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1.  Lateral Epitaxial Overgrowth of GaN on Sapphire and Silicon Substrates for Ultraviolet Photodetector Applications
M. Razeghi, P. Sandvik, P. Kung, D. Walker, K. Mi, X. Zhang, V. Kumar, J. Diaz, and F. Shahedipour
-- May 1, 2000
 
2.  The Development of Nitride-based UV Photodetector
D. Walker and M. Razeghi
-- March 1, 2000
 
3.  Solar-Blind AlxGa1-xN p-i-n Photodetectors grown on LEO and non-LEO GaN
P. Sandvik, D. Walker, P. Kung, K. Mi, F. Shahedipour, V. Kumar, X. Zhang, J. Diaz, C. Jelen, and M. Razeghi
SPIE Conference, San Jose, CA, Vol. 3948, pp. 265 -- January 26, 2000
The III-Nitride material system is an excellent candidate for UV photodetector applications due to its wide, direct bandgaps and robust material nature. However, despite many inherent material advantages, the III-Nitride material system typically suffers from a large number of extended defects which degrade material quality and device performance. One technique aimed at reducing defect densities in these materials is lateral epitaxial overgrowth (LEO). In this work, we present a preliminary comparison between AlGaN UV, solar-blind p-i-n photodiodes fabricated form LEO GaN and non-LEO GaN. Improvements in both responsivity and rejection ratio are observed, however, further device improvements are necessary. For these, we focus on the optimization of the p- i-n structure and a reduction in contact resistivity to p- GaN and p-AlGaN layers. By improving the structure of the device, GaN p-i-n photodiodes were fabricated and demonstrate 86 percent internal quantum efficiency at 362 nm and a peak to visible rejection ratio of 105. Contact treatments have reduced the contact resistivity to p-GaN and p-AlGaN by over one order of magnitude form our previous results. reprint
 
4.  Solar-blind AlGaN photodiodes with very low cutoff wavelength
D. Walker, V. Kumar, K. Mi, P. Sandvik, P. Kung, X.H. Zhang, and M. Razeghi
Applied Physics Letters 76 (4)-- January 24, 2000
We report the fabrication and characterization of AlxGa1–xN photodiodes (x~0.70) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The peak responsivity for –5 V bias is 0.11 A/W at 232 nm, corresponding to an internal quantum efficiency greater than 90%. The device response drops four orders of magnitude by 275 nm and remains at low response for the entire near-ultraviolet and visible spectrum. Improvements were made to the device design including a semitransparent Ni/Au contact layer and a GaN:Mg cap layer, which dramatically increased device response by enhancing the carrier collection efficiency. reprint
 
5.  Future Trends of III-Nitrides Using Lateral Epitaxial Overgrowth
M. Razeghi, P. Kung, P. Sandvik, X. Zhang, K. Mi, D. Walker, V. Kumar, and J. Diaz
-- December 14, 1999
 
6.  Pulse Autocorrelation Measurements Based on Two- and Three-Photon Conductivity in a GaN Photodiode
A. Streltsov, K.D. Moll, A. Gaeta, P. Kung, D. Walker, and M. Razeghi
Applied Physics Letters 75 (24)-- December 13, 1999
We characterize the performance of a GaN p-i-n photodiode as a nonlinear sensor for second- and third-order femtosecond pulse autocorrelation measurements in the visible and near-infrared regimes, respectively. The two- and three-photon absorption coefficients for GaN are also determined. reprint
 
7.  Development of High-performance III-Nitride-based Semiconductor Devices
M. Razeghi, P. Kung, D. Walker, E. Monroy, M. Hamilton, and P. Sandvik
-- June 1, 1999
 
8.  High-quality visible-blind AlGaN p-i-n photodiodes
E. Monroy, M. Hamilton, D. Walker, P. Kung, F.J. Sanchez, and M. Razeghi
Applied Physics Letters 74 (8)-- February 22, 1999
We report the fabrication and characterization of AlxGa1−xN p-i-n photodiodes (0 < x < 0.15) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The devices present a visible rejection of six orders of magnitude with a cutoff wavelength that shifts from 365 to 338 nm. Photocurrent decays are exponential for high load resistances, with a time constant that corresponds to the RC product of the system. For low load resistances, the transient response becomes non-exponential, with a decay time longer than the RC constant. This behavior is justified by the strong frequency dependence of the device capacitance. By an admittance analysis, we conclude that speed is not limited by deep levels, but by substitutional Mg capture and emission time. reprint
 
