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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.

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 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.

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.

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.

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 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.

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.

High-quality epitaxial ZnO films were grown on α-Al2O3 substrates using Cr2O3 buffer layers by plasma-assisted molecular beam epitaxy (PA MBE). The Cr2O3 buffer layer was formed through the oxidation of a thin CrN layer pre-deposited on the (0 0 0 1) Al2O3 substrate, using an O-plasma treatment. From the reflection high-energy electron diffraction (RHEED) and grazing incident X-ray diffraction (GIXRD) analyses, the epitaxial relationship between sequential layers was determined to be [2 1¯ 1¯0]ZnO//[1 1¯ 0 0]Cr2O3//[0 1 1¯]CrN//[1 1¯ 0 0]Al2O3 and [1 1¯ 0 0]ZnO//[1 2¯ 1 0]Cr2O3//[1 2¯ 1]CrN//[1 2¯ 1 0]Al2O3, respectively. The atomic force microscope (AFM) image of ZnO that had grown on the Cr2O3 buffer layer clearly showed steps and terraces on the surface. The Cr2O3 buffer layer was found to be very effective in improving crystal quality, as expressed by the narrowing of the ω-rocking curve full-width at half-maximum of the (0 0 0 2) ZnO diffraction peak (85 arcsec) and the smooth surface morphology (root-mean-square=0.47 nm) of the ZnO films.

Nitrogen-doped ZnO films (ZnO:N) were grown on (0 0 0 1) and (0 0 0 1¯) ZnO templates by plasma-assisted molecular beam epitaxy (PA-MBE). High nitrogen concentration (>5×10 20 /cm 3 ) was achieved from ZnO:N/(0 0 0 1)ZnO films, while relatively low nitrogen concentration (∼10 19 /cm 3 ) was observed from ZnO:N/(0 0 0 1¯)ZnO layers at the same growth temperature ( T g ). High-resolution X-ray diffraction (HRXRD) revealed considerable lattice deformation. At T g =300 °C, the c -lattice expansion in ZnO:N/(0 0 0 1)ZnO film is almost 36% larger than that in ZnO:N/(0 0 0 1¯)ZnO due to the high nitrogen concentration in the film. Electron concentration of the ZnO:N/(0 0 0 1)ZnO films gradually decreases down to ∼10 16 /cm 3 as the nitrogen concentration increases. Moreover, donor–acceptor-pair emission dominates the photoluminescence (PL) spectrum (3.23 eV) of ZnO:N/(0 0 0 1)ZnO films grown at 300 °C and that emission is 3.1 times stronger than that of ZnO:N/(0 0 0 1¯)ZnO.

Nitrogen-doped ZnO films grown by plasma-assisted molecular beam epitaxy (P-MBE) have been investigated in terms of growth temperatures (300–800 °C). The growth rate of nitrogen-doped zinc oxide (ZnO) film decreases as growth temperature increases from 500 up to 700 °C since Zn–N bonds decompose. When nitrogen atoms are incorporated to be ∼10 21 /cm 3 at low growth temperature of 300 °C, it is observed the significant decrease of carrier concentration down to ∼10 16 /cm 3 and the abrupt increase of resistivity up to ∼60 Ω cm. However, p-type conductivity is not realized due to the formation of N–H bond.

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.

Microstructure and defects in nonpolar ZnO ( 1 1 2 ¯ 0 ) films with different thicknesses were studied by transmission electron microscopy. The ZnO films were grown on Al 2 O 3 ( 1 1 ¯ 0 2 ) substrates by plasma-assisted molecular beam epitaxy. The misfit dislocations were observed at the interface with regularly spaced configurations, which were well agreed with the equilibrium spacing of the misfit dislocations calculated based on the lattice misfits. The diagonal defect along the ZnO [ 1 0 1 ¯ 0 ] and [ 0 1 1 ¯ 0 ] directions and the misfit dislocations were mainly observed in the 30 nm-thick ZnO film. As increasing the film thickness, the diagonal defect was seldom observed and the threading dislocations (in addition to the misfit dislocations) were being the major defects. The dislocation densities of the 240 nm-thick ZnO film were determined to be ∼7.3×10 10 cm −2 for the dislocations with 〈0 0 0 1〉 Burgers vector and ∼6.1×10 9 cm −2 for the 1 3 〈 1 1 2 ¯ 0 〉 Burgers vector, resulting in the total dislocation density of ∼7.9×10 10 cm −2 . In addition to the perfect threading dislocations, stacking faults on (0 0 0 1) planes were observed. The type of stacking fault was determined to be a type-I 1 intrinsic stacking fault having the stacking sequence of (AB‘ABC’BC), which has the Frank partial dislocations with the Burgers vector of 1 6 [ 0 2 2 ¯ 3 ] at the end. The stacking fault density of the 240 nm-thick ZnO film was determined to be ∼1.2×10 5 cm −1 .

