The solid-phase epitaxial regrowth of a III–V compound semiconductor by a two-stage reaction between a two-layer metallization and a compound semiconductor substrate is described. The regrowth process begins with a low-temperature reaction between a metal M (e.g. Ni, Pd, or Pt) and a compound semiconductor substrate, AB, to produce an intermediate M, AB or MB, phase. A subsequent reaction at a higher temperature between an overlayer of Si, Ge, Al, or In and the intermediate phase results in the decomposition of the intermediate phase and the epitaxial regrowth of a layer of the compound semiconductor. This regrowth mechanism is verified experimentally for the specific case of the Si/Ni/GaAs system. Rutherford backscattering spectrometry and transmission electron microscopy data show that the ternary phase Nix GaAs, formed during the initial stage of the reaction, decomposes toNiSi and GaAs by reaction with the Si overlayer. The incorporation of the overlayer element into the regrown semiconductor layer is proposed as a mechanism to explain the formation of Ohmic contacts in Si/Pd/n-GaAs, In/Pd/n-GaAs, In/Pt/n-GaAs, and similar two-layer metallization systems on n-GaAs.
A low resistance nonalloyed ohmic contact to n-GaAs is formed which utilizes the solid-phase epitaxy of Ge through PdGe. Discussion focuses on the conditions necessary to attain low specific contact resistivity (∼10−6 Ω cm2 on 1018 cm−3 n-GaAs) and on the interfacial morphology between the contact metallization and the GaAs substrate. MeV Rutherford backscattering spectrometry and channeling show the predominant reaction to be that of Pd with amorphous Ge to form PdGe followed by the solid-phase transport and epitaxial growth of Ge on 〈100〉 GaAs. Cross-sectional transmission electron microscopy and lattice imaging show a very limited initial Pd-GaAs reaction and a final interface which is planar and structurally abrupt to within atomic dimensions. The presence of excess Ge over that necessary for PdGe formation and the placement of Pd initially in contact with GaAs are required to result in the lowest contact resistivity. The experimental data suggest a replacement mechanism in which an n+-GaAs surface region is formed when Ge occupies excess Ga vacancies.
The high-temperature stability of Schottky barriers on GaAs has been correlated with the thermodynamic driving force for chemical reaction between the metallic contacts and the substrate. The chemical stability of a gate metallurgy can result in the stability of the electrical characteristics of the contact after high-temperature anneal. Since single element metal contacts on GaAs are chemically unstable, thermally stable Schottky barriers are not expected from these systems. Alternatively, most of the common metal silicides are chemically stable on GaAs and hence are more likely to form Schottky barriers which are stable against high-temperature annealing. The chemically stable silicides of Ni and Co, which exhibit low electrical resistivities, are suggested as improved gate metallurgies in self-aligned metal-semiconductor field-effect transistor technologies.
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