Selective wet etching of GaAs on AlxGa1-xAs for AlGaAs/InGaAs/AlGaAs pseudomorphic high electron mobility transistor

Selective wet etching of GaAs over AlxGa1-xAs (x=0.2, 0.23, 0.3, and 1.0) for AlGaAs/InGaAs/AlGaAs pseudomorphic high electron mobility transistor (PHEMT) was studied using current-voltage, etched step measurement, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy. The volume ratio of 50% citric acid solution (citric acid:H2O=1:1) to H2O2 was changed from 0.7:1 to 3:1, and the etching selectivity of GaAs over AlxGa1-xAs was examined. In the solution with the ratio of 1.5:1 of the 50% citric acid/H2O2 the best selectivities for GaAs on AlxGa1-xAs with x=0.2, x=0.23, x=0.3, and x=1.0 are measured to be 137, 143, 159, and 2700, respectively. The AFM measurements show that the AlxGa1-xAs surface etched by the selective etching solution is much smoother than that by a nonselective one, consisting of H3PO4:H2O2:H2O=4:1:180. The amount of Al-O bond at the selectively etched surface of AlxGa1-xAs increases with the Al composition in the AlxGa1-xAs. This means that the insoluble Al2O3 mainly plays a role in suppressing the dissolution of the AlGaAs in the etching solution. The selective etching solution is applied to the fabrication of PHEMT. The PHEMT exhibits excellent and reliable performance, demonstrating the applicability of this solution to gate recess process. (C) 1998 American Institute of Physics. [S0021-8979(98)03619-6].