Materials Science Forum Vols. 717-720

Abstract: Synchrotron White Beam X-ray Topography (SWBXT) imaging of wafers cut parallel to the growth axis from 4H-SiC boules grown using Physical Vapor Transport has enabled visualization of the evolution of the defect microstructure. Here we present observations of the propagation and post-growth mutual interaction of threading growth dislocations with c-component of Burgers vector. Detailed contrast extinction studies reveal the presence of two types of such dislocations: pure c-axis screw dislocations and those with Burgers Vector n₁c+n₂a, where n₁ is equal to 1 and n₂ is equal to 1 or 2. In addition, observations of dislocation propagation show that some of the threading dislocations with c-component of Burgers adopt a curved, slightly helical morphology which can drive the dislocations from adjacent nucleation sites together enabling them to respond to the inter-dislocation forces and react. Since all of the dislocations exhibiting such helical configurations have significant screw component, and in view of the fact that such dislocations are typically not observed to glide, it is believed that such morphologies result in large part from the interaction of a non-equilibrium concentration of vacancies with the originally approximately straight dislocation cores during post-growth cooling. Such interactions can lead to complete or partial Burgers vector annihilation. Among the reactions observed are: (a) the reaction between opposite-sign threading screw dislocations with Burgers vectors c and –c wherein some segments annihilate leaving others in the form of trails of stranded loops comprising closed dislocation dipoles; (b) the reaction between threading dislocations with Burgers vectors of -c+a and c+a wherein the opposite c-components annihilate leaving behind the two a-components; (c) the similar reaction between threading dislocations with Burgers vectors of -c and c+a leaving behind the a-component.

Abstract: A review is presented of Synchrotron White Beam X-ray Topography (SWBXT) studies of stacking faults observed in PVT-Grown 4H-SiC crystals. A detailed analysis of various interesting phenomena were performed and one such observation is the deflection of threading dislocations with Burgers vector c/c+a onto the basal plane and associated stacking faults. Based on the model involving macrostep overgrowth of surface outcrops of threading dislocations, SWBXT image contrast studies of these stacking faults on different reflections and comparison with calculated phase shits for postulated fault vectors, has revealed faults to be of basically four types: (a) Frank faults; (b) Shockley faults; (c) Combined Shockley + Frank faults with fault vector s+c/2; (d) Combined Shockley + Frank faults with fault vector s+c/4.

Abstract: Solution growth is considered to be a powerful method for high quality SiC crystals. This work reports that the conversion process from a threading screw dislocation into a few Frank partial dislocations in basal planes was investigated by synchrotron X-ray topography. This process was effectively assisted by step-flow growth on off-oriented (0001) seed crystals. The Frank partials were not extended into the crystal grown toward the [0001] direction perpendicular to the basal plane. Thus, the conclusion of this study suggests the use of off-oriented seed crystal is important to improve crystal quality.

Abstract: Structures and propagating behaviors of threading dislocations (TDs) in PVT-grown 4H-SiC single crystals were both investigated using Synchrotron monochromatic X-ray topography. Comparative studies by examining images obtained for the crystals with different diffraction geometries of (0004) and (11-20) of 4H-SiC revealed that a large amount of TDs are likely to be mixed in character, i.e., dislocations with Burgers vector components of both <0004> and <11-20>. Closer observations of topography images has revealed that, although TDs lie largely along the c-axis direction, some of the TDs show quite a complex propagating behavior: not extending in a straight line but meandering along the growth direction.

Abstract: The origins of certain types of micrometer-scale surface morphological defects on SiC epitaxial layers are clarified using X-ray topography. Two types of surface morphological defects are commonly observed on Si- and C-face epitaxial layers. Relatively large pits (around 4μm×2μm) originate from threading screw dislocations (TSDs). Relatively small pits (around 1.5μm×1μm) originate from threading edge dislocations (TEDs). The shapes and depths of these surface morphological pits depend on the fabrication history of the epitaxial wafers.

Abstract: We report our investigation results on triangular-defects formed on 4deg. off 4H-SiC epi- taxial wafers. Triangular-defects that had neither down-falls nor basal-plane dislocations previously reported as origins of triangular-defects at the tips of triangle were investigated by TEM. Our TEM results revealed that foreign materials contamination that were different from well-known down- -falls in size and in composition caused one of the defect formations and abnormal domain forma- tions were implied to occur and thought to relate to defect formations. We also report that several types of microstructure existed in the isosceles of defect during dislocation analyses around triangular-defects by X-ray topography.

Abstract: The wide size distribution of the hexagonal etch pit of screw dislocations (SD) in 4H-SiC wafer was found in spite of the narrow size distribution of the SD pit in epitaxial film. Calculation on the basis of the strain energy equation indicated that etch pit size depends on the Burgers vector and dislocation tilt. Size variation of SD etch pits in 4H-SiC wafer fabricated by sublimation method is explained to be caused by the dislocation tilt by observing the sizes and the positions of etch pits from the surface of the epitaxial film to the inside of 4H-SiC wafer. The SDs in 4H-SiC wafer fabricated by sublimation method propagate to c-axis direction in macroscopic but changing tilt in microscopic.

Abstract: We investigate the influence of SiC surface morphology on increase and variability in reverse leakage current of SiC Schottky barrier diodes using device simulation. It is found that etch pits with only a few tens of nm in depth has a large influence on leakage current and is also shown that leakage current is sensitive to both etch pit shape and density. From these results, we suggest the critical density of nanoscale pit, which is suppressing the variability of leakage current, at various drift layer thickness t_drift and doping concentration N_drift.

Abstract: The leakage current sources of 4H-SiC Schottky barrier diodes (SBDs) were analyzed using atomic force microscopy (AFM) to determine the surface morphology. Nanosized circular cone shaped pits (nanopits), which depth were distributed from 5 to 70 nm, were observed at the leakage current sources. The leakage currents of 4H-SiC SBDs generate at the nanopits due to the concentration of the electrical field strength. The positions of nanopits correspond to the positions of threading dislocations (TDs), which were identified from molten potassium hydroxide (KOH) etching.

Abstract: The etch pit density produced on the C-face 4H-SiC substrate using chlorine trifluoride gas at various temperatures was evaluated. Because the etch pit density formed at the substrate temperature of 713 K showed the comparable value to the current dislocation level of the Si-face 4H-SiC, the etch pit density obtained by this technique is considered to have a relationship with the crystal quality.