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1.  Scaling in back-illuminated GaN avalanche photodiodes
K. Minder, J.L. Pau, R. McClintock, P. Kung, C. Bayram, M. Razeghi and D. Silversmith
Applied Physics Letters, Vol. 91, No. 7, p. 073513-1-- August 13, 2007
Avalanche p-i-n photodiodes of various mesa areas were fabricated on AlN templates for back illumination for enhanced performance through hole-initiated multiplication, and the effects of increased area on device performance were studied. Avalanche multiplication was observed in mesa sizes up to 14,063 µm^2 under linear mode operation. Uniform gain and a linear increase of the dark current with area were demonstrated. reprint
 
2.  Geiger-mode operation of back-illuminated GaN avalanche photodiodes
J. L. Pau, R. McClintock, K. Minder, C. Bayram, P. Kung, M. Razeghi, E. Muñoz, and D. Silversmith
Applied Physics Letters, Vol. 91, No. 04, p. 041104 -1-- July 23, 2007
We report the Geiger-mode operation of back-illuminated GaN avalanche photodiodes fabricated on transparent AlN templates specifically for back illumination in order to enhance hole-initiated multiplication. The spectral response in Geiger-mode operation was analyzed under low photon fluxes. Single photon detection capabilities were demonstrated in devices with areas ranging from 225 to 14,063 µm2. Single photon detection efficiency of 20% and dark count rate < 10 kHz were achieved in the smallest devices. reprint
 
3.  Hole-initiated multiplication in back-illuminated GaN avalanche photodiodes
R. McClintock, J.L. Pau, K. Minder, C. Bayram, P. Kung and M. Razeghi
Applied Physics Letters, Vol. 90 No. 14, p. 141112-1-- April 2, 2007
Avalanche p-i-n photodiodes were fabricated on AlN templates for back illumination. Structures with different intrinsic layer thicknesses were tested. A critical electric field of 2.73 MV/cm was estimated from the variation of the breakdown voltage with thickness. From the device response under back and front illumination and the consequent selective injection of holes and electrons in the junction, ionization coefficients were obtained for GaN. The hole ionization coefficient was found to be higher than the electron ionization coefficient as predicted by theory. Excess multiplication noise factors were also calculated for back and front illumination, and indicated a higher noise contribution for electron injection. reprint
 
4.  Materials characterization of n-ZnO/p-GaN:Mg/c-Al(2)O(3) UV LEDs grown by pulsed laser deposition and metal-organic chemical vapor deposition
D. Rogers, F.H. Teherani, P. Kung, K. Minder, and M. Razeghi
Superlattices and Microstructures-- April 1, 2007
n-ZnO/p-GaN:Mg hybrid heterojunctions grown on c-Al2O3 substrates showed 375 nm room temperature electroluminescence. It was suggested that the high materials and interface quality obtained using pulsed laser deposition for the n-ZnO growth and metal–organic chemical vapor deposition for the p-GaN:Mg were key factors enabling the injection of holes and the radiative near band edge recombination in the ZnO. In this paper we present the materials characterization of this structure using x-ray diffraction, scanning electron microscopy and atomic force microscopy. reprint
 
5.  Etching of ZnO Towards the Development of ZnO Homostructure LEDs
K. Minder, F.H. Teherani, D. Rogers, C. Bayram, R. McClintock, P. Kung, and M. Razeghi
SPIE Conference, January 25-29, 2007, San Jose, CA Proceedings – Zinc Oxide Materials and Devices II, Vol. 6474, p. 64740Q-1-6-- January 29, 2007
Although ZnO has recently gained much interest as an alternative to the III-Nitride material system, the development of ZnO based optoelectonic devices is still in its infancy. Significant material breakthroughs in p-type doping of ZnO thin films and improvements in crystal growth techniques have recently been achieved, making the development of optoelectonic devices possible. First, a survey of current ZnO processing methods is presented, followed by the results of our processing research. reprint
 
