Preparation, structural and electrical properties of zinc oxide grown on silicon nanoporous pillar array

Polycrystalline thick film of zinc oxide （ZnO） is grown on a unique silicon substrate with a hierarchical structure, silicon nanoporous pillar array （Si-NPA）, by using a vapour phase transport method. It is found that as-grown ZnO film is composed of closely packed ZnO crystallites with an average size of -10 μm. The film resistivity of ZnO/SiNPA is measured to be -8.9Ωcm by the standard four probe method. The lengthwise Ⅰ-Ⅴ curve of ZnO/Si-NPA heterostructure is measured. Theoretical analysis shows that the carrier transport across ZnO/Si-NPA heterojunction is dominated by two mechanisms, i.e. a thermionic process at high voltages and a quantum tunnelling process at low voltages.     

L lo FePt thin films with [001] crystalline growth fabricated by ZnO addition and rapid thermal annealing

FePt films with a high degree of order S of the L10 structure （S 〉 0.90） and well defined [001] crystalline growth perpendicular to the film plane are fabricated on thermally oxidized Si substrates by the addition of ZnO and a successive rapid thermal annealing （RTA） process. The optimum condition to prepare high-ordering L10 FePtZnO films is 20 vol% ZnO addition and 450 ℃ annealing. The effect of the ZnO additive on the ordering process of the L10 FePtZnO films is discussed. In the annealing process, Zn atoms move to the film surface and evaporate. The motion of the Zn atoms accelerates the intergrain exchange and decreases the ordering temperature.     

Enhanced ultraviolet emission from ZnO thin film covered by TiO_2 nanoparticles

A ZnO thin film covered by TiO2 nanoparticles is prepared by electron beam evaporation. The structure and surface morphology of the sample are analyzed by X-ray diffraction （XRD） and atomic force microscopy （AFM）, respectively. Photoluminescence is used to investigate the fluorescent property of the ample. The results show that the ultraviolet （UV） emission of the ZnO thin film is greatly enhanced after it is covered by TiO2 nanoparticles while the green emission is suppressed. The enhanced UV emission mainly results from the fluorescence resonance energy transfer （FRET） between ZnO thin film and TiO： nanoparticles. This TiO2-ZnO composite thin film can be used to fabricate high-efficiency UV emitters.     

Magneto-optical and Microwave Properties of LuBiIG Thin Films Prepared by Liquid Phase Epitaxy Method from Lead-Free Flux

Lu2.1Bi0.9Fe5O12 （LuBiIG） garnet films are prepared by liquid phase epitaxy （LPE） method on gadolinium gallium garnet （GGG） substrates from lead-free flux. Three-inch single crystal garnet films with （444） orientation and good surface are successfully fabricated. The lattice mismatch to the GGG（111） substrate is as small as 0.08%. The ferromagnetic resonance （FMR） linewidth of the film is 2AH = 2.8-5.10e, the Faraday rotation is 1.64 deg/μm at 633nm at room temperature and the optical absorption coefficient of the film is 600cm-1 in visible range and about 100-170 cm-1 when the wavelength is larger than 800 nm. The epitaxy film possesses dominating in-plane magnetization with a saturation magnetization of about 1562G. These superior optical, magnetic-optical （MO） and microwave properties of our garnet films have potential applications in both MO and microwave devices.     

Depth Dependence of Tetragonal Distortion of a ZnO/Mg0.1Zn0.90/ZnO Heterostructure Studied by Rutherford Backscattering/Channeling

Rutherford backscattering and channeling are used to characterize the structure of a ZnO/Mg0.1Zn0.90/ZnO heterostructare grown on a sapphire （0001） substrate by rf plasma-assisted molecular beam epitaxy. The results show that the Mg0.1Zn0.90 layer has the same hexagonal wurtzite structure as the underlying ZnO layer, and the heterostructure has a good crystalline quality with xmin = 5%, which is the ratio of the backscattering yields of aligned and random spectra in the near-surface region. Using the channeling angular scan around an off-normal （1213） axis in the {1010} plane of both ZnO and MgZnO layer, the tetragonal distortion εT, which is caused by the elastic strain in the epilayer, is determined. The depth dependence of εT is obtained by using this technique. It can clearly be seen that the elastic strain rapidly decreases with the increase in thickness of the ZnO film in the early growth stage and becomes slightly larger in the region of the Mg0.1Zn0.9O layer.     

