Search

We have synthesized inverse-perovskite-type oxysilicides and oxygermanides represented by R3SiO and R3GeO (R = Ca and Sr) and studied their characteristics in the search for nontoxic narrow band gap semiconductors. These compounds exhibit a sharp absorption edge around 0.9 eV and a luminescence peak in the same energy range. These results indicate that the obtained materials have a direct-band electronic structure, which was confirmed by hybrid DFT calculations. These materials, made from earth abundant and nontoxic elements and with a relatively light electron/hole effective mass, represent strong candidates for nontoxic optoelectronic devices in the infrared range.

Metal oxide semiconductor gas sensors have been widely studied for the selective detection of various gases with trace concentrations. The identification of the reaction scheme governing the gas sensing response is crucial for further development; however, the mechanism of ethanol (EtOH) gas sensing by ZnO is still controversial despite being one of the most intensively studied target gas and sensing material combinations. In this work, for the first time, the detailed mechanism of EtOH sensing by ZnO is studied by using a bulk single-crystalline substrate, which has a well-defined stoichiometry and atomic arrangement, as the sensing material. The sensing response is substantial on the ZnO substrate even with a millimeter-size thickness, and it becomes larger with resistance of the substrate. The large sensing response is described in terms of the adsorption/desorption of the oxygen species on the substrate surface, namely, oxygen ionosorption. The valence state of the ionosorbed oxygen involved in EtOH sensing is identified to be O2– regardless of the temperature. The increase in the sensing response with the temperature is attributed to the enhanced oxidation rate of the EtOH molecule on the surface as analyzed by pulsed-jet temperature-programmed desorption mass spectrometry, which has been newly developed for analyzing surface reactions in simulated working conditions.

ScN films were grown on MgO(110) substrates and α-Al 2 O 3 ( 10 1 ¯ 0 ) substrates by a molecular beam epitaxy method, and their crystalline orientation, crystallinity, and electric properties were examined. (110)-oriented ScN films were epitaxially grown on MgO(110) substrates with the same crystal orientations, and ScN films with an orientation relationship (110)ScN || ( 10 1 ¯ 0 )α-Al 2 O 3 and [001]ScN || [ 1 2 ¯ 10 ]α-Al 2 O 3 were epitaxially grown on α-Al 2 O 3 ( 10 1 ¯ 0 ) substrates. Remarkably, electric-resistivity anisotropy was observed for ScN films grown on MgO(110) substrates, and the anisotropy depended on the growth temperature. The carrier concentration and Hall mobility of the ScN films grown on α-Al 2 O 3 ( 10 1 ¯ 0 ) substrates ranged from 10 19 –10 21 cm -3 and 10–150 cm 2 V -1 s -1 , respectively. The crystallinity, crystalline-orientation anisotropy, and electric properties of the films were strongly affected by growth conditions. For the growth of ScN films with high mobility on α-Al 2 O 3 ( 10 1 ¯ 0 ) substrates, a high temperature and an appropriate ratio of source materials were necessary.

Low-energy He+ ion scattering spectroscopy (He+ LEIS) was combined with the pulsed jet technique to analyze the outermost surface of a Zn-ZnO (0 0 0 1) substrate that is sensing trace ethanol (EtOH, 50 ppmv) contained in the air. The gas sensing response to EtOH by ZnO under static atmospheric pressure was successfully simulated by the periodically pulsed gas jet irradiation onto the surface while keeping the background pressure low enough to operate He+ LEIS. It was found that the gas sensing response to EtOH is due to the surface compositional change analyzed by He+ LEIS. Thus, the gas sensing response to EtOH is caused by adsorption of EtOH-related molecules on the surface, which governs the oxygen ionosorption. It was demonstrated that the combination of He+ LEIS and the pulsed jet technique is a useful approach to analyze the outermost surface of a gas sensor in the realistic working condition.

