the nanopipes are revealed to terminate within the p-type GaN capping layer of the DUV-LEDs. The circuit model is modified to account for another p-n junction between the n-type nanopipes and the p-type GaN, and an excellent fit to the forward-bias IV characteristics of the leaky DUV-LED is achieved.}, number={9}, journal={Journal of Applied Physics}, author={Moseley, Michael W. and Allerman, Andrew A. and Crawford, Mary H. and Wierer, Jonathan J., Jr. and Smith, Michael L. and Armstrong, Andrew M.}, year={2015} } @article{armstrong_moseley_allerman_crawford_wierer_2015, title={Growth temperature dependence of Si doping efficiency and compensating deep level defect incorporation in Al0.7Ga0.3N}, volume={117}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000354984500042&KeyUID=WOS:000354984500042}, DOI={10.1063/1.4920926}, abstractNote={The growth temperature dependence of Si doping efficiency and deep level defect formation was investigated for n-type Al0.7Ga0.3N. It was observed that dopant compensation was greatly reduced with reduced growth temperature. Deep level optical spectroscopy and lighted capacitance-voltage were used to understand the role of acceptor-like deep level defects on doping efficiency. Deep level defects were observed at 2.34 eV, 3.56 eV, and 4.74 eV below the conduction band minimum. The latter two deep levels were identified as the major compensators because the reduction in their concentrations at reduced growth temperature correlated closely with the concomitant increase in free electron concentration. Possible mechanisms for the strong growth temperature dependence of deep level formation are considered, including thermodynamically driven compensating defect formation that can arise for a semiconductor with very large band gap energy, such as Al0.7Ga0.3N.}, number={18}, journal={Journal of Applied Physics}, author={Armstrong, Andrew M. and Moseley, Michael W. and Allerman, Andrew A. and Crawford, Mary H. and Wierer, Jonathan J., Jr.}, year={2015} } @article{koleske_fischer_bryant_kotula_wierer_2015, title={On the increased efficiency in InGaN-based multiple quantum wells emitting at 530-590 nm with AlGaN interlayers}, volume={415}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000349603500011&KeyUID=WOS:000349603500011}, DOI={10.1016/j.jcrysgro.2014.12.034}, abstractNote={InGaN/AlGaN/GaN-based multiple quantum wells (MQWs) with AlGaN interlayers (ILs) are investigated, specifically to examine the fundamental mechanisms behind their increased radiative efficiency at wavelengths of 530–590 nm. The AlzGa1−zN (z~0.38) IL is ~1–2 nm thick, and is grown after and at the same growth temperature as the ~3 nm thick InGaN quantum well (QW). This is followed by an increase in temperature for the growth of a ~10 nm thick GaN barrier layer. The insertion of the AlGaN IL within the MQW provides various benefits. First, the AlGaN IL allows for growth of the InxGa1−xN QW well below typical growth temperatures to achieve higher x (up to~0.25). Second, annealing the IL capped QW prior to the GaN barrier growth improves the AlGaN IL smoothness as determined by atomic force microscopy, improves the InGaN/AlGaN/GaN interface quality as determined from scanning transmission electron microscope images and x-ray diffraction, and increases the radiative efficiency by reducing non-radiative defects as determined by time-resolved photoluminescence measurements. Finally, the AlGaN IL increases the spontaneous and piezoelectric polarization induced electric fields acting on the InGaN QW, providing an additional red-shift to the emission wavelength as determined by Schrodinger-Poisson modeling and fitting to the experimental data. The relative impact of increased indium concentration and polarization fields on the radiative efficiency of MQWs with AlGaN ILs is explored along with implications to conventional longer wavelength emitters.}, journal={Journal of Crystal Growth}, author={Koleske, D. D. and Fischer, A. J. and Bryant, B. N. and Kotula, P. G. and Wierer, J. J.}, year={2015}, pages={57–64} } @article{king_armstrong_dickerson_vizkelethy_fleming_campbell_wampler_kizilyalli_bour_aktas_et al._2015, title={Performance and Breakdown Characteristics of Irradiated Vertical Power GaN P-i-N Diodes}, volume={62}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000367732600074&KeyUID=WOS:000367732600074}, DOI={10.1109/tns.2015.2480071}, abstractNote={Electrical performance and defect characterization of vertical GaN P-i-N diodes before and after irradiation with 2.5 MeV protons and neutrons is investigated. Devices exhibit increase in specific on-resistance following irradiation with protons and neutrons, indicating displacement damage introduces defects into the p-GaN and n- drift regions of the device that impact on-state device performance. The breakdown voltage of these devices, initially above 1700 V, is observed to decrease only slightly for particle fluence <; 1013 cm-2. The unipolar figure of merit for power devices indicates that while the on-resistance and breakdown voltage degrade with irradiation, vertical GaN P-i-Ns remain superior to the performance of the best available, unirradiated silicon devices and on-par with unirradiated modern SiC-based power devices.}, number={6}, journal={Ieee Transactions on Nuclear Science}, author={King, M. P. and Armstrong, A. M. and Dickerson, J. R. and Vizkelethy, G. and Fleming, R. M. and Campbell, J. and Wampler, W. R. and Kizilyalli, I. C. and Bour, D. P. and Aktas, O. and et al.}, year={2015}, pages={2912–2918} } @article{wierer_allerman_skogen_tauke-pedretti_vawter_montano_2015, title={Selective layer disordering in intersubband Al0.028Ga0.972N/AlN superlattices with silicon nitride capping layer}, volume={8}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000358071400004&KeyUID=WOS:000358071400004}, DOI={10.7567/APEX.8.061004}, abstractNote={Selective layer disordering in an intersubband Al0.028Ga0.972N/AlN superlattice using a silicon nitride (SiNx) capping layer is demonstrated. The SiNx capped superlattice exhibits suppressed layer disordering under high-temperature annealing. Additionally, the rate of layer disordering is reduced with increased SiNx thickness. The layer disordering is caused by Si diffusion, and the SiNx layer inhibits vacancy formation at the crystal surface and ultimately, the movement of Al and Ga atoms across the heterointerfaces. Patterning of the SiNx layer results in selective layer disordering, an attractive method to integrate active and passive III–nitride-based intersubband devices.}, number={6}, journal={Applied Physics Express}, author={Wierer, Jonathan J., Jr. and Allerman, Andrew A. and Skogen, Erik J. and Tauke-Pedretti, Anna and Vawter, Gregory A. and Montano, Ines}, year={2015} } @article{koleske_wierer_fischer_lee_2014, title={Controlling indium incorporation in InGaN barriers with dilute hydrogen flows}, volume={390}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000335770000007&KeyUID=WOS:000335770000007}, DOI={10.1016/j.jcrysgro.2013.12.037}, abstractNote={InxGa1−xN multiple quantum wells (MQWs) with InyGa1−yN barriers were grown by adding dilute hydrogen flows to the QW growth conditions in order to modify the barrier indium composition. With the H2 flow off, the indium concentration in the InxGa1−xN QWs were x=0.183, x=0.163, and x=0.096 for growth temperatures of 730, 750 and 770 °C respective. Using these same QW growth conditions, the H2 flow was increased up to 3 SLM resulting in a gradual decrease in the indium concentration in the InyGa1−yN barriers. Kinetic analysis suggests that hydrogen enhances indium surface desorption through the formation of more volatile indium-hydride species, thereby decreasing the surface indium concentration available for incorporation into the InyGa1−yN barriers. For the MQW structures grown at 750 °C, the photoluminescence (PL) wavelength blue-shifts from 477 to ~450 nm as the indium in the InyGa1−yN barriers increases as expected from the reduced influence of the piezoelectric fields. While a corresponding increase of spontaneous emission from the increasing overlap of electron states within the QW is also expected, the PL intensity of the QW instead decreases. This conflicting expectation of increased spontaneous emission and the observation of decreased PL intensity suggest either decreases in the QW-barrier height or increases in non-radiative defects offset any efficiency gains obtained when InGaN barriers to reduce polarization fields within the QWs.