Issues in ZnO homoepitaxy

Zinc oxide (ZnO) bulk single crystals, which are of high purity and transparency with a large size of 2 in., are successfully grown by the hydrothermal method. The sliced substrates are chemomechanically polished to form an epi-ready surface. The impurities existing on the as-polished substrate surface are characterized before and after annealing by SIMS (secondary-ion mass spectroscopy), and a damaged surface layer due to chemomechanical polishing is evaluated by an optical method. We attempt to remove the layer damaged due to chemomechanical polishing with two approaches, chemical etching and thermal annealing in N₂, O₂ or high vacuum. The improvement of the surface morphology and crystallinity is evaluated by means of high resolution X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM). In the PL measurements, the relative intensity of the first-order longitudinal optical phonon replica of the free exciton (FX-1LO) is compared against varying etching depth. The relative intensity becomes weak with increasing etch depth and finally saturates at the etch depth of 5 μm. After the annealing process, we grow ZnO thin films on these ZnO(0001) substrates by plasma-assisted molecular beam epitaxy. Films grown directly on the substrate show a 3D growth mode in the initial stage of growth with various surface treatments. To overcome this problem, we employ a low temperature grown ZnO buffer layer (LT-ZnO), and a two-dimensionally grown high quality ZnO film is attained.     

Grain boundary wetting in the Al-Mg system and synthesis of magnesium diboride in contact with melt

The interaction between a Mg-containing melt and B under conditions of partial and complete wetting of Al/Al grain boundaries by Al-Mg melt has been investigated. The study was performed on Al polycrystals with Mg contents of 5, 10, 15, 18, and 25 wt %. Correspondingly, the Mg content in the melt was determined by the liquidus line and was in the range from 5 to 30 wt %. The obtained metal-matrix composites were investigated by light and scanning electron microscopy, electron-probe microanalysis, and X-ray diffraction. The possibility of synthesizing MgB₂ in the contact with a melt having a relatively low Mg content (from 15 to 30 wt %) has been demonstrated.     

Chemical etchant dependence of surface structure and morphology on GaN(0001) substrates

We studied the procedure of cleaning GaN(0001) substrate surfaces by wet etching and subsequent annealing in ultrahigh vacuum for two different types of freestanding GaN wafers: hydride vapor phase epitaxy (HVPE) crystal and Na flux liquid phase epitaxy (LPE) crystal wafers. A flat surface containing GaN(0001)2 × 2 reconstruction was successfully achieved on both HVPE and LPE surfaces by etching in HF and subsequent annealing at ～ 550 °C but was not achieved by etching in HCl, NaOH, and HNO₃.     

Role of lateral interaction in the homoepitaxy of GaAs on the (001)-β(2 × 4) surface

The homoepitaxy of GaAs on the (001)-β(2 × 4) surface during molecular beam epitaxy is considered as a twodimensional first-order phase transition from the lattice gas of adsorbed growth components to the two-dimensional crystalline phase. In the context of the mean field theory of phase transitions, the parameters of lateral interaction between filled cells of the lattice gas are determined. The causes for the completion of the growth of a particular monolayer (self-ordering) before the beginning of growth of a new monolayer are clarified. The oscillations observed in the reflection high-energy electron diffraction experiments support the conclusions of the suggested theory of the phase transition for homoepitaxy.     

Determination of the orientation parameters of grains from the contrast of an image of a multisilicon polished surface using backscattered electrons

The grain structure of multisilicon crystals are investigated by scanning electron microscopy and electron backscatter diffraction. It is found that the contrast of an image obtained by scanning polished multisilicon surfaces in the mode of backscattered electrons by electron-probe microanalysis is caused by the fact that the contrasting grains on the test site of the surface belong to different crystallographic orientations. It is revealed that high-angle grain boundaries are areas where the contrast varies, whereas small-angle boundaries are not observed on the polished surfaces. Consequently, the degree of contrast of the image obtained in this scan mode can be used to qualitatively assess the degree of misorientation of neighboring grains.     

