ZnO has recently attracted a great deal of attention as a material for transparent contacts in light emitters and adsorbers. ZnO films heavily doped with Ga (carrier concentration in the range of 1020 - 1021 cm-3) were grown on a-plane sapphire substrates by RF plasma-assisted molecular beam epitaxy. Oxygen pressure during growth (i.e. metal (Zn+Ga)–to–oxygen ratio) was found to have a crucial effect on structural, electrical, and optical properties of the ZnO:Ga films. As-grown layers prepared under metal-rich conditions exhibited resistivities below 3×10-4 Ω-cm and an optical transparency exceeding 90% in the visible spectral range. In contrast, the films grown under the oxygen-rich conditions required thermal activation and showed inferior structural, electrical, and optical characteristics even after annealing.

Zinc oxide (ZnO) nanowires have been widely investigated and various different methods of their synthesis have been suggested. This work is devoted to the optimisation of the growth conditions for uniform in terms of structure and evenly distributed ZnO nanowire arrays. The nanowire growth process includes two steps: 1. Radio-frequency (RF) magnetron sputtering of a ZnO nucleation layer onto a substrate; 2. A hydrothermal growth step of ZnO nanowires using the aforementioned sputtered layer as a template. The optimisation process was divided into two sets of experiments: (i) the deposition of different thicknesses of the ZnO nucleation layer and the subsequent nanowire growth step (using the same conditions) for each thickness. The results revealed a strong dependence of the nanowire size upon the seed layer thickness and structural properties; (ii) a second set of experiments were based on growth solution temperature variation for the nucleation layers of the same thicknesses. This also showed nanowire size and distribution change with solution temperature variation.

Aluminium-doped zinc oxide (ZnO) films have been prepared by spray pyrolysis technique using the mixed solution of zinc acetate dihydrate and aluminium nitrate nonahydrate in methanol. Concentration of aluminum in the solution was varied in a range of 1, 3 and 5 atomic percents. The results from X-ray diffraction showed that the preferred orientation of ZnO films changed to the [002] direction when the concentration of aluminum in the solution exceeded 1 atomic percents. ZnO films deposited from the 3 atomic percent Al containing solution had the largest grains and showed the lowest resistivity of 75 Ω-cm. Addition of aluminum into the precursor solution shifted the absorption edge towards longer wavelengths.

Nanoparticles Fe (x wt. %)-doped Zn-TiO2 rutile powders, with x between 0 an 10 wt. %, were prepared using a solution chemistry route based on the wet-gel stirring method. Using the TEM images we found that the powder samples exhibit nanorods and nanosheets with nanorods oriented in different directions and accompanied by an amorphous Zn on the surface. The average length of these nanorods is about 60 nm and they have an average diameter of 7 nm. The x-ray diffraction patterns revealed the formation of the nanocrystalline particles with the rutile phase, which is characterized by the (101) diffraction peak. The magnetic properties of the samples were studied using a vibrating sample magnetometer (VSM) in magnetic filed up to 13.5 kOe and in the temperature range of 100 K to 300 K. We found that the magnetization of the samples does not saturate in the maximum available field. The magnetization (M) at an applied magnetic field of 13.5 kOe is found to increase with increasing the Fe percentage at room temperature and at 100 K. TEM measurements and atomic-force microscopy (AFM) were used to image the samples.

Excitons in semiconductor alloys feel a random disorder potential leading to inhomogeneous line broadening and a lack of knowledge about the dominating recombination processes. Nevertheless, we demonstrate competing localization effects due to disorder (random potential fluctuations) and shallow point defects. We were able to spectrally separate donor-bound and quasi-free excitons within the whole wurtzite-type composition range of MgxZn1-xO (0 ≤ x ≤ 0.33) using spectrally resolved (x ≤ 0.06) and time-resolved photoluminescence (x ≥ 0.08). We found out that donor-bound excitons dominate photoluminescence spectra even for Mg-contents up to x = 0.18 and still appear for x = 0.33.

