Thin anodic oxides formed on GaAs in aqueous solutions

Thin (0-250 Angstrom) anodic oxides were formed on p-GaAs (100) in aqueous solutions (berate buffer, pH 8.4 and 0.3 M NH4H2PO4, pH 4.4). The thickness, composition, and electrochemical behavior were characterized by x-ray photoelectron spectroscopy, secondary ion mass spectrometry, ellipsometry, and electrochemical impedance spectroscopy. In berate buffer, oxide growth results in a two-layer structure with an outer As2O3-rich layer and ion transfer is mainly diffusion controlled. From the potential dependence of the film thickness a growth rate of 25 Angstrom/V was determined. Due to the formation of a Ga-phosphate precursor layer in NH4H2PO4 films grow under field control which results in a time dependence of oxide thickness and composition. Under pseudosteady state conditions a growth rate of 28 Angstrom/V was obtained. Films formed in NH4H2PO4 have, except for the outer phosphate layer, a fairly uniform composition in depth with a significant enrichment of Ga2O3, and show a 40 times higher specific charge transfer resistance than films formed in berate buffer. The superior quality of the oxide formed in NH4H2PO4 is also reflected in its high stability against H2O etch. In addition, substrate pretreatment and the effects of oxide washing on composition were investigated and the importance of an appropriate preparation of samples for ex situ analysis is demonstrated.