Adsorption-desorption studies of Zn on GaAs

Abstract

By employing a wide range of techniques to study adsorption-desorption behaviour in the ostensibly simple system of the metal Zn on the semiconductor GaAs it has been found that many complicating factors can occur, and reliance on any one of the techniques would have given a totally misleading picture. The methods used were temperature programmed thermal desorption, modulated atomic beam adsorption measurements, AES, high resolution UHV SEM combined with AES and high resolution (300 Å) AES, and RHEED. GaAs substrate surfaces were cleaned in-situ by thermal treatment or inert gas ion bombardment followed by annealing. It was established by SEM and RHEED observations that surface topographic and compositional changes could occur at this stage. Zn sticking coefficient measurements by modulated beam and AES techniques showed that it could vary widely for minor changes in surface composition, and that it was also a strong function of deposition time and substrate temperature. However, initial growth of the Zn deposit was always two dimensional (within the resolution limits of the SEM) and epitaxial, for the range of substrate conditions used. Thermal desorption spectra were also found to depend rather critically on the substrate surface, with very pronounced difference between the Ga and As stabilized forms. An attempt has been made at a systematic interpretation of the kinetic data based on reasonably simple models, and also to relate it to previously published work on this system, but the profound influence of substrate surface effects makes a fully quantitative evaluation extremely difficult. Nevertheless, the value, and perhaps the necessity, of employing a wide range of techniques to investigate metal-semiconductor systems is clearly demonstrated.