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.
A thermally stab ie, low-resistance Pdln ohmic contact to n-GaAs has been developed based on the solid phase regrowth mechani.sm [T. Sands, E. D. Marshall, and L. C. Wang, J. Mater. Res. 3, 914 (1988)]. Rapid thermal annealing of a Pd-In/Pd metallization induces a two-stage reaction resulting in the formation of a uniform single-phase film of Pdln, an intermetalHc with a melting point greater than 1200°C. A thin (-5 nm) layer of average composition IUoAGao.6As uniformly covers the interface between the Pdln layer and the GaAs substrate. Specific contact resistivities and contact resistances of ~ 1 X 10-. 6 n cm 2 and 0.14 n mm, respectively, were obtained for samples annealed at temperatures in the 600-650 °C range. The addition of a thin layer of Ge (2 urn) to the first Pd layer extends the optimum annealing temperature window down to 500°C. Specific contact resistivities remained in the low 10-6 n cm 2 range after subsequent annealing at 400 °C for over two
A low-resistance nonspiking Ohmic contact to «-GaAs is formed via solid-state reactions utilizing the Si/Pd/GaAs system. Samples with Si to Pd atomic ratios greater than 0.65 result in specific contact resistivity of the order of 10^6 O cm2, whereas samples with atomic ratios less than 0.65 yield higher specific contact resistivities or rectifying contacts. Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy, and electron diffraction patterns show that a Pd2Si layer is in contact with GaAs with excess Si on the surface after the Ohmic formation annealing. This observation contrasts with that on a previously studied Ge/Pd/GaAs contact where Ohmic behavior is detected after transport of Ge through PdGe to the interface with GaAs. Comparing the Ge/Pd/GaAs system with the present Si/Pd/ GaAs system suggests that a low barrier heterojunction between Ge and GaAs is not the primary reason for Ohmic contact behavior. Low-temperature measurements suggest that Ohmic behavior results from tunneling current transport mechanisms. A regrowth mechanism involving the formation of an n * GaAs surface layer is proposed to explain the Ohmic contact formation.
High-resolution SIMS (secondary ion mass spectrometry) depth profiles of Ge/Pd ohmic contacts on InP are obtained by sputter-etching from the back (semiconductor) side. The samples contain an InGaAs-etch stop layer, to allow chemical thinning, and InGaAsP marker layers, which allow alignment and calibration of the depth profiles on the nm scale. At 200 °C, a Pd-In-P alloy layer is observed to form at the contact interface. The thickness of this layer is dependent on the amount of metallic Pd available for reaction. Subsequent processing at 325 °C results in the partial dissolution of this alloy layer, as PdGe forms at the contact interface, and regrowth of the liberated InP. Ge is detected in the regrown region but is not observed to diffuse into the substrate. Ge epitaxy is not observed at the contact interface at 325 °C, in contrast to the behavior of the Ge/Pd-GaAs contact. The experimental evidence suggests that regrowth is a key step in the formation of the ohmic contact.
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