Junji Murata

Atomic-scale flattening of SiC surfaces by electroless chemical etching in HF solution with Pt catalyst

The authors present a method for flattening SiC surfaces with Pt as a catalyst in HF solution. The mechanism for flattening SiC surfaces is discussed. The flattened 4H-SiC(0001) surface is composed of alternating wide and narrow terraces with single-bilayer-height steps, which are induced by the rate difference of the catalytic reactions between adjacent terraces. Scanning tunneling microscopy images reveal a 1×1 phase on the terraces. The 1×1 phase is composed of coexisting of F- and OH-terminated Si atoms, which originate from the polarization of the underlying Si–C bonds.

Enhancement of photoluminescence efficiency from GaN(0001) by surface treatments

We investigated the photoluminescence (PL) efficiency of GaN(0001) single crystals with clean and well-defined surfaces using the PL technique in ultrahigh vacuum in situ. We found typical degradation factors: native oxides at the top surface, damaged layers in the subsurface, and hydrogenated non-radiative states inside bulk GaN. By eliminating the degradation factors, a band-to-band PL intensity of approximately 120 times higher than that of the as-received samples was achieved. The PL efficiency enhancement mechanism is discussed, and the role of hydrogen in GaN crystals is proposed.

Improvement of Removal Rate in Abrasive-Free Planarization of 4H-SiC Substrates Using Catalytic Platinum and Hydrofluoric Acid

We used catalyst-referred etching, which is an abrasive-free planarization method, to produce an extremely smooth surface on a 4H-SiC substrate. However, the removal rate was lower than that obtained by chemical mechanical polishing, which is the planarization method generally used for SiC substrates. To improve the removal rate, we investigated its dependence on rotational velocity and processing pressure. We found that the removal rate increases in proportion to both rotational velocity and processing pressure. A lapped 4H-SiC substrate was planarized under conditions that achieved the highest removal rate of approximately 500 nm/h. A smooth surface with a root-mean square roughness of less than 0.1 nm was fabricated within 15 min. Because the surface, which was processed under conditions of high rotational velocity and high processing pressure, consisted of a step–terrace structure, it was well ordered up to the topmost surface.

Planarization of GaN(0001) Surface by Photo-Electrochemical Method with Solid Acidic or Basic Catalyst

A novel planarization technique for the GaN(0001) surface has been developed. In this method, the surface is oxidized by a photo-electrochemical reaction and the resulting oxide is removed using a solid acidic/basic catalyst. Smooth surfaces that are free from scratches and etch pits are obtained. Photoluminescence analysis shows that the intensity of the band-edge luminescence markedly increases after the planarization.

Damage-Free Planarization of 2-Inch 4H-SiC Wafer Using Pt Catalyst Plate and HF Solution

We report a damage-free and efficient planarization process for silicon carbide (SiC) using platinum as a catalyst in hydrofluoric acid (HF) solution. In previous studies, 4H-SiC (0001) on-axis wafers were planarized by this process and an extremely flat surface was obtained. However, electronic device substrates require off-axis wafers. In the present study, 4H-SiC (0001) 8° off-axis Si-face wafers were planarized using a Pt catalyst plate and HF solution. In the first trial using these wafers, the surface roughness worsened and a diagonal pattern was observed by phase-shift interference microscopy. The pattern seemed to have been formed when the Pt plate morphology was transcribed onto the wafer. The removal rate of the 8° off-axis Si-face wafer is much greater than that of the on-axis Si-face wafer. Thus, we concluded that the use of a smoother catalyst plate would be necessary to obtain a smooth 8° off-axis Si-face wafer surface. Improving the Pt plate morphology by hand lapping also improved the surface roughness of the processed wafer as compared with the preprocessed surface. The maximum height of the surface irregularity (peak-to-valley, P-V) and root-mean-square roughness were improved to 0.513 nm and 0.044 nm, respectively, as determined by atomic force microscopy (2×2 μm2).

Defect-Free Planarization of 4H–SiC(0001) Substrate Using Reference Plate

In this paper, a new defect-free planarization technique for 4H–SiC(0001) substrate is described. This technique uses hydroxyl radicals (OH radicals) that are generated on an Fe metal surface in a hydrogen peroxide (H2O2) solution. First, the oxidation of a 4H–SiC substrate by OH radicals is investigated by X-ray photoelectron spectroscopy (XPS) analysis. Next, the planarization of the 4H–SiC substrate is conducted. A very flat and smooth surface without any scratches and etch pits is obtained. The planarized surface has a step-terrace structure.

Damage-Free Planarization of 4H-SiC (0001) by Catalyst-Referred Etching

We report the damage-free planarization of 4H-SiC (0001) wafers using a new planarization technique we named CAtalyst-Referred Etching (CARE). The CARE setup equipped with a polishing pad made of a catalyst is almost the same as a lapping setup. Since the catalyst generates reactive species that activate only when they are next to the catalyst surface, SiC can be chemically removed in contact with the catalyst surface with a pressure noticeably lower than that in a conventional polishing process. The processed surfaces were observed by optical interferometry and AFM. These observations presented a marked reduction in surface roughness. A step-terrace structure was observed with a step height of approximately 3み, corresponding to one-bilayer thickness of Si and C, in the AFM images. To estimate the crystallographic properties of the CARE-processed surface, the surfaces were observed by cross-sectional TEM. The TEM images showed that a more crystallographically well-ordered surface was realized in comparison with the conventional CMP-processed surface.

Reduction of Surface Roughness of 4H-SiC by Catalyst-Referred Etching

Flat and well-ordered surfaces of silicon carbide (SiC) substrates are important for electronic devices. Furthermore, researchers have reported that 4H-SiC surface roughness increases by step-bunching during epitaxial growth and annealing. Degradation of device properties induced by surface roughening is of great concern. Therefore, a method to reduce this surface roughening is requested. We have developed a damage-free planarization method called catalyst-referred etching (CARE). In this paper, we planarized 4H-SiC substrates and evaluated the processed surface before and after the epitaxial growth. Then, we reduced the step-bunching on the epi-wafer surface and determined the electrical properties of the Schottky barrier diodes (SBD) on the processed surface.

Catalyst-referred etching of silicon

Abstract A Si wafer and polysilicon deposited on a Si wafer were planarized using catalyst-referred etching (CARE). Two apparatuses were produced for local etching and for planarization. The local etching apparatus was used to planarize polysilicon and the planarization apparatus was used to planarize Si wafers. Platinum and hydrofluoric acid were used as the catalytic plate and the source of reactive species, respectively. The processed surfaces were observed by optical interferometry, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results indicate that the CARE-processed surface is flat and undamaged.

Novel Abrasive-free Planarization of Si and SiC using Catalyst

We propose a new chemical planarization method using a catalyst as a polishing plate. A sample is placed on the polishing plate in a solution that is a source of reactive species. Since the catalyst generates reactive species that activate only next to the catalyst surface, this method can efficiently planarize. This processed surface is not damaged by chemical removal. We named this method CAtalyst-Referred Etching (CARE). CARE was applied to SiC planarization. The processed surfaces were observed by atomic force microscopy and optical interferometry. These observations presented a marked reduction in surface roughness.