MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.     

Enhancement of near-band edge photoluminescence of ZnO thin films by employing MgF2 buffer layer

ZnO/MgF₂/ZnO sandwich structure films were fabricated. The effects of a buffer layer on structure and optical properties of ZnO films were investigated by X-ray diffraction, photoluminescence, optical transmittance and absorption measurements. Measurement results showed that the buffer layer had the effects of improving the quality of ZnO films and releasing the residual stresses in the films. The near-band edge emissions of ZnO films deposited on the MgF₂ buffer layer were significantly enhanced compared with those deposited on bare substrate due to the smaller lattice mismatch between MgF₂ and ZnO than that between fused silica and ZnO.     

Investigations on the effect of InSb and InAsSb step-graded buffer layers in InAs0.5Sb0.5 epilayers grown on GaAs (0 0 1)

We report the structural and electrical properties of InAsSb epilayers grown on GaAs (0 0 1) substrates with mid-alloy composition of 0.5. InSb buffer layer and InAs_xSb_1−x step-graded (SG) buffer layer have been used to relax lattice mismatch between the epilayer and substrate. A decrease in the full-width at half-maximum (FWHM) of the epilayer is observed with increasing the thickness of the InSb buffer layer. The surface morphology of the epilayer is found to change from 3D island growth to 2D growth and the electron mobility of the sample is increased from 5.2×10³ to 1.1×10⁴ cm²/V s by increasing the thickness of the SG layers. These results suggest that high crystalline quality and electron mobility of the InAs_0.5Sb_0.5 alloy can be achieved by the growth of thick SG InAsSb buffer layer accompanied with a thick InSb buffer layer. We have confirmed the improvement in the structural and electrical properties of the InAs_0.5Sb_0.5 epilayer by quantitative analysis of the epilayer having a 2.09 μm thick InSb buffer layer and 0.6 μm thickness of each SG layers.     

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO₃ (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr₄Ti₃O₁₀ second phase at the STO/Si interface in a Sr-rich film.     

Growth of epitaxial ZnO films on Si (1 1 1) substrates with Cr compound buffer layer by plasma-assisted molecular beam epitaxy

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.     

The photoluminescence of ZnO thin films grown on Si (1 0 0) substrate by plasma-enhanced chemical vapor deposition

High-quality ZnO thin films have been grown on a Si(1 0 0) substrate by plasma-enhanced chemical vapor deposition (PECVD) using a zinc organic source (Zn(C₂H₅)₂) and carbon dioxide (CO₂) gas mixtures at a temperature of 180°C. A strong free exciton emission with a weak defect-band emission in the visible region is observed. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption peak in the absorption spectra, are closely related to the gas flow rate ratio of Zn(C₂H₅)₂ to CO₂. Full-widths at half-maximum of the free exciton emission as narrow as 93.4 meV have been achieved. Based on the temperature dependence of the PL spectra from 83 to 383 K, the exciton binding energy and the transition energy of free excitons at 0 K were estimated to be 59.4 meV and 3.36 eV, respectively.     

Epitaxial growth of ZnO thin films on Si substrates by PLD technique

Epitaxial ZnO thin films have been grown on Si(1 1 1) substrates at temperatures between 550 and 700 °C with an oxygen pressure of 60 Pa by pulsed laser deposition (PLD). A ZnO thin film deposited at 500 °C in no-oxygen ambient was used as a buffer layer for the ZnO growth. In situ reflection high-energy electron diffraction (RHEED) observations show that ZnO thin films directly deposited on Si are of a polycrystalline structure, and the crystallinity is deteriorated with an increase of substrate temperature as reflected by the evolution of RHEED patterns from the mixture of spots and rings to single rings. In contrast, the ZnO films grown on a homo-buffer layer exhibit aligned spotty patterns indicating an epitaxial growth. Among the ZnO thin films with a buffer layer, the film grown at 650 °C shows the best structural quality and the strongest ultraviolet (UV) emission with a full-width at half-maximum (FWHM) of 86 meV. It is found that the ZnO film with a buffer layer has better crystallinity than the film without the buffer layer at the same substrate temperature, while the film without the buffer layer shows a more intense UV emission. Possible reasons and preventive methods are suggested to obtain highly optical quality films.     

