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A bonding method utilizing redox reactions of metallic oxide microparticles achieves metal-to-metal bonding in air, which can be alternative to lead-rich high-melting point solder. However, it is known that the degree of the reduction of metallic oxide microparticles have an influence on the joint strength using this bonding method. In this paper, the reduction behavior of CuO paste and its effect on Cu-to-Cu joints were investigated through simultaneous microstructure-related x-ray diffraction and differential scanning calorimetry measurements. The CuO microparticles in the paste were gradually reduced to submicron Cu2O particles at 210–250°C. Subsequently, Cu nanoparticles were generated instantaneously at 300–315°C. There was a marked difference in the strengths of the joints formed at 300°C and 350°C. Thus, the Cu nanoparticles play a critical role in sintering-based bonding using CuO paste. Furthermore, once the Cu nanoparticles have formed, the joint strength increases with higher bonding temperature (from 350°C to 500°C) and pressure (5–15 MPa), which can exceed the strength of Pb-5Sn solder at higher temperature and pressure.

We report on the p-type conductivity control using N and Te codoping and thermal annealing in the ZnO films, heteroepitaxially grown on Al2O3 substrates and homoepitaxially grown on ZnO substrates by molecular-beam epitaxy, respectively. The N and Te codoping and the homoepitaxy effectively reduce the background electron concentration in ZnO films due to the suppression of various defect generation, and the thermal annealing causes the conductivity conversion from n-type to p-type due to the activation of N-related defects and the annihilation of donor-type defects. The p-type conductivity with a hole concentration of 1.61016 cm-3 and a hole mobility of 16 cm2/Vsec is obtained in the ZnO:N+T film grown on the Al2O3 substrate and the p-type conductivity with a hole concentration of 4.01016 cm-3 and a hole mobility of 11 cm2/Vsec is obtained in the ZnO:N+Te film grown on the ZnO substrate.

The room-temperature 2.2 eV photoluminescence intensity of n-type GaN grown by metal-organic vapor phase epitaxy was found at lower excitation intensities to be proportional to excitation intensity and at higher excitation intensities to be proportional to the 1/3 power of the excitation intensity. That of undoped bulk GaN grown by hydride vapor phase epitaxy, however, was proportional to the excitation intensity throughout the excitation region. The luminescence is thought from the intensity proportional to the 1/3 power of the excitation intensity not to be the result of a conventional process. The observed proportionality is well explained by exciton–exciton annihilation in one-dimensional structures.

We report on the structural and optical properties of non polar a-plane MgxZn1−xO (0 ⩽ x ⩽ 0.57) films on r-plane sapphire substrates grown by plasma-assisted molecular beam epitaxy. Reflection high energy electron diffraction (RHEED) revealed a formation of cubic MgO phase when an Mg concentration increases. Room temperature (RT) photoluminescence (PL) and transmission electron microscopy consistently revealed the formation of cubic MgO phase from the Mg0.21Zn0.79O film. The Mg0.11Zn0.89O film showed a band edge emission at ∼360 nm, which is a shorter wavelength than the ZnO (∼373 nm), from the RT PL measurements. Photoluminescence excitation (PLE) measurements at RT showed that band-gap energies of MgxZn1−xO films could be tuned up to ∼4.65 eV (∼270 nm) although cubic MgO phase were mixed for high Mg concentration. For the single phase wurtzite MgZnO film, band-gap energy of 3.48 eV was obtained from the Mg0.11Zn0.89O film.

Remarkable reduction of the threading dislocation (TD) density has been achieved by inserting a GaN layer grown at an intermediate temperature (900 °C) (IT-GaN layer), just prior to the growth of GaN at 1040 °C by using a hydride vapor phase epitaxy. The variation in the dislocation density variation along the growth direction was observed by using cathodoluminescence (CL) and transmission electron microscopy (TEM). A cross-sectional CL image revealed that the reduction of the TD density happened during the growth of IT-GaN layer. The TEM measurement provided the proof that the TD reduction could be ascribed to the masking of the TD by stacking faults in the IT-GaN layer.

