Shun Sadakuni

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.

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.

Influence of the UV Light Intensity on the Photoelectrochemical Planarization Technique for Gallium Nitride

We have developed a novel planarization technique for gallium nitride (GaN) substrates using a photo-electro chemical process and solid acid catalyst. In this method, a GaN surface is oxidized by ultraviolet (UV) light irradiation, and the oxide layer is chemically removed by a solid acid catalyst. In the current work, the dependence of the removal rate on the UV light intensity was investigated.

4H-SiC Planarization Using Catalyst-Referred Etching with Pure Water

A novel abrasive-free polishing method called catalyst-referred etching (CARE) has been developed. CARE can be used to chemically planarize a silicon carbide (SiC) surface with an etching agent activated by a catalyst. Platinum (Pt) and hydrofluoric (HF) acid are used as the catalyst and etchant, respectively. CARE can produce an atomically flat and crystallographically highly ordered surface of 4HSiC (0001) with a root-mean-square roughness of less than 0.1 nm regardless of the cut-off angle. However, industrial use of CARE is difficult because of HF acid usage. In this study, pure water was investigated as an alternative etchant to HF acid. We examined CARE using pure water by applying it to the planarization of a 4HSiC substrate and observed a feasible performance. The removal mechanism is considered to be the dissociative adsorption of water molecules to the SiC bonds of the topmost Si atom, namely the hydrolysis of the back bond, and the catalysis of Pt is considered to enhance the reaction. CARE with pure water is expected to represent a breakthrough method for surface processing of SiC, and will be widely applied in industrial processes such as planarization after high temperature processing in device fabrication.

Study of Terminated Species on 4H-SiC (0001) Surfaces Planarized by Catalyst-Referred Etching

Our group has developed a novel abrasive-free planarization technique known as catalyst-referred etching (CARE). It can produce flat, undamaged, and smooth SiC surfaces with a root-mean-square roughness of less than 0.1 nm over a whole wafer. This study investigates the etching mechanism of CARE by performing X-ray photoelectron spectroscopy (XPS) measurements to determine the termination species of CARE-processed SiC surfaces. We compared XPS spectra of a CARE-processed surface with those of an as-received SiC surface that had been treated with 50% HF solution. XPS spectra of the CARE-processed wafer contain the F 1s core level, whereas those of an as-received SiC wafer surface did not. This indicates that F anions play an important role in the etching process of CARE.

Bias-Assisted Photochemical Planarization of GaN(0001) Substrate with Damage Layer

An electrochemical polishing process for an n-GaN(0001) surface with a subsurface damage layer has been developed that involves irradiating with ultraviolet (UV) light and applying a voltage. In this method, a positively biased GaN substrate is exposed to UV light to oxidize its surface. The oxide layer does not dissolve in solution; rather it is chemically removed from the protruding region by a solid acid catalyst, which functions as a polishing pad. The wafer was prepared by mechanical polishing with diamond particles. Without a bias, the removal rate is quite low because photoinduced carriers are rapidly depleted through recombination at crystallographic defects. In contrast, when a bias is applied, photoinduced electrons and holes are forcibly separated so that they contribute to surface oxidation. Consequently, the damaged surface was effectively planarized when a bias was applied.

High-Resolution TEM Observation of 4H-SiC (0001) Surface Planarized by Catalyst-Referred Etching

A novel abrasive-free planarization method “called catalyst-referred etching (CARE)” has been invented. After the CARE process, a flat and well-ordered surface is obtained as observed by atomic force microscopy (AFM). To determine the atomic structure at the topmost surface, in this study, CARE-processed surfaces of a standard commercial 2-inch n-type 4H-SiC (0001) wafer cut 8o off-axis toward the [1-100] direction were observed by high-resolution transmission electron microscopy (HRTEM). The HRTEM images showed alternating wide and narrow terraces and a single-bilayer step height. The relationship between the width of the terraces and the 4H-SiC crystal structure has been clarified.

TEM Observation of 8 Deg Off-Axis 4H-SiC (0001) Surfaces Planarized by Catalyst-Referred Etching

We have developed a novel abrasive-free planarization method called catalyst-referred etching (CARE). A CARE-processed 8 deg off-axis 4H-SiC (0001) surface is investigated by cross-sectional transmission electron microscopy (TEM). The surface is composed of alternating wide and narrow terraces with single-bilayer-height steps, which are similar to the structure observed on a CARE-processed on-axis 4H-SiC (0001) surface. These results indicate that the structure appears on CARE-processed surfaces regardless of the off-cut angle.