Search

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)

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.

We propose a rocksalt (RS) structured chromium nitride (CrN) layer as a novel buffer for the growth of gallium nitride (GaN) films on c-sapphire substrate. A RS-CrN buffer has good advantages for growing GaN films on c-sapphire from the viewpoints of lattice mismatch and thermal expansion coefficient. The RS-CrN layer has mid value of both the lattice constant and the thermal expansion coefficient of GaN and c-sapphire. It is verified that the lattice mismatch between CrN and c-sapphire of 6.2–6.7%, which is experimentally estimated from reflection high-energy electron diffraction (RHEED) characterization, about 6.2–6.7%, agree well with the calculated value of 6.6%. By these advantages, N-polarity GaN epilayers grown on RS-CrN buffer layer show remarkably improved crystal quality when compared with those of grown on conventional low temperature (LT) GaN buffer using molecular beam epitaxy (MBE). For a further improvement of GaN crystallinity, we performed the surface treatment of Ga-exposure on CrN buffer layer. The Ga-exposure treatment on the RS-CrN buffer layer drastically improved the crystal quality of upper GaN epilayer, which was identified by measuring structural and optical properties using X-ray diffraction (XRD) and Photoluminescence (PL), respectively. The improved crystallinity is resulted not from the polarity conversion but from the stress relaxation effect by a metallic Ga interlayer. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ordering of In and Ga in In 0.53 Ga 0.47 As films grown at 573 K, 673 K, and 773 K by molecular beam epitaxy was investigated by electron diffraction pattern analysis using a transmission electron microscope equipped with an Ω-filter and imaging plates. In addition, high-resolution electron microscopy on the specimens and fast Fourier transformation analyses were performed to identify the short-range ordering. In the EDPs obtained from the specimen grown at 573 K,the diffuse scattering corresponding to short-range ordering was observed only when the film was investigated at [110] beam incidence, whereas for the specimens grown at 673 and 773 K, diffuse scattering was observed only at [110] beam incidence. The ordering of 573 K specimen has a triple period and those of 673 K and 773 K have a double period. Through the processing of the HREM images and comparison of calculated and observed diffuse-scattering distribution, models of short-range ordered structures were proposed on the basis of the triple-A type ordering at the specimen grown at the 573 K and the CuPt-B type ordering at the specimen grown at 773 K.

By using implant source growth, an ion beam synthesis technique where implanted ions are used as a growth source for nanostructures, we have fabricated self-assembled GaN nanodots on SiO 2 . The fabrication method consists of implanting Ga ions into a 60 nm thick thermally grown SiO 2 layer by using a focused ion beam system followed by annealing under an NH 3 flux. The morphology, crystal structure, and optical properties of the GaN nanodots are also shown and discussed.

We report on GaN growth on Zn-polar ZnO substrates using plasma-assisted molecular-beam epitaxy (P-MBE). Before GaN growth, ZnO substrate annealing conditions were optimized. Reflection high-energy electron diffraction (RHEED) patterns after low-temperature GaN buffer layer annealing changed from streaky to spotty, suggesting that zinc and oxygen atoms interdiffuse from the ZnO substrate into the GaN epilayer. This interdiffusion results in a mix-polar GaN epilayer.

We report growth optimization of molecular beam epitaxy (MBE) grown ZnSe on GaAs (0 0 1) substrate tilted by 15° toward [1 1 0] in terms of beam equivalent pressure (BEP) ratio and growth temperature. A LT-ZnSe buffer was grown to reduce the formation of Ga–Se bonding, a well-known source of defect generation, due to interdiffusion through the heterointerface in the initial stage of growth. The ZnSe layer was further optimized by low-temperature-grown (LT-ZnSe) buffer. The optical and structural properties of the ZnSe film with LT-ZnSe and GaAs buffer are also analyzed by photoluminescence spectroscopy (PL), X-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS), which show very large intensity ratio of near-band-edge emission to deep level emission, narrow XRD peak width of (0 0 4) rocking curves, and abrupt ZnSe/GaAs heterointerface under the optimum growth condition, respectively. The optimum growth conditions are BEP ratio ( P Se / P Zn ) of 3 and growth temperature of 310°C with an LT-ZnSe buffer grown at 250°C.

