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

Abstract

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 depthprofiles are analysed by secondary ion mass spectrometry (SIMS). Contact resistivities ρ c of the proposed metallization are measured utilizing a conventional transmission line model(TLM) method. Annealing at 360 °C for 30 min yielded the lowest ρ c of approximately 3.26 × 10 −5 Θ cm 2 on 2 × 10 18 cm −3 n-GaAs with a Pd(300 Å)/Sn(1500 Å) contact, whereas a Pd(300 Å)/Sn(900 Å) contact showsa lowest ρ c value of 6.05 × 10 −5 Θ cm 2 under the same annealing condition. Two-step annealing improves the contact properties. A lowest ρ c of approximately 1.49 × 10 −5 Θ cm 2 is obtained with a Pd(300 Å)/Sn(1500 Å) contact aftertwo-step annealing. The mass spectrometer analysis confirms the formation of PdGa and SnGa compounds at the lowest ρ c. Energy dispersive analysis of X-ray (EDAX) spectra show a correlation among Ga signal peaks and measured ρ 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.