Michael Cocivera

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.

Optical and luminescent properties of undoped and rare-earth-doped Ga2O3 thin films deposited by spray pyrolysis

Spray pyrolysis was used to prepare polycrystalline thin films of undoped Ga2O3, as well as those doped separately with Eu3+, Tb3+ and Tm3+. The energy gap values of the as-grown films and those annealed at 600\r{}C and 900\r{}C were found to be 4.75, 4.48 and 4.44 eV, respectively. Films containing Eu3+ and Tb3+ exhibited red and green cathodoluminescence (CL), respectively. The CL of Ga2O3 : Tm mainly showed a broad band in the blue-green region, which resulted from the emission by both the Ga2O3 host and Tm3+. The broad blue-green emission band was divided into three Gaussian peaks at 424 nm (2.92 eV), 497 nm (2.49 eV) and 526 nm (2.36 eV). The CL intensity of undoped Ga2O3 thin films depended on the annealing ambient and temperature, suggesting that it is associated with the presence of oxygen vacancies. Mechanisms responsible for the broad blue-green emission of undoped Ga2O3 thin films were explored.

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.

Tuning of the blue emission from europium-doped alkaline earth chloroborate thin films activated in air

Thin films of M2B5O9Cl:Eu (M=Ca, Sr, Ba) were prepared on glass substrates using spray pyrolysis. Blue cathodoluminescence due to the abnormal reduction of Eu3+→Eu2+ was obtained by annealing films in air. The cation of the host lattice was found to affect the effectiveness of the reduction process, which could result in influencing the emission band. By selecting types and composition of alkaline cation, it was possible to tune the dominant emitting wavelength between 435 to 465 nm. Activation of the films occurred at temperatures suitable for the use of glass substrates.

Green, blue, and yellow cathodoluminescence of Ba2B5O9Cl thin-films doped with Tb3+, Tm3+, and Mn2+

Green, blue, and yellow cathodoluminescence (CL) have been obtained for thin-films of Ba2B5O9Cl doped with Tb3+, Tm3+, and Mn2+, respectively. These phosphor films were deposited by spray pyrolysis, and then activated in air at temperatures suitable for use on glass substrates. The CL characteristic peaks of the films correspond to transitions between electronic energy levels of Tm3+, Tb3+, and Mn2+ ions. The chromaticity coordinates, dominant wavelength, and color purity were determined for each phosphor. Saturation effects were observed as the beam current density increased. The more severe current saturation of Mn-doped film may be due to a greater ground state deplection of Mn luminescent centers than that of the Tb and Tm.

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.