Nickolay Golego

Fabrication of green and orange photoluminescent, undoped ZnO films using spray pyrolysis

Photoluminescent, undoped ZnO films have been fabricated using spray pyrolysis of zinc nitrate solution. The luminescent films had a polycrystalline hexagonal wurtzite type structure with no preferred orientation. Photoluminescence intensity was critically dependent on substrate temperature during spray pyrolysis and on post-annealing temperature. Green, photoluminescent films possessed a porous structure while orange films possessed a close packed granular morphology. Green luminescence appears to be due to oxygen vacancies in a layer just below the crystallite surface.

Optical and electrical properties of undoped ZnO films grown by spray pyrolysis of zinc nitrate solution

Undoped ZnO films were deposited by spray pyrolysis using aqueous zinc nitrate solution at different substrate temperatures. The effect of the growth temperature on the structural, optical, electrical, and relaxation properties has been studied. It was found that there was a critical temperature Tc=180 °C below which the thermal decomposition to ZnO did not occur or was incomplete. Films grown above Tc showed strong preferred orientation of polycrystals along the c-axis, while the films grown at Tc or below showed a powder-like, non-oriented polycrystalline structure when they were converted afterwards to zinc oxide by annealing. A slight increase of the optical band gap was observed for as-prepared films as the substrate temperature was decreased near the critical temperature. Annealing brought all the samples to the same band gap 3.30 eV measured at a half height of the maximum absorption. After illumination, the steady-state photoconductivity decayed very slowly with a time constant of about a week for...

Carrier mobility and density contributions to photoconductivity transients in polycrystalline ZnO films

Slow photoconductivity transients were comprehensively studied in ZnO films prepared by spray pyrolysis of the zinc-nitrate solution. Surface charge controlled the film conductivity, and it was possible to reversibly change the conductivity by many orders of magnitude using short-term annealing in hydrogen and oxygen. Under illumination, the conductivity of as-grown films may increase by several orders of magnitude, depending on the dark conductivity. Photoconductivity was due to the capture of nonequilibrium holes at surface oxygen states to produce an equivalent number of excess electrons in the conduction band. Reverse process of the photoconductivity relaxation is determined by an electron tunneling mechanism to the surface oxygen states.

Sensor Photoresponse of Thin‐Film Oxides of Zinc and Titanium to Oxygen Gas

Response of steady-state photoconductivity to changes in oxygen partial pressure (10{sup {minus}3} to 1 atm) has been quantitatively studied in thin-film polycrystalline TiO{sub 2}:Nb and ZnO at 80--120 C. The magnitude of photoconductivity varied as a square root of illumination intensity regardless of oxygen pressure. Both materials showed fast response to oxygen, although in different pressure ranges. Zinc oxide was more sensitive to lower oxygen pressures while titanium dioxide worked better at pressures close to 1 atm.

Density of band-gap traps in polycrystalline films from photoconductivity transients using an improved Laplace transform method

Slow relaxation of the photoconductivity over a period of days was studied in polycrystalline ZnO and TiO2 films prepared by spray pyrolysis. The phenomenon is described by a model involving deep sensitizing hole traps in the forbidden gap. The trap state distribution based on this model was calculated using an improved Laplace transform method. For ZnO and TiO2 films, the density of states was shown to have a peak-like distribution with a maximum near the lower third of the energy gap.

Spray pyrolysis preparation of porous polycrystalline thin films of titanium dioxide containing Li and Nb

Titanium dioxide thin films prepared with and without lithium and niobium were as follows: uniform, crack-free, and stoichiometric, amorphous as-deposited at 300 °C and below; polycrystalline anatase when deposited at 400 °C or annealed at 500−800 °C; and rutile when annealed at 900 °C. Films prepared around 200 °C were very porous, but the porosity decreased as the substrate temperature increased. Optical absorption spectra revealed an indirect bandgap of 3.0 eV for amorphous and anatase films, and a direct bandgap of the same value in rutile. Dark dc conductivity of undoped films was lower than 10 −10 (Ω · cm) −1 ; Hall effect measurements indicated that effective electron mobility was below 1 cm 2 /(V · s). The presence of Nb and Li increased the conductivity by 2–3 orders of magnitude, similar to the effect of hydrogen annealing. Illumination increased the conductivity by several orders of magnitude, and the decay followed a multiexponential law extending into the 10 6 second range after irradiation was stopped. The electronic properties of the films were determined by oxygen-related surface states at grain boundaries. Samples containing Li exhibited considerable sensitivity to water vapor.

Thin‐Film BaMgAl10 O 17 : Eu Phosphor Prepared by Spray Pyrolysis

For the first time, polycrystalline thin films of europium-doped barium magnesium aluminate (BAM) have been prepared by spray pyrolysis of an aqueous solution of the corresponding metal nitrates. Stoichiometric BAM films were obtained at temperatures as low as 350°C, in sharp contrast to the commonly used high-temperature powder route. The deposition procedure can be applied to other phosphor materials as well, allowing for efficient one-step thin-film phosphor preparation. The prepared films were comprehensively characterized. Their luminescent properties are discussed.

Improved Laplace transform method to determine trap densities from transients: application to ZnO and films

A simple method is proposed for analysis of non-exponential transients based on the Laplace transform of a transient. It is useful for analysis of any non-exponential transient to obtain information on the density of localized states distribution. The improvement involved a correction of the tail regions of the distribution to provide a close fit between the simulated and experimental transient. By testing it on a number of different types of simulated transients, it was possible to ascertain the scope of this method. In principle this analysis can be applied to a variety of physical processes. In the present case it was applied to experimental photoconductivity transients for polycrystalline ZnO and films deposited by spray pyrolysis.

Characterization of KTiOPO4 Thin Films Grown by Spray Pyrolysis

A spray pyrolysis (SP) procedure has been developed to prepare thin films of KTiOPO 4 (KTP), and details of the mechanism were explored. The key to the successful application of this procedure was the choice of a titanium precursor capable of coexisting with orthophosphate and nitrate anions in aqueous solution without the presence of chloride anion. The absence of chloride anion permitted a lower pyrolysis temperature. The nearly stoichiometric KTP films were grown on several types of substrates and characterized by several analytical techniques. The thickness of the films could be varied from 0.1 to 7.0 μm, and the apparent density was calculated to be about 90 to 95% of the single-crystal value. The effects of the deposition parameters on film composition and morphology were studied by energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM), and the optical quality was determined spectroscopically. As-deposited films were amorphous and annealing at 700 to 800°C in air was necessary to convert them to the polycrystalline state. The effects of annealing films on different substrates were investigated using x-ray diffraction (XRD), SEM, and EDS. The XRD results indicated that annealing caused interdiffusion at the interface between KTP films and either the quartz or silicon substrates. The resistivities of as-deposited films were close to 2.5 x 10 9 Ω cm, while those of the annealed samples were higher than 10 10 Ω cm. Hall voltage on such high resistivity samples could not be measured with our equipment.

Colossal magnetoresistance in La0.7Ba0.3MnO3 thin films prepared by spray pyrolysis

Barium–lanthanum–manganite (La0.7Ba0.3MnO3) thin films that exhibit a colossal magnetoresistance (CMR) effect have been prepared by spray pyrolysis of solutions of aqueous nitrates. Structural, optical, and magnetotransport properties of the films were studied as a function of growth conditions and postgrowth treatments. In a magnetic field of 600 mT, the maximum CMR effect amounted to a 15% decrease in resistivity relative to zero field value. Instead of decreasing at lower temperature, the value of the CMR continued to increase as the temperature decreased to 100 K. This behavior is attractive for practical applications.