}, journal={Journal of Crystal Growth}, author={Koleske, D. D. and Wierer, J. J. and Fischer, A. J. and Lee, S. R.}, year={2014}, pages={38–45} } @article{wierer_montano_crawford_allerman_2014, title={Effect of thickness and carrier density on the optical polarization of Al0.44Ga0.56N/Al0.55Ga0.45N quantum well layers}, volume={115}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000335643700647&KeyUID=WOS:000335643700647}, DOI={10.1063/1.4874739}, abstractNote={The thickness and carrier density of AlGaN quantum well (QW) layers have a strong influence on the valence subband structure, and the resulting optical polarization and light extraction of ultraviolet light-emitting diodes. An ultraviolet-emitting (270–280 nm) multiple quantum well heterostructure consisting of 3 periods of Al0.44Ga0.56N/Al0.55Ga0.45N with individual layer thicknesses between 2–3.2 nm is studied both experimentally and theoretically. The optical polarization changes to preferentially polarized perpendicular to the QW plane as the QW thickness increases or the carrier density increases. Calculations show these trends are due to (a) a larger decrease in overlap of conduction band to light and heavy hole envelope functions compared to crystal-field split-off envelope functions, and (b) coupling between the valence subbands where higher heavy hole subbands couple to lower light hole and crystal-field split-off subbands. These changes in the valence band have a profound effect on the optical polarization, emission patterns, and eventual light extraction for ultraviolet emitters at these compositions and thicknesses, and need to be controlled to ensure high device efficiency.}, number={17}, journal={Journal of Applied Physics}, author={Wierer, J. J. and Montano, I. and Crawford, M. H. and Allerman, A. A.}, year={2014}, pages={10} } @article{moseley_allerman_crawford_wierer_smith_biedermann_2014, title={Electrical current leakage and open-core threading dislocations in AlGaN-based deep ultraviolet light-emitting diodes}, volume={116}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=INSPEC&KeyUT=INSPEC:14482172&KeyUID=INSPEC:14482172}, DOI={10.1063/1.4891830}, abstractNote={Electrical current transport through leakage paths in AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) and their effect on LED performance are investigated. Open-core threading dislocations, or nanopipes, are found to conduct current through nominally insulating Al0.7Ga0.3N layers and limit the performance of DUV-LEDs. A defect-sensitive phosphoric acid etch reveals these open-core threading dislocations in the form of large, micron-scale hexagonal etch pits visible with optical microscopy, while closed-core screw-, edge-, and mixed-type threading dislocations are represented by smaller and more numerous nanometer-scale pits visible by atomic-force microscopy. The electrical and optical performances of DUV-LEDs fabricated on similar Si-doped Al0.7Ga0.3N templates are found to have a strong correlation to the density of these nanopipes, despite their small fraction (<0.1% in this study) of the total density of threading dislocations.}, number={5}, journal={Journal of Applied Physics}, author={Moseley, M. and Allerman, A. and Crawford, M. and Wierer, J. J., Jr. and Smith, M. and Biedermann, L.}, year={2014}, pages={053104 (7 pp.)} } @article{coltrin_armstrong_brener_chow_crawford_fischer_kelley_koleske_lauhon_martin_et al._2014, title={Energy Frontier Research Center for Solid-State Lighting Science: Exploring New Materials Architectures and Light Emission Phenomena}, volume={118}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000338184300002&KeyUID=WOS:000338184300002}, DOI={10.1021/jp501136j}, abstractNote={The Energy Frontier Research Center (EFRC) for Solid-State Lighting Science (SSLS) is one of 46 EFRCs initiated in 2009 to conduct basic and use-inspired research relevant to energy technologies. The overarching theme of the SSLS EFRC is the exploration of energy conversion in tailored photonic structures. In this article we review highlights from the research of the SSLS EFRC. Major research themes include: studies of the materials properties and emission characteristics of III-nitride semiconductor nanowires; development of new phosphors and II–VI quantum dots for use as wavelength downconverters; fundamental understanding of competing radiative and nonradiative processes in current-generation, planar light-emitting diode architectures; understanding of the electrical, optical, and structural properties of defects in InGaN materials and heterostructures; exploring ways to enhance spontaneous emission through modification of the environment in which the emission takes place; and investigating routes such...}, number={25}, journal={Journal of Physical Chemistry C}, author={Coltrin, Michael E. and Armstrong, Andrew M. and Brener, Igal and Chow, Weng W. and Crawford, Mary H. and Fischer, Arthur J. and Kelley, David F. and Koleske, Daniel D. and Lauhon, Lincoln J. and Martin, James E. and et al.}, year={2014}, pages={13330–13345} } @article{wierer_allerman_montano_moseley_2014, title={Influence of optical polarization on the improvement of light extraction efficiency from reflective scattering structures in AlGaN ultraviolet light-emitting diodes}, volume={105}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=INSPEC&KeyUT=INSPEC:14502570&KeyUID=INSPEC:14502570}, DOI={10.1063/1.4892974}, abstractNote={The improvement in light extraction efficiency from reflective scattering structures in AlGaN ultraviolet light-emitting diodes (UVLEDs) emitting at ∼270 nm is shown to be influenced by optical polarization. Three UVLEDs with different reflective scattering structures are investigated and compared to standard UVLEDs without scattering structures. The optical polarization and therefore the direction of light propagation within the various UVLEDs are altered by changes in the quantum well (QW) thickness. The improvement in light extraction efficiency of the UVLEDs with reflective scattering structures increases, compared to the UVLEDs without scattering structures, as the fraction of emitted light propagating parallel to the QW plane increases. Additionally, the light extraction efficiency increases as the average distance to the reflective scattering structures decreases.}, number={6}, journal={Applied Physics Letters}, author={Wierer, J. J., Jr. and Allerman, A. A. and Montano, I. and Moseley, M. W.}, year={2014}, pages={061106 (4 pp.)