Papers by Author: Mark E. Twigg

Abstract: Electron channeling contrast imaging (ECCI) has been utilized to evaluate the surface morphology and crystalline quality of 3C-SiC films grown by chemical vapor deposition (CVD) on (100) and (111) Si substrates. ECCI in this study was performed using an electron backscatter diffraction (EBSD) system equipped with forescatter diode detectors and mounted inside a commercial scanning electron microscope (SEM). This nondestructive method permits direct dislocation imaging through local fluctuations in forescattered electron yield attributable to lattice strain. Coordinated ECCI, SEM, and EBSD analysis of film surfaces allowed correlations between film orientation, surface morphology, and dislocation behavior. Evidence of lateral dislocations parallel to <110> directions and atomic step pinning by dislocations was observed.

Abstract: Silicon carbide (SiC) has become the substrate of choice for III-N epilayers applied to electronic devices due to the lack of a native III-N substrate. This is particularly true for high power applications, since the thermal conductivity of the substrate enhances device performance. Although the GaN lattice match is slightly better for SiC than for sapphire, the dislocation densities that result are still very high (generally in the high 108 cm-2 range) and often deleterious to device performance. Screw-component dislocations are especially critical since they serve as leakage paths in vertically conducting III-N devices. In this paper efforts to reduce the extended defect density in III-N films grown on SiC will be reviewed. Details on recent efforts to use step-free SiC mesa surfaces arrayed on commercial 4HSiC substrates will then be highlighted showing dramatic reductions in extended defect densities and the virtual elimination of critical defects for vertically conducting devices. In these experiments, SiC surfaces that are homoepitaxially grown step-free or of very low step density have been used as growth templates for thin (<3 μm) GaN films deposited on a novel 1000 Å AlN nucleation layer characterized by a total dislocation density two orders of magnitude lower than the previous state-of-the-art, and with no evidence of screw-component dislocations.

Abstract: Through the use of specially-prepared on-axis SiC substrates with patterned mesa tops completely free of atomic-scale surface steps, we have previously reported the growth of highquality GaN heteroepitaxial films with greatly reduced threading dislocation densities on the order of 107/cm2. In these films, we reported a defect substructure in which lateral a-type dislocations are present in the nucleation layer but do not bow into threading dislocations during the subsequent GaN growth. This study focuses further on the role of SiC substrate surface steps in the generation of misfit, a-type, and threading dislocations at the heteroepitaxial interface. By using weak-beam imaging (both to eliminate Moiré effects and to observe narrow dislocation images) from plan-view transmission electron microscopy (TEM), we identify dislocations generated on stepped and unstepped mesas and compare their geometries. We observe that misfit dislocations nucleated on an unstepped SiC mesa are confined to one set of a-type Burgers vectors of the form g=1/3 [2110] _ _ , straight and well-ordered so that they are less likely to interact with each other. On the other hand, misfit dislocation structures on a stepped SiC mesa surface are not nearly as well-ordered, having bowed structure with threading dislocations that appear to nucleate at SiC surface steps.

Abstract: Using light emission imaging (LEI), we have determined that not all planar defects in 4H-SiC PiN diodes expand in response to bias. Accordingly, plan-view transmission electron microscopy (TEM) observations of these diodes indicate that these static planar defects are different in structure from the mobile stacking faults (SFs) that have been previously observed in 4H-SiC PiN diodes. Bright and dark field TEM observations reveal that such planar defects are bounded by partial dislocations, and that the SFs associated with these partials display both Frank and Shockley character. That is, the Burgers vector of such partial dislocations is 1/12<4-403>. For sessile Frank partial dislocations, glide is severely constrained by the need to inject either atoms or vacancies into the expanding faulted layer. Furthermore, these overlapping SFs are seen to be fundamentally different from other planar defects found in 4H-SiC.

Abstract: In this work, we have investigated the 3C-SiC re-growth on planarized 3C-SiC epitaxial layers, grown on (001)Si, after the application of a chemical mechanical polishing (CMP) process. A specific polishing process was developed for 3C-SiC to achieve a flat, high-quality surface. The interface between the deposited 3C-SiC and the polished 3C-SiC on Si film was studied by TEM characterization to determine if defects appear at this interface. It was observed that no additional defects were nucleated at the interface. The resulting re-grown film roughness, as a function of film thickness, was studied and is reported along with recommendations for future work.