The pulsed jet technique was applied to measure the electrical resistance response of a Zn-terminated ZnO(0001) substrate to trace ethanol in the air to simulate the realistic working surface of a semiconductor gas sensor in a vacuum. The resistance response to the trace ethanol under the static atmospheric pressure air was well reproduced by the periodic pulsed jet technique with the optimal irradiation conditions, in which the pressure in the vicinity of the surface transiently reached 2 kPa. The behavior of the resistance upon the pulsed jet irradiation was discussed in terms of the ionosorption of oxygen. Because the background pressure was kept low enough during the pulsed jet irradiation, the reaction products during the ethanol sensing, such as acetaldehyde, has been successfully detected at the elevated temperature above 400 K by temperature programmed desorption-mass spectrometry (TPD-MS).

The effect of crystalline polarity on the microstructure of gallium-doped zinc oxide (GZO) deposited using magnetron sputtering onto glass substrates was investigated. X-ray photoelectron spectroscopy was used to determine the crystalline polarity of c-axis textured GZO films. Grains whose radii were more than 1 μm grew abnormally in 0.2 mol% doped GZO when the film was thicker than ~ 1 μm, and the radius of the grains was much smaller than 100 nm in the heavily (i.e., 4 mol%) doped GZO, regardless of the film thickness. Such abnormal growth of the grains in the 0.2 mol% doped GZO films coincided with a change in the crystalline polarity: the surfaces of unusually large GZO grains were terminated with the (000 1 ¯ ) face, and those of normal GZO grains were terminated with the (0001) face. The results indicated that polarity flipping is a very important event for controlling the texture of doped zinc oxide films.

Scandium nitride (ScN) films were grown on α-Al2O3( 1 1 ¯ 02 ) substrates using the molecular beam epitaxy method, and the heteroepitaxial growth of ScN on α-Al2O3( 1 1 ¯ 02 ) and their electric properties were studied. Epitaxial ScN films with an orientation relationship (100)ScN || ( 1 1 ¯ 02 )α-Al2O3 and [001]ScN || [ 11 2 ¯ 0 ]α-Al2O3 were grown on α-Al2O3( 1 1 ¯ 02 ) substrates. Their crystalline orientation anisotropy was found to be small. In addition, [100] of the ScN films were tilted along [ 1 ¯ 101 ] of α-Al2O3( 1 1 ¯ 02 ) in the initial stage of growth. The tilt angle between the film growth direction and [100] of ScN was 1.4–2.0° and increased with growth temperature. The crystallinity of the ScN films also improved with the increasing growth temperature. The film with the highest Hall mobility was obtained at the boundary growth conditions determined by the relationship between the crystallinity and the nonstoichiometric composition because the film with the highest crystallinity was obtained under the Sc-rich growth condition. The decreased Hall mobility with a simultaneous improvement in film crystallinity was caused by the increased carrier scattering by the ionized donors originating from the nonstoichiometric composition.

The a-axis oriented ZnO thin films deposited on sapphire substrates by pulsed laser deposition were studied to investigate the effects of pre-annealing on oxygen diffusion. The effect was as follows: the oxygen diffusion coefficient decreased, and the oxygen concentration in the tailing regions of the profiles reduced. Ion images of an oxygen tracer revealed the high-diffusivity paths for oxygen tracer diffusion. The temperature dependence of oxygen tracer diffusion coefficients (Db) in as-deposited and pre-annealed thin films were determined to be Db [cm2/ = 9.2x102 exp (- 405 [kJ/mo / RT) and Db [cm2/ = 1.8x103 exp (- 418 [kJ/mo / RT), respectively. On basis of these results, the crystal orientation on Db and the mechanism for oxygen diffusion were discussed.