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 report the observation of two emissions from the (0 0 0 1) face of GaN grown by the acidic ammonothermal method, which appears rather unusual. Excitonic emission at 3.357 eV (called Y4) by photoluminescence was observed which is possibly due to an exciton bound to neutral donors related to structural defects of GaN. Deep level luminescence at 1.93 eV was also observed, which had two transition possibilities of a simple two-level transition and donor–acceptor pairs.

High-quality ZnO layers are grown on Zn-polar ZnO substrates by surfactant-mediated plasma-assisted molecular-beam epitaxy (P-MBE) using atomic hydrogen as a surfactant. Careful investigation with atomic force microscopy (AFM) and reflection high-energy electron diffraction (RHEED) reveals that two-dimensional growth is preserved down to 400 °C by irradiating atomic hydrogen during growth, while the low-temperature limit of two-dimensional growth is 600 °C without atomic hydrogen irradiation. The crystal quality of ZnO layers grown at 400 °C by surfactant-mediated MBE is evaluated to be the same as those grown at 600 °C by conventional MBE in terms of X-ray diffraction and photoluminescence properties.

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.

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.

The magnetic properties of ferromagnetic GaMnNAs films grown by plasma-assisted molecular beam epitaxy (p-MBE) have been studied. All samples showed p-type conductivity in the present experiment. The 1.0% nitrogen content samples showed metallic behavior, whereas the 1.5% nitrogen content samples showed insulating behavior. The Curie temperature T c showed the saturation with the Mn content increasing, and the critical Mn concentration for the saturation depended on the nitrogen content.

Photoluminescence (PL) line width broadening of Zn 1− x Cd x Te/ZnTe multiple quantum well (MQW), grown on ZnTe (0 0 1) substrates by molecular beam epitaxy (MBE), has been investigated. During the growth, Cd-composition and thickness of QW are controlled by reflection high energy electron diffraction (RHEED) intensity oscillations. PL line width broadening due to an alloy/interface disorder is calculated and compared with experimental results. The PL line width varies closely related to the exciton radius and shows considerable agreement with theoretical values.

Polarity-controlled ZnO films with an MgO buffer layer were grown on c -plane sapphire by plasma-assisted molecular beam epitaxy (MBE). The effect of the thickness of the MgO buffer layer on the MBE growth of ZnO was investigated. The growth rate of ZnO increased from 250 to 500 nm/h when the MgO layer thickness was increased from 1 to 3 nm. Convergent beam electron diffraction results showed that Zn-polarity (+ c ) growth occurred when the MgO layer was thicker than 3 nm, whereas O-polarity (− c ) growth occurred when the layer was less than 2 nm. Polarity conversion apparently occurred due to the difference in atomic structure between the MgO wetting layer and MgO islands. The minimum line-widths of (0 0 0 2) and (1 0 1¯ 0) ω -rocking curves were 70 and 722 arcsec, respectively, for O-polar ZnO, and 289 and 851 arcsec, respectively, for Zn-polar ZnO. The O-polar ZnO with a 1-nm-thick MgO buffer layer had a residual carrier concentration of 4.6×10 16 cm −3 with an electron mobility of 155 cm 2 /V s, whereas Zn-polar ZnO with an MgO layer thicker than 3 nm showed semi-insulating properties. In conclusion, control of the polarity of ZnO films grown on c -plane sapphire substrates by plasma-assisted MBE was achieved by inserting an MgO buffer layer of specific thickness between a ZnO film and a nonpolar substrate.

