Publications by    
Page 1  (16 Items)

1.  
On the interface properties of ZnO/Si electroluminescent diodes
On the interface properties of ZnO/Si electroluminescent diodes
J.L. Pau, J. Piqueras, D.J. Rogers, F. Hosseini Teherani, K. Minder, R. McClintock, and M. Razeghi
Journal of Applied Physics, Vol. 107, No. 3, p. 033719-1-- February 1, 2010
ZnO layers grown on n–Si(100), n+–Si(100), and n–Si(111) substrates by pulsed-laser deposition were found to give electroluminescence. Light emission was observed in the form of discrete spots for currents over 1 mA with a white appearance to the naked eye. The intensity of these spots showed an erratic behavior over time, appearing and disappearing at random, while showing an associated random telegraph noise in the current signal. Regardless the substrate used, the electroluminescence spectra had a main broadband emission centered at about 600 nm and a relatively small peak at around 380 nm which corresponds to the energy of ZnO near band edge emission. Furthermore, the devices exhibited rectifying characteristics, whose current blocking direction depended on the substrate orientation. Optimization of ZnO conductivity and performing sample growth in N2 ambient were found to be critical to enhance the emission intensity. Rutherford backscattering characterization revealed the existence of an intermixed region at the interface between ZnO and Si. To study the electronic properties at the interface, frequency dependent capacitance measurements were carried out. The junction capacitance became frequency dependent at the bias voltages at which light emission occurs due to the relatively slow trapping and generation processes at deep centers. These centers are believed to play an important role in the mechanism of light emission. reprint
 
2.  
III-Nitride avalanche photodiodes
III-Nitride avalanche photodiodes
R. McClintock, J.L. Pau, C. Bayram, B. Fain, P. Giedratis, M. Razeghi and M. Ulmer
SPIE Proceedings, San Jose, CA Volume 7222-0U-- January 26, 2009
Research into avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet (UV) detectors in numerous civilian and military applications. By designing photodetectors to utilize low-noise impact ionization based gain, GaN APDs operating in Geiger mode can deliver gains exceeding 1×107. Thus with careful design, it becomes possible to count photons at the single photon level. In this paper we review the current state of the art in III-Nitride visible-blind APDs, and present our latest results regarding linear and Geiger mode III-Nitride based APDs. This includes novel device designs such as separate absorption and multiplication APDs (SAM-APDs). We also discuss control of the material quality and the critical issue of p-type doping - demonstrating a novel delta-doping technique for improved material quality and enhanced electric field confinement. The spectral response and Geiger-mode photon counting performance of these devices are then analyzed under low photon fluxes, with single photon detection capabilities being demonstrated. Other major technical issues associated with the realization of high-quality visible-blind Geiger mode APDs are also discussed in detail and future prospects for improving upon the performance of these devices are outlined. reprint
 
3.  
GaN-based nanostructured photodetectors
GaN-based nanostructured photodetectors
J.L. Pau, C. Bayram, P. Giedraitis, R. McClintock, and M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7222-14-- January 26, 2009
The use of nanostructures in semiconductor technology leads to the observation of new phenomena in device physics. Further quantum and non-quantum effects arise from the reduction of device dimension to a nanometric scale. In nanopillars, quantum confinement regime is only revealed when the lateral dimensions are lower than 50 nm. For larger mesoscopic systems, quantum effects are not observable but surface states play a key role and make the properties of nanostructured devices depart from those found in conventional devices. In this work, we present the fabrication of GaN nanostructured metal-semiconductor-metal (MSM) and p-i-n photodiodes (PIN PDs) by e-beam lithography, as well as the investigation of their photoelectrical properties at room temperature. The nanopillar height and diameter are about 520 nm and 200 nm, respectively. MSMs present dark currents densities of 0.4 A/cm2 at ±100 V. A strong increase of the optical response with bias is observed, resulting in responsivities higher than 1 A/W. The relationship between this gain mechanism and surface states is discussed. PIN PDs yield peak responsivities (Rpeak) of 35 mA/W at -4 V and show an abnormal increase of the response (Rpeak > 100 A/W) under forward biases. reprint
 