Tunable strain effect on the charge/orbital ordering state in electron-doped La0.9Hf0.1MnO3 films

The impact of the lattice strain on the charge/orbital ordering state was studied by using a heterostructure composed with electron-doped La 0.9 Hf 0.1 MnO 3(LHMO) and ferro-and piezoelectric 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3.The ferroelectric poling induces a significant reduction in the biaxial tensile strain in the LHMO film,leading to a decrease in the film resistance over the whole temperature range and an increase in the insulator to metal transition temperature T IM.The resistance of LHMO film exhibits different responses to the external electric fields and lattice deformation,which is attributed to the coactions of converse piezoelectric effect and ferroelectric polarity effect.The modification of charge/orbital ordering phase by the electric fields and ferroelectric polarization suggests that the unstable states in the manganites are sensitive to strain effects.     

Structural and electrical properties of single crystalline and bi-crystalline ZnO thin films grown by molecular beam epitaxy

C-oriented ZnO epitaxial thin films are grown separately on the a-plane and c-plane sapphire substrates by using a molecular-beam epitaxy technique.In contrast to single crystalline ZnO films grown on a-plane sapphire,the films grown on c-plane sapphire are found to be bi-crystalline;some domains have a 30 rotation to reduce the large mismatch between the film and the substrate.The presence of these rotation domains in the bi-crystalline ZnO thin film causes much more carrier scatterings at the boundaries,leading to much lower mobility and smaller mean free path of the mobile carriers than those of the single crystalline one.In addition,the complex impedance spectra are also studied to identify relaxation mechanisms due to the domains and/or domain boundaries in both the single crystalline and bi-crystalline ZnO thin films.     

High-sensitivity long-period fiber grating sensor with SAN/cryptophane A for coal mine gas detection

A high-sensitivity long-period fiber grating （LPFG） methane sensor that contains a compact and uniform styrene-acrylonitrile （SAN）/cryptophane A nanofilm is presented. The sensor is prepared by using an automatic dip-coater in a solution of cryptophane A, SAN resin dissolved in ortho-dichlorobenzene, a low- volatile solvent. The effect of film thickness on the LPFG＇s resonant wavelength is thoroughly investigated. The optimum sensor among the three LPFGs with different film thicknesses is directly used to detect the methane concentration in a coal mine gas sample. The results indicate that the sensors with film thicknesses of 484 to 564 nm exhibit a redshifted resonant wavelength when the methane concentration is increased from 0% to 3.5% （vol）. The data demonstrates that the sensor with a film thickness of 484 nm has remarkable sensitivity （~0.633 nm%-X）, and its detection limit can reach 0.2%. The methane concentrations determined by our sensor are consistent with those obtained by gas chromatography.     

Design of a Simple Integrated Coupler for SPP Excitation in a Dielectric Coated Ag Thin Film

A simple integrated coupler is proposed for the efficient excitation of surface plasmon polariton （SPP） mode in a thin metal film. The SPP mode is generated in a single Ag-dielectric interface by the incident field and coupled with an Ag thin film. The coupling efficiency at different wavelengths using two different dielectrics, gallium lanthanum sulfide （GLS） and aluminum gallium arsenide （AIGaAs） is calculated by analyzing the SPP propagation dynamics with the finite difference time domain method. A maximum coupling efficiency of 70~ is obtained at a wavelength of 46Ohm when GLS is used, whereas the corresponding value obtained for A1GaAs is 60~o at 56Onto. The proposed structure can be used to excite SPPs in a nano-thin film from an external bulky source and is easier to fabricate since a single interface metal-dielectric configuration is used to excite the metal-thin film.     