Optical and electrical properties of fresh dislocations in GaN bulk crystals deformed plastically at elevated temperatures were reviewed. A dislocation band model was proposed. The fresh dislocations of ( a /3) [ 1 2 ¯ 10 ] -type edge dislocations on the ( 10 1 ¯ 0 ) prismatic plane induced several photoluminescence peaks at around 1.8, 1.9 and 2.4 eV, which implies the formation of radiative recombination centers of the dislocations. Simultaneously, near-band-edge (3.48 eV) photoluminescence intensity decreased remarkably for a high-density of non-radiative recombination centers originating in deformation-induced abundant Ga-vacancy related clusters. The intensity variation of yellow luminescence with plastic deformation and subsequent annealing did not relate to the native property of dislocations. Variation of optical absorption dependent on the strain in plastically deformed GaN was understood in a model of the Franz–Keldysh effect by the electric fields associated with charged dislocations (~5.8 e/nm). Scanning spreading resistance microscopic images showed many spots with high conductivity around the induced dislocations, showing electrical conduction along dislocations according to the Frenkel–Poole mechanism.

InGaN layers with controlled In composition up to 30 at.% are grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. By considering the growth rate differences in GaN and InN caused by different vapor pressure and sticking coefficient, factors of the Ga and In source fluxes for the targeted In composition are determined. By applying the factors, InGaN layers with the almost same In compositions are grown. Before the growth the substrates were nitrided by rf-nitrogen plasma, which resulted in the formation of epitaxial AlN layer. The growth of thin GaN on this AlN surface shows strong streaky reflection high energy electron diffraction pattern with a specular spot, however, InGaN layers on the GaN layer show spotty patterns. Surface morphology of the InGaN layers shows island-like granules and the granule-like morphology is getting clear as the In composition and roughness are increased, too. The InGaN layers with In composition up to 30 at.% do not show formation of InN and only InGaN peaks are detected from the X-ray diffraction. Crystal quality of the InGaN layer with In composition of 15 at.% is worse than that of 30 at.%-In layer as addressed by larger broadening of X-ray rocking curves.

The growth of high-quality indium (In)-rich InXGa1−XN alloys is technologically important for applications to attain highly efficient green light-emitting diodes and solar cells. However, phase separation and composition modulation in In-rich InXGa1−XN alloys are inevitable phenomena that degrade the crystal quality of In-rich InXGa1−XN layers. Composition modulations were observed in the In-rich InXGa1−XN layers with various In compositions. The In composition modulation in the InXGa1−XN alloys formed in samples with In compositions exceeding 47%. The misfit strain between the InGaN layer and the GaN buffer retarded the composition modulation, which resulted in the formation of modulated regions 100 nm above the In0.67Ga0.33N/GaN interface. The composition modulations were formed on the specific crystallographic planes of both the {0001} and {0114} planes of InGaN.

The growth of N-polar GaN films on (0001) Al 2 O 3 substrates by plasma-assisted molecular beam epitaxy was demonstrated using a CrN buffer layer. Analysis of reflection high energy electron diffraction (RHEED) patterns and a chemical etching method were used in order to confirm the polarity of GaN on CrN. RHEED patterns show the 3 × 3 pattern characteristic of GaN. Chemical etching significantly changes the GaN surface morphology which implies that the N-polar GaN was grown on the CrN buffer layer. In addition, an improvement in the crystal properties of GaN was achieved using the annealing process for the CrN buffer layers.

We report on the use of the photoresponse characteristics of polarity- controlled ZnO films for determining ZnO polarity. ZnO films were grown on CrN and Cr2O3 buffer layers in order to produce Zn- and O-polar films, respectively, and Ti/Au metal contacts were formed. All samples showed Ohmic behavior, but the Zn-polar ZnO film showed lower contact resistance than the O-polar ZnO film. The O-polar ZnO film showed higher photocurrent and longer decay time than the Zn-polar film by photoresponse measurement. These phenomena can be explained by the expansion of the depletion layer into the bulk ZnO surface. Compared with current methods, this method of determination of the polarity of ZnO films through the measurement of photoresponse characteristics is very easy and simple to implement.

