The properties of AlGaN films and AlGaN/GaN heterostructures grown on (11\bar{2}0) sapphire substrates

Al _x Ga_1-x N films and Al _x Ga_1-x N/GaN heterostructures were prepared on the ( ) sapphire substrates at elevated temperatures by alternate supply of trimethylgallium (TMG)/trimethylaluminum (TMA) and ammonia (NH₃) in an inductively heated quartz reactor. X-ray studies reveal the monocrystalline nature of these Al-containing structures. The results of absorption measurements of the Al _x Ga_1-x N films exhibit clear cut-off energies of the films. Based on the investigations of transmission electron microscopy (TEM), Al _x Ga_1-x N films and Al _x Ga_1-x N/GaN structures were found to deposit on the ( ) sapphire substrates with < 0001 > _AlGaN and being parallel to and , respectively.     

Deposition of GaN films on (1 1 1) Si substrates by alternate supply of TMG and NH3

GaN films were grown on (1 1 1) Si substrates at 1000 °C by separate admittances of trimethylgallium (TMG) and ammonia (NH₃). To achieve high quality GaN films, the optimization in growth temperature and layer thickness of AlN buffer layer between GaN film and Si substrate is required. Cross-sectional transmission electron microscopic observations of the GaN/(1 1 1)Si samples show a nearly parallel orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. Room temperature photoluminescence spectra of high quality GaN films show a strong near band edge emission and a weak yellow luminescence. The achievement of high quality GaN films on (1 1 1) Si substrates is believed to be attributed to enhancement in surface mobilities of the adsorbed surface species and adequate accommodation of lattice mismatch between high temperature AlN buffer layer and Si substrate.     

Defect structure in GaN pyramids

High-quality GaN/AlN layers grown on (111) Si substrates have been used as the seeding layer for lateral epitactic overgrowth of GaN. The selective overgrowth was controlled by depositing a Si₃N₄ mask on the GaN seed layer. Growth of additional GaN resulted in the formation of GaN pyramids above the apertures in the patterned Si₃N₄ mask. Transmission electron microscopy showed that the GaN pyramids, the GaN seed layer, and the AlN buffer layer in the samples have the following epitactic relationship with respect to the silicon substrate: and . The pyramids were found to consist of a defective core region and a nearly defect-free outer region. In the core of the pyramid (at, or above, the aperture in the mask), numerous dislocations thread through the pyramid perpendicular to the interface plane. Some of these threading dislocations, which originated from the GaN/AlN seed layer, bend abruptly through 90° at the edge of this core region. In the outer part of the GaN pyramid, the density of vertically propagating dislocations was much lower. Most of the dislocations in this region are closely parallel to the original (0001) substrate plane. The top few microns of material are found to be essentially defect-free. The growth mechanism of the GaN pyramids is discussed in light of this defect structure.     

Synthesis of vertically oriented GaN nanowires on a LiAlO2 substrate via chemical vapor deposition

Vertically oriented nanowires (NWs) of single-crystalline wurtzite GaN have been fabricated on γ-LiAlO₂ (100) substrate coated with a Au layer, via a chemical vapor deposition process at 1000 °C using gallium and ammonia as source materials. The GaN NWs grow along the nonpolar [100] direction with steeply tapering tips, and have triangular cross-sections with widths of 50–100 nm and lengths of up to several microns. The GaN NWs are formed by a vapor-liquid-solid growth mechanism and the tapering tips are attributed to the temperature decrease in the final stage of the synthesis process. The aligned GaN NWs show blue-yellow emission originating from defect levels, residual impurities or surface states of the GaN NWs, and have potential applications in nanotechnology.     

Characteristics of nanostructure and electrical properties of Ti thin films as a function of substrate temperature and film thickness

Titanium films of different thickness at different substrate temperatures are prepared using PVD method. The nanostructure of these films was obtained using X-ray diffraction (XRD) and AFM, while the thicknesses were measured by means of Rutherford back scattering (RBS) technique. Resistivity, Hall coefficient, concentration of carriers and the mobility in these films are obtained. The results show that, the rutile phase of TiO₂ is formed which is initially amorphous and as the film thickness increases it tends to become textured in (020) direction, which is more pronounced at higher temperatures and possibly transforms to anatase TiO₂ with (112) orientation for thickest films of 224 nm. The conductivity and concentration of carriers increase with thickness, while the Hall coefficient and the mobility decrease. The activation energies in these samples were obtained from the Arrhenius plots of σ and R _H. For thinner films ( eV) and for thickest film (224 nm) a break point is observed at about 500 K, which is consistent with the idea of more processes becoming activated at higher temperatures.     