Isotopic ZnO thin films were deposited on the c -plane of ZnO single crystals by pulsed laser deposition. The isotopic abundance of Zn in the films was determined with a secondary ion mass spectrometry before and after the films was diffusion annealed. The diffusion profiles across the film/substrate interface behaved smooth features. The zinc diffusion coefficient ( D Zn ) was obtained by analyzing the slope of the profile in the annealed sample. The temperature dependence of D Zn was determined to be D Zn (cm 2 /s)=8.0×10 4 exp(−417[kJ/mol])/RT, where R and T are gas constant and temperature. The zinc ion diffusion coefficients were of the same order as that in a ZnO single crystal. A comparison of the experimental and theoretical values indicated that the zinc ions diffused in the thin film and the single crystal through a vacancy mechanism.

The crystalline polarity of undoped and impurity-doped ZnO films grown on SiO2 glass substrates was investigated with the goal of achieving polarity-selective growth of ZnO films on non-crystalline substrates. We first demonstrated that hard X-ray photoelectron spectroscopy (HX-PES) is an appropriate method for determining the crystalline polarity of ZnO. We then characterized the ZnO films grown by pulsed laser deposition using HX-PES. The resulting films deposited with a 1 mol% Al-doped ZnO target had the (0001) surface, whereas films grown with nominally undoped, 0.1 mol% Al-doped, 1 mol% Ga-doped, and 1 mol% In-doped ZnO targets had the (000 1 ¯ ) surface. Since a clear polarity change due to Al-doping was seen at the ZnO/glass structure, we conclude that the essential parameter governing the polarity of the ZnO films is unlikely lattice matching (alignment of the lattice on the atomic scale) at the heterointerface between the ZnO films and substrates.

Zinc self-diffusion along the growth direction was analyzed for the isotopic multilayer ZnO thin film (( 64 ZnO/ 68 ZnO) 6 64 ZnO) deposited by pulsed laser deposition. The isotopic distribution was measured using a secondary ion mass spectrometry. The amplitude of the 64 Zn abundance in the depth profile was reduced by annealing at 993 K for several hours due to interdiffusion between the 64 ZnO and 68 ZnO layers. The diffusion profiles at the isotopic interfaces were analyzed using a periodic equation. The obtained zinc self-diffusion coefficients at several isotopic interfaces along the growth direction showed that the self-diffusion coefficients increased towards the film/substrate interface. A similar trend was also found in the lateral direction. The variation among the self-diffusion coefficients was related to the film thicknesses at the analysis positions. Since zinc self-diffusion is controlled by a vacancy-mediated mechanism, the variation in zinc diffusivity along the growth direction can be attributed to the effect of compressive biaxial stress. These findings are useful for producing high-quality ZnO devices.

SnO2 thin films were grown on silica glass substrates using the pulsed laser deposition (PLD) method, and their structures, electric properties, and sensor performances were examined to investigate the sensing mechanism of thin-film gas sensors. Single-phase SnO2 films with rutile-type structures were obtained at 650 °C. All the SnO2 films had many columnar grains, and the grain size increased with film thickness. Measurements of the Hall coefficient at room temperature revealed that the Hall mobility of the films was independent of the film thickness. In contrast, the sensing performances of the films for NO2 and H2 gases respectively in an air atmosphere drastically improved for film thicknesses under 100 nm. These results for the film properties and sensing performances of SnO2 thin films are discussed in terms of a space-charge layer formed on the columnar grains.

The relationship between Al and Li during diffusion was studied using Al-implanted ZnO. The Al donor in ZnO acts to increase the concentration of Li contamination from the atmosphere during the annealing. It is difficult to decompose the relationship formed by diffusion between Al and Li during high-temperature annealing. The most effective method to decompose the relationship is to anneal the as-implanted ZnO at a pressure of 5×10-3 torr. This annealing increases the Al solubility limit because the ZnO surface evaporates.

Ga 14 N and Ga 15 N were grown to study the isotopic effect on GaN lattice constants and band gap energy. It was shown that Ga 15 N has smaller lattice constants a and c at room temperature. However, the thermal expansion coefficients of Ga 15 N are larger than those of Ga 14 N up to 600 °C. Photoluminescence at 4.5–50 K implies a wider band gap in Ga 15 N, presumably due to its smaller unit cell at low temperature, and a weaker band gap renormalization effect via phonon–electron interaction.

