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A method for preparing atomically smooth gallium nitride (GaN) surfaces that involves chemical etching with a platinum catalyst in water is presented. The flattened GaN(0001) surface exhibits periodic structures composed of straight-edged steps the height of a single bilayer. Atomic step and terrace structures containing an extremely small number of scratches and pits can be achieved on wafer scale (2 inch diameter). Photoluminescence analysis revealed that the intensity of band-edge luminescence increases significantly after flattening. Low energy electron diffraction indicated that the flattened GaN surface is crystallographically well-ordered. Current density between the wafer and Pt is observed when they are rotated, suggesting that the GaN surface is etched by an electrochemical reaction initiated by the tribo effect between Pt and the wafer surface. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Atrial enlargement is thought to provide arrhythmogenic substrates, leading to the induction of atrial fibrillation (AF). In this study, we investigated the anatomical, molecular biological, and electrophysiological characteristics of remodeled atria in an animal model with chronic volume overload. We used rats that underwent abdominal aorto-venocaval shunt (AVS) surgery. In the in vivo studies, marked changes in electrocardiogram parameters, such as the P-wave duration, PR interval, and QRS width, as well as prolongation of the atrial effective refractory period were observed 12 weeks after the creation of AVS (AVS-12W), which were undetected at 8 weeks postoperative (AVS-8W) despite obvious atrial and ventricular enlargement. Moreover, the duration of AF induced by burst pacing in the AVS-12W rats was significantly longer than that in the Sham and AVS-8W rats. In the isolated atria, a longer action potential duration at 90% repolarization was detected in the AVS-12W rats compared with that in the Sham group. The mRNA levels of the K v and K ir channels in the right atrium were mostly upregulated in the AVS-8W rats but were downregulated in the AVS-12W rats. These results show that chronic volume overload caused by abdominal AVS provides arrhythmogenic substrates in the rat atrium. The difference in gene expression in the right atrium between the AVS-8W and AVS-12W rats may partly explain the acquisition of arrhythmogenicity.

Rationale: Sirolimus has been shown in a randomized, controlled clinical trial to stabilize lung function in patients with lymphangioleiomyomatosis (LAM) treated for a 12-month time period; however the pretreatment decline in lung function after the drug was discontinued indicated that continued exposure is required to suppress disease progression., Objectives: To elucidate the durability and tolerability of long-term sirolimus treatment in Asian patients with LAM., Methods: We conducted a single-arm, open-label, investigator-initiated safety and efficacy study of sirolimus in 63 women with LAM at 9 sites in Japan. Subjects received sirolimus for 2 years at doses adjusted to maintain a trough blood level of 5-15 ng/ml., Measurements and Main Results: Fifty-two subjects (82.5%) completed the trial with mean drug compliance of more than 80% overall during the study. The number of adverse events was greatest during the initial 6 months of therapy, but they continued to occur with declining frequency throughout the 2-year study period. Of the 1,549 adverse events, 27 were classified as serious, including reversible sirolimus pneumonitis in 3 patients. New hypercholesterolemia occurred in 30 patients (48%); microcytosis in 10 patients; loss of body weight in 33 patients; and increase in blood pressure that required treatment in 5 patients. FEV 1 , FVC, and quality-of-life parameters were stable in the overall study cohort during the study period, but baseline to 2-year improvements in lung function occurred in the subset of patients with a prior history of chylothorax., Conclusions: Although long-term sirolimus treatment of Asian patients with LAM was associated with a large number of adverse events, including three episodes of pneumonitis, most patients completed the 2-year course of medication with good drug compliance and stable quality of life and lung function.

Photoenhanced chemical (PEC) etching is applicable for processing an n-GaN (0001) surface rapidly. In this process, the surface oxidation is enhanced by photo-generated holes and the resulting oxide can dissolve into solutions. In current work, we conduct bias-assisted PEC etching in a KOH solution with a positively biased wafer, to remove the crystallographically highly damaged layer. The employed substrate was mechanically polished with diamond slurry of sub-micrometer particle size. Without the positive bias, the rate of PEC etching was quite low because the photogenerated holes were quickly depleted by the recombination process at the crystallographic defects and they could not contribute to the oxidation. On the other hand, in the case where the bias was applied, the photogenerated holes and electrons are separated forcibly in the band-bended surface, which effectively contributed to surface oxidation. As a result, a high removal rate was realized even on the damaged surface.

The optical and electrical properties of GaN(0001) surfaces treated by a novel chemical polishing method are described. Scanning microscopic photoluminescence images reveal that the polished GaN surface shows improved luminescence properties compared to the untreated surface. Current-voltage measurements of Schottky barriers formed using the GaN substrates show that the polished GaN surface has a lower reverse leakage current, and that the barrier height and ideality factor are improved after the polishing treatment.

Catalyst-referred etching (CARE) is a novel abrasive-free planarization method. CARE-processed 4H-SiC(0001) surfaces are extremely flat and undamaged over the whole wafer. They consist of single-bilayer-height atomic steps and atomically flat terraces. This suggests that the etching properties depend principally on the atomic-step density of the substrate surface. We used on-axis and 8 degrees off-axis substrates to investigate the processing characteristics that affect the atomic-step density of these substrates. We found a strong correlation between the removal rate and the atomic-step density of the two substrates. For the on-axis substrate, the removal rate increased with increasing surface roughness, which increases with an increasing atomic-step density. The removal rate ratio is approximately the same as the atomic-step density ratio of the two substrates.