InN-based dilute magnetic semiconductor DMS In 1-x Mn x N and In 1-x Cr x N films were prepared on Al 2 O 3 substrates at low temperature by RF-plasma-assisted molecular beam epitaxy. Reflection high-energy electron diffraction, X-ray diffraction and atomic force microscopy show Mn was homogeneously incorporated into InN up to 4% and 10%, respectively, at 300°C and 200°C, while Cr was incorporated up to 4% at 300°C. In 1-x Mn x N films with Mn composition x = 0.04 showed a paramagnetic behavior at low temperature, whereas a paramagnetic to spin-glass transition was observed at 3K in In 0.9 Mn 0.1 N films grown at lower temperature (200°C). Homogeneous In 0.98 Cr 0.02 N films grown at 300°C showed clear ferromagnetic properties with Curie temperature higher than 350K.

High-quality ZnO epilayers have been grown on Zn-face ZnO substrates by plasma-assisted molecular beam epitaxy. With increasing O/Zn flux ratio from the stoichiometric to the O-rich, the growth mode changed from three- to two-dimensional growth and the line widths of (0 0 0 2) and (1 0 1 0) ω -rocking curves showed dramatic narrowing from 471 to 42 arcsec and from 1635 to 46 arcsec, respectively. These values are narrower compared to those of ZnO on sapphire and also those of device-grade MOCVD-grown GaN. Moreover, A-, B-excitons (FE A , FE B ), and the n =2 state of FE A at 3.378, 3.393, and 3.424 eV, respectively, were clearly observed in the low-temperature (4.2 K) photoluminescence spectrum of ZnO grown under O-rich flux conditions. Our results show that growth under high O-rich flux conditions is required to produce high-quality Zn-polar ZnO films.

High-electron-mobility ZnO epilayers are grown on c -plane sapphire with ZnO/MgO double-buffer layers by plasma-assisted molecular beam epitaxy. Reflection high-energy electron diffraction and transmission electron microscopy analysis showed the growth mode of ZnO buffer layers (LT-ZnO) grown at low temperature significantly affected the structural properties of the ZnO epilayers grown at high temperature, thereby affecting the electrical properties of the epilayers. When LT-ZnO was grown at a high-growth-rate, three-dimensional growth dominated and threading dislocation (TD) density was as high as ca. 1×10 10 cm −2 . By using the low growth rate of LT-ZnO, two-dimensional growth dominated and TD density was reduced by one order of magnitude, down to ca. 2×10 9 cm −2 , yielding significantly improved electrical properties of the ZnO epilayers. The highest electron mobility in as-grown undoped ZnO film, 145 cm 2 V −1 s −1 , was achieved at room temperature, comparable to the mobility previously reported for high-quality bulk ZnO.

Short period AlN/GaN quantum cascade (QC) laser structures that utilize a polarization field for electron injection were fabricated by hot wall epitaxy. A (GaN) n /(AlN) 1 short period superlattice with several molecular layers of AlN was designed in order to realize a mid-infrared laser. The QC structures were analyzed by X-ray diffraction (XRD) measurements, atomic force microscopy and cross-sectional transmission electron microscopy observation. The XRD patterns and cross-sectional TEM images showed that a well-controlled quantum cascade structure could be prepared by hot wall epitaxy without inter-diffusion of the layers.

We investigate the wet chemical cleaning process of GaAs substrate for ready-to-use. To prepare epi-ready GaAs wafer, we used NH4OH mixture to remove particles and metals as a first step of wet-chemical cleaning and acidic peroxide mixture to remove residual metallic contaminants and control the components of oxide layer as a second step of wet-chemical cleaning. The particle-free, metal-free, and thin As-oxide rich surface is achieved by the proposed ratio of NH4OH mixture followed by HCl mixture among the second step of wet-chemical cleaning solution used in this experiment. Metal-contamination was analyzed by total reflection X-ray fluorescence spectroscopy. Characterization of the oxide composition was carried out by X-ray photoelectron spectroscopy. Surface morphology and thickness of oxide layer were observed by atomic force microscopy and ellipsometer, which show very flat surface roughness of 0.95 nm, and uniform oxide thickness of about 2 nm, respectively.

