Al/Ni And Al/Ti Ohmic Contact To P-type SiC Diffused Layer
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Al/Ni And Al/Ti Ohmic Contact To P-type SiC Diffused Layer
Xaiobin Wang, Stanislav Soloviev, Ying Gao, G.Straty, Tangali Sudarshan, John R. Williams*, John Crofton** Univ. of South Carolina, Dept. of Electrical Engineering, Columbia, SC. *Auburn Univ., Leach Nuclear Science center, Auburn, AL ** Physics Department, Murray State University, KY
ABSTRACT Ohmic contacts to p-type SiC were fabricated by depositing Al/Ni and Al/Ti followed by high temperature annealing. A p-type layer was fabricated by Al or B diffusion from vapor phase into both p-type and n-type substrates. The thickness of the diffused layer was about 0.1-0.2 µm with surface carrier concentration of about 1.0x1019cm-3. Metal contacts to a p-type substrate with a background doping concentration of 1.2x1018cm-3, without a diffusion layer, were also formed. The values of specific contact resistance obtained by Circular Transmission Line Method (CTLM) and Transfer Length Method (TLM) for the n-type substrate, and by Cox & Strack method for p-type substrate, respectively, varied from 1.3x10-4 FP2 to 8.8x10-3 FP2. The results indicate that the specific contact resistance could be significantly reduced by creating a highly doped diffused surface layer.
INTRODUCTION Silicon carbide is an ideal semiconductor material for high power, high temperature and high frequency electronic devices owing to its wide band gaps, high breakdown fields, high thermal conductivity and high electron saturation velocities. The operating temperature of SiC devices is expected to be as high as 1000K[1], thus a thermally stable ohmic contact with low specific contact resistance, ρc, is one of the key issues that needs to be addressed for commercial applications. For n-type SiC, Ni-based ohmic contacts are commonly used and yield a specific contact resistance less than 10-5Ω cm2 [2, 3]. For p-type SiC, ohmic contacts based on Alcompositions such as co-deposited aluminum and titanium thin films [4] or Al-Ti alloy [1, 5] are mostly used. A systematic research about the influence of doping concentration on specific ohmic contact resistance of p-type SiC was reported in [1], where the experimental results of Al-Ti ohmic contact to epitaxial p-type SiC are presented. It has been shown that the specific contact resistance significantly drops when the surface carrier concentration is above 1019 cm-3. In real device processing, in order to obtain a good ohmic contact, the highly doped p+ regions are formed either by selective implantation [6] or by selective CVD epi-growth [7]. However, it has been reported recently that highly doped p-type regions could also be realized by selective diffusion of boron or aluminum [8,9]. In this work, we present the results of a study of Al/Ti and Al/Ni ohmic contacts that were fabricated on both the p-type and n-type diffused SiC samples. Also, for comparison, the same metal contacts were formed on a p-type substrate with no diffusion layer. The influence of the H5.19.1
type of acceptor impurity, diffusion time, as well as annealing technique on the co
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