Surface composition changes in GaAs due to low-energy ion bombardment

Abstract

Auger spectroscopy has been used to study the changes in composition produced in the surface of crystalline GaAs by low energy (0.5 < E < 5.0 keV) ion bombardment as a function of substrate temperature (−180 < T < 150° C). Changes in the composition of the surface (∼0.6 nm) and the subsurface (−2.0 nm) layers were monitored by observing the low energy (MNN) and the high energy (LMM) Ga and As lines, respectively. When ions bombarded a crystal at high substrate temperatures, the Ga As ratio showed very little dependence on ion energy, flux (current density), or temperature. When the crystal was held below a certain annealing temperature, the Ga As ratios of the bombarded surface increased. The amplitude of the low energy Ga/As ratio increased by approximately 20% from the high temperature value, which suggests that As was preferentially sputtered. The high energy Ga As ratio increased from ∼1.6 to a maximum of ∼2.1 with increasing ion energy and decreasing temperature, indicating that As depletion also extended to the subsurface layers. In addition, the implanted Ar concentration, which ranged from 0.4 (at E = 5.0 keV) to 1.2 (at E = 0.5 keV) at% increased by up to 50% in the As-depleted surface. The surface composition changes took place at a temperature, T a, which depended strongly on ion energy and flux (current densities of 1 and 100 μ A cm 2 ). At a fixed ion energy. T a increased with increasing ion flux; at a fixed flux, with increasing energy. The dependence of T a on the ion flux is discussed in terms of a simple model in which the rate of defect production equals the rate of defect annealing. An activation energy of φ = 1 eV was found for annealing of defects produced by 5 and 2 keV Ar + ion bombardment of GaAs. As depletion is attributed to preferential As sputtering from a surface disordered by ion bombardment. The depth of the disordered layer shrunk to near zero as T reached T a.