Oxidation of AlAs films under ultrahigh vacuum conditions: interaction of H 2O and O 2 with the AlAs(001) surface

Abstract

The initial stages of oxidation of AlAs(001) (using H 2O and O 2 as the oxidants) have been investigated using Auger electron spectroscopy, temperature-programmed desorption and high-resolution electron energy-loss spectroscopy. We have found that O 2 has a low probability of dissociative chemisorption on AlAs(001), rendering it ineffective as an oxidant of AlAs under ultrahigh vacuum conditions. In contrast, water, by virtue of overlapping desorption and dissociation temperatures, is able to dissociate readily on the AlAs(001) surface. Water dissociation within the subsurface is observed, indicative of Al oxide/hydroxide formation. Arsenic is found to be depleted within the oxide film by arsine desorption, a process which is shown to enhance the oxide growth. By identifying the various reaction steps which occur (with annealing) after the low-temperature adsorption of water on AlAs(001), we are able to propose a mechanism for the initial stages of wet AlAs oxidation. Briefly, the initial H 2O adsorption occurs molecularly at 100 K. Upon annealing to 160 K, both desorption and dissociation from the molecularly adsorbed state occurs. Both AlO and AlOH, as well as AsH, are produced by the dissociation of H 2O (via partial and total dehydrogenation, respectively). The AlOH species disproportionate and/or rehydrogenate between 400 and 650 K resulting in H 2O desorption. The remaining AlOH dehydrogenate above 550 K to produce AlO and desorbing hydrogen. The AsH species desorb as AsH 3 in a broad peak at 500–550 K with an additional low-temperature desorption peak appearing below 200 K after higher H 2O exposures.