The authors report on the p-type conductivity in the ZnO films, grown on c-Al2O3 substrates by molecular-beam epitaxy, using N and Te codoping and thermal annealing. In the electrical properties, the N and Te codoping effectively suppresses the background electron concentration of ZnO films, and the thermal annealing causes the conductivity conversion from n-type to p-type. In the structural properties, the N and Te codoping deteriorates the crystalline quality of ZnO films, even if Te doping contributes to improve the crystallinity, and the thermal annealing recovers the degraded crystalline quality again. In the optical properties, the N and Te codoping simultaneously increases the donor-related emission, the acceptor-related emission, and the nonradiative recombination in ZnO films, while the thermal annealing relatively enhances the acceptor-related emission. It is proposed that (i) the N and Te codoping simultaneously induces N-related defects and donor-type defects along with free acceptors in the ZnO films, but the amount the N-related defects is larger than the donor-type defects and the free acceptors; and (ii) the thermal annealing relatively activates the N-related defects and suppresses the donor-type defects.

We report on the evolution of the surface morphology induced by thermal annealing in N2 ambient over a wide temperature range of 500–1200 °C in single-crystalline ZnO films and ZnO bulks. The surface morphology is seriously changed by the annealing temperature, and the evolution can be categorized into three regions: island growth, island agglomeration, and pit formation. Island growth at low temperatures below 700 °C, is ascribed to the atomic migration to reduce surface energy, which causes surface roughening. Island agglomeration at intermediate temperatures of 700–900 °C is ascribed to the migration and the evaporation of surface atoms, which causes surface flattening. Pit formation at high temperatures above 900 °C is ascribed to the atomic evaporation by high vapor pressure, which causes surface destruction. On the other hand, the bulk lattice is continuously improved with increasing annealing temperature in the temperature regions before the surface-destruction region, which is attributed to the reduction in the numbers of point and line defects caused by recrystallization. As a result, the best surface morphology and the best bulk lattice are obtained at an annealing temperature of 900 °C. The common surface-morphology evolution of ZnO films and ZnO bulks with increasing annealing temperature can be summarized using the three steps of surface roughening by island growth, surface flattening by island agglomeration, and surface destruction by pit formation.

The authors report on a comparative study of photoluminescences for the Ga-/N-faces of a freestanding GaN bulk, fabricated by using hydride vapor-phase epitaxy and self-separation technique. It is found that the Ga-face and the N-face of the same bulk have apparently different optical properties: i) the Ga-face has well-defined excitionic emission lines while the N-face has just broad emission peaks at similar energy levels and ii) the Ga-face is characterized by a transition crossover between DoX (neutral donor-bound exciton) and FXA (A free exciton) in changing temperature while the N-face is characterized by changes in the dominant emission mechanism in the order of DoX, 3.43-eV emission, DoAo (neutral donor-acceptor) pair and eAo (neutral electron-acceptor), and FXA with increasing temperature from 10 K to room temperature. On the other hand, the N-face is shown to have a rougher surface morphology and a larger density of edge dislocations, which is due to structural imperfections generated during the lift-off process. Also, the N-face is shown to have a higher conductivity and a larger electron concentration, which indicates that the N face contains a larger density of donor-type defects. It is suggested that the different emission mechanism of the Ga-/N-faces in the free-standing GaN bulk is ascribed to the fact that the N-face contains a larger density of electrically active radiative and nonradiative recombination centers, which are generated during the self-separation process.

We report on the fabrication and characterization of periodically polarity inverted (PPI) ZnO heterostructures on (0 0 0 1) Al{sub 2}O{sub 3} substrates. For the periodically inverted array of ZnO polarity, CrN and Cr{sub 2}O{sub 3} polarity selection buffer layers are used for the Zn- and O-polar ZnO films, respectively. The change of polarity and period in fabricated ZnO structures is evaluated by diffraction patterns and polarity sensitive piezo-response microscopy. Finally, PPI ZnO structures with subnanometer scale period are demonstrated by using holographic lithography and regrowth techniques.

The growth and annihilation mechanism of pit type defects in HVPE grown thick GaN films was studied using a cross sectional cathodoluminescence (CL) technique. Two kinds of pit type defects were distinguished by their morphology: the hexagonal V-pit surrounded by {10−11} facets and the U-pit with {10−11} facets having a blunt bottom. It was found that the V-pit originated from different growth rates between (0001) plane and {10−11} facet, and was filled and annihilated by {10−12} facets’ growth, namely U-pit generation. The formation of U-pit may play an important role in annihilation of pit type defects.