The homogeneity of the Schottky barrier potential of reactively sputtered PdOy/ZnO Schottky contacts has been investigated by light beam-induced current measurements on the micrometer scale. It is found that a metallic capping layer, acting as an equipotential surface, is not necessary for PdOy/ZnO Schottky contacts in contrast to AgxO/ZnO Schottky diodes. Further, we probed the generated photocurrent of a ZnO-based metal-semiconductor field-effect transistor for a closed and open channel, respectively. The photocurrent is, in general, one order of magnitude larger for closed channel conditions. The position of maximum photocurrent generation shifts towards the drain for higher source drain voltages for closed channel conditions, whereas it is nearly independent of the source drain potential for an open channel.

Effects of H2O partial pressure on ZnO crystal growth by chemical vapor transport (CVT) have been investigated. The use of H2O causes the increase in growth rate of ZnO, indicating that H2O acts as a dominant oxygen source in ZnO growth by CVT. The use of H2O also improves structural, electrical, and optical properties of the CVT-grown ZnO crystals. A sharp X-ray rocking curve for the ZnO (0002) reflection was obtained, and the full width at half maximum value was 38 arcsec. Strong near band edge emission was observed in photoluminescence spectra at room temperature. Both carrier concentration and Hall mobility increased with partial pressure of H2O. The dependence of the carrier concentration on temperature indicates that there exist two donors in the CVT-grown ZnO crystals. The estimated ionization energy for the shallow donor was 35±5 meV and that for the deep donor was 115±5 meV.

In many instances the quality of the surface in ZnO nanoscale systems is a key performance-defining parameter. The surface itself could be a very significant source of lattice defects as well as contaminating impurities, and this influence may extend into the sub-surface vicinity. In our work, key element of the surface analysis is the surface photovoltage (SPV) spectroscopy known for its advantages, such as: identification of conduction vs. valence band nature of the defect-related transitions and the defect level positions within the band gap, ability to measure relatively low densities of surface defects as well as their cross sections. Additional information can be obtained from the SPV transient measurements. In our system, SPV characterization is run in high vacuum, complemented by in situ remote plasma treatment. This combination of surface-sensitive and surface-specific tools is well-suited for studying surface properties with a high degree of reliability since there is no exposure to common air contaminants between processing and characterization cycles. We employed O/He remote plasma treatments of ZnO nanocrystalline surfaces. In situ SPV spectra and transient measurements of the as-received and processed samples revealed, on the one hand, a number of common spectral features in different ZnO nanopowder specimens, and, on the other hand, a noticeable plasma-driven changes in the surface defect properties, as well as in the overall electronic and optical surface characteristics.

Nitrogen and tellurium co-doped ZnO (ZnO:[N+Te]) films have been grown on (0001) ZnO substrate by plasma-assisted molecular beam epitaxy. The electron concentration of tellurium doped ZnO (ZnO:Te) gradually increases, compared that of undoped ZnO (u-ZnO). On the other hand, conductivity of ZnO:[N+Te] changes from n-type to p-type characteristic with a hole concentration of 4×1016 cm-3. However, nitrogen doped ZnO film (ZnO:N) still remain as n-type conductivity with a electron concentration of 2.5×1017 cm-3. Secondary ion mass spectroscopy reveals that nitrogen concentration ([N]) of ZnO:[N+Te] film (2×1021 cm-3) is relatively higher than that of ZnO:N film (3×1020 cm-3). 10 K photoluminescence spectra shows that considerable improvement of emission properties of ZnO:[N+Te] with an emergence of narrow acceptor bound exciton (A°X, 3.359 eV) and donor-acceptor pair (DAP, 3.217 eV), compared with those of u-ZnO. Consequently, high quality p-type ZnO with high N concentration is realized by using Te and N co-doping technique due to reduction of Madelung energy.

