Behavior of W and WSi x Contact Metallization on n- and p-Type GaN
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BEHAVIOR OF W AND WSi,, CONTACT METALLIZATION ON n- AND p- TYPE GaN X. A. Cao,* F. Ren,** J. R. Lothian, *** S. J. Pearton,* C. R. Abernathy,* J. C. Zolper,**** M. W. Cole,***** A. Zeitouny,****** M. Eizenberg******* and R. J. Shul******* *Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA "**Departmentof Chemical Engineering, University of Florida, Gainesville, FL 32611, USA ***Multiplex Inc., South Plainfield, NJ 07080, USA ****Office of Naval Research, Arlington, VA 22217, USA *****US Army Research Laboratory, WMRD, Aberdeen Proving Ground, MD 21105, USA "******Department of Materials Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel *******Sandia National Laboratories, Albuquerque, NM 87185, USA Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G6.39(1999) ABSTRACT Sputter-deposited W-based contacts on p-GaN (NA-101 8 cm 3 ) display non-ohmic behavior independent of annealing temperature when measured at 25 TC. The transition to ohmic behavior occurs above -250 °C as more of the acceptors become ionized. The optimum annealing temperature is -700 TC under these conditions. These contacts are much more thermally stable than the conventional Ni/Au metallization, which shows a severely degraded morphology even at 700 TC. W-based contacts may be ohmic as-deposited on very heavily doped n-GaN, and the specific contact resistance improves with annealing up to -900 °C. INTRODUCTION One of the limiting factors in GaN device technology are the relatively high ohmic contact resistances. In the search for improved contact characteristics, a wide variety of metallizations have been investigated on p-GaN besides the standard Ni/Au[l-7], including Ni[4,5,8], Au[4,7,9,10], Pd[4], Pd/Au[ 11,12], Pt/Au[6], Au/Mg/Au[9,13], Au/C/Ni[14], Ni/Cr/Au[12,15] and Pd/Pt/Au[6]. This area has been reviewed recently by Mohney and Lau[16], and by Liu and Lau[ 17]. Typically Ni, Pd or Pt is the metal in direct contact with the GaN, and the structure is annealed at 400 - 750 'C. This produces contact resistances in the 10.1 - 10-3 "2.cm 2 range. For higher temperatures severe degradation in contact morphology is observed, usually resulting from the formation of the metal gallides[161. For n-type ohmic contacts, the most popular metallization schemes have been those based on AIITi/n-GaN, often with overlayers of Au/Ni[16,17]. It is believed that diffusion of Al to the metal/GaN interface is a critical step in forming the lowest specific contact resistance, and that TiN formation produces a high concentration of nitrogen vacancies that lead to n' doping level and lower R, values[17]. We have previously reported that both W and WSix on n' epi GaN layers (n - 1019 cm-3) produce reasonable contacts (R1 - 8x 10-5 .cm2), but extremely stable behavior[18,19] - annealing at 1000 'C led to a shallow reacted region of _
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