In situ magneto-optical Kerr effect study of uncovered Fe films on ZnSe(0 0 1)

The magnetic properties of uncovered Fe/ZnSe/GaAs(1 0 0) ultrathin films have been determined in situ by magneto-optical Kerr effect (MOKE). Fe films up to 10 monolayers (ML) thick were deposited on c(2×2) Zn-rich ZnSe/GaAs(0 0 1) surfaces at 180 °C. We have studied the thickness dependence of the in-plane lattice parameter of the Fe films and of the MOKE hysteresis loops in the longitudinal geometry, at 150 K, under magnetic fields up to 0.1 T applied along the [1 1 0] and [1-1 0] directions of the ZnSe(0 0 1). Reflection high energy electron diffraction show that in the low thickness regime the Fe films present an in-plane structural anisotropy characterized by an expansion along the [1 1 0] direction. Hysteretic loops were obtained only starting from ∼5 ML Fe. We found the onset of an uniaxial magnetic anisotropy with [1 1 0] magnetic easy axis at 7 ML Fe.     

The photoluminescence properties of TiO2-sheathed ZnSe nanowires

ZnSe nanowires have been synthesized by thermal evaporation of ZnSe powders on gold-coated Al₂O₃(0 0 0 1) substrates and then sheathed with TiO₂ by sputtering. Our results show that sheathing Zn nanowires with thin TiO₂ layers can significantly enhance the photoluminescence (PL) emission intensity. XPS analysis results suggest that the PL enhancement is attributed to increases in the concentrations of deep levels such as oxygen and titanium interstitials as well as the density of interface states. The PL emission of ZnSe nanowires is also enhanced by thermal annealing. Annealing in an argon atmosphere is more efficient in enhancing the PL emission than annealing in an oxygen atmosphere.     

Band-gap engineering of ZnSe quantum dots via a non-TOP green synthesis by use of organometallic selenium compound

Single-step green synthesis of ZnSe quantum dots (QDs) from 1,2,3-selenadiazole and zinc acetate resulted in formation of high-quality mono-disperse ZnSe with engineered band-gap. The present method is a non-TOP green route where oleic acid is used as a surfactant. The size quantization effect can be monitored by UV–visible spectroscopy which shows in the range 370–387 nm (3.20–3.35 eV), a blue shift of about 70–90 nm with respect bulk ZnSe. Photoluminescence measurement revealed band-gap emission at ∼390 nm (3.18 eV, Stokes shift of <10 nm with FWHM <30 nm). Broadened XRD pattern indicated formation of cubic ZnSe and the estimation of particle size from the line broadening at 〈1 1 1〉 matched well the TEM analysis.     

Optical investigation of InAs/InP(1 0 0) quantum dots grown by gas source molecular beam epitaxy

We report on the optical characteristics of InAs quantum dots based on the InP(1 0 0) substrate grown by gas source molecular beam epitaxy without assisting any other methods. The photoluminescence was carefully investigated by adjusting the thickness of InAs layers and the growth temperature. A wide range of emitting peaks is obtained with the increase in the thickness of InAs layers. In addition, we find that the morphology and shape of quantum dots also greatly depend on InAs layers. The images of atomic force microscopy show that the quantum dots like forming into quantum dashes elongated along the [0 1 ?1] direction when the thickness of InAs layers increased. A critical thickness of formation quantum dots or quantum dash is obtained. At the same time, we observe that the growth temperature also has a great impact on the emission wavelength peaks. High qualities of InAs/InP(1 0 0) quantum dots providing their emission wavelength in 1.55 μm are obtained, and good performances of quantum dots lasers are fabricated.     

Room-temperature deposition of group III metals on Si(1 0 0): A comparative study of nucleation behavior

In this paper, we compare and contrast the processes of nucleation and subsequent growth of single-atom wide metal chains formed when group III metals (Al, Ga, In) are deposited onto Si(1 0 0) at room-temperature (RT). Employing Density Functional Theory (DFT) calculations, diffusion pathways on Si(1 0 0) surface are identified and their associated activation barriers are calculated. Then, the relative stabilities of various C-defect-pinned chains are examined by comparing the relevant adsorption energies. We also account for the observation that defect-nucleated chains grow on only one side of a C-defect by showing that the latter’s presence breaks the symmetry between the two previously equivalent binding sites on either side and rendering one much more stable than the other. Next, a growth model tailored for each group III metal/Si(1 0 0) system and incorporating the above results was simulated using Kinetic Monte Carlo (KMC) techniques to show that the surface morphologies generated by this model accurately reflect the observed ratio of homogeneously to heterogeneously nucleated chains. Finally, we examine through KMC simulations the consequences of the contrasting roles of a defect on In/Si(1 0 0) and Al/Si(1 0 0) – it captures adatoms in the former while it merely blocks direct adatom diffusion in the latter – on key quantities such as the mean island density.     

