Design and surface chemistry of nonalloyed ohmic contacts to pseudomorphic InGaAs on n+GaAs

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Abstract
Pseudomorphic layers of molecular‐beam‐epitaxy (MBE) grown InxGa1−xAs (xIn =.25 and 0.35) on heavily doped GaAs were studied for nonalloyed emitter and collector contacts to heterojunction bipolar transistors. Since the InGaAs layers in this study are coherent to the GaAs lattice, subsequent epitaxial layers can be grown after the ohmic contact with high quality. Contact resistances as low as 5×10−7 Ω cm2 have been obtained for a 60‐Å layer of n+ InGaAs (x=0.35). These contact resistances are achieved through a combination of heavy silicon doping, which leads to a greater net donor concentration in InGaAs than in GaAs, and a low Schottky barrier height. The InGaAs layers, which are grown at a temperature of 380 °C, are heavily silicon doped to 2.4×1019 cm−3 by lowering the MBE growth rate. The chemical composition of the InGaAs surface was analyzed by X‐ray photoelectron spectroscopy after selective etching in NH4OH:H2O2:H2O and after various surface treatments. The selectivity of the etch arises from the formation of In(OH)3, which has a low solubility in ammonium hydroxide solutions. The contact resistances after several surface treatments will be summarized.