Yuzuru Narita

Adsorption Density Control of N719 on TiO2 Electrodes for Highly Efficient Dye-Sensitized Solar Cells

N719 dye adsorption on anatase TiO 2 surfaces with different adsorption densities was investigated by IR-absorption spectroscopy using a multiple internal reflection technique. When the N719 dye was adsorbed on the TiO 2 surface by immersing it in an N719 solution consisting of acetonitrile and tertiary-butyl alcohol, chemisorbed and physisorbed N719 dyes coexisted on the surface. The ratio of chemisorbed and physisorbed dye densities was dependent on the N719 exposure, and excessive exposure led to the generation of the physisorbed dye. Test fabrications of the dye-sensitized solar cells (DSCs) with various N719 exposures suggested that we need to optimize the dye adsorption density to achieve a higher power conversion efficiency in DSCs.

Low-Temperature Heteroepitaxial Growth of SiC on (100) Si Using Hot-Mesh Chemical Vapor Deposition

The crystal growth of 3C-SiC films on (100) Si substrates by the hot-mesh chemical vapor deposition (HMCVD) method using monomethylsilane (MMS) as a source gas was investigated. A mesh structure of hot tungsten (W) wire was used as a catalyzer. At substrate temperatures above 750°C and at a mesh temperature of 1600°C, 3C-SiC crystal was epitaxially grown on Si substrates. From the X-ray rocking curve spectra of the (311) peak, SiC was also epitaxially grown in the substrate plane. From the dependence of growth rate on substrate temperature and W-mesh temperature, the growth mechanism of SiC films by HMCVD was considered.

Thin-Film Deposition of Silicon-Incorporated Diamond-Like Carbon by Plasma-Enhanced Chemical Vapor Deposition Using Monomethylsilane as a Silicon Source

We have deposited Si-incorporated diamond-like carbon (DLC) films by radio-frequency plasma-enhanced chemical vapor deposition using methane, argon, and monomethylsilane (MMS; CH3SiH3) as a silicon source, and have investigated the structural and mechanical properties of the films. The deposition rate and Si atomic fraction [Si/(Si+C)] in the DLC films increased with increasing MMS flow ratio. The Si fraction was approximately 13% at a MMS flow ratio [MMS/(MMS+CH4)] of 3%, showing that the deposition using a combination of CH4 and MMS produces films with high Si content compared with those deposited using conventional C and Si sources. The Si fraction was also found to increase with a decrease in Ar flow rate under a constant MMS flow ratio. Many particles composed mainly of Si, whose size was 0.3–1 µm in diameter, were observed on the surface when deposition was carried out at MMS flow ratios of 15 and 30%. Compressive internal stress in the films decreased with the MMS flow ratio and/or with the Ar flow rate. The decrease in internal stress is probably due to the relaxation of a three-dimensional rigid network by the formation of Si–C and Si–H bonds in the films as well as Ar+ ion bombardment.

Real-Time Observation of Initial Thermal Oxidation on Si(110)-16×2 Surfaces by O 1s Photoemission Spectroscopy Using Synchrotron Radiation

The initial oxidation on a Si(110)-16×2 surface at room temperature and 540 °C has been investigated by real-time X-ray photoemission spectroscopy (O 1s) using 687 eV photons. At both temperatures, the initial oxidation of Si(110) is characterized by its unique rapid oxidation regime immediately after the introduction of oxygen molecules. O 1s spectra are shown to consist of at least four oxidation states. It is likely that oxidation at or around the adatoms of pentagon pairs, reportedly present on the Si(110)-16×2 reconstructed surface, is the predominant process in the very early stage of oxidation.

Effects of Silicon Source Gas and Substrate Bias on the Film Properties of Si-Incorporated Diamond-Like Carbon by Radio-Frequency Plasma-Enhanced Chemical Vapor Deposition

We have deposited Si-incorporated diamond-like carbon (DLC) films by radio-frequency plasma-enhanced chemical vapor deposition using methane, argon, and organosilanes, and investigated the effects of Si source gas (monomethylsilane, dimethylsilane) and substrate bias (negative dc bias, negative pulse bias) on the structure and the mechanical and tribological properties of the films. The Si-DLC films deposited using monomethylsilane as a Si source gas tended to have a higher Si atomic fraction ratio [Si/(Si+C)] than the films deposited using dimethylsilane. Friction coefficient and internal stress decreased by the incorporation of Si into the films. However, many particles composed mainly of Si were observed on the film surfaces when deposition using a dc bias was carried out at higher monomethylsilane or dimethylsilane flow ratios. It was found that for both the Si source gases, the use of a pulse bias was effective in suppressing the formation of particles and further decreasing friction coefficient and internal stress. Additionally, the pulse-biased Si-DLC films were found to have a higher wear resistance than the dc-biased Si-DLC films.

Hydrogen-Controlled Crystallinity of 3C-SiC Film on Si(001) Grown with Monomethylsilane

Heteroepitaxial growth of 3C-SiC has been conducted on Si(001) substrate using monomethylsilane as a single source gas. By evaluating the crystalliniy of the film as a function of the growth temperature T and pressure P, a process window for a good epitaxy has been obtained, which is expressed as Pc1(T)<P<Pc2(T). Both of the two critical pressures increase with T, and are both successfully expressed with a single analytical function derived from the hydrogen adsorption/desorption balance on the surface.