This article demonstrates that interdiffusion, which is inherent in heteroepitaxy, can be well suppressed by inserting a thin buffer layer grown at sufficiently lower temperature with taking a ZnSe/GaAs heteroepitaxy as an example. Two types of samples are prepared by molecular beam epitaxy (MBE): a 1114 A thick ZnSe epilayer grown on a 150 A thick low-temperature ZnSe buffer layer on a GaAs (0 0 1) substrate, and a 1356 A thick ZnSe layer grown directly on GaAs. The composition profile at the heterointerface measured by secondary ion mass spectrometry (SIMS) clearly shows that the Ga diffusion is greatly suppressed in the ZnSe/GaAs system prepared with the low-temperature buffer layers, while considerable Ga interdiffusion from the GaAs substrate into the ZnSe epilayer is observed in the sample without the low-temperature buffer layer. The suppression of interdiffusion is also evidenced by low-temperature photoluminescence (PL) spectroscopy which shows no trace of donor–acceptor pair (DAP) emission at around 2.7 eV normally observed in ZnSe epilayers. In addition to the suppression of interdiffusion, Se reaction with GaAs surfaces is suppressed, which leads to the reduction of built-in defect generation due to heterovalency. Consequently, the ZnSe layer grown on a thin low-temperature ZnSe buffer layer shows high crystallinity as indicated by strong and sharp exciton emission in low-temperature photoluminescence spectrum and narrow X-ray line width.

The control and determination of Zn 1− x − y Mg x Be y Se quaternary alloy composition grown by molecular beam epitaxy was investigated. Compositions x and y of Zn 1− x − y Mg x Be y Se were measured by inductive coupled plasma atomic emission spectrometry (ICP-AES). The energy band gap of Zn 1− x − y Mg x Be y Se can be controlled in a range of 2.9 eV E g

We study the electrical properties of heavily Al-doped ZnSe grown by molecular beam epitaxy, whose electron concentration is saturated. Hall measurements show that electron mobility decreases with the increase of Al cell temperature, while it is almost independent of measurement temperature. These phenomena can be explained in terms of scattering mechanisms by ionized impurities and point defects. The point defects, which are induced by the incorporation of excess Al atoms, result in carrier compensation in heavily Al-doped ZnSe and decrease the electron mobility. As Al concentration increases, deep level emission around 2.25 eV, associated with a point defect such as a self-activated center [Al Zn –V Zn ], develops in photoluminescence spectrum. The line width of (0 0 4) X-ray rocking curve is broadened due to degradation of crystal quality, which is induced by the increase of point defects with an increase of Al concentration. As a result, it is suggested that the carrier compensation in highly Al-doped ZnSe is dominated by generation of complex point defects such as self-activated centers [Al Zn –V Zn ].

Highly efficient luminescence at room temperature is achieved from a novel CdTe(thickness ≤2 ML)/ ZnSe fractional monolayer heterostructures (FMS) grown by atomic layer epitaxy. Various optical spectroscopic methods such as, photoluminescence (PL), PL-excitation, spatially-resolved PL and temperature-dependent PL are used to investigate the origin of the luminescence. Structural inhomogeneities such as alloying and thickness fluctuation affect the line width and line shape of luminescence, also the luminescence intensity is greatly enhanced by the localization of excitons at disorder induced localized states. The radiative recombination mechanism is explained in terms of a simple model which includes localized states originating from the inhomogeneity of CdTe FMS.

Since the discovery of carrier-induced ferromagnetism in InMnAs and GaMnAs, diluted magnetic semiconductors (DMS) have been of much interest from the industrial viewpoint because of their potential application to new classes of devices based on spin-polarized transport or spin injection into semiconductors. However the Curie temperature Tc of GaMnAs (110 K) is relatively low and from the practical consideration it is desirable to find material with higher values. Recent theoretical prediction showed the Mn-doped GaN exhibits a Tc above room temperature. Very recently, several groups have reported the growth and magnetic properties of GaMnN films. But their results are quite different, and the origin of the magnetism is still controversial.

