A. K. Akhmedov

Effect of growth temperature on properties of transparent conducting gallium-doped ZnO films

Transparent conducting gallium-doped ZnO films are deposited on glass substrates by magnetron sputtering of conducting ceramic targets. The dependences of structural, electric, and optical characteristics of ZnO:Ga films on the substrate temperature are investigated during the deposition. Stability of resistivity of films is considered during annealing in air. It is found that the films deposited at the substrate temperature of 250°C have the lowest resistivity of 3.8 × 10−4 Ω cm, while those deposited at 200°C have the highest thermal stability.

Transparent conducting ZnO-Based thin films deposited by magnetron sputtering of a ZnO:Ga-C composite target

We have studied the structure, electrical, and optical properties of thin transparent conducting films based on gallium-doped zinc oxide (ZnO:Ga), which were deposited by magnetron sputtering of a ZnO:Ga-C composite target. The ion-bombardment-induced interaction of ZnO with carbon in a thin surface layer of the target leads to an increase in the excess zinc content in the reagent flow. The formation of transparent conducting ZnO:Ga films at a substrate temperature above 100°C in the presence of excess zinc near the growth surface leads to improvement in the structure and conductivity of films without decreasing their transmission in the visible spectral range.

Vapor phase synthesis of ZnO structures

We observed zinc oxide structures formed in an oxygen-containing atmosphere as a result of oxidation of the surface of zinc droplets. The gas-phase oxidation leads to the formation of hollow ZnO whiskers on the metal surface, which grow due to the transport of zinc vapor through their channels. It was found that high partial pressures of zinc and atomic oxygen give rise to fractal structures, which appear in a cascade process involving the sequential formation of zinc oxide vapor, ZnO clusters, and cluster aggregates as a result of the cluster-cluster interaction. A deposit of ZnO synthesized on the cathode surface exhibits a columnar structure.

Effects of the mechanoactivation treatment on the structure of ZnO–Zn cermet mixture

ZnO–Zn сermet composite powder with a nanocrystalline structure has been obtained by grinding a mixture of initial powders in a ball mill under inert atmosphere conditions. A study of the transformation of the structural state of this powder as a result of long-term mechanical activation has shown the formation of a cermet composite, in which ZnO nanocrystallites with an average size less than 50 nm are incorporated into an almost completely amorphous metallic matrix.

UV-assisted growth of transparent conducting layers based on zinc oxide

A comparative study of the microstructure and optical and electrical characteristics of gallium-doped ZnO films synthesized by magnetron sputtering with and without assistance to the growth process by UV radiation has been carried out. It was found that the UV assistance to the growth process of ZnO-based transparent conducting layers improves their electrical characteristics because of the formation of additional donor centers and the lower carrier scattering at grain boundaries, without any strong inf luence on the layer morphology and on the average optical transmittance in the visible spectral range.

Growth Mechanism of ZnO Layers

The formation processes of clusters in gas phase in the Zn - O2 system were studied. It was shown that Zn12O clusters are formed in a surplus of zinc vapor. We studied dc-magnetron sputtering of ZnO layers by co-sputtering with ZnO:Ga ceramic and Zn metal targets method. We observe that at surplus of zinc vapor in the reagent stream leads to synthesis of homogeneous layers without columnar structure. We propose that growth of ZnO layers at surplus of zinc vapor is governed by the vapor-liquid-solid mechanism.

Growth of hollow annular zinc oxide structures

The conditions and mechanisms of the growth of hollow crystals, dendrite structures, and whiskers of zinc oxide during the annealing of pressed zinc oxide-carbon mixtures were studied. The growth of hollow structures is due to the interaction of reagents in pressed tablets, which is accompanied by the formation of a gas-impermeable external ZnO shell and the transport of Zn vapor and CO2 along the growing tube. Orientation of the hollow crystals is determined by the direction of Zn vapor transport. The process was modeled by the Zn vapor emission from a Knudsen effusion cell.

Transformation of the Structure of ZnO–C Powders during Mechanical Activation and Spark Plasma Sintering

The processes of grinding and transformation of the structure of the ZnO–C powder mixture at the mechanical activation by ball milling in an inert atmosphere have been investigated. The mechanical activation of the ZnO–C powder mixture is shown to lead to the formation of graphene oxide shells on the surface of crushed zinc oxide particles. The influence of the duration of activation of powder mixtures on the dynamics of their compaction during spark plasma sintering and structure of the ceramics obtained has been investigated. It is established that an increase in the activation duration to 100 h leads to a monotonic decrease in the density of sintered ceramics. The carbon shell formed during sintering at ZnO grain boundaries is shown to have the structure of reduced graphene oxide.

Synthesis of TiO x -based ceramics by plasma spraying

Structure of ceramics obtained by the plasma spray deposition of spherical TiO2 powders has been investigated. An electron microscopy study of the surfaces and cross sections of particles in the initial powder and of the deposited ceramic coatings was performed. X-ray diffraction and Raman scattering data proved that the coatings were mainly structured as rutile. In addition, Raman and X-ray diffraction data have revealed an amorphous phase, an anatase phase, and non-stoichiometric phases Ti8O15, Ti10O19, Ti7O13, etc. being present in the coatings. The observed suppression of (011) and (111) XRD peaks and an increased intensity of (110) peak are indicative of a predominant orientation of grains in the synthesized ceramics. Mechanisms of formation of the complex coating structure are discussed.

Variation of the Structure and Stoichiometry of a Zinc Oxide Ceramic in the Course of Sintering in an Open Atmosphere

Sintering of the ZnO ceramic in open atmosphere in the temperature range 200–1400°C has been studied. The relationship is found between the processes of shrinkage and loss of mass by the ceramic, on the one hand, and changes in the structure, electrical conductivity, and luminescent characteristics, on the other. It is shown that low-temperature sintering of undoped compacted powders of zinc oxide is due to the desorption of oxygen from the surface of ZnO grains in the temperature range of 200–600°C and to the formation of the low-melting ZnO1 − x phase at grain boundaries, which accelerates mutual-diffusion processes. The conductivity is due to oxygen vacancies VO for the ceramic synthesized at temperatures of up to 900°C and to oxygen vacancies VO and interstitial zinc Zni for that obtained at temperatures higher than 1100°C.