9.  High-speed, low-noise metal-semiconductor-metal ultraviolet photodetectors based on GaN
D. Walker, E. Monroy, P. Kung, J. Wu, M. Hamilton, F.J. Sanchez, J. Diaz, and M. Razeghi
Applied Physics Letters 74 (5)-- February 1, 1999
We present the fabrication and characterization of nonintentionally doped GaN and GaN:Mg Schottky metal–semiconductor–metal (MSM) photodetectors, grown on sapphire by metalorganic chemical vapor deposition. Low-leakage, Schottky contacts were made with Pt/Au. The devices are visible blind, with an ultraviolet/green contrast of about five orders of magnitude. The response times of the MSM devices were <10 ns and about 200 ns for GaN and GaN:Mg, respectively. The noise power spectral density remains below the background level of the system (10−24  A²/Hz) up to 5 V, for the undoped GaN MSM detector. reprint
 
10.  AlxGa1-xN p-i-n Photodiodes on Sapphire Substrates
D. Walker, P. Kung, P. Sandvik, J. Wu, M. Hamilton, I.H. Lee, J. Diaz, and M. Razeghi
SPIE Conference, San Jose, CA, -- January 27, 1999
We report the fabrication and characterization of AlxGa1-xN p-i-n photodiodes (0.05 ≤ to X ≤ 0.30) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The devices present a visible-rejection of about four orders of magnitude with a cutoff wavelength that shifts from 350 nm to 291 nm. They also exhibit a constant responsivity for five decades (30 mW/m² to 1 kW/m²) of optical power density. Using capacitance measurements, the values for the acceptor concentration in the p-AlxGa1-xN region and the unintentional donor concentration in the intrinsic region are found. Photocurrent decays are exponential for high load resistances, with a time constant that corresponds to the RC product of the system. For low load resistances the transient response becomes non-exponential, with a decay time longer than the RC constant. reprint
 
11.  Electrical Characterization of AlxGa1-xN for UV Photodetector Applications
A. Saxler, M. Ahoujja, W.C. Mitchel, P. Kung, D. Walker, and M. Razeghi
SPIE Conference, San Jose, CA, -- January 27, 1999
Ultraviolet photodetectors have many military and commercial applications. However, for many of these applications, the photodetectors must be solar blind. This means that the photodetectors must have a cutoff wavelength of less than about 270 nm. Semiconductor based devices would then need energy gaps of over 4.6 eV. In the AlxGa1-xN system, the aluminum mole fraction, x, required is over 40%. As the energy gap is increased, doping becomes much more difficult, especially p-type doping. This report is a study of the electrical properties of AlxGa1-xN to enable better control of the doping. Magnesium doped p-type AlxGa1-xN has been studied using high-temperature Hall effect measurements. The acceptor ionization energy has been found to increase substantially with the aluminum content. Short-period superlattices consisting of alternating layers of GaN:Mg and AlGaN:Mg were also grown by low-pressure organometallic vapor phase epitaxy. The electrical properties of these superlattices were measured as a function of temperature and compared to conventional AlGaN:Mg layers. It is shown that the optical absorption edge can be shifted to shorter wavelengths while lowering the acceptor ionization energy by using short- period superlattice structures instead of bulk-like AlGaN:Mg. Silicon doped n-type films have also been studied. reprint
 
