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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

A ZnO nanorod (NR) bridge structure has been synthesized across geminated Si substrates (on which various metals were pre-deposited) by one-step thermal evaporation. The photoresponse and current–voltage ( I – V ) properties of a ZnO nanorod detector were observed at room temperature. By photocurrent spectroscopy, strong near-band-edge (NBE) absorption at the UV region was observed. The presented results demonstrate the feasibility of spontaneous formation of a novel device based on ZnO NRs without a high-cost complex process.

In doped ZnO nanorods ( ZnO : In NRs) were grown on Si (111) substrates by using AuGe as a catalyst. In-concentration was controlled as high as 0.33 at.%. The length L to diameter D relationship of NRs revealed that the growth mechanism should be explained in terms of not a simple vapor–liquid–solid (VLS) mechanism but a diffusion-induced growth model. In-induced changes have been investigated from the viewpoint of photoluminescence (PL) property. At room temperature, an increase of extrinsic carrier concentration and interaction with excitons were estimated from the blueshift of peak position and decrease of luminescence intensity. Also, narrow linewidth of low-temperature PL spectra characterized the high structural quality of ZnO : In NRs. We have assigned several PL lines and confirmed the assignment by using excitation power and temperature dependence PL. Considerable evidence of crystal quality degradation was not observed from both measurements; however, we could observe that a defect and donor-bound-exciton complex dominate the PL spectrum. ZnO : In NRs shows good optical quality within our experimental range, and this result strongly supports the feasibility of In -doping to enhance the electrical property of ZnO -based nanostructure.

Recently, an innovative method of fabricating vertical light-emitting diode (LED) was introduced by the chemically lift-off (CLO) process. Such fabrication of the vertical LED could be realized by inserting the CrN interlayer between sapphire substrate and gallium nitride (GaN) layer. In this paper, by using off-cut sapphires, we tried to find out the influence of off-angles on the overgrown GaN films with respect to crystal quality and surface roughness. The CrN is formed by nitridizing the deposited Cr layer on the sapphire substrate at 1080 °C, and then GaN was grown on the nitrided Cr. By X-ray diffraction system (XRD) measurements, the crystal quality tendency of the GaN film on the variation of the off-angles was similar with that of the CrN interlayer. Through high-resolution transmission electron microscopy (HRTEM) measurement, it was found out that AlN layer with a thickness of a few nanometers also formed between sapphire and CrN during the nitridation treatment of Cr layer. Compared with the previous reports, such tendency of the crystal quality for the vicinal sapphires with CrN interlayer was well matched with the tendency of the previous results in the vicinal sapphire without the CrN interlayer. Therefore, it can be surmised that the AlN layer between sapphire and CrN was precedently influenced by the vicinal substrate and then, the AlN layer had an influence on the upper layers of GaN as well as CrN. In addition, it was found that the vicinal sapphires with the off-angles of 0.2–0.3° had the minimum full-width at half-maximum (FWHM) values by XRD measurements, which were lower by 20% than the others.

Existence of a p-n junction in a depletion layer is expected to change band energy distribution. The structures of thin p-type GaN on n-type GaN were investigated for the working electrode of the photoelectrochemical reactions. When the p-type layer thickness was thinner than the depletion layer, lower flatband potential was observed. The photocurrent density and the H2 gas generation also increased when the n-type GaN working electrode with thin p-type layer was used.

We have studied the molecular beam epitaxy (MBE) of GaSb films on GaAs (0 0 1) substrates by using ZnTe as a new buffer layer. GaSb films were grown on two distinct ZnTe surfaces and the influence of surface chemical composition of ZnTe on the morphological and structural properties of GaSb films has been investigated. Initial 2-dimensional (2D) growth of GaSb films is obtained on Zn-terminated surface consequently smooth morphology and high crystal quality GaSb films are achieved. The thin GaSb film (0.4 μm) grown on Zn-terminated ZnTe surface reveals considerably narrow X-ray diffraction linewidth (113 arcsec) along with small residual strain, which strongly supports the availability of ZnTe buffer for the growth of high-quality GaSb film.

Nanostructured ZnO films were fabricated on nanopored Si(001) surfaces through reactive ion etching and plasma-assisted molecular-beam epitaxy techniques. Nanowall-like nanostructures were formed on the ZnO film surfaces depending on the thickness of the ZnO films. Significant enhancement of the photoluminescence intensity up to 15 fold was observed from the nanostructured ZnO films. We found that the emission properties of the film changed very sensitively with the nanostructure on the surface and the enhancement is closely related to the formations of nanostructures on the surface of the ZnO films, not at the interface between the films and the substrate. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Periodically polarity inverted (PPI) ZnO templates were fabricated using molecular beam epitaxy by employing MgO buffer layers. The polarity of ZnO film was controlled by the transformation of crystal structure from hexagonal to rocksalt due to the thickness of the MgO buffer layers. The polarity of ZnO in the PPI template was confirmed by AFM and PRM measurement. Higher growth rate and lower current value under positive supplied voltage in the region of Zn-polar were measured with comparing to that of O-polar. Holographic lithographic technique was employed for the realization of submicron pattern of periodical inverted polar ZnO over large area. After reaction using a carbothermal reduction, spatially well-separated ZnO nanorods with pitch of submicron were only observed in the Zn-polar regions. The possible reason for the difference of surface characteristics was considered as being due to the configuration of dangling bonds according to polarity.

