Microstructural and surface property variations due to the amorphous region formed by thermal annealing in Al-doped ZnO thin films grown on n-Si (1 0 0) substrates

X-ray diffraction (XRD) patterns revealed that the as-grown and annealed Al-doped ZnO (AZO) films grown on the n-Si (1 0 0) substrates were polycrystalline. Transmission electron microscopy (TEM) images showed that bright-contrast regions existed in the grain boundary, and high-resolution TEM (HRTEM) images showed that the bright-contrast regions with an amorphous phase were embedded in the ZnO grains. While the surface roughness of the AZO film annealed at 800 °C became smoother, those of the AZO films annealed at 900 and 1000 °C became rougher. XRD patterns, TEM images, selected-area electron diffraction patterns, HRTEM images, and atomic force microscopy (AFM) images showed that the crystallinity in the AZO thin films grown on the n-Si (1 0 0) substrates was enhanced resulting from the release in the strain energy for the AZO thin films due to thermal annealing at 800 °C. XRD patterns and AFM images show that the crystallinity of the AZO thin films annealed at 1000 °C deteriorated due to the formation of the amorphous phase in the ZnO thin films.     

Existence and atomic arrangement of microtwins in CdTe epilayers grown on GaAs (211) B substrates

Selected area electron diffraction pattern (SADP) and high-resolution transmission electron microscopy (HRTEM) measurements were carried out to investigate the existence and the atomic arrangement of microtwins in CdTe epilayers grown on GaAs (211) B substrates by using molecular beam epitaxy. The SADP results showed that an epitaxial relationship between the CdTe epilayer and the GaAs substrate was formed. The lattice of the CdTe (211) tilts about 2° with respect to the GaAs (211) B substrate about the CdTe [110]∥GaAs [110] common zone axis. The HRTEM images showed that microtwins were formed in the CdTe epilayers. A possible atomic arrangement of the microtwins is presented on the basis of the HRTEM result. The present observations can help to improve understanding of the microstructural properties in CdTe epilayers grown on GaAs substrates.     

Formation mechanisms of GaN nanorods grown on Si(1 1 1) substrates

Scanning electron microscopy (SEM) images, transmission electron microscopy (TEM) images, and selected-area electron diffraction (SAED) patterns showed that vertically well aligned GaN nanorods with c-axis-oriented crystalline wurzite structures were grown on Si(1 1 1) substrates by using hydride vapor phase epitaxy. The high-resolution TEM (HRTEM) images showed that the crystallized GaN nanorods contained very few defects and that they were consisted of , {0 0 0 1}, and { } facets. The formation mechanisms for the GaN nanorods grown on Si(1 1 1) substrates are described on the basis of the SEM, TEM, SAED pattern, and HRTEM results.     

Magnetoresistance of magnetite thin films grown by pulsed laser deposition on GaAs(1 0 0) and Al2O3(0 0 0 1)

Magnetotransport properties of magnetite thin films deposited on gallium arsenide and sapphire substrates at growth temperatures between 473 and 673 K are presented. The films were grown by UV pulsed laser ablation in reactive atmospheres of O₂ and Ar, at working pressure of 8 × 10⁻² Pa. Film stoichiometry was determined in the range from Fe_2.95O₄ to Fe_2.97O₄. Randomly oriented polycrystalline thin films were grown on GaAs(1 0 0) while for the Al₂O₃(0 0 0 1) substrates the films developed a (1 1 1) preferred orientation. Interfacial Fe³⁺ diffusion was found for both substrates affecting the magnetic behaviour. The temperature dependence of the resistance and magnetoresistance of the films were measured for fields up to 6 T. Negative magnetoresistance values of ∼5% at room temperature and ∼10% at 90 K were obtained for the as-deposited magnetite films either on GaAs(1 0 0) or Al₂O₃(0 0 0 1).     

