The Formation Mechanism of Carrot Defects in SiC Epifilms
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0911-B05-24
The Formation Mechanism of Carrot Defects in SiC Epifilms Hui Chen, Guan Wang, Yi Chen, Xiaoting Jia, Jie Bai, and Michael Dudley Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY, 11794-2275 ABSTRACT Carrot-like defects in a 7° off-cut (from [0001] toward < 1210 > direction) 4H-SiC wafer with a 36µm thick 4H-SiC epilayer have been investigated using Nomarski optical microscopy, synchrotron white beam x-ray topography (SWBXT), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray topographs confirm that threading screw dislocations are often associated with the carrots. Cross-sectional TEM observation confirms that a prismatic stacking fault exists below the carrot. This fault was found to show contrast in all observed diffraction geometries except for g=0004. A model for the mechanism of formation of this type of defect during epitaxial growth is proposed. INTRODUCTION The growth of SiC using CVD has experienced great progress in the last decade. Defect densities in PVT grown 4H-SiC substrates have dramatically decreased and as a consequence so have those in CVD grown homoepitaxial layers particularly when grown on off-axis substrates. However, when growth is carried out on off-axis substrates, triangular 3C inclusions have been found to be a persistent problem1-3 as have surface morphological defects that have a shape similar to a “carrot” (hereafter simply referred to as carrots)3-6 and both are found to have significantly adverse effects on device performance7. Several groups4-6 have studied carrot defects and found that they comprise complicated intersections of stacking faults on the prismatic and basal planes. However, while the general morphology of the defect may be understood to a certain extent, the formation mechanism of the carrots remains unclear. In this paper, we report on observations of these defects using SWBXT, SEM, and TEM and propose a model for their formation during epitaxial growth. EXPERIMENTAL DETAILS SWBXT experiments were carried out at the Stony Brook Topography Facility (Beamline X-19C) at the National Synchrotron Light Source, at Brookhaven National Laboratory. The white x-ray spectrum ranges from 0.01Å to 2.00Å, with peak intensity at around 0.8 Å. The specimen-film distances were set to be 25cm to reduce background noise. Three cross-sectional TEM samples containing carrot defects were made from a 2-inch 4H-SiC wafer with a 36µmthick 4H-SiC homoepitaxial layer (substrate grown at II-VI WBG Inc, epilayer grown at SemiSouth). They were cut with angles of 90o, 60o and 30o, respectively, to the long axis of the carrot. Samples were mechanically ground using a T-tool and 30 µm, 15 µm, 6 µm, 3 µm, 1 µm and 0.5 µm diamond lapping films and then ion milled to electron transparency. TEM observation was carried out on a Philips CM12 transmission electron microscope with an accelerating voltage of 120KV.
RESULTS AND DISCUSSION Fig. 1 shows SEM images of the surface morphology of typical carrot defects. Fig. 2
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