Very high selective etching of GaAs/Al_0.2Ga_0.8as for gate recess process to pseudomorphic high electron mobility transistors (PHEMT) applications using citric buffer solution

Highly selective wet etching of GaAs on Al_0.2Ga_0.8As can be realized using citric buffer (citric acid/H₂O ₂/H₂O) solutions. The selectivity can be up to 256, which is distinct from the previous non-selectivity inference, due to consulting the role of H₂O₂ in this etching system. The etch stop mechanism is demonstrated by X-ray photoelectron spectroscopy (XPS) and indicates the formation of dense oxide Al₂O₃ on Al _0.2Ga_0.8As to stop etching. The applications to pseudomorphic high electron mobility transistors (PHEMT) gate recess processes, no obvious drain current decrease can be perceived even after 8 min overetching. The etched surface is very smooth with the roughness of only 1.97 Å. The devices also show excellent etched uniformity with a small threshold voltage deviation of 28 mV and good electrical performance, which is useful to the heterostructure device fabrications.