Growth of thin Fe(0 0 1) films for terahertz emission experiments

The electrical and magnetic properties of thin iron (Fe) films have sparked significant scientific interest. Our interest, however, is in the fundamental interactions between light and matter. We have discovered a novel application for thin Fe films. These films are sources of terahertz (THz) radiation when stimulated by an incident laser pulse. After intense femtosecond pulse excitation by a Ti:sapphire laser, these films emit picosecond, broadband THz frequencies. The terahertz emission provides a direct measure of the induced ultrafast change in magnetization within the Fe film. The THz generation experiments and the growth of appropriate thin Fe films for these experiments are discussed. Several criteria are used to select the substrate and film growth conditions, including that the substrate must permit the epitaxial growth of a continuous, monocrystalline or single crystal film, yet must also be transparent to the emitted THz radiation. An Fe(0 0 1) film grown on the (0 0 1) surface of a magnesium oxide (MgO) substrate makes an ideal sample. The Fe films are grown by physical vapor deposition (PVD) in an ultrahigh vacuum (UHV) system. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) are used to characterize the Fe(0 0 1) films. Two substrate surface preparation methods are investigated. Fe(0 0 1) films grown on MgO(0 0 1) substrates that are used as-received and films grown on MgO(0 0 1) substrates that have been UV/ozone-cleaned ex vacuo and annealed in vacuo produce the same results in the THz generation experiments. Either substrate preparation method permits the growth of samples suitable for the THz emission experiments.     

Spin wave excitations in Fe films grown on GaAs(1 1 3)A substrates

The spin wave excitation and its size effect has been studied in Al-capped Fe films grown on low-symmetry GaAs(1 1 3)A substrates. The temperature dependence of saturation magnetization follows an effective Bloch's law as long as magnetization remains larger than about 70% of its saturation value. A significant increase of the spin wave parameter B is found in Al-capped ultrathin Fe films grown on GaAs(1 1 3)A compared to bulk Fe, Fe films on GaAs(0 0 1) and other systems. This is explained as a result of the reduction in uniaxial magnetic anisotropy observed in this orientation for the same thickness range. However, this observed uniaxial magnetic anisotropy is found to be a likely reason for stabilizing the ferromagnetism.     

Effects of different annealing atmospheres on the surface and microstructural properties of ZnO thin films grown on p-Si (1 0 0) substrates

Effects of different annealing atmospheres on the surface and microstructural properties of ZnO thin films grown on Si (1 0 0) substrates were investigated. X-ray diffraction results showed that the crystallinity of the ZnO thin film annealed in an oxygen atmosphere was better than that annealed in a nitrogen atmosphere. Atomic force microscopy and transmission electron microscopy (TEM) images showed that the surfaces of the ZnO thin films annealed in a nitrogen atmosphere became very rough in contrast to those annealed in an oxygen atmosphere. High-resolution TEM images showed that many stacking faults and tilted grains could be observed in the ZnO thin films annealed in a nitrogen atmosphere in contrast to those annealed in an oxygen atmosphere. Surface morphology and microstructural property variations due to different annealing atmospheres in ZnO thin films are also described on the basis of the experimental results.     

Role of crystal orientation on the magnetic properties of CoFe2O4 thin films grown on Si (1 0 0) and Al2O3 (0 0 0 1) substrates using pulsed laser deposition

We report the influence of crystal orientation on the magnetic properties of CoFe₂O₄ (CFO) thin films grown on single crystal Si (1 0 0) and c-cut sapphire (Al₂O₃) (0 0 0 1) substrates using pulsed laser deposition technique. The thickness was varied from 200 to 50 nm for CFO films grown on Si substrates, while it was fixed at 200 nm for CFO films grown on Al₂O₃ substrates. We observed that the 200 and 100 nm thick CFO-Si films grew in both (1 1 1) and (3 1 1) directions and displayed out-of-plane anisotropy, whereas the 50 nm thick CFO-Si film showed only an (1 1 1) orientation and an in-plane anisotropy. The 200 nm thick CFO film grown on an Al₂O₃ substrate was also found to show a complete (1 1 1) orientation and a strong in-plane anisotropy. These observations pointed to a definite relation between the crystalline orientation and the observed magnetic anisotropy in the CFO thin films.     

Epitaxial growth of Nd2Hf2O7(1 1 1) thin films on Ge(1 1 1) substrates by pulsed laser deposition

Nd₂Hf₂O₇ (NHO) thin films have been epitaxially grown by pulsed laser deposition (PLD) on Ge(1 1 1) substrates. In situ reflection high-energy electron diffraction (RHEED) evolution of the (1 1 1)-oriented NHO during the deposition has been investigated and shows that the epilayer has a twin-free character with type-B stacking. Interfacial structure of NHO/Ge has been examined by high-resolution transmission electron microscopy (HRTEM). The results indicate a highly crystalline film with a very thin interface, and the orientation relationship between NHO and Ge can be denoted as (1 1 1)_NHO//(1 1 1)_Ge and . Finally, twin-free epitaxial growth of NHO with type-B orientation displays temperature dependence and the type-B epitaxy is favored at high temperature.     