Thick GaN films were grown on two different low-temperature GaN (LT-GaN)-buffer layers, i.e., one- and two-step LT-GaN buffer, by hydride vapor-phase epitaxy (HVPE). NH 4 Cl layer was included in two-step LT-GaN, which was evaluated by X-ray diffraction patterns. Many voids were observed at the interface for both samples. We will discuss the mechanism of void formation for both samples. The full-width at half-maximum (FWHM) values of (0 0 0 2) ω -rocking curves were 390 and 440 arcsec for 200-μm-thick GaN on one- and two-step LT-GaN, respectively. The 200-μm-thick high-temperature GaN (HT-GaN) film on two-step LT-GaN was self-separated without any cracks after cooling down. NH 4 Cl layer included into two-step LT-GaN buffer was effectively contributed to the voids formation and self-separation in realizing a stress-free free-standing GaN (FS-GaN) substrate.

We have investigated optical properties and electrical properties of electrically degraded ZnSe layers by heavy Al doping, where their electron concentrations lie in the range of 7×1017–6×1018cm−3. Low-temperature photoluminescence exhibits two dominant radiative trap centers of 1.97eV (RD1) and 2.25eV (RD2), which are ascribed to VZn-related complex defects. Deep-level-transient spectroscopy shows two electron-trap centers at 0.16eV (ND1) and 0.80eV (ND2) below the conduction-band minimum. On the other hand, it is found that RD2 is dominant in relatively lightly doped ZnSe:Al layers below 7×1018cm−3 and RD1 is dominant in more heavily doped layers near 1×1019cm−3, while ND1 and ND2 are independent of Al doping concentration and their trap densities are estimated be below 3×1016cm−3. This indicates that RD1 and RD2 cause the carrier compensation in heavily doped ZnSe:Al layers. Their electron transport mechanism can be explained by ionized-impurity scattering mechanism.

Low-temperature growth of high quality homoepitaxial ZnO is realized by using hydrogen irradiation and annealing processes (modified method). By modified method, two-dimensional growth and atomically smooth surface with steps (terrace length ∼75 nm) are achieved at 400 °C. Furthermore, FWHMs of high resolution X-ray rocking curves for (0 0 0 2) and (10–11) reflections are evaluated as narrow as 21 arcsec, which indicates that high crystallinity of the ZnO grown at 400 °C by modified method is almost similar to that grown at 600 °C by conventional method. Photoluminescence results show the considerable improvement of optical properties, such as an emersion of free exciton (FX) and a decrease of donor bound exciton (D°X) linewidth, by using modified method, even at growth temperature as low as 400 °C.

ZnO has hexagonal wurtzite structure that is characterized by a mixture of ionic and covalent bondings between Zn and O atoms. A Zn atom is surrounded by four O atoms, or vice versa, which are located at the edges of a tetrahedron. Such coordination gives rise to polar symmetry along the c-axis of the hexagonal structure. The polarity affects electrical, optical, mechanical, and chemical properties of ZnO. In 2000, we have reported the successful selective growth of polarity-controlled ZnO films by molecular-beam epitaxy (MBE) using either GaN templates or MgO buffer. We confirmed different growth behaviors on the surfaces of ZnO layers with different polarities. Recently, we have explored the effect of crystal polarity on the formation of ZnO nanostructures. The periodically polarity-inverted (PPI) ZnO templates with submicron line-pitch over a large area were fabricated using MBE by employing MgO buffer layers and lithography technique. We grew ZnO nanorods using chemical transport and condensation method. The ZnO nanorods grew only on Zn-polar regions with the same periodicity as that of PPI template, which eventually produces well-aligned ZnO nanorod arrays.

