Materials Science Forum Vols. 717-720

Abstract: Annealing of high purity semi-insulating (HPSI) 4H-SiC is investigated as a method to improve bulk photoconductive semiconductor switches through recombination lifetime modification. Five samples of HPSI 4H-SiC were annealed at 1810 °C for lengths of time ranging from 3 to 300 minutes. The recombination lifetime of the unannealed and annealed samples was measured using a contactless microwave photoconductivity decay (MPCD) system. The MPCD system consists of a 35 GHz continuous microwave probe and a tripled Nd:YAG pulsed laser. The recombination lifetime was increased from 6 ns, as received, up to 185 ns by annealing for 300 minutes. To experimentally verify switch improvements, identical switches from unannealed and annealed material were fabricated and tested at low voltage. The unannealed device generated a 15 ns pulse with a 2 ns rise-time. The annealed device conducted for upwards of 300 ns with a comparable 2 ns rise-time. The increased recombination lifetime resulted in lower on-state resistance and increased energy transfer.

Abstract: We obtained excess carrier lifetime maps by the microwave photoconductivity decay (µ-PCD) method in a free-standing n-type 3C-SiC wafer, and then we compared the lifetime maps with distributions of strains and defects observed by the optical microscopy and the Raman spectroscopy. We found that the excess carrier lifetimes are short in a strained region in 3C-SiC, which indicates that structural defects exist around a strained region.

Abstract: Optical monitoring of diffusivity in wide bandgap semiconductors was performed by using a picosecond light-induced transient grating technique. The bandgap renormalization and carrier-carrier scattering manifested itself at room temperature as two-fold decrease of the ambipolar diffusion coefficient Da in cubic SiC and 5-fold decrease of Da in diamond at excess carrier density N > 10¹⁷ cm^-3, while for GaN the impact was observed only at T 19 cm^-3, the plasma degeneracy led to enhanced D_a values in SiC and GaN and compensated the diffusivity decrease in diamond.

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: Morphological features, such as the orientation and linearity of basal plane dislocations (BPDs) in SiC crystals, were analyzed by applying a two-dimensional fast Fourier transform (2D-FFT) to X-ray topographic images of the BPDs. An SiC crystal fabricated by an improved repeated a-face (RAF) method and an SiC crystal fabricated by an conventional RAF method discussed in a previous study were evaluated. In the 2D-FFT images of the improved crystal, streaks along the directions were observed, indicating that the BPDs were highly oriented along the directions. The degree of orientation of the BPDs, which may reflect their linearity, was calculated, and the improved RAF crystal had a much higher degree of orientation than the conventional RAF crystal.

Abstract: This paper demonstrates the X-ray three-dimensional (3D) topography of basal-plane dislocations (BPDs) and threading edge dislocations (TEDs) in 4H-SiC for the first time. Stereographic topographs are obtained for BPDs and TEDs, showing the propagation of BPDs from a substrate to an epilayer and the conversion of BPDs into TEDs near the epilayer/substrate interface. Strain analysis is also demonstrated for a TED, providing the image of strains in the order of ±10-5. It is verified that the 3D topography is successfully applicable to BPDs and TEDs.

Abstract: In this paper, we report on the synchrotron white beam topographic (SWBXT) observation of “hopping” Frank-Read sources in 4H-SiC. A detailed mechanism for this process is presented which involves threading edge dislocations experiencing a double deflection process involving overgrowth by a macrostep (MP) followed by impingement of that macrostep against a step moving in the opposite direction. These processes enable the single-ended Frank-Read sources created by the pinning of the deflected basal plane dislocation segments at the less mobile threading edge dislocation segments to “hop” from one slip plane to other parallel slip planes. We also report on the nucleation of 1/3< >{ } prismatic dislocation half-loops at the hollow cores of micropipes and their glide under thermal shear stress.

Abstract: Using 6H SiC wafers including regions with a varying residual stress, the birefringence pattern of almost vertical dislocations is measured and modeled. We show that it is possible to identify the basal plane component of "small" dislocations by a quantitative fit of the birefringence pattern. Combining birefringence and etch pit detection after KOH etching shows that most of the vertical dislocations are of a mixed nature, exhibiting both an edge and a screw component.

Abstract: Morphologies of BPDs in 4H-SiC epilayers with different nitrogen doping concentrations are explained in detail. While BPDs in low-doped epilayers have the typical morphology of gliding dislocations responding to stress, BPDs in highly doped ([N]≥1.0×10¹⁸ cm^-3) epilayers are straight and tilt away from [11-20]. Structures of BPDs are further studied by weak-beam TEM.

Abstract: 4H-SiC intrinsic homoepitaxied single crystals have been nano indented at room temperature using a spherical indentor and the related deformation microstructures have been analyzed by Transmission Electron Microscopy. Dislocations are lying in the basal plane but have been found to be perfect, in contrast with observations made at higher temperature. Although such a change in deformation mechanism has been observed in other semiconductors such as Silicon and Indium Antimonide, it was unexpected in a very low stacking fault material such as SiC.