4.  
GaN nanostructured p-i-n photodiodes
GaN nanostructured p-i-n photodiodes
J.L. Pau, C. Bayram, P. Giedraitis, R. McClintock, and M. Razeghi
Applied Physics Letters, Vol. 93, No. 22, p. 221104-1-- December 1, 2008
We report the fabrication of nanostructured p-i-n photodiodes based on GaN. Each device comprises arrays of ~200 nm diameter and 520 nm tall nanopillars on a 1 µm period, fabricated by e-beam lithography. Strong rectifying behavior was obtained with an average reverse current per nanopillar of 5 fA at −5 V. In contrast to conventional GaN diodes, nanostructured devices reproducibly show ideality factors lower than 2. Enhanced tunneling through sidewall surface states is proposed as the responsible mechanism for this behavior. Under backillumination, the quantum efficiency in nanostructured devices is partly limited by the collection efficiency of holes into the nanopillars. reprint
 
5.  
Comprehensive study of blue and green multi-quantum-well light-emitting diodes grown on conventional and lateral epitaxial overgrowth GaN
Comprehensive study of blue and green multi-quantum-well light-emitting diodes grown on conventional and lateral epitaxial overgrowth GaN
C. Bayram, J.L. Pau, R. McClintock and M. Razeghi
Applied Physics B: Lasers and Optics, Vol. 95, p. 307-314-- November 29, 2008
Growths of blue and green multi-quantum wells (MQWs) and light-emitting diodes (LEDs) are realized on lateral epitaxial overgrowth (LEO) GaN, and compared with identical structures grown on conventional GaN. Atomic force microscopy is used to confirm the significant reduction of dislocations in the wing region of our LEO samples before active-region growth. Differences between surface morphologies of blue and green MQWs are analyzed. These MQWs are integrated into LEDs. All devices show a blue shift in the electroluminescence (EL) peak and narrowing in EL spectra with increasing injection current, both characteristics attributed to the band-gap renormalization. Green LEDs show a larger EL peak shift and a broader EL spectrum due to larger piezoelectric field and more indium segregation in the MQWs, respectively. Blue LEDs on LEO GaN show a higher performance than those on conventional GaN; however, no performance difference is observed for green LEDs on LEO GaN versus conventional GaN. The performance of the green LEDs is shown to be primarily limited by the active layer growth quality. reprint
 
6.  
High quantum efficiency back-illuminated GaN avalanche photodiodes
High quantum efficiency back-illuminated GaN avalanche photodiodes
C. Bayram, J.L. Pau, R. McClintock, M. Razeghi, M.P. Ulmer, and D. Silversmith
Applied Physics Letters, Vol. 93, No. 21, p. 211107-1-- November 24, 2008
Back-illuminated avalanche photodiodes (APDs) composed of heterojunctions of either p-GaN/i-GaN/n-AlGaN or p-GaN/i-GaN/n-GaN/n-AlGaN were fabricated on AlN templates. At low voltage, an external quantum efficiency of 57% at 352 nm with a bandpass response was achieved by using AlGaN in the n-layer. Dependency of gain and leakage current on mesa area for these heterojunction APDs were studied. Back-illumination via different wavelength sources was used to demonstrate the advantages of hole-initiated multiplication in GaN APDs. reprint
 
7.  
Delta-doping optimization for high qualityp-type GaN
Delta-doping optimization for high qualityp-type GaN
C. Bayram, J.L. Pau, R. McClintock and M. Razeghi
Journal of Applied Physics, Vol. 104, No. 8-- October 15, 2008
Delta-doping is studied in order to achieve high quality p-type GaN. Atomic force microscopy, x-ray diffraction, photoluminescence, and Hall measurements are performed on the samples to optimize the delta-doping characteristics. The effect of annealing on the electrical, optical, and structural quality is also investigated for different delta-doping parameters. Optimized pulsing conditions result in layers with hole concentrations near 1018 cm−3 and superior crystal quality compared to conventional p-GaN. This material improvement is achieved thanks to the reduction in the Mg activation energy and self-compensation effects in delta-doped p-GaN. reprint
 
8.  
Performance enhancement of GaN ultraviolet avalanche photodiodes with p-type delta-doping
Performance enhancement of GaN ultraviolet avalanche photodiodes with p-type delta-doping
C. Bayram, J.L. Pau, R. McClintock and M. Razeghi
Applied Physics Letters, Vol. 92, No. 24, p. 241103-1-- June 16, 2008
High quality delta-doped p-GaN is used as a means of improving the performance of back-illuminated GaN avalanche photodiodes (APDs). Devices with delta-doped p-GaN show consistently lower leakage current and lower breakdown voltage than those with bulk p-GaN. APDs with delta-doped p-GaN also achieve a maximum multiplication gain of 5.1×104, more than 50 times higher than that obtained in devices with bulk p-GaN. The better device performance of APDs with delta-doped p-GaN is attributed to the higher structural quality of the p-GaN layer achieved via delta-doping. reprint
 
