Microstructure analysis of ohmic contacts to GaAs
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I.
INTRODUCTION
HIGH-performance
ohmic contacts to GaAs are essential in the fabrication of microwave and optoelectronic devices. 0,21 The metal-semiconductor contact becomes ohmic by reducing the barrier height, by forming a heavily doped layer under the metal contact, or by forming a damaged layer in the semiconductor, t3] The most general approach is to form a heavily doped layer in the semiconductor under the contact so that the resulting metalsemiconductor tunnel junction forms the ohmic contact. The heavily doped layer in the semiconductor can be formed (1)by alloying of multicomponent metallization, t41 (2)by diffusion,t51 and (3)by molecular beam epitaxy, t6] The simplest approach for the implementation of ohmic contact to GaAs is by alloying multicomponent metallization. Au-Zn-Au is the most common metaUization used for the fabrication of ohmic contact to p-GaAs and AuGe-Ni for n-GaAs.[3] In the conventional ohmic contact fabrication process, furnace heating is used for the alloying operation. Because of long heating times, furnace alloying of multicomponent metallization gives rise to undesirable indiffusion of metallization components into GaAs as well as outdiffusion of Ga and As into metallization with uncontrolled penetration of metallization components into GaAs. Transient heating techniques like scanned electron beam, t7] laser beam, [8j and pulsed laser and electron beams tg,t~ have been used to minimize the problems associated with furnace annealing, but the applicability of these techniques for large volume production of devices is still doubtful. Rapid thermal alloying with high-intensity lamps as heat sources seems to be the best technique even for large size wafers and large volume production of GaAs devices. In the case of GaAs integrated circuits, gold-base contact metaUization to n-GaAs reduces the reliability of the contacts due to the formation of low-temperature fl-AuGa phase. [H,12] Recently, more effort has been concentrated in the develT.S. KALKUR, Assistant Professor, and C. ARAUJO, Associate Professor, are with the Microelectronics Research Laboratories, Deparanent of Electrical Engineering, University of Colorado, Colorado Springs, CO 80933-7150. Y.C. LU, formerly with the University of Colorado, is Assistant Professor, Rutgers University, Piscataway, NJ 08855-0909. This paper is based on a presentation made in the symposium "Interface Science and Engineering" presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26-29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee. METALLURGICAL TRANSACTIONS A
opment of nongold-based ohmic contact metallization for n-GaAs.~ Aluminum-base ohmic contacts to GaAs are attractive, because the eutectic temperature for A1-Ge is 424 ~ which is higher than that of AuGe (356 ~ and aluminum has low energy absorption compared to gold in a radiation environment. Therefore, in this paper, we are presenting the results of rapid thermal
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