12.  Schottky MSM Photodetectors on GaN Films Grown on Sapphire by Lateral Epitaxial Overgrowth
P. Kung, D. Walker, P. Sandvik, M. Hamilton, J. Diaz, I.H. Lee and M. Razeghi
SPIE Conference, San Jose, CA, -- January 27, 1999
We report the growth and characterization of Schottky based metal-semiconductor-metal ultraviolet photodetectors fabricated on lateral epitaxially overgrown GaN films. The lateral epitaxial overgrowth of GaN was carried out on basal plane sapphire substrates by low pressure metalorganic chemical vapor deposition and exhibited lateral growth rates more than 5 times as high as vertical growth rates. The spectral responsivity, the dependence on bias voltage, on incident optical power, and the time response of these photodetectors have been characterized. Two detector orientations were investigated: one with the interdigitated finger pattern parallel and the other perpendicular to the underlying SiOx mask stripes. reprint
 
13.  Lateral epitaxial overgrowth of GaN films on sapphire and silicon substrates
P. Kung, D. Walker, M. Hamilton, J. Diaz, and M. Razeghi
Applied Physics Letters 74 (4)-- January 25, 1999
We report the lateral epitaxial overgrowth of GaN films on (00.1) Al2O3 and (111) Si substrates by metalorganic chemical vapor deposition. The lateral epitaxial overgrowth on Si substrates was possible after achieving quasi-monocrystalline GaN template films on (111) Si substrates. X-ray diffraction, photoluminescence, scanning electron microscopy, and atomic force microscopy were used to assess the quality of the lateral epitaxial overgrown films. Lateral growth rates more than five times as high as vertical growth rates were achieved for both lateral epitaxial overgrowths of GaN on sapphire and silicon substrates. reprint
 
14.  Development of High-performance III-Nitride-based Semiconductor Devices
M. Razeghi, P. Kung, D. Walker, M. Hamilton, and P. Sandvik
-- November 6, 1998
 
15.  High quality LEO growth and characterization of GaN films on Al2O3 and Si substrates
M. Razeghi, P. Kung, D. Walker, M. Hamilton, and J. Diaz
SPIE International Conference on Solid State Crystals, Zakopane, Poland; Proceedings 3725-- October 12, 1998
We report the lateral epitaxial overgrowth (LEO) of GaN films on (00.1) Al2O3 and (111) Si substrates by metalorganic chemical vapor deposition. The LEO on Si substrates was possible after achieving quasi monocrystalline GaN template films on (111) Si substrates. X-ray diffraction, photoluminescence, scanning electron microscopy and atomic force microscopy were used to assess the quality of the LEO films. Lateral growth rates more than 5 times as high as vertical growth rates were achieved for both LEO growths of GaN on sapphire and silicon substrates. reprint
 
16.  Solar blind GaN p-i-n photodiodes
D. Walker, A. Saxler, P. Kung, X. Zhang, M. Hamilton, J. Diaz and M. Razeghi
Applied Physics Letters 72 (25)-- June 22, 1998
We present the growth and characterization of GaN p-i-n photodiodes with a very high degree of visible blindness. The thin films were grown by low-pressure metalorganic chemical vapor deposition. The room-temperature spectral response shows a high responsivity of 0.15 A/W up until 365 nm, above which the response decreases by six orders of magnitude. Current/voltage measurements supply us with a zero bias resistance of 1011  Ω. Lastly, the temporal response shows a rise and fall time of 2.5 μs measured at zero bias. This response time is limited by the measurement circuit. reprint
 
17.  Continuous-wave room-temperature operation of InGaN/GaN multiquantum well lasers grown by low-pressure metalorganic chemical vapor deposition
M. Razeghi, A. Saxler, P. Kung, D. Walker, X. Zhang, A. Rybaltowski, Y. Xiao, H.J. Yi and J. Diaz
SPIE Conference, San Jose, CA, Vol. 3284, pp. 113-- January 28, 1998
Continuous-wave (CW) room temperature operation of InGaN/GaN multi-quantum well (MQW) lasers is reported. Far-field beam divergence as narrow as 13 degrees and 20 degrees for parallel and perpendicular directions to epilayer planes were measured, respectively. The MQW lasers showed strong beam polarization anisotropy as consistent with QW laser gain theory. Dependencies of threshold current on cavity-length and temperature are also consistent with conventional laser theory. No significant degradation in laser characteristics was observed during lifetime testing for over 140 hours of CW room temperature operation. reprint
 
