Materials Science Forum Vols. 717-720

Abstract: Shallow defects, which were induced by mechanical treatment, on 4H-SiC wafers were investigated. The density and the distribution in depth of shallow defects on the wafers were depended on wafer venders. Most of serious defects such as dislocation array (DA), triangular stacking fault (TRSF) and triangular defect (TRD) in epitaxial film were demonstrated to be caused by shallow dislocations on the surface of the wafers. Revised mechanical polish can reduce the densities of DA, TRSF and TRD in epitaxial film.

Abstract: The effects of Shockley stacking faults (SSFs) that originate from half loop arrays (HLAs) on the forward voltage and reverse leakage were measured in 10 kV 4H-SiC PiN diodes. The presence of HLAs and basal plane dislocations in each diode in a wafer was determined by ultraviolet photoluminescence imaging of the wafer before device fabrication. The SSFs were expanded by electrical stressing under forward bias of 30 A/cm², and contracted by annealing at 550 °C. The electrical stress increased both the forward voltage and reverse leakage. Annealing returned the forward voltage and reverse leakage to nearly their original behavior. The details of SSF expansion and contraction from a HLA and the effects on the electrical behavior of the PiN diodes are discussed.

Abstract: Shockley stacking fault (SSF) contraction in 4H-SiC was investigated, in-situ, under varying temperature and ultraviolet (UV) intensity. Contraction of single SSFs at room temperature was observed for the first time under low power UV excitation of 0.04 W/cm². At temperatures above 150 °C, complete SSF contraction occurred for UV power at 0.2 W/cm². In contrast to expansion, SSF contraction occurred in discrete jumps between pinning sites along existing C-core partials. Luminescence from the pinning sites suggest they may be local concentrations of point defects. Additionally, a change in the line direction of the Si-core partials by ~25o off the direction was observed.

Abstract: We investigated expansion velocities of Shockley stacking faults (SSFs) in 4H-silicon carbide under laser illumination using photoluminescence methods. The experiments showed that the velocity of SSF expansion or the glide velocity of SSF-bounding 30°-Si(g) partial dislocations (PD) is supralinearly dependent on the excitation intensity. We estimated sample temperature by analyzing the broadening of band-edge emission and concluded that the lattice heating by laser illumination is not the cause of the enhanced dislocation glide. The supralinear dependence can be accounted for by a photo-induced sign reversal of the effective formation energy of SSF acting as the driving force of SSF expansion under the illumination.

Abstract: Room temperature photoluminescence was obtained by UV excitation of homoepitaxially grown 4H-SiC thin films. A broad band emission from boron deep levels centered at 517nm was observed along with the band-edge emission of 4H-SiC at 391 nm. The wavelength of the excitation was varied and the change in the relative intensity of the two emission peaks was observed. The variation of the relative intensity was correlated with the in-grown stacking fault density in the epilayer. A physical model was developed to explain the correlation in terms of carrier diffusion length. For epilayers with very high density of in-grown stacking faults, a sharp emission was observed at 480nm.

Abstract: Over the past decade, improvements in silicon carbide growth and materials has led to the development of commercialized unipolar devices such as Schottky diodes and MOSFETs, however, much work remains to realizing the goal of wide-scale commercialization of both unipolar and bipolar devices such as pin diodes or IGBTs, for high applications requiring high powers, operating in elevated temperatures or radiation environments or for many fast switching applications. Despite the great strides that have been made in reducing extended and point defect densities during this period, such defects still remain and with the push to lower off-cut angle substrates are in many cases seeing increases in prevalence. Thus, spectroscopic and imaging techniques for locating and identifying these defects are in high demand. Luminescence imaging and spectroscopy have both been utilized heavily in such work, yet simultaneously obtaining corresponding spectroscopic and spatial information from such defects is problematic. Here we report on hyperspectral imaging of electroluminescence from SiC pin diodes, whereby a stack of luminescence images are collected over a wide spectral range (400-900 nm), thereby providing the ability to both image distinct features and identify their corresponding spectral properties. This process is also equally applicable to collecting either photo- or electroluminescence from other materials or devices emitting in either the UV-Vis or NIR spectral range, as well as to reflectance, transmission or other imaging techniques.

Abstract: The radiative recombination spectra of 6H-SiC epilayers grown on low angle (1.4° off-axis) substrates have been investigated by low temperature photoluminescence spectroscopy. Four different types of stacking faults have been identified, together with the presence of 3C-SiC inclusions. From the energy of the momentum-conserving phonons, four excitonic band gap energies have been found with Egx equal to 2.837, 2.698, 2.600 and 2.525 eV. These photoluminescence features, which give a rapid and non-destructive approach to identify stacking faults in 6H-SiC, provide a direct feedback to improve the material growth.

Abstract: 6H-type stacking faults (SFs) observed in PVT-grown 4H-SiC ingle crystals were investigated using Photoluminescence (PL) microscopy at room temperature. Structural analyses using high resolution X-ray topography have revealed that there exist no (n=4, 8) component in Burger’s vectors of the 6H-type SFs we observed, strongly suggesting that the 6H-type SFs are constructed either by insertions of very thin 6H-type foreign polytype inclusions or by successive repetitions of Shockley-type in-plane glides.

Abstract: We carry out ab initio density functional theory calculations to investigate the fundamental mechanical properties of stacking faults in 3C-SiC, including the effect of stress and doping atoms (substitution of C by N or Si). Stress induced by stacking fault (SF) formation is quantitatively evaluated. Extrinsic SFs containing double and triple SiC layers are found to be slightly more stable than the single-layer extrinsic SF, supporting experimental observation. Effect of tensile or compressive stress on SF energies is found to be marginal. Neglecting the effect of local strain induced by doping, N doping around an SF obviously increase the SF formation energy, while SFs seem to be easily formed in Si-rich SiC.

Abstract: The present study reports on the propagation of twin boundaries in (111) 3C-SiC by means of conventional (CTEM) and high resolution transmission electron microscopy (HRTEM). The investigated 3C-SiC layers were homoepitaxially grown by Chemical Vapour Deposition (CVD) on layers previously grown by Vapor Liquid Solid (VLS) mechanism on 6H-SiC substrates. At the initial stages of growth the usual twin boundary that occurs is an incoherent {-211} Σ3 one. It transforms to more energetically favorable cases by several ways: (i) The initial {-211} boundary turns 90º, to a fully coherent (111) interface, forming microtwins; (ii) A step-like interface occurs with facets along the (111) and the {-211} planes; (iii) It transforms in a fourfold twin complex propagating to the surface.