Molecular beam epitaxy (MBE) growth and structural properties of GaN and AlN on 3C-SiC(0 0 1) substrates

AlN and GaN was deposited by molecular beam epitaxy (MBE) on 3C-SiC(0 0 1) substrates on low-temperature (LT) GaN and AlN buffer layers. It is shown that not only GaN but also epitaxial AlN can be stabilized in the metastable zincblende phase. The zincblende AlN is only obtained on a zincblende LT-GaN buffer layer; on the other hand, AlN crystallizes in the wurtzite phase if it is grown directly on a 3C-SiC(0 0 1) substrate or on a LT-AlN buffer layer. The structural properties of the layers and in particular the orientation relationship of the wurtzite AlN on the 3C-SiC(0 0 1) were analyzed by conventional and high-resolution transmission electron microscopy.     

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.     

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.     

Er-doped hydrogenated amorphous silicon: structural and optical properties

The effect of erbium-doping on the structural and optical properties of hydrogenated amorphous silicon (a-Si:H) is investigated. Optical absorption and Raman spectra indicate that erbium doping introduces defect states, and that above a concentration of 0.27 at.%, induces strong structural disorder. The photoluminescence measurements show that erbium doping introduces non-radiative decay paths for carriers in a-Si:H, leading to decrease in both the Er³⁺ and intrinsic a-Si:H luminescence intensity when the Er concentration is increased to more than 0.04 at.%. The results are compared to that of Er-doped crystalline Si, and the possible excitation mechanisms of Er in a-Si:H are discussed.     

Enhanced two-dimensional growth of MOVPE InN films on sapphire (0 0 0 1) substrates

MOVPE growth of InN on sapphire substrates is compared using two different designs of horizontal reactor. The major difference between the two designs is a variation in the reactant-gas flow-spacing between the substrate and the ceiling of the quartz chamber: 33 mm for the Type A reactor and 14 mm for Type B. Compared with the Type A reactor, the Type B reactor brings about InN films with a larger grain size. This is especially true when InN is grown at 600°C using the Type B reactor, in which case the two-dimensional (2D) growth of InN is found to be extremely enhanced. An investigation of the NH₃/TMIn molar ratio dependence of the surface morphology of grown InN films using the two reactors suggests that the enhanced 2D growth is attributed to the decrease in the effective NH₃/TMIn ratio in the growth atmosphere. Even using the Type A reactor, a film with enhanced 2D growth can be obtained when the NH₃/TMIn ratio is considerably low (1.8×10⁴). The enhanced 2D growth results in a smaller XRC-FWHM (full-width at half maximum of the X-ray rocking curve) (1500 arcsec), than that for a 3D-grown film (5000 arcsec).     

The growth of Pd thin films on a 6H-SiC(0 0 0 1) substrate

Pd thin films, grown on Si-rich 6H-SiC(0 0 0 1) substrates, were studied by atomic force microscopy, electron diffraction and high-resolution transmission electron microscopy. It is concluded that the growth is successful only when all the growth process takes place at room temperature. Under these conditions a very good epitaxial growth of Pd is achieved, despite the large misfit (about 8.6%) between Pd and the substrate and the existence of a semi-amorphous layer between the thin film and the substrate. A large number of twins appear in these films.     

Defect reduction in GaN epilayers and HFET structures grown on (0 0 0 1)sapphire by ammonia MBE

The quality of GaN epilayers grown by molecular beam epitaxy on substrates such as sapphire and silicon carbide has improved considerably over the past few years and in fact now produces AlGaN/GaN HEMT devices with characteristics among the best reported for any growth technique. However, only recently has the bulk defect density of MBE grown GaN achieved levels comparable to that obtained by MOVPE and with a comparable level of electrical performance. In this paper, we report the ammonia-MBE growth of GaN epilayers and HFET structures on (0 0 0 1)sapphire. The effect of growth temperature on the defect density of single GaN layers and the effect of an insulating carbon doped layer on the defect density of an overgrown channel layer in the HFET structures is reported. The quality of the epilayers has been studied using Hall effect and the defect density using TEM, SEM and wet etching. The growth of an insulating carbon-doped buffer layer followed by an undoped GaN channel layer results in a defect density in the channel layer of 2×10⁸ cm⁻². Mobilities close to 490 cm²/Vs at a carrier density of 8×10¹⁶ cm⁻³ for a 0.4 μm thick channel layer has been observed. Growth temperature is one of the most critical parameters for achieving this low defect density both in the bulk layers and the FET structures. Photo-chemical wet etching has been used to reveal the defect structure in these layers.     