} } @article{wierer_allerman_skogen_tauke-pedretti_alford_vawter_montano_2014, title={Layer disordering and doping compensation of an intersubband AlGaN/AlN superlattice by silicon implantation}, volume={105}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=INSPEC&KeyUT=INSPEC:14615894&KeyUID=INSPEC:14615894}, DOI={10.1063/1.4896783}, abstractNote={Layer disordering and doping compensation of an Al0.028Ga0.972N/AlN superlattice by implantation are demonstrated. The as-grown sample exhibits intersubband absorption at ∼1.56 μm which is modified when subject to a silicon implantation. After implantation, the intersubband absorption decreases and shifts to longer wavelengths. Also, with increasing implant dose, the intersubband absorption decreases. It is shown that both layer disordering of the heterointerfaces and doping compensation from the vacancies produced during the implantation cause the changes in the intersubband absorption. Such a method is useful for removing absorption in spatially defined areas of III-nitride optoelectronic devices by, for example, creating low-loss optical waveguides monolithically that can be integrated with as-grown areas operating as electro-absorption intersubband modulators.}, number={13}, journal={Applied Physics Letters}, author={Wierer, J.J., Jr. and Allerman, A.A. and Skogen, E.J. and Tauke-Pedretti, A. and Alford, C. and Vawter, G.A. and Montano, I.}, year={2014}, pages={131107 (4 pp.)} } @article{benz_campione_moseley_wierer_allerman_wendt_brener_2014, title={Optical Strong Coupling between near-Infrared Metamaterials and Intersubband Transitions in III-Nitride Heterostructures}, volume={1}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000343276800001&KeyUID=WOS:000343276800001}, DOI={10.1021/ph500192v}, abstractNote={We present the design, realization, and characterization of optical strong light–matter coupling between intersubband transitions within a semiconductor heterostructures and planar metamaterials in the near-infrared spectral range. The strong light–matter coupling entity consists of a III-nitride intersubband superlattice heterostructure, providing a two-level system with a transition energy of ∼0.8 eV (λ ∼1.55 μm) and a planar “dogbone” metamaterial structure. As the bare metamaterial resonance frequency is varied across the intersubband resonance, a clear anticrossing behavior is observed in the frequency domain. This strongly coupled entity could enable the realization of electrically tunable optical filters, a new class of efficient nonlinear optical materials, or intersubband-based light-emitting diodes.}, number={10}, journal={Acs Photonics}, author={Benz, Alexander and Campione, Salvatore and Moseley, Michael W. and Wierer, Jonathan J., Jr. and Allerman, Andrew A. and Wendt, Joel R. and Brener, Igal}, year={2014}, pages={906–911} } @article{wierer_tsao_sizov_2014, title={The potential of III‐nitride laser diodes for solid‐state lighting}, url={http://dx.doi.org/10.1002/pssc.201300422}, DOI={10.1002/pssc.201300422}, abstractNote={Abstract}, journal={physica status solidi c}, author={Wierer, Jonathan J. and Tsao, Jeffrey Y. and Sizov, Dmitry S.}, year={2014}, month={Feb} } @article{wang_li_wierer_koleske_figiel_2014, title={Top-down fabrication and characterization of axial and radial III-nitride nanowire LEDs}, volume={211}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000333911800005&KeyUID=WOS:000333911800005}, DOI={10.1002/pssa.201300491}, abstractNote={Complex axial and radial type III‐nitride InGaN/GaN nanowire LEDs are realized using a recently developed top–down fabrication approach which enables high quality GaN‐based nanowires with independently controlled height, pitch, and diameter. In this paper, we report on the fabrication, structural characterization, and luminescence of these two different structures and discuss their relative merits, weaknesses, and prospects in the context of the field. Axial (left) and radial (right) nanowire LEDs fabricated by a two‐step top–down method.}, number={4}, journal={Physica Status Solidi a-Applications and Materials Science}, author={Wang, G. T. and Li, Q. M. and Wierer, J. J. and Koleske, D. D. and Figiel, J. J.}, year={2014}, pages={748–751} } @article{tsao_crawford_coltrin_fischer_koleske_subramania_wang_wierer_karlicek_2014, title={Toward Smart and Ultra-efficient Solid-State Lighting}, volume={2}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000344171800001&KeyUID=WOS:000344171800001}, DOI={10.1002/adom.201400131}, abstractNote={Solid‐state lighting has made tremendous progress this past decade, with the potential to make much more progress over the coming decade. In this article, the current status of solid‐state lighting relative to its ultimate potential to be “smart” and ultra‐efficient is reviewed. Smart, ultra‐efficient solid‐state lighting would enable both very high “effective” efficiencies and potentially large increases in human performance. To achieve ultra‐efficiency, phosphors must give way to multi‐color semiconductor electroluminescence: some of the technological challenges associated with such electroluminescence at the semiconductor level are reviewed. To achieve smartness, additional characteristics such as control of light flux and spectra in time and space will be important: some of the technological challenges associated with achieving these characteristics at the lamp level are also reviewed. It is important to emphasise that smart and ultra‐efficient are not either/or, and few compromises need to be made between them. The ultimate route to ultra‐efficiency brings with it the potential for smartness, the ultimate route to smartness brings with it the potential for ultra‐efficiency, and the long‐term ultimate route to both might well be color‐mixed RYGB lasers.}, number={9}, journal={Advanced Optical Materials}, author={Tsao, Jeffrey Y. and Crawford, Mary H. and Coltrin, Michael E. and Fischer, Arthur J. and Koleske, Daniel D. and Subramania, Ganapathi S. and Wang, G. T. and Wierer, Jonathan J. and Karlicek, Robert F., Jr.}, year={2014}, pages={809–836} } @article{wierer_tsao_sizov_2013, title={Comparison between blue lasers and light-emitting diodes for future solid-state lighting}, volume={7}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000328150300016&KeyUID=WOS:000328150300016}, DOI={10.1002/lpor.201300048}, abstractNote={Abstract}, number={6}, journal={Laser & Photonics Reviews}, author={Wierer, J. J. and Tsao, J. Y. and Sizov, D. S.}, year={2013}, pages={963–993} } @inbook{tsao_wierer_rohwer_coltrin_crawford_simmons_hung_saunders_sizov_bhat_et al._2013, title={Introduction Part B. Ultra-efficient Solid-State Lighting: Likely Characteristics, Economic Benefits, Technological Approaches}, volume={126}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000348907300003&KeyUID=WOS:000348907300003}, DOI={10.1007/978-94-007-5863-6_2}, abstractNote={Technologies for artificial lighting, as illustrated on the left side of Fig. 2.1, have made tremendous progress over the centuries: from fire, with an efficiency of about a tenth of a percent; to incandescent lamps, with an efficiency of about 4 %; to gas discharge lamps, with an efficiency of about 20 %; and soon to solid-state lighting (SSL), with efficiencies that in principle could approach 100 %. At this point in time, there is virtually no question that SSL will eventually displace its predecessor technologies. A remaining question, however, is what the final efficiency of SSL will be. Will it be, as illustrated on the right side of Fig. 2.1, 50 %, which is what the community (Haitz and Tsao in Phys. Status Solidi A 208:17–29, 2011) has long targeted as its “efficient” lighting goal? Will it be 70 % or higher, which is what some (Phillips et al. in Laser Photon. Rev. 1:307–333, 2007) have called the “ultra-efficient” lighting goal? Or will it be even beyond an effective efficiency of 100 %, something that might be enabled by smart lighting (Kim and Schubert in Science 308:1274–1278, 2005), in which one doesn’t just engineer the efficiency with which light is produced, but the efficiency with which light is used? In this chapter, we give a perspective on the future of SSL, with a focus on ultra-high efficiencies. We ask, and sketch answers to, three questions. First, what are some of the likely characteristics of ultra-efficient SSL? Second, what are some of the economic benefits of ultra-efficient SSL? And, third, what are some of the challenges associated with the various technological approaches that could be explored for ultra-efficient SSL?