Papers by Author: Yong Zhao Yao

Abstract: In order to understand the crystal defects of beta-gallium oxide (β-Ga₂O₃) in more detail, we classified the crystal defects of a 2-inch substrate of β-Ga₂O₃ (001) single crystal. As a result of observing the etch pits formed by molten alkali etching using scanning electron microscope (SEM) and atomic force microscope (AFM), we succeeded in observing six different etch pit shapes. These etch pit shapes are categorized into “Cicada I type”, “Cicada II type”, “Cannonball type”, “Trapezoid type”, “Bar type”, and “Shell type”. We consider that “Cicada I type” and “Cicada II type” are etch pit shapes caused by planar defects, and “Cannon ball type” is etch pit shapes due to dislocations. In addition, “Trapezoid type”, “Bar type”, and “Shell type” are deduced the result of surface morphology.

Abstract: The expansion behavior of basal plane dislocations (BPDs) in a 4H-SiC epitaxial layer on the (110) A-plane under electron beam (EB) (//[110]) irradiation was observed. BPD expanded and formed a single Shockley stacking fault (SSSF) between a partial dislocation (PD) pair. The width of the SSSF was proportional to the EB current. The dependence of the expansion velocity on the irradiation position was observed with a fixed EB spot. It was found that the electron-hole pair migration to the PD and/or SSSF can expand the SSSF. The velocity of SSSF expansion by direct SSSF excitation with an EB was much smaller than that by the preferential excitation of a PD with migrated electron-hole pairs.

Abstract: We have studied threading dislocations (TDs) in 4H-SiC by means of X-ray topography (XRT) taken under 6 equivalent g-vector of 11-28 and two different chemical etching methods. Threading screw and mixed-type dislocations (TSDs and TMDs) can be distinguished and the direction of the a-components of TMDs can be determined by XRT. Efforts have been made to examine if there are features of etch pits that can be used to distinguish TMDs from TSDs.

Abstract: Threading dislocations (TDs) in an HVPE-grown free-standing GaN substrate have been studied by means of etch pit method using molten KOH+Na₂O₂ solution, cathodoluminescence (CL) mapping and transmission electron microscopy (TEM). We have focused on the correlation between (1) Burgers vectors of TDs, (2) their appearances as dark spots in CL image, and (3) geometries of corresponding etch pits. Based on the above results, dislocation categorization for GaN by using etch pits or CL is discussed.

Abstract: Threading dislocations (TDs) in 4H-SiC have been studied by comparing etch pits formed by NaOH vapor etching with results of synchrotron monochromatic-beam X-ray topography (XRT) taken under different g-vectors. Burgers vectors determined based on XRT results were utilized to investigate the etch pit characteristics of edge (TED), screw (TSD) and mixed (Burgers vector b=c+a, TMD) threading dislocations. It has been found that pit formation by NaOH vapor etching was very different to that by conventional molten KOH etching. We discuss the possibility of using NaOH vapor etching to distinguish TMDs from TSDs, and report a variety of characteristic etch pits formed by this method and their correlations to dislocation behavior.

Abstract: To remove the surface damages induced during mechanical polishing (MP) of 4H-SiC, a variety of wet etching recipes and etching conditions were studied. By evaluating the epilayers grown on these etching-treated wafers, it has been found that triangular defects (TRDs) are the main defects originated from the MP-induced damages in these samples. High temperature molten KCl etching at 1100 °C with KOH additive is very effective to remove the damaged surface while keeping a relatively flat surface. Epilayer grown on the KCl+KOH etched wafer showed a TRD density <0.9 cm^-2.

Abstract: High temperature (>1000 °C) chemical etching using molten KCl or molten KCl+KOH as the etchant has been carried out to remove the mechanical-polishing (MP) induced damage layer from 4H-SiC surface. Atomic force microscopy observations have shown that line-shaped surface scratches that have appeared on the as-MPed surface could be completely removed by KCl-only etching or by KCl+KOH etching (KCl:KOH=99:1 in weight) at ~1100 °C. Between the two recipes, KCl+KOH etching has shown a higher etch rate (6~7 times) and is able to remove ~9 μm and ~36 μm-thick damage layer from the Si (0001) and the C(000-1) surface, respectively. Besides, KCl+KOH etching seems to have formed a Si (0001) surface covered with atomic steps while KCl-only etched surface is featured with nanometer-scale pores.

Abstract: The damage induced at the cut surface of SiC crystal by slicing were investigated by Raman scattering method and transmission electron microscopy. Electric discharge machining (EDM) predominately forms cracks, silicon, carbon and 3C-SiC by 6H-SiC pyrolysis and wiresawing with loose abrasive (WSLA) induces triangular crystal disordered area, stacking faults and dislocation loop bundles by stressing at the cut surfaces of SiC crystal.

Abstract: A threading dislocation (TD) in 4H-SiC, which was currently interpreted as a perfect threading edge dislocation (TED) by synchrotron monochromatic-beam X-ray topography (SMBXT) and molten KOH etching with Na₂O₂ additive, was performed comparative characterization using weak-beam dark-field (WBDF) and large-angle convergent-beam electron diffraction (LACBED) methods. The TD was suggested to be dissociated into a dislocation pair which can be observed in the WBDF image of g=-12-10. The TD, which was identified as b//[-12-10] by SMBXT observation, was unambiguously determined as b=1/3[-12-10] by LACBED analysis. In the case of perfect TED, it was found that the direction of Burgers vector derived from SMBXT observation corresponds to LACBED analysis.

Abstract: In this paper, we report a newly developed dislocation-revealing etch pit method for 4H-SiC single crystal, which can distinguish edge (TED, Burgers vector b=a), elementary screw (TSD, b=1c) and mixed (TMD, b=c+a) threading dislocations. In this method, vaporized NaOH gas was used to etch the Si-face of a SiC wafer at substrate temperature around 950 °C. By a side-by-side comparison between the optical images of the etch pits and the X-ray topographic (XRT) images, it has been found that threading dislocations (TDs) in SiC could be revealed as hexagonal etch pits with distinct geometrical features (shape, size and facet orientation) depending on their Burgers vectors. Based on these results, we consider this etch pit method as an easily-operated and inexpensive technique to categorize TDs, and it may help to promote our understanding on the different roles that these types of TDs have played in the performance degradation of SiC power devices.