@article{saripalli_pei_biggerstaff_ramachandran_duscher_johnson_zeng_dandu_jin_barlage_2007, title={Transmission electron microscopy studies of regrown GaN Ohmic contacts on patterned substrates for metal oxide semiconductor field effect transistor applications}, volume={90}, ISSN={["1077-3118"]}, DOI={10.1063/1.2741123}, abstractNote={Contact selected area regrowth of GaN was performed by metal organic chemical vapor deposition using a silicon nitride dielectric hard mask to define plasma etched recesses and to define source-drain regions. A low temperature regrowth process at 750–850°C was adopted to limit lateral overgrowth. High resolution electron microscopy images and selected area diffraction confirmed the regrowth selectivity and revealed that the low temperature regrown GaN is epitaxial and has a wurtzite crystal structure. I-V characteristics of the fabricated metal oxidesemiconductor field effect transistor show enhancement mode operation.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Saripalli, Y. N. and Pei, L. and Biggerstaff, T. and Ramachandran, S. and Duscher, G. J. and Johnson, M. A. L. and Zeng, C. and Dandu, K. and Jin, Y. and Barlage, D. W.}, year={2007}, month={May} }
 @article{gollakota_dhawan_wellenius_lunardi_muth_saripalli_peng_everitt_2006, title={Optical characterization of Eu-doped β-Ga2O3 thin films}, volume={88}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2208368}, DOI={10.1063/1.2208368}, abstractNote={Europium-doped β-Ga2O3 thin films were grown on double-side polished c-axis (0001) sapphire substrates by pulsed laser deposition at 850°C. Transmission measurements of the films revealed a sharp band edge with a band gap at 5.0eV. The films exhibited intense red emission at 611nm (2.03eV) due to the transitions from D05 to F27 levels in europium, with intensities that increased with the concentration of europium. Time-resolved photoluminescence measurements revealed a temperature-insensitive lifetime of 1.4ms, which is much longer than the lifetimes of europium luminescence observed in GaN hosts.}, number={22}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Gollakota, P. and Dhawan, A. and Wellenius, P. and Lunardi, L. M. and Muth, J. F. and Saripalli, Y. N. and Peng, H. Y. and Everitt, H. O.}, year={2006}, month={May}, pages={221906} }
 @article{wang_park_saripalli_johnson_zeng_barlage_long_2006, title={Optical spectroscopic analysis of selected area epitaxially regrown n(+) gallium nitride}, volume={99}, ISSN={["1089-7550"]}, DOI={10.1063/1.2204755}, abstractNote={Gallium nitride (GaN) metal-insulator-semiconductor field-effect transistor with regrown by selected area metal organic vapor-phase-epitaxy n+ layer has been analyzed by micro-Raman and microphotoluminescence (micro-PL) spectroscopy. The material properties of the regrown n+ layer and the intrinsic layer in the gate region were extracted by using both spectroscopies. The free-carrier concentration of the regrown GaN layer and the intrinsic layer were determined by line shape analysis of the coupled plasmon-phonon mode to be 4.7×1017 and <3×1016cm−3, respectively. The inefficient substitutions of Ga vacancy (VGa) by Si result in relatively low carrier concentration in the regrown GaN layer. From the shift of E2(2) Raman peak and the near-band-edge (NBE) PL peak, the biaxial compressive stress in the intrinsic layer was found to be 0.4GPa. The residual stress was found to be fully relaxed in the regrown layer. The Si doping concentration in the regrown layer was determined to be 2×1019cm−3 based on the potential fluctuations introduced redshift of its NBE PL peak.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Wang, D. and Park, M. and Saripalli, Y. N. and Johnson, M. A. L. and Zeng, C. and Barlage, D. W. and Long, J. P.}, year={2006}, month={Jun} }