We report that the nonlinear optical response of polarity controlled ZnO films grown by selective growth technique of Zn-polar and O-polar ZnO layers on sapphire substrate using Cr-compound buffer layers. ZnO layers grown on CrN/sapphire show Zn polar, while those grown on Cr2O3/sapphire result in O-polar ZnO films. In order to verify the origin of nonlinear optical response of ZnO, the polarity-controlled ZnO thin films grown on different buffer layers were investigated as nonlinear optical materials for second harmonic generation (SHG). The effective nonlinear optical coefficient (deff) of ZnO grown on Cr-compound buffer layers showed a higher value than that of ZnO grown on MgO buffer layers. Finally, by combining suggested in-situ polarity control technique with photolithography technique, we have fabricated 1D and 2D periodically-polarity-inverted (PPI) hetro-structures with periodicity ranging from 60 μm to 2 μm. The lateral polarity inversion is confirmed by piezo response microscopy. Such PPI ZnO heterostructures show the enhancement of SHG intensity comparing with the ZnO films.

n-ZnO/i-CdZnO thin film was grown on p-type Si substrate by plasma-assisted molecular-beam epitaxy (MBE). Rectifying I-V curves show typical diode characteristics. Cyan electroluminescence emissions at around 473 nm were observed when the diodes were forward-biased at room temperature. The emission intensity increases with the increase of the injection current. Room temperature photoluminescence verifies the electroluminescence emissions come from CdZnO layer.

Nanoparticles were incorporated into poly(methyl methacrylate) matrix by the mean of in situ bulk polymerization. Particle chemistry, size, shape, and percent loading were experimental variables in the synthesis and mechanical properties were assessed, particularly impact resistance, which showed improvement at the optimal particle loading. In assessing the mechanisms of this improvement, the elongated shape of zinc oxide particles appears to promote crack deflection processes to introduce a pull-out mechanism similar to that observed in fiber composite systems. Raman spectroscopy was performed to examine the effect of polymer chain conformation and configuration with the addition of nanoparticles.

We report the formation of p-type nitrogen doped ZnO (ZnO:N) grown by thermal-evaporation deposition. The effects of nitrogen precursors on the electrical and optical properties of ZnO:N were investigated. This study shows that growth process plays a critical role in the electrical properties of ZnO:N. The chemical reaction mechanism was analyzed.

Zn0.90Co0.10O particles, synthesized by mechanical milling and thermal treatment, were pressed at 25 tons to form a 2” target for a radio frequency (r. f.) magnetron sputtering system. Using this target, thin films were deposited on (0001) oriented sapphire (α-Al2O3) substrates under 30W, 60W and 120W r. f. powers. Structural analyses of these films were done with X-Ray Diffractometer (XRD), Energy Dispersive X-Ray Spectrometry (EDS), X-Ray Photo Spectroscopy (XPS) and Atomic Force Microscopy (AFM). The ZnO films were deposited with (0002) preferred direction, which was coherent to (0001) ordered α-Al2O3. Impurity phases, such as Co clusters, CoO and Co3O4, were not detected with the surface analyses of Zn0.90Co0.10O thin films. Substituted Co atoms in the host ZnO matrix were identified by the binding energy peak of Co2p3/2, 781.3±0.4eV, and the energy difference of ∼15.61±0.03eV between Co2p1/2 and Co2p3/2. These results also proved that there were no Co clusters or Co3O4 phases in the lattice. Homogeneity of Co atoms in the lattice was shown by EDS spectra. It was understood that the higher r. f. power caused the more homogeneous distribution of Co and Zn atoms in thin films. Distributions of Co and Zn on the film surface, deposited under 120W, were found as 8.1±0.1% (normalized atomic ratio) and 91.7±0.7% (normalized atomic ratio), respectively, and the surface roughness of thin film was demonstrated by AFM figures as 14.2±0.1nm.

In the present work we study Zn+O divacancies filled up with varying amount of hydrogen atoms. Besides the structure and energy-related properties of such defects, we also investigate their capability to trap positrons taking into account positron induced forces. We show that the Zn+O divacancy may trap positrons when up to two hydrogen atoms are located inside the divacancy. The calculated properties are discussed in the context of other computational and experimental studies of ZnO.

