Papers by Author: Toshiyuki Isshiki

Abstract: Two types of carrot defects with and without a shallow pit were found by mirror projection electron microscopy (MPJ) inspection in 4H-SiC epi wafer. Surface morphology and cross-sectional structure of prismatic stacking faults (PSFs) were investigated using MPJ and atomic force microscopy (AFM), transmission electron microscopy (TEM) and high-resolution scanning transmission electron microscopy (STEM). The depths of the surface grooves due to the PSFs, the stacking sequences around the PSFs and the structure of the Frank-type stacking faults which were connected to the PSFs were different. We discuss the difference between the two types of carrot defects.

Abstract: Stacking faults (SFs) in 4H-SiC epitaxial wafers were inspected by using a mirror projection electron microscope (MPJ) [1, 2]. Dark and bright line contrasts of SFs in MPJ images represent surface morphology and crystal defects located in the epitaxial layer. Inspected SFs were classified into three types of SFs on the basis of the MPJ images. After classification, a cross section of each type of SF was observed by transmission electron microscopy (TEM) to verify the classification result. Complex SFs classified by MPJ images consisted of prismatic plane and basal plane SFs.

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 defect structure of Mg implanted GaN substrate was evaluated by TEM observations, AFM surface observations and Raman scattering spectroscopic analysis. Mg ions were implanted at room temperature (RT) and 500 °C. TEM results showed that the defect distribution along depth scale is different between RT and 500 °C condition. The several peaks originated from ion implantation were found from Raman scattering spectra and the characteristics of the defects by implantation were discussed. The crystal quality of the sample implanted at 500 °C was found to be better than that of RT by comparing the FWHM of the E₂ peak.

Abstract: Dislocations and stacking faults in 4H-SiC (0001) _si epitaxial wafer was inspected by mirror projection electron microscopy (MPJ) with the aid of low-energy SEM and FIB-STEM. MPJ observation found dislocation conversion near the wafer surface, and the conversion was confirmed by micro etch pit and low energy SEM method. Another conversion occurred in the epitaxial layer on array of TED half loops, which were detected by MPJ, was also observed by cross-sectional STEM.

Abstract: Band structure calculations for radiofrequency-sputtered AlN-films doped with various 3d-transition-metals (TM: V, Cr, and Mn) were conducted to investigate the origin of the characteristic optical absorption structures. Experimentally evaluated crystal structures and lattice constants of the synthesized films were adopted for supercells. The model calculations showed that additional energy bands mainly consisting of 3d e and t states of TMs are formed in the band gap of AlN (6.2 eV), and that their potentials depend on the TM species. It was also shown that the Fermi levels of Cr- and Mn-doped AlN lie within the spin-up t band, while the Fermi level of V-doped AlN lies between the spin-up e and t bands. These findings imply that the materials have TM species-dependent, multiple absorption paths with lower energy than the band gap energy of AlN, resulting in optical absorption in the near-ultraviolet, visible, and infrared regions.

Abstract: The dislocation analysis of latent scratch induced chemical mechanical polishing process on 4H-silicon carbide (SiC) using the multi directional scanning transmission electron microscopy (STEM) method and elastic stain measurement were performed. A scanning electron microscope image shows that a latent scratch extended toward the [30] direction and the width is about 50 nm. Cross sectional STEM images shows that the depth of latent scratch due to distortion is about 20 nm. From the result of plan view STEM observation along [000] direction, it was observed that a latent scratch had two defect lines toward the [30] direction, which were a loop type on upper side and a linear type on the lower side. The Burgers vector of each defect have a component in basal plane. Elastic strain mapping was performed using transmission electron microscope equipped with a procession diffraction system. Inside the latent scratch indicates stain-free field, however around latent scratch indicates compressive strain field. About 1.5 % compressive strain field x, y direction and shear strain along latent scratch exists on typical area. As a results of STEM and elastic strain analysis, the atomic arrangement in basal plane seems to be related with the compressive strain around latent scratch.

Abstract: A latent scratch which is an extremely shallow scratch induced on a SiC wafer during chemo-mechanical polishing (CMP) has been investigate by mirror projection electron microscopy (MPJ), low-energy scanning electron microscopy (LESEM), atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM). The latent scratch, which is difficult to detect by using optical microscopes, was easily visualized by MPJ as a high contrast dark line. The morphology of detected latent scratch is less than 1nm in depth and about 30nm in full width at half depth by AFM evaluation. The STEM observation revealed the latent scratch was accompanied two dislocation arrays. One contains loop-like dislocations and the other contains spiky dislocations, both lying in the wafer at a few ten nm in depth.

Abstract: Basal plane dislocation (BPD) with dislocation lines in shallow areas near the surface in 4H-SiC epitaxial wafer was observed by mirror projection electron microscopy (MPJ) and low-energy scanning electron microscopy (LE-SEM). A contrast of dislocation line of BPD appeared in a MPJ observation as gradually weakened dark line toward the upper stream of offcut of wafer, and the contrast almost agreed with the LE-SEM image taken at the same BPD by not a morphology-sensitive imaging method but a potential-sensitive imaging method. Thus an origin of the contrast corresponding to BPD in MPJ is considered to surface potential change due to charging on dislocation line. MPJ observation can gives a BPD image with same quality as a potential-sensitive image by LE-SEM, in extremely short time and damage-and contamination-free condition at no electron irradiation on wafer.

Abstract: The recently developed multi directional scanning transmission electron microscopy (MD-STEM) technique has been applied to exactly determine the Burgers vector (b) and dislocation vector (u) of a threading mixed dislocation in a silicon carbide (SiC) as-epitaxial wafer. This technique utilizes repeated focused ion beam (FIB) milling and STEM observation of the same dislocation from three orthogonal directions (cross-section, plan-view, cross-section). Cross section STEM observation in the [1-100] viewing direction showed that the burgers vector have a and c components. Subsequent plan view STEM observation in the [000-1] direction indicated that the b=[u -2u u w] (u≠0 and w≠0). Final cross section STEM observation in the [11-20] direction confirmed that the dislocation was an extended dislocation, with the Burgers vector experimentally found to be b = [1-210]a/3 + [0001]c which decomposes into two partial dislocations of b_p1 = [0-110]a/3 + [0001]c/2 and b_p2 = [1-100]a/3 + [0001]c/2. The dislocation vector u is [-12-10]a/3 + [0001]c. This technique is an effective method to analyze the dislocation characteristics of power electronics devices.