We report on the growth and structural, electrical and magnetotransport characteristics of epitaxial manganese perovskite (Pr 1-x Sr x ) y MnO 3-δ (PSMO) thin films by oxygen plasma assisted molecular beam epitaxy (MBE). Special attention was given to the non-stoichiometric phase, which exhibits interesting properties such as different phase transition temperatures and marked low field magnetoresistance. Under appropriate growth conditions, 2D step-flow growth mode has been realized with super smooth film surfaces. The structure characterization and magnetoresistance properties were studied and analyzed.

Low dislocation density ZnTe heteroepitaxial layers are achieved by inserting a thin ZnTe buffer layer at low growth temperature (LT-buffer) followed by annealing. The ZnTe heteroepitaxy layer with a LT-buffer shows a narrow line width of X-ray rocking curve as narrow as 42 arcsec in (0 0 4) reflection and the dislocation density is estimated as 1.3×10 6 cm −2 . A cross-sectional view of transmission electron microscopy shows high structural quality of ZnTe layer with LT-buffers.

We investigate doping effects in ZnO layers using Li 3 N as a doping source by plasma-assisted molecular-beam epitaxy. Li and N are simultaneously doped into ZnO layers through decomposition of Li 3 N into Li and N. Li- and N-doped ZnO layers grow three dimensionally at 400 o C, while undoped layers grow two dimensionally. The secondary ion mass spectroscopy of Li- and N-doped samples shows high incorporation of Li impurities at around the surface, which is due to surface segregation of Li impurities. A new emission at 3.321 eV is observed in PL spectrum of a heavily Li- and N-doped ZnO. The new emission can be assigned to exciton emission bound to neutral N acceptor. Li- and N-doped ZnO layers show n-type conductivity with high electron concentration of 6.8×10 19 /cm 3 , while the electron concentration of an undoped ZnO layer is 1.0×10 17 /cm 3 .

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.

ZnSe:Sb bulk crystals were grown by a physical vapor transport technique and subsequently annealed under Se2 partial pressure. Conduction type and net doping concentrations have been determined by capacitance–voltage measurements. All the as-grown crystals are semi-insulating. After annealing, they become p-type conducting with maximum net acceptor concentrations of (7±3)×1016 cm−3. The low-temperature photoluminescence of the as-grown (semi-insulating) crystals is dominated by a deep neutral acceptor bound exciton emission line and its phonon replica. After annealing, the p-type conducting samples show an intense donor–acceptor pair recombination and a shallow neutral acceptor bound exciton line appears with a binding energy of about 9.5 meV.

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.

Gallium-doped ZnO epitaxial layers were grown on a-plane sapphire substrates by molecular beam epitaxy (MBE) at various Ga cell temperatures from 350°C to 450°C. The ZnO layers grown on a-plane sapphire were c-oriented without any trace of the 30° rotation domains often observed in ZnO on c-plane sapphire. The Ga concentration in Ga-doped ZnO increased from 4 x 10 16 to 7 × 10 18 cm -3 with increasing Ga cell temperature. The activation ratio of Ga was about unity when the Ga concentration exceeded 3 x 10 17 cm -3 . The photoluminescence (PL) spectra of Ga-doped ZnO were dominated by an emission at 3.362eV which can be assigned to emission of exciton bound to Ga-related neutral donors. The intensity of this emission was maximum when the Ga concentration was 2 × 10 18 cm -3 . The high crystalline quality of the Ga-doped ZnO epilayers was confirmed by X-ray diffraction (XRD). Hall effect measurement and Rutherford backscattering spectrometry. Our results show that high-quality Ga-doped n-type ZnO can be grown on a-plane sapphire substrates by using MBE.

Optical properties of fresh dislocations, ( a /3)[1 1 2¯ 0]-type edge dislocations on the (1 1¯ 0 0) prismatic plane, introduced into GaN by plastic deformation at elevated temperatures were investigated by photoluminescence and optical absorption measurements. Plastic deformation acts as an effective passivation, leading to remarkable reduction of near-band-edge photoluminescence intensity centered at 3.48 eV and noticeable red-shift of the optical absorption edge. In a model of the Franz–Keldysh effect, the induced edge dislocations posses nonradiative trap sites around 3 e / c along their core, resulting in the reduction of free-carrier concentration. Also, the induced dislocations give rise to some luminescence peaks in the energy range 1.7–2.4 eV, differing from the yellow luminescence, which implies the formation of radiative recombination centers by the dislocations.

