Materials Science Forum Vols. 600-603

Abstract: Defect signatures, such as deep level positions, hyperfine parameters, local vibrational modes and optical transitions characterize a defect and enable the identification of defect centers. This identification is a key to an understanding of complex phenomena like the defect kinetics. Albeit density functional theory enabled the identification of several defects and their kinetic properties, a new approach is needed to address the optical excitation of defect. Within a quasiparticle theory and taking into account excitonic effects we analyze the excited states of VC +.

Abstract: The atomic and electronic structure of 4H-SiC(1 1 02) surfaces were investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and photoemission (PES). Two well ordered phases existing on this surface, i.e. (2×1) and c(2×2) are discussed. The (2×1) phase consists of a Si adlayer which is topped by an array of ordered Si-nanowires with electronic states confined to one dimension. For the c(2×2) phase STM indicates the presence of adatoms and PES a surface composition close to bulk SiC stoichiometry. A detailed atomic model for this c(2×2) phase is proposed.

Abstract: Grazing-incidence synchrotron topography studies of micropipes (MPs) and closed-core threading screw dislocations (TSDs) have been carried out and the results compared with ray-tracing simulations. Simulations indicate that both MPs and TSDs appear as roughly elliptically shaped white features which are canted to one side or the other of the g-vector depending on the dislocation sense and which have asymmetric perimeters of dark contrast which are greatly enhanced on the side towards which the feature is canted (again depending on the dislocation sense). For MPs, observations are generally consistent with this although the cant of the features is more obviously discerned than the asymmetry in the perimeter contrast. Sense assignment for MPs has been validated using back-reflection reticulography. For TSDs, observation are again generally consistent with the simulations although the smaller feature size and the variability in the line direction of the TSDs make the asymmetry of perimeter contrast a more obvious and reliable way to determine the dislocation sense than the sense of cant. TSD dislocation senses so obtained were validated using back-reflection images of same-sign and opposite-sign pairs.

Abstract: The density and sense distribution of elementary threading screw dislocations in a physical vapor transport grown 3-inch 4H silicon carbide wafer have been studied. The density of TSDs ranges between 1.6×103/cm2 and 7.1×103/cm2 and the lowest density is observed at positions approximately half radius off the wafer center. The dislocation sense of elementary threading screw dislocations can be readily revealed by the asymmetric contrast of their images in grazing-incidence x-ray topographs using pyramidal plane reflections. The circumferential and radial distributions of the sense of elementary threading screw dislocations have been studied and no clear trends are observed in either distribution.

Abstract: Threading edge dislocations (TEDs) in a 4H-SiC epitaxial layer are investigated using high-resolution synchrotron topography. Six types of TED image are confirmed to correspond to the Burgers vector directions by a comparison of computer simulated images and observed topography images in crystal boundaries. Using a mapping method, a wide spatial distribution of the six types of TED is examined in a quarter section of a 2-inch wafer.

Abstract: 4H-SiC substrate wafers with epi-layers were observed using monochromatic synchrotron X-ray topography in grazing incidence geometries, to investigate the defects in the epi-layer. Misfit dislocations with b=+1/3[11 2 0] caused by the difference in lattice parameter between the epi-layer and the substrate were observed. The misfit dislocations are located near the interface as edge dislocations, and appear at the top surface as screw dislocations on basal planes. It was observed that more than half of them were introduced from the growing epi-layer surface. The misfit dislocations and some screw dislocations with b=+1/3[11 2 0] are observed to remain as basal plane dislocations at the surface, while other basal plane dislocations were converted to threading edge dislocations in the epi-layer.

Abstract: We have investigated dislocation image of 4H-SiC wafers projected on synchrotron X-ray topographs taken under different positions in the rocking curve of a diffraction peak. The diffraction geometry was grazing-incidence extremely asymmetric and the diffraction vectors were g = 1 1 2 8 and 112 8. The weak-beam images were demonstrated for basal-plane dislocations and threading-screw dislocations. The basal-plane dislocation images became narrower in width at the off-Bragg conditions, and they were decomposed to separate lines under the weak-beam condition. The threading-screw dislocations showed changes in their shape and contrast as the crystal set was tilted from the rocking-curve peak, and finally the characteristic images near the dislocation core were observed under the weak-beam condition. The origin of these weak-beam images is unclear, but it will offer detailed analysis of the dislocations.

Abstract: The evolution of basal plane dislocations (BPDs) in 4H-SiC epitaxy during its growth is investigated by using two types of interrupted growth in conjunction with ultraviolet photoluminescence (UVPL) imaging of the dislocations. For the first, each epitaxial growth was stopped after 10-20 μm and a UVPL map was collected. For the second, changing the gas flow interrupted the growth and the BPDs were imaged at the end. The first sequence made it possible to track the formation of half-loop arrays and show that they arise from BPDs that glide perpendicular to the offcut direction. For both types, each interruption causes between 30 – 50% of the BPDs to be converted to threading edge dislocations (TEDs). This result suggests that using interrupted growth may be an alternate method to producing epitaxial layers with low BPD concentration.

Abstract: Dislocations in a substrate wafer of 4H-SiC with an epi-layer were observed using technique of monochromatic synchrotron X-ray topography in a grazing incidence geometry. Six different Burgers vectors of basal plane dislocations and threading edge dislocations were identified by changing the Bragg reflections, and by analysis of images of dislocation. We identify some relations of the Burgers vector and the dislocation contrast observed for g=11 2 8. Some of these relationships are discussed in this report.

Abstract: Slip of basal plane dislocations in 4H-SiC epitaxy is observed by synchrotron reflection X-ray topography. It is verified that average slip distance and slip thickness increase with decreasing donor concentration of epilayer. Since N atom, which is incorporated at carbon site in 4H-SiC and acts as a donor, has a smaller radius than that of C atom, lattice contraction effect is expected due to incorporation of N. Considering difference of lattice contraction between the substrate and the epilayer owing to difference of donor concentration, strain in the epilayer is estimated. The strain is expected to increase with decreasing the donor concentration of epilayer and the results are explained from this point of view.