Influence of cooling rate on the liquid-phase epitaxial growth of Zn3P2

We report the liquid-phase epitaxial growth of Zn₃P₂ on InP (1 0 0) substrates by conventional horizontal sliding boat system using 100% In solvent. Different cooling rates of 0.2–1.0 °C/min have been adopted and the influence of supercooling on the properties of the grown epilayers is analyzed. The crystal structure and quality of the grown epilayers have been studied by X-ray diffraction and high-resolution X-ray rocking measurements, which revealed a good lattice matching between the epilayers and the substrate. The supercooling-induced morphologies and composition of the epilayers were studied by scanning electron microscopy and energy dispersive X-ray analysis. The growth rate has been calculated and found that there exists a linear dependence between the growth rate and the cooling rate. Hall measurements showed that the grown layers are unintentionally doped p-type with a carrier mobility as high as 450 cm²/V s and a carrier concentration of 2.81×10¹⁸ cm⁻³ for the layers grown from 6 °C supercooled melt from the cooling rate of 0.4 °C/min.     

Large-size β-Ga2O3 single crystals and wafers

Large-size single crystals of β-Ga₂O₃ with 1 inc in diameter have been grown by the floating zone technique. The stable growth conditions have been determined by the examination of the crystal structure. Wafers have been cut and fine polished in the (1 0 0), (0 1 0) and (0 0 1) planes. These were highly transparent in the visible and near UV, as well as electrically conductive, indicating the potential use of β-Ga₂O₃ as a substrate for optoelectic devices operating in the visible/near UV and with vertical current flow.     

Morphology transition of one-dimensional ZnO grown by metal organic vapour phase epitaxy on (0 0 0 1)-ZnO substrate

Metal organic vapour phase epitaxy (MOVPE) has been used to successfully grow one-dimensional (1D) ZnO deposits on (0 0 0 1)-ZnO substrate. Dimethylzinc–triethylamine and nitrous oxide were used as zinc and oxygen sources, respectively, with nitrogen as the carrier gas. Vertically aligned 1D ZnO structures were observed along the c-axis by using lower VI/II mole ratio R_VI/II<2025 and/or high growth temperatures (Tg>800 °C). The diameter, length, density and the mechanism of formation could be controlled with the growth time. Scanning electron microscopy (SEM) shows different structures, i.e., sharp-top, flat-top and open-top with slim bottom and large-top one-dimensional ZnO. A good structural quality was revealed by X-ray diffraction rocking curve with a full-width at half-maximum (FWHM) varying from 40 to 92 arcsec with increasing growth time.     

Preparation and characterization of fiber-textured SrAl2O4:Eu films grown using a homo-buffer layer

This paper describes the preparation and characterization of fiber-textured SrAl₂O₄:Eu films on a quartz glass substrate using a homo-buffer layer. The effect of the buffer layer on the crystallinity and adhesion was investigated by cross-section transmission electron microscopy and X-ray diffraction (XRD). The results show that the prepared film was not only well crystallized but also highly textured. The preferred orientation of this textured film was confirmed to be (0 3 1) by pole figure measurement. In addition, this film exhibits excellent optical transparency, with an average transmittance of more than 80% in the visible range.     

Synthesis and characteristics of Na-doped Bi4Ti3O12 thin films on Si substrate

Bi_3.25Na_2.25Ti₃O₁₂ thin films were prepared on p-Si(1 1 1) substrate by a metalorganic solution decomposition (MOSD) method. The structural characteristic and crystallization of the films were examined by X-ray diffraction. The current–voltage characteristic shows ohmic conductivity in the lower voltage range and space-charge-limited conductivity in the higher voltage range. The dielectric constant is 53 at a frequency of 100 kHz at room temperature and the dissipation factor exists at a minimal value of 0.02 at a frequency of 200 kHz. The retention time estimated by measuring capacitance is about 10⁶ s. Nonhysteretic C–V curves at various frequencies were also collected.     

