Formation of anodic oxides on Al0.2Ga0.8As and Al0.8Ga0.2As in tungstate electrolytes

The study examines the formation of amorphous anodic films on Al0.2Ga0.8As and Al0.8Ga0.2As substrates, using transmission electron and atomic force microscopies and Rutherford backscattering spectroscopy. The anodizing conditions consist of a constant current density, of 0.25 mA cm−2, with either 2 mM or 0.1 M aqueous sodium tungstate electrolyte, and a temperature of 293 K. The resultant films are of relatively uniform composition, except for the presence of about 1.5 at% tungsten species, which are derived from the electrolyte, in the outer ∼36% of their thickness. The average compositions of the films are 0.24Al2O3·0.8Ga2O3·1.2As2O3 and 0.96Al2O3·0.2Ga2O3·1.2As2O3 for the films grown on Al0.2Ga0.8As and Al0.8Ga0.2As substrates, respectively. The reduced amounts of gallium in the film with respect to the substrate is due to fast outward migration of gallium ions relative to that of Al3+ and As3+ ions, and dissolution of gallium species at the film surface. The dissolution reduces the anodizing efficiency by about 10%. The formation ratios are approximately 1.92 and 1.44 for films grown on Al0.2Ga0.8As and Al0.8Ga0.2As substrates, respectively, consistent with an increased amount of Al3+ ions in the latter films.