Effects of growth temperature modulated by HCl flow rate on the surface and crystal qualities of thick GaN by HVPE

We studied the influence of the growth temperature and HCl flow rate on the morphological evolution of crack-free thick GaN films by using a home-made horizontal hydride vapor phase epitaxy on sapphire substrates. Optical difference microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and cathodoluminescence (CL) were carried out to reveal the surface property of the GaN epilayer. It was found that the higher growth temperature is a key factor to obtain mirror, colorless and flat GaN surface. However, this key effect of temperature was modulated by HCl flow rate (HCl > 15 sccm). The surface RMS roughness was reduced from 206 to 2.51 nm for 10 μm × 10 μm scan area when GaN was grown at 1070 °C with HCl flow rate up to 30 sccm. These samples also reduced their (0 0 0 2) FWHM result from 1000 to 300 arcsec and showed a strong near-band-edge peak in CL spectra. Results indicated that growth temperature influence growth velocities on different crystalline planes, which will lead to the different morphologies obtained. High growth temperature can improve the lateral growth rate of vertical {1 1 ? 2 0} facets and reduce the vertical growth rate of top {0 0 0 1} facet combined with higher HCl flow rate, which leads to completely coalescence of surface.     

Growth and photoemission studies of Ag nanofilms on pseudomorphic fcc Fe(1 0 0)

The successful growth of Ag nanofilms on pseudomorphic metastable-fcc-phase Fe(1 0 0) substrates is reported. We have carried out a photoemission study of the Ag nanofilms prepared on pseudomorphic fcc Fe(1 0 0). From these results, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. From the detailed photoemission spectra, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

A competition between (0 0 1) and (0 1 1) alignments in yttria stabilized zirconia thin films fabricated by ion beam assisted deposition

Biaxially textured yttria stabilized zirconia (YSZ) thin films, were deposited on glass substrates by ion beam assisted deposition method with different deposition time. As contrasts, films were also fabricated without assisting ion beam. The orientation properties of the films were characterized by X-ray diffraction. A comparative study shows that there is a competition between (0 0 1) and (0 1 1) alignments during the growth process. Assisting ions make the films (0 0 1)-advantaged and biaxially textured. The competitive growth and the orientation development are explained by selective resputtering and anisotropic damage on growing films induced by assisting ions.     

p-type characteristics of ZnSe:Li3N grown by a closed Bridgman method

We intend to search a new method to prepare high-quality and large-size p-ZnSe single crystal. In this study, ZnSe:Li₃N single crystal is grown by a vertical Bridgman method using a closed double-crucible. The photoluminescence (PL) spectrum of the as-grown ZnSe:Li₃N crystal at 8 K shows very strong donor–acceptor pair (DAP) and very weak exciton emissions. In order to activate doped Li₃N, ZnSe:Li₃N single crystal is annealed at high temperature in Zn-saturated atmosphere. By selecting suitable annealing conditions, a very strong I₁ emission line related to shallow acceptor is observed. The capacitance–voltage (C–V) characteristics indicate that the annealed ZnSe:Li₃N single crystal is a p-type conduction. Furthermore, the acceptor concentration and ionization energy are estimated by examining the temperature dependences of the free-to-acceptor (FA) emission, the behaviors of Li and N are investigated, and the new emission at 2.34 eV is discussed.     

Growth of Nb thin films on SiO2

Nb thin films have been prepared by electron beam evaporation under ultrahigh vacuum conditions on fused silica substrates at various temperatures, and their structural and morphological evolutions have been investigated using X-ray diffraction and atomic force microscopy. The crystallographic texture of the Nb films is found to depend on the growth temperature. At room temperature, the [1 1 0] texture is dominant. However, at 200°C, the [3 1 0] oriented growth is favored, co-existing with [1 1 0] and [2 0 0] oriented grains. At 400–600°C, a completely [1 1 0] textured film is formed. At even higher temperature (800°C), a complex texture of [1 1 0] (dominant), [2 0 0] and [3 1 0] is observed again. It is also found that the single [1 1 0] textured Nb films have smooth surfaces, and the complex textured Nb films have rough surfaces.     