6.  III-Nitride Avalanche Photodiodes
P. Kung, R. McClintock, J. Pau Vizcaino, K. Minder, C. Bayram and M. Razeghi
SPIE Conference, January 25-29, 2007, San Jose, CA Proceedings – Quantum Sensing and Nanophotonic Devices IV, Vol. 6479, p. 64791J-1-12-- January 29, 2007
Wide bandgap III-Nitride semiconductors are a promising material system for the development of ultraviolet avalanche photodiodes (APDs) that could be a viable alternative to photomultiplier tubes. In this paper, we report the epitaxial growth and physical properties of device quality GaN layers on high quality AlN templates for the first backilluminated GaN p-i-n APD structures on transparent sapphire substrates. Under low bias and linear mode avalanche operation where they exhibited gains near 1500 after undergoing avalanche breakdown. The breakdown electric field in GaN was determined to be 2.73 MV/cm. The hole impact ionization coefficients were shown to be greater than those of electrons. reprint
 
7.  Electroluminescence at 375 nm from a Zn0/GaN:Mg/c-Al2O3 heterojunction light emitting diodes
D.J. Rogers, F.Hosseini Teherani, A. Yasan, K. Minder, P. Kung, and M. Razeghi
Applied Physics Letters, 88 (14)-- April 13, 2006
n-ZnO/p-GaN:Mg heterojunction light emitting diode (LED) mesas were fabricated on c-Al2O3 substrates using pulsed laser deposition for the ZnO and metal organic chemical vapor deposition for the GaN:Mg. Room temperature (RT) photoluminescence (PL) showed an intense main peak at 375 nm and a negligibly low green emission indicative of a near band edge excitonic emission from a ZnO layer with low dislocation/defect density. The LEDs showed I-V characteristics confirming a rectifying diode behavior and a RT electroluminescence (EL) peaked at about 375 nm. reprint
 
8.  Investigations of p-type signal for ZnO thin films grown on (100) GaAs substrates by pulsed laser deposition
D.J. Rogers, F. Hosseini Teherani, T. Monteiro, M. Soares, A. Neves, M. Carmo, S. Periera, M.R. Correia, A. Lusson, E. Alves, N.P. Barradas, J.K. Morrod, K.A. Prior, P. Kung, A. Yasan, and M. Razeghi
Phys. Stat. Sol. C, 3 (4)-- March 1, 2006
n this work we investigated ZnO films grown on semi-insulating (100) GaAs substrates by pulsed laser deposition. Samples were studied using techniques including X-ray diffraction (XRD), scanning electron microscopy, atomic force microscopy, Raman spectroscopy, temperature dependent photoluminescence, C-V profiling and temperature dependent Hall measurements. reprint
 
9.  Solar-blind avalanche photodiodes
R. McClintock, K. Minder, A. Yasan, C. Bayram, F. Fuchs, P. Kung and M. Razeghi
SPIE Conference, San Jose, CA, Vol. 6127, pp. 61271D-- January 23, 2006
There is a need for semiconductor based UV photodetectors to support avalanche gain in order to realize better performance and more effectively compete with existing photomultiplier tubes. However, there are numerous technical issues associated with the realization of high-quality solar-blind avalanche photodiodes (APDs). In this paper, APDs operating at 280 nm, within the solar-blind region of the ultraviolet spectrum, are investigated. reprint
 
10.  Fabrication of GaN nanotubular material using MOCVD with aluminum oxide membrane
W.G. Jung, S.H. Jung, P. Kung, and M. Razeghi
SPIE Conference, San Jose, CA, Vol. 6127, pp. 61270K-- January 23, 2006
GaN nanotubular material is fabricated with aluminum oxide membrane in MOCVD. SEM, XRD, TEM and PL are employed to characterize the fabricated GaN nanotubular material. An aluminum oxide membrane with ordered nano holes is used as template. Gallium nitride is deposited at the inner wall of the nano holes in aluminum oxide template, and the nanotubular material with high aspect ratio is synthesized using the precursors of TMG and ammonia gas. Optimal synthesis condition in MOCVD is obtained successfully for the gallium nitride nanotubular material in this research. The diameter of GaN nanotube fabricated is approximately 200 ~ 250 nm and the wall thickness is about 40 ~ 50 nm. GaN nanotubular material consists of numerous fine GaN particulates with sizes ranging 15 to 30 nm. reprint
 
