Scanning tunneling microscopy study of the ambient oxidation of passivated GaAs(100) surfaces

Abstract

A scanning tunneling microscope (STM) operating in air has been used to study GaAs(100) surfaces which have been passivated in aqueous P2S5/(NH4)2S and P2S5/(NH4)2Sx solutions. The growth of native oxides under ambient conditions on these passivated surfaces has been monitored by STM. A spatial analysis of the oxide growth over 3×3 μm2 areas indicates that the native oxide initiates as isolated islands across the surface which coalesce to form a uniform oxide layer. Evaluation of these surfaces in terms of peak-to-valley and root-mean-square roughness measurements indicates that surfaces treated in the P2S5/(NH4)2Sx solution oxidize at a slower rate than those treated with the P2S5/(NH4)2S solution. The repeated scanning of a particular area by the STM results in a significant increase in the local oxidation rate compared with surrounding areas which are scanned only once. The application of short high voltage pulses to the tip during an interruption in the scanning routine results in a greatly enhanced oxidation rate directly beneath the tip. Scanning tunneling spectroscopy characteristics, both I/V and (dI/dV)/(I/V), reveal considerable differences in the band bending and consequently the density of surface states, present on the passivated and unpassivated surfaces.