Photoluminescence of ZnO and Mn-Doped ZnO Polycrystalline Films Prepared by Plasma Enhanced Chemical Vapour Deposition

ZnO and Mn-doped ZnO polycrystalline films are prepared by plasma enhanced chemical vapour deposition at low temperature （220℃）, and room-temperature photoluminescence of the films is systematically investigated. Analysis from x-ray diffraction reveals that a11 the prepared films exhibit the wurtzite structure of ZnO, and Mndoping does not induce the second phase in the films. X-ray photoelectron spectroscopy confirms the existence of Mn^2＋ ions in the films rather than metalic Mn or Mn^4＋ ions. The emission efficiency of the ZnO film is found to be dependent strongly on the post-treatment and to degrade with increasing temperature either in air or in nitrogen ambient. However, the enhancement of near band edge （NBE） emission is observed after hydrogenation in ammonia plasma, companied with more defect-related emission. Furthermore, the position of NBE shifts towards to high-energy legion with increasing Mn-doped concentration due to Mn incorporation into ZnO lattice.     

Controlled Growth of Zn-Polar ZnO Films on Al-Terminated α-Al2O3(0001) Surface by Using Wurtzite MgO Buffer

The controlled growth of Zn-polar ZnO fihns on Al-terminated α-Al203 （0001） substrates is investigated by the radio-frequency plasma-assisted molecular beam epitaxy method. Prior to the growth, α-Al2O3 （0001） surface is modified by an ultrathin MgO layer, which serves as a uniform template for epitaxy of Zn-polar ZnO films. The microstructures of ZnO/MgO/Al2O3 interface are investigated by in-situ reflection high-energy electron diffraction observations and ex-situ high-resolution transmission electron microscopy characterization. It is found that under Mg-rich condition, the achievement of the wurtzite MgO ultrathin layer plays a key role in the subsequent growth of Zn-polar ZnO. An interracial atomic model is proposed to explain the mechanism of polarity selection of both MgO and ZnO films.     

A mild reduction-phosphidation approach to nanocrystalline GaP

Nanocrystalline gallium phosphide (GaP) has been prepared through a reduction-phosphidation by using Ga, PCl₃ as gallium and phosphorus sources and metallic sodium as reductant at 350 °C. The XRD pattern can be indexed as cublic GaP with the lattice constant of a=5.446 Å. The TEM image shows particle-like polycrystals and flake-like single crystals. The PL spectrum exhibits one peak at 330 nm for the as-prepared nanocrystalline GaP.     

Effects of diffraction conditions and processes on rheed intensity oscillations during the MBE growth of GaAs

The RHEED intensity oscillation technique has received wide-spread attention for the study of MBE growth dynamics, but insufficient consideration has been given to the diffraction conditions and processes involved. We report here a systematic investigation of the intensity oscillation behaviour as a function of diffraction parameters (azimuth, incidence angle, specular and non-specular beams), with constant growth conditions for GaAs films on GaAs (001) substrates.We show that many reported anomalies attributed to growth effects, such as phase differences and periodicity variations, can be accounted for entirely by diffraction events, provided it is realised that multiple scattering processes are the dominant cause of RHEED intensity variations during growth.The technique can provide valuable information on growth behaviour, but only if diffraction-dependent effects are first eliminated.     

The application of RHEED intensity effects to interrupted growth and interface formation during MBE growth of GaAs/(Al,Ga)As structures

There is a diffraction-induced phase effect in the RHEED intensity oscillation technique used in MBE, whereby intensity maxima only correspond to monolayer completion for very restricted conditions. In particular, the angle of incidence of the primary beam is extremely critical. The effect occurs because the total intensity at the measured position of the specular beam is always derived from at least two different diffraction processes, which do not have the same phase relation to monolayer formation. It can be accomodated either by a systematic series of measurements to establish an empirical relationship between incidence angle and phase, or by Fourier transform techniques. Unless full account is taken of this purely diffraction-induced effect, very misleading results can be obtained for the time constants of the recovery period following cessation of growth and this is illustrated for GaAs. The effect also has important implications for the growth-interrupt technique. In addition, it is shown that for heterojunction formation in the GaAs/(Al, Ga)As system, adatom (Ga and Al) migration lengths are of greater importance than the position in the monolayer at which the composition is changed, and that RHEED can provide only limited information on the interface structure.     

LEED structure analysis of a Co{001} surface

A {001} surface of face-centered-cubic cobalt was cleaned to the point of elimination of all impurities except carbon and oxygen, which were reduced to minimum terminal amounts. A LEED structure analysis of this surface, using 12 intensity spectra at 3 angles of incidence, reveals that the atomic arrangement corresponds to truncation of the bulk structure but with about 4% contraction of the first interlayer spacing along 〈001〉 with respect to the bulk.     

