Microstructure and electrical characteristics of tungsten and WSi_x contacts to GaAs

Contacts consisting of tungsten, W_0.79Si_0.21, W_0.67Si_0.33 and W_0.57Si_0.43 have been produced by sputtering on GaAs(001) and annealed isochronally for 20 min in an arsenic overpressure at temperatures ranging from 500 to 950°C. Subsequently, solid state reaction products, microstructure and phase morphology were characterized by cross-section transmission electron microscopy and glancing-angle X-ray diffraction, whereas carrier concentration profiles, Schottky barrier heights and corresponding ideality factors were measured by capacitance-voltage and current-voltage techniques. Contacts consisting of W/GaAs are characterized by an equilibrium microstructure (β-W and/or α-W/GaAs) under all conditions, but corresponding electrical properties degrade substantially during annealing at 750°C and above. In contrast, W_0.79Si_0.21/GaAs and W_0.67Si_0.33/GaAs contacts are characterized by an amorphous structure after annealing up to 750°C, but revert to an equilibrium microstructure (α-W and W₅Si₃ on GaAs) during annealing at higher temperatures. Corresponding electrical properties degrade less severely during annealing at 750°C and above. Finally, W_0.57Si_0.43/GaAs contacts are characterized by an amorphous structure after annealing up to 500°C, but revert to an equilibrium microstructure (W₅Si₃ and WSi₂ on GaAs) during annealing at higher temperatures. Corresponding electrical properties degrade during annealing at 500°C and above. In general, degradation of electrical properties is attributed to a combination of interdiffusion across and solid state reactions at the WSi_x-GaAs interface. A previously unreported cubic phase is also observed to form at the W_0.57Si_0.43-GaAs interface during annealing at 750°C and above.