Materials Science Forum Vols. 556-557

Abstract: The origin of dislocation evolution during SiC crystal growth is usually related to lattice relaxation mechanisms caused by thermal stress. In this paper we discuss dislocation generation and dislocation propagation related to doping and suppression of basal plane dislocations, the latter being of particular interest for bipolar electronic devices. We have prepared alternating p-/n-/pdoped SiC crystals using the donor nitrogen and the acceptors aluminum or boron. In addition we determined the mechanical properties of n-type and p-type SiC; in particular we measured the critical shear stress by nano-indentation on c-plane and a-plane 6H-SiC surfaces. A considerably lower basal plane dislocation density is found in aluminum as well as in boron doped p-type SiC compared to nitrogen doped n-type SiC. It is concluded that the explanation of the reduced basal plane dislocation density in p-type SiC needs the consideration of electronic as well as mechanical effects.

Abstract: In this paper we report, based on analysis of dislocation statistics, on the influence of growth temperature on the nucleation, propagation and annihilation mechanisms of dislocations. Using KOH defect etching and optical microscopy we have conducted dislocation tracking along lengths of crystals grown under various process temperature regimes to study their evolution and propagation mechanisms statistically. We further present the influence of growth temperature on the step structure of the growth front using AFM measurements. From the analysis of dislocation statistics and step structure in relation to temperature we derive the role of surface kinetics of the SiC gas species on the growth surface in dislocation evolution during PVT growth of bulk SiC.

Abstract: Synchrotron reflection X-ray topography and KOH etching were applied to investigate the effects of the ion implantation/annealing process on the existing dislocations in the 4H-SiC epilayers and second epitaxial growth on the ion implanted layer. No systematic generation of dislocations or stacking faults caused by the second epitaxial growth on the implanted layer was observed, while BPDs were confirmed to migrate in the epilayer during the implantation/annealing process. The BPDs bend markedly near the bottom of the implanted layer and tend to lie along the <1-100> (perpendicular to the off-cut direction) after the implantation/annealing process. The lattice mismatch strain created by the implantation is a possible driving force of the glide motion of the BPDs.

Abstract: The advantage of room-temperature photoluminescence (PL) mapping was demonstrated for nondestructive detection of stacking faults (SFs) in off-oriented 4H-SiC epitaxial and bulk wafers. In mapping of the SF-related emission at 2.9 eV on the wafers, the SFs in the surface region appeared as a bar-shaped pattern with the long side perpendicular to the off-cut direction. The use of 266 nm light excitation is essential to detect the SF pattern in the bulk wafers because of its shallow penetration depth. The dark lines crossing the bar-shaped patterns in the epitaxial wafers are ascribable to the basal plane dislocation located close to the SF-planes.

Abstract: First-principles calculations are used to investigate the partial dislocations in 4H-SiC. We have shown that the Peierls barriers are strongly dependent on the dislocation core structures. Our results have revealed that the asymmetric reconstruction does not possess midgap states while the symmetric reconstructions, characterized by dangling bond on like atoms along the dislocation line, are always electrically active. We suggested that under forward bias, the free energies of the symmetric reconstructions are dynamically lowered by continuous electron-hole transitions between the respective deep levels and valence/conduction bands.

Abstract: By the use of Cl2-O2 thermal etching method, the etching rates of 4H-SiC were reached to about 1μm/h for Si and 40μm/h for C face at 950oC. Etch pits only appeared over 0.25-μm-etched depth on the 4H-SiC (0001) Si face. The shapes and density of etch pits are similar tendencies in the case of molten KOH etched surface. To study the relationship between thermally etched surface features and crystal defects, the planar mapping electron-beam-induced current (EBIC) technique was carried out. Almost dark areas in the EBIC image correspond to the etch pits. From the EBIC image, a shell-like pit formed by the Cl2-O2 etching on the (0001) Si face is a basal plane dislocation.

Abstract: Using TEM we show that defective regions are formed in SiC by ion implantation, and that some of the regions grow at the expense of others. Using HRTEM we show that these regions contain a large number of stacking faults. It is proposed that these stacking faults are Frank intrinsic stacking faults formed by condensation of divacancies, and it is this defect that is associated with the DI defect.

Abstract: Device quality SiC wafers are extremely expensive and available from only a limited number of vendors. This has limited the ability of researchers to compare and evaluate quality from various vendors. This paper surveys some properties and characteristics of SiC wafers purchased in the commercial market place and describes the product variability among vendors as a method to highlight the areas where improvements in substrate quality are desirable.

Abstract: A non-destructive technique to image the dislocations and other extended defects in SiC epitaxial layers has been developed. Basal plane dislocations (BPDs) and threading dislocations (TDs) are imaged. Photoluminescence from the dislocations is excited with the 364 and/or 351 nm lines of an argon ion laser and near-infrared light is collected. A computer controlled probe station takes multiple images and the mm-sized images are stitched together to form whole-wafer maps. The technique is applied to a set of four n+ wafers from the same boule with 50 um n- epitaxial layers. The epitaxy was grown with Cree’s low-BPD process. BPDs form as either single, isolated dislocations or as clusters encircling micropipes. The concentration of TDs is on the order 104/cm2 and the local concentration varies more than an order of magnitude. The advantages of mapping dislocations by UV-PL imaging compared to other techniques are discussed.