Growth and characterization of GaN and AlN films on (1 1 1) and (0 0 1) Si substrates

GaN and AlN films were grown on (1 1 1) and (0 0 1) Si substrates by separate admittances of trimethylgallium (or trimethylaluminum) and ammonia (NH₃) at 1000°C. A high temperature (HT) or low temperature (LT) grown AlN thin layer was employed as the buffer layer between HT GaN (or HT AlN) film and Si substrate. Experimental results show that HT AlN and HT GaN films grown on the HT AlN-coated Si substrates exhibit better crystalline quality than those deposited on the LT AlN-coated Si substrates. Transmission electron microscopy (TEM) of the HT GaN/HT AlN buffer layer/(1 1 1)Si samples shows a particular orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. High quality HT GaN films were achieved on (1 1 1) Si substrates using a 200 Å thick HT AlN buffer layer. Room temperature photoluminescence spectra of the high quality HT GaN films show strong near band edge luminescence at 3.41 eV with an emission linewidth of ∼110 meV and weak yellow luminescence.     

Effects of vicinal angles from (0 0 1) surface on the Boron-doping features of high-quality homoepitaxial diamond films grown by the high-power microwave plasma chemical-vapor-deposition method

Vicinal surface effects on homoepitaxial growth and boron-doping processes have been studied in case of single-crystalline diamond (0 0 1) surfaces grown using the high-power microwave plasma chemical-vapor-deposition (MWPCVD) method. The off-angles inclined from the on-orientation (0 0 1) surfaces ranged to 5° along the [1 1 0] or [1 0 0] direction, while the concentration of doping B(CH₃)₃ gas was kept constant with a B/C ratio of 50 ppm. Although a number of square-like growth hillocks often appeared, depending substantially on the crystalline quality of the high-pressure/high-temperature-synthesized (HPHT) Ib diamond substrates employed, the number and shape of the hillocks changed significantly with the increasing off-angle. For the vicinal surfaces with off-angles of ≈3° inclined along the [1 1 0] direction, macroscopically flat surfaces were obtained, compared with the other off-angle cases examined. Furthermore, the growth rate and acceptor density of substitutional boron atoms in the homoepitaxial layers were found to substantially increase with the increasing off-angle. These indicate that the step density can play important roles not only in the homoepitaxial growth but also in the boron-incorporation process during the high-power MWPCVD growth.     

Defects in a mixed-habit Yakutian diamond: Studies by optical and cathodoluminescence microscopy,infrared absorption,Raman scattering and photoluminescence spectroscopy

Widespread occurrences in the crystallisation history of natural diamonds are epochs of mixed-habit growth in which normal {1 1 1}-faceted growth is accompanied by non-faceted growth on curved surfaces of mean orientation ∼{1 0 0}, termed ‘cuboid’. This paper analyses mixed-habit-related phenomena in a near-central, (1 1 0)-polished slice of an octahedron from the Mir pipe, previously studied principally by SIMS probes analysing N impurity content and C and N isotope composition. In the present work, newly studied features include dislocation content, fine structure in cathodoluminescence (CL) patterns, refined IR absorption data, Raman and photoluminescence (PL) microspectroscopy and microscopy of internal non-diamond bodies. Topographic imaging and spectroscopic techniques traced the specimen's morphological evolution from a cubo-octahedral core containing complex relative development of {1 1 1} and cuboid sectors, both populated by graphite crystallites, diameters up to ∼5 μm, lying on all diamond host {1 1 1}. Coherently overgrowing the core was a zone of widely but smoothly varying relative development of {1 1 1} and cuboid sectors, both on birefringence evidence dislocation-free, emitting strongly from cuboid sectors the PL spectra associated with Ni–N-vacancy complexes. An enclosing octahedral shell of solely {1 1 1} lamellae terminated mixed-habit growth. High-resolution FTIR absorption measurements of I(B′), the integrated absorption due to {1 0 0}-platelet defects, showed from its absence or weakness that total or substantial platelet degradation had taken place in the mixed-habit zones, indicating that these had undergone conditions close to the diamond–graphite phase boundary in their history.     

