The Anodization of GaAs and GaP in Aqueous Solutions

Abstract

The anodic oxidation of and in properly conductivity‐ and/orpH‐adjusted water has been successfully demonstrated under both constant voltage and constant current conditions. With as the acidic conductivity/pH modifier, uniform, well‐controlled oxides have been grown in thepH range 2.5–3.5. The oxide thickness‐voltage relationships for both and are linear, with slopes of approximately 20 and 12 Å/V, respectively. At room temperature, oxides as thick as 3600 and 2000Å can reproducibly be grown on and , respectively; at 100°C, an oxide as thick as 5000Å has been grown on . Under basic conditions, with as the conductivity/pH modifier, oxides have been grown in the pH range of 10–11, but dissolution of the oxide in the bath results in much poorer control than in the acidic system; this dissolution effect can be utilized more in the line of electroetching than in simple oxide formation. Anodic oxidation, with relatively little oxide dissolution, has also been accomplished in near neutral‐pH water (i.e., ∼7) with as the conductivity/pH modifier. Restricted‐area oxidation and/or etching has been demonstrated whereby a prefabricated photoresist pattern is used to define restricted areas for anodization; lines as narrow as 5μ wide are readily delineated. Anodization in aqueous solutions containing either Cl− or ions is shown to result in simple electroetching, and current densities in the range 10–20 mA/cm2 are demonstrated to be most effective for controlled electroetching. The grown oxides are soluble, in , , and , and are affected by prolonged contact with water. If properly baked, however, the oxide grown on shows no evidence of change in months of storage in laboratory air; the oxide grown on GaP does show evidence of moisture absorption after 1 month of equivalent storage. A preliminary analysis of some of the controlling electrical factors during anodization shows that under constant voltage conditions, the current varies asand under constant current conditions, the time derivative of the voltage is a constant.