11.  Fabrication of GaN Nanotubular Material using MOCVD with an Aluminium Oxide Membrane
W.G. Jung, S.H. Jung, P. Kung, and M. Razeghi
Nanotechnology 17-- January 1, 2006
GaN nanotubular material is fabricated with an aluminium oxide membrane in MOCVD. SEM, XRD, TEM and PL are employed to characterize the fabricated GaN nanotubular material. An aluminium oxide membrane with ordered nanoholes is used as a template. Gallium nitride is deposited at the inner wall of the nanoholes in the aluminium oxide template, and the nanotubular material with high aspect ratio is synthesized using the precursors of TMG and ammonia gas. Optimal synthesis conditions in MOCVD are obtained successfully for the gallium nitride nanotubular material in this research. The diameter of the GaN nanotube fabricated is approximately 200–250 nm and the wall thickness is about 40–50 nm. reprint
 
12.  Avalanche multiplication in AlGaN based solar-blind photodetectors
R. McClintock, A. Yasan, K. Minder, P. Kung, and M. Razeghi
Applied Physics Letters, 87 (24)-- December 12, 2005
Avalanche multiplication has been observed in solar-blind AlGaN-based p-i-n photodiodes. Upon ultraviolet illumination, the optical gain shows a soft breakdown starting at relatively low electric fields, eventually saturating without showing a Geiger mode breakdown. The devices achieve a maximum optical gain of 700 at a reverse bias of 60 V. By modeling the device, it is found that this corresponds to an electric-field strength of 1.7 MV/cm. reprint
 
13.  Short-wavelength ultraviolet light-emitting diodes based on AlGaN
M. Razeghi; A. Yasan; R. McClintock; K. Mayes; P. Kung
2005 Conference on Lasers and Electro-Optics, CLEO. 153-155 [CMI5] (2005)-- May 22, 2005
We review our progress toward realization of highly-efficient ultraviolet light-emitting diodes (UV LEDs) based on high Al-composition AlxGa1-xN. Milliwatt level optical output powers have been measured at wavelengths as short as 247 nm.
 
14.  Back-illuminated solar-blind photodetectors for imaging applications
R. McClintock, A. Yasan, K. Mayes, P. Kung, and M. Razeghi
SPIE Conference, Jose, CA, Vol. 5732, pp.175-- January 22, 2005
Back-illuminated solar-blind ultraviolet p-i-n photodetectors and focal plane arrays are investigated. We initially study single-pixel devices and then discuss the hybridization to a read-out integrated circuit to form focal plane arrays for solar-blind UV imaging. reprint
 
15.  AlGaN-based deep UV light emitting diodes with peak emission below 255 nm
A. Yasan, R. McClintock, K. Mayes, P. Kung, and M. Razeghi
SPIE Conference, Jose, CA, Vol. 5732, pp. 197-- January 22, 2005
We report on the growth and fabrication of AlGaN-based deep ultraviolet light-emitting diodes (LEDs) with peak emission of below 255 nm. In order to achieve such short wavelength UV LEDs, the Al mole fractions in the device layers should be greater than ~60%. This introdues serious challenges on the growth and doping of AlxGa1-xN epilayers. However, with the aid of a high-quality AlN template layer and refinement of the growth conditions we have been able to demonstrate UV LEDs emitting below 255 nm. reprint
 
16.  ZnO Thin Film Templates for GaN-based Devices
D.J. Rogers, F. Hosseini Teherani, A. Yasan, R. McClintock, K. Mayes, S.R. Darvish, P. Kung, M. Razeghi and G. Garry
SPIE Conference, Jose, CA, Vol. 5732, pp. 412-- January 22, 2005
GaN-based optoelectronic devices are plagued by a tendency to non-radiative transitions linked to defects in the active layers. ZnO is promising as a substrate material for GaN because it has the same wurtzite structure and a relatively small lattice mismatch (~1.8%). In this paper, we discuss use of ZnO thin films as templates for GaN based LED. reprint
 