Effect of annealing on the structural and optical properties of (3 1 1)B GaAsBi layers

The influence of post-growth annealing on the microstructure and photoluminescence (PL) of GaAsBi alloys grown on (3 1 1)B GaAs is analyzed. Conventional transmission electron microscopy (TEM) performed on as-grown samples evidence the presence of structural defects and a mosaic structure in the GaAsBi layer. A sequence of stacking faults at regions close to the GaAs/GaAsBi interface are observed in high resolution TEM images. After annealing at 473 K during 3 h the mosaic structure disappears, the presence of defects is reduced and the PL peak intensely enhances.     

Structure and homoepitaxial growth of GaAs(6 3 1)

We have studied the surface atomic structure of GaAs(6 3 1), and the GaAs growth by molecular beam epitaxy (MBE) on this plane. After the oxide desorption process at 585 °C reflection high-energy electron diffraction (RHEED) showed along the [−1 2 0] direction a 2× surface reconstruction for GaAs(6 3 1)A, and a 1× pattern was observed for GaAs(6 3 1)B. By annealing the substrates for 60 min, we observed that on the A surface appeared small hilly-like features, while on GaAs(6 3 1)B surface pits were formed. For GaAs(6 3 1)A, 500 nm-thick GaAs layers were grown at 585 °C. The atomic force microscopy (AFM) images at the end of growth showed the self-formation of nanoscale structures with a pyramidal shape enlarged along the [5−9−3] direction. Transversal views of the bulk-truncated GaAs(6 3 1) surface model showed arrays of atomic grooves along this direction, which could influence the formation of the pyramidal structures.     

ZnTe/GaAs(2 1 1)B heterojunction valence band discontinuity measured by X-ray photoelectron spectroscopy

Thin ZnTe layers were grown by molecular beam epitaxy on single crystal GaAs(2 1 1)B substrates. Reflection high energy electron diffraction monitored the deoxidation of substrate and entire growth process. Valence band offset was calculated with X-ray photoelectron spectroscopy. Also interface formation of the ZnTe/GaAs was studied. Analysis shows that interface is abrupt and calculated valance band offset is 0.25 ± 0.1 eV and indicates type I alignment. The experimental result agrees well with the theoretical predictions involving interface dipole effect as well as electron affinity rule.     

Highly oriented (1 0 0) ZnO thin films by spray pyrolysis

Undoped ZnO thin films have been deposited onto glass substrates by spray pyrolysis. The structural, electrical and optical properties were studied on thin films, prepared from precursor solutions with varying the ethanol concentrations. X-ray diffraction studies have shown polycrystalline nature of the films with a hexagonal wurtzite-type structure. The preferential orientation plane (1 0 0) of the ZnO thin film is found to be sensitive to ethanol concentration. The texture coefficient (TC) and grain size value have been calculated. Also ethanol concentration was found to have significant effect on sheet resistivity of the films.     

Atypical grain growth for (2 1 1) CdTe films deposited on surface reconstructed (1 0 0) SrTiO3 substrates

The (1 0 0) SrTiO₃ substrate has emerged as the oxide substrate of choice for the deposition of a wide variety of materials. The substrate's unavoidable miscut leads to a step-terrace morphology when heated to high temperatures. This morphological transition is accompanied by an atomic scale repositioning of the uppermost terrace atoms, the nature of which is strongly dependent on the substrate temperature and ambient atmosphere used. Here, we report the deposition of CdTe films on the as-received and reconstructed surfaces of (1 0 0) SrTiO₃. The as-received substrate gives rise to a [1 1 1] CdTe film with four equally distributed in-plane grain orientations. The surface reconstruction, on the other hand, gives rise to an unprecedented reorientation of the film's grain structure. For this case, a [2 1 1] CdTe film emerges having twelve unevenly distributed in-plane orientations. We attribute the film's grain structure to an atomic scale surface reconstruction, with the anisotropic distribution of grain-types arising from a preferential formation due to the step edges.     