Gas-sensing properties of spin-coated indium oxide film on various substrates

Indium oxide thin films were spin-coated uniformly on alumina and silicon substrates with different roughness for the fabrication of gas-sensing films by a novel method of aqueous acetic acid solution dissolving ln( and ammonium carboxymethyl cellulose. The effects of material properties and surface morphologies of the substrate on film formation and gas-sensing properties of the spin-coated films were studied for various kinds of substrates. Gas-sensing properties including sensitivity, selectivity and the rates of response and recovery were found to be strongly dependent on the kind of substrate and the surface roughness of the substrate as well as film thickness and operating temperature.     

Microstructure and ionic conductivity of alternating-multilayer structured Gd-doped ceria and zirconia thin films

Multilayer thin film of Gd-doped ceria and zirconia have been grown by sputter-deposition on α-Al₂O₃ (0001) substrates. The films were characterized using X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The Gd-doped ceria and zirconia layers had the fluorite structure and are highly textured such that the (111) plane of the films parallel to the (0001) plane of the α-Al₂O₃. The epitaxial relationship can be written as and , respectively. The absence of Ce³⁺ features in the XPS spectra indicates that the Gd-doped ceria films are completely oxidized. The ionic conductivity of this structure shows great improvement as compared with that of the bulk crystalline material. This research provides insight on designing of material for low temperature electrolyte applications.     

Hydrothermal processing of barium strontium titanate sol-gel composite thin films

Stoichiometric barium strontium titanate (BST) films of composition with thickness >2 μm have been fabricated on Si/SiO₂/Pt substrates by hydrothermal sol-gel composite processing. This film deposition technique involves the treatment of a spun-on sol-gel composite film, formed from a suspension of a powder in an aqueous BST sol-gel, at temperatures from 100–200°C at a pressure of 1–15 atm. An initial hydrolysis procedure eliminates dissolution of the dried sol-gel during the hydrothermal treatment. Glancing angle X-ray diffraction shows excellent crystallinity and stoichiometry in the BST films with no evidence of new phases created during processing. Scanning electron micrography and atomic force microscopy show densification of the film structure and the development of a bridging microstructure. Transmission electron micrography indicates that while much of the sol-gel derived matrix phase is amorphous a more crystalline interface occurs with the powder particles. The relative permittivity and loss tangent of the films are measured using a parallel plate capacitor technique in the frequency range 1–100 kHz. At 100 kHz relative permittivities of the films range from ɛ_r = 400–1200 and loss tangents lie in the range 0.05 < tan δ < 0.10, depending on the parameters of film preparation. The film structure and morphology and the electrical studies suggest that the microstructure of the films evolves by deposition of the sol-gel derived BST on the underlying powder, resulting in an electrically interconnected microstructure in which the sol-gel derived material bridges between the high permittivity powder particles.     

Fabrication of conductive SrRuO<Subscript>3</Subscript> thin film and Ba<Subscript>0.60</Subscript>Sr<Subscript>0.40</Subscript>TiO<Subscript>3</Subscript>/SrRuO<Subscript>3</Subscript> bilayer films on MgO substrate