Non-polar, A-plane ( 1 1 2 ¯ 0 ) ZnO films are epitaxially grown on R-plane ( 1 1 ¯ 0 2 ) Al2O3 substrates by plasma-assisted molecular-beam epitaxy and structural and optical properties are investigated. Epitaxial relationships between ZnO films and Al2O3 substrates are determined to be ZnO ( 1 1 2 ¯ 0 ) / / Al 2 O 3 ( 1 1 ¯ 2 0 ) , ZnO [ 1 ¯ 1 0 0 ] / / Al 2 O 3 [ 1 1 2 ¯ 0 ] . The misfit is relaxed by the regularly spaced misfit dislocations. Structural properties are anisotropic and surfaces of films show stripes running along the ZnO [ 0 0 0 1 ] direction. Full width at half maximums of X-ray rocking curves for the on-axis ( 1 1 2 ¯ 0 ) with φ=0° and 90°, and the off-axis ( 1 0 1 ¯ 1 ) reflections are 0.41°, 0.36°, and 0.39°, respectively, for the 300 nm-thick ZnO film. Near-band edge emissions are dominant in low-temperature (12 K) photoluminescence while emissions from deep levels are negligible indicating a high optical quality of the layers. The near-band edge emissions are strongly polarized perpendicular to the [ 0 0 0 1 ] axis. Transitions of neutral-donor bound excitons (D0X) and allowed free excitons are observed in photoluminescence and photoreflectance spectra, which are significantly blue-shifted compared to the transitions in C-plane ZnO.

The photoelectrochemical and electrical properties of nonpolar (1120)-oriented and semipolar (1122)-oriented GaN were compared with those of (0001)-oriented GaN. Flatband potentials were obtained in the order of (1120)

Deep-level luminescence at 1.93 eV (red luminescence) was observed from GaN grown by the acidic ammonothermal method. Typical deep emission at 2.2 eV (yellow luminescence) was not observed. The intensity decreased linearly with increasing temperature, which is different from the red and yellow luminescence reported thus far. The time decay of the luminescence appeared partially exponential, i.e. with the both possibilities of a simple two-level transition and donor–acceptor pairs. The origin of the red luminescence is probably different from the previously reported one [J. Appl. Phys. 97, 061301 (2005)]. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the structure study of MgxZn1−xO films grown by plasma-assisted molecular beam epitaxy (P-MBE) on ZnO/MgO/c-sapphire template in a wide range of 0

We report on the MBE growth of GaSb layer by using three-step ZnTe buffer layer: a thin ZnTe buffer layer was grown at 250 °C (LT-buffer), then annealed at 330 °C (HT-annealing) and a thick buffer layer was grown at 310 °C (HT-buffer). The effect of a three-step buffer on the surface and structural quality of the GaSb layer has been investigated. Two-dimensional reciprocal space mapping (RSM) results of the GaSb layers clearly indicate that the structural deformation of the GaSb layer is greatly reduced by introducing the ZnTe buffer, which remarkably improves the crystallinity of the GaSb layer.

Using ion implantated material as a growth source, implant source growth, is an alternative growth method where ion implanted atoms diffuse to the surface and can be used to form nanostructures. By introducing a small amount of a steel based contaminate we were able to grow high quality and high density GaN nanodots on Si(111) which showed strong excitonic UV luminescence. Furthermore by varying the implanted Ga dose and annealing conditions, the size of the nanodots can be controlled. This method for producing GaN nanostructures may be useful for optical or electronic device applications. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thick GaN films were grown by HVPE (Hydride Vapor Phase Epitaxy) on c-sapphire substrates with GaN nano-rod buffer layers. Lateral epitaxial growth mode was adapted to grow GaN thick films on nano-structure buffer. Thick GaN films were self-separated during cooling down by thermal stress caused by the difference of thermal expansion coefficient (TEC) between GaN and sapphire. Since the nano-rod buffer consists of nano-rods and voids, it is mechanically weaker than planar GaN layers and contributes to the self-separation of GaN thick films. 200 μm-thick free-standing GaN substrates show smooth surface morphology without any microcracks. The full width at half maximum (FWHM) of (0002) X-ray rocking curve is 619 [arcsec]. Donor-bound exciton emission is observed at 3.4718 eV in low-temperature photoluminescence spectra, which is nearly the same peak position as bulk GaN. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In order to understand the origin of leakage current, light emitting devices were grown on two different templates with apparently different dislocation density: one on thin GaN template (∼2 μm) with higher dislocation density (low × 109 cm–2) prepared by metal-organic vapor-phase epitaxy (sample A), and the other on thick GaN template (∼20 μm) with comparatively low dislocation density (high × 108 cm–2) by hydride vapor-phase epitaxy (sample B). Especially, the template B showed very low value of the dislocation density for a screw component, 2.2 × 107 cm–2 evaluated by transmission electron microscope and 2.3 × 107 cm–2 approximated by the Williamson-Hall plot which was evaluated by high resolution X-ray diffraction, respectively. On the other hand, sample A showed one order higher, low × 108 cm–2, than that of sample B for a screw component. Sample A showed the larger leakage current (more than two orders of magnitude) than sample B in a forward-biased region and a reverse-biased region also. It is expected that the screw dislocation were strongly contributed to the leakage current of forward and reverse I-V regions in LEDs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The ZnO-based oxide and GaN-based nitride semiconductors have been explored for applications to photonic devices in the UV wavelength region and high power electronic devices, where the control of crystal polarity is one of the key issues. This paper will deal with the control of crystal polarity in ZnO/GaN heterostructures and ZnO/sapphire heterostructures by interface engineering.

