Study of Encapsulants for Annealing GaAs

ABSTRACT Low temperature photoluminescence and Auger electron spectroscopy have been used to study chemical-vapor deposited SiO2 and SigN4 layers as en- capsulants for high temperature annealing of GaAs. Silicon dioxide or silicon oxynitride layers allow out-diffusion of Ga, while suitably prepared rf plasma deposited SisN4 layers can be used to anneal GaAs with negligible Ga out- diffusion. Ion implantation is a versatile method of doping semiconductors and is an established fabrication step for many silicon devices (1). Although the use of implantation in the III-V compounds is less wide- spread, important device applications have emerged. These include the fabrication of light emitting diodes (2), GaAs field-effect transistors, optical waveguid.es, and detectors (3). Because of the difficulties of selec- tively doping GaAs or other compound semiconductors by standard diffusion processes as compared with Si, it now appears that implantation will play an increas- ing role in future device applications of compound semiconductors. In view of the potential for this doping method, it is important to develop reliable procedures for im- plantation and annealing in these materials. Ion im- plantation produces considerable lattice damage which must be annealed out in order to restructure the lattice and activate the implanted impurities. While this pro- cedure is relatively straightforward in Si, annealing a compound semiconductor such as GaAs is more difficult. The incongruent evaporation of Ga and As from GaAs at temperatures ~ in excess of 600~ (4) makes it impossible to anneal bare GaAs samples without surface degradation. It is thus necessary to encapsulate the sample with a suitable dielectric layer or to perform the anneal in a carefully controlled ambient (5. 6). There have been numerous discussions of implantation and annealing in GaAs using encapsu- lants such as sputtered or chemical,vapor deposited SiO2 (7), thermally o r reactively deposited or sput- tered Si3N4 (8-10), or sputtered A1N (11). In the present work we have used low temperature photo- luminescence (PL) and Auger electron spectroscopy _(AES) to investigate the annealing-encapsulation properties of chemical vapor deposited SiO2 and rf plasma deposited Si.~N4 layers on GaAs. It is well known from diffusion experiments in Si that Ga has a very high diffusion coefficient in SiO2 (12). Gyulai et