Growth of CdS/ZnS strained layer superlattices on GaAs(0 0 1) by molecular-beam epitaxy with special reference to their structural properties and lattice dynamics

Strained layer CdS/ZnS superlattices have been grown on GaAs(0 0 1) by molecular-beam epitaxy using CdS and ZnS compound sources. The samples were investigated with special reference to their structural properties and lattice dynamics by means of X-ray diffraction and Raman spectroscopy. The results of four superlattices with different period length are discussed in detail. X-ray diffraction profiles show superlattice satellite peaks up to the fourth order indicating a high degree of periodicity. The lateral and in-depth homogeneity of the period length is also confirmed by Raman investigations. Folded longitudinal acoustic phonons in CdS/ZnS superlattices were observed for the first time. The experimental values agree very well with theoretical calculations based on the Rytov model and show the expected dependency on the superlattice period. The behaviour of the optical phonons is mainly determined by strain induced shifts caused by the high lattice mismatch (−7%) for this system. A good agreement between theoretical predictions and detected frequencies is obtained.     

Epitaxial variations of Ni films grown on MgO(0 0 1)

Epitaxial Ni films were deposited on (0 0 1)MgO by DC magnetron sputtering under ultra-high vacuum conditions for studies involving magnetic-multilayer applications. The deposition temperatures of the Ni films studied in this work were 100 and 400°C. Examination by transmission electron microscopy (TEM) and electron diffraction revealed that the film deposited at the lower temperature was predominately Ni[0 0 1]MgO[0 0 1] and Ni(0 1 0)MgO(0 1 0) oriented and smooth, as expected. However, the higher temperature films were predominately of the Ni MgO[0 0 1] and Ni MgO(1 0 0) orientation and facetted. The orientation has been confirmed by X-ray diffraction, where this orientation was observed to be four-fold degenerate. For each of these four orientations there also existed a twin orientation, reflected about the MgO(1 0 0) planes, giving eight possible orientations for the Ni crystallites on MgO. This epitaxial relationship was studied by dark-field TEM and electron diffraction. Because these films were polycrystalline and hence produced many diffraction spots from both the Ni and MgO with similar lattice spacings, electron diffraction patterns of the films were indexed using an electron diffraction image processing (EDIP) technique. In this technique, the polycrystalline electron diffraction pattern was converted into a graph, with the x-axis displaying lattice spacings and the y-axis, integrated intensity.     

Geometric modeling of homoepitaxial CVD diamond growth: I. The {1 0 0}{1 1 1}{1 1 0}{1 1 3} system

Plasma-assisted CVD homoepitaxial diamond growth is a process that must satisfy many stringent requirements to meet industrial applications, particularly in high-power electronics. Purity control and crystalline quality of the obtained samples are of paramount importance and their optimization is a subject of active research. In the process of such studies, we have obtained high purity CVD diamond monocrystals with unusual morphologies, namely with apparent {1 1 3} stable faces. This phenomenon has led us to examine the process of CVD diamond growth and build up a 3D geometrical model, presented here, describing the film growth as a function of time. The model has been able to successfully describe the morphology of our obtained crystals and can be used as a predictive tool to predetermine the shape and size of a diamond crystal grown in a given process configuration. This renders accessible control of desirable properties such as largest usable diamond surface area and/or film thickness, before the cutting and polishing manufacture steps take place. The two latter steps are more sensitive to the geometry of the growth sectors, which will be addressed in a companion paper.Our model, applicable to the growth of any cubic lattice material, establishes a complete mapping of the final morphology state of growing diamond, as a function of the growth rates of the crystalline planes considered, namely {1 0 0}, {1 1 1}, {1 1 0}, and {1 1 3} planes, all of which have been observed experimentally in diamond films. The model makes no claim as to the stability of the obtained faces, such as the occurrence of non-epitaxial crystallites or twinning. It is also possible to deduce transient behavior of the crystal morphology as growth time is increased. The model conclusions are presented in the form of a series of diagrams, which trace the existence (and dominance) boundaries of each face type, in presence of the others, and where each boundary crossing represent a topology change in terms of number of faces, edges and vertices. We validate the model by matching it against crystals published in the literature and illustrate its predictive value by suggesting ways to increase usable surface area of the diamond film.     

Transport properties of Se80−xTe20Bix (0 ⩽ x ⩽ 1) system

Thermal studies using differential scanning calorimetry under non-isothermal conditions with the samples heated at different rates (5, 10, 15 and 20 K/min) have been reported and discussed. The glass transition activation energy and crystallization activation energy have been determined. Thermal stability and glass-forming tendency have also been studied. The activation energy in dark and optical bandgap have been calculated. It has been found that optical bandgap is equal to twice the dark thermal activation energy.     

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.     

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.     

Influence of AlN nucleation layer growth conditions on quality of GaN layers deposited on (0 0 0 1) sapphire

The influence of AlN nucleation layer (NL) growth conditions on the quality of GaN layer deposited on (0 0 0 1) sapphire by organometallic chemical vapor phase epitaxy (OMVPE) has been investigated by X-ray diffraction, atomic force microscopy and transmission electron microscopy. Growth pressure, temperature and time were varied in this study. Results indicate that there exists an optimal thickness of the NL is required for optimal growth. Both thin and thick NLs are not conducive to the growth of high-quality GaN layers. Arguments have been developed to rationalize these observations.