Conductive SrRuO₃ (SRO) thin films have been grown on (100) MgO substrates by pulsed laser deposition (PLD) technique. Effects of oxygen pressure and deposition temperature on the orientation of SRO thin film were investigated. X-ray diffraction (XRD) θ/2θ patterns and the temperature dependent resistivity measurements indicated that oxygen pressure of 30 Pa and deposition temperature of 700 °C were the optimized deposition parameters. A parallel-plate capacitor structure was prepared with the SRO films deposited under optimized condition as an electrode layer and Ba_0.60Sr_0.40TiO₃ (BST) thin film as the dielectric layer. XRD Φ scans indicated a epitaxial relationship between BST and SRO on MgO substrate. The dielectric constant and loss tangent measured at 10 kHz and 300 K was 427 and 0.099 under 0 V bias, and 215 and 0.062 under 8 V bias, respectively. A tunability of 49.6% has been achieved with DC bias as low as 8 V. The C–V hysteresis curve and the P–E hysteresis loop suggested that the BST films epitaxially grown on SRO/MgO have ferroelectricity at room temperature. The induced ferroelectricity was believed to originate from the compressive strain between the epitaxial BST and SRO thin films. These results show the potential application of the BST/SRO heterostructures in microelectronic devices.     

Frictional Effects in Thin 3He Films

The recent high-precision torsional oscillator experiments of Casey et al. involving thin films of normal liquid ³He showed that the film decouples from the substrate with a time constant which is proportional to T ⁻¹ where T is the absolute temperature. We interpret this experiment by adapting a theory due to Meyerovich which was developed for dilute ³He-⁴He mixtures flowing between two relatively smooth plates. The analysis of the experiment confirms the central idea that varies as T ⁻¹. The variation of with film thickness, d, is affected by the change in the shape of the free surface of the film, due to van der Waals forces, as the film becomes thinner.     

Orientation relations between carbon nanotubes grown by chemical vapour deposition and residual iron-containing catalysts

Orientation relationships between the growth direction of carbon nanotubes and encapsulated residual iron-containing particles have been determined using transmission electron microscopy. The nanotubes that are prepared by Fe-catalysed chemical vapour deposition on sol–gel Fe(NO₃)₃-tetraethyl orthosilicate substrates are the helical multiwall type. Nanoscale particles of both the low-temperature α-Fe (ferrite) and high-temperature γ-Fe (austenite) were found in the cavity of the carbon nanotubes with , and parallel to the tube growth direction, respectively. Cementite Fe₃C, the most abundant Fe-containing phase in present samples was also found to be entrapped in nanotubes with or parallel to the tube axis. The metastable retention of γ-Fe particles at room temperature is ascribed to the strain energy induced at the particle-nanotube interface due to volume expansion upon the γ- → α-Fe phase transformation. The decomposition of initially high aspect-ratio, rod-shape particles into a string of ovulation, while encapsulated in carbon nanotubes is accounted for by the Rayleigh instability. Ovulation leading to reduced particle size has also contributed to increase the surface energy term that counterbalances the total free energy change of phase transformation from γ- to α-Fe and further aids to the metastable retention of γ-Fe.     

Growth mechanism of ZnO nanowires via direct Zn evaporation

Zinc oxide (ZnO) nanowire synthesized from direct Zinc (Zn) vapor transport in O₂ environment has been studied. The results show that the first step is the formation of ZnO film on the substrate. Then anisotropic abnormal grain growth in the form of ZnO platelets takes place. Subsequently, single-crystalline ZnO platelets grow in [0001] direction to form whiskers. During whisker growth, transformation from layer-by-layer growth to simultaneous multilayer growth occurs when the two-dimensional (2D) Ehrlich–Schwoebel (ES) barrier at the ZnO island edge is sufficiently large and the monolayer island diameter is smaller than the island spacing. As multilayered islands grows far away from the base, isotropic mass diffusion (spherical diffusion) will gradually displace anisotropic diffusion (linear diffusion), which contributes to the formation of pyramid on the top plane of the whisker. When the pyramid contains enough atomic layers, the 2D ES barrier transits to 3-dimensional ES barrier, which contributes to repeated nucleation and growth of multilayered islands or pyramids on the old pyramids. The pyramids play a critical role to taper the whisker to nanorod with a diameter less than 100 nm. The nanorod then grows to nanowire via repeated growth of epitaxial hexagonal-pyramid shape-like islands on the (0001)-plane with facets as the slope planes. During coarsening, the breakage of step motion of facets and the appearance of facets on the base of pyramids may result from the step bunching of {0001} facets, which is consistent with the existence of “2D” Ehrlich–Schwoebel barrier on the edge of (0001) facets. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Growth process from amorphous GaN to polycrystalline GaN on Si (111) substrates

