G. N. Panin

Postgrowth annealing effect on structural and optical properties of ZnO films grown on GaAs substrates by the radio frequency magnetron sputtering technique

High-resolution scanning electron microscopy and cathodoluminescence spectroscopy measurements were performed to study the effect of postgrowth annealing on properties of ZnO films grown on GaAs substrates by rf sputtering. The films annealed at 550 °C show a well-oriented columnar structure and strong exciton emission at room temperature. Outdiffusion of gallium and arsenic from substrate into a ZnO film has been found to result in a different secondary electron dopant contrast, measured by the through-the lens secondary electron detector. Extended structural defects such as subgrain boundaries in ZnO assist Ga outdiffusion from the GaAs substrate and show a reduced secondary electron (SE) emission after annealing, while As doped ZnO adjacent to the ZnO/GaAs interface demonstrates an enhanced SE emission and the enhanced luminescence associated with donor–acceptor pairs and exciton bound to acceptors.

Vertical ZnO nanorod/Si contact light-emitting diode

Blue-white light emission was obtained from glass/indium tin oxide (ITO)/n-ZnO nanorod array (NRA)/p+-Si vertical contact light emitting diodes (VCLEDs). The nanoscale p-n heterojunction VCLEDs were formed by direct engagement n−-tips of n-ZnO NRA grown vertically on an ITO/glass substrate with p+-Si wafer. Proposed configuration of the VCLED allows creating a high density (∼109 cm−2) of self-assembled ZnO/Si nanodiodes with point junctions of high quality due to structural perfection of the Si wafer and the tips of ZnO nanorods as well as providing a high injection current and light emission from the NRA VCLED required for solid state lighting.

Luminescence of bound excitons in epitaxial ZnO thin films grown by plasma-assisted molecular beam epitaxy

Luminescence properties of ZnO films, which have been epitaxially grown on c-sapphire (0001) substrates by plasma-assisted molecular beam epitaxy, are investigated by means of different excitation sources and their measurement conditions. With the increase of measurement temperature, photoluminescence spectra clearly present, the appearance of different bound-exciton peaks (I10 line) with an abrupt increase of emission intensity at the measurement temperature of 30-50K. Hypothetical explanations on the basis of thermalization effects, vibronic/rotational resonance states, and the involvement of the B-valence level in emission are given. In cathodoluminescence (CL), the deep level emission intensity was enlarged with the electron beam penetration depth due to the higher defect density near the interface between ZnO and the sapphire. From the CL image of the ZnO film, the dislocation density was estimated as 6×108-3×109∕cm2. The lasing phenomenon was observed at the threshold power density of 1.3MW∕cm2 at 300K.

Electrical and optical properties of ZnO thin films grown on Si substrates

Undoped and nitrogen doped ZnO films were deposited on Si substrates by ultrasonic spray pyrolysis at ambient atmosphere. The p type of conductivity for the nitrogen doped ZnO films has been observed. The concentration of free holes in ZnO films has been found to depend strongly on the resistivity of Si substrate. With decreasing of the substrate resistivity the concentration of free holes in ZnO films increases. The hole concentration of 1.1×1018cm−3 and the hole mobility of 24cm2V−1s−1 were observed in the ZnO films grown on the Si substrates with the resistivity of 0.001Ωcm.

Resistive Switching in Al/Graphene Oxide/Al Structure

We report resistive switching behaviors in an Al/graphene oxide/Al planar structure. Graphene oxide was synthesized by a modified Hummers method from graphite rods. The planar structures were fabricated on a Si/SiO2 substrate by spin-coating graphene oxide suspensions and patterning Al electrodes by photolithography. Both diode-like (rectifying) and resistor-like (nonrectifying) behaviors were observed in the device switching characteristics. Electrical characterization of the Al/graphene oxide interface using the induced current identified a potential barrier near the interface and its spatial modulation, caused by local changes of resistance at a bias voltage, which correlated well with the resistive switching of the whole structure. The mechanism of the observed local resistance changes near the electrode and the associated resistive switching of the entire structure is associated with the electrodiffusion of oxygen and the formation of sp2 graphene clusters in an sp3 insulating graphene oxide layer formed near the electrode by a pre-forming process.

Spatially resolved investigations of the emission around 3.31 eV (A-line) from ZnO nanocrystals

ZnO nanocrystals grown by chemical solution deposition were studied by means of photoluminescence and cathodoluminescence spectroscopy. A postgrown annealing treatment significantly improved the UV emission efficiency and resulted in the clear appearance of a low temperature emission band around 3.31 eV (so-called A-line). Spatially and wavelength resolved cathodoluminescence measurements revealed a spotlike distribution of the A-line emission on a nanocrystal surface. It is found that there is a strong correlation between the emission around 3.31 eV and the specklike defects on the nanocrystal surface that appeared after annealing. The origin of the A-line and the specklike defects are discussed.