Polarities of AlN films and underlying 3C-SiC intermediate layers grown on (1 1 1) Si substrates

The hydride vapor phase epitaxy (HVPE) of {0 0 0 1} AlN films on {1 1 1} Si substrates covered with epitaxial {1 1 1} cubic SiC (3C-SiC intermediate layers) was carried out. 3C-SiC intermediate layers are essential to obtain high-quality AlN films on Si substrates, because specular AlN films are obtained with 3C-SiC intermediate layers, whereas rough AlN films are obtained without 3C-SiC intermediate layers. We determined the polarities of AlN films and the underlying 3C-SiC intermediate layers by convergent beam electron diffraction (CBED) using transmission electron microscopy. For the first time, the polarities of the AlN films and the 3C-SiC intermediate layers were determined as Al and Si polarities, respectively. The AlN films were hardly etched by aqueous KOH solution, thereby indicating Al polarity. This supports the results obtained by CBED. The result is also consistent with electrostatic arguments. An interfacial structure was proposed. The 3C-SiC intermediate layers are promising for the HVPE of AlN films on Si substrates.     

Thickness-dependent structural transition in GaAs nanocrystals grown on Si (1 1 1) surface

Structural stabilities in GaAs nanocrystals grown on the Si (1 1 1) substrate have been studied by transmission electron microscopy in order to see the structure and growth mechanism. The GaAs nanocrystals grown epitaxially on the Si (1 1 1) surface kept at 573 K have thin shapes consisting of a flat surface which is parallel to the Si (1 1 1) surface. The crystalline structure of the initial growth layer approximately below 5 nm in thickness is the zincblend structure, but with increasing thickness the structure changes to the wurtzite structure by formation of orderly-arranged stacking faults. The small difference in the driving force between the wurtzite structure and the zincblende structure could bring about a situation, where the kinetic rate of nucleus formation is high for the wurtzite structure than for the zincblende structure. It would highly increase the probability that the wurtzite structure is formed as a non-equilibrium state.     

Beryllium doping and silicon amphotericity in (1 1 0) GaAs-based heterostructures: structural and optical properties

The use of beryllium as an acceptor at high doping levels in (1 1 0)GaAs-based heterostructures is found to be deleterious to the structural and optical properties of these epi-layers. This may limit the use of beryllium as a p-type dopant on the (1 1 0) surface. Because silicon is amphoteric on the (1 1 0), it can be used as an alternative p-type dopant, in addition to its traditional role as an n-type dopant. Transmission electron microscopy, optical absorption, and luminescence data indicate that high quality multiple quantum well structures with p-type GaAs buffer layers doped with silicon, rather than beryllium, can be grown.     

Growth and characterization of GaSb/AlGaSb multi-quantum well structures on Si (0 0 1) substrates

GaSb/AlGaSb multi-quantum well (MQW) structures with an AlSb initiation layer and a relatively thick GaSb buffer layer grown on Si (0 0 1) substrates were prepared by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated definite MQW structures. The photoluminescence (PL) emission around 1.55 μm wavelength was observed for 10.34 nm GaSb/30 nm Al_0.6Ga_0.4Sb MQW structure at room temperature. Dependence of PL emission energy on GaSb well width was well explained by finite square well potential model.     

Desymmetrization of the polyhedral crystal shape of tetragonal lysozyme due to face-growth rate fluctuations

We have studied the desymmetrization of the polyhedral crystalline shape of tetragonal lysozyme crystals due to the growth rate differences of the equivalent {1 0 1} planes. Using atomic force microscopy, we have observed the evolution of the multifaceted structures composed of four equivalent {1 0 1} faces during growth. In our growth condition, lateral step flow, where a large density of dislocations acts as a source of steps, is the dominant growth mechanism. The measured step flow velocities are almost independent of the separation between the neighboring steps, revealing that the local face normal growth rate is determined by the local step density. By tracing the motion of the vertex surrounded by the {1 0 1} faces, we have found that the desymmetrization of the crystalline shape is due to the large fluctuation of the local face normal growth rate, which is comparable in magnitude to the average growth rate.     