Amorphous GaN (a-GaN) films on Si (111) substrates have been deposited by RF magnetron sputtering with GaN powder target. The growth process from amorphous GaN to polycrystalline GaN is studied by XRD, SEM, PL and Raman. XRD data mean that annealing under flowing ammonia at 850-950 °C for 10 min converts a-GaN into polycrystalline GaN (p-GaN). The growth mechanism can be mostly reaction process through N³⁻ in amorphous GaN replaced by N³⁻ of NH₃. Annealing at 1000 °C, the appearance of GaN nanowires can be understood based on the vapor-liquid-solid (VLS) mechanism. In addition, XRD, PL and Raman measurement results indicate that the quality of GaN films increases with increasing temperature. The tensile stress in the films obtained at 1000 °C is attributable to the expansion mismatch between GaN and Si, with the gallium in the film playing a negligible role.     

Voltage tunable epitaxial Pb x Sr(1− x )TiO3 films on sapphire by MOCVD: Nanostructure and microwave properties

Frequency and phase agile microwave components such as tunable filters and phase shifters will require ferroelectric thin films that exhibit a nonlinear dependence of dielectric permittivity (ɛ _r ) with dc electric bias, as well as a high material (Δɛ _r /tan δ) and device (or K-factor in phase shift/dB) figure of merits (FOM). Therefore, voltage tunable (Pb_0.3Sr_0.7)TiO₃ (PST) thin films (90–150 nm) on (0001) sapphire were deposited by metal-organic chemical vapor deposition at rates of 10–15 nm/min. The as-deposited epitaxial PST films were characterized by Rutherford backscattering spectroscopy, X-ray methods, field emission scanning electron microscope, high resolution transmission electron microscopy, Raman spectroscopy, and electrical methods (7–17 GHz) using coplanar waveguide test structures. The epitaxial relationships were as follows: out-of-plane alignment of [111] PST//[0001] sapphire, and orthogonal in-plane alignments of [ ] PST//[ ] sapphire and [ ] PST//[ ] sapphire. The material FOM and device FOM (or K-factor) at 12 GHz were determined to be 632 and ∼13 degrees/dB, respectively. The results are discussed in light of the nanostructure and stress in epi-PST films. Finally, a rational basis for the selection of PST composition, substrate, and process parameters is provided for the fabrication of optimized coplanar waveguide (CPW) phase shifters with very high material and device FOMs.     

The growth of GaN films by alternate source gas supply hot-mesh CVD method

Gallium nitride (GaN) films and Aluminium nitride (AlN) layers were deposited on SiC/Si (111) substrates by an alternating source gas supply or an intermittent supply of a source gas such as ammonia (NH₃), trimethylgallium (TMG) or trimethylaluminum (TMA) in a hot-mesh chemical vapor deposition (CVD) apparatus. The AlN layer was deposited as a buffer layer using NH₃ and TMA on a SiC layer grown by carbonization on Si substrates using propane (C₃H₈). GaN films were grown on an AlN layer by a reaction between NH_x radicals generated on a ruthenium (Ru) coated tungsten (W)-mesh and TMG molecules. An alternating source gas supply or an intermittent supply of one of the source gases during the film growth are expected to be effective for the suppression of gas phase reactions and for the enhancement of precursor migration on the substrate surface. By the intermittent supply of alkylmetal gas only during the growth of the AlN layer, the defect generation in the GaN films was reduced. GaN film growth by intermittent supply on an AlN buffer layer, however, did not lead to the improvement of the film quality.     

Highly conductive and transparent In-doped zinc oxide thin films deposited by chemical spray using Zn(C5H7O2)2

Highly conductive and transparent indium-doped zinc oxide, ZnO, thin films were deposited on sodocalcic glass substrates by chemical spray, using zinc acetylacetonate, Zn(C₅H₇O₂)₂, and doped with indium chloride. Substrate temperature, dopant concentration in the starting solution and the kind of alcohol used as a solvent (methanol, ethanol and isopropanol) were varied in order to optimize deposition conditions. The lowest resistivity value of was obtained with ethanol at a substrate temperature of 475 °C and a [In]/[Zn]=2.5 at % ratio in the starting solution. The mobility and the carrier concentration values were in the order of and , respectively. For optimal deposition conditions no preferential growth was found. Surface morphology was altered depending on the kind of alcohol used, producing a rough surface with isopropanol than with methanol or ethanol. Transmittance average was of the order of 85%, at 550 nm.     

