@article{hagar_routh_abdelhamid_colter_muth_bedair_2024, title={GaN-based tunnel junction with negative differential resistance by metalorganic chemical vapor deposition}, volume={125}, ISSN={["1077-3118"]}, DOI={10.1063/5.0208759}, abstractNote={We present metalorganic chemical vapor deposition-grown III-nitride tunnel junction (TJ) devices showing negative differential resistance (NDR) under forward bias with a peak to valley ratio of 1.3 at room temperature. Previously, NDR in GaN material systems has only been achievable utilizing molecular beam epitaxy or polarization enhanced AlGaN interlayers. The TJ devices presented here utilize structures based on p+InGaN/n+InGaN materials with the n-side of the junction doped with both Si and Mg and with electron and hole concentrations roughly in the 1019 cm−3 range. The Mg precursor flow is maintained at a constant rate during the whole TJ growth. This co-doped technique can eliminate several Mg-related issues such as delayed incorporation, the memory effect, and Mg solid-state diffusion. Structures grown on relaxed InGaN semibulk templates show enhanced hole concentrations and improved TJ performance.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Hagar, B. G. and Routh, E. L. and Abdelhamid, M. and Colter, P. C. and Muth, J. and Bedair, S. M.}, year={2024}, month={Aug} } @article{abdelhamid_routh_hagar_bedair_2022, title={Improved LED output power and external quantum efficiency using InGaN templates}, volume={120}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0084273}, DOI={10.1063/5.0084273}, abstractNote={InGaN templates have recently attracted interest due to their ability to reduce strain in the quantum wells and to induce a red shift in the emission wavelength. For such technology to be competitive, it should outperform the traditional technology for LEDs grown on GaN substrates and offer improved output characteristics. InGaN based LEDs on InyGa1−yN templates with varying In-content of 8% ≤ y ≤ 12% are studied for the same emission wavelength. The electroluminescence, optical output power, and external quantum efficiency of the LEDs are investigated as a function of the In-content in the templates. LEDs on InGaN templates with In-content of 8–10% show better performance than LEDs grown on GaN. This enhancement is attributed to improved radiative recombination as a result of the reduced strain in the quantum wells. However, templates with In-content of ∼10.5% and ∼11% show inferior performance to the LEDs on GaN because the deterioration from the increased defects from the template is stronger than the improvement in the radiative recombination. It can be concluded that the InGaN templates with 8–10% offer a technology for LEDs that is outperforming the traditional GaN technology.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Abdelhamid, Mostafa and Routh, Evyn L. and Hagar, Brandon and Bedair, S. M.}, year={2022}, month={Feb} } @article{hagar_abdelhamid_routh_colter_bedair_2022, title={Ohmic co-doped GaN/InGaN tunneling diode grown by MOCVD}, volume={121}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0103152}, DOI={10.1063/5.0103152}, abstractNote={Tunnel junctions (TJs) have recently been proposed as a solution for several III-nitride current problems and to enhance new structures. Reported III-nitride TJs grown by metalorganic chemical vapor deposition (MOCVD) resulted in backward diodes with rectifying behavior in forward bias, even with Mg and Si doping in 1020 cm−3. This behavior limits applications in several device structures. We report a TJ structure based on p+In0.15Ga0.85N/n+In0.05Ga0.95N, where the n-side of the junction is co-doped with Si and Mg and with electron and hole concentrations in the mid-1019 cm−3 for both the n and p dopants. Co-doping creates deep levels within the bandgap that enhances tunneling under forward biased conditions. The TJ structure was investigated on both GaN substrates and InGaN templates to study the impact of strain on the TJ I–V characteristics. The resulting TJ I–V and resistivities reported indicate the potential for this TJ approach in several device structures based on III-nitrides. We are not aware of any previous MOCVD grown TJs that show Ohmic performance in both forward and reverse biases.}, number={5}, journal={APPLIED PHYSICS LETTERS}, author={Hagar, B. G. and Abdelhamid, M. and Routh, E. L. and Colter, P. C. and Bedair, S. M.}, year={2022}, month={Aug} } @article{hagar_sayed_colter_bedair_2020, title={Multi-junction solar cells by Intermetallic Bonding and interconnect of Dissimilar Materials: GaAs/Si}, volume={215}, ISSN={["1879-3398"]}, DOI={10.1016/j.solmat.2020.110653}, abstractNote={We present a novel, low temperature approach to multijunction solar cell fabrication combining the high efficiency multi-junction concept with the low cost of thin film technology in one solar cell structure. The intermetallic bonding approach presented is based on joining indium metal which has been deposited on the metal contact grid of the respective solar cells. This approach avoids the problems of lattice mismatch and tunnel junction limitations, connecting solar cells of potentially any material with patterned contacts. No measurable increase in resistance has been measured between bonded materials. This method allows the independent development of each cell technology for use in multijunction solar cells. This technique can be applied to any commercial off-the-shelf solar cells, if available. A GaAs/Si multijunction solar cell bonded using this approach is demonstrated. The silicon cell is off-the-shelf with textured surface and commercial metal contacts. This is integrated with an in-house grown thin film GaAs cell. The GaAs/Si device is demonstrated in both two and three terminal configurations.}, journal={SOLAR ENERGY MATERIALS AND SOLAR CELLS}, author={Hagar, Brandon and Sayed, Islam and Colter, Peter C. and Bedair, S. M.