Metallurgical Study of Contacts to Gallium Nitride
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t Department of Electrical Engineering The Pennsylvania State University, University Park, PA 16802 M. A. KHAN, APA Optics, Blaine, MN 55449
ABSTRACT Thermally stable contacts to GaN that also have desirable electrical characteristics are required for the further development of optoelectronic and high temperature devices based on GaN. To make improvements on existing contacts or to develop new ones, information on the metallurgy of potential contact systems is needed. In this work, the Metal-Ga-N ternary phase equilibria and the contact metallurgy are examined for Ti, Re, and Ni. Annealed contacts of these metals have been examined with x-ray diffraction and/or x-ray photoelectron spectroscopy, and the observed metallurgical reactions are discussed in light of estimated or experimentally determined Metal-Ga-N phase diagrams. Particular attention is paid to the gas phase equilibria and the role of the annealing environment on the metallurgical reactions. Finally, the consequences of this work for the design of thermally stable contacts are considered. INTRODUCTION Researchers of contacts to the other III-V semiconductors have long recognized the utility of Metal-III-V phase diagrams for designing thermally stable electrical contacts and for better understanding the reactions that occur when metal contacts are not in thermodynamic equilibrium with the semiconductor [1-4]. Unfortunately, no Metal-Ga-N phase diagrams are available in the literature. Thus, there is a lack of information on the metallurgy of the Metal/GaN systems just when it could prove extremely valuable: during the early stages of development of electrical contacts to GaN. We have previously estimated the phase equilibria for the Transition Metal-Ga-N systems through thermodynamic calculations [5]. In this work, we discuss in greater detail the contact metallurgy of one metal from each of three groups in the periodic table: Ti from the early transition metals, Re from the middle transition metals, and Ni from the late transition metals. The features of the contact metallurgy that may be important for the development of electrical contacts to GaN are emphasized, including the role of the gas phase equilibria and the effect of the annealing environment on the reactions in Metal/GaN contacts. Ti/GaN CONTACTS The early transition metals all form metal nitrides of considerable thermodynamic stability, some of which exist over a wide compositional range. They also form metal gallides, and Metal-Ga-N ternary phases have been reported for Ti [6], V [7-9], Nb [7], and Ta [7]. For those metals for which no ternary phases have been reported (Sc, Y, Zr, La, and Hf), sufficient thermodynamic data is available to estimate the M-Ga-N phase diagrams. The common feature of these diagrams is a tie-line between GaN and a metal nitride of nominal composition MN; thus, the MN phases are predicted to be in thermodynamic equilibrium with GaN over at least a portion of their ranges of homogeneity. Experimentally we have confirmed the existence of the TiN-GaN tie through a p
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