}, booktitle={Iii-Nitride Based Light Emitting Diodes and Applications}, author={Tsao, Jeff Y. and Wierer, Jonathan J., Jr. and Rohwer, Lauren E. S. and Coltrin, Michael E. and Crawford, Mary H. and Simmons, Jerry A. and Hung, Po-Chieh and Saunders, Harry and Sizov, Dmitry S. and Bhat, Raj and et al.}, year={2013}, pages={11–26} } @article{howell_padalkar_yoon_li_koleske_wierer_wang_lauhon_2013, title={Spatial Mapping of Efficiency of GaN/InGaN Nanowire Array Solar Cells Using Scanning Photocurrent Microscopy}, volume={13}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000327111700021&KeyUID=WOS:000327111700021}, DOI={10.1021/nl402331u}, abstractNote={GaN-InGaN core-shell nanowire array devices are characterized by spectrally resolved scanning photocurrent microscopy (SPCM). The spatially resolved external quantum efficiency is correlated with structure and composition inferred from atomic force microscope (AFM) topography, scanning transmission electron microscope (STEM) imaging, Raman microspectroscopy, and scanning photocurrent microscopy (SPCM) maps of the effective absorption edge. The experimental analyses are coupled with finite difference time domain simulations to provide mechanistic understanding of spatial variations in carrier generation and collection, which is essential to the development of heterogeneous novel architecture solar cell devices.}, number={11}, journal={Nano Letters}, author={Howell, S. L. and Padalkar, S. and Yoon, K. and Li, Q. M. and Koleske, D. D. and Wierer, J. J. and Wang, G. T. and Lauhon, L. J.}, year={2013}, pages={5123–5128} } @article{riley_padalkar_li_lu_koleske_wierer_wang_lauhon_2013, title={Three-Dimensional Mapping of Quantum Wells in a GaN/InGaN Core-Shell Nanowire Light-Emitting Diode Array}, volume={13}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000330158900056&KeyUID=WOS:000330158900056}, DOI={10.1021/nl4021045}, abstractNote={Correlated atom probe tomography, cross-sectional scanning transmission electron microscopy, and cathodoluminescence spectroscopy are used to analyze InGaN/GaN multiquantum wells (QWs) in nanowire array light-emitting diodes (LEDs). Tomographic analysis of the In distribution, interface morphology, and dopant clustering reveals material quality comparable to that of planar LED QWs. The position-dependent CL emission wavelength of the nonpolar side-facet QWs and semipolar top QWs is correlated with In composition.}, number={9}, journal={Nano Letters}, author={Riley, J. R. and Padalkar, S. and Li, Q. M. and Lu, P. and Koleske, D. D. and Wierer, J. J. and Wang, G. T. and Lauhon, L. J.}, year={2013}, pages={4317–4325} } @book{ultra-efficient solid-state lighting: likely characteristics, economic benefits, technological approaches_2013, journal={Springer}, year={2013} } @article{lee_koleske_crawford_wierer_2012, title={Effect of interface grading and lateral thickness variation on x-ray diffraction by InGaN/GaN multiple quantum wells}, volume={355}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000307121900011&KeyUID=WOS:000307121900011}, DOI={10.1016/j.jcrysgro.2012.06.048}, abstractNote={We develop a method for simulating the effects of interface grading and lateral variation in layer thickness on x-ray diffraction by InGaN/GaN multiple quantum wells (MQWs). Using the resulting simulation scheme, we perform detailed fitting of symmetric (0002) ω/2θ scans measured for a selection of typical InGaN/GaN MQW heterostructures. We find that incorporation of the combined effects of interface grading and thickness variation substantially improves the goodness of fit relative to a conventional model that assumes ideal MQW structure. The improved simulations of experiments reveal that the examined InxGa1−xN/GaN MQWs (0.17≤x≤0.24) grown on the basal plane of GaN have graded heterointerface widths, w, in the range 0.5≤w≤1.1 nm concomitant with lateral variations in total MQW thickness of 0.7–6.3 nm rms. Atomic force microscopy of 10×10 μm2 areas of the as-grown MQWs finds surface roughnesses of 1.0–5.6 nm rms in agreement with corresponding rms thickness variations found by simulating the XRD measurements. For samples with smaller thickness variations, higher order MQW satellites are observed in high-dynamic-range diffraction experiments and best-fit simulations find evidence for asymmetric MQW interface widths. The lower interfaces are narrower than the upper interfaces in agreement with recent transmission electron microscopy and atom-probe studies of MQW interfaces by other groups. These under-recognized structural features—heterointerface grading and lateral film-thickness variation—will influence not only x-ray diffraction, but also polarization, bandstructure, and carrier localization within InGaN-based MQW heterostructures.}, number={1}, journal={Journal of Crystal Growth}, author={Lee, S. R. and Koleske, D. D. and Crawford, M. H. and Wierer, J. J., Jr.}, year={2012}, pages={63–72} } @article{kim_jung_song_kim_li_kim_song_wierer_pao_huang_et al._2012, title={High-Efficiency, Microscale GaN Light-Emitting Diodes and Their Thermal Properties on Unusual Substrates}, volume={8}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000304817700002&KeyUID=WOS:000304817700002}, DOI={10.1002/smll.201200382}, abstractNote={A method for forming efficient, ultrathin GaN light-emitting diodes (LEDs) and for their assembly onto foreign substances is reported. The LEDs have lateral dimensions ranging from ~1 mm × 1 mm to ~25 μm × 25 μm. Quantitative experimental and theoretical studies show the benefits of small device geometry on thermal management, for both continuous and pulsed-mode operation, the latter of which suggests the potential use of these technologies in bio-integrated contexts.}, number={11}, journal={Small}, author={Kim, Tae-il and Jung, Yei Hwan and Song, Jizhou and Kim, Daegon and Li, Yuhang and Kim, Hoon-sik and Song, Il-Sun and Wierer, Jonathan J. and Pao, Hsuan An and Huang, Yonggang and et al.}, year={2012}, pages={1643–1649} } @article{wierer_li_koleske_lee_wang_2012, title={III- nitride core-shell nanowire arrayed solar cells}, volume={23}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000303534600008&KeyUID=WOS:000303534600008}, DOI={10.1088/0957-4484/23/19/194007}, abstractNote={A solar cell based on a hybrid nanowire–film architecture consisting of a vertically aligned array of InGaN/GaN multi-quantum well core–shell nanowires which are electrically connected by a coalesced p-InGaN canopy layer is demonstrated. This unique hybrid structure allows for standard planar device processing, solving a key challenge with nanowire device integration, while enabling various advantages by the nanowire absorbing region such as higher indium composition InGaN layers by elastic strain relief, more efficient carrier collection in thinner layers, and enhanced light trapping from nano-scale optical index changes. This hybrid structure is fabricated into working solar cells exhibiting photoresponse out to 2.1 eV and short-circuit current densities of ∼1 mA cm−2 under 1 sun AM1.5G. This proof-of-concept nanowire-based device demonstrates a route forward for high-efficiency III-nitride solar cells.