High Temperature Stable Titanium Carbide Ohmic and Schottky Contacts to SiC
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C.-M. ZETTERLING*, M. OSTLING*, L. NORIN**, U. JANSSON** *KTH, Royal Institute of Technology, Dept. of Electronics, Box 229, S-164 40 Kista, SWEDEN **Uppsala University, Angstrtm Laboratory, Dept. of Inorganic Chemistry, Uppsala, SWEDEN
ABSTRACT Epitaxial titanium carbide is investigated as a low resistivity, high temperature stable Ohmic and Schottky contact to 4H and 6H SiC. The titanium carbide films were deposited at 500 'C using co-evaporation of titanium (e-beam) and C60 (Knudsen cell) in a UHV system. Schottky diodes and TLM structures were fabricated on low and high doped SiC material respectively. The samples were annealed at 700 'C in a vacuum furnace, and electrical measurements were performed up to 300 'C. INTRODUCTION Silicon carbide has been suggested as a material for high performance electronic devices. The wide bandgap and high thermal conductivity allows high operating temperatures, whereas the high critical field offers high blocking voltage and low on-resistance [1]. A limiting factor for achieving high temperature stable devices is the Ohmic and Schottky contacts. In silicon technology, metal silicides [2] have been used to great advantage due to their low resistivity and high stability, and these materials are also available for SiC technology [3]. Further possibilities for improvement are offered using metal carbides, including metals forming both silicides and carbides. Tungsten [4] and tungsten carbide [5] has been found to be highly stable material choices with good electrical characteristics for Schottky diodes, although very difficult to pattern. A materials choice that allows wet etching and offers good electrical characteristics is titanium carbide, TiC. This material forms at the interface when Ti films evaporated onto SiC have been annealed at temperatures above 700 'C [6, 7]. However, at these high temperatures titanium silicides or titanium carbosilicides may be formed as well [8]. Another way to form stoichiometric TiC films that are more stable is by CVD growth at 1260 'C [9, 10]. In this paper we investigate Ohmic and Schottky contacts of TiC to SiC formed by a novel method at low temperatures (500 'C), which has previously been reported for TiC on MgO [11] and promises easier process integration. EXPERIMENT The investigation was performed on SiC wafers from CREE of various doping and polytypes. For the Schottky contacts both n- and p-type 4H SiC with 4 gim thick low doped epitaxial layers (2 - 8x1016 cm-3 ) of the same doping type were grown by CREE. The Ohmic contacts were made on highly doped SiC material: 6H n-type, 6H p-type and 4H p-type. The 1 jm thick n-type layer was grown by CREE with a doping of 2x10' 9 cm-3 (on a p-type wafer). The I Rim thick p-type layers with doping > 1020 cm3 were grown by the Industrial Microelectronics Center (IMC) in Sweden [12] (on n-type wafers).
125 Mat. Res. Soc. Symp. Proc. Vol. 512 ©1998 Materials Research Society
For the Ohmic contact evaluation, TLM structures (for transfer length measurement [13]) were manufactured by mesa etching
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