We investigated the nucleation and growth behavior of GaN on various powders by hydride vapor phase epitaxy. In relative comparison, the nucleation tendency of GaN on each powder can be summarized as AlN > LaN, TiN, NbN > ZrN > ZrB2 > VN, BeO, indicating that the number of nucleation sites increased from right to left. LaN and NbN have not yet been reported as buffer materials for GaN growth. Of these, NbN is expected to be a good buffer material because the interatomic distance on the NbN (111) plane has only 2% difference from that on the GaN (0001) plane.

Department of Nano-semiconductor Engineering, National Korea Maritime University, Busan 606-791, Korea*Department of Offshore Plant Management, National Korea Maritime University, Busan 606-791, Korea**Division of Electrical and Electronics Engineering, National Korea Maritime University, Busan 606-791, Korea***PAN-Xal Co., Ltd., Suwon 443-380, Korea****Center for Interdisciplinary Research, Tohoku University, Sendai 980-8577, Japan

Position controlled single crystalline ZnO nanowires are grown on periodically polarity-inverted (PPI) ZnO heterostructures without catalyst. PPI ZnO templates were fabricated by using selective growth method of polarity with Cr-compound intermediate layers by plasma assisted molecular beam epitaxy. In order to control the position, we used the in-plane two-dimensionally discrete PPI ZnO templates. The lateral polarity inversion in PPI template was confirmed by piezo response microscopy. After syntheses of ZnO nanowires, vertically aligned ZnO nanowires were grown only onto the Zn-polar regions without the O-polar regions. The results clearly show that the position control of ZnO nanowires is possible using the PPI ZnO templates.

Annealed low-temperature GaN layers grown on r-plane sapphire substrates were examined by X-ray diffraction pole figure measurements. The GaN layers were mainly a-plane oriented, in which misoriented grains with four different orientations were detected. The c-axes of the misoriented grains are tilted from the surface normal by about 35°, which are along the bonds not parallel to the c-axis of the a-plane oriented layers. There was a difference among the peak intensities corresponding to the c-planes of the misoriented grains with the four different orientations. Considering the difference, the relative amount of misoriented grains with each orientation can be predicted. The evolution of the misoriented grains is expected to be controlled by the major polarity. [doi:10.2320/matertrans.MA201221]

Photoelectrochemical conversion from light to chemical energy is one of the most promising candidates for renewable energy generation. The most crucial of the many serious problems to be solved for this conversion is the slow carrier transfer at the semiconductor–electrolyte interface. We propose one of solutions using quantum wells, and in the work reported here we evaluated the electrochemical and photoelectrochemical properties of InxGa1–xN/GaN multi quantum well (MQW) structures in the depletion layer. The structures were grown by conventional metal–organic vapor phase epitaxy on (0001) sapphire substrates. Their electrochemical and photoelectrochemical properties differ from those of the bulk materials.

The microstructure of nonpolar GaN layers grown on r -plane sapphire substrates was investigated using transmission electron microscopy (TEM). The structure of the GaN layers was predominantly a -plane oriented single-crystal. In the a -plane oriented layers, misoriented grains about 200 nm in size with a preferential orientation were detected by selecting a proper observation direction. The preferential orientation relationship between the a -plane oriented layers and the misoriented grains was analyzed using atomic structure models, and two possible formation mechanisms of the misoriented grains were suggested.

The composition modulation of in In-rich InxGa1−xN layers with an indium content as high as ∼67% was suppressed in plasma-assisted molecular beam epitaxy on c-sapphire substrates. It was found that the higher nitrogen plasma flow rate was very effective in suppression of composition modulation in In-rich InGaN layers. X-ray diffraction, X-ray energy dispersive spectroscopy, and transmission electron microscopic images clearly showed disappearance of composition modulation with increasing the nitrogen flow rate.