Parametrization of turbulent fluxes over inhomogeneous landscapes

Reasons for the nonclosure of the heat balance in the atmospheric boundary layers over natural land surfaces are analyzed. Results of measuring the heat-balance components over different land surfaces are used. The Cabauw (Netherlands) data (obtained throughout 1996 over a grass surface with intermittent shrubs and single trees) and the data from the Anchor station in Germany (measured over coniferous forest in 2000–2001) are analyzed. In all, the analysis involves about fifty thousand independent values of the heat-balance components measured in the experiments, which should be indicative of the reliability of the results obtained in the paper. The data have shown that the heat balance is not closed and the imbalance is 50–250 W/m2. The sum of the latent and sensible heat fluxes λE + H = STF is found to be systematically smaller than the difference between the net radiation and the heat flux into the ground Rn − G. It is shown that the main cause of a systematic heat imbalance in the atmospheric boundary layers over inhomogeneous land surfaces is that the methods of surface-flux measurement and estimation are based on the theory that requires the hypothesis of stationarity and horizontal homogeneity. Direct data analysis has shown that the heat imbalance increases with landscape inhomogeneity. In the paper, a parametrization of the heat imbalance is carried out and the coefficient kf(z0ef/Lef) is introduced as a measure of inhomogeneity. For this, data from the experiments FIFE, KUREX, TARTEX, SADE, etc., are also used. Empirical formulas are presented to refine the results of direct measurements and calculations of surface fluxes over natural (inhomogeneous) land surfaces from profile and standard (using bulk parametrizations) data. These formulas can also be used to determine surface fluxes over inhomogeneous underlying land surfaces in order to take into account so-called subgrid-scale effects in constructing prediction models.

In situ study of the ZnO?NaCl system during the growth of ZnO nanorods

Synchrotron x-ray diffraction and high temperature SEM examinations were used to study the crystallochemical and morphological evolution of a ZnO–NaCl system during thermal processing. Decomposition of ZnO precursor (Zn2(OH)2CO3 ∗ xH2O) at T < 400 ◦ Ci saccompanied by shrinkage and destruction of its fibre-lik ep articles into nanosized isotropic ZnO crystallites. Intensive intergrowth of ZnO nanoparticles and its interaction with coarse NaCl crystallites leads at T = 650–700 ◦ Ct o the formation of a continuous sponge-like framework with an elementary unit size which is far below the initial size of NaCl crystallites. The higher temperature processing at T 700 ◦ Ci saccompanied by the appearance of nanorods. The role of the metastable liquid phase in the formation of ZnO nanorods at T < 800 ◦ Ci s discussed. Luminescence properties of ZnO nanorods synthesized at 500 and 700 ◦ Cw ere demonstrated.

Electric field switching between blue-green and red cathodoluminescence in poly(4,4′- diphenylene diphenylvinylene) mixed with ZnO:Mg nanoparticles

We report the effect of reversible switching between blue-green and red cathodoluminescence (CL) in poly(4,4′-diphenylene diphenylvinylene) (PDPV), mixed with the 12–60nm size ZnO:Mg nanoparticles by applying an electric field. We found that without electric field the CL emission maximum is in the blue-green region for a PDPV–ZnO:Mg composite deposited on Si–SiO2 substrate with gold electrodes. Application of positive bias suppressed the blue-green emission and shifted the emission maximum to the red region. The mechanism for the formation of the exited states in PDPV–ZnO:Mg structure implies the presence of radiative recombination channels, which can be controlled by an electric field.

Ferroelectricity in Mn-implanted CdTe

CdTe:Mn was prepared by implantation of CdTe with 200 keV Mn ions with a dose of 5×1016 cm−2, which produced an insulating layer in the surface region. Ferroelectric characterization of the implanted sample revealed a clear hysteresis in its polarization–voltage curves. The remnant polarization of CdTe:Mn amounts to 0.64 μC/cm2 at 400 Hz, and it decreases with increasing temperature in a continuous and diffusive manner. Capacitance measurement demonstrated a maximum capacitance at 140 °C, and fitting of the data with Curie–Weiss law shows the occurrence of a second-order-type phase transition. The ferroelectricity is accounted for by the cation size difference in CdTe:Mn.