RHEED and XPS observations of trimethylgallium adsorption on GaAs (001) surfaces—Relevance to atomic layer epitaxy

Abstract

A series of experiments under UHV conditions have been performed to determine the surface reacted species and surface structure that results from trimethylgallium (TMGa) adsorption on a GaAs (001) As-stabilized surface. In these experiments, the conditions were chosen to simulate typical ALE growth conditions. Thus, the substrate temperature was varied between 320 and 530° and an admittance of 10⁻⁷ to 5 × 10⁻⁶ Torr of TMGa with exposure time of 5 to 15 sec were applied. X-ray photoelectron spectroscopy (XPS) was used to identify the chemical species on the surface after TMGa adsorption. The XPS intensity associated with the Ga 2p _3/2 level was used to monitor the quantity of adsorbed Ga and RHEED was used to monitor the surface structure. Below 440°, the Ga intensity was saturated at a level close to 1 ML and no definite Ga-stabilized 4 ×X RHEED pattern was observed. At 320° and an exposure of 200 L, a 2 × 4 As-stabilized RHEED pattern still existed, which suggests that the reaction between impinging TMGa and the (001) GaAs surface is very slow at this temperature. When the substrate temperature was between 440 and 530° exposure to greater than 6 L of TMGa resulted in saturation of surface Ga atoms to one monolayer (ML) and a successive change of surface reconstruction from 2×4 As-stabilized to 4 ×X (X = 1 or 2) Ga-stabilized surface. In all runs no carbon related species were observed within the XPS detection limit. This observation suggests that adsorption and decomposition of TMGa on As sites goes to completion very rapidly in this temperature range. From these observations we conclude that the self limiting mechanism in ALE occurs because of the differential chemisorption and decomposition rates of TMGa on As and Ga sites and that the dominant surface adsorbate is atomic Ga.