Papers by Author: Tatsuo Fujimoto

Abstract: The defect structure at the growth front of 4H-SiC boules grown using the physical vapor transport (PVT) method has been investigated using high resolution x-ray diffraction and x-ray topography. The crystal parameters such as the c-lattice constant exhibited characteristic variations across the growth front, which appeared to be caused by variation in surface morphology of the as-grown surface of the boules rather than the defect structure underneath the surface. X-ray topography also revealed that basal plane dislocations are hardly nucleated at the growth front during PVT growth of 4H-SiC crystals.

Abstract: We investigated the run-to-run fluctuation in growth conditions of physical vapor transport growth of 4H-SiC boules through observations of surface morphology on the (000-1) facet of the boules. The boules, which were grown under the same macroscopic growth conditions, exhibited slightly different surface morphologies. This indicates that some microscopic growth parameters that influence the surface morphology fluctuate between growth runs. We have considered the C/Si ratio of the vapor sublimed from the source material as a major parameter and discussed the associated variations in the physical and surface properties of the grown crystals.

Abstract: The annealing behavior of electrical resistivities perpendicular and parallel to the basal plane of heavily nitrogen-doped 4H-SiC crystals was investigated. The temperature dependencies of the resistivities exhibited characteristic behaviors after multiple rounds of high-temperature annealing (1100°C, 30 min). High-temperature annealing induced stacking fault formation to various extents in heavily nitrogen-doped 4H-SiC crystals. Based on these results, we discuss the cause and mechanism of the observed annealing-induced changes in electrical resistivities of the crystals.

Abstract: The strain fields in a 4H-SiC homo-epitaxial layer deposited on a nitrogen-doped 4H-SiC substrate were studied using Raman scattering microscopy. The cross-sectional (1-100) and (11-20) surfaces of the epitaxial substrate were examined through the peak shifts of several Raman-active phonon modes for 4H-SiC, and tensile strain was found along the direction of 4° off the c-axis at the epilayer/substrate interface. The effect of the facet trace in the substrate, which has a higher nitrogen concentration than the other parts of the substrate, was also studied. The tensile strain at the epilayer/substrate interface was found to be hardly enhanced for the epilayer deposited on the facet trace.

Abstract: Step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface was investigated using low-voltage electron scanning microscopy (LVSEM) and electron channeling contrast (ECC) imaging. LVSEM observations revealed that the step bunching resulted in the formation of atomically flat wide (~250 nm) terraces on the surface, and the terraces tended to form in pairs. The two terraces in paired terraces often showed the same electron channeling contrast as each other, and the contrast of the two terraces, either bright or dark, appeared to be determined by the orthogonal misorientation of substrates. On the basis of these results, the formation mechanism of the step-bunched structure on a vicinal 4H-SiC (0001) surface is discussed.

Abstract: The stacking fault formation during physical vapor transport growth of heavily nitrogen-doped (mid-10¹⁹ cm⁻³) 4H-SiC crystals was investigated. Low-voltage scanning electron microscopy (LVSEM) observations detected the stacking fault formation on the (000-1) facet of heavily nitrogen-doped 4H-SiC crystals. Stacking faults showed characteristic morphologies, and atomic force microscopy (AFM) studies revealed that these morphologies of stacking faults stemmed from the interaction between surface steps and stacking faults. Based on these results, the stacking fault formation mechanism in heavily nitrogen-doped 4H-SiC crystals is discussed.

Abstract: 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 Zadanov's 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.