Characterization of homoepitaxial diamond for ionizing radiation detectors

Synthetic diamond has been proven as an important material for advanced electronic applications, such as those encountered in high energy physics and astrophysics. In fact, diamond transparency to visible light, its high carrier mobility, high breakdown field and strong resistance to chemical attack and radiation damage have suggested the potential application of this material for ‘solar-blind’ UV detectors which have to operate in extreme environments. To avoid the possible problems connected with the presence of grain boundaries in polycrystalline diamond films, a great effort has been devoted to the optimization of the growth process leading to high-quality single-crystal diamond on diamond substrates (homoepitaxy). In this view, characterization studies play a crucial role, because they provide the feedback for the optimization of the deposition process, in order to obtain the best quality material. In this work, a characterization study of homoepitaxial diamond grown by chemical vapor deposition (CVD) on synthetic diamond substrates is presented. The samples have been deposited in a CVD tubular reactor using a CH₄–H₂ gas mixture (1–7%) at approximately 560 °C substrate temperature. The growth rate ranged between 0.9 μm/h and 2.2 μm/h, microwave powers between 520 W and 720 W. The crystalline quality of the diamond layer has been studied by means of Raman spectroscopy. Photoluminescence has been used to study the nature and the distribution of impurities, having energy levels in the diamond band gap, which influence negatively the electronic quality of the material. The results have been compared with electro-optical characterization of UV detectors for astrophysics based on the analyzed diamond samples. The growth parameters which guarantees both high material quality and optimal device response have been determined.     

Self-limiting nature in atomic-layer epitaxy of rutile thin films from TiCl4 and H2O on sapphire (0 0 1) substrates

The atomic-layer epitaxy of rutile thin films on sapphire (0 0 1) substrates was studied in the controlled growth of titanium oxide films by sequential surface chemical reactions using sequentially fast pressurized titanium tetrachloride (TiCl₄) and water (H₂O) vapor pulses. Optical constants and thicknesses of these rutile films were investigated in terms of vapor pressure using a variable-angle spectroscopic ellipsometer. As a result, the self-limiting nature in the atomic-layer epitaxy of rutile thin films was demonstrated clearly under various conditions of dosing reactant vapors, where growth rates were almost constant at approximately 0.077 nm/cycle (0.77 nm/min) and refractive indices were also constant at 2.59.     

Heteroepitaxy of InSb films grown on a Si(0 0 1) substrate with AlSb buffer layer

The growth of InSb films on a Si(0 0 1) substrate with AlSb buffer layer was performed in an ultra high vacuum chamber (UHV) by a co-evaporation of elemental Indium (In) and antimony (Sb) sources. The samples were characterized by Auger electron spectroscopy (AES), X-ray diffraction (XRD) and atomic force microscopy (AFM). The surface morphology and the crystal quality of the grown films strongly depend on the flux ratio of Sb/In. It is found that the optimized flux ratio for the one-step growth procedure is about 2.9 to obtain the InSb films with smooth surface and good crystal quality, for the growth temperature of 300 °C. The two-step growth procedure was also used to further improve the crystal quality of the films.     

Crystallization kinetics of 1Na2O · 2CaO · 3SiO2 · glass monitored by electrical conductivity measurements

We investigated the kinetics of crystal nucleation, growth, and overall crystallization of a glass with composition close to the stoichiometric 1Na₂O · 2CaO · 3SiO₂. The nucleation and subsequent growth of sodium-rich crystals in this glass decreases the sodium content in the glassy matrix, drastically hindering further nucleation and growth. Compositional changes of the crystals and glassy matrix at different stages of the crystallization process were determined by EDS. These compositional variations were also monitored by electrical conductivity measurements, carried out by impedance spectroscopy, in glassy, partially, and fully crystallized samples. The electrical conductivity of both crystalline and glassy phases decreases with the increase of the crystallized volume fraction. Starting at a crystallized volume fraction of about 0.5, the crystalline phase dominates the electrical conductivity of the sample. This behavior was corroborated by an analysis of the activation energy for conduction. We show that electrical conductivity is highly sensitive and can indicate compositional shifts, changes in the spatial distribution of mobile ions in the glassy matrix. Conductivity measurements are thus a powerful tool for the investigation of complex heterogeneous systems, such as partially crystallized glasses and glass-ceramics.     

