Abstract

During the last years significant efforts have been made to increase the Schottky barrier height in various InP devices such as high-speed photodetectors and transistors. Chu et al.¹ found that the barrier height ϕ_Bn of contacts to n-type InAlAs is independent of the choice of the metal and given by ϕ_Bn = 0.61 eV. Oppositely, Sadwick et al.² observed a pronounced influence of the kind of the metal on the barrier height with a highest ϕ_Bn. value of 0.72 eV for Pt contacts. Such data dispersing imply that the treatment of the semiconductor surface is of primary importance. We demonstrate that the quality of Au/Pt-n-AlInAs Schottky barrier diodes can be improved, if low energy (30 eV) Ar⁺ bombardment for 1 min prior to the metal evaporation and proper rapid thermal processing (RTP) for 20 s are used. Experiments were performed onto 1.5 µm-thick Si-doped AlInAs layers with a donor concentration of 2 × 10¹⁶cm⁻³ grown by MBE. Some wafers were subjected to shallow (20 nm) Zn diffusion. Pt/Au Schottky contacts with diameters of 500 µm were fabricated in a procedure similar to the method described elsewhere.³ Figure 1 presents barrier heights of Au/Pt-n-AlInAs contacts as-deposited and after subsequent RTP at temperatures ranging from 230 to 430°C. Prior to any heat treatment the chemically cleaned and ion-etched contacts exhibit barrier heights of ϕ_Bn = 0.82 eV and ϕ_Bn = 0.85 eV, respectively. The annealing has no significant effect on ϕ_Bn if the surface is only chemically precleaned, but gives a drastic enhancement of ϕ_Bn for ion-etched diodes. The highest value of ϕ_Bn = 1.09 eV is obtained with RTP carried out at 400°C. In our opinion, this is the largest ever reported value of ϕ_Bn for AlInAs. The distinguished increase of ϕ_Bn is due to the formation of a thin interfacial PtAs₂ layer,⁴ which results from thermally stimulated As outdiffusion towards the metal. This subtle reaction is obviously suppressed by native oxides in chemically precleaned samples. The ion process used introduces negligible subsurface damages since the starting ϕ_Bn values for chemically and ion-cleaned wafers are similar.

© 1994 IEEE