Investigation of Factors Limiting the High Temperature Stability of W/WC/TaC/SiC Ohmic Contacts to n-type 6H-SiC
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Investigation of Factors Limiting the High Temperature Stability of W/WC/TaC/SiC Ohmic Contacts to n-type 6H-SiC T. Jang, B. Odekirk1 and L.M. Porter Dept. of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A. 1 Caldus Semiconductor Corporation, 1356 Parkview Ct. NE, Keizer, OR 97303, U.S.A.
ABSTRACT The electrical properties and thermal stability of TaC ohmic contacts with W/WC overlayers were investigated on n-type 6H-SiC (0001) substrates. The specific contact resistance (SCR) was measured after annealing at 600 °C and 1000 °C for 50 ~ 1000 h. The SCRs (3.0 ~ 3.6 ×10-5 Ω cm2) of contacts annealed at 600 °C for 1000 h remained constant within the experimental error. Significant increases in the SCR were not observed until the samples were annealed at 1000 °C for several hundred hours. No reaction of the film with the SiC substrate was observed after annealing at 600 °C for 1000 h; however, atomic-scale reactions appeared to be concentrated adjacent to grain boundaries in the reacted film. From Auger depth profiles, it was found that W and WC reacted to form W2C on TaC after annealing. According to SIMS analysis, after annealing at 1000 °C for 600 h, small but measurable changes in the electrical characteristics were associated with O incorporation at the interface between TaC and SiC. Investigation of the W/WC/TaC/SiC interface by TEM indicated that a reaction between the metal layers had occurred, but there was no observed reaction with the SiC substrate. After annealing for 1000 h, substantial changes in the contacts were observed. The findings indicate that both oxidation and metallurgical reactions have important implications on the current operating limits for SiC high temperature devices. INTRODUCTION Silicon carbide is a wide band gap semiconductor for high power and high temperature devices because of its wide band gap and other beneficial intrinsic properties such as its high bond strength, high electric field strength, and high saturation electron velocity1,2. However, electrical contacts that are chemically and microstructurally stable at high temperature (> 600 °C) and that have low specific contact resistances (SCR’s) must be developed for reliable high temperature devices. Based on its thermodynamic compatibility (high melting temperature and stability with SiC) and other properties (e.g., low work function3), TaC was chosen as an ohmic contact on n-type SiC. Thermodynamic stability between the contact layer and the SiC is critical if devices are to be used in continuous operation at high temperatures. The W/WC overlayer scheme was investigated based on its reported stability with TiC/SiC contacts4. In this study electrical measurements were correlated with microstructural and chemical analyses of the W/WC/TaC/SiC contacts to understand the conditions associated with the stability and the mechanisms for degradation. Limits for stability with respect to annealing conditions in vacuum along with observations of reactions and oxygen incorporation are presented.
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