Chemical etching characteristics of GaAs(100) surfaces in aqueous HF solutions

It is well known 1-3 that etching of silicon in HF or buffered-HF solutions results in the removal of the native oxide and leaves behind silicon surfaces terminated by atomic hydrogen. A more recent study on HF processing of silicon reports that a completely flat, hydrogenterminated surface with enough fluorine bonds is chemically stable against oxidation. Ingrey 11 reported that HF solution selectively removes native oxides to create temporary hydrogen-passivated III-V surfaces. He measured Auger O/In or O/As intensity ratios from a series of experiments designed to evaluate the chemical passivation by different surface treatments of InP and GaAs(100) polished and etched and (110) cleaved surfaces exposed to air. He obtained that effective surface passivation occurs when concentrated HF solution is used to remove native oxides from (100) surfaces, as well as from the cleaved (110) surfaces. However, he did not show any concrete data in his report. More recently, Schmuki et al. 12 have shown, using X-ray photoelectron spectroscopy (XPS), that the dip in a 1 wt % HF solution removes an air-formed oxide film from the sample surface, but an extensive exposure to air after the HF dip leads to a reoxidation of the sample surface. The aim of this paper is to study chemical treatment effects of GaAs(100) surfaces in aqueous (1 and 50 wt %) HF solutions using spectroscopic ellipsometry (SE), atomic force microscopy (AFM), XPS, and contact-angle measurement techniques. SE is a highly surface-sensitive technique which enables detection of not only submonolayer coverage of a surface by adsorbed species, 9,13 but also the surface roughness of its size smaller than the wavelength of light. 14 To our knowledge, however, no SE study has been performed on the HF-treated GaAs surfaces. We adopt an effective medium approximation (EMA) Experimental The GaAs wafers used in this study were grown by the horizontal Bridgman method and were (100)-oriented, electrically semiinsulating. They were mirror-finished and supplied by Sumitomo Electric Co., Ltd. The as-received GaAs(100) wafers were first degreased with organic solvents in an ultrasonic bath and then rinsed with deionized (DI) water. No further cleaning of the wafer surface was performed. Then the wafer surfaces to be studied were covered with a ϳ5 nm thick native oxide film. It should be noted that this native oxide was naturally grown in room air. The native oxide thickness (ϳ5 nm) was determined by SE and, thus, was of the equivalent dielectric layer of GaAs oxide, including possible surface microroughness. The aqueous HF solutions studied here were diluted (1 wt %) and concentrated (50 wt %) solutions. A large quantity of the solution was prepared to prevent the etching solution composition from varying during the experiment. All etching experiments were done at 20ЊC and room light without stirring. After chemical treatment, each sample was rinsed in DI water. The HF-etched GaAs(100) surfaces were characterized by measuring SE spectra in the 1.5-5.3 eV photon-energy range at room temperature. SE measurements were performed with a rotating-analyzer-type ellipsometer (DVA-36VW-A, Mizojiri Optical, Co., Ltd.) operating at an angle of incidence of 70Њ with a polarization azimuth of 45Њ. A xenon arc lamp was used as the light source. Since SE is sensitive not only to the native oxide but also to the surface microroughness, we tried to deduce information on the surface microroughness from SE data using an EMA-LRA simulation program. The microstructures of the HF-treated GaAs(100) surfaces were investigated by AFM using a Digital Instruments Nanoscope III. The AFM images were acquired in the tapping mode and in the repulsive force regime. The wettability measurements were made on a commercially available apparatus, Kyowa Interface Science Co., Ltd. The mass of the DI water drop was about 0.004 g. The contact angle was then determined by viewing its drop from the side through an optical microscope. Results and Discussion Spectroscopic ellipsometry.- Chemical Etching Characteristics of GaAs(100) Surfaces in Aqueous HF Solutions Sadao Adachi z and Daitetsu Kikuchi Department of Electronic Engineering, Faculty of Engineering, Gunma University, Japan Chemical etching characteristics of GaAs(100) surfaces in aqueous HF solutions have been studied using spectroscopic ellipsometry (SE), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and wettability measurements. The SE data clearly indicate that the HF solutions cause the efficient removal of the GaAs native oxide upon immersing the samples in the solutions. When the native oxide is completely etch-removed, the resulting surface has a roughened overlayer about 1.3 nm thick, which is about five times as large as the AFM rms value (ϳ0.2-0.3 nm). The discrepancy of this is due to the SE technique being sensitive to both the surface microroughness and the adsorbed chemical species, but AFM is sensitive only to the surface microroughness. The XPS data reveal that etching in the concentrated (50 wt %) solution selectively removes gallium while leaving elemental arsenic on the GaAs surfaces. The XPS data also indicate the presence of fluorine on the HF-treated surfaces. The as-degreased surface is hydrophilic, while the HF-treated surfaces are highly hydrophobic

Umbilical Cord Blood as an Alternative Source of Reduced-Intensity Hematopoietic Stem Cell Transplantation for Chronic Epstein-Barr Virus–Associated T or Natural Killer Cell Lymphoproliferative Diseases

AbstractChronic Epstein-Barr virus–associated T/natural killer cell lymphoproliferative diseases represented by chronic active Epstein-Barr virus infection are lethal but are curable with several courses of chemotherapy and allogeneic hematopoietic stem cell transplantation (HSCT). Recently, we reported that reduced-intensity conditioning (RIC) provided better outcomes than myeloablative conditioning because RIC was less toxic. However, it was unclear whether cord blood transplantation (CBT) works in the context of RIC. We retrospectively analyzed 17 patients who underwent RIC followed by bone marrow transplantation (RIC-BMT) and 15 patients who underwent RIC followed by CBT (RIC-CBT). The representative regimen was fludarabine and melphalan based. The overall survival rates with RIC-BMT and RIC-CBT were 92.9% ± 6.9% and 93.3% ± 6.4%, respectively (P = .87). One patient died of lung graft-versus-host disease after RIC-BMT, and 1 patient died of multiple viral infections after RIC-CBT. Although cytotoxic chemotherapy was also immunosuppressive and might contribute to better donor cell engraftment after RIC-HSCT, the rate of engraftment failure after RIC-CBT was still higher than that after RIC-BMT (not significant); however, patients who had experienced graft failure were successfully rescued with a second HSCT. Unrelated cord blood can be an alternative source for RIC-HSCT if a patient has no family donor

TRIAL OPERATION OF A SIMPLE AUTOMATIC WEATHER STATION AT ASUKA CAMP, ANTARCTICA

A tentative unmanned observation was carried out at Asuka Camp in 1985 by means of a data logger equipped with a thermometer and an anemometer. The data logger, run by Lithium batteries, was buried in the surface snow without any heater. Although the sensors were covered by heavily drifting snow during the later period of observation, the desired recordings were performed successfully. Analyses of the obtained data provided information on diurnal variations of air temperature, periods and behavior of disturbances, monthly mean air temperature and depth of snow drift at Asuka Camp in 1985