}, number={19}, journal={Nanotechnology}, author={Wierer, Jonathan J., Jr. and Li, Qiming and Koleske, Daniel D. and Lee, Stephen R. and Wang, George T.}, year={2012} } @article{wierer_koleske_lee_2012, title={Influence of barrier thickness on the performance of InGaN/GaN multiple quantum well solar cells}, volume={100}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000302204900019&KeyUID=WOS:000302204900019}, DOI={10.1063/1.3695170}, abstractNote={The performance of InGaN/GaN multiple quantum well (MQW) solar cells containing 15 periods of 2.7 nm thick In0.21Ga0.79N wells and three different GaN barriers thicknesses of 3.0 nm, 6.3 nm, and 10.0 nm is investigated. Increasing barrier thickness results in absorption at lower energies, consistent with piezoelectric polarization induced electric fields tilting the energy bands of the MQW and changing the transition energy of well states. The internal quantum efficiency and leakage currents are additionally affected by GaN barrier thickness, resulting in the 6.3 nm barrier structure achieving the highest power conversion efficiency (1.66%, 1 sun AM1.5G).}, number={11}, journal={Applied Physics Letters}, author={Wierer, J. J., Jr. and Koleske, D. D. and Lee, S. R.}, year={2012} } @article{wang_li_wierer_figiel_wright_luk_brener_streubel_jeon_tu_et al._2012, title={Top-down fabrication of GaN-based nanorod LEDs and lasers}, volume={8278}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000301055700017&KeyUID=WOS:000301055700017}, DOI={10.1117/12.909377}, abstractNote={Although planar heterostructures dominate current optoelectronic architectures, 1D nanowires and nanorods have distinct and advantageous properties that may enable higher efficiency, longer wavelength, and cheaper devices. We have developed a top-down approach for fabricating ordered arrays of high quality GaN-based nanorods with controllable height, pitch and diameter. This approach avoids many of the limitations of bottom-up synthesis methods. In addition to GaN nanorods, the fabrication and characterization of both axial and radial-type GaN/InGaN nanorod LEDs have been achieved. The precise control over nanorod geometry achiveable by this technique also enables single-mode single nanowire lasing with linewidths of less than 0.1 nm and low lasing thresholds of ~250kW/cm2.}, journal={Light-Emitting Diodes: Materials, Devices, and Applications For Solid State Lighting Xvi}, author={Wang, George T. and Li, Qiming and Wierer, Jonathan and Figiel, Jeffrey and Wright, Jeremy B. and Luk, Ting S. and Brener, Igal and Streubel, KP and Jeon, H and Tu, LW and et al.}, year={2012} } @article{neumann_wierer_davis_ohno_brueck_tsao_2011, title={Four-color laser white illuminant demonstrating high color-rendering quality}, volume={19}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000292876500041&KeyUID=WOS:000292876500041}, number={14}, journal={Optics Express}, author={Neumann, A. and Wierer, J. J., Jr. and Davis, W. and Ohno, Y. and Brueck, S. R. J. and Tsao, J. Y.}, year={2011}, pages={A982–A990} } @article{wang_li_huang_wierer_armstrong_lin_upadhya_prasankumar_bardwell_hunter_et al._2011, title={III-Nitride Nanowires: Emerging Materials for Lighting and Energy Applications}, volume={35}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000305936300001&KeyUID=WOS:000305936300001}, DOI={10.1149/1.3570840}, abstractNote={The aligned growth of III-nitride nanowires, along with results providing insights into the nanowire properties obtained using electrical, optical and structural characterization techniques, are discussed. A new "top-down" approach for fabricating ordered arrays of high quality GaN-based nanorods with controllable height, pitch and diameter is also presented, along with results from preliminary LEDs grown on these nanorod arrays. Additionally, a novel application of aligned nanowire arrays as strain-relief templates for the growth of high quality GaN is demonstrated.}, number={6}, journal={Wide Bandgap Semiconductor Materials and Devices 12}, author={Wang, George T. and Li, Qiming and Huang, Jianyu and Wierer, Jonathan and Armstrong, Andrew and Lin, Yong and Upadhya, Prashanth and Prasankumar, Rohit and Bardwell, JA and Hunter, GW and et al.}, year={2011}, pages={3–11} } @article{wierer_allerman_li_2010, title={Silicon impurity-induced layer disordering of AlGaN/AlN superlattices}, volume={97}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000281059500023&KeyUID=WOS:000281059500023}, DOI={10.1063/1.3478002}, abstractNote={Impurity-induced layer disordering is demonstrated in Al0.1Ga0.9N/AlN superlattices grown by metal-organic vapor phase epitaxy. During growth at temperatures as low as 885 °C and under post growth annealing at 1000 °C in N2 the heterointerfaces of Si-doped (Si concentration >8×1019 cm−3) superlattices exhibit layer disordering (intermixing) while the unintentionally doped superlattices remain stable. Shifts in the intersubband energy transitions and scanning transmission electron microscope images showing changes in the layer abruptness are used to verify layer disordering due to Si diffusion in Al0.1Ga0.9N/AlN superlattices.}, number={5}, journal={Applied Physics Letters}, author={Wierer, J. J., Jr. and Allerman, A. A. and Li, Q.}, year={2010} } @article{wierer_fischer_koleske_2010, title={The impact of piezoelectric polarization and nonradiative recombination on the performance of (0001) face GaN/InGaN photovoltaic devices}, volume={96}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000274319500007&KeyUID=WOS:000274319500007}, DOI={10.1063/1.3301262}, abstractNote={The impact of piezoelectric polarization and nonradiative recombination on the short-circuit current densities (Jsc) of (0001) face GaN/InGaN photovoltaic devices is demonstrated. P-i-n diodes consisting of 170 nm thick intrinsic In0.09Ga0.91N layers sandwiched by GaN layers exhibit low Jsc∼40 μA/cm2. The piezoelectric polarization at the GaN/InGaN heterointerfaces creates drift currents opposite in direction needed for efficient carrier collection. Also, nonradiative recombination centers produce short carrier lifetimes, limiting Jsc. Alternative structures with intrinsic InGaN layers sandwiched by n-type InGaN or graded InyGa1−yN (y=0–0.09) layer and a p-type In0.015Ga0.985N layer have favorable potentials, longer carrier lifetimes, and improve Jsc to ∼0.40 mA/cm2.}, number={5}, journal={Applied Physics Letters}, author={Wierer, J. J., Jr. and Fischer, A. J. and Koleske, D. D.}, year={2010} } @article{wierer_david_megens_2009, title={III-nitride photonic-crystal light-emitting diodes with high extraction efficiency}, volume={3}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000264289600018&KeyUID=WOS:000264289600018}, DOI={10.1038/nphoton.2009.21}, number={3}, journal={Nature Photonics}, author={Wierer, Jonathan J., Jr. and David, Aurelien and Megens, Mischa M.