Comparative studies on low-resistance Pd-based ohmic contacts on p-GaAsSb

Ohmic contacts based on PdAu, PdIrAu, PdPtAu, PdIrMoAu, and PdPtMoAu metallizations have been formed on highly doped (5× 1019 cm-3) 500 nm thick p-type Ga As0.5 Sb0.5. All the ohmic contacts with Pd as a bottom layer exhibited better electrical characteristics than conventional TiPtAu metallization at as-deposited conditions. It shows that Pd forms a more favorable metal-semiconductor junction with the Ga As0.5 Sb0.5 layer compared with Ti. When Ir or Pt was incorporated on top of the bottom Pd layer, ohmic characteristics of PdIrAu and PdPtAu were improved by annealing at 250°C by the effect of Ir and Pt with high work function. When PdAu, PdIrAu, and PdPtAu ohmic metallizations were annealed at temperatures higher than 300°C, gradual degradation of ohmic properties was observed. To improve the thermal properties of the Pd-based ohmic contacts, an Mo layer was inserted beneath the top Au layer as a diffusion-barrier layer. The PdIrMoAu metallization scheme withstood higher annealing temperatures than the ohmic metallizations without the Mo layer. X-ray photoelectron spectroscopy was also carried out to investigate the interactions between the multilayer metallizations and the ternary GaAsSb compound semiconductor. The Mo layer was found to be effective in blocking Au diffusion into the semiconductor layer when it was combined with an Ir layer.