Influence of surface preparation on CdZnTe nuclear radiation detectors

High resolution X-ray diffraction (HRXRD), Atomic Force Microscope (AFM), Scanning Electron Microscope (SEM) techniques were used to characterize the surface of CdZnTe (CZT) samples treated by mechanical lapping, polishing and chemical etching processes. The results confirm that the etching process produces the highest intensity diffraction peak, and the best full-width-at-half-maximum (FWHM). Fourier Transform Infrared (FTIR) spectroscopy shows that fine polishing increases the infrared transmission of the CZT sample, while etching with 2% bromine methanol (BM) etching decreases the infrared transmission. Different etchants and concentrations were investigated by comparing the surface morphology and roughness. The bromine methanol etching has shown more flat surface with lower roughness than the other etchants.     

Formation and magnetic properties of mechanically alloyed Al65Cu20Fe15 quasicrystal

The formation of icosahedral phase by mechanical alloying of crystalline elemental powder of Al, Cu and Fe has been investigated. The effect of milling time on the formation of icosahedral phase of nominal composition of Al₆₅Cu₂₀Fe₁₅ has been studied using the X-ray diffraction technique. Further studies have been carried out by scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDAX), particle size, magnetisation and ferromagnetic resonance studies. All these studies indicate that the icosahedral alloy shows soft ferromagnetic behaviour.     

SERS study of Ag nanoparticles electrodeposited on patterned TiO2 nanotube films

In this work, Ag nanoparticles (NPs) were deposited on patterned TiO₂ nanotube films through pulse‐current (PC) electrodeposition, and as a result patterned Ag NPs films were achieved. Scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X‐ray diffraction (XRD) were used, respectively, to study the morphology, uniformity, and phase structure of the patterned Ag NP films. The size and density of the as‐deposited Ag NPs could be controlled by changing the deposition charge density, and it was found that the patterned Ag NP films produced under a charge density of 2.0 C cm⁻² gave intense UV–vis and Raman peaks. Two‐dimensional surface‐enhanced Raman scattering (SERS) mapping of rhodamine 6G (R6G) on the patterned Ag NP films demonstrated a high‐throughput, localized molecular adsorption and micropatterned SERS effect. Copyright © 2010 John Wiley & Sons, Ltd.     

Formation of diffractive microrelief on diamond film surface

The fabrication of diamond-based optical elements for high-power CO₂ lasers is of particular interest because of low optical absorption, high thermal conductivity and weak temperature dependence of the refractive index of diamond. In earlier publications, we presented the results of the implementation of ion-chemical etching technology for generating a diffraction grating on the surface of a diamond film. The results on generating diamond diffractive optical elements and a sub-wavelength antireflection structure by plasma etching were also presented. In this paper, we study the generation of the grating on the diamond surface by ion-chemical etching. The choice of (Ar+O₂) mixture as a working gas and use of niobium as a masking layer are substantiated. The results of studying a diffraction microrelief generated on the diamond-film surface through ion-chemical etching are discussed.     

Chemical etching investigation of polycrystalline p-type 6H-SiC in HF/Na2O2 solutions

In this work, an experimental study on the chemical etching reaction of polycrystalline p-type 6H-SiC was carried out in HF/Na₂O₂ solutions. The morphology of the etched surface was examined with varying Na₂O₂ concentration, etching time, agitation speed and temperature. The surfaces of the etched samples were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) Fourier transform infrared spectroscopy (FT-IR) and photoluminescence. The surface morphology of samples etched in HF/Na₂O₂ is shown to depend on the solution composition and bath temperature. The investigation of the HF/Na₂O₂ solutions on 6H-SiC surface shows that as Na₂O₂ concentration increases, the etch rate increases to reach a maximum value at about 0.5 M and then decreases. A similar behaviour has been observed when temperature of the solution is increased. The maximum etch rate is found for 80 °C. In addition, a new polishing etching solution of 6H-SiC has been developed. This result is very interesting since to date no chemical polishing solution has been developed on the material.     

(001) textured PbTiO3 thin films grown on redopingn-Si by metalorganic chemical vapor deposition under reduced pressure

(001) textured PbTiO₃ thin films have been deposited on (001) redopingn-Si substrates by metalorgnic chemical vapor deposition (MOCVD) under reduced pressure, and the film ferroelectricity has been measured using the substrate as bottom electrode directly. Besides this investigation, a set of analysis including AFM surface morphology, SEM cross section morphology, electron-probe element analysis, XRD 0-20 scan and high temperature X-ray diffraction have been carried out to study the microstructure and phase transition process of the PbTiO₃ thin film.     