Four types of GaN-based light-emitting diodes (LEDs) with V-pits formed in different regions were grown by metal-organic chemical vapor deposition. The position of the V-pits embedded in the layers of the LED structures was controlled by varying the growth temperature. We achieved the highest output power and lowest leakage current values with the LED structures comprising V-pits embedded in active regions and the p-GaN textured surface. The V-pit formation enhances the light output power and reverse voltage values by 1.3 times the values of the conventional LED owing to the enhancement of the light scattering probability and the effective filtering of threading dislocations.

This article demonstrates the effect of the thin low-temperature-grown ZnSe buffer layer (LT-ZnSe) in improving crystallinity of ZnSe-based films grown on GaAs substrate. Especially, the density of stacking fault defects, which is normally observed in ZnSe-based films grown on GaAs substrate, can be well reduced by inserting a thin ZnSe buffer layer grown at sufficiently low temperature between ZnSe/GaAs heterostructure. A ZnSe film with stacking fault densities as low as ∼2×10 6 /cm 2 was obtained by growing on the LT-ZnSe/GaAs buffer layers, in contrast, ∼8×10 8 /cm 2 was obtained by directly growing on GaAs substrate. The Frank-type stacking faults was dominant for films grown with LT-ZnSe, meanwhile, Shockley-type stacking faults was dominant for films grown without LT-ZnSe buffer. At the initial stages of ZnSe growth, three-dimensional growth mode is considerably suppressed with LT-ZnSe buffer. The improvement of crystallinity in the ZnSe film with LT-ZnSe and GaAs buffer is also evidenced by low-temperature photoluminescence spectroscopy and high-resolution X-ray diffraction, which show very large intensity ratio of near-band-edge emission to deep level emission and narrow X-ray diffraction peak width of (0 0 4) rocking curve with 92 arcsec, respectively.

We have studied the growth and properties of ZnSe/BeTe superlattices by molecular beam epitaxy for two interface configurations, which are basically characterized by a “ZnTe” and a “BeSe” interface layer. The interface properties for the two interface configurations have been studied by means of reflection high energy electron diffraction, high resolution X-ray diffraction, and photoluminescence. It is shown that the optical and structural properties of ZnSe/BeTe superlattices are improved with a ZnTe interface compared to a BeSe interface.

We report a novel structure of a selectively grown buried-ridge (SGBR) type nitride-based laser diode structure grown on sapphire and lateral epitaxial overgrowth (LEO) substrates by metal-organic chemical vapor deposition (MOCVD). The AlGaN layers were designed for current confinement into ridge as well as to enhance lateral optical confinement by introducing an index difference between MQW and AlGaN layers in the lateral direction. The I–V characteristics of SGBR structure on LEO substrate showed no leakage current upto a reverse bias of −10 V. From an EL image through transparent metal, we demonstrated that the current path was well defined into the ridge region.

Photoelectrochemical water reduction using n-type GaN in electrolytes with/without alcohol was investigated. For both cases, hydrogen evolution from counterelectrode was observed without bias. Hydrogen gas amount using 1.0 mol/L NaOH electrolyte with alcohol was approximately double compared to that without alcohol where the total current flow charges are similar at 300 min. The amount of hydrogen evolution was almost coincidence with the amount calculated from the total charge flow. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The power dependence and the temperature dependence of the luminescence intensity of ultra-thin pseudomorphic ZnSe quantum wells, grown on high index GaP substrates were investigated. A blue shift of PL with power excitation is mainly related to the band filling of interfacial ZnSe clusters while it is found that the piezoelectric field has only a small influence. Luminescence spectra of ultra-thin ZnSe/ZnS single quantum wells are characterized by an excitation dependent excitonic localization phenomenon.

