Tomoya Moribayashi

Structural evaluation of defects in β-Ga2O3 single crystals grown by edge-defined film-fed growth process

We have structurally evaluated β-Ga2O3 crystals grown by edge-defined film-fed growth process using etch pitting, focused ion beam scanning ion microscopy, transmission electron microscopy, and related techniques. We found three types of defects: arrays of edge dislocations corresponding to etch pit arrays on -oriented wafers, platelike nanopipes corresponding to etch pits revealed on the (010)-oriented wafers, and twins including twin lamellae.

Determination of the type of stacking faults in single-crystal high-purity diamond with a low dislocation density of <50 cm−2 by synchrotron X-ray topography

The properties of stacking faults in a single-crystal high-purity diamond with a very low dislocation density of <50 cm?2 and a very low impurity concentration of <0.1 ppm were investigated by synchrotron X-ray topography. We found stacking faults on the {111} plane and determined the fault vector f of the stacking faults to be on the basis of the f g extinction criteria. Furthermore, we have found that the partial dislocations are of the Shockley type on the basis of the b g extinction criteria. Consequently, we concluded that the stacking faults are of the Shockley type and formed because of the decomposition of dislocations with into dislocations with and .

Observation of nanometer-sized crystalline grooves in as-grown β-Ga2O3 single crystals

On the surface of as-grown β-Ga2O3 single crystals that are cut and polished, we found nanometer-sized grooves elongated in the [001] direction. We confirmed that these grooves terminate within the crystals in the [010] direction. This proves that the grooves are different from micropipes penetrating crystals. Their typical length and width are 50–1200 nm in the [001] direction and ~40 nm in the [100] direction, respectively. The grooves tend to form an array in the [001] direction. The type of nanometer-sized grooves should be essentially different from etch pits.

Origins of etch pits in β-Ga2O3(010) single crystals

Etch pits of various shapes were observed on etched ?-Ga2O3(010) single crystals and classified into types A?F according to shape. Type-A etch pits changed in shape in the order of types B, C, and D by etching. Groove-shaped pits observed on as-grown ?-Ga2O3(010) single crystal surfaces [K. Hanada et al., Jpn. J. Appl. Phys. 55, 030303 (2016)] were classified into type G. Type-G pits, which were determined to be void defects because of three-dimentional spaces in single crystals, existed before etching and changed to type A after etching. Therefore, after etching, void defects must change in shape as follows: Type G ? A ? B ? C ? D. The exposed facets change with etching time. Types-E and -F etch pits were observed to be parallelograms and hexagons, respectively. Types-E and -F etch pits must include dislocations along the [010] direction because they did not change in shape after etching.

Electrical properties of Schottky barrier diodes fabricated on (001) β-Ga2O3 substrates with crystal defects

The electrical properties of Schottky barrier diodes (SBDs) on a (001) β-Ga2O3 substrate were characterized and correlated with wet etching-revealed crystal defects below the corresponding Schottky contacts. The etching process revealed etched grooves and etched pits, indicating the presence of line-shaped voids and small defects near the surface, respectively. The electrical properties (i.e., leakage currents, ideality factor, and barrier height) exhibited almost no correlation with the density of the line-shaped voids. This very weak correlation was reasonable considering the parallel positional relation between the line-shaped voids extending along the [010] direction and the (001) basal plane in which the voids are rarely exposed on the initial surface in contact with the Schottky metals. The distribution of small defects and SBDs with unusually large leakage currents showed similar patterns on the substrate, suggesting that these defects were responsible for the onset of fatal leak paths. These results will encourage studies on crystal defect management of (001) β-Ga2O3 substrates for the fabrication of devices with enhanced performance using these substrates.

Crystal defects observed by the etch-pit method and their effects on Schottky-barrier-diode characteristics on

A pixel array of vertical Schottky-barrier diodes (SBDs) was fabricated and measured on the surface of a β-Ga2O3 single crystal. Subsequently, etch pits and patterns were observed on the same surface. Three types of etch pits were discovered: (1) a line-shaped etch pattern originating from a void and extending toward the [010] direction, (2) an arrow-shaped etch pit whose arrows head faces toward the [102] direction and, (3) a gourd-shaped etch pit whose point head faces toward the [102] direction. Their average densities were estimated to be 5 × 102, 7 × 104, and 9 × 104 cm−2, respectively. We confirmed no clear relationship between the leakage current in SBDs and these crystalline defects. Such results are obtained because threading dislocations run mainly in the [010] growth direction and do not go through the sample plate.

(\bar{2}01)

We fabricated Schottky barrier diodes (SBDs) on the entire surface of a β-Ga2O3 single crystal, and investigated the leakage current in both forward and reverse directions. Subsequently, we investigated the distribution of dislocation and void etch pits on the entire surface. The dislocation etch pit density on the surface ranged from <1 × 103 to 6 × 104, and its average was 1.1 × 104 cm−2. The void etch pit density on the surface ranged from <5 × 102 to 7 × 103, and its average was 6 × 103 cm−2. From a comparison between the SBD leakage current and the dislocation and void etch pit densities, we found that dislocations are closely related to the SBD reverse leakage current, and that not all voids produce the leakage current.