The importance of the Pd to Sn ratio and of annealing cycles on the performance of Pd/Sn ohmic contacts to n-GaAs

A novel Pd/Sn ohmic contact system has been developed for n-GaAs. The Pd to Sn ratio and annealing cycles have a significant effect on the performance of the contacts. Scanning electron microscopy (SEM) is employed to investigate the surface morphology of the contacts. Contact depth profiles are analysed by secondary ion mass spectrometry (SIMS). Contact resistivities rho(c) of the proposed metallization are measured utilizing a conventional transmission line model (TLM) method. Annealing at 360 degrees C for 30 min yielded the lowest rho(c) of approximately 3.26 x 10(-5) Omega cm(2) on 2 x 10(18) cm(-3) n-GaAs with a Pd(300 Angstrom)/Sn(1500 Angstrom) contact, whereas a Pd(300 Angstrom)/Sn(900 Angstrom) contact shows a lowest rho(c) value of 6.05 X 10(-5) Omega cm(2) under the same annealing condition. Two-step annealing improves the contact properties. A lowest rho(c) of approximately 1.49 X 10(-5) Omega cm(2) is obtained with a Pd(300 Angstrom)/Sn(1500 A) contact after two-step annealing. The mass spectrometer analysis confirms the formation of PdGa and SnGa compounds at the lowest rho(c). Energy dispersive analysis of X-ray (EDAX) spectra show a correlation among Ga signal peaks and measured rho(c) values at various annealing temperatures. The experimental data suggest a replacement mechanism in which an n(+)-GaAs surface region is formed by solid-phase epitaxy of Sn when Sn occupies excess Ga vacancies.