Georgia Institute of Technology Materials Science and Engineering

Committee Members:

Prof. Russell D. Dupuis, Advisor, MSE/ECE

Prof. Eric Vogel, MSE

Prof. Ajeet Rohatgi, ECE

Prof. A. Nepomuk Otte, SoP

Prof. Shyh-Chiang Shen, ECE

Development of High-performance Front-illumination III-N Avalanche Photodiodes by Metalorganic Chemical Vapor Deposition

Abstract:

Gallium nitride (GaN) avalanche photodiodes (APDs) having large direct band-gap energy of ~3.5 eV at 300 K can exhibit a high sensitivity in the ultraviolet (UV). They are appropriate candidates for many applications because of their capability of operation in the near-UV and visible-blind spectral regions, high detection sensitivity, high breakdown electric field (~3.3 MV/cm), high optical gain, and low dark current. Furthermore, their high structural, chemical, and thermal stability make GaN APDs attractive for operation in harsh environments. Their high breakdown electric field and high operating frequency properties can play an important role in efficient electrical conversion and transmission for many applications such as in-space communications, detection of missile plumes, portable biochemical threat detectors, flame detectors, and environmental monitoring. Metalorganic chemical vapor deposition (MOCVD) is a growth technique well-suited to the growth of advanced III-N devices which has been developed for high-quality epitaxial layer growth with high throughput and flexibility.

The incorporation of both intentional and unintentional impurities in GaN impacts the electrical conductivity and optical properties of the films grown MOCVD. Unintentional impurity reduction is required for properly designed drift layers in GaN avalanche photodetectors. Thus, it is critical to understand how to control or reduce background impurities. The first phase of this presentation will be focused on control of the background impurities in epitaxial growth, their effects on device performance. In the second phase of this study, front-illuminated GaN-based UV APD structures were grown by MOCVD on free-standing (FS) GaN substrates. The growth parameters of each layer were separately studied and optimized. Devices were fabricated from these wafers by my colleagues. The APD device data will be discussed. GaN UV-APD arrays are well-suited for many applications in UV imaging systems to improve the collection efficiency and sensitivity for low-level UV light detection. However, increasing the detection area of UV-APDs either by increasing the individual device area or by increasing the number of APDs in the array, increases the probability of the presence of individual APDs having crystalline defects or threading dislocations which reduces the performance and uniformity of the electrical properties of the array. So, in the third phase, the device performance of front-illuminated p-i-n 6 × 6 UV-APD arrays will be studied. Evaluation of the performance of the devices in these arrays confirms the uniformity of the epitaxial materials as well as the device processing. In the last phase of this study, front-illuminated p-i-n UV-APD heterostructures having a p-Al0.15Ga0.85N “window” layer was grown on n-GaN ammonothermal bulk substrates to improve light detection of APD. The details of growth challenges as well as device performance will be discussed.