}, year={2009}, pages={163–169} } @inbook{wierer_krames_epler_gardner_wendt_sigalas_brueck_li_shagam_2005, title={III-Nitride LEDs with photonic crystal structures}, volume={5739}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000228826900013&KeyUID=WOS:000228826900013}, DOI={10.1117/12.591218}, abstractNote={Electrical operation of III-Nitride light emitting diodes (LEDs) with photonic crystal structures is demonstrated. Employing photonic crystal structures in III-Nitride LEDs is a method to increase light extraction efficiency and directionality. The photonic crystal is a triangular lattice formed by dry etching into the III-Nitride LED. A range of lattice constants is considered (a ~ 270 - 340nm). The III-Nitride LED layers include a tunnel junction providing good lateral current spreading without a semi-absorbing metal current spreader as is typically done in conventional III-Nitride LEDs. These photonic crystal III-Nitride LED structures are unique because they allow for carrier recombination and light generation proximal to the photonic crystal (light extraction area) yet displaced from the absorbing metal contact. The photonic crystal Bragg scatters what would have otherwise been guided modes out of the LED, increasing the extraction efficiency. The far-field light radiation patterns are heavily modified compared to the typical III-Nitride LED’s Lambertian output. The photonic crystal affects the light propagation out of the LED surface, and the radiation pattern changes with lattice size. LEDs with photonic crystals are compared to similar III-Nitride LEDs without the photonic crystal in terms of extraction, directionality, and emission spectra.}, booktitle={Light-Emitting Diodes: Research, Manufacturing, and Applications IX}, author={Wierer, J. J. and Krames, M. R. and Epler, J. E. and Gardner, N. F. and Wendt, J. R. and Sigalas, M. M. and Brueck, S. R. J. and Li, D. and Shagam, M.}, editor={Stockman, S. A. and Yao, H. W. and Schubert, E. F.Editors}, year={2005}, pages={102–107} } @article{gardner_kim_wierer_shen_krames_2005, title={Polarization anisotropy in the electroluminescence of m-plane InGaN-GaN multiple-quantum-well light-emitting diodes}, volume={86}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000228050700001&KeyUID=WOS:000228050700001}, DOI={10.1063/1.1875765}, abstractNote={InGaN – GaN multiple-quantum-well light-emitting diodes were fabricated on (101¯0) m plane GaN films grown on (101¯0) m plane 4H–SiC substrates. The [0001] axis of the epitaxial film is parallel to the [0001] axis of the substrate. The surface is striated, with features running perpendicular to the c axis and a maximum surface height difference of 45nm. Electroluminescence shows strong polarization anisotropy, with 7× more light emitted with polarization perpendicular to the c axis compared to parallel to the c axis. An Ahrrenius fit of the polarization ratio indicates that there is a 49meV difference in the energy gap between the two polarization states. This suggests that a high polarization ratio can be maintained at the high temperatures (>150°C) and drive current densities required for high-power light-emitting diode applications.}, number={11}, journal={Applied Physics Letters}, author={Gardner, N. F. and Kim, J. C. and Wierer, J. J. and Shen, Y. C. and Krames, M. R.}, year={2005} } @article{wierer_krames_epler_gardner_craford_wendt_simmons_sigalas_2004, title={InGaN/GaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures}, volume={84}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000221210100055&KeyUID=WOS:000221210100055}, DOI={10.1063/1.1738934}, abstractNote={Electrical operation of InGaN/GaN quantum-well heterostructure photonic crystal light-emitting diodes (PXLEDs) is demonstrated. A triangular lattice photonic crystal is formed by dry etching into the top GaN layer. Light absorption from the metal contact is minimized because the top GaN layers are engineered to provide lateral current spreading, allowing carrier recombination proximal to the photonic crystal yet displaced from the metal contact. The chosen lattice spacing for the photonic crystal causes Bragg scattering of guided modes out of the LED, increasing the extraction efficiency. The far-field radiation patterns of the PXLEDs are heavily modified and display increased radiance, up to ∼1.5 times brighter compared to similar LEDs without the photonic crystal.}, number={19}, journal={Applied Physics Letters}, author={Wierer, J. J. and Krames, M. R. and Epler, J. E. and Gardner, N. F. and Craford, M. G. and Wendt, J. R. and Simmons, J. A. and Sigalas, M. M.}, year={2004}, pages={3885–3887} } @article{shen_wierer_krames_ludowise_misra_ahmed_kim_mueller_bhat_stockman_et al._2003, title={Optical cavity effects in InGaN/GaN quantum-well-heterostructure flip-chip light-emitting diodes}, volume={82}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000182018800009&KeyUID=WOS:000182018800009}, DOI={10.1063/1.1566098}, abstractNote={Optical cavity effects have a significant influence on the extraction efficiency of InGaN/GaN quantum-well-heterostructure flip-chip light-emitting diodes (FCLEDs). Light emitted from the quantum well (QW) self-interferes due to reflection from a closely placed reflective metallic mirror. The interference patterns couple into the escape cone for light extraction from the FCLED. This effect causes significant changes in the extraction efficiency as the distance between the QW and the metallic mirror varies. In addition, the radiative lifetime of the QW also changes as a function of the distance between the QW and the mirror surface. Experimental results from packaged FCLEDs, supported by optical modeling, show that a QW placed at an optimum distance from the mirror provides a ∼2.3× increase in total light output as compared to a QW placed at a neighboring position corresponding to a minimum in overall light extraction.}, number={14}, journal={Applied Physics Letters}, author={Shen, Y. C. and Wierer, J. J. and Krames, M. R. and Ludowise, M. J. and Misra, M. S. and Ahmed, F. and Kim, A. Y. and Mueller, G. O. and Bhat, J. C. and Stockman, S. A. and et al.}, year={2003}, pages={2221–2223} } @article{steranka_bhat_collins_cook_craford_fletcher_gardner_grillot_goetz_keuper_et al._2002, title={High power LEDs - Technology status and market applications}, volume={194}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000180101800004&KeyUID=WOS:000180101800004}, DOI={10.1002/1521-396x(200212)194:2<380::aid-pssa380>3.0.co;2-n}, abstractNote={High power light emitting diodes (LEDs) continue to increase in output flux with the best III-nitride based devices today emitting over 150 lm of white, cyan, or green light. The key design features of such products will be covered with special emphasis on power packaging, flip-chip device design, and phosphor coating technology. The high-flux performance of these devices is enabling many new applications for LEDs. Two of the most interesting of these applications are LCD display backlighting and vehicle forward lighting. The advantages of LEDs over competing lighting technologies will be covered in detail.}, number={2}, journal={Physica Status Solidi a-Applied Research}, author={Steranka, F. M. and Bhat, J. and Collins, D. and Cook, L. and Craford, M. G. and Fletcher, R. and Gardner, N. and Grillot, P. and Goetz, W. and Keuper, M. and et al.}, year={2002}, pages={380–388} } @article{krames_collins_gardner_gotz_lowery_ludowise_martin_mueller_mueller-mach_rudaz_et al._2002, title={High-power III-nitride emitters for solid-state lighting}, volume={192}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000177750700001&KeyUID=WOS:000177750700001}, DOI={10.1002/1521-396x(200208)192:2<237::aid-pssa237>3.0.co;2-i}, abstractNote={High-power, large-area InGaN/GaN quantum-well heterostructure light-emitting diodes based on an inverted, or flip-chip, configuration are described. These devices are mounted in specially designed high-power (1-5 W) packages and exhibit high extraction efficiency and low operating voltage. In the blue wavelength regime, output powers greater than 250 mW (1 x 1 mm 2 device) and 1 W (2 x 2 mm 2 device) are delivered at standard operating current densities (50 A/cm 2 ), corresponding to wall-plug efficiencies of 22%-23%. Employing phosphors for the generation of white light, these same devices achieve luminous efficiencies greater than 30 lm/W.