CrO2 (1 0 0) and TiO2 (1 0 0) film heteroepitaxy on a BaF2 (1 1 1)/Si (1 0 0) substrate

Multi-domained heteroepitaxial rutile-phase TiO₂ (1 0 0)-oriented films were grown on Si (1 0 0) substrates by using a 30-nm-thick BaF₂ (1 1 1) buffer layer at the TiO₂–Si interface. The 50 nm TiO₂ films were grown by electron cyclotron resonance oxygen plasma-assisted electron beam evaporation of a titanium source, and the growth temperature was varied from 300 to 600 °C. At an optimal temperature of 500 °C, X-ray diffraction measurements show that rutile phase TiO₂ films are produced. Pole figure analysis indicates that the TiO₂ layer follows the symmetry of the BaF₂ surface mesh, and consists of six (1 0 0)-oriented domains separated by 30° in-plane rotations about the TiO₂ [1 0 0] axis. The in-plane alignment between the TiO₂ and BaF₂ films is oriented as [0 0 1] TiO₂ || BaF₂ or [0 0 1] TiO₂ || BaF₂ . Rocking curve and STM analyses suggest that the TiO₂ films are more finely grained than the BaF₂ film. STM imaging also reveals that the TiO₂ surface has morphological features consistent with the BaF₂ surface mesh symmetry. One of the optimally grown TiO₂ (1 0 0) films was used to template a CrO₂ (1 0 0) film which was grown via chemical vapor deposition. Point contact Andreev reflection measurements indicate that the CrO₂ film was approximately 70% spin polarized.     

The influence of the Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate

The influence of Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition (MOCVD) has been systematically studied. Compared with the sample without Al pre-deposition, optimum Al pre-deposition time could improve the AlN buffer layer crystal quality and reduce the root mean square (RMS) roughness. Whereas, overlong Al-deposition time deteriorated the AlN crystal quality and Al-deposition patterns could be found. Cracks and melt-back etching patterns appeared in the GaN layer grown without Al pre-deposition. With suitable Al-deposition time, crack-free 2.0 μm GaN was obtained and the full-width at half-maximum (FWHM) of (0 0 2) plane measured by double crystal X-ray diffraction (DCXRD) was as low as 482 arcsec. However, overlong Al-deposition time would result in a great deal of cracks, and the crystal quality of GaN layer deteriorated. The surface of GaN layer became rough in the region where the Al-deposition patterns were formed due to overlong Al-deposition time.     

Controllable growth of ZnO nanowires with different aspect ratios and microstructures and their photoluminescence and photosensitive properties

ZnO nanowires with variable aspect ratios and microstructures have been prepared by a hydrothermal reaction of Zn foil and Na₂C₂O₄ solution at 140 °C. The ZnO nanowires are single crystalline with the wurtzite structure and grow in the [0 0 0 1] direction, and their aspect ratios and microstructures can be changed by tuning the reaction time and the Na₂C₂O₄ concentration. UV and blue-green emissions that depended on the Na₂C₂O₄ concentration are observed from the ZnO nanowires with different aspect ratios. The photosensitivity of ZnO ultralong nanowires with honeycomb-like micropatterns is found to be about 10 at 5 V.     

Sn3O4 single crystal nanobelts grown by carbothermal reduction process

This article reports on the growth of single crystal Sn₃O₄ nanobelts and SnO by a carbothermal reduction process in two different regions of a furnace tube. Even though intermediate tin oxide compounds (Sn₃O₄) have been observed experimentally, the study of structures based on them is a challenging task. Characterization data allowed us to propose that Sn₃O₄ nanobelts grew by vapor–solid mechanism while SnO grew by self-catalyst vapor–liquid–solid mechanism. Electrical measurements of a single Sn₃O₄ nanobelt were performed at different temperatures, revealing undoped semiconductor characteristics.     