Ion beam as a probe to study the behavior of hydrogen on silicon surfaces

Hydrogen adsorption and its behavior on Si surfaces is studied by ion beam techniques in the energy range of MeV–keV. Elastic recoil detection analysis employing MeV ion beams is one of the most reliable experimental techniques for direct determination of absolute hydrogen coverages on Si surfaces. Results of its application to Si(1 0 0) and Si(1 1 1) clean surfaces are described. Important new results of the role of adsorbed hydrogen on the growth process or structures of metallic thin films on Si(1 1 1) surfaces also are described. Characterization of the growth process or structure of the thin films, as well as the characterization of hydrogen, is performed by ion beam techniques.     

Growth and characterization of CoSi2 films on Si (1 0 0) substrates

In the present work, a special solid phase epitaxy method has been adapted for the preparation of CoSi₂ film. This method includes an epitaxial growth of Co films on Si (1 0 0) substrate, and in situ annealing of the Co/Si films in vacuum. It has been found that at the substrate temperature of 360°C, fcc cobalt film grows epitaxially on the Si (1 0 0) surface. The crystallographic orientation relations between fcc Co film and Si substrate determined from the electron diffraction result are: (0 0 1) Co//(0 0 1) Si, [1 0 0] Co//[1 1 0]Si. Upon annealing at temperatures range from 500 to 600°C, Co film reacts with Si substrate and transforms into CoSi₂. The CoSi₂ films prepared by this way are characterized by XTEM, XPS and AFM.     

Study on the formation mechanism of isoniazid crystal defects and defect elimination strategy based on ultrasound

In crystallization, crystal growth defects may reduce the strength and purity of crystals, which are not welcomed in the industry. Herein, isoniazid (INH) crystals were chosen as an example to investigate the formation of crystal defects at the molecular scale by combining experiments and molecular dynamics simulations. It was found that the strong interaction between the solvent and the crystal surface, high temperature, small stirring rate, and low supersaturation can lead to more pronounced crystal defects. The bulk severity of INH crystal defects was reflected by N₂ adsorption–desorption measurement. Besides, the single-crystal growth experiments manifested the rough growth mechanism for the (1 1 0) surface in the axial direction and the stepwise growth mechanism for the (0 0 2) surface in the radial direction. For the (1 1 0) surface, cavities occurred under the condition where the growth rate of the crystal edges and corners was greater than that of the surface center due to the starvation phenomenon of diffusion. While for the (0 0 2) surface, when the solvent removal rate was lower than the solute insertion rate, liquid inclusions were formed, which was verified by Raman microscopy. Furthermore, the ultrasonic strategy was successfully proposed to eliminate INH crystal defects and prepare perfect INH crystals. Moreover, the mechanism of ultrasound to reduce the crystal defect was proposed. We believe this work can provide insights into the design and preparation of defect-free crystals in crystallization.     

Investigation of hillocks formation on (1 0 0) HgCdTe layers grown by MOCVD on GaAs epi-ready substrates

In this paper we present the recent progress in the growth of (1 0 0) HgCdTe epilayers using metal organic chemical vapour deposition on GaAs epi-ready substrates. Particular progress has been achieved in the reduction of macro-defects known as “hillocks”, revealed on the surface of HgCdTe epilayers with (1 0 0) crystallographic orientation. The large-scale defects can arise from such sources as poor substrate processing, dust and remnants from previous deposition, and non optimal parameters of nucleation and growth process. In our experiment, hillocks density was decreased to <10² cm⁻² by proper choice of the growth parameters.Obtained epilayers are suitable for device fabrication. So far, significant improvements has been obtained in photoconductors operated at near-room temperatures. Devices fabricated from (1 0 0) HgCdTe have about one order of magnitude higher voltage responsivity than their (1 1 1) B counterparts.     

Exploring surface processes by coaxial impact-collision ion scattering spectroscopy and time-of-flight elastic recoil detection analysis

The usefulness of coaxial impact-collision ion scattering spectroscopy and time-of-flight elastic recoil detection analysis for in situ surface analysis, especially for elucidation of surface processes, is demonstrated by taking the following studies as examples: (1) self-restoration processes of oxygen vacancies at vacuum-annealed TiO₂(1 1 0) surface, (2) effects of Mn incorporation on MBE growth of GaMnN film, (3) growth process in hydrogen-surfactant mediated epitaxy of Ge/Si(0 0 1), and (4) structure analysis of the Si(0 0 1)2×3-Ag surface.     

Preparation and characterization of highly conducting and transparent Al doped CdO thin films by pulsed laser deposition

Highly conducting and transparent aluminum doped CdO thin films were deposited using pulsed laser deposition technique. The effect of growth temperature on structural, electrical, and optical properties was studied. It is observed that the film orientation changes from preferred (1 1 1) plane to (2 0 0) plane with increase in growth temperature. The electrical resistivity of the films was found to increase with increase in growth temperature. The low resistivity of 4.3 × 10⁻⁵ Ω cm and high transparency (∼85%) was obtained for the film grown at 150 °C. The band gap of the films varies from 2.74 eV to 2.84 eV.     