}, year={2020}, month={Sep} } @misc{colter_hagar_bedair_2018, title={Tunnel Junctions for III-V Multijunction Solar Cells Review}, volume={8}, ISSN={["2073-4352"]}, DOI={10.3390/cryst8120445}, abstractNote={Tunnel Junctions, as addressed in this review, are conductive, optically transparent semiconductor layers used to join different semiconductor materials in order to increase overall device efficiency. The first monolithic multi-junction solar cell was grown in 1980 at NCSU and utilized an AlGaAs/AlGaAs tunnel junction. In the last 4 decades both the development and analysis of tunnel junction structures and their application to multi-junction solar cells has resulted in significant performance gains. In this review we will first make note of significant studies of III-V tunnel junction materials and performance, then discuss their incorporation into cells and modeling of their characteristics. A Recent study implicating thermally activated compensation of highly doped semiconductors by native defects rather than dopant diffusion in tunnel junction thermal degradation will be discussed. AlGaAs/InGaP tunnel junctions, showing both high current capability and high transparency (high bandgap), are the current standard for space applications. Of significant note is a variant of this structure containing a quantum well interface showing the best performance to date. This has been studied by several groups and will be discussed at length in order to show a path to future improvements.}, number={12}, journal={CRYSTALS}, author={Colter, Peter and Hagar, Brandon and Bedair, Salah}, year={2018}, month={Dec} } @inproceedings{sayed_hagar_carlin_colter_bedair_2016, title={Extending the absorption threshold of InGaP solar cells to 1.60 eV using quantum wells: experimental and modeling results}, DOI={10.1109/pvsc.2016.7750063}, abstractNote={Strain balanced multiple quantum wells (SBMQWs) lattice matched to GaAs consisting of InGaAsP wells balanced with InGaP barriers have been used to extend the absorption of In0.49Ga0.51P subcells to longer wavelengths for use in five and six junction photovoltaic devices. Thin layers of InGaAsP quantum wells that absorb beyond 760 nm, have been grown with compositions within the miscibility gap of InGaAsP while maintaining thermodynamic stability. External quantum efficiency and current-voltage measurements reveal that InGaAsP/InGaP SBMQWs extend absorption beyond the InGaP band-edge and improve the short circuit current with minimal degradation of open circuit voltage. We study the effect of barrier height on the carrier transport through altering the Indium percentage in the InGaP barrier. Three samples of different barrier heights are fabricated and compared with each other. Results indicate that with proper design of the layers thicknesses and compositions, the absorption threshold of InGaP can be extended up to 780 nm (∼1.59 eV). The promising results of InGaAsP/InGaP SBMQWs in this work offer tremendous potential to alleviate current matching restrictions in next generation and current photovoltaic devices.}, booktitle={2016 ieee 43rd photovoltaic specialists conference (pvsc)}, author={Sayed, I. E. H. and Hagar, B. G. and Carlin, C. Z. and Colter, P. C. and Bedair, S. M.}, year={2016}, pages={2366–2370} } @inproceedings{sayed_hagar_carlin_colter_bedair_2016, title={InGaP-based quantum well solar cells}, DOI={10.1109/pvsc.2016.7749566}, abstractNote={Quantum well structures hold tremendous potential in taking next step beyond current photovoltaic structures in achieving solar conversion efficiencies beyond 50%. In this paper we investigate p-i-n InGaP solar cells incorporating InGaAsP/InGaP strain balanced multiple quantum wells (SBMQWs) to tune the absorption threshold beyond the In0.49Ga0.51P cut-off (∼ 1.85 eV). The effects of quantum well number and thickness on the optoelectronic properties of InGaAsP/InGaP SBMQWs are investigated. Specifically, we investigate the bandgap tunability of these SBMQW devices by varying well and barrier thickness. Spectral response measurements reveal that longer excitonic absorption with efficient carrier transport can be realized if proper materials compositions and thicknesses are realized. In addition, InGaP pi-n solar cells including various numbers of InGaAsP/InGaP SBMQWs with an effective bandgap of 1.65 eV in the intrinsic (i) layer were fabricated and characterized. With up to 30 quantum wells, spectral response and light I-V measurements reveal an improvement in the excitonic absorption and short circuit current in comparison to the standard device. The promising results in this work provide an alternative path for realizing 1.5–1.8 eV subcells in next-generation multi-junction solar cells.}, booktitle={2016 ieee 43rd photovoltaic specialists conference (pvsc)}, author={Sayed, I. E. H. and Hagar, B. G. and Carlin, C. Z. and Colter, P. C. and Bedair, S. M.}, year={2016}, pages={147–150} } @article{hashem_carlin_hagar_colter_bedair_2016, title={InGaP-based quantum well solar cells: Growth, structural design, and photovoltaic properties}, volume={119}, ISSN={["1089-7550"]}, DOI={10.1063/1.4943366}, abstractNote={Raising the efficiency ceiling of multi-junction solar cells (MJSCs) through the use of more optimal band gap configurations of next-generation MJSC is crucial for concentrator and space systems. Towards this goal, we propose two strain balanced multiple quantum well (SBMQW) structures to tune the bandgap of InGaP-based solar cells. These structures are based on InxGa1−xAs1−zPz/InyGa1−yP (x > y) and InxGa1−xP/InyGa1−yP (x > y) well/barrier combinations, lattice matched to GaAs in a p-i-n solar cell device. The bandgap of InxGa1−xAs1−zPz/InyGa1−yP can be tuned from 1.82 to 1.65 eV by adjusting the well composition and thickness, which promotes its use as an efficient subcell for next generation five and six junction photovoltaic devices. The thicknesses of wells and barriers are adjusted using a zero net stress balance model to prevent the formation of defects. Thin layers of InGaAsP wells have been grown thermodynamically stable with compositions within the miscibility gap for the bulk alloy. The growth conditions of the two SBMQWs and the individual layers are reported. The structures are characterized and analyzed by optical microscopy, X-ray diffraction, photoluminescence, current-voltage characteristics, and spectral response (external quantum efficiency). The effect of the well number on the excitonic absorption of InGaAsP/InGaP SBMQWs is discussed and analyzed.}, number={9}, journal={JOURNAL OF APPLIED PHYSICS}, author={Hashem, Islam E. and Carlin, C. Zachary and Hagar, Brandon G. and Colter, Peter C. and Bedair, S. M.}, year={2016}, month={Mar} } @article{sayed_carlin_hagar_colter_bedair_2016, title={Strain-Balanced InGaAsP/GaInP Multiple Quantum Well Solar Cells With a Tunable Bandgap (1.65-1.82 eV)}, volume={6}, ISSN={["2156-3381"]}, DOI={10.1109/jphotov.2016.2549745}, abstractNote={Currently available materials for III–V multijunction solar cells lattice matched to GaAs covering the spectral range from 1.65 to 1.82 eV are composed of either immiscible quaternary alloys or contain aluminum. We report the fabrication of a novel aluminum-free In