Space-selective phase separation inside a glass by controlling compositional distribution with femtosecond-laser irradiation

Space-selective phase separation was induced inside Na₂O–SiO₂ glasses by changing the glass composition locally from a miscible composition to an immiscible one via femtosecond-laser irradiation at a high repetition rate. The formation of a nanoscale co-continuous structure due to phase separation was confirmed by scanning electron microscopy after heat treatment, polishing until the laser-modified area was exposed, and subsequent etching. The configuration of the structure could be controlled by changing the heat-treatment time.     

Non-equilibrium and crystalline phases on the Mg-Ga-Sn system

Samples in the eutectic composition were obtained with the piston and anvil technique and the crystallization towards a metastable phase was observed upon annealing at room temperature. Small amounts of Sn were added to the binary MgGa in order to analyze its influence on the glass forming ability. Cast and rapid quenched ternary samples were studied employing X-ray diffraction, Mössbauer spectroscopy and electron probe microanalysis. The solubility of the third element was analyzed in the equilibrium phases Mg₅Ga₂, Mg₂Ga and Mg₂Sn. As far as we know, these are the first contributions to the knowledge of the MgGaSn system.     

Removal of scratch on the surface of MgO single crystal substrate in chemical mechanical polishing process

Etching and chemical mechanical polishing (CMP) experiments of the MgO single crystal substrate with an artificial scratch on its surface are respectively performed with the developed polishing slurry mainly containing 2 vol.% phosphoric acid (H₃PO₄) and 10-20 nm colloidal silica particles, through observing the variations of the scratch topography on the substrate surface in experiments process, the mechanism and effect of removing scratch during etching and polishing are studied, some evaluating indexes for effect of removing scratch are presented. Finally, chemical mechanical polishing experiments of the MgO substrates after lapped are conducted by using different kinds of polishing pads, and influences of the polishing pad hardness on removal of the scratches on the MgO substrate surface are discussed.     

Structural and compositional analyses of nanocrystalline diamond/β-SiC composite films

Nanocrystalline diamond/β-SiC composite films are synthesized by microwave plasma chemical vapor deposition using a gas mixture of H₂, CH₄, and tetramethylsilane (Si(CH₃)₄, TMS) in a single process step. Structural and compositional analyses revealed that the films consist of a mixture of diamond and β-SiC nanocrystalline phases in a desired volume fraction combinatorial form. Transmission electron microscopy analysis confirmed the X-ray diffraction results and showed that the major diffraction lines corresponded to a two-component nanocrystalline composite film. Infrared spectroscopic analysis showed that the content of β-SiC in the films can be increased by increasing the TMS concentration. This correlated very well with electron probe microanalysis and Rutherford backscattering analysis that showed an almost linear correspondence of β-SiC content in the films with the TMS concentration in the gas phase. The phase purity of the diamond crystallites decreased with increase in the β-SiC content in the films, as shown by micro Raman scattering studies. Smooth surface morphologies are measured for these films by using atomic force microscopy; the root mean square roughness was 12 ± 1 nm. The β-SiC volume fraction (vol. %) was identified as an important compositional factor to determine any mechanical and frictional properties of these films. PACS 68.55.-a; 68.55.Nq; 68.60.-p     

Controlling the nanodot formation on GaAs surface during focused ion beam processing

GaAs processed using gallium-focused ion beams for the fabrication of photonic devices mostly results in gallium nanodots on the surface. These gallium nanodots may produce unwanted effects and deteriorate the optical and electrical properties of the devices. We have investigated the FIB processing of GaAs with and without exposure to an insulator-enhanced etching precursor gas (XeF₂) to explore the use of XeF₂ during GaAs processing. It is reported that without the gas, FIB processing results in nanodots on the surface that vary in size and density depending on processing parameters such as incident energy, beam current, angle and dwell time. Processing with insulator (XeF₂)-enhanced etching gas irrespective of the process parameters eliminates the nanodots and results in a smooth surface, as characterized by scanning electron microscopy and atomic force microscopy. This method will be useful for surfaces which require dry processing without exposure to any wet chemical etching.     

