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26.  Top-emission ultraviolet light-emitting diodes with peak emission at 280 nm
A. Yasan, R. McClintock, K. Mayes, S.R. Darvish, P. Kung, and M. Razeghi
Virtual Journal of Nanoscale Science & Technology, 5-- August 5, 2002reprint
 
27.  Top-emission ultraviolet light-emitting diodes with peak emission at 280 nm
A. Yasan, R. McClintock, K. Mayes, S.R. Darvish, P. Kung, and M. Razeghi
Applied Physics Letters 81 (5)-- July 29, 2002
We demonstrate light emission at 280 nm from UV light-emitting diodes consisting of AlInGaN/AlInGaN multiple quantum wells. Turn-on voltage of the devices is ~5 V with a differential resistance of ~40 Ω. The peak emission wavelength redshifts ~1 nm at high injection currents. reprint
 
28.  Future of AlxGa1-xN Materials and Device Technology for Ultraviolet Photodetectors
P. Kung, A. Yasan, R. McClintock, S. Darvish, K. Mi, and M. Razeghi
SPIE Conference, San Jose, CA, Vol. 4650, pp. 199-- May 1, 2002
Design of the photodetector structure is one of the key issues in obtaining high performance devices; especially the thickness of the intrinsic region for p-i-n photodiodes is a crucial value and needs to be optimized. We compare the performance of the p-i-n photodiodes with different widths for the depletion region, which shows a trade-off between speed and responsivity of the devices. reprint
 
29.  Characteristics of high quality p-type AlxGa1-xN/GaN superlattices
A. Yasan, R. McClintock, S.R. Darvish, Z. Lin, K. Mi, P. Kung, and M. Razeghi
Applied Physics Letters 80 (12)-- March 18, 2002
Very-high-quality p-type AlxGa1–xN/GaN superlattices have been grown by low-pressure metalorganic vapor-phase epitaxy through optimization of Mg flow and the period of the superlattice. For the superlattice with x = 26%, the hole concentration reaches a high value of 4.2×1018 cm–3 with a resistivity as low as 0.19 Ω · cm by Hall measurement. Measurements confirm that superlattices with a larger period and higher Al composition have higher hole concentration and lower resistivity, as predicted by theory. reprint
 
30.  280 nm UV LEDs Grown on HVPE GaN Substrates
A. Yasan, R. McClintock, K. Mayes, S.R. Darvish, P. Kung, M. Razeghi, and R.J. Molnar
Opto-Electronics Review, 10 (4)-- January 1, 2002
We report on the enhancement of optical and electrical properties of 280 nm UV LEDs using low dislocation density HVPE-grown GaN substrate. Compared with the same structure grown on sapphire, these LEDs show ~30% reduction in current-voltage differential resistance, ~15% reduction in turn-on voltage, more than 200% increase in output power slope efficiency and saturation at higher currents. Lower density of defects due to higher material quality and better heat dissipation are believed to be the reason behind these improvements. reprint
 
31.  AlxGa1-xN Materials and Device Technology for Solar Blind Ultraviolet Photodetector Applications
R. McClintock, P. Sandvik, K. Mi, F. Shahedipour, A. Yasan, C. Jelen, P. Kung, and M. Razeghi
SPIE Conference, San Jose, CA, Vol. 4288, pp. 219-- January 22, 2001
There has been a growing interest for the development of solar blind ultraviolet (UV) photodetectors for use in a variety of applications, including early missile threat warning, flame monitoring, UV radiation monitoring and chemical/biological reagent detection. The AlxGa1-xN material system has emerged as the most promising approach for such devices. However, the control of the material quality and the device technology are still rather immature. We report here the metalorganic chemical vapor deposition, the n-type and the p-type doping of high quality AlxGa1-xN thin films on sapphire substrates over a wide range of Al concentration. reprint
 
32.  AlxGa1-xN for Solar-Blind UV Detectors
P. Sandvik, K. Mi, F. Shahedipour, R. McClintock, A. Yasan, P. Kung, and M. Razeghi
Journal of Crystal Growth 231 (2001)-- January 1, 2001
We report on the metalorganic chemical vapor deposition of high quality AlGaN thin films on sapphire substrates over a wide range of Al concentrations. The quality of these AlGaN materials was verified through a demonstration of high performance visible and solar-blind UV p–i–n photodiodes with peak cutoff wavelengths ranging from 227 to 364 nm. External quantum efficiencies for these devices reached as high as 69% with over five orders rejection ratio from the peak to visible wavelengths. reprint
 
33.  III-Nitride Wide Bandgap Semiconductors: A Survey of the Current Status and Future Trends of the Material and Device technology
P. Kung and M. Razeghi
Opto-Electronics Review 8 (3)-- September 1, 2000
During the past decade, group III - Nitride wide bandgap semiconductors have become the focus of extremely intensive reearch because of their exceptional physical properties and their high potential for use in countless numbers of applications. Nearly all aspects have been investigated, from the fundamental physical understanding of these materials to the development of the fabrication technology and demonstration of commercial devices. The purpose of this paper is to review the physical properties of III-Nitrides, their areas of application, the current status of the material technology (AlN, AlGaN, GaN, GaInN) including synthesis and processing. The state-of-the-art of III-Nitride material quality, as well as the devices which have been demonstrated, including electronic devices, AlGaN ultraviolet photoconductors, ultraviolet photodiodes, visible light emitting diodes (LEDs) and ultraviolet - blue laser diodes, will also be presented.
 