17.  320x256 Solar-Blind Focal Plane Arrays based on AlxGa1-xN
R. McClintock, K. Mayes, A. Yasan, D. Shiell, P. Kung, and M. Razeghi
Applied Physics Letters, 86 (1)-- January 3, 2005
We report AlGaN-based back-illuminated solar-blind ultraviolet focal plane arrays operating at a wavelength of 280 nm. The electrical characteristics of the individual pixels are discussed, and the uniformity of the array is presented. The p–i–n photodiode array was hybridized to a 320×256 read-out integrated circuit entirely within our university research lab, and a working 320×256 camera was demonstrated. Several example solar-blind images from the camera are also provided. reprint
 
18.  Review of III-Nitride Optoelectronic Materials for light Emission and Detection
M. Razeghi, A. Yasan, R. McClintock, K. Mayes, D. Shiell, S. Darvish, and P. Kung
Physica Status Solidi C S141 - S148-- September 10, 2004
We review the significant achievements relating to optoelectronic devices based on III-nitrides at the center for quantum devices (CQD). Based on GaN/InGaN multiple-quantum well structures, we demonstrated blue laser diodes at a wavelength of 405 nm. This achievement was particularly significant at the time, because while no defect reduction technique was used, a fairly low threshold current density was achieved (3.8 kA/cm²). In the past few years, however, the interest has shifted towards shorter wavelength light emitters, i.e. ultraviolet LEDs and LDs. Lower crystalline quality and unsatisfactory doping levels of AlGaN compound semiconductors posed serious challenges en route to the realization of UV light emitters. However, steady progress in the growth of AlGaN and AlN epilayers made it possible to overcome some of the difficulties. To date, we have been able to demonstrate UV LEDs at wavelengths as short as 265 nm (corresponding to 45% Al in AlxGa1-xN) with optical output powers of over 5 mW. We have addressed the n-type AlGaN doping problem by using a Si-In co-doped scheme. We also employed high-quality AlGaN/AlN superlattice templates for the reduction of defects. We have also demonstrated 280 nm UV LEDs with output powers of over 6 mW and external quantum efficiencies of over 0.25%. Despite all the success in the realization of short-wavelength UV LEDs, UV laser diodes at these short wavelengths are yet to be realized. The main difficulties are the low material quality, high device resistance leading to excessive heating of the device, realization of smooth cavity mirrors, and issues related to the cracking of the material. We have also demonstrated different types of photodetectors in the UV range of the spectrum: photoconductors, MSM photodetector, Schottky barrier photodetectors, and p-i-n photodiodes to name a few. The most promising type of photodetector for realization of UV imaging focal plane arrays is the p-i-n photodiode. Realization of high-efficiency AlGaN-based p-i-n photodiodes becomes more difficult when considering the need for the collection of the light from the backside of the substrate. However, similar to our back-emission UV LED structure, we have demonstrated back-illuminated p-i-n solar-blind photodiodes with external quantum efficiencies as high as 68% under no applied bias and 74% under -5 V of bias. reprint
 
19.  High Quantum Efficiency AlGaN Solar-Blind Photodetectors
R. McClintock, A. Yasan, K. Mayes, D. Shiell, S.R. Darvish, P. Kung and M. Razeghi
Applied Physics Letters, 84 (8)-- February 23, 2004
We report AlGaN-based back-illuminated solar-blind ultraviolet p-i-n photodetectors with a peak responsivity of 136 mA/W at 282 nm without bias. This corresponds to a high external quantum efficiency of 60%, which improves to a value as high as 72% under 5 V reverse bias. We attribute the high performance of these devices to the use of a very-high quality AlN and Al0.87Ga0.13N/AlN superlattice material and a highly conductive Si–In co-doped Al0.5Ga0.5N layer reprint
 
20.  High Power 280 nm AlGaN Light Emitting Diodes Based on an Asymmetric Single Quantum Well
K. Mayes, A. Yasan, R. McClintock, D. Shiell, S.R. Darvish, P. Kung, and M. Razeghi
Applied Physics Letters, 84 (7)-- February 16, 2004
We demonstrate high-power AlGaN-based ultraviolet light-emitting diodes grown on sapphire with an emission wavelength of 280 nm using an asymmetric single-quantum-well active layer configuration on top of a high-quality AlGaN/AlN template layer. An output power of 1.8 mW at a pulsed current of 400 mA was achieved for a single 300 µm×300 µm diode. This device reached a high peak external quantum efficiency of 0.24% at 40 mA. An array of four diodes produced 6.5 mW at 880 mA of pulsed current. reprint
 