The effects of various buffer layers on the InN growth were studied by using atomic force microscopy (AFM), X-ray diffraction (XRD), KOH wet etching and photoluminescence (PL). GaN or InN buffers with various temperatures and conditions were prepared for the InN growth by using plasma-assisted molecular beam epitaxy (PAMBE). For GaN buffers, the InN polarity was controlled by the GaN polarity. Namely, high temperature buffers (HT, 765°C-880°C) led to -c polar InN with better quality, while intermediate-temperature buffers (IT, 500°C-650°C) to +c polar InN with lower quality. When InN buffer was used, the quality of main InN was lower than that on GaN buffer. This reason is attributed to the twist misorientation (rotation of InN unit cell along c-axis) in InN/InN-buffer/sapphire structure. This drawback has been effectively suppressed by a longer substrate nitridation, or by inserting a GaN buffer between sapphire and InN buffer. By comparing the InN grown on GaN, InN or (GaN + InN) buffer, we concluded that GaN grown at about 800°C was the optimal buffer because it had atomically flat surface and led to -c polar InN with small misorientation.

ZnO samples added with MgO of 1.4–11.7 mol % were used for studying oxygen diffusion by the vapor–solid exchange method. The analyses of oxygen diffusion profiles and impurities were performed by secondary-ion mass spectrometry (SIMS). The results indicate that the increase of MgO concentration enhances oxygen diffusion. The difference between the bulk-diffusion coefficients of oxygen for ZnO samples added with MgO of 1.4 and 11.7 mol % was about two orders of magnitude. It was found that the grain boundary diffusion coefficients of oxygen in ZnO with 11.7 mol % of MgO were larger than those in other samples, by a factor of about 10. Above results indicate that MgO addition into ZnO increases the concentration of effective defects for that promote oxygen diffusion. As the effective defects, the possibilities of vacancy and interstitial of oxygen are discussed.

In order to perform a high throughput exploration of sensor materials using surface plasmon resonance (SPR), the gas sensing property of a ZnO/Au/SiO2 chip with SPR and the enhancing effect of UV irradiation on the desorption rate of NO2 from the ZnO surface were investigated. When the ZnO/Au/SiO2 chip was exposed to a high concentration of NO2 (1000 ppm), a large peak shift was observed in the SPR curve. However, this sensing signal for NO2 gas did not recover to the baseline. In the case of low-concentration NO2 (10 ppm), the peak shift of the SPR curve was lower than that in the case of the high-concentration gas, but recovery to the baseline was observed. From the X-ray photoelectron spectra for N 1s of the ZnO thin films exposed to 1000and 10-ppm NO2, two chemisorption states—NO2 (403.7 eV) and NO3 (407 eV)—were confirmed. After the ZnO film was irradiated by UV rays, exposed to 10-ppm NO2, all peaks related to N 1s disappeared. However, in the case of the ZnO film exposed to 1000-ppm NO2, adsorbed NO3 remained on the surface of ZnO. From these results, it was found that UV irradiation effectively assisted NO2 desorption from the surface of the ZnO thin film exposed to 10-ppm NO2. ©2010 The Ceramic Society of Japan. All rights reserved.

Zinc isotopic heterostructured zinc oxide thin films of 64ZnO/68ZnO/64ZnO were synthesized using pulsed laser deposition. The pulsed laser was first irradiated onto a polycrystalline target of 64ZnO to deposit the 64ZnO layer, then onto the 68ZnO target to prepare the 68ZnO layer and finally, the 64ZnO target was used again. The 64ZnO/68ZnO/64ZnO layered thin film was thus obtained. The thin films were annealed at various diffusion annealing temperatures. Diffusion profiles of the zinc isotopes due to the annealing were evaluated using secondary ion mass spectrometry (SIMS). The diffusion coefficients were slightly higher near the interface between the thin film and the substrate (the inner region) compared to the near surface (the outer region).