Low-Temperature Heteroepitaxial Growth of 3C-SiC(111) on Si(110) Substrate Using Monomethylsilane

To realize the forthcoming ubiquitous society, electronics is now required to integrate various non-Si devices, acting as sensors or actuators, onto a Si chip. To this goal, we propose use of an ultrathin SiC film on a Si substrate (SiC/Si), which is expected to act as a common platform in the formation of various non-Si materials. We further propose use of methylsilanes (SiH4-x(CH3)x) as a suitable precursor for such SiC growth on Si substrates, in that it is a single source containing both Si and C atoms within a molecule and that it decomposes at relatively low temperatures (<900°C) as compared to conventional hydrocarbons. By using monomethylsilane (MMS, SiH3CH3) as the precursor, we have demonstrated that a high quality, single-domain 3C-SiC film can be grown on a Si(001) substrate at 900-1000°C, with a thickness of as thin as 0.1 μm [1,2]. One of the largest challenges confronted in the heteroepitaxy of SiC on Si is the lattice mismatch between Si and SiC, which is as large as ~20%. To solve this problem, we have tried growth of 3C-SiC(111) crystal on Si(110) substrate, which is expected to show much reduced lattice mismatch (a few %)[3]. 3C-SiC(111) growth on Si(110) was first reported by Nishiguchi et al.[3] by using Si2H6 + C3H8[3], but no studies with metylsilanes have ever been attempted to reduce the growth temperature below 1000°C. The growth was conducted in an UHV chamber with a gas-source-molecular-beam-epitaxy (GSMBE) mode. Figure 1 shows an XRD pattern obtained from a 3C-SiC film grown at T=1000°C using MMS (2.7×10 Pa). It is clearly seen that a 3C-SiC(111) film grows on a Si(110) substrate. The crystallinity of the film, as evaluated by the in-plane (Figs. 2(a) and (b)) and the out-ofplane (Figs. 2(c) and (d)) XRD rocking curves (Rigaku, SuperLab), shows clear betterment in the 3C-SiC(111) film on Si(110) ((a) and (c)) from that of 3C-SiC(111) film on Si(111) ((b) and (d)). This improvement is understood in terms of the reduced lattice mismatch between the film and the substrate. The 3C-SiC(111)/Si(110) films showed presence of double domains with equivalent areas, which may affect the performance of the devices to be fabricated on this surface. Despite this open problem, the high quality of the film demonstrated in the present study is sufficient to show the high potential of the MMS-GSMBE-grown 3C-SiC(111)/Si(110) film in its use in the fabrication of integrated ubiquitous devices.

Low-temperature-atomic-layer-deposition of SiO2 with Tris(dimethylamino)Silane (TDMAS) and Ozone using Temperature Controlled Water Vapor Treatment

SiO2 ALD with precursors of tris(dimethylamino)silane (TDMAS) and ozone on Si(100) surfaces at room temperature were investigated by infrared absorption spectroscopy with a multiple internal reflection geometry. TDMAS dissociatively adsorbs on OH sites of hydroxylated Si surfaces and ozone irradiation is effective to remove the hydroaminocarbon adsorbates introduced in the course of the TDMAS adsorption. After the ozone treatment, H2O vapor treatments at substrate temperatures around 160 aC allow generation of OH sites for the TDMAS adsorption. The TDMAS adsorption and the ozone treatment at room temperature followed by the H2O treatment at 160 aC enable the cyclic deposition of SiO2. V-I measurements of SiO2 grown by the present 160 aC ALD indicated the deposited film has breakdown electric fields from 3 to 11 MV/cm. C-V measurements indicated that the present ALD is available for MOS capacitors.

(100)-Oriented 3C–SiC Polycrystalline Film Grown on SiO2 by Hot-Mesh Chemical Vapor Deposition Using Monomethylsilane and Hydrogen

To fabricate a SiC-on-insulator (SiCOI) structure, the growth of cubic silicon carbide (3C–SiC) polycrystalline films on thermally oxidized Si (SiO2/Si) substrates by hot-mesh chemical vapor deposition (HMCVD) utilizing high-density hydrogen radicals was investigated. The SiC films were grown at low temperatures below 900°C using monomethylsilane (MMS) as a source gas. On the basis of growth activation energy, the rate-determining step of SiC growth below 850°C was considered to be the supply of hydrogen radicals from a tungsten (W) mesh surface. From X-ray diffraction measurements, the growth of (100)-oriented 3C–SiC films was determined to be achieved at a substrate temperature of 750°C, while that of polycrystalline SiC films, at substrate temperatures above 850°C.

Growth of GaN on SiC/Si substrates using AlN buffer layer under low III/V source gas ratio by hot-mesh CVD

GaN is a widegap compound semiconductor, which is useful for optoelectronic devices operating in short wavelengths and at high-temperatures. The GaN films are usually grown on sapphire substrates at growth temperatures higher than 1000°C using MOCVD method [1–3]. For the growth of GaN films with excellent crystallinity and optical property, high V/III source gas ratio (NH3/TMG>10,000) is required due to the decomposition-resistant property of nitrogen source-gas such as NH3. The reduction of the source gas consumption is strongly desired from the viewpoint of the resource savings. For the GaN growth at low V/III source gas ratios, excitation of NH3 is required. Among several methods, catalytic reaction of hot tungsten (W) wire surface with NH3 is very promising, because it produces high-density NHx radicals, in particular when the W fine wires with a mesh structure are used [4]. Recently, heteroepitaxial growth of GaN films on Si substrates has been attempted, aiming at fabrication of various GaN electronic devices at a low cost [5–8]. A large lattice mismatch, however, also exists between GaN and Si, which is similar to the case between GaN and sapphire. In order to overcome this problem, insertion of a thin SiC buffer layer between them is useful [9]. In our previous studies, GaN films were grown on SiC/Si(111) substrates by hot-mesh CVD using TMG and NH3. Photoluminescence spectra of GaN films grown on the SiC buffer layer, however, showed relatively strong yellow luminescence at RT [10]. In this study, in order to further improve the crystallinity and the optical property, the insertion of AlN buffer layer between GaN and SiC layer was attempted.