Selective Doping of 4H-SiC by Aluminum/Boron Co-diffusion
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Selective Doping of 4H-SiC by Aluminum/Boron Co-diffusion
Ying Gao, S. I. Soloviev, X. Wang, C. C. Tin1 and T. S. Sudarshan University of South Carolina, Department of Electrical Engineering, Columbia, SC 29208, U.S.A. 1 Auburn University, Physics Department, Auburn, AL 36849, U.S.A. ABSTRACT Based upon graphite mask, selective aluminum/boron doping of SiC by thermal diffusion has been successfully realized in a temperature range of 1800 to 2100 oC. Secondary ion mass spectrometry (SIMS) was used to identify the doping profiles, which showed very high aluminum concentration (5×1019 cm-3) near the surface and linearly graded boron profile up to several micrometers in depth. Hall-effect measurement was also employed to obtain the carrier concentration, which showed more than 1019 cm-3 carrier concentration at room temperature. Cathodoluminescence (CL) image clearly illustrated the locally diffused pattern. In addition, planar p-n diodes based upon this technique were fabricated and current-voltage (I-V) characteristics were measured. Excellent rectification property has been obtained. Built-in voltage of 2.9 V in the formed p-n junction was obtained by capacitance-voltage (C-V) measurement.
INTRODUCTION In recent years, with the considerable improvement in the quality of SiC wafers, more attention has been paid for device fabrication, where selective doping plays a significant role. Due to the lack of a suitable protective mask for thermal diffusion, selective doping is currently realized by ion implantation followed by high temperature annealing to reduce the lattice damage and to obtain reasonable percentage of electrical activation of impurities. However, the inherent drawbacks of ion implantation limit the potential features offered by SiC [1,2]. Using a high temperature mask developed in our laboratory we have successfully fabricated p-n diodes based on selective boron diffusion [3]. The good rectification characteristics confirmed the feasibility of this process except that the forward voltage drop is poor mostly due to the high ionization energy (300~340 meV) of boron acceptors. Considering that Al (which is also a commonly used acceptor for SiC) has lower ionization energy (160~200 meV) and a much lower diffusion coefficient compared with boron [4], in this work we attempted to carry out selective codiffusion of boron and aluminum in order to form a shallow p+-layer (Al) and a deep linearly graded p-layer (B) simultaneously.
EXPERIMENTS The samples used in experiments were n-type 4H-SiC with epilayer thickness, epilayer concentration and substrate resistivity of 10 µm, 1.1×1015 cm-3 and 0.045 Ωcm, respectively, H6.9.1
purchased from Cree Research Inc. A vertical double wall water-cooled quartz chamber with inductive heating was used to perform the diffusion process. The diffusion was carried out at temperatures varying from 1800 to 2100 oC in argon. A mixture of elemental boron, Al4C3 and silicon carbide powder was used as the source of acceptor dopants. RCA cleaning procedure was used to remove organic a
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