4H-SiC Epitaxial Layers Grown on on-axis Si-face Substrate

We report on the growth of 4H-SiC epitaxial layer on Si-face polished nominally on-axis 2” full wafer, using Hot-Wall CVD epitaxy. The polytype stability has been maintained over the larger part of the wafer, but 3C inclusions have not been possible to avoid. Special attention has given to the mechanism of generation and propagation of 3C polytype in 4H-SiC epilayer. Different optical and structural techniques were used to characterize the material and to understand the growth mechanisms. It was found that all 3C inclusions were generated at the interface between the substrate and the epitaxial layer, and no 3C inclusions were initiated at later stages of the growth. Introduction The availability of increasing diameter of single crystal 4H-SiC wafer has opened up the possibilities for many power applications in recent years. The (00.1) Si-face polished, off-cut (4o or 8o) substrates are normally used to grow active layer for SiC electronic devices. With increasing wafer diameter this off-angle results in material losses when wafers are sliced from a boule [1]. Epitaxial layer grown on off-cut wafer on one hand easily replicates the polytype of the substrate, but on other hand makes it possible for basal plane dislocations to penetrate into the epilayer from off-cut substrate. It has been reported before that after long operation of bipolar electronic devices under heavy load, basal plane dislocations in the epilayer dissociate into two partials, one stationary and one moving thus resulting in the formation of stacking faults which ultimately degrade forward voltage [2]. The replication of basal plan dislocations into epilayer can be avoided through growth on on-axis substrate. One of the major issue with on-axis growth on (00.1) Si-face is the nucleation of 3C-SiC inclusions which reduces the effective useable area on full wafer for device purpose [3]. Therefore, it is crucial to investigate the origin and propagation of 3C-SiC on 4H-SiC on-axis substrate. Experimental A horizontal hot-wall CVD reactor [4] was used for the growth of n-type low doped, thick epilayers on Si-face on-axis substrate. The growth temperature and pressure were 1560 o C and 200 mbar, respectively. Hydrogen purified through heated palladium membrane was used as carrier gas while silane and propane were used as the sources for Si and C, keeping C/Si=1. Nitrogen was used as ntype dopant. In situ high temperature treatment was performed on all samples for 10 minutes in a mixture of hydrogen and propane before growth. In order to reveal 3C parts in the epilayer different methods like molten KOH etching, high resolution X-ray diffraction (HRXRD) and illumination of epilayer under UV laser light at 77K was performed. Surface morphology was observed under optical microscope with Nomarski interface contrast while the surface roughness was measured with tapping mode atomic force microscopy (AFM). Minority carrier lifetime mapping was Materials Science Forum Vols. 556-557 (2007) pp. 53-56 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 136.206.1.17-25/05/07,16:26:59) performed on full wafer at room temperature [5]. Synchrotron white beam X-ray topography (SWBXT) was performed with the synchrotron radiation from the bending magnet source of the DORIS III storage ring at HASYLAB-DESY, in Hamburg. SWBXT was made in back reflection mode on some selected areas of the sample while full wafer maps were recorded using a Bede Lang X-ray topograph. In order to observe the lateral expansion of 3C nuclei, wafer was cut around 3C inclusion and polished from sides parallel to growth direction to reach as close as possible to the area on substrate where 3C nucleation had started. Also a part of wafer, with 3C inclusions in it, was polished from epilayer top to the substrate surface in steps of 20μm and images were recorded with optical microscope after each step. Results and Discussion A typical result from an epitaxial growth on (00.1) Siface on-axis substrate is shown in Fig. 1. The figure shows a full wafer mapping using different experimental techniques. Image taken with UV illumination at 77K is shown in Fig.1a where black areas are related to 3C inclusions while the remaining part is 4H-SiC. Fig. 1b shows a X-ray topographic image of full wafer where black area corresponds to 3C parts while gray area is 4H-SiC. Figure 1c, shows a mapping of the photoluminescence decay time at room temperature. The white areas correspond to the 3C inclusions where no optical signal at the 4H band gap is present. The presence of 3C polytype inclusions in 4H was further confirmed by HRXRD. 3C inclusions spread randomly on the wafer with typically higher density at the edges of the wafer which could be due to bad crystal quality of the wafer at edges and rougher surface because of polishing scratches. The surface morphology of epilayer observed under optical microscope indicates that 4H parts grew in columns while 3C parts are more flat as shown in Fig. 2. Screw dislocations are the main nucleation center where growth occurs through the spiral growth mechanism. The spiral wounds and forms a pyramid structure around dislocation. This columnar growth of 4H parts could be due to low ad-atom mobility [6] which results in slow lateral growth and hence macro step bunching. 3C inclusion formation on 4H substrate strongly depends on the face polarity with (a) (b) (c) Fig. 1 Full wafer images of 60 μm thick 4H on-axis grown epitaxial layer. a) Image taken with UV light illumination at 77K. The black areas correspond to 3C-SiC while the rest is 4H-SiC. b) X-ray topographic map of full wafer. The black areas are 3C while gray areas are 4H-SiC. C) Carrier life time map at room temperature. White spots are 3C while the rest is 4H-SiC. Fig. 2 Optical image taken from as grown epilayer showing 3C inclusions in 4H-SiC. 3C parts are more flat as compared to 4H parts. Silicon Carbide and Related Materials 2006 54