TEM studies of multilayer ohmic contacts to n-type AlGaN/GaN

M W Fay,P D Brown,I Harrison,K P Hilton,B T Hughes,T Martin,R S Balmer,D E J Soley,M J Uren,G Moldovan

doi:10.1201/9781351074636-111

Abstract

Ti and Pd barrier layers between the Al/Ti diffusion couple and the Au capping layer of multilayer ohmic contacts to n-type AlGaN/GaN field effect transistors were found to be ineffective in preventing the diffusion of Au to the AlGaN following high temperature rapid thermal annealing. The formation of a band of TiN grains at the contact/AlGaN interface is responsible for the activation of the contact. The presence of interfacial Au and threading dislocations are implicated in the formation of additional Ti-nitride inclusions into the AlGaN, although these do not appear to disrupt the Ti-nitride layer at the original contact/nitride interface, nor significantly influence the contact resistance.

Highlights

It was considered undesirable to have Au diffusing to the contact/nitride interface, diffusion barrier layers were incorporated into the Au/x/Al/Ti contact scheme where x corresponds to materials such as Ti, Pd, Ni or Pt. [Fan et al 1996, Cai et al 1998, Mohammad et al 1996, Chor et al 2001]
Characterisation was performed using a Jeol JEM-2010F field emission gun TEM operating at 200keV equipped with a Gatan Imaging Filter (GIF) and an Oxford Instruments Energy Dispersive XRay (EDX) detection system, and a Jeol-4000FX operating at 400keV equipped with a GIF
The onset of ohmic behaviour of AuTiAlTi and AuPdAlTi diffusion couple contacts to n-type AlGaN/GaN is related to the formation of a thin TiN band at the original contact/nitride interface, which is not disrupted by the formation of additional Ti-nitride inclusions into the AlGaN

Summary

INTRODUCTION

Devices based on AlGaN/GaN heterostructures are of interest in the area of high power, high frequency applications. For such devices to become commercially viable, ohmic contacts must fulfil the requirements of reproducibility, low resistance and good thermal and mechanical stability. It was considered undesirable to have Au diffusing to the contact/nitride interface, diffusion barrier layers were incorporated into the Au/x/Al/Ti contact scheme where x corresponds to materials such as Ti, Pd, Ni or Pt. TEM investigations have indicated that such barrier layers are ineffective at preventing the diffusion of Au through to the semiconductor under the condition of rapid thermal annealing needed to activate the contact. Complementary TEM techniques have been used to investigate the effect of varying the metallic layer construction, the annealing temperature, and the substrate on which the nitride layers are grown, on the development of the contact microstructure and the evolution of the contact/nitride interface in particular

EXPERIMENTAL DETAILS

RESULTS AND DISCUSSION

SUMMARY