I. Introduction
Single-Photon counting detectors can be utilized in a wide range of applications, including optical computing, high-speed communications, military, and biological agent detection [1]. Conventionally, photomultiplier tubes (PMTs), which offer a high sensitivity and a fast response time, have fulfilled these needs. However, they are generally expensive and bulky, and require very high operating voltages and an external cooling system for low-noise performance. High-performance solid-state photodetectors with high internal gain have many advantages over PMTs, especially in the areas of portability and power consumption. Avalanche photodiodes (APDs) operating in Geiger-mode are expected to replace PMTs in many critical applications with comparable sensitivity and speed. For APDs operating in the ultraviolet (UV) spectral region, III–nitride materials are promising candidates, as these devices can provide an “intrinsic” detection capability with their wide bandgap and expected long-term stability in harsh and radiation environments [2], [3]. For the growth of III–nitride-based APDs on lattice-mismatched foreign substrates, such as sapphire and SiC, by strained heteroepitaxy, a high density of crystalline defects, mainly threading dislocations, is usually introduced during the epitaxial growth, resulting in premature microplasma breakdown of the APD before the electric field reaches the avalanche breakdown field strength. Although there are many reports of III–nitride APDs grown on the foreign substrates [4], [5], the demonstration of devices with a high yield, low dark current density at large reverse bias, and high multiplication gain still remains a serious technical challenge due to the high defect density originating from the strained heteroepitaxy. The growth of epitaxial structures by homoepitaxy on low-dislocation-density free-standing “bulk” GaN substrates is a powerful alternative method to minimize the dislocation density. These authors have previously demonstrated GaN APDs grown by homoepitaxy on free-standing GaN substrates with very high avalanche multiplication gains [6], [7]. The devices showed stable avalanche gain and a peak absorption wavelength at 364 nm. In addition, Al0.05 Ga0.95N APDs grown on bulk GaN substrates showed a peak absorption wavelength at 350nm, confirming the feasibility for the operation AlxGa1-xN APDs in the deep-UV spectral region [8]. However, definitive Geiger-mode operation of III–nitride APDs grown on bulk GaN substrates in the UV spectral region has not been reported. In this article, we report the GaN UV APDs grown on high quality GaN substrates and the Geiger-mode operation of these devices in the deep UV spectral region.