Self-assembled InAs quantum dots (QDs) on In 0.52 Al 0.48 As lattice matched to (1 0 0) InP substrates were grown by solid-source molecular beam epitaxy. We present in situ reflection high-energy electron diffraction studies of the initial growth processes and strain relaxation behaviors of the InAs QDs. The growth process of an InAs layer at the initial stage is clearly divided into two-dimensional (2D) growth and QD formation. It was found that the critical thickness for the 2D–3D transition is about two monolayers. The photoluminescence peak energies from the QDs were found to depend strongly on the InAs coverage, and the emission wavelength can be controlled over the 1.3–1.55 μm range.

We report on plasma-assisted molecular beam epitaxy of ZnO on Al 2 O 3 (0 0 0 1) substrates with an MgO buffer layer. We found a thin MgO buffer greatly influence the initial growth of ZnO by facilitating the adhesion and promoting the lateral epitaxy of ZnO. Consequently, a flat surface with a (3×3) reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Both in situ investigations and structural analysis revealed that the MgO buffer has a rocksalt structure and consists of a 2D wetting layer and 3D islands above. The change in surface energy by covering the substrate with an MgO wetting layer results in a drastic suppression of columnar growth and layer-by-layer epitaxy of ZnO is achieved. The resulting improvement in crystal quality of the ZnO epilayers is confirmed by a large decrease in the X-ray rocking curve widths of both symmetrical and asymmetrical diffraction peaks, as well as the presence of free exciton recombinations in low temperature photoluminescence and the low n -type background carrier concentration.

Oxygen plasma assisted molecular beam epitaxial (MBE) growth of Pr 1-x Sr x MnO 3 thin films has been carried out on NdGaO 3 (110) substrates. The growth parameters have been optimized to realize nearly 2D layer-by-layer growth. XRD results of the epi-films show that the Pr 1-x Sr x MnO 3 /NGO (110) thin films are of high crystal quality, as clear ('clean') diffraction peaks can be observed belonging to the film and the substrate, respectively. Based on analysis of the peaks, it was concluded that epitaxial relation is PrSrMnO3 (110)∥NdGaO 3 (110). i.e., the c-axis being parallel to the surface. It is a different epitaxial relation from that of other growth methods.

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 on the structural characteristics and magnetic properties of Mn oxide films grown on MgO (001) substrates by plasma-assisted molecular beam epitaxy (MBE). Under well-controlled growth conditions, three types of Mn oxide films were obtained. They are Mn3O4, lambda -MnO2 and their mixed structure (Mn3O4 + lambda -MnO2). A structural phase diagram about the films is depicted as a function of growth rate and substrate temperature. It is found that at lower growth rate, a new phase of Mn oxide lambda -MnO2 was obtained, and two transition temperatures were observed in its magnetization. At a higher growth rate, the Mn3O4 film was obtained. Its coercive force and transition temperature were different from its bulk sample. The atomic force microscopy (AFM) images exhibited the surfaces of the three films are composed of pyramids, flat surfaces and truncated-pyramids, respectively. (C) 2001 Elsevier Science B.V. All rights reserved.

GaN single crystals were synthesized by heating a Na–Ga melt placed in a BN crucible at 750°C and 5 MPa of N2 for 200 h. The yields and morphology of the single crystals were varied with Na/(Na+Ga) molar ratio (rNa) in the starting composition of the melts. Colorless transparent bulk single crystals with a size of 3 mm in the longest direction were obtained at rNa=0.60. The full-width at half-maximum (FWHM) of the rocking curve measured for 0 0 0 4 X-ray diffraction peak was 25 arcsec. The electrical resistivity of the platelet crystals was 0.04 Ω cm with a carrier concentration (n-type) of 1–2×1018 cm−3 and a mobility of 100 cm2 V−1 s−1 at room temperature. No emission peaks or bands except at 365 nm, corresponding to near-band emission, were observed at room temperature in the cathodoluminescence spectrum.