Self-organized in-plane incorporation of Si atoms in GaAs by molecular beam epitaxy

The preferential attachment of Si atoms at misorientation steps on vicinal GaAs(001) surfaces has been studied by RHEED. By analysing the time evolution of the specular beam intensity and the change in surface reconstruction during Si deposition we show that a self-organized Si incorporation along the step edges takes place. The observed (3×2) structure is due to an ordered array of dimerized Si atoms with missing dimer rows. Taking into account the structure of the (3×2) unit mesh and its orientation with respect to the As-terminated or Ga-terminated steps, a characteristic minimum in the RHEED intensity recording corresponds to the number of Ga step-edge sites. Since the preferential path for Ga as well as for Si adatom diffusion is along the [110] direction, the critical terrace width for wirelike Si attachment is much larger for a misorientation toward (111)As than for a misorientation toward (111)Ga. Despite the high local impurity concentration, the Si-modified surface can be overgrown with GaAs without adverse effects on the growth front. This is promising for the fabrication of doping wires.     

Short range order in high-T c superconductors investigated by electron diffraction

Observation of modulated diffuse streaks in electron diffraction patterns of Tl₂Ba₂CuO₆ and YBa₂Cu₃O_7– ceramic superconductors is reported. These streaks are parallel with the c-axis and the intensity modulation has a period in reciprocal space which is larger than the c*-parameter. This behaviour is interpreted in terms of disordered sublattices. It is shown that the fine structure results from short range order in two successive homologous layers forming one bi-layer and that the intensity modulation in the diffuse streaks therefore allows one to determine which of the bi-layers in the material exhibit disorder.Research assistant of the National Fund for Scientific Research, Belgium     

Quantum Theory of Electronic Double-Slit Diffraction

Phenomena of electron, neutron, atomic and molecular diffraction have been studied in many experiments, and these experiments have been explained by some theoretical works. We study electronic double-slit diffraction with a quantum mechanical approach and obtain the following results： （1） When the ratio of d ＋ a /a=n （n = 1,2, 3,...）, orders n, 2n, 3n,... are missing in diffraction pattern. （2） When the ratio of d＋a/a≠ n （n =1,2, 3,...）, there is a not missing order in diffraction pattern. （3） The slit thickness c has a large affect on the electronic diffraction pattern, which is a new quantum effect. We believe that all the predictions in our work can be tested by the electronic double slit diffraction experiment.     

Refit Silver Nanostructures Using a Convergent Electron Beam

Using a superionic conductor AgI thin film and a direct current electric field, we synthesize silver nanowires in diameter of about lOOnm. In order to refit the prepared nanowires, the samples are irradiated by a convergent electron beam （200 k V） inside a transmission electron microscope to prepare new small silver nanostructures. The new nanostructures are investigated in situ by high-resolution transmission electron microscope. This electron- induced crystal growth method is useful for technical applications in fabrication of nanodevices.     

SPA-LEED studies of growth of Ag on Ag(111) at low temperatures

The surface morphology after deposition of Ag on Ag(111) at low temperatures (130–200 K) has been studied in detail with SPA-LEED (Spot-Profile Analysis of Low-Energy Electron Diffraction). The surface roughness and the mean terrace size have been quantitatively determined under various conditions. At 130 K the surface roughness increases with coverage exactly according to the relation = ^1/2, which indicates that the inter-layer diffusion can be neglected at 130 K. Although the mean terrace length decreases with increasing coverage (following an approximate power law of ^–2/3) for all studied coverages, it is much larger than expected for a pure random or Poisson-growth mode without any diffusion of the adatoms. Therefore, Ag grows on Ag(111) at this temperature without interlayer diffusion but with intra-layer diffusion. The intralayer diffusion barrierE _d has been determined by measuring the temperature dependence of the two-dimensional island density according to the nucleation theory (supposing a critical nucleus size of one). The obtained valueE _c=0.18 eV agrees with the theoretical calculations and previous measurements. Furthermore, from comparing measured and Monte-Carlo-simulated (MC) surface roughness at different deposition temperatures we obtain E=0.05 eV as a lower limit for the additional barrier at steps.