Chemical properties of structurally well-defined PdRu/Ru(0 0 0 1) surface alloys – Interaction with CO

The chemical properties of structurally well-defined PdRu/Ru(0 0 0 1) monolayer surface alloys [H. Hartmann, T. Diemant, A. Bergbeiter, J. Bansmann, H.E. Hoster, R.J. Behm, Surf. Sci. in press, doi:10.1016/j.sucs.2008.10.055.] and a Pd monolayer on Ru(0 0 0 1) were studied by temperature programmed desorption and infrared reflection absorption spectroscopy using CO as probe molecule. IR experiments on the PdRu/Ru(0 0 0 1) surface alloys demonstrate that CO adsorption on Ru sites resembles that on pure Ru(0 0 0 1) (on-top adsorption), while adsorption on the Pd sites occurs on both multifold coordinated and on-top sites, similar to CO on Pd(1 1 1). A significant destabilization of CO adsorption on Pd sites for both, surface alloys and the Pd monolayer film, compared to pure Pd(1 1 1) surfaces is attributed to a combination of geometric strain and vertical electronic ligand effects; an additional variation in the CO adsorption bond strength in the surface alloys is attributed to changes in the neighboring surface atom shell (lateral ligand effects). The chemical modifications introduced by PdRu surface alloy formation are compared with findings for deuterium adsorption on the same surface alloys; effects of the two-dimensional (2D) distribution of surface atoms are illustrated by comparison with CO adsorption on PtRu/Ru(0 0 0 1) surface alloys, where in contrast to the pronounced 2D phase segregation in PdRu/Ru(0 0 0 1) the surface atoms are essentially randomly distributed.     

Atomic and nanostructures of monolayer c(2 × 2)NiN on Cu(0 0 1)

Structures of monolayer nickel nitride (NiN) on Cu(0 0 1) surface are studied by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Formations of Ni–N chemical bonds and NiN monolayer at the surface are confirmed by XPS on the N-adsorbed Cu(0 0 1) surfaces after Ni deposition and subsequent annealing to 670 K. A c(2 × 2) structure is always observed in the LEED patterns, which is a quite contrast to the (2 × 2)p4g structure observed usually at the N-adsorbed Ni(0 0 1) surface. Atomic images by STM indicate the mixture of Ni–N and Cu–N structures at the surface. Density of the trenches on the N-saturated surface decreases and the grid pattern on partially N-covered surfaces becomes disordered with increasing the Ni coverage. These results are attributed to the decrease of the surface compressive stress at the N-adsorbed Cu surface by mixing Ni atoms.     

Mechanical characteristics of solution grown potassium zinc chloride crystals doped with lithium ions

Results of indentation-induced hardness testing studies on potassium zinc chloride crystals doped with Li⁺ ions, leading to an understanding of their mechanical behaviour, are presented. The Vickers hardness of these crystals for (1 0 0), (0 1 0) and (0 0 1) planes in the load range 20–160 g were studied. Load-independent values of hardness are estimated for the three crystallographic planes by applying Hays-Kendall’s and Li-Bradt models. The results showed that: (1) for the three crystallographic planes the load-independent hardness obtained by Li-Bradt model is higher than that predicted by Hays-Kendall’s, approach; (2) the load independent hardness of the (0 0 1) plane is higher than that of both (1 0 0) and (0 1 0) planes, (3) the values of load-independent hardness depend on Li⁺ concentrations in the K₂ZnCl₄ crystals, (4) the variations of crack length and crack morphology are described for studied crystal planes.     

Microstructures and nano mechanical properties of the metal tungsten film

The W film was prepared on 1045 steel by magnetron sputtering, with the thickness of 2 μm, its surface and cross-section morphologies were investigated with SEM, and the phase structure was analyzed with XRD. X-ray stress determinator was utilized to measure its residual stress, and the nano-hardness and elastic modulus of the film were surveyed by nano-indentation tester. The results show that the surface of W film is very compact and smooth; the particles arranged regularly, the granularity of the thin film is about 1 μm. The microcracks, cavities and desquamation were not found in the film and interface, and the bonding between the W film and substrate is well. The XRD results showed that the W film had a body-centered cubic structure, the lattice constant: a = 0.316 nm, the growth preferred orientations are (1 1 0) and (2 2 0). The compressive stress (−169 MPa) was found on the surface. The average nano-hardness and elastic modulus of W film are 15.22 GPa, 176.64 GPa, respectively, and the mechanical properties of W film are well.