GaN films were grown on ZnO substrate under various beam equivalent pressure ratios by plasma-assisted molecular beam epitaxy (P-MBE). We theoretically calculated the thermal stress caused by the difference of thermal expansion coefficients between GaN and ZnO. The changes of stress and critical thickness were evaluated by measurement of XRD for HT GaN and LT GaN buffer grown under Ga-rich and N-rich conditions. From this study, we observed that GaN grown under Ga-rich condition causes GaN film to under compressive-stress, while GaN grown under N-rich condition was tensile-stressed. Consequently, interdiffusion has no effect on the variation of the critical thickness.

Optical properties of ZnO thin films with/without MgO-buffer annealing were investigated by low and room temperature photoluminescence measurements. The ZnO films were grown on c -sapphire substrates by plasma-assisted molecular-beam epitaxy employing a thin MgO-buffer layer. Dislocation density of ZnO layer was reduced from 5.3 × 10 9 to 1.9 × 10 9 cm −2 by annealing MgO-buffer prior to the growth of ZnO. The intensity of free exciton emission from the sample with MgO-buffer annealing was almost twice of that from the sample without annealing, while the deep level emission from the sample with MgO-buffer annealing was about 1/3 of that without annealing. The MgO-buffer annealing improves optical quality of overgrown ZnO films.

In this paper GaNxAs1−x surfaces during growth are observed using reflectance difference or reflectance anisotropy spectroscopy (RDS or RAS). The epi-layer was grown by solid-source molecular beam epitaxy (MBE) system with a RF nitrogen prasma source. RD spectra showed broader structure and reduced amplitude compared to those of GaAs surfaces; GaAs(2 × 4)-like features were still observed with weak and blue-shifted peaks. In the low growth temperature region, an extra structure was also observed around 3.02 eV. We proposed that GaNxAs1−x surface can be classified into three types of the surface.

A laser diode which includes II–VI device structure directly grown on III–V device structure is proposed. This idea makes possible one-chip-multiple-wavelength operation in the visible wavelength region by the vertical integration of individual light emitters, which is demonstrated by a ZnCdSe/ZnSe/ZnMgBeSe quantum-well structure for a blue-green light emitter grown by molecular beam epitaxy directly on a metalorganic chemical vapor deposition grown InGaP/InGaAlP device wafer for a red light emitting device. The feasibility of the II–VI/III–V complex light emitter is demonstrated by optical-pumping experiments. Optically pumped lasing at 504 and 664 nm is achieved from II–VI and III–VI laser structures on one chip simultaneously at room temperature with a threshold power of 115 and 84 kW/cm2, respectively. The present results clearly show the feasibility of epitaxial integration of II–VI and III–V laser structures, which will stimulate the investigation of multifunctional optical devices.

Large-sized and high-quality free standing GaN are required with the development of GaN-based devices. We have developed new techniques to reduce the price of GaN substrates. In this paper, we introduce a simple fabrication way of freestanding GaN substrate using hydride vapor phase epitaxy (HVPE). An evaporable buffer layer was applied for the fabrication of 2inch freestanding GaN to separate from a sapphire substrate, in other words, a freestanding GaN was fabricated only by HVPE (one-stop process) without any process.

A 150 μm-thick GaN layer was grown by halide vapor phase epitaxy utilizing selective lateral overgrowth on a SiO2-prepatterned sapphire substrate. A series of optically active regions above the SiO2 mask was observed in cross sectional monochromatic cathodoluminescence images taken at 367 nm. These bright regions were, however, consistently terminated by triangular shaped domains at 60 to 80 μm thickness, leaving no sign of luminescence nonuniformity beyond the thickness. In conjunction with the recent results on the characteristics of inversion domains in GaN, we proposed that these triangular regions might be inversion domains.

ZnO nanorods were synthesized by chemical solution method at low temperature. The surface of nanorods was changed to porous by using thermal annealing and chemical etching. Surface morphology and their optical properties were changed according to annealing and etching condition. Photoluminescence from as-grown ZnO nanorods almost showed defect related emission in wide range from 450∼900 nm. After annealing at 500°C, the band-edge emission of ZnO was observed and emission at visible range was changed to green with decreasing red-orange. The surface morphology of ZnO nanorods was transformed to porous by chemical etching and it led to increase the intensity of band-edge emission about three times. The internal quantum efficiency for porous ZnO nanorods, which was calculated from ratio PL intensity at 10K and 300K, is about 21%. Also, the random lasing at porous ZnO nanorods was occurred at high optical excitation by a photon with traveling inside or outside of porous ZnO nanorod gets amplified by injection second photon before leaving porous ZnO nanorods.