Metal-semiconductor-metal photodetectors (MSM-PDs) on ZnO:N thin films deposited by radiofrequency (RF) sputtering and with post N+ ion implantation processing were fabricated using a ZnO/Si structure. A 10 times reduction in dark current was observed compared to the devices on an as-deposited ZnO thin film without ion implantation. These MSM-PDs gave performances of a photo-to-dark current ratio of 2030 and responsivity (R) = 2.7 A/W; the pulse response was a 12.3 ns rise time and 15.1 ns fall time using a femto-second pulse. Temperature-dependent current -voltage (I-V-T) characteristics of the MSM-PDs were observed and the space charge limited current (SCLC) theory was applied to determine the current transport mechanisms. In the SCLC region, J∼Vm gave m to determine the current transport mechanism and the value of m changes with temperatures and applied voltages. Current transport is governed by the ZnO structure rather than the electrodes.

A FirstNano EasyTube 3000 CVD system with a 3” diameter horizontal tube furnace was used to investigate the optimization of both APCVD (Atmospheric Pressure Chemical Vapor Deposition) and LPCVD (Low Pressure Chemical Vapor Deposition) processes to grow both boron and fluorine doped ZnO films with a sheet resistance, slice resistance and haze suitable for their potential utilizations as TCO (Transparent Conductive Oxide) layers for photovoltaic applications. Growth rates as high as 100 nm per minute have been obtained in some parameter regions for both processes. In both cases the resulting material property parameters were the same or better than reported in the literature. Although the horizontal hot wall CVD R&D reactor is not optimum for uniform TCO thin film deposition it allowed us to investigate the interrelationship of the most critical parameters with the resulting material properties.

The driving force for this work is the ultimate goal of demonstrating a process parameter solution suggesting that ZnO films (usable for either display system manufacturing and/or photovoltaic applications) can be deposited with optimized material properties that are comparable to LPCVD or sputtering processes, but that the APCVD solution could be more economical for large scale thin film ZnO coating implementation. Ultimately our desire is to transfer such a ZnO deposition process to our proprietary, APCVD CVDgCoat™ platform, which has the ability to coat up to 4 meter wide glass sheets and metal foils.

In this work, size-classified substrate-free Zn nanocrystals (NCs) are prepared and investigated for their oxidation kinetics using an in-flight tandem ion-mobility method. The first mobility characterization size selects the NCs, while the second mobility characterization measures changes in mass resulting from a controlled oxidation of the NCs. This method allows for a direct measurement of mass change of individual particles and thus enables us to explore the intrinsic reactivity of NCs while minimizing the sampling error introduced by mass and heat transfer. Two reaction regimes were observed for Zn NC oxidation. A shrinking core model is used to extract the size-dependent oxidation activation energies. We also observed a strong anisotropy effect in the oxidation process as imaged by electron microscopy. An oxidation mechanism is proposed that qualitatively explains the oxidation anisotropy and its relationship to the surface energy of the Zn NCs.

The wide bandgap polar semiconductors GaN and ZnO and their related alloys exhibit fascinating properties in terms of bandgap engineering, carrier confinement, internal polarisation fields, and surface terminations. With a small lattice mismatch of ∼1.8 % between GaN and ZnO and the possibility to grow MgZnO lattice-matched to GaN, the system AlGaN/MgZnO offers the opportunity to design novel optoelectronic devices circumventing the problem of p-type doping of ZnO. In such AlGaN/MgZnO heterostructures with either hetero- or isovalent interfaces, tuning of band offsets is possible in various ways by polarisation fields, surface termination, strain, and composition. These aspects need to be fully understood to be able to make full use of this class of heterostructures. We report on the growth of ZnO films by chemical vapor deposition on p-type GaN and AlGaN grown by metal-organic vapor deposition on sapphire templates and on the fabrication of corresponding light-emitting diode (LED) structures. Electrical and optical properties of the n ZnO/p-GaN and n-ZnO/p-AlGaN LEDs will be compared and the observed differences will be discussed in terms of the band alignment at the heterointerface.

We perform density functional theory calculations to determine equilibrium lattice parameters of wurtzite Zn1-xMgxO alloys for Mg concentrations x ranging from 0 to 31.25 %. We use the local density approximation (LDA) as well as the generalized gradient approximation (GGA) for the exchange correlation functional. For the lattice constants a and c we find a deviation from Vegard's law and a constant unit cell volume independent of the Mg concentration.