The effects of growth temperatures (in a wide range from 100 to 800 °C) on properties of a-plane (11 2 ¯ 0) ZnO films grown on r-plane (1 1 ¯ 02) sapphire substrates by plasma-assisted molecular beam epitaxy were investigated. The film was grown as polycrystalline at 100 °C, but grown as single crystalline at the temperatures from 200 to 800 °C without any mixture of c-plane (0001) ZnO and m-plane (10 1 ¯ 0) ZnO. The single crystalline ZnO films showed anisotropic surface morphology with the conglomerated granules or striations along the [0001]ZnO direction. The ZnO film grown at 400 °C showed better crystal quality than others. It showed the smallest full width at half maximums of 0.450° and 0.297° for (11 2 ¯ 0) x-ray omega rocking curves under the measuring configuration with the omega axis parallel and perpendicular to the ZnO and directions, respectively, and 0.387° for (10 1 ¯ 1) omega rocking curve. The threading dislocation and the stacking fault densities were determined to be ~ 3.7 × 1010 cm−2 and ~ 8.5 × 104 cm−1 for the ZnO film grown at 400 °C.

We have investigated the role of low temperature (LT)-MgO buffer layer on the quality of ZnO films grown on c-sapphirebyplasma-assistedmolecularbeamepitaxy(P-MBE)method. EffectofMgObufferthicknessand its annealing were evaluated. We found that surface morphology and crystalline quality of the ZnO layers were improved by controlling of the buffer layers. There is no improvement in morphology and crystalline quality of the ZnO layers if the buffer thickness is less than the critical thickness. The critical thickness is determined to be 1.5 nm. Furthermore, surface, structural, optical, and electrical qualities of the ZnO layers were improved by annealing MgO buffer at high temperature. Dislocation density of the ZnO layer was reduced from 5.3 x109 cm-2 to 1.9 x109 cm-2 by annealing the MgO buffer layer. The results indicate that we can engineer defect in highly mismatched heteroepitaxial using buffer layer.

Herein is a discussion of how the structural and optical properties of m-plane GaN (m-GaN) films are affected by the inclination direction of vicinal m-plane sapphire substrate. The m-GaN films were grown on three different types of substrates inclined toward the a-axis direction, the c-axis direction, and with no inclination. We found that m-GaN film grown on an m-plane sapphire inclined in the a-axis direction showed the highest quality with a smooth surface and a low stacking fault density. A model is proposed that shows how the surface step of a substrate can reduce the generation of stacking fault in m-GaN film.

Free-standing GaN thick films are successfully fabricated by employing intentional mechanical fracturing of Al2O3 substrates during HVPE growth followed by chemical lift-off of Al2O3 substrates after growth. To do so, thermal stress induced in the Al2O3 substrates and overgrown GaN layers are controlled by adjusting the thickness of the Al2O3 substrates. The grown layers however consist of multi domains, which deteriorate crystal quality. Domains coalesce to form larger domains as the growth proceed, which results in an improvement in crystal quality. Bending of free-standing GaN thick layers is of concave shape and its typical bending radius is 2.5 m (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the annealing effects of CrN buffer layers on the crystal quality of Zn-polar ZnO films grown by plasma assisted molecular beam epitaxy. The high-temperature (HT) annealing of CrN buffer layer improved the crystallinity of ZnO films. The full width at half maximums of (0002) and (10–11) ZnO ω-scan X-ray diffraction show 574 and 1296 arcsec, respectively, which show the 3 and 2 times narrower values than those of ZnO films without the annealing process. Moreover, the HT annealing can be effective method to improve the surface smoothness of ZnO film and reduce the crystal tilting.

Photoelectrochemical water splitting is expecting a new energy source to produce hydrogen from water. The photoilluminated electrode using a uniform semiconductor single crystal has been well analyzed, but the effect of that using a structural semiconductor is still obscure. The properties of one of the structural semiconductors with a p−n junction in and near a depletion region were investigated using the samples with thin p-type layers on n-type GaN in this article. The properties were also evaluated by the calculation based on the Poisson equation. Not only the band-edge energy of a semiconductor at the semiconductor−electrolyte interface but also the length of the depletion region and the band-edge energy profile affected the photocurrent density from the evaluation. The effect of the energy profile of the depletion region is also discussed.