}, number={2}, journal={Physica Status Solidi a-Applied Research}, author={Krames, M. R. and Collins, J. B. D. and Gardner, N. F. and Gotz, W. and Lowery, C. H. and Ludowise, M. and Martin, P. S. and Mueller, G. and Mueller-Mach, R. and Rudaz, S. and et al.}, year={2002}, pages={237–245} } @article{high-power algainn flip-chip light-emitting diodes_2001, volume={78}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000168885000001&KeyUID=WOS:000168885000001}, DOI={10.1063/1.1374499}, abstractNote={Data are presented on high-power AlGaInN flip-chip light-emitting diodes (FCLEDs). The FCLED is “flipped-over” or inverted compared to conventional AlGaInN light-emitting diodes (LEDs), and light is extracted through the transparent sapphire substrate. This avoids light absorption from the semitransparent metal contact in conventional epitaxial-up designs. The power FCLED has a large emitting area (∼0.70 mm2) and an optimized contacting scheme allowing high current (200–1000 mA, J∼30–143 A/cm2) operation with low forward voltages (∼2.8 V at 200 mA), and therefore higher power conversion (“wall-plug”) efficiencies. The improved extraction efficiency of the FCLED provides 1.6 times more light compared to top-emitting power LEDs and ten times more light than conventional small-area (∼0.07 mm2) LEDs. FCLEDs in the blue wavelength regime (∼435 nm peak) exhibit ∼21% external quantum efficiency and ∼20% wall-plug efficiency at 200 mA and with record light output powers of 400 mW at 1.0 A.}, number={22}, journal={Applied Physics Letters}, year={2001}, pages={3379–3381} } @article{kim_gotz_steigerwald_wierer_gardner_sun_stockman_martin_krames_kern_et al._2001, title={Performance of high-power AlInGaN light emitting diodes}, volume={188}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000172686900004&KeyUID=WOS:000172686900004}, DOI={10.1002/1521-396x(200111)188:1<15::aid-pssa15>3.0.co;2-5}, abstractNote={The performance of high-power AlInGaN light emitting diodes (LEDs) is characterized by light output-current-voltage (L-I-V) measurements for devices with peak emission wavelengths ranging from 428 to 545 nm. The highest external quantum efficiency (EQE) is measured for short wavelength LEDs (428 nm) at 29%. EQE decreases with increasing wavelength, reaching 13% at 527 nm. With low forward voltages ranging from 3.3 to 2.9 V at a drive current density of 50 A/cm 2 , these LEDs exhibit power conversion efficiencies ranging from 26% (428 nm) to 10% (527 nm).}, number={1}, journal={Physica Status Solidi a-Applied Research}, author={Kim, A. Y. and Gotz, W. and Steigerwald, D. A. and Wierer, J. J. and Gardner, N. F. and Sun, J. and Stockman, S. A. and Martin, P. S. and Krames, M. R. and Kern, R. S. and et al.}, year={2001}, pages={15–21} } @inbook{yao_ferguson_schubert_2000, title={High-brightness AlGaInN light-emitting diodes}, volume={3938}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000087782000001&KeyUID=WOS:000087782000001}, DOI={10.1117/12.382822}, abstractNote={Currently, commercial LEDs based on AlGaInN emit light efficiently from the ultraviolet-blue to the green portion of the visible wavelength spectrum. Data are presented on AlGaInN LEDs grown by organometallic vapor phase epitaxy (OMVPE). Designs for high-power AlGaInN LEDs are presented along with their performance in terms of output power and efficiency. Finally, present and potential applications for high-power AlGaInN LEDs, including traffic signals and contour lighting, are discussed.}, booktitle={Light-Emitting Diodes: Research, Manufacturing, and Applications Iv}, year={2000}, pages={2–12} } @article{wierer_kellogg_holonyak_1999, title={Tunnel contact junction native-oxide aperture and mirror vertical-cavity surface-emitting lasers and resonant-cavity light-emitting diodes}, volume={74}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000078571400010&KeyUID=WOS:000078571400010}, DOI={10.1063/1.123452}, abstractNote={Vertical-cavity surface-emitting lasers (VCSELs) and resonant-cavity light-emitting diodes (RCLEDs) are demonstrated with high index contrast distributed Bragg reflectors (DBRs) on either side of a λ-thickness cavity (λ∼980 nm). The devices, with tunnel contact junctions making possible lateral electron current excitation, have a lower 6.5 period native-oxide-based AlxOy/GaAs DBR and an upper reflector that is either a 2–4 period AlxOy/GaAs DBR, a 1–2 period SiO2/ZnSe DBR, a λ/4-thickness layer of AlxOy (antireflecting), or no mirror at all. The AlxOy/GaAs DBRs and a buried-oxide-defined current aperture are formed by selective oxidation of the high Al composition AlxGa1−xAs layers. Device characteristics are observed as a function of the upper DBR periodicity (reflectivity). Devices with upper reflectivities of R≳99% operate as VCSELs while those with less reflectivity R≲96% operate as RCLEDs, some with external differential quantum efficiencies as high as η∼27% and narrow spectral emission (Δλ∼50 Å).}, number={7}, journal={Applied Physics Letters}, author={Wierer, J. J. and Kellogg, D. A. and Holonyak, N.}, year={1999}, pages={926–928} } @article{evans_wierer_holonyak_1998, title={AlxGa1-xAs native-oxide-based distributed Bragg reflectors for vertical cavity surface emitting lasers}, volume={84}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000076669100005&KeyUID=WOS:000076669100005}, DOI={10.1063/1.368857}, abstractNote={The problems associated with constructing AlxGa1−xAs native-oxide-based distributed Bragg reflectors (DBRs) for vertical cavity surface emitting lasers are investigated. Reflection and stability measurements are performed on structures with central λ/2 cavities (λ∼980 nm) of GaAs surrounded by two periods of native-oxide-based DBRs on the top and 2.5 periods on the bottom. Prior to crystal oxidation (H2O vapor+N2, 430 °C) a period of the DBRs consists of a ∼λ/4 optically thick layer of GaAs and a thicker (oxidation) layer of AlxGa1−xAs (x=0.95, 0.96, 0.97, 0.98, 1.00) surrounded by thinner (∼100 Å) buffer layers that are AlyGa1−yAs (y=0, 0.25, 0.50, 0.65, 0.070, 0.75, 0.80, 0.85). The DBRs are formed after oxidation of the high Al composition AlxGa1−xAs layers, and to some extent the AlyGa1−yAs buffer layers, forming a ∼λ/4 optically thick layer of the native oxide. For comparison, more complicated DBRs are created by oxidizing superlattice layers. It is found that the AlxGa1−xAs composition, x, of the oxidation layer, choice of oxidizing or nonoxidizing AlyGa1−yAs buffer layers (y), oxidation parameters, and post-processing parameters determine the DBR quality and stability, as well as the possibility of reoxidation.}, number={10}, journal={Journal of Applied Physics}, author={Evans, P. W. and Wierer, J. J. and Holonyak, N.