Abstract
This study of the thermal desorption of zinc from GaAs was carried out in ultra-high-vacuum using molecular beam and mass spectrometric techniques. Preliminary experiments showed that the GaAs substrate may be heated to 930K without altering greatly the stoichiometry of the surface, while above this temperature it will evaporate with preferential loss of arsenic. The sticking coefficient of zinc on GaAs was found to increase with coverage, reaching a value between 0.8 and 1 at high coverages, and the pumping speed of the vacuum system for zinc was found to be very high. Auger electron spectroscopy of the GaAs substrates was carried out to determine the contaminants present on the surface. The desorption spectra of zinc from GaAs were studied as a function of initial zinc coverage. Care was taken to match the experimental conditions with those assumed in the theory of thermal desorption. On all GaAs substrates used, a low temperature zinc desorption peak was seen which grew with zinc coverage without saturating. For the cleanest specimen used, this peak was analysed, and, from Arrhenius plots, the desorption energy of the zinc was found to be about 16kcal/mole over a range of coverage from about 5 x 10[15] to 1.4 x 10[17] atoms/sq. cm. This main peak was probably due to evaporation from the nuclei of zinc on the surface. During the last stages of desorption the desorption energy of the zinc fell from about 16kcal/mole at a coverage of 5 x 10[15] atoms/sq. cm to 6-8 kcal/mole at about 5 x 10[14] atoms/sq. cm. As the number of desorption runs increased the nature of the GaAs became permanently changed: this may have been due to diffusion of zinc into the top few atomic layers of the substrate. The nature of the adsorbate-adsorbent and adsorbate-adsorbate interactions complicated the analysis and this allowed comparisons to be made between the various methods of analysing thermal desorption curves.