Many characteristics of epitaxially grown films depend sensitively on their orientations. In most cases, orientation is uniquely determined by that of the substrate, a correlation established by energy minimization. Here, we show that substrate-independent control of orientation is possible by paths available in kinetically limited growth regimes for materials exhibiting polytypism. Demonstrated for GaN, our intriguing results provide fundamentally new insight into controlling one of the most important structural parameters of epitaxial films. This approach might be readily extended to other III-nitrides and ZnO, which also exhibit polytypism. Our procedures can be easily applied to tailor characteristics of epitaxial films, allowing investigations of new properties such as unipolarity.

We investigated the structural properties of Zn-polar ZnO films with low temperature (LT) ZnO and MgO buffer layers grown by plasma-assisted molecular beam epitaxy on (0001) c-Al2O3 substrates using X-ray diffraction and transmission electron microscopy (TEM). The effects of MgO buffer layer thickness and LT ZnO buffer layer thickness were also examined. The optimum thicknesses for better crystal quality were 8 and 40 nm. One-pair and two-pair LT ZnO/MgO buffer layers were employed, and the changes in the structural properties of the high-temperature (HT) ZnO films using such buffer layers were studied. Contrary to the general tendency of c-ZnO films, the HT ZnO films on the LT ZnO/MgO buffer layers showed higher full width at half maximum (FWHM) values for X-ray rocking curves (XRCs) with (0002) reflection than those with (1011) reflection. Compared with the one-pair LT ZnO/MgO buffer layers, the FWHM values of (0002) XRCs markedly decreased, whereas those of (1011) XRCs slightly increased due to the insertion of one more pair of LT ZnO/MgO buffer layers into the previous film with one-pair LT ZnO/MgO buffer layers. The cross-sectional TEM observations with the two-beam condition confirmed that the screw dislocation was the dominant threading dislocation type—a finding that agreed well with the XRC results. On the basis of the plan-view TEM observations, the densities of the total threading dislocations for the HT ZnO films with the one- and two-pair LT ZnO/MgO buffer layers were determined as 2.3 × 109 cm− 2 and 1.6 × 109 cm− 2, respectively. The results imply that the crystal quality of Zn-polar ZnO films can be improved by two-pair LT ZnO/MgO buffer layers, and types of threading dislocations can be modified by adjusting the buffer system.

A new hydride vapor phase epitaxy (HVPE)-based approach for the fabrication of freestanding GaN (FS-GaN) substrates was investigated. For the direct formation of low-temperature GaN (LT-GaN) layers, the growth parameters were optimized: the polarity of ZnO, the growth temperature, and the V/III ratio. The FS-GaN layer was achieved by gas etching in an HVPE reactor. A fingerprint of Zn out-diffusion was detected in the photoluminescence measurements, especially for the thin (80 μm) FS-GaN film; however, a thicker film (400 μm) was effectively reduced by optimization of GaN growth.

Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1)ZnO‖(1 1 1)Si and [1 1 2¯ 0]ZnO‖[0 1 1]Si. Full-width at half-maximums (FWHMs) of (0 0 0 2) and (1 0 1¯ 1) X-ray rocking curves (XRCs) were 1.379° and 3.634°, respectively, which were significantly smaller than the FWHMs (4.532° and 32.8°, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be ∼5×109 cm−2. Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.

Structural and optical properties of nonpolar a -plane ZnO films grown with different II/VI ratios on r -plane sapphire substrates by plasma-assisted molecular beam epitaxy were investigated. Even by increasing the II/VI ratio across the stoichiometric flux condition a consistent surface morphology of striated stripes along the ZnO 〈0 0 0 1〉 direction without any pit formation was observed, which is contrary to polar c -plane ZnO films. Root mean square surface roughness, full width at half maximum values of X-ray rocking curves, defect densities, and photoluminescence were changed with the II/VI ratio. The sample grown with stoichiometric flux condition showed the lowest value of rms roughness, the smallest threading dislocation and stacking fault densities of ∼4.7×10 8 cm −2 and ∼9.5×10 4 cm −1 , respectively, and the highest intensity of D o X peak. These results imply that the stoichiometric flux growth condition is suitable to obtain superior structural and optical properties compared to other flux conditions.

