Applied Surface Science

Effects of interfacial microstructure on ohmic contacts to GaAs

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Thermally stable, low resistance ohmic contacts on n-type GaAs which withstand high temperature cycles without degrading their electrical properties are critical to fabrication of high performance GaAs integrated circuits. Analysis of the microstructure between the metal and GaAs was carried out for the universally used AuNiGe ohmic contacts and the recently developed NiInW ohmic contacts. Both contacts provided low contact resistances. For AuNiGe ohmic contacts, poor thermal stability after contact formation was concluded to be due to formation of low-melting-point β-AuGa phases. Since formation of the β-AuGa phases is essential to yield low contact resistances, improvement of thermal stability of the AuNiGe contacts without degrading the electrical properties is extremely difficult. For NiInW ohmic contacts, the contacts were observed to contain only refractory materials of InxGa1-xAs, Ni3In, NiAs, and W, which provided the excellent thermal stability at 400°C after contact formation. In addition, the excellent thermal stability of the NiInW contacts was observed after depositing directly Al-Cu wiring which is commonly used in the current Si technology. The stability was concluded to be due to lack of chemical interaction between Al and In which is the key element for low contact resistance in the NiInW ohmic contacts. © 1989.