In this study, we successfully fabricated the Schottky junctions consisting of Pt electrode and high concentration Nb-doped (0.5 wt%) SrTiO3 (001) single crystal by sputtering process. The carrier concentrations of Nb-0.5wt%-doped SrTiO3 were determined as 1020 /cm3 order by Hall effect measurement. The electrical properties of junctions were investigated by measuring their current-voltage (I-V), capacitance-voltage (C-V) characteristics at temperature range from 80K to 400K. The hysteresis feature was observed that indicating the alteration of barrier height in junctions especially at lower temperature. The donor concentration and built-in potentials calculated from C-2-V data showed large discrepancy from Hall effect measurement indicating that the junctions deviate from the ideal Schottky diode model.

Gallium nitride (GaN) films were grown with and without lattice-matched (In,Ga)N buffer layers on the c(+) and c(-) faces of ZnO single-crystal substrates using molecular beam epitaxy, and their interface structures, including the relationship between the crystallinity of the GaN film and the polarity of the ZnO substrate, were investigated. Growth at a high temperature (e.g., 800°C) was made possible by using an (In,Ga)N buffer layer, which improved the crystallinity of the GaN films compared with that of GaN films grown directly on a ZnO substrate. Ion scattering, i.e., coaxial impact-collision ion scattering spectroscopy, and electron beam channeling, i.e., convergent beam electron diffraction, profiles revealed that the GaN films grown on c(+)-and c(-)-ZnO substrates with an (In,Ga)N buffer layer had a (0001) surface (i.e., c(+) polarity). That is, polarity inversion occurs when GaN film is grown with an (In,Ga)N buffer layer on a c(-)-ZnO substrate.

Oxygen isotopic heterostructural zinc-oxide thin films, i.e., Zn16O/Zn18O/Zn16O, were synthesized. Pulsed-laser deposition was used to deposit the films. First, only 18O2 gas was leaked into the deposition chamber, then 16O-enriched ZnO thin film was deposited, and after this 18O-enriched layer was obtained using 18O radicals as a source of isotopes. Finally, the 16O-enriched layer was deposited by turning off the radical source. The resulting thin films were annealed at various diffusion-annealing temperatures. The change in 18O-diffusion profiles due to annealing was evaluated with secondary ion mass spectrometry (SIMS). The diffusion coefficients were slightly higher near the interface between the thin film and the substrate (inner region) than those near the surface (outer region). The dependencies of oxide ion diffusion on temperature for the outer and inner regions areexpressed as $D_{outer} = 3.2 \times 10^1 \left({_{5.5 \times 10^{-1}}^{1.9 \times 10^3}} \right)\exp \left({-\frac{{397 \pm 42(kJ/mol)}}{{RT}}} \right)cm^2/s$ and $D_{inner} = 6.7 \times 10^1 \left({_{4.2 \times 10^{-5}}^{1.0 \times 10^4}} \right)\exp \left({-\frac{{346 \pm 96(kJ/mol)}}{{RT}}} \right)cm^2/s$. The activation energy is concluded to consist of the enthalpy of the oxygen migration and the oxygen vacancy formation, comparing the present data with the reported theoretical results.

As part of an effort to develop low-voltage surge filters, zinc oxide (ZnO) bicrystals were investigated to clarify the electronic state of grain boundaries in ZnO varistors. The varistors had a ZnO/glass/ZnO-type sandwich structure, and the interfacial glass layer was composed of oxide glass of the Bi-B-O system. A highly nonlinear current-voltage relationship was obtained for a sandwich structure composed of Co-doped single crystal ZnO, and the current through the junction was proportional to roughly the 30th power of the bias voltage at the breakdown stage. In contrast, the junction using undoped single crystal ZnO showed insignificant nonlinearity. Dielectric measurements revealed that a double-sided depletion layer was formed at the interface and the high nonlinearity was ascribed to the corresponding potential barrier formed at the interface.

