Comparison of ultraviolet APDs grown on free-standing GaN ...

Comparison of ultraviolet APDs grown on free-standing GaN and sapphire substrates

Erdem Ciceka, Zahra Vashaeia, Can Bayrama, Ryan McClintocka, Manijeh Razeghi*a, and Melville P. Ulmerb

aCenter for Quantum Devices, Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL USA 60208

bDepartment of Physics and Astronomy, Northwestern University, Evanston, IL USA 60208

ABSTRACT

There is a need for semiconductor-based ultraviolet photodetectors to support avalanche gain in order to realize better performance andmore effective compete with existing technologies. Wide bandgap III-Nitride semiconductors are the promising material system for the development of avalanche photodiodes (APDs) that could be a viable alternative to current bulky UV detectors such as photomultiplier tubes. In this paper, we review the current state-of-the-art in IIINitride visible-blind APDs, and present our latest results on GaN APDs grown on both conventional sapphire and low dislocation density free-standing c- and m-plane GaN substrates. Leakage current, gain, and single photon detection efficiency (SPDE) of these APDs were compared. The spectral response and Geiger-mode photon counting performance of UV APDs are studied under low photon fluxes, with single photon detection capabilities as much as 30% being demonstrated in smaller devices. Geiger-mode operation conditions are optimized for enhanced SPDE.

Keywords: APD, GaN, polar and nonpolar GaN substrates, Geiger Mode, avalanche photodiode, single photon detection

1. INTRODUCTION

Wide bandgap semiconductors have been the subject of intense scientific and technological developments since the 1990's, primarily driven by the quest for blue lasers and high brightness visible light emitting diodes. In parallel, ultraviolet (UV) photodetectors have also been studied extensively due to their potential to offer visible- or solar-blind detection, which is highly desirable for numerous applications in the defense, commercial, and scientific arenas. These include covert space-to-space communications, early missile threat detection, chemical and biological threat detection and spectroscopy, flame detection and monitoring, UV environmental monitoring, and UV astronomy.1,2,3 Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 to 400 nm, and energies from 3 to 124 eV. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet. Current high-sensitivity UV detectors, such as photomultiplier tubes (PMTs), present high detectivity due to their large internal gain (typically 106). However, these detectors are not without their drawbacks: they are bulky, fragile glass vacuum tubes that require large biases (typically 1000 V) to operate effectively.4 Thus there is an interest in developing semiconductor based alternatives, however silicon and silicon-carbide baed alternatives require additional filtering to operate in the UV making them less favorable.5 Using a visible- or solar-blind detector dramatically relaxes the system requirements, eliminating the need for expensive and efficiency-limiting optical filters to remove undesired out-of-band photons. III-Nitride?based UV APDs are the most promising candidates to replace conventional PMTs due to the tunability of their band-gap (3.4 < AlxGa1-xN < 6.2 eV) to cover the entire UV spectrum and their ability to achieve low-noise internal gain.

Great strides have been made in the realization of UV avalanche photodiodes based on III-Nitrides 6,7,8,9,10. Most of the early GaN APD devices were designed for front-illumination operation with photons reaching the p-layer first. This configuration has been mostly used historically because one would get fewer defects by growing a device structure on several-micron thick n-type GaN templates on sapphire or SiC substrates. However, there is a strong scientific and technological desire to investigate back-illuminated GaN avalanche photodiodes for a number of reasons. Backilluminated GaN p-i-n diodes benefit from hole-initiated multiplication which yields gain and noise characteristics with superior performances due to the higher hole impact ionization coefficients.4 Back-illumination also allows easier integration and packaging of APDs through flip-chip bonding technology. However, growing GaN based APDs on

Detectors and Imaging Devices: Infrared, Focal Plane, Single Photon, edited by Eustace L. Dereniak, John P. Hartke, Paul D. LeVan, Randolph E. Longshore, Ashok K. Sood, Manijeh Razeghi, Rengarajan Sudharsanan,

Proc. of SPIE Vol. 7780 ? 77801P ? ? 2010 SPIE ? CCC code: 0277-786X/10/$18 ? doi: 10.1117/12.863905

Proc. of SPIE Vol. 7780 77801P-1

Downloaded from SPIE Digital Library on 21 Sep 2010 to 129.105.215.146. Terms of Use:

conventional UV transparent substrates like sapphire5-7 results in an effective lattice mismatch as high as 16%, which leads to dislocation densities on the order of 109 cm-2.11 These lattice-mismatch?induced dislocations increase the leakage current, disrupt the electric field distribution, and can result in premature microplasma breakdown -- all of which limit the avalanche gain, making the realization of high performance APDs challenging. The epitaxial growth on low?dislocation density free-standing (FS) GaN substrates rather than sapphire or SiC substrates is preferable, as it results in a considerably lower dislocation density ( ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download