Oxidation of AlAs films under ultrahigh vacuum conditions: Interaction of H₂O and O₂ with the AlAs(001) surface

The initial stages of oxidation of AlAs(001) (using H₂O and O₂ as the oxidants) have been investigated using Auger electron spectroscopy, temperature-programmed desorption and high-resolution electron energy-loss spectroscopy. We have found that O₂ 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 H₂O 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 H₂O (via partial and total dehydrogenation, respectively). The Al-OH species disproportionate and/or rehydrogenate between 400 and 650 K resulting in H₂O desorption. The remaining Al OH dehydrogenate above 550 K to produce Al-O and desorbing hydrogen. The As-H species desorb as AsH₃ in a broad peak at 500-550 K with an additional low-temperature desorption peak appearing below 200 K after higher H₂O exposures.