Papers by Keyword: Interfacial Dislocations

Abstract: As SiC power devices are being developed toward ultrahigh-voltage bipolar structures, the density of basal plane dislocations in SiC epilayers has to be minimized. In this work, a special category of basal plane dislocations, i.e. interfacial dislocations, was investigated. Their etch pits were detected at the interface and the microstructure was revealed by cross-section transmission electron microscope analysis.

Abstract: During 4H silicon carbide (4H-SiC) homoepitaxy and post-growth processes, the development of stress relaxation has been observed, in which interfacial dislocations (IDs) are formed at the epilayer/substrate interface, relaxing the misfit strain induced by the nitrogen doping concentration difference between the epilayer and substrate. It is widely believed that an interfacial dislocation is created by the glide of a mobile segment of a basal plane dislocation (BPD) in the substrate or epilayer towards the interface, leaving a trailing edge component right at the interface. However, direct observation of such mechanisms has not been made in SiC before. In this work, we present an in situ study of the stress relaxation process, in which a specimen cut from a commercial 4H-SiC homoepitaxial wafer undergoes the stress relaxation process during a high-temperature heat treatment while sequential synchrotron white beam X-ray topographs were recorded simultaneously. Based on the dynamic observation of this process, it can be concluded that thermal stress plays a role in the relaxation process while the increased misfit strain at elevated temperature most likely drives the formation of an interfacial dislocation.

Abstract: Dislocation behavior during homo-epitaxy of 4H-SiC on offcut substrates by Chemical Vapor Deposition (CVD) has been studied using Synchrotron X-ray Topography and KOH etching. Studies carried out before and after epilayer growth have revealed that, in some cases, short, edge oriented segments of basal plane dislocation (BPD) inside the substrate can be drawn towards the interface producing screw oriented segments intersecting the growth surface. In other cases, BPD half-loops attached to the substrate surface are forced to glide into the epilayer producing similar screw oriented surface intersections. It is shown that the initial motion of the short edge oriented BPD segments that are drawn from the substrate into the epilayer is caused by thermal stress resulting from radial temperature gradients experienced by the wafer whilst in the epi-chamber. This same stress also causes the initial glide of the surface half-loop into the epilayer and through the advancing epilayer surface. These mobile BPD segments provide screw oriented segments that pierce the advancing epilayer surface that initially replicate as the crystal grows. Once critical thickness is reached, according to the Mathews-Blakeslee model, these screw segments glide sideways under the action of the mismatch stress leaving IDs and HLAs in their wake.

Abstract: Synchrotron X-ray Beam Topography (SWBXT) and KOH etching observations are presented of interfacial dislocations (IDs) and half-loop arrays (HLAs) which can form under certain growth conditions during homoepitaxy of 4H-SiC on off-cut substrates. The HLAs and IDs are observed to form from pairs of opposite sign basal plane dislocations in the substrate which intersect the substrate surface in screw orientation. These dislocations glide in opposite direction in the epilayer once critical thickness has been exceeded. Half-loop arrays are formed at the same time as the screw-type basal plane dislocations (BPDs) side-glide inside the epilayer. From knowledge of the formation mechanism of the HLAs [, if the line of the HLA is extended to intersect the original threading dislocation line direction, then the distance between this intersection point and the ID along the line direction of the original BPD provides a measure of the critical thickness. It is also calculated that the critical thickness in this case is largely determined by the mutual attractive force between the pairs of opposite sign threading BPDs in the substrate. In addition we observed both interfacial dislocations and HLAs generated from: (a) surface sources of BPDs; (b) micropipes; (c) 3C inclusions; and (d) substrate/epilayer interface scratches.

Abstract: Thermal annealing experiments were performed to determine the critical conditions of misfit dislocation formation in 4H-SiC epilayers in a temperature range of 1400-1800 °C. Misfit dislocations were observed to form at a given annealing temperature if the temperature gradient across the epi-wafer exceeded a critical value. It was also found that two types of interfacial dislocations could form under different stress conditions. Their formation mechanisms are discussed.

Abstract: Interfacial dislocations are frequently observed to form during 4H-SiC epitaxy and thermal annealing. This report attempts to establish the correlation between the distribution of interfacial dislocations and the thermal stress induced by a radial temperature gradient. In addition, it is argued that they are misfit dislocations formed by the interaction between thermal strain and misfit strain.

Abstract: Internal elastic strain, and its change accompanied with the raft formation during creep deformation in the Ni-based single crystal superalloy (TMS-26) have been investigated by X-ray diffractometry. The elastic strain caused by the lattice misfit between g and g' phases has markedly been changed by creep deformation especially in the directions perpendicular to the [001] tensile axis. The change in the elastic strain can be explained by the effect of creep dislocations stacked at g/g' interfaces. The evolution of the elastic stress field estimated from the elastic strain has explained well the transition from primary creep stage to the second one.