High-quality diamond films grown at high deposition rates using high-power-density MWPCVD method with conventional quartz-type chamber

Undoped and boron-doped homoepitaxial diamond films with high quality have been successfully grown on high-pressure/high-temperature-synthesized type-Ib single-crystalline diamond (1 0 0) substrates. In the growth process, a conventional microwave-plasma (MWP) chemical-vapor-deposition (CVD) system with an easily-exchangeable 36-mm-inner-diameter quartz-tube growth chamber was employed under a condition of high MW power densities while a rather high methane concentration (4%) and high substrate temperatures (>1000 °C) were used. The growth conditions applied to the undoped and B-doped diamond thin films were separately optimized by controlling the MW plasma density and substrate temperatures. The homoepitaxial films thus grown yielded strong exciton-related luminescence even at room temperature, meaning that their crystalline quality was good and roughly comparable with that of homoepitaxial films deposited using a high-power MWPCVD system with a stainless steel chamber having a rather large diameter. This indicates that by using such a conventional deposition system with inexpensive and easily-exchangeable exclusive-use quartz-tube chambers, various growth experiments can be performed under different process conditions without any severe interference among the different experiments.     

Hydrothermal epitaxy of KNbO3 thin films and nanostructures

Heteroepitaxial KNbO₃ thin films and nanostructures were grown hydrothermally on (1 0 0)-oriented single-crystal SrTiO₃ substrates at 125–200 °C in 15 M KOH solutions. The KNbO₃ film grew with the orthorhombic structure and displayed both {1 1 0)_o and (0 0 1)_o orientations as anisotropic lattice expansion reduced the difference in lattice mismatch seen by each orientation. After an initial period of approximately planar growth, the film gave rise to an array of nano-sized tower-like structures apparently growing by a dislocation-assisted mechanism. It is suggested that the anisotropic growth is further promoted by a combination of decreasing supersaturation and the increased effect of adsorbed impurities on the growth front.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

Ternary chalcogenides LiBC2 (B = In, Ga; C = S, Se, Te) for mid-IR nonlinear optics

We review on the growth improvement and optical characterization of a new family of ternary lithium-based chalcogenide crystals of generic formula ${\text{LiB}}^{\text{III}}{\text{C}}_2^{\text{VI}}$ (B = In, Ga; C = S, Se, Te) which displays improved thermo-mechanical properties for mid-IR nonlinear optical applications. Some of these compounds are now produced in sufficiently large size, single-domain quality to allow their implementation in optical parametric oscillators.     

Synthesis and luminescent properties of novel RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) micro-crystalline phosphors

Using rare earth coordination polymers with aromatic carboxylic acids as the precursors of rare earth oxide components, with polyethylene glycol (PEG) as the dispersing media, micro-crystalline phosphors RENbO₄:Ln³⁺ (RE = Y, Gd, Lu; Ln = Eu, Tb) have been synthesized by an in situ co-precipitation method. Both X-ray diffraction and scanning electron microscopy have shown that the resultant samples present are crystalline with ‘rice glue ball’ micro-morphology and crystalline grain sizes in the range of 1–2 μm. The luminescent properties of these phosphors have been studied, which show that the best photoluminescent performance is achieved for GdNbO₄:Tb³⁺ or Eu³⁺. This was because Gd³⁺ plays an important role to enhance the luminescence of Tb³⁺ or Eu³⁺ in an energy transfer process. In addition, the influence of the doping concentration on the fluorescence behaviors has been examined. With increase of the doping concentration from 1 mol% to 5 mol%, both the red emission intensity of Eu³⁺ and the green emission intensity of Tb³⁺ increase.