$_x$

$_{1-x}$

$_{1-z}$

$_z$

/Ga

$_{1-y}$

$_y$

P (x > y ) strain-balanced multiple quantum-well (SBMQW) p-i-n solar cell structure lattice matched to GaAs, grown by metal–organic chemical vapor deposition. SBMQWs consist of alternating layers of In

$_x$

$_{1-x}$

$_{1-z}$

$_z$

wells and Ga

$_{1-y}$

$_y$

P barriers (x > y) under compressive and tensile strain, respectively. When compared with standard GaInP devices, SBMQW structures exhibit longer photoluminescence wavelength (680–780 nm) emission and enhanced light absorption with improved short-circuit current density. In this study, the SBMQW emission and absorption wavelength is controlled by adjusting the layer thickness of InGaAsP wells, while the arsenic and indium compositions are fixed. We show that carriers generated in QWs are extracted via thermionic emission. The proposed SBMQWs allow more flexibility in the design of current multijunction solar cells and future cells with more than four junctions. InGaAsP/GaInP SBMQWs may also be used in applications other than solar cells, such as light-emitting diodes (LEDs) and lasers, with the advantages of tuning the emission and absorption processes.}, number={4}, journal={IEEE JOURNAL OF PHOTOVOLTAICS}, author={Sayed, Islam E. Hashem and Carlin, Conrad Zachary and Hagar, Brandon G. and Colter, Peter C. and Bedair, S. M.}, year={2016}, month={Jul}, pages={997–1003} } @inproceedings{sayed_carlin_hagar_colter_bedair_2015, title={Tunable GaInP solar cell lattice matched to GaAs}, DOI={10.1109/pvsc.2015.7356081}, abstractNote={A new strain-balanced multiple quantum well (MQW) approach to tune the Ga_0.51In_0.49P bandgap is demonstrated. This approach is based on Ga_1-xIn_xP/Ga_1-yIn_yP (x > y) or Ga_1-xIn_xAs_zP_1-z/Ga_1-yIn_yP (x > y) structures, strain balanced and lattice matched to GaAs in a p-i-n solar cell structure. A red shift in the absorption edge and an increase in the short circuit current were observed. Carriers generated in quantum wells due to transitions between the quantum levels are transported across the barriers via thermionic emission. The proposed structure allows more flexibility in the design of current multi-junction solar cells and future cells with more than four junctions.}, booktitle={2015 ieee 42nd photovoltaic specialist conference (pvsc)}, author={Sayed, I. E. H. and Carlin, C. Z. and Hagar, B. and Colter, P. C. and Bedair, S. M.}, year={2015} }