Takayuki Yano

Theoretical investigation of the formation of basal plane stacking faults in heavily nitrogen-doped 4H-SiC crystals

The formation of basal plane stacking faults in heavily nitrogen-doped 4H-SiC crystals was theoretically investigated. A novel theoretical model based on the so-called quantum well action mechanism was proposed; the model considers several factors, which were overlooked in a previously proposed model, and provides a detailed explanation of the annealing-induced formation of double layer Shockley-type stacking faults in heavily nitrogen-doped 4H-SiC crystals. We further revised the model to consider the carrier distribution in the depletion regions adjacent to the stacking fault and successfully explained the shrinkage of stacking faults during annealing at even higher temperatures. The model also succeeded in accounting for the aluminum co-doping effect in heavily nitrogen-doped 4H-SiC crystals, in that the stacking fault formation is suppressed when aluminum acceptors are co-doped in the crystals.

Growth of Low Basal Plane Dislocation Density 4H-SiC Crystals in Controlled Temperature Distribution inside the Crucible

For bulk growth of SiC crystal with higher quality, it is important to control the temperature distribution inside the crucible. We have performed numerical calculations of the temperature distribution inside the growing crystal, and discussed the relationship between the calculated sheer stress and the basal plane dislocation densities. We found that growth with lower basal plane dislocation defect densities, specifically at the front edge of the crystal, is possible by lowering the temperature gradient toward the growth direction.

Formation of Epitaxial Defects by Threading Screw Dislocations with a Morphological Feature at the Surface of 4º Off-Axis 4H-SiC Substrates

In this paper, we present the formation of extended epitaxial defects, such as carrot defects, from threading screw dislocations (TSDs) with a morphological feature at the surface of the substrates. It was confirmed using highly sensitive surface observation, atomic force microscopy (AFM) and KOH etching that the surface roughness around a TSD was observed as the morphological feature and TSDs with such a morphological feature formed extended epitaxial defects with high frequency of appearance compared to usual TSDs without any features. The density of TSDs with such morphological feature depended on the polishing methods. Furthermore, we observed that the formation and shapes of extended defects from TSDs with such morphological feature were affected by step-bunching at the surface of the epilayers.

A Possible Mechanism for Hexagonal Void Movement Observed during Sublimation Growth of SiC Single Crystals

A possible mechanism of hexagonal void movement during Physical vapor transport (PVT)-growth is proposed in terms of quasi-equilibrium phase transition process based upon the Si-C binary phase diagram. The hexagonal void movement can be realized when two different reactions occurs simultaneously: (1) SiC(s) solidification and (2) decomposition without graphitization. Further, the kinetic instability of the void movement observed is also discussed, and found to be explainable if the effect of the temperature gradient existing in the crystal grown in conventional PVT-process is included.

Temperature Dependent Stability of Stacking Fault in Highly Nitrogen-Doped 4H-SiC Crystals

The formation of basal plane stacking faults in highly nitrogen-doped 4H-SiC crystals was theoretically investigated. A novel theoretical model based on the so-called quantum well action (QWA) mechanism was proposed; the model considers several factors, which were overlooked in a previously proposed model, and explains well the annealing-induced formation of double layer Shockley-type stacking faults in highly nitrogen-doped 4H-SiC crystals. We further revised the model to consider the carrier distribution in the depletion regions adjacent to the stacking fault and were successful in explaining the shrinkage of stacking faults during annealing at even higher temperatures.

Structural Transformation from TSDs to Frank-Type Stacking Faults by Giant Bunched Steps in PVT-Grown 4H-SiC Single Crystals

Structural transformation from threading screw dislocations (TSDs) to stacking faults (SFs) has been investigated for PVT-grown 4H-SiC single crystals using X-ray topography and transmission electron microscopy (TEM). The transformation of TSDs is induced by the structural interference with bunched surface macrosteps over 100 nm in height. The stacking sequence of a SF was determined to be (433) in Zadanovs notation by using high-resolution TEM. Our detailed analyses revealed that the (433) stacking structure can be constructed by a combination of five faults including both four Frank type faults and one Shockley type fault.

Evolution of threading edge dislocations at earlier stages of PVT growth for 4h-SiC single crystals

Dislocation structures at the seed/grown-crystal interface in PVT-grown 4H-SiC crystals are investigated. The dislocation density is found to show a sharp increase at the interface and its main contribution is probably ascribable to TEDs which stem from BPDs generating at the interface through the structural transformation. Intense TEM observations reveal an intriguing in-plane distribution structure of the interface BPDs; the BPDs form a two-dimensional dislocation network comprising of {-1100} partial dislocations associated with expanded areas of stacking faults at the nodes of the network.

Characterization of Lattice Plane Bending and Stress Distribution in Physical Vapor Transport-Grown 4H-SiC Crystals

Basal plane bending and stress distribution in physical vapor transport-grown n-type 4H-SiC crystals were investigated. High resolution X-ray diffraction measurements were performed on commercially available 3-inch-diameter 4H-SiC substrates and along the growth front surface of as-grown 1-inch-diameter 4H-SiC boules. The measurements revealed that structural parameters such as the c-lattice constant, basal plane tilting, and FWHM showed characteristic variations across the substrates and as-gown boules, indicating that the crystals had a non-uniform distribution of dislocations comprising domain structures. Residual stress measured by micro Raman spectroscopy showed a similar behavior, which was an oscillatory spatial variation. On the basis of these results, defect structures in the crystals are elucidated.

Observation of the Surface Morphology on the (0001)C Facet of 4H-SiC Boules

The surface morphology on the (000-1)C facet of 4H-SiC boules grown by the physical vapor transport method was examined in various scales (from millimeter to nanometer) using different types of microscopies such as differential interference contrast (DIC) optical microscopy and atomic force microscopy (AFM). The DIC optical microscope observation revealed that there exist three distinct morphological regions at the growth front of the 4H-SiC boules; they are facetted, non-facetted, and the intermediate region between them. The local inclination of the facet surface from the (000-1) basal plane increases toward the facet edge and then decreases over the intermediate region. AFM observations revealed characteristic step structures in these two regions and also that they are significantly influenced by nitrogen-doping. Based on the results, the formation mechanism of the facet morphology on 4H-SiC boules is discussed.

Structural and Electrical Characterization of the Initial Stage of Physical Vapor Transport Growth of 4H-SiC Crystals

Defect formation during the initial stage of physical vapor transport (PVT) growth of 4H-SiC crystals in the [000-1] and [11-20] directions was investigated by x-ray diffraction, defect-selective etching, and micro Raman scattering imaging. X-ray diffraction studies showed that the growths in the [000-1] and [11-20] directions exhibited markedly different behaviors with respect to the defect formation during the initial stage of growth. While a characteristic lattice plane bending was observed for the PVT growth along [000-1], a tilted domain structure was revealed near the grown crystal/seed interface for the growth in the [11-20] direction. Micro Raman scattering imaging revealed that nitrogen enrichment occurred near the grown crystal/seed interface and was associated with compressive stress parallel to the interface. Based on the results, the defect formation mechanisms during the initial stage of PVT growth of 4H-SiC are discussed.