Nonalloyed ohmic contacts to n-GaAs by solid-phase epitaxy of Ge

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Abstract
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.