9.  
High Optical Response in Forward Biased (In,Ga)N-GaN Multiquantum-Well Diodes Under Barrier Illumination
High Optical Response in Forward Biased (In,Ga)N-GaN Multiquantum-Well Diodes Under Barrier Illumination
J.L. Pau, R. McClintock, C. Bayram, K. Minder, D. Silversmith and M. Razeghi
IEEE Journal of Quantum Electronics, Vol. 44, No. 4, p. 346-353.-- April 1, 2008
The authors report on the current–voltage (I–V) characteristic under forward biases obtained in low leakage, small size p-(In,Ga)N–GaN-n multiquantum well diodes. Under barrier illumination, the devices present a high optical response with capabilities to detect optical powers in the pW range without further amplification. This response is attributed to the screening of the internal electric fields. Recombination times of a few seconds are found to be associated to this mechanism. Moreover, a step-like feature is found in the I– V characteristic before the diode turn-on voltage. Our model proposes tunneling current through the multi-quantum-well structure as responsible of this feature. Fast modulation of the tunneling effect under barrier illumination is used to evaluate the detection of low photon fluxes. reprint
 
10.  
Back-illuminated separate absorption and multiplication GaN avalanche photodiodes
Back-illuminated separate absorption and multiplication GaN avalanche photodiodes
J.L. Pau, C. Bayram, R. McClintock, M. Razeghi and D. Silversmith
Applied Physics Letters, Vol. 92, No. 10, p. 101120-1-- March 10, 2008
The performance of back-illuminated avalanche photodiodes with separate absorption and multiplication regions is presented. Devices with an active area of 225 µm2 show a maximum multiplication gain of 41,200. The calculation of the noise equivalent power yields a minimum value of 3.3×10−14 W·Hz−1/2 at a gain of 3000, increasing to 2.0×10−13 W·Hz−1/2 at a gain of 41,200. The broadening of the response edge has been analyzed as a function of bias. reprint
 
11.  
III-Nitride photon counting avalanche photodiodes
III-Nitride photon counting avalanche photodiodes
R. McClintock, J.L. Pau, K. Minder, C. Bayram and M. Razeghi
SPIE Conference, January 20-25, 2008, San Jose, CA Proceedings – Quantum Sensing and Nanophotonic Devices V, Vol. 6900, p. 69000N-1-11.-- February 1, 2008
In order for solar and visible blind III-Nitride based photodetectors to effectively compete with the detective performance of PMT there is a need to develop photodetectors that take advantage of low noise avalanche gain. Furthermore, in certain applications, it is desirable to obtain UV photon counting performance. In this paper, we review the characteristics of III-nitride visible-blind avalanche photodetectors (APDs), and present the state-of-the-art results on photon counting based on the Geiger mode operation of GaN APDs. The devices are fabricated on transparent AlN templates specifically for back-illumination in order to enhance hole-initiated multiplication. The spectral response and Geiger-mode photon counting performance are analyzed under low photon fluxes, with single photon detection capabilities being demonstrated in smaller devices. Other major technical issues associated with the realization of high-quality visible-blind APDs and Geiger mode APDs are also discussed in detail and solutions to the major problems are described where available. Finally, future prospects for improving upon the performance of these devices are outlined. reprint
 
12.  
Advances in APDs for UV astronomy
Advances in APDs for UV astronomy
Melville P. Ulmer; Ryan M. McClintock; Jose L. Pau; Manijeh Razeghi
Proc. SPIE 6686, UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XV, 668605 (September 13, 2007)-- November 13, 2007
We report the most recent work of our group of the development of avalanche photo diodes based on (Al)GaN. The goal of this group is to achieve single photon counting. In this paper we first give the scientific motivation for making such a device in the context of UV astronomy and then describe current work and plans for future development. The development includes improving the sensitivity to be able to carry out single photon detection and the fabrication of arrays. reprint
 
13.  
Scaling in back-illuminated GaN avalanche photodiodes
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
 
14.  
Geiger-mode operation of back-illuminated GaN avalanche photodiodes
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
 
15.  
Hole-initiated multiplication in back-illuminated GaN avalanche photodiodes
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
 
16.  
III-Nitride Avalanche Photodiodes
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
 

Page 1  (16 Items)