Surface Evaluation of 6H-SiC after Doping by Diffusion
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Surface Evaluation of 6H-SiC after Doping by Diffusion
Ying Gao, S. I. Soloviev, and T. S. Sudarshan University of South Carolina, Department of Electrical Engineering, Columbia, SC 29208, U.S.A.
ABSTRACT Doping by diffusion of aluminum into 6H-SiC has been carried out in the temperature range of 1800-2100oC. Aluminum carbide (Al4C3) is thought to be one of the best candidates for a ptype diffusion source material. A thin layer graphite mask was developed to protect the wafer surface from deteriorating by sublimation/epigrowth during high temperature diffusion. Highresolution optical microscope (HROM) and atomic force microscopy (AFM) were employed to evaluate the surface morphology of the diffused samples. The protective mask significantly decreased the surface roughness. In addition, secondary ion mass spectroscopy (SIMS) was used to identify the influence of the thin graphite mask on the diffusion properties in SiC. There were no significant differences in doping profiles in the samples with and without the graphite mask. INTRODUCTION Surface morphology is very critical for device fabrication. Considerable research has been done regarding this topic, particularly relating to post-annealing (>1500oC) after ionimplantation in SiC [1-3]. Recently, thermal diffusion (>1800oC) in SiC has been shown to be a possible alternative approach to realize selective doping, based upon graphite masks with superior properties [4-6]. However, this extreme temperature may cause severe surface deterioration of SiC, due to epigrowth, sublimation or chemical reactions. Thus, the evaluation of the wafer surface is one of the aspects essential in determining the applicability of diffusion in practice. In this paper, we demonstrate that a very thin layer graphite mask covering the wafer surface eliminates surface damage from high temperature. EXPERIMENTS The 5×5 mm2 samples were cut from n-type 6H-SiC substrates from Cree Research Inc. A vertical quartz chamber with inductive heating served to implement diffusion. The diffusion temperature varied from 1800 to 2100 oC in argon ambient. Silicon carbide powder was mixed with elemental aluminum and aluminum carbide (Al4C3), respectively and then was charged in diffusion crucibles as the sources of acceptor dopants. Prior to use, standard cleaning procedure was implemented to remove organic and ionic impurities from the wafer surface. The samples used for the diffusion experiments are listed in Table 1. I-10 is a virgin sample used as a reference for surface analysis. AD-01 and AD-03 were used to check the feasibility of elemental aluminum and Al4C3 as impurity materials. For this purpose, graphite masks with circle patterns were deposited on the sample surfaces. AF-01 and AF-03 were used to investigate the influence of graphite masks on the diffusion properties, particularly in terms of doping concentration distribution. A very thin blanket graphite mask was deposited on sample AF-03. After diffusion, E5.10.1
the graphite mask was removed by burning at around 1000oC in air. Further, with
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