Kanji Yasui

Growth of High-Density Zinc Oxide Nanorods on Porous Silicon by Thermal Evaporation

The formation of high-density zinc oxide (ZnO) nanorods on porous silicon (PS) substrates at growth temperatures of 600–1000 °C by a simple thermal evaporation of zinc (Zn) powder in the presence of oxygen (O2) gas was systematically investigated. The high-density growth of ZnO nanorods with (0002) orientation over a large area was attributed to the rough surface of PS, which provides appropriate planes to promote deposition of Zn or ZnOx seeds as nucleation sites for the subsequent growth of ZnO nanorods. The geometrical morphologies of ZnO nanorods are determined by the ZnOx seed structures, i.e., cluster or layer structures. The flower-like hexagonal-faceted ZnO nanorods grown at 600 °C seem to be generated from the sparsely distributed ZnOx nanoclusters. Vertically aligned hexagonal-faceted ZnO nanorods grown at 800 °C may be inferred from the formation of dense arrays of ZnOx clusters. The formation of disordered ZnO nanorods formed at 1000 °C may due to the formation of a ZnOx seed layer. The growth mechanism involved has been described by a combination of self-catalyzed vapor-liquid-solid (VLS) and vapor-solid (VS) mechanism. The results suggest that for a more precise study on the growth of ZnO nanostructures involving the introduction of seeds, the initial seed structures must be taken into account given their significant effects.

Seed/catalyst-free vertical growth of high-density electrodeposited zinc oxide nanostructures on a single-layer graphene

We report the seed/catalyst-free vertical growth of high-density electrodeposited ZnO nanostructures on a single-layer graphene. The absence of hexamethylenetetramine (HMTA) and heat has resulted in the formation of nanoflake-like ZnO structure. The results show that HMTA and heat are needed to promote the formation of hexagonal ZnO nanostructures. The applied current density plays important role in inducing the growth of ZnO on graphene as well as in controlling the shape, size, and density of ZnO nanostructures. High density of vertically aligned ZnO nanorods comparable to other methods was obtained. The quality of the ZnO nanostructures also depended strongly on the applied current density. The growth mechanism was proposed. According to the growth timing chart, the growth seems to involve two stages which are the formation of ZnO nucleation and the enhancement of the vertical growth of nanorods. ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics such as photovoltaic devices, sensing devices, optical devices, and photodetectors.

Seed/catalyst-free growth of zinc oxide nanostructures on multilayer graphene by thermal evaporation

We report the seed/catalyst-free growth of ZnO on multilayer graphene by thermal evaporation of Zn in the presence of O2 gas. The effects of substrate temperatures were studied. The changes of morphologies were very significant where the grown ZnO structures show three different structures, i.e., nanoclusters, nanorods, and thin films at 600°C, 800°C, and 1,000°C, respectively. High-density vertically aligned ZnO nanorods comparable to other methods were obtained. A growth mechanism was proposed based on the obtained results. The ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics.

Chemical Vapor Deposition of Low Hydrogen Content Silicon Nitride Films Using Microwave-Excited Hydrogen Radicals

The chemical vapor deposition method using hydrogen radicals excited by microwave plasma has been applied to obtain silicon nitride (SiN) films of low hydrogen content. In this method, silane and monomethylamine were used as source gases. Various properties of deposited films such as the composition of SiN, the residual contents of carbon and oxygen, and the deposition rate were influenced by growth conditions. In the growth conditions, the distance between the center of the waveguide and the substrate was the most critical parameter because the distribution of the H radical strongly depended upon it in our experiment. For a suitable substrate position, carbon and oxygen contents could be reduced to a small value. On the other hand, residual hydrogen content was less than ~1×1022 cm-3. This value was a quarter of that in the films grown by silane and ammonia gases under similar conditions.

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.

Amorphous SiN films grown by hot‐filament chemical vapor deposition using monomethylamine

Hot‐filament chemical vapor deposition (hot‐filament CVD) of silicon nitride films has been studied using silane and monomethylamine as source gases for the deposition temperature 600–800 °C. The deposition rate was about one order larger than that of thermochemical vapor deposition (thermo‐CVD) using the same gases. The activation energy of the growth rate was 0.2 eV smaller than that of thermo‐CVD. The hydrogen content was below the detection limit of infrared absorption measurements even in the film deposited at 600 °C. The film surface deposited at 700 °C had a smooth specular surface and the flatness of the hot‐filament CVD films was the same as that of thermo‐CVD films deposited at 100–200 °C higher temperatures.

Electromagnetically induced transparency like transmission in a metamaterial composed of cut-wire pairs with indirect coupling

(Received 21 October 2013; revised manuscript received 22 January 2014; published 18 February 2014)We theoretically and numerically investigate metamaterials composed of coupled resonators with indirectcoupling. First, we theoretically analyze a mechanical model of coupled resonators with indirect coupling.The theoretical analysis shows that an electromagnetically induced transparency (EIT)-like phenomenon with atransparency bandwidth narrower than the resonance linewidths of the constitutive resonators can occur in themetamaterial with strong indirect coupling. We then numerically examine the characteristics of the metamaterialcomposedofcoupledcut-wirepairsusingafinite-differencetime-domain(FDTD)method.TheFDTDsimulationconfirmsthatanEIT-liketransparencyphenomenonoccursinthemetamaterialowingtoindirectcoupling.Finally,we compare the results of the theoretical and numerical analyses. The behavior of the EIT-like metamaterial isfound to be well described by the mechanical model of the coupled resonators.DOI: 10.1103/PhysRevB.89.075120 PACS number(s): 78

High electron mobility and low carrier concentration of hydrothermally grown ZnO thin films on seeded a-plane sapphire at low temperature

Hydrothermal zinc oxide (ZnO) thick films were successfully grown on the chemical vapor deposition (CVD)-grown thick ZnO seed layers on a-plane sapphire substrates using the aqueous solution of zinc nitrate dehydrate (Zn(NO3)2). The use of the CVD ZnO seed layers with the flat surfaces seems to be a key technique for obtaining thick films instead of vertically aligned nanostructures as reported in many literatures. All the hydrothermal ZnO layers showed the large grains with hexagonal end facets and were highly oriented towards the c-axis direction. Photoluminescence (PL) spectra of the hydrothermal layers were composed of the ultraviolet (UV) emission (370 to 380 nm) and the visible emission (481 to 491 nm), and the intensity ratio of the former emission (IUV) to the latter emission (IVIS) changed, depending on both the molarity of the solution and temperature. It is surprising that all the Hall mobilities for the hydrothermal ZnO layers were significantly larger than those for their corresponding CVD seed films. It was also found that, for the hydrothermal films grown at 70°C to 90°C, the molarity dependences of IUV/IVIS resembled those of mobilities, implying that the mobility in the film is affected by the structural defects. The highest mobility of 166 cm2/Vs was achieved on the hydrothermal film with the carrier concentration of 1.65 × 1017 cm−3 grown from the aqueous solution of 40 mM at 70°C.

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.

Improvement in Crystallinity of ZnO Films Prepared by rf Magnetron Sputtering with Grid Electrode

ZnO films were prepared on Si and quartz substrates by rf magnetron sputtering with a mesh grid electrode. The influence of a negative grid bias on the crystallinity and optical property of ZnO films was investigated. At an appropriate dc grid bias, ZnO films with good crystallinity were grown from room temperature to 300°C. The number of stacking faults, which were observed in the X-ray diffraction spectra of the films deposited without the grid electrode, decreased in the films deposited with the grid electrode. All ZnO films showed a high transmittance in the visible region.