The present study describes an optical resonant cavity in a nanotaper with scale reduction from micro to several nanometers. Both experimental results and a finite-difference time-domain (FDTD)-based simulation suggested that the nanometer-scale taper with a diameter similar to the wavelength of light acted as a mirror, which facilitated the formation of a laser cavity and caused lasing in ZnO nanotapers. As the light inside the nanotaper propagated toward the apex, the lateral mode was reduced and reflection occurred. This report suggests that use of the resonant optical cavities in nanotapers might result in novel active and passive optical components, which will broaden the horizons of photonic technology.

Two-dimensional periodically polarity-inverted (PPI) ZnO structures were fabricated on (0001) Al 2 O 3 substrates using plasma assisted molecular beam epitaxy. Holographic lithography was carried out for the realization of nanometer-scale patterns of CrN which was obtained by nitridation of Cr metal. The PPI ZnO structures consisted of Zn-polar regions grown on CrN and O-polar regions grown on Zn-terminated AlN/Al 2 O 3 . The periodical polarity inversion was confirmed by voltage-piezo response curves and contrast in the piezo response microscopy images. A diffraction pattern of the fabricated PPI ZnO sample was observed when a He–Ne laser beam was incident on the sample surface.

The authors have studied the photoluminescence (PL) intensities of a-plane (11-20) GaN films (a-GaN) as a function of x-ray rocking-curve (XRC) linewidth values measured in both c- and m-axis directions. PL intensity of well-known luminescence lines such as 3.47 eV (bound exciton emission), 3.41 eV (basal-plane stacking fault related emission), and 3.29 eV (defect induced emission) are discussed in terms of XRC linewidth values. PL intensities reveal a close relationship with XRC linewidth measured in the c-axis direction, while an unusual relationship was observed between PL intensity and XRC linewidth in the m-axis direction. Inhomogeneous strain along the m-axis direction of a-GaN film is discussed as a cause of XRC linewidth broadening, rather than the formation of structural defects.

The present study focused on the effect of an intermediate-temperature (IT; ∼900 °C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29×107 cm−2) and a higher edge-dislocation density (8.157×109 cm−2) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.

Microstructural analyses of of ZnO films on (1 1 1) Si substrates grown by plasma-assited molecualr beam epitaxy were performed in this study. Zn pre-deposition and its subsequent oxidation, in which either oxygen gas or oxygen-plasma was used as the oxygen source, were employed before ZnO growth. Both reflection high energy electron diffraction and x-ray pole figure showed the single crystalline features in the ZnO films with both post-oxidation of deposited Zn. Detailed transmission electron microscopy (TEM), however, revealed a locally multi-crystalline feature with 30 degrees-rotated domians at the near-interface regions in the ZnO film with oxidation by oxygen gas. ZnO film with oxidation of pre-deposited Zn by oxygen-plasma was observed to be single crystalline through the whole thickness by TEM. We observed a new epitaxial relationship, (0 0 0 1)ZnO//(1 1 1)Si and [0 1 1¯ 0]ZnO//[1 1¯ 0]Si, with a crystallographic rotation of ZnO with respect to Si by 30 degrees, which is energitically more favorable because of a lower lattice misfit (2.2%). No cracks were observed from the ZnO film with a thickness of 1.5 μm, supporting the mechanical integrity of the film prepared in this study.

The authors investigated the evolution of surface morphology of AlN films grown on ZnO substrates at low temperature (LT) (400°C) as a function of anion/cation supplying ratio (V/III ratio). Unlike the well-known favorable growth conditions for high-temperature growth, smooth-surface LT-AlN layers were obtained under the O-polar surface, stoichiometric, and N-rich conditions. LT-AlN layers revealed smooth surface (roughness in root mean square=0.20nm for AlN on O-polar ZnO and 0.44nm for AlN on Zn-polar ZnO) and quite low etch-pit density (∼2×106cm−2 for AlN∕Zn-polar ZnO).

A c -oriented GaN film was grown on a r -plane sapphire substrate by utilizing hydride vapor phase epitaxy. This non-typical orientational relation between the film and the substrate was possible by spontaneous transition in preferred orientation, and voids were spontaneously formed in an orientation-transition layer. This c -oriented thick GaN film was then self-separated from the substrate during cool-down process to room temperature after the growth. This intriguing phenomenon of the self-separation of crack- and strain-free GaN is attributed to the spontaneous formation of voids during the preferred orientation-transition, and suggests a novel method to fabricate crack- and strain-free GaN free-standing templates.

