Characterization of Porous SiC Substrates and of the Epilayer Structures Grown on Them
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Characterization of Porous SiC Substrates and of the Epilayer Structures Grown on Them Balaji Raghothamachar1, Jie Bai1, William M. Vetter1, Perena Gouma1, Michael Dudley1, Marina Mynbaeva2, Matthew T. Smith3, Stephen E. Saddow3 1
Department of Materials Science & Engineering, SUNY at Stony Brook, Stony Brook, NY 11794-2275, U.S.A. 2 A. F. Ioffe Physico-Technical Institute, St. Petersburg 194021, Russia 3 Center for Microelectronics Research, University of South Florida, Tampa, FL 33620, U.S.A. ABSTRACT Porous 6H-SiC and 4H-SiC wafers formed by anodization have been characterized in this study prior to and following the CVD deposition of SiC epitaxial layers, using a combination of synchrotron white beam x-ray topography (SWBXT), SEM, TEM and optical microscopy. Under the high temperatures employed during epitaxial growth, a significant change in pore morphology occurs. While no evidence of reduced screw dislocation density in the epilayers is obtained, a small tilt of the epilayers with respect to the porous substrate observed on x-ray topographs could play a role in limiting penetration of defects from the substrate. INTRODUCTION Currently, the application of SiC towards fabricating high temperature, high frequency and high power electronic devices is limited by the quality of SiC epilayers grown on SiC bulk substrates. Recently, a novel growth technique has been discovered to reduce defect density in and improve the quality of SiC epilayers involving their growth on porous SiC (PSC) buffer layers [1,2]. PSC substrates formed by anodization in an electrochemical cell [3] have already been developed for other applications such as power device passivation [4] and room temperature visible luminescence [5]. In SiC epitaxial growth, the porous layer acts as a buffer for defects that are likely to penetrate into the epilayer from the substrate. To better understand the role of porous layers in SiC epitaxial growth, pore structures in PSC substrates (4H & 6H) have been studied before and after epitaxial growth using a number of characterization techniques including SWBXT, SEM and TEM. Preliminary results and analysis are presented here. EXPERIMENTAL DETAILS Off-axis (0001) oriented 6H-SiC and 4H-SiC samples cleaved from 2” commercial wafers purchased from Cree, Inc. were made porous by surface anodization on the (0001)Si face in an electrochemical cell at a current density of 8mA/cm2 for 3 minutes [6]. About 3-4µm thick, clear SiC epitaxial layers were grown by chemical vapor deposition (CVD) in a newly designed cold wall CVD system with a silicon to carbon ratio of 0.13 at 1600°C [7]. Characterization studies were carried out before and after CVD growth. SWBXT in the backreflection geometry was used to map the distribution of axial screw dislocations for both 4H-SiC and 6H-SiC wafers using the highly collimated beam of synchrotron white x-rays obtained at beamline X19C at the National Synchrotron Light Source, Brookhaven National Laboratory.
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Cleaved edges of the samples were examined under a Schottk
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