34.  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
 
35.  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
 
36.  LEO of III-Nitride on Al2O3 and Si Substrates
M. Razeghi, P. Kung, P. Sandvik, K. Mi, X. Zhang, V.P. Dravid, J. Freitas, and A. Saxler
SPIE Conference, San Jose, CA, -- January 26, 2000
Lateral epitaxial overgrowth (LEO) has recently become the method of choice to reduce the density of dislocations in heteroepitaxial GaN thin films, and is thus expected to lead to enhanced performance devices. We present here the LEO growth and characterization of GaN films by low pressure metalorganic chemical vapor deposition. Various substrates were used, including basal plane sapphire and oriented Si substrates. The steps in the LEO growth technology will be briefly reviewed. The characterization results will be discussed in detail. The structural, electrical and optical properties of the films were assessed through scanning, atomic and transmission electron microscopy, x-ray diffraction, capacitance-voltage, deep level transient spectroscopy, photoluminescence, and scanning cathodoluminenscence measurements. Single-step and double- step LEO GaN was achieved on sapphire. Similarly high quality LEO grown GaN films were obtained on sapphire and silicon substrates. Clear and dramatic reduction in the density of defects are observed in LEO grown materials using the various characterization techniques mentioned previously. reprint
 
37.  Ultraviolet Detector Materials and Devices Studied by Femtosecond Nonlinear Optical Techniques
M. Wraback, H. Shen, P. Kung, M. Razeghi, J.C. Carrano, T. Li, and J.C. Campbell
SPIE Conference, San Jose, CA, -- January 26, 2000
Femtosecond nonlinear optical techniques have been employed in the study of carrier dynamics and transport in UV detector materials. Visible femtosecond pulses derived from the signal beam of a 250 kHz regenerative amplifier-pumped optical parametric amplifier were frequency doubled to obtain pulses tunable from 250 nm to 375 nm. Time-resolved reflectivity experiments indicate that the room-temperature carrier lifetime in GaN grown by double lateral epitaxial overgrowth is about 3 times longer than that of GaN grown on sapphire without benefit of this technique. The electron velocity-field characteristics and saturation velocity in GaN have been obtained form time-resolved studies of electroabsorption in a GaN p-i-n diode. The peak steady- state velocity of 1.9x107 cm/s in this device occurs at 225 kV/cm. Time-resolved transmission measurements have been used to monitor ultrafast carrier relaxation phenomena in a thin AlGaN layer with bandgap in the solar blind region of the spectrum. Excitation intensity and wavelength dependent studies of the photoinduced bleaching decays suggest that they are primarily governed by trapping in a high density of sub-bandgap defect levels. reprint
 
38.  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
 
39.  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
 
40.  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
 
41.  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
 
42.  Aluminum gallium nitride short-period superlattices doped with magnesium
A. Saxler, W.C. Mitchel, P. Kung and M. Razeghi
Applied Physics Letters 74 (14)-- April 9, 1999
Short-period superlattices consisting of alternating layers of GaN:Mg and AlGaN:Mg were 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. reprint
 
43.  Phase-matched optical second-harmonic generation in GaN and AlN slab waveguides
D.N. Hahn, G.T. Kiehne, G.K.L. Wong, J.B. Ketterson, P. Kung, A. Saxler and M. Razeghi
Journal of Applied Physics 85 (5)-- March 1, 1999
Phase-matched optical second-harmonic (SH) generation was observed in GaN and AlN slab waveguides. Phase matching was achieved by waveguide modal dispersion. By tuning the output wavelength of an optical parametric amplifier, several phased-matched SH peaks were observed in the visible spectrum covering blue to red wavelengths. The peak positions are in agreement with the values calculated using the dispersive refractive indices of the film and substrate materials. reprint
 
44.  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
 
45.  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
 
46.  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
 
47.  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
 
48.  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
 
49.  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
 
50.  Band-gap narrowing and potential fluctuation in Si-doped GaN
I.H. Lee, J.J. Lee, P. Kung, F.J. Sanchez, and M. Razeghi
Applied Physics Letters 74 (1)-- January 4, 1999
We investigate the optical properties of two sets of Si-doped GaN epitaxial layers with different degree of compensation. The electron concentration dependence of the band-gap energy measured by photoluminescence is interpreted as band-gap narrowing effect and evaluated by a simple relation. The photoluminescence peak positions of heavily compensated samples are shifted downward with respect to those of moderately compensated samples, and the down shift becomes larger at higher electron density. Based on analysis of photoluminescence spectra, these prominent behaviors are accounted for by band-edge potential fluctuation associated with inhomogeneous residual impurities. reprint
 

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