We have investigated the polarity of zinc oxide (ZnO) and Al-doped ZnO films grown on (11¯20) and (0001) sapphire substrates, using coaxial impact collision ion scattering spectroscopy. The films grown by pulsed laser deposition with a nominally undoped ZnO ceramic target had a (000¯1) surface, whereas the films prepared with a 1 mol% Al-doped ZnO ceramic target had a (0001) surface. The usage of Al-doped and undoped targets caused no difference in the in-plane lattice orientation. Electron microscope observations revealed that polarity change due to doping occurred without the formation of any interfacial phase between ZnO and sapphire.

Photocatalytic machining of the organic molecular layer of octadecyl-trichlorosilane (OTS) coated on a Si wafer was investigated in the atmosphere by use of a porous TiO2 micro wire which was prepared by phase separation and selective leaching process. The OTS coated on the Si wafer surface was clearly machined within 1 min under UV irradiation. From the examination of the effect of humidity on the OTS machining rate, the ambient air was found to have sufficient condition for this machining procedure. On the contrary, the OTS in UV non-irradiated area was not machined at all. This indicated “double excitation mechanism”, which the photo induced oxygen radicals on TiO2 were further photo excited by the irradiation of UV light, proceeded. This process is capable of on-demand machining of organic molecular layer in normal atmospheric condition.

The effects of hydrogen doping on the optical and electrical properties of zinc oxide (ZnO) films were investigated. We prepared about 50-nm thick ZnO films on a sapphire substrate by using a pulsed laser deposition method and used a pulse-modulated inductively coupled plasma technique (PM-ICP) to intentionally insert hydrogen into the films. An increase in the electron concentration by hydrogen doping indicated that hydrogen is actually a cause of shallow donors. Moreover, the excitation intensity threshold for optically pumped stimulated emissions was significantly reduced after hydrogen doping.

Indium nitride (InN) films were grown on the c-face of zinc oxide (ZnO) single crystals by molecular beam epitaxy, and their interfacial structure and crystalline polarity were examined. The c-parameter and the crystallinity of the films strongly depended on the substrate polarity and growth temperature. In particular, the low-temperature-grown films on the oxygen terminated c(-)-face of ZnO (c(-)-ZnO) were single crystalline films with a shorter c-axis than that of bulk InN, while the films grown on the zinc terminated c (+) -face of ZnO (c(+)-ZnO) and the high-temperature-grown films had low crystallinity. Coaxial impact-collision ion scattering spectroscopy (CAICISS) revealed that the low-temperature-grown films on c(-)-ZnO had (0001) In-faces, namely c (+) -polarity (c (+) -InN), and TEM observations suggested the existence of an interfacial layer a few atomic thick in the c (+) -InN/c (-) -ZnO heterostructure. These results on the lattice restriction and the polarity of InN films are discussed in terms of the interfacial structure at the InN/ZnO boundary.

In order to achieve fine phosphor materials with high efficiency, phase-separated glasses and glass-ceramics in Mn-doped ZnO-SiO2 system, of which composition was xZnO-(100-x)SiO2 mol% (x=10, 20, 30) in the immiscibility region, were prepared by melting and subsequent heat-treatment process. All the melt-quenched glasses were phase-separated into two phases, i.e. a ZnO-rich phase and a SiO2-rich one. The SiO2-rich particles were formed into the ZnO-rich phase in the composition of x=20 and x=30, while the ZnO-rich particles were formed in the SiO2-rich phase in the composition of x=10. The melt-quenched glass showed the orange-emission at around 625 nm of its peak wavelength. After the heat-treatment on the melt-quenched glasses, the Zn2SiO4 (willemite) phase were precipitated, and the glass-ceramics gave strong green-emission at around 525 nm. The intensity of the green-emission of the glass-ceramics in the composition of x=10 was higher than those in the composition of x=20 and x=30 in spite of the same amount of precipitated Zn2SiO4 phase.