}, year={1998}, pages={5436–5440} } @article{wierer_evans_holonyak_1998, title={Transition from edge to vertical cavity operation of tunnel contact AlGaAs-GaAs-InGaAs quantum well heterostructure lasers}, volume={72}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000073126000016&KeyUID=WOS:000073126000016}, DOI={10.1063/1.120869}, abstractNote={Al x Ga 1−x As–GaAs–In y Ga 1−y As quantum well heterostructure lasers with thin sharply defined cavities that operate longitudinally or vertically are demonstrated. Longitudinally, the cavity is defined by cleaving (edge emission) while vertical definition is provided by a lower AlxOy/GaAs distributed Bragg reflector (DBR) and an upper SiO2/Si DBR that form a resonant structure. A reverse-biased tunnel contact junction provides lateral electron current to support hole injection through a native-oxide-defined aperture. This ultrathin cavity configuration produces low threshold high efficiency (η∼91%, L=140 μm) edge emission (longitudinal operation) in the case of long lasers (L≳90 μm), and vertical emission for shorter lasers (L≲65 μm). The transition from longitudinal to vertical laser operation as a function of cavity length, L, is demonstrated.}, number={7}, journal={Applied Physics Letters}, author={Wierer, J. J. and Evans, P. W. and Holonyak, N.}, year={1998}, pages={797–799} } @article{wierer_evans_holonyak_kellogg_1998, title={Vertical cavity surface emitting lasers utilizing native oxide mirrors and buried tunnel contact junctions}, volume={72}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000075273500039&KeyUID=WOS:000075273500039}, DOI={10.1063/1.121445}, abstractNote={Vertical cavity surface emitting lasers (VCSELs) are demonstrated with high-index-contrast native-oxide-based (AlxOy) distributed Bragg reflectors (DBRs) on both sides of a “2λ” cavity, thus creating a compact (thin, ∼2.8 μm) laser structure. Selective oxidation of high Al composition AlxGa1−xAs layers yields a structure with a four period upper AlxOy/GaAs DBR, a 5.5 period lower AlxOy/GaAs DBR, and a buried oxide current aperture. A reverse-biased tunnel contact junction provides hole injection via lateral electron current between the upper DBR and the oxide aperture layer. These VCSELs operate with submilliampere thresholds, high spontaneous efficiencies, and excellent polarization control.}, number={21}, journal={Applied Physics Letters}, author={Wierer, J. J. and Evans, P. W. and Holonyak, N. and Kellogg, D. A.}, year={1998}, pages={2742–2744} } @article{wierer_evans_holonyak_1997, title={Buried tunnel contact junction AlGaAs-GaAs-InGaAs quantum well heterostructure lasers with oxide-defined lateral currents}, volume={71}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:A1997YB43600021&KeyUID=WOS:A1997YB43600021}, DOI={10.1063/1.120071}, abstractNote={Al x Ga 1−x As-GaAs-In y Ga 1−y As quantum well heterostructure (QWH) lasers with p+-n+ GaAs–InyGa1−yAs reverse-biased tunnel junctions (hole sources) located in the upper cladding of standard lasers and in oxide-defined cavities (requiring lateral bias currents) are demonstrated. The tunnel junctions, introduced to aid lateral current spreading, are grown at different distances from the waveguide active region in a standard QWH structure to determine first the effect of heavily doped tunnel layers on laser threshold currents. Other QWH laser crystals are oxidized to form oxide-aperture devices with, in addition, either a top confining oxide or a top and bottom oxide confining layer. Hole injection is provided between the oxide layers with the aid of the tunnel contact junction and lateral electron current. The buried tunnel contact junction is shown to be an effective method to make possible hole injection via a lateral electron current, with only a modest increase (a small penalty) in voltage drop and series resistance compared to standard devices.}, number={16}, journal={Applied Physics Letters}, author={Wierer, J. J. and Evans, P. W. and Holonyak, N.}, year={1997}, pages={2286–2288} } @article{wierer_evans_holonyak_kellogg_1997, title={Lateral electron current operation of vertical cavity surface emitting lasers with buried tunnel contact hole sources}, volume={71}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:A1997YL27300002&KeyUID=WOS:A1997YL27300002}, DOI={10.1063/1.120400}, abstractNote={Vertical cavity surface emitting lasers (VCSELs) are demonstrated with reverse-biased tunnel contact junctions allowing low-loss lateral electron current to support hole injection. A compact hybrid vertical cavity is employed consisting of a lower 6.5 period AlxOy/GaAs distributed Bragg reflector (DBR) formed by selective oxidation of high Al composition AlxGa1−xAs, and an electron-beam deposited 5 period SiO2/Si upper DBR. The cavity (active region) is defined also by selectively oxidizing a current-confining aperture. Lateral electron current drives a tunnel contact junction providing hole injection underneath the upper DBR through the oxide-defined current aperture. The p-type crystal in the VCSEL is reduced to a minimum, thus reducing resistive loss and device voltage.}, number={24}, journal={Applied Physics Letters}, author={Wierer, J. J. and Evans, P. W. and Holonyak, N. and Kellogg, D. A.}, year={1997}, pages={3468–3470} } @article{evans_wierer_holonyak_1997, title={Photopumped laser operation of an oxide post GaAs-AlAs superlattice photonic lattice}, volume={70}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:A1997WL14700020&KeyUID=WOS:A1997WL14700020}, DOI={10.1063/1.118480}, abstractNote={Data are presented on the laser operation of photoexcited active hexagonal photonic lattices consisting of a GaAs–AlAs superlattice slab waveguide patterned with Zn-disordered AlGaAs posts that are converted to oxide. The semiconductor-oxide-post photonic lattice structure lases without the benefit of cleaved edges or other reflecting interfaces owing to strong local optical feedback provided by the high refractive index contrast between oxide posts and the active GaAs–AlAs superlattice. As the pump area is increased at constant pump power, the threshold intensity decreases as higher Q modes in an effectively larger cavity are excited. Similar hexagonal photonic lattices with nonoxidized posts (disordered AlGaAs posts) operate as lasers, but only with the assistance of cleaved edges and by shifting to longer wavelength. The oxide post photonic lattice is compatible with current-driven photonic lattice lasers or active filters.}, number={9}, journal={Applied Physics Letters}, author={Evans, P. W. and Wierer, J. J. and Holonyak, N.}, year={1997}, pages={1119–1121} } @article{wierer_maranowski_holonyak_evans_chen_1996, title={Double injection and negative resistance in stripe-geometry oxide-aperture AlyGa1-yAs-GaAs-InxGa1-xAs quantum well heterostructure laser diodes}, volume={69}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:A1996VQ17000030&KeyUID=WOS:A1996VQ17000030}, DOI={10.1063/1.117350}, abstractNote={Data are presented demonstrating double injection and negative resistance in stripe-geometry oxide-aperture AlyGa1−yAs–GaAs–InxGa1−xAs quantum well heterostructure lasers. The buried oxide laser structures are defined, in current and cavity, by laterally oxidizing the higher Al composition upper and lower cladding layers from a mesa edge (a ridge), thus, forming a narrow oxide-defined buried aperture (∼2μm). Post fabrication annealing (425 °C in N2) removes the negative resistance, indicating that the crystal growth and oxidation processes introduce products such as H and OH in the active region that compensate the dopants.}, number={19}, journal={Applied Physics Letters}, author={Wierer, J. J. and Maranowski, S. A. and Holonyak, N. and Evans, P. W. and Chen, E. I.}, year={1996}, pages={2882–2884} } @article{coleman_wierer_1995, title={ESTABLISHMENT OF A DYNAMIC-MODEL FOR THE P-GE FAR IR LASER}, volume={16}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:A1995QE09400001&KeyUID=WOS:A1995QE09400001}, DOI={10.1007/bf02085845}, number={1}, journal={International Journal of Infrared and Millimeter Waves}, author={Coleman, P. D. and Wierer, J. J.}, year={1995}, pages={3–32} }