Direct production of carbon nanotubes decorated with Cu2O by thermal chemical vapor deposition on Ni catalyst electroplated on a copper substrate

Carbon nanotubes (CNTs) decorated with Cu₂O particles were grown on a Ni catalyst layer deposited on a Cu substrate by thermal chemical vapor deposition from liquid petroleum gas. Ni catalyst nanoparticles with different sizes were produced in an electroplating system at 45 °C using the corrosive effect of H₂SO₄ which was added to solution. These nanoparticles provide the nucleation sites for CNT growth avoiding the need for a buffer layer. The surface morphology of the Ni catalyst films and CNT growth over this catalyst was studied by scanning electron microscopy (SEM). High temperature surface segregation of the Cu substrate into the Ni catalyst layer and its exposition to O₂ at atmospheric environment, during the CNTs growth, lead to the production of CNTs decorated with about 6 nm Cu₂O nanoparticles. We used SEM to study the surface characteristics of Ni catalyst films and characteristic of grown CNTs. Raman spectroscopy, transmission electron microscopy (TEM), electron diffraction (EDX), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) revealed the formation of CNTs. The selected area electron diffraction pattern, EDX, and XPS studies show that these CNTs were decorated with Cu₂O nanoparticles. This way of fabrication is the easiest and lowest cost method.     

Investigation of Ceramic-Like Coatings Formed on Aluminum Composites by Microarc Oxidation

Ceramic-like coatings with a thickness of up to 40 μm are formed on aluminum composites without additives and with copper additives (1 and 4.5%) in a silicate-alkaline electrolyte by microarc oxidation. The composites are prepared by powder metallurgy (cold pressing and sintering in forevacuum). An increase in the copper concentration in the composites to 4.5% leads to the retardation of anode voltage growth on the initial stage of oxidation corresponding to the formation of a barrier layer. The coatings are studied by scanning electron microscopy, X-ray microanalysis, X-ray photoelectron spectroscopy, and X-ray diffraction. The morphology of their surface corresponds to the morphology of the surface of coatings on compact aluminum alloys. According to X-ray photoelectron spectroscopy, a thin 1-μm layer forms on the surface. It consists predominantly of electrolyte components. X-ray diffraction analysis shows that the coatings mainly consist of γ-Al₂O₃ oxide as well as the η-Al₂O₃ phase, the peaks of which are broadened. This broadening is characteristic of the amorphous component and may be due to the presence of nanocrystalline regions in the coating structure. In the coatings on the composite Al + 4.5% Cu, mullite Al₂SiO₅ and copper oxide CuO are also found. The excess aluminum content may be associated with residual unoxidized aluminum inclusions in the structure of the coatings.     

Growth of bulk GaN in a vertical hydride vapour phase epitaxy reactor

Using the hydride vapour phase epitaxy technique, we have grown 2-inch diameter bulk GaN material with a thickness up to 2 mm. The growth was performed in a vertical hot-walled reactor at atmospheric pressure. In this geometry, the process gases are distributed from the bottom upwards through the reactor. We present recent results on growth and characterization of the bulk GaN material. The structural and optical properties of the layers have been studied using decorative etching, optical microscopy, scanning electron microscopy, X-ray diffraction, cathodoluminescence, and low temperature photoluminescence.     

A surface sensitive optical method for the evaluation of processed ZnO: exploitation of LO phonon interaction

We report on a surface property of bulk ZnO crystals and an optical method to evaluate it. Bulk ZnO crystals have a damaged surface layer due to chemomechanical polishing. We prepared the ZnO crystals by etching, and evaluated the improvement of the surface by high-resolution X-ray diffraction (XRD) and photoluminescence (PL). In PL measurements, the relative intensity of the first order longitudinal optical phonon replica of free exciton (FX-1LO) to second order process (FX-2LO) was compared. The relative intensity becomes weak with increasing etched depth and finally saturates at the etched depth of 5 μm. This result agrees well with XRD results.