S. A. Studenikin

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...

Time-resolved luminescence and photoconductivity of polycrystalline ZnO films

The relative intensities of the green and blue luminescence of a ZnO film was shown to depend on the excitation regime. Time-resolved and steady-state luminescence were studied along with photoconductivity transients. Under continuous excitation the film emitted green light, while under pulsed excitation the luminescence was either blue or green, depending on the intensity of the excitation pulse. The intensity of the blue component depended linearly on the pulse intensity while the green intensity followed a sublinear power law dependence with the exponent α=1/3. The transient luminescence exhibited fast (below nanosecond) and slow (microsecond) decay components at room temperature. The fast component was ascribed to interband exciton recombination, and the slow component was attributed to an electron-hole recombination involving a donor-acceptor complex, which most likely consisted of oxygen and zinc vacancies. In this model, the complex can emit light only when it is activated, i.e., oxygen vacancy is ...

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.

Transient photoconductivity properties of tungsten oxide thin films prepared by spray pyrolysis

Tungsten oxide (WO3) thin films were deposited by spray pyrolysis of an ammonium tungsten oxide solution. The effect of postannealing on the structural, transport and optical properties of the films has been studied. Under steady-state illumination, slow photoconductivity growth and relaxation transients were observed at room temperature. The contributions of carrier concentration and mobility to the photoconductivity were determined from photo-Hall and photoconductivity data. The transient photoconductivity was found to be mainly due to photoinduced excess electrons over a wide time range from 0.0 to 104 s. This slow relaxation was characterized by two exponential decays indicating two discrete subband gap levels were involved. The fast photoconductivity relaxation over the time range from 10−8 to 10−1 s was more complicated and probably involved a distribution of subband gap states.

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.

Luminescent properties of Eu2+ ion in BaMg(1+x)SixAl10Oy films prepared by spray pyrolysis

A new host material BaMg(1+x)SixAl10Oy has been prepared and studied for use with Eu2+ ions as a thin-film phosphor. Films were prepared by a spray pyrolysis method at ambient pressure. Magnesium hexafluorosilicate was used as the precursor for the silicon component. In comparison with BaMgAl10O17 (BAM), this host required a lower annealing temperature to convert Eu3+ to Eu2+. In the crystalline matrix, Eu2+ emitted a narrow line at 450 nm. In amorphous films, Eu2+ emitted two broad lines at 440 and 500 nm and several narrow red lines around 612 nm that made the luminescence look white. Deposition and post-deposition conditions were determined for maximum luminance. Heat treatment at 650°C achieved essentially the same intensity of the blue-green luminescence as that of BAM films, which were annealed at 1000°C.