The defect structure of undoped ZnO and (Zn1-x,Mgx)O solid-solution films were deposited on YSZ substrate with pulsed laser deposition (PLD) to investigate defect equilibria in those films. In particular, the effects of thermal treatment on the structures and prosperities of (Zn1-x,Mgx)O solid-solution films were examined. The films with high MgO concentration (x>0.12) decomposed to the wurtzite-type and rock-salt-type phase after thermal treatment, indicating that the solubility limit of Mg was about x=0.12 and the wurtzite-type (Zn,Mg)O films with x>0.12 were indicated to be non-equilibrium ones. The subsequent analyses of oxygen diffusivity in those films revealed that the films under non-equilibrium state, i.e., wurtzite-type (Zn1-x,Mgx)O with x>0.12, contained significantly high concentration of anion defects.

Heteroepitaxial growth of indium nitride (InN) film crystal on ZnO single crystals was examined. InN films were grown on c(+)- and c(−)-ZnO single crystal substrates by molecular beam epitaxy using an RF plasma cell as a nitrogen source. InN films on c(+)-ZnO had a columnar structure with low crystallinity, while those on c(−)-ZnO were single crystalline films in which the full-width-at-half-maxima of their 0 0 0 2 rocking curves were about 150 arcsec. The polarity dependence of the film crystallinity is discussed in terms of the reactivity at the InN/ZnO interface.

The defect structure of undoped and Al-doped ZnO films deposited by pulse laser deposition was investigated to understand the charge compensation mechanism in those films. Particularly, the effect of oxidation assist, i.e., O2 gas or oxygen radicals, on the defect structure of the resultant films was examined. The examination indicated that the defect structure of undoped ZnO was not affected by the oxidation assist, whereas the properties of Al-doped ZnO obviously varied with the method of oxidation assist. An analyses of oxygen diffusion in these films revealed that Al-doping enhanced formation of oxygen defects in Al-doped ZnO.

Gallium nitride (GaN) and indium nitride (InN) films were grown on a zinc oxide (ZnO) single crystalline substrate with a (0001) orientation using molecular beam epitaxy. The interfacial structure and relaxation mechanism of the lattice mismatch at the nitride/oxide interface were investigated, particularly the effects of an (In,Ga)N alloy buffer layer on the interfacial structure of the GaN films. This layer significantly improved the crystallinity of the GaN films by gradually relaxing the lattice mismatch between the GaN and ZnO. In spite of the large lattice mismatch between InN and ZnO, InN films with high crystallinity were grown without an (In,Ga)N buffer layer. Structural analysis revealed that an InN layer with low crystallinity formed spontaneously during the initial growth stage, and this amorphous-like layer likely contributed to relaxation of the interfacial stress caused by the lattice mismatch.

The organic polymers, polyimide thin films and PMMA plate, were photo-catalytically machined by use of the porous Ti02 micro wire which was prepared by the phase separation-selective-leaching method. The photocatalytic decomposition of a nanometer range in depth occurred in all the polymer surfaces. The photo decomposition rate was different among the samples. This difference was attributed to that of the chemical bond strength of the polymers, and this mechanism of the photo decomposition by Ti02 micro wire was discussed from the results of the IR-ATR spectroscopy measurements.

GaN films were grown on ZnO single crystal substrates with various orientations by a molecular beam epitaxy method. Particularly, applicability of ZnO substrates for growth of GaN along non-polar axis was examined by using ZnO substrates with (1 1 2¯ 0) or (1 0 1¯ 0) orientation and effect of lattice matched (In,Ga)N buffer layer on crystallinity of GaN films was also investigated. Epitaxial GaN films were obtained regardless of the absence of (In,Ga)N buffer layer, when ZnO substrates with c-face were used. In contrast, GaN films deposited on (1 1 2¯ 0) or (1 0 1¯ 0) face of ZnO exhibited complicated manner of lattice orientation. Epitaxial growth of GaN films along non-polar axis was achieved only when the GaN films were grown on (In,Ga)N buffer layer deposited on ZnO substrates of (1 1 2¯ 0) orientation.