Self-assembled InAs island formation on GaAs (1 1 0) by metalorganic vapor phase epitaxy

Formation of self-assembled InAs 3D islands on GaAs (1 1 0) substrate by metal organic vapor phase epitaxy has been investigated. Relatively uniform InAs islands with an average areal density of 10⁹ cm⁻²are formed at 400 ° C using a thin InGaAs strain reducing (SR) layer. No island formation is observed without the SR layer. Island growth on GaAs (1 1 0) is found to require a significantly lower growth temperature compared to the more conventional growth on GaAs (1 0 0) substrates. In addition, the island height is observed to depend only weakly on the growth temperature and to be almost independent of the V/III ratio and growth rate. Low-temperature photoluminescence at 1.22 eV is obtained from the overgrown islands.     

High-resolution electron microscopy study of SiGeC thin films grown on Si(1 0 0) by laser-assisted techniques

PLIE was used for rapid crystallisation of a-SiGeC films deposited by LCVD on Si(1 0 0) substrates. HRTEM study of thin films grown with several laser energies shows that the combination of the two laser techniques gives an almost completely crystallised alloy, even for the lowest laser fluence. Island formation is observed below a certain threshold of fluence (about 450 mJ/cm²). In the case of the lowest energy (100 mJ/cm²) the material was partially crystallised (with the crystalline material being the predominant state), to a nanocrystalline alloy with a considerable amount of epitaxialy grown grains and with grain sizes of several tens of nanometers. Above the threshold of 450 mJ/cm² a rather smooth thin film is grown. The crystallisation is almost complete and the alloy is grown in an almost perfect epitaxial way.     

Preparation and structural properties of MgO films grown on GaAs substrate

Epitaxial MgO thin films have been grown on semiinsulating GaAs (0 0 1) substrates using electron beam (e-beam) evaporation. X-ray diffraction indicates c-axis oriented MgO with (0 0 2) reflection only and rocking curve widths ∼2.2-3°. Transmission electron microscopy (TEM) analyses confirm an epitaxial growth of the MgO films. We study the microstructure and the defects at the interface between the MgO film and the GaAs substrate. Auger electron Spectroscopy (AES) concentration depth profiles reveal no contamination of the MgO films by As and Ga at different temperatures of the deposition process.     

Characteristics of Ni films deposited on SiO2/Si(1 0 0) and MgO(0 0 1) by direct current magnetron sputtering system with the oblique target

280 nm-thick Ni films were deposited on SiO₂/Si(1 0 0) and MgO(0 0 1) substrates at 300 K, 513 K and 663 K by a direct current magnetron sputtering system with the oblique target. The films deposited at 300 K mainly have a [1 1 0] crystalline orientation in the film growth direction. The [1 1 0]-orientation weakens and the [1 1 1]- and [1 0 0]-orientations enhance with increasing deposition temperature. The lattice constant of the Ni films is smaller than that of the Ni bulk, except for the film grown on MgO(0 0 1) at 663 K. Furthermore, as the deposition temperature increases, the lattice constant of the films grown on the SiO₂/Si(1 0 0) decreases whereas that of the films grown on the MgO(0 0 1) increases. The films deposited at 300 K and 513 K grow with columnar grains perpendicular to the substrate. For the films deposited at 663 K, however, the columnar grain structure is destroyed, i.e., an about 50 nm-thick layer consisting of granular grains is formed at the interface between the film and the substrate and then large grains grow on the layer. The Ni films deposited at 300 K consist of thin columnar grains and have many voids at the grain boundaries. The grains become thick and the voids decrease with increasing deposition temperature. The resistivity of the film decreases and the saturation magnetization increases with increasing deposition temperature.     

Fabrication of rubrene nanowires on vicinal (0 0 0 1) sapphire surfaces

Morphology of high-vacuum deposited rubrene thin films on the annealed (0 0 0 1) vicinal sapphire surfaces was studied by atomic force microscopy in non-contact mode. Atomic force microscopy images of rubrene thin films indicate that a regular array of steps on the sapphire surface acts as a template for the growth of the arrays of rubrene nanosize wires. To further demonstrate that morphological features of a substrate are crucial in determining the morphology of rubrene layers we have grown rubrene on the sapphire surfaces that were characterized by the terrace-and-step morphology with islands. We have found preferential nucleation of rubrene molecules at the intersection between a terrace and a step, as well as around the islands located on terraces.