Sol-gel ZnO Research Articles

Nanocrystalline ZnO thin films on porous silicon/silicon substrates obtained by sol–gel technique

In this study, nanocrystalline ZnO thin films deposited onto silicon substrates by sol–gel technique were studied. The starting material for ZnO sol–gel thin films was Zinc acetate. Single crystalline silicon wafers with different orientations and the porous silicon substrates were used as the substrates. XRD was employed to investigate the evolution of the crystalline orientation during the thermal treatment, and SEM was used to observe the surface morphology of the ZnO films. The effects of the anneal temperature and the substrate doping type on the deposited ZnO thin films were studied in detail. The results indicated that the highly oriented ZnO could be generated on Si substrates by controlling the process conditions. It was found that the substrates types had less influence on the obtained ZnO thin films compared with the anneal temperatures. Thin porous silicon substrates introduced is beneficial to enhance the bonding strength between the films and the substrates. These results are helpful to deposit the highly orientation-controlled ZnO thin films for different kinds of research and application.

ZnO sol–gel derived porous film for CO gas sensing

Fine powder of ZnO was synthesized by the sol–gel method, which was then calcined followed by spin coating on an alumina substrate and tested for CO gas sensing. With variation in the calcination temperature, the shape of gel powder changed into various morphologies: sheet, needle, and sphere. The grain growth rate increased above 700°C and the larger grains showed higher degree of crystallization. The organic element in gel powders evaporated below 300°C. The temperature dependence of the electrical conductance showed the sigmoidal shape, but the temperature range of the constant conductance narrowed with the decrease in the calcination temperature. The optimum sensing properties were observed for the specimen calcined at 500°C, and it degraded with the increase in calcination temperature because of the larger grain size that limits the surface area for gas–solid reaction.

Optical characterization of Er-implanted ZnO films formed by sol–gel method

In this paper, we report on the 1.54 μm photoluminescence (PL) of Er-implanted ZnO thin films formed by a sol–gel method on Si substrates. In spite of the polycrystalline structure of the sol–gel ZnO thin films, they showed strong PL emissions due to the near band edge recombination at 375 nm as well as the Er-related luminescence at 1.54 μm. The Er-related luminescence showed no decrease (quench) in the intensity up to the Er concentration of 1.5 × 10 21 cm −3. The PL intensity of Er-implanted ZnO at 1.54 μm was found to be as strong as Er-doped PS (porous Si) at 20 K, and the intensity reduced to 1/3 at room temperature.

Surface Morphology of Sol-Gel ZnO Films

Surface Morphology of Sol-Gel ZnO Films

Effects of Li and Mg doping on microstructure and properties of sol-gel ZnO thin films

Zinc oxide thin films doped with Li and Mg were prepared by the sol-gel method, and effects of doping on microstructure and electrical properties were examined. The doped films exhibited c-axis-orientation after final heating at 500°C for 30 min in flowing oxygen. The ZnO crystallite size increased by doping and the surface of the films became rougher. The current density of the films was reduced by doping probably due to the formation of acceptor levels (Li-doping) and the reduction of oxygen defects (Mg-doping). The film with a nominal composition of Zn0.85Li0.10Mg0.05O showed the lowest current density of 1.7×10−6 A cm−2 in the present study.

Optical, electrical and structural properties of sol gel ZnO:Al coatings

Single and multilayer transparent conducting aluminium doped zinc oxide films have been prepared on DESAG AF45 substrates by the sol gel dip coating process. Zinc acetate solutions of 0.1–0.5 M in isopropanol stabilised by diethanolamine and doped with a concentrated solution of aluminium nitrate in ethanol were used. Each layer was fired at 600°C in a conventional furnace for 15 min. The final coatings were then tempered under a flux of forming gas (N2/H2) at 400°C for 2 h. The coatings have been characterised by surface stylus profiling, optical spectroscopy (UV-NIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and combined Hall and van der Pauw techniques. Single layers with thickness, d, ranging from 25 to 186 nm are polycrystalline with the zincite structure and exhibit a small preferential orientation with the (002) direction perpendicular to the surface (texture coefficient, TC, ranging from 1.8 to 2.8). They consist of almost spherical particles with size ranging from 15 to 25 nm (thin sample) to 40 nm (thick sample) with crystallite sizes ranging from 23 to 40 nm (002), respectively. The film resistivity decreases from 5×10−1 Ω cm (d=25 nm) to 4×10−2 Ω cm (d=186 nm). Multilayer coatings built with about 20 nm thick layers are also slightly textured along the (002) direction (TC ranging from 1.9 to 2.8). A structural evolution in morphology from spherical to columnar growth was observed. The crystallite size calculated from the (002) reflex increases with the number of layers from 23 nm for a single layer to over 100 nm for ten layers. The resistivity decreases from 5×10−1 Ω cm for a single layer to 5×10−3 Ω cm for ten layers (d=174 nm). A model for the growth of the crystallites in sol gel multilayer coatings is presented.