21.  High Quantum Efficiency Solar-Blind Photodetectors
R. McClintock, A. Yasan, K. Mayes, D. Shiell, S. Darvish, P. Kung and M. Razeghi
SPIE Conference, Jose, CA, Vol. 5359, pp. 434-- January 25, 2004
We report AlGaN-based back-illuminated solar-blind p-i-n photodetectors with a record peak responsivity of 150 mA/W at 280 nm, corresponding to a high external quantum efficiency of 68%, increasing to 74% under 5 volts reverse bias. Through optimization of the p-AlGaN layer, we were able to remove the out-of-band negative photoresponse originating from the Schottky-like p-type metal contact, and hence significantly improve the degree of solar-blindness reprint
 
22.  Growth of Deep UV Light Emitting Diodes by Metalorganic Chemical Vapor Deposition
A. Yasan, R. McClintock, K. Mayes, D. Shiell, S. Darvish, P. Kung and M. Razeghi
SPIE Conference, Jose, CA, Vol. 5359, pp. 400-- January 25, 2004
We demonstrate high power AlGaN based ultraviolet light-emitting diodes (UV LEDs) with an emission wavelength of 280 nm using an asymmetric single quantum well active layer configuration on top of a high-quality AlGaN/AlN template layer grown by metalorganic chemical vapor deposition (MOCVD). An output power of 1.8 mW at a pulsed current of 400 mA was achieved for a single 300 µm × 300 µm diode. This device reached a high peak external quantum efficiency of 0.24% at 40 mA. An array of four diodes produced 6.5 mW at 880 mA of pulsed current. reprint
 
23.  4.5 mW Operation of AlGaN-based 267 nm Deep-Ultraviolet Light-Emitting Diodes
A. Yasan, R. McClintock, K. Mayes, D. Shiell, L. Gautero, S.R. Darvish, P. Kung and M. Razeghi
Applied Physics Letters, 83 (23)-- December 8, 2003
We demonstrate 4.5 mW output power from AlGaN-based single quantum well ultraviolet light-emitting diodes at a very short wavelength of 267 nm in pulsed operation mode. The output power in continuous-wave mode reaches a value of 165 µW at an injected current of 435 mA. The measurements were done on arrays of four devices flip chip bonded to AlN submounts for thermal management. reprint
 
24.  Photoluminescence Study of AlGaN-based 280 nm Ultraviolet Light-Emitting Diodes
A. Yasan, R. McClintock, K. Mayes, D.H. Kim, P. Kung, and M. Razeghi
Applied Physics Letters, 83 (20)-- November 17, 2003
We investigated optical properties of single quantum well AlGaN-based UV 280 nm light-emitting diodes using temperature-dependent photoluminescence (PL) measurement. We found an "S-shaped" temperature dependence of the peak energy. From the Arrhenius plot of integrated PL intensity, we speculate that dislocations as well as thermal emission of carriers out of the quantum well are responsible for the PL quenching behavior. Also a second nonradiative channel with much lower activation energy was found, the origin of which we believe to be quenching of the bound excitons reprint
 
25.  Comparison of ultraviolet light-emitting diodes with peak emission at 340 nm grown on GaN substrate and sapphire
A. Yasan, R. McClintock, K. Mayes, S.R. Darvish, H. Zhang, P. Kung, M. Razeghi, S.K. Lee and J.Y. Han
Applied Physics Letters, 81 (12)-- September 16, 2002
Based on AlInGaN/AlInGaN multiquantum wells, we compare properties of ultraviolet light-emitting diodes (LED) with peak emission at 340 nm grown on free-standing hydride vapor phase epitaxially grown GaN substrate and on sapphire. For the LED grown on GaN substrate, a differential resistance as low as 13 Ω and an output power of more than one order of magnitude higher than that of the same structure grown on sapphire are achieved. Due to higher thermal conductivity of GaN, output power of the LEDs saturates at higher injection currents compared to the devices grown on sapphire. reprint
 

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