Conductive and transparent ZnO:Al thin films obtained by chemical spray

Electrical, structural, morphological and optical characteristics of ZnO:Al thin films obtained by chemical spray are presented in this paper. The dependence of the resistivity on the substrate temperature and the film thickness is reported. For the optimized conditions with no post-annealing the lowest resistivity values obtained for ZnO:Al thin films were for films with thicknesses of 1500 and 600 nm, respectively. Preferential growth in the (0 0 2) direction was observed in all cases. The surface morphology was analyzed by using atomic force and scanning electron microscopy (AFM and SEM) techniques. High transmittance, 85%, was obtained in all cases. The band gap was of the order of 3.35 eV.     

Amorphous carbon buffer layers for separating free gallium nitride films

The possibility of using amorphous diamond-like carbon (DLC) films for self-separation of gallium nitride (GaN) layers grown by hydride vapor-phase epitaxy has been analyzed. DLC films have been synthesized by plasma-enhanced chemical vapor deposition under low pressure on sapphire (Al₂O₃) substrates with a (0001) crystallographic orientation. The samples have been studied by the methods of Raman scattering and X-ray diffraction analysis. It is shown that thin DLC films affect only slightly the processes of nucleation and growth of gallium nitride films. Notably, the strength of the “GaN film–Al₂O₃” substrate interface decreases, which facilitates separation of the GaN layers.     

Effect of annealing temperature on void coalescence in 5086 aluminium alloy formed under different stress conditions

In this paper, the influence of annealing temperature on formability, fracture behavior, void nucleation, its growth, and coalescence are studied. The voids and fracture behavior are studied as a function of various void parameters, namely δd-factor (ligament thickness between consecutive voids), d factor (ratio of δd and radius of the void), void area fraction (V_a) and L/W ratio (ratio of length to width of the void). The L/W ratio of the oblate or prolate voids at fracture is correlated with the mechanical properties, microstructure, minor strain at fracture (ε₂), Mohr’s circle shear strains, stress, and strain triaxiality factors. The Lode factor (θ) is determined and correlated with the Stress triaxiality factor (T), which is the ratio of mean stress (σ_m) to effective stress (σ_e). In addition, the Void area fraction (V_a), which is the ratio of void area to the representative sample area, is determined correlated with the Strain triaxiality factor (T_o). It is found that the sheets annealed at 300°C, possesses better formability due to lower d-factor, higher and -values, greater void area fraction and lower L/W ratio of void accommodating more plastic deformation.     

Initial stages of the ion-beam assisted epitaxial GaN film growth on 6H-SiC(0001)

Ultra-thin gallium nitride (GaN) films were deposited using the ion-beam assisted molecular-beam epitaxy technique. The influence of the nitrogen ion to gallium atom flux ratio (I/A ratio) during the early stages of GaN nucleation and thin film growth directly, without a buffer layer on super-polished 6H-SiC(0001) substrates was studied. The deposition process was performed at a constant substrate temperature of 700 °C by evaporation of Ga and irradiation with hyperthermal nitrogen ions from a constricted glow-discharge ion source. The hyperthermal nitrogen ion flux was kept constant and the kinetic energy of the ions did not exceed 25 eV. The selection of different I/A ratios in the range from 0.8 to 3.2 was done by varying the Ga deposition rate between 5 × 10¹³ and 2 × 10¹⁴ at. cm^− 2 s^− 1. The crystalline surface structure during the GaN growth was monitored in situ by reflection high-energy electron diffraction. The surface topography of the films as well as the morphology of separated GaN islands on the substrate surface was examined after film growth using a scanning tunneling microscope without interruption of ultra-high vacuum. The results show, that the I/A ratio has a major impact on the properties of the resulting ultra-thin GaN films. The growth mode, the surface roughness, the degree of GaN coverage of the substrate and the polytype mixture depend notably on the I/A ratio.