 @article{saripalli_zeng_long_barlage_johnson_braddock_2006, title={Properties of III-N MOS structures with low-temperature epitaxially regrown ohmic contacts}, volume={287}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2005.10.075}, abstractNote={A significant limitation in the fabrication of III-N MOSFET relates to the formation of ohmic contacts for enhancement-mode MOSFET structures. Unlike existing III-N HFET devices, which include a high free-carrier density two-dimensional electron gas (2DEG) in the semiconductor substrate, a MOSFET in either accumulation or inversion mode require low free-carrier concentrations for the semiconductor channel to have an off-state. The applied gate bias enhances the free-carrier density in the channel, turning on the FET. Unfortunately, a low free-carrier density substrate is problematic for the formation of ohmic contacts, a problem usually dealt with in silicon MOS through self-aligned ion implantation. The high annealing temperatures associated with activating implanted dopants to substitutional sites limits the use of ion implantation for III-N MOSFET fabrication. To overcome this difficulties, selected area epitaxial re-growth of doped III-N materials was developed to form source-drain contacts on otherwise low-doped III-N epitaxial substrates, yielding the needed N+/n−/N+ or N+/p−/N+ structures. Contact re-growth was performed by MOVPE using a silicon nitride dielectric mask defining plasma-etched recesses in the source-drain region. A significant acceleration in the growth rate and surface roughening was observed following re-growth relative to a non-selective area epitaxial growth due to the reduced fill-factor, motivating a general change in MOVPE-operating conditions during re-growth. As the re-growth was intentionally designed to limit the lateral extent of the source-drain regions, the MOVPE re-growth process was performed under conditions limiting lateral overgrowth. III-N MOSFET structures with epitaxial regrown contacts are projected to provide a pathway for low threshold voltage devices suitable for amplifier or logic applications.}, number={2}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Saripalli, YN and Zeng, C and Long, JP and Barlage, DW and Johnson, MAL and Braddock, D}, year={2006}, month={Jan}, pages={562–565} }
 @article{gu_stevie_hitzman_saripalli_johnson_griffis_2006, title={SIMS quantification of matrix and impurity species in AlxGa1-xN}, volume={252}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsusc.2006.02.148}, abstractNote={The quantification in Al x Ga 1-x N with different AlN mole fraction (x) is challenging because of matrix effects and charging effects. For quantitative characterization of both matrix and impurity elements in Al x Ga 1-x N, a novel charge neutralization method was employed and calibration curves were created using an O 2 + primary beam with positive secondary ion detection and a Cs + primary beam with negative and MCs + secondary ion detection. Over the range of 0 < x < 0.58, the matrix ion intensity ratios of Al + /Ga + and AlCs + /GaCs + appear linear with the mole fraction ratio x/(1-x), and the ratio of AlN - /GaN - is linear with AlN mole fraction (x). The sputter rate decreases as AlN mole fraction increases, while the relative sensitivity factors (RSF's) of impurities have an exponential relationship with AlN mole fraction. These calibration curves allow the quantification of both matrix and impurity species in AlGaN with varying AlN mole fraction. The technique can be employed for impurity control, composition and growth rate determination, as well as structural analysis of the finished optoelectronic and electronic devices.}, number={19}, journal={APPLIED SURFACE SCIENCE}, author={Gu, C. J. and Stevie, F. A. and Hitzman, C. J. and Saripalli, Y. N. and Johnson, M. and Griffis, D. P.}, year={2006}, month={Jul}, pages={7228–7231} }
 @article{dandu_saripalli_braddock_johnson_barlage_2005, title={Characterization and modeling of AlGaN/GaN MOS capacitor with leakage for large signal transistor modeling}, volume={15}, ISSN={["1531-1309"]}, DOI={10.1109/LMWC.2005.856680}, abstractNote={Dual mode AlGaN/GaN metal oxide semiconductor (MOS) heterostructure field-effect transistor (HFET) devices were fabricated and characterized. In HFET mode of operation the devices showed an f/sub t//spl middot/L/sub g/ product of 12GHz/spl middot//spl mu/m at Vgs=-2 V. The AlGaN devices showed formation of an accumulation layer under the gate in forward bias and a f/sub t//spl middot/L/sub g/ product of 6GHz/spl middot//spl mu/m was measured at Vgs=5 V. A novel piecewise small signal model for the gate capacitance of MOS HFET devices is presented and procedures to extract the capacitance in presence of gate leakage are outlined. The model accurately fits measured data from 45MHz to 10GHz over the entire bias range of operation of the device.}, number={10}, journal={IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS}, author={Dandu, K and Saripalli, Y and Braddock, D and Johnson, M and Barlage, DW}, year={2005}, month={Oct}, pages={664–666} }