Improved Ohmic Contacts to n-Type GaAs, using Diboride Diffusion Barriers
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IMPROVED OHMIC CONTACTS TO n-TYPE GaAs, USING DIBORIDE DIFFUSION BARRIERS Joel R. Shappirio, Robert A. Lux, John J. Finnegan, Donald C. Fox, and Joseph H. Kwiatkowski, Electronics Technology and Devices Laboratory, LABCOM, Fort Monmouth, New Jersey 07703-5302 ABSTRACT Ohmic contacts employing Au/Metal/Au/Ge/Ni (Metal=Ni or. TiB 2 ) to n-type Alloying of the contacts, performed by both GaAs have been investigated. optical (rapid thermal) and strip heater methods, resulted in very low specific contact resistivities for the samples employing the TiB 2 diffusion Such contacts are shown to be stable on aging at 350C for 180 barrier. hours. INTRODUCTION The ability to produce a stable, low resistivity alloyed ohmic contact to n-type GaAs is an experimentally elusive but technologically important Such contacts are conventionally fabricated by the vacuum problem. deposition of an appropriate combination of metal layers, followed by an alloying process at a 3 temperature and time sufficient to produce a highly The doped (>1019 atoms/cm ) region at the semiconductor/metal interface. most extensively studied ohmic metallization consists of the eutectic Au-Ge alloy (88 wt % Au) combined with Ni which is believed to improve the homogeneity of the contact by assisting in the wetting of the GaAs surface After alloying, the Ge and thus preventing ball-up of the eutectic alloy. atoms are generally believed to form donor atoms occupying Ga sites in the GaAs host lattice. The typical alloying process is conducted in the temperature range 400-500C for times on the order of seconds to minutes. The appropriate alloy cycle is empirically chosen to obtain a minimum in the specific contact resistivity rc vs. time/temperature profile. Either underor over-alloying results in an rc value which is higher than this minimum. Kuan et al. [1], in a study of the Ni/Ge/Au alloy system combining transmission electron microscopy with energy dispersive x-ray spectroscopy, have investigated the relationship between the metallurgical evolution of the contact region and rc. They correlate the rc minimum with the development of the high conductivity ternary phase, Ni2 GeAs, and the subsequent increase in rc at higher alloying temperatures with the development and finally the dominance of Au/GaAs areas at the contact interface, developed as the result of continued in-migration of Au from the outer contact. Aging of these once-formed contact structures, typically at sub-eutectic temperatures (
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