The Structure and Electrical Properties of AU Contacts to GaAs
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* Lawrence Berkeley Laboratory, 62-203, University of California, Berkeley, CA 94720 ** Department of Materials Science, University of California, Berkeley, CA 94720 *** Stanford Electronics Laboratories, Stanford University, Stanford, CA 94305 ABSTRACT The structure and electrical properties of Au contacts to GaAs have been studied by analytical and high-resolution transmission electron microscopy (TEM) combined with electrical characterization and photoemission spectroscopy (PES). The contacts were prepared by in-situ deposition of Au onto The effects of annealing ultra-high vacuum cleaved n-GaAs (110) surfaces. on the electrical, morphological and chemical properties of the Au:GaAs contacts were studied. It was shown that the formation of Schottky contacts is correlated with the change in stoichiometry of the GaAs substrate near The change to ohmic behavior for the samples the Au/GaAs interface. annealed above Au-Ga eutectic was explained by leakage currents at the TEM micrographs of the structures revealed the periphery of the devices. existence of elongated Au crystallites on the GaAs surface at the periphery of the contacts. These leakage currents were eliminated by removing the current path at the periphery using a mesa-etch. INTRODUCT ION Metal-semiconductor contacts, both ohmic and Schottky, are crucial to semiconductor devices, but the physical mechanisms involved in their formation are not well understood. The simplest explanation by Schottky [1] was that the barrier height is the difference between the metal work function and the electron affinity (for n-type semiconductors), and therefore the Schottky barrier height should be linearly dependent on the metal work function. This is not the case for GaAs and many other semiconductors [2]. Observations show that the pinning position of the Fermi level for many metals as well as for oxygen falls within a narrow range of -0.95 * 0.1 eV below the conduction-band minimum (CBM) for p-type GaAs and -0.7 * 0.1 eV Many models have been developed to below the CBM for n-type GaAs [3-5]. There were many explain these metal-independent midgap pinning levels. attempts to explain the measured barrier heights in terms of semiconductor surface states, metal-induced gap states, anion clusters, or other defects near the interface [3-8]. Interesting observations were made for thick metal film Au:n-GaAs devices annealed above the Au-Ga eutectic temperature (3600C). Leakage currents were found to dominate the I-V characteristics for annealed devices. This is believed to be caused by the ohmic-like contacts that were found to occur at the periphery of the device [9-11]. Because Au has a high work function and is not known to form a shallow donor level in GaAs, the physical mechanism responsible for this "ohmic" behavior is still unclear. These structures are of particular interest, as they allow one to study barriertype behavior as well as ohmic behavior in the same Au-GaAs system. Therefore, in this study electron microscopy and microscopic analytical methods were applied to a
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