18.  GaN p-i-n photodiodes with high visible-to-ultraviolet rejection ratio
P. Kung, X. Zhang, D. Walker, A. Saxler, and M. Razeghi
SPIE Conference, San Jose, CA, -- January 28, 1998
UV photodetectors are critical components in many applications, including UV astronomy, flame sensors, early missile threat warning and space-to-space communications. Because of the presence of strong IR radiation in these situations, the photodetectors have to be solar blind, i.e. able to detect UV radiation while not being sensitive to IR. AlxGa1-xN is a promising material system for such devices. AlxGa1-xN materials are wide bandgap semiconductors, with a direct bandgap whose corresponding wavelength can be continuously tuned from 200 to 365 nm. AlxGa1-xN materials are thus insensitive to visible and IR radiation whose wavelengths are higher than 365 nm. We have already reported the fabrication and characterization of AlxGa1-xN-based photoconductors with a cut-off wavelength tunable from 200 to 365 nm by adjusting the ternary alloy composition. Here, we present the growth and characterization of GaN p-i- n photodiodes which exhibit a visible-to-UV rejection ratio of 6 orders or magnitude. The thin films were grown by low pressure metalorganic chemical vapor deposition. Square mesa structures were fabricated using dry etching, followed by contact metallization. The spectral response, rejection ratio and transient response of these photodiodes is reported. reprint
 
19.  New Developments in III-Nitride Material and Device Applications
M. Razeghi, A. Saxler, P. Kung, D. Walker, X. Zhang, K.S. Kim, H.R. Vydyanath, J. Solomon, M. Ahoujja, and W.C. Mitchel
-- January 1, 1998
 
20.  GaInN/GaN Multi-Quantum Well Laser Diodes Grown by Low-Pressure Metalorganic Chemical Vapor Deposition
P. Kung, A. Saxler, D. Walker, A. Rybaltowski, X. Zhang, J. Diaz, and M. Razeghi
MRS Internet Journal of Nitride Semiconductor Research 3 (1)-- January 1, 1998
We report the growth, fabrication and characterization of GaInN/GaN multi-quantum well lasers grown on (00·1) sapphire substrates by low pressure metalorganic chemical vapor deposition. The threshold current density of a 1800 µm long cavity length laser was 1.4 kA/cm² with a threshold voltage of 25 V. These lasers exhibited series resistances of 13 and 14 Ω at 300 and 79 K, respectively. reprint
 
21.  Comparison of Trimethylgallium and Triethylgallium for the Growth of GaN
A. Saxler, D. Walker, P. Kung, X. Zhang, M. Razeghi, J. Solomon, W. Mitchel, and H.R. Vydyanath
Applied Physics Letters 71 (22)-- December 1, 1997
GaN films grown by low-pressure metalorganic chemical vapor deposition using trimethylgallium and triethylgallium as gallium precursors are compared. The films were characterized by x-ray diffraction, Hall effect, photoluminescence, secondary ion mass spectroscopy, and etch pit density measurements. GaN layers grown using triethylgallium exhibited superior electrical and optical properties and a lower carbon impurity concentration. reprint
 
22.  GaN Grown Using Trimethylgallium and Triethylgallium
A. Saxler, P. Kung, X. Zhang, D. Walker, J. Solomon, W.C. Mitchel and M. Razeghi
-- July 1, 1997
 
23.  GaN Doped with Sulfur
A. Saxler, P. Kung, X. Zhang, D. Walker, J. Solomon, M. Ahoujja, W.C. Mitchel, H.R. Vydyanath, and M. Razeghi
-- July 1, 1997
 
24.  Electroluminescence of III-Nitride Double Heterostructure Light Emitting Diodes with Silicon and Magnesium Doped InGaN
A. Saxler, K.S. Kim, D. Walker, P. Kung, X. Zhang, G.J. Brown, W.C. Mitchel and M. Razeghi
-- July 1, 1997
 
25.  Structural and Microstructural Characterization of GaN Thin Films and GaN-based Heterostructures Grown on Sapphire Substrates
M. Razeghi, P. Kung, X. Zhang, D. Walker, A. Saxler, K.Y. Lim and K.S. Kim
-- June 30, 1997
 

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