Growth and Raman spectroscopic characterization of As4S4 (II) single crystals

As described by Kutoglu (1976 [16]), single crystals of As₄S₄ (II) phase have been grown using a new two-step synthesis that drastically increases the reproducibility that is attainable in synthetic experiments. First, through photo-induced phase transformation, pararealgar powder is prepared as a precursor instead of AsS melt. Then it is dissolved and recrystallized from CS₂ solvent. Results show that single crystals of the As₄S₄ (II) phase were obtained reproducibly through the dissolution–recrystallization process. Single crystals of As₄S₄ (II) obtained using this method were translucent and showed a uniform yellow-orange color. The crystal exhibits a platelet-like shape as a thin film with well-developed faces (0 1 0) and (0 1¯ 0). The grown crystals are as large as 0.50×0.50×0.01 mm. They were characterized using powder and single crystal X-ray diffraction techniques to confirm the phase identification and the lattice parameters. The As₄S₄ (II) phase crystallizes in monoclinic system with cell parameters a=11.202(4) Å, b=9.954(4) Å, c=7.142(4) Å, β=92.81(4)°, V=795.4(6) Å³, which shows good agreement with the former value. Raman spectroscopic studies elucidated the behavior of the substance and the relation among phases of tetra-arsenic tetrasulfide.     

Anisotropic vapor phase growth of Ga2O3 crystalline nanobelts

Ga₂O₃ nanobelts were synthesized by gas reaction at high temperature in the presence of oxygen in ammonia. X-ray diffraction and chemical microanalysis revealed that the nanostructures were Ga₂O₃ with the monoclinic structure. Electron microscopy study indicated the nanobelts were single crystalline with broad (0 1 0) crystallographic planes. The nanostructures grew anisotropically with the growth direction of . Statistical analysis of the anisotropic morphology of the nanobelts and electron microscopy investigation of the nanobelt tips indicated that both vapor–solid and vapor–liquid–solid mechanisms controlled the growth process. The anisotropic nature of crystallographic morphology is explained in terms of surface energy.     

Properties of ZnO films grown on (0 0 0 1) sapphire substrate using H2O and N2O as O precursors by atmospheric pressure MOCVD

In this paper, we compare the properties of ZnO thin films (0 0 0 1) sapphire substrate using diethylzinc (DEZn) as the Zn precursor and deionized water (H₂O) and nitrous oxide (N₂O) as the O precursors, respectively in the main ZnO layer growth by atmospheric pressure metal–organic chemical vapor deposition (AP-MOCVD) technique. Surface morphology studied by atomic force microscopy (AFM) showed that the N₂O-grown ZnO film had a hexagonal columnar structure with about 8 μm grain diameter and the relatively rougher surface compared to that of H₂O-grown ZnO film. The full-widths at half-maximum (FWHMs) of the (0 0 0 2) and () ω-rocking curves of the N₂O-grown ZnO film by double-crystal X-ray diffractometry (DCXRD) measurement were 260 and 350 arcsec, respectively, indicating the smaller mosaicity and lower dislocation density of the film compared to H₂O-grown ZnO film. Compared to H₂O-grown ZnO film, the free exciton A (FX_A) and its three phonon replicas could be clearly observed, the donor-bound exciton A⁰X (I₁₀):3.353 eV dominated the 10 K photoluminescence (PL) spectrum of N₂O-grown ZnO film and the hydrogen-related donor-bound exciton D⁰X (I₄):3.363 eV was disappeared. The electron mobility (80 cm²/V s) of N₂O-grown ZnO film has been significantly improved by room temperature Hall measurement compared to that of H₂O-grown ZnO film.