M. Mollar

Structural, electrical and optical properties of ZnO thin films deposited by sol-gel method

Thin films of intrinsic and Al-doped ZnO were prepared by the sol-gel technique associated with spin coating onto glass substrates. Zinc acetate dehydrate, ethanol and monoethanolamine were used as a starting material, solvent and stabiliser, respectively. Structural, electrical and optical characterizations of the films have been carried out. All films are polycrystalline with a hexagonal wurtzite structure with a preferential orientation according to the direction . The four-points technique was used to characterize thin films electrically. All films exhibit a transmittance above 80-90% along the visible range up to 650nm and a sharp absorption onset about 375nm corresponding to the fundamental absorption edge 3.3eV. Intense UV photoluminescence is observed for undoped and 1% Al-doped ZnO films.

Optical properties of nanocolumnar ZnO crystals

Oriented nanocolumnar ZnO single crystals were prepared by the electrodeposition technique on conducting glass substrates by using different growth parameters. A factorial design at two levels of three growth parameters such as current density, exposition time and temperature of the bath has been used to study their influence on the height of ZnO columns. The average height of the Zn nanocolumns was obtained by means of atomic force microscopy. X-ray diffraction and scanning electron microscope were used to characterize the morphology of the nanocolumnar ZnO crystals. The optical properties were investigated by means of transmittance and photoluminescence. It was found that further annealing of the electrodeposited samples was necessary in order to achieve good optical properties which are very sensitive to the duration and temperature of the annealing.

Nanostructured hybrid ZnO thin films for energy conversion

We report on hybrid films based on ZnO/organic dye prepared by electrodeposition using tetrasulfonated copper phthalocyanines (TS-CuPc) and Eosin-Y (EoY). Both the morphology and porosity of hybrid ZnO films are highly dependent on the type of dyes used in the synthesis. High photosensitivity was observed for ZnO/EoY films, while a very weak photoresponse was obtained for ZnO/TS-CuPc films. Despite a higher absorption coefficient of TS-CuPc than EoY, in ZnO/EoY hybrid films, the excited photoelectrons between the EoY levels can be extracted through ZnO, and the porosity of ZnO/EoY can also be controlled.

p- and n-type doping of zinc oxide through electrochemical methods

Cu- and Cl-doped ZnO thin films were electrodeposited on conductive glass substrates in a non-aqueous electrolyte containing Cu, Cl and Zn salts. The Cu content of the films was found to be proportional to the [Cu2+]/[Zn2+] precursor ratio in the deposition electrolyte. Electrodeposited ZnO:Cu thin films exhibit hexagonal wurtzite structure with (002) preferred orientation. A transition from n-type to p-type was observed for ZnO:Cu films with a [Cu2+]/[Zn2+] ratio higher than 2% as inferred from the change in the direction of the photocurrent. Further, by the addition of different amounts of chlorine ions in the starting electrolyte results in an effective n-type doping of electrochemically synthesized ZnO thin films. The ratio between chlorine and zinc cations was varied between 0 and 2 while the zinc concentration in the solution was kept constant. When the concentration of chloride in the bath increases an effective n-type doping of ZnO films takes place. n-type doping is evidenced by the rise of donors concentration obtained from Mott-Schottky measurements as well as from the blue shift observed in the optical gap owing to the Burstein-Moss effect. When electrodepositing ZnO:Cu on ZnO:Cl layers a homojunction device is formed so evidencing the p- and n-type respective character of the layers. The rectifying characteristics of the ZnO:Cu/ZnO:Cl homojunction further confirm the p- and n-type behavior of ZnO:Cu and ZnO:Cl layers, respectively.