Structural and electrical investigation of ohmic contacts to gallum arsenide and indium phosphide

Abstract

This dissertation is concerned with the fabrication and characterization of ohmic contacts to n-type GaAs and InP. The main aim was to correlate the interdiffusional and metallurgical reactions that occurred between the metal and the semiconductor substrates during the annealing step, with the corresponding electrical observations. Following a review of the fabrication techniques and ohmic metallization contact systems used on n-type GaAs and InP, present metal-semiconductor junction theories are discussed. This includes a discussion of the various techniques used for specific contact resistance measurements. Furthermore, the experimental procedures and equipment used are also presented. Auger electron spectroscopy (AES) was utilized to investigate GaAs and InP surfaces after different chemical cleaning and etching treatments. These results are used as background for the fabrication of ohmic contacts. By comparing Ni and In as wetting agents, together with Au-Ge on n-type GaAs, the deduction was made that Ni acts chemically and physically to produce a better contact than In. The morphology of Ni/Au-Ge contacts on GaAs was improved by introducing an ion implantation step. From structural investigations it follows that the non-uniformity of the Au-Ge based contacts can be related to an Au-Ge-As phase that was formed at the metal-GaAs interface region. In parallel with the work on GaAs, contacts ton-type InP have also been studied. Noticeable lateral spreading of annealed Au and Au-based contacts to InP was observed. As a result, alternative fabrication techniques and ohmic contact systems were investigated. Two such systems were the Ti/Pt and Ti/Ni metallization schemes. AES, morphology studies and specific contact resistance (re) measurements of these contacts to Ar+ sputtered and non-sputtered InP substrates were carried out. These results were compared with the widely used Ni/Au-Ge contact system. The latter system yielded the lowest specific contact resistance values, while the best surface morphology was observed with the Ti/Ni contact system.