Interface and defect structures in ZnO films grown on ( 1 0 1 ¯ 0 ) m -plane sapphire substrates by plasma-assisted molecular beam epitaxy were investigated by transmission electron microscopy (TEM). Distribution of misfit dislocations (MDs) at the ZnO/Al 2 O 3 interfaces was orientation dependent and quite anisotropic. In addition, type-І 1 stacking faults (SFs) bounded by the Frank dislocations with the Burgers vector of 1 / 6 [ 2 0 2 ¯ 3 ] were observed to be predominant. Many SFs initiated from the interface to the film surface were observed in addition to the MDs at the interface. The analysis of the high-resolution TEM revealed no correlation between the SFs and the MDs. The ZnO films were not found to be pure m -plane ZnO and ( 1 0 1 ¯ 3 ) domains were frequently observed in the film with otherwise mostly ( 1 0 1 ¯ 0 ) m -plane in nature. These ( 1 0 1 ¯ 3 ) domains were not nucleated at the interface but initiated inside the ZnO film above the interface and reached the top surface.

A single crystal and polycrystals of GaN grown by the Na-flux method were photoelectrochemically characterized. The flatband potential of the GaN single crystal was approximately 0.2 V anodic of the potential of the GaN layer grown by metal-organic vapor phase epitaxy. Clear photo-electrochemical response was observed for the GaN single crystal. The turn-on slopes of the photocurrent for the poly-crystalline samples were gentler than the slope for the single crystal. The turn-on slopes were probably affected by the high resistivity of grain boundaries in the polycrystalline samples.

Individual ZnO wires were deposited on two closely spaced electrodes in a microfluidic channel by using the positive dielectrophoretic (p-DEP) alignment technique. By using the p-DEP technique in the presence of a microfluidic force, efficient and rapid alignment of single wires was achieved; this was a result of spatial confinement, uniform wire orientation in the channel, and the competition between microfluidic and DEP forces. The technique helped achieve a capturing efficiency of up to 86% within a few tens of seconds. In addition, we fabricated an electrochemically gated field-effect transistor (EC-FET) using single ZnO wires. The conductivity of the as-aligned wire was enhanced by carrying out post-treatments such as the deposition of metal layers at both ends of the wire, annealing, and functionalization with dodecanedioic acid. By applying a voltage across the electrolyte, it was confirmed that the fabricated EC-FET had stable n-channel FET characteristics.

We have studied on the polarity selection procedure of ZnO grown on CrN buffer by using X-ray photoelectron spectroscopy (XPS). XPS studies have been performed on the O- and Zn-treated CrN/Al 2 O 3 surfaces and revealed that Cr 2 O 3 and Zn-chromate-like structures are formed on O- and Zn-treated CrN surfaces, respectively. The polarity selection procedure is explained in terms of the variation of bonding coordination by the formation of ZnO on the topmost O- and Zn-atoms of each surface.

In-doped ZnO (ZnO:In) nanocrystals with various In-contents have been successfully synthesized by vapor phase condensation of mixed source. Hexagonal phase ZnO:In samples with high In-concentration (∼25 at.%) were easily obtained and structural phase transformation was observed at ZnO:In nanocrystal with an In-concentration of ∼35 at.%. Near band edge (NBE) emission shifted to lower energy side and increase of emission intensity were observed from photoluminescence (PL) measurement, which was explained in terms of the increase of extrinsic carrier concentration. Diffuse-reflectance spectra (DRS) show free-carrier absorption band at 0.6-2.0 eV, which corresponds with the PL results.

The role of a NH4Cl layer grown between GaN and a sapphire substrate for self-separation of GaN films was investigated in detail. Microstructural analysis of the NH4Cl layer reveals that self-separation of a GaN film was effectively assisted by the sub-micrometer-sized voids formed due to etching of GaN by evaporated NH4Cl. An areal fraction of nonetched GaN in contact with sapphire substrate was found to be linearly dependent on the thickness of NH4Cl layer and to have two different proportionality constants below and above an NH4Cl thickness at which GaN begins to be self-separated. This intriguing behavior was well explained by a simple model that takes account of void merging.