Epitaxial ZnO films are successfully grown on Al2O3 substrates with phase controlled CrN buffer layer using Zn and O-plasma pre-exposures on CrN layers by plasma assisted molecular beam epitaxy (P-MBE). The Zn exposures on CrN layers prior to ZnO film growth result in the formation of rocksalt CrN without surface oxidation. On the other hand, the surface of the initially deposited CrN layers with rocksalt structure changes into hexagonal structured Cr2O3 after O-plasma exposure as confirmed by reflection high-energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HR TEM). Etching studies show that the ZnO films grown on CrN have +C polarity, while the polarity of ZnO on Cr2O3/CrN double buffer is -C polarity. The interdiffusion of Zn and Cr occurs at the ZnO/CrN interface, while the interdiffusion is negligible at the ZnO/ Cr2O3 interface. The interdiffusion of Cr and Zn can be suppressed by inserting a low-temperature ZnO buffer layer in between ZnO and CrN layers, which helps improve the crystal quality of ZnO layers grown with CrN buffer.

High crystalline a-plane (112¯0) GaN epitaxial layers with smooth surface morphology were grown on r-plane (11¯02) sapphire substrate by metalorganic chemical vapor deposition. The full width at half maximum of x-ray rocking curve was measured as 407 arc sec along c-axis direction, and the root mean square roughness was 1.23 nm. Nonpolar a-plane InGaN/GaN light emitting diodes were subsequently grown on a-plane GaN template, and the optical output power of 0.72 mW was obtained at drive current of 20 mA (3.36 V) and 2.84 mW at 100 mA (4.62 V) with the peak emission wavelength of 477 nm.

We report the electrical characteristics of vertical and lateral type light emitting diodes(LEDs) grown with CrN buffer layer. The LED with CrN buffer showed lower reverse leakage current than the reference sample grown with conventional low-temperature GaN buffer. It was also observed that the density of open core screw dislocation was smaller by one order of magnitude, which was thought to relate to the leakage current of devices. The vertical type LED fabricated by chemical etching of CrN buffer showed lower series resistance, lower turn-on voltage, and larger light output power than those of the conventional LEDs.

not Available.

This letter presents a reliable and very easy method for selective growth of polarity controlled ZnO films on (0001) Al2O3 substrates by plasma-assisted molecular-beam epitaxy. Cr-compound intermediate layers are used to control the crystal polarity of ZnO films on (0001) Al2O3. ZnO films grown on rocksalt structure CrN/(0001) Al2O3 shows Zn polarity, while those grown on rhombohedral Cr2O3∕(0001) Al2O3 shows O polarity. Possible interface atomic arrangements for both heterostructures are proposed.

The Si/SiO2 one-dimensional photonic crystals of heterostructural multilayers with two periods, ΛA and ΛB, have great potential for multiple-wavelength-transmission filters. These structures were prepared by inserting N pairs of ΛB (as the defect region) in the middle of two sets of two pairs of ΛA, so that the structure becomes air→[(2∙ΛA)→(N∙ΛB)→(2∙ΛA)]→substrate. N means the number of ΛB pairs in the defect region. The complex refractive indices of Si and SiO2 are assumed to be 3.7+i0 and 1.5+i0 in the transfer matrix calculation. The number of transmission channels or defect branches m is given by 2N, that is, m=2N. For large N(>10), the photonic band gap exists in a normalized frequency range ω of 0.0846–0.3838, which corresponds to the wavelength range of 0.84–6.67 μm. The defect branches are placed on a branch band between two symmetric flat bands. For a filling factor η=0.406, a matching condition of optical length in two alternating layers, the branches at the center of the branch band are divide...