Zinc oxide (ZnO) films were grown on sapphire (1120) substrates by molecular beam epitaxy under oxygen radical irradiation. The effect of the growth conditions, including the Zn/O ratio supplied to the film surface, on the electrical properties of ZnO films was studied in relation to the film morphology. We found that the growth rate strongly depended on the Zn flux from the Knudsen cell and the optimum condition for high growth rate was very narrow. The grain size in the lateral direction increased with increasing growth rate in the thickness direction. The increase in growth rate, especially in the lateral direction, resulted in the carrier mobility increasing up to 42 cm2 V−1 s−1. The carrier concentration N was sensitive to the substrate temperature, while the value of N was not sensitive to the source supplying ratio Zn/O. We discuss the decrease of the carrier concentration with increasing substrate temperature in regard to the formation of nonequilibrium defects.

Silver implantation (75 keV, 2 × 1014 ions/cm2) in ZnO ceramics was carried out at room temperature to characterize the diffusion phenomenon. Annealing caused Ag evaporation from the sample. Results suggest that it is difficult to incorporate Ag into Zn site by annealing. The Ag profile with the monotonous decrease in concentration below 3 × 1017/cm3 was observed in ZnO annealed at 900°C. Above results become the basic results for producing the ZnO sensor used the reaction at surface and the grain boundary.

Undoped and aluminum-doped ZnO epitaxial films were grown on (001) sapphire substrates by an ion-beam sputtering method with or without the irradiation of oxygen radicals. The effect of oxygen-radical irradiation was notable in the undoped ZnO films when the growth temperature was relatively low. The irradiation improved the crystallinity and decreased the oxygen-vacancy concentration, while it induced internal stress into the films. The carrier concentration of the undoped ZnO films was decreased by the oxygen-radical irradiation, which was attributable to a decrease in the oxygen-vacancy concentration. The Hall mobility of the undoped ZnO films was as low as 1–3 cm2 V-1 s-1. The low mobility was explained by carrier scattering due to the potential barriers at the grain boundaries. The height of the potential barriers at the grain boundaries decreased with increasing carrier concentration. This behavior was well explained by a simple model assuming a single defect state at grain boundaries.

We report on the effect of sample non-uniformity on the results of Hall-effect measurements. False positive Hall coefficients were obtained from an evidently n-type ZnO single crystal, although four electrodes with low contact resistance were made and the Van der Pauw parameter for this electrode configuration was close to 1.00. Further position-sensitive characterization revealed that the false positive Hall coefficient was due to non-uniform electrical properties of the sample. To demonstrate a false positive sign of the Hall coefficient due to sample non-uniformity, we devised a model structure made from evident n-type ZnO thin film and successfully reproduced a false positive Hall coefficient from n-type ZnO.

The role of the crystalline polarity of zinc oxide (ZnO) in the interfacial properties was investigated with semiconductor–insulator–semiconductor structures prepared from ZnO single crystals and an oxide glass insulator. The insulator/ZnO(0001) interface showed highly nonlinear current–voltage (I–V) relationships, characteristic of ZnO varistors, with clear breakdown behavior at a bias voltage of 3 V. Whereas, the insulator/ZnO(0001) interface was characterized by a relatively large leakage current in the pre-breakdown region and very slow decay behavior in the I–V relationships. The dielectric properties of the insulator/ZnO interface also exhibited crystalline polarity dependence.

Zinc oxide (ZnO) crystals joined by using an intergranular glass phase were investigated in order to develop surge filters for low-voltage applications and to clarify the current transport mechanism through the junction. The junctions having a ZnO/glass/ZnO sandwich structure were synthesized by using an interfacial glass phase of the Bi–B–O system. A highly nonlinear current–voltage curve was obtained for the ZnO/glass/ZnO junction with Co-doped ZnO single crystals, and the current through the junction at the breakdown voltage was proportional to the 30th power of the bias voltage. Dielectric measurements revealed that a double-sided depletion layer was formed at the interface, and the observed high nonlinearity was ascribed to the corresponding potential barrier formed at the interface.