The Effect of Doping Concentration and Conductivity Type on Preferential Etching of 4H-SiC by Molten KOH
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J5.20.1
The Effect of Doping Concentration and Conductivity Type on Preferential Etching of 4H-SiC by Molten KOH Ying Gao1, Zehong Zhang, Robert Bondokov, Stanislav Soloviev and Tangali Sudarshan Dept. of Electrical Engineering, Univ. of South Carolina, Columbia, South Carolina, 29208 1 Bandgap Technologies, Inc. Columbia, South Carolina, 29203 ABSTRACT Molten KOH etchings were implemented to delineate structural defects in the n- and ptype 4H-SiC samples with different doping concentrations. It was observed that the etch preference is significantly influenced by both the doping concentrations and the conductivity types. The p-type Si-face 4H-SiC substrate has the most preferential etching property, while it is least for n+ samples. It has been clearly demonstrated that the molten KOH etching process involves both chemical and electrochemical processes, during which isotropic etching and preferential etching are competitive. The n+ 4H-SiC substrate was overcompensated via thermal diffusion of boron to p-type and followed by molten KOH etching. Three kinds of etch pits corresponding to threading screw, threading edge, and basal plane dislocations are distinguishably revealed. The same approach was also successfully employed in delineating structural defects in (000 1 ) C-face SiC wafers. INTRODUCTION Molten KOH etching is a simple and effective method for revealing structural defects in SiC. However, it has been reported that molten KOH etching is better able to identify specific defects in medium and low doped n-type wafers than in highly doped SiC wafers [1]. In highdoped n-type wafers, the etch pit pattern is not distinguishable. In addition, it is very difficult to reveal structural defects on the C-face SiC wafers since isotropic etching is dominant on this face [2]. In this work, molten KOH etching was conducted to delineate defects in 4H-SiC samples of different conductivity types and doping concentrations and an explanation for the molten KOH etching mechanism is proposed based on the experimental observations. Furthermore, an approach to delineate structural defects on the Si- and C-face of highly doped n-type 4H-SiC substrates (currently not possible) by a combination of thermal diffusion of boron and molten KOH etching is developed. EXPERIMENTAL DETAILS Samples 1, 2, 3 and 5, 6, 7 in this study were cut from an n-type 8° off-axis 4H-SiC (0001) wafer with a doping concentration of 6×1018 cm-3, and sample 4 was cut from a p-type 8° offaxis 4H-SiC (0001) wafer with a doping concentration of 1×1018 cm-3. Both wafers were purchased from CREE Inc.. Sample 1, 2, 3, 4, 5 were used to study the etching on Si-face SiC, and samples 6, 7 were used to study the etching on C-face SiC. A 10 µm thick epilayer was grown on the Si-faces of sample 2 and sample 3 in a low pressure hot-wall CVD system, with ntype 1×1015 cm-3 and p-type 7×1015 cm-3, respectively. The Si-face of sample 5 and the C-face of sample 7 were overcompensated, by thermal diffusion of boron, to p-type with a doping concentration of 1×1018 cm-3 measured
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