Well-aligned ZnO nanorods have been achieved using new alloy (AuGe) catalyst. Zn powder was used as a source material and it was transported in a horizontal tube furnace onto an AuGe deposited Si substrates. The structural and optical properties of ZnO nanorods were characterized by scanning electron microscopy, high resolution X-ray diffraction, and photoluminescence. ZnO nanorods grown at 650 °C on 53 nm thick AuGe layer show uniform shape with the length of 8±0.5 μm and the diameter of 150±5 nm. Also, the tilting angle of ZnO nanorods (±5.5°) is confirmed by HRXRD. High structural quality of the nanorods is conformed by the photoluminescence measurement. All samples show strong UV emission without considerable deep level emission. However, weak deep level emission appears at high (700 °C) temperature due to the increase of oxygen desertion.

Tetrapod-shape ZnO nanostructures are formed on Si substrates by vapor phase transportation method. The effects of two important growth parameters, growth temperature and VI/II ratio, are investigated. The growth temperature is varied in the range from 600 °C to 900 °C to control the vapor pressure of group II-element and the formation process of nanostructures. VI/II ratio was changed by adjusting the flux of carrier gas which affects indirectly the supplying rate of group VI-element. From the scanning electron microscopy (SEM), systematic variation of shape including cluster, rod, wire and tetrapod was observed. ZnO tetrapods, formed at 800 °C under the carrier gas flux of 0.5 cc/mm2 min, show considerably uniform shape with 100 nm thick and 1 ∼ 1.5 μm long legs. Also stoichiometric composition (O/Zn ∼ 1) was observed without any second phase structures. While, the decrease of growth temperature and the increase of carrier gas flux, results in the irregular shaped nanostructures with non-stoichiometric composition. The excellent luminescence properties, strong excitonic UV emission at 3.25 eV without deep level emission, indicate that the high crystalline quality tetrapod structures can be formed at the optimized growth conditions.

Recently, we have shown that hard X-ray photoemission spectroscopy using undulator X-rays at SPring-8 is quite feasible with both high resolution and high throughput. Here we report an application of hard X-ray photoemission spectroscopy to the characterization of electronic and chemical states of thin solid films, for which conventional PES is not applicable. As a typical example, we focus on the problem of the scatter in the reported band-gap values for InN. We show that oxygen incorporation into the InN film strongly modifies the valence and plays a crucial role in the band gap problem. The present results demonstrate the powerful applicability of high resolution photoemission spectroscopy with hard X-rays from a synchrotron source.

Ultraviolet anti-Stokes photoluminescence (ASPL) is observed under continuous visible light excitation in a GaN single crystal at 4 and 77 K. The ASPL spectra are almost the same as the Stokes photoluminescence (SPL) spectra under band-to-band excitation. Namely, we observe ASPL due to bound and free excitons at 4 and 77 K, respectively. Based on the temporal decay, the excitation spectrum and the excitation intensity dependence of the ASPL intensity, we propose a two-step two-photon absorption mechanism through an electron trapped at a deep ionized donor with a lifetime of 1.45 ms.

We report on the electrical properties of ZnO/GaN heterostructures and the photoresponsivity of ZnO Schottky barrier diodes for the applications of heterojunction transistors and ultraviolet photodetectors, respectivly. ZnO/GaN heterostructures exhibit a large plateau region of 6.5 V in 10 kHz capacitance-voltage curves. Moreover, it is found that a high electron density of ∼1018 cm–3 is accumulated at the heterointerface in depth profile. ZnO Schottky barrier diodes show a rapid increase of photocurrent of ∼102 A at the long-wavelength cutoff of 390 nm with maitaining stable diode characteristics. And, it is observed that ZnO Schottky barrier diodes respond to photosignal within 1 ∼ 2 msec with a time constant of 0.35 msec. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

物質によっては，X線を照射すると紫外•可視域のX線ルミネセンス光が生じる．本研究では，ZnOエピタキシャル薄膜（ZnO/MgO/Al2O3）を試料に用い，X線ルミネセンス強度の角度依存性の測定を行った．同時にZn Kα線測定による蛍光X線強度（蛍光X線ホログラム）の測定も行い，それとの比較を行った．X線ルミネセンス測定においては，Znの蛍光X線ホログラムと異なり，サファイア基板の構造を反映したホログラムパターンが得られた．この結果より，X線ルミネセンスを用いれば，従来困難であった軽元素の原子分解能ホログラムを測定できる可能性が示された．