Electrical and optical properties of Ga doped zinc oxide thin films deposited at room temperature by continuous composition spread

Ga doped ZnO (GZO) thin films were deposited on glass substrates at room temperature by continuous composition spread (CCS) method. CCS is thin films growth method of various Ga_xZn_1−xO(GZO) thin film compositions on a substrate, and evaluating critical properties as a function position, which is directly related to material composition. Various compositions of Ga doped ZnO deposited at room temperature were explored to find excellent electrical and optical properties. Optimized GZO thin films with a low resistivity of 1.46 × 10⁻³ Ω cm and an average transmittance above 90% in the 550 nm wavelength region were able to be formed at an Ar pressure of 2.66 Pa and a room temperature. Also, optimized composition of the GZO thin film which had the lowest resistivity and high transmittance was found at 0.8 wt.% Ga₂O₃ doped in ZnO.     

Effects of H2 ambient annealing in fully 0 0 2-textured ZnO:Ga thin films grown on glass substrates using RF magnetron co-sputter deposition

Gallium doped zinc oxide (ZnO:Ga) thin films were grown on glass substrates using RF magnetron co-sputtering, followed by H₂ ambient annealing at 623 K to explore a possibility of steady and low-cost process for fabricating transparent electrodes. While it was observed that the ZnO:Ga thin films were densely packed c-axis oriented self-textured structures, in the as-deposited state, the films contained Ga₂O₃ and ZnGa₂O₄ which had adverse effect on the electrical properties. On the other hand, post-annealing in H₂ ambient improved the electrical properties significantly via reduction of Ga₂O₃ and ZnGa₂O₄ to release elemental Ga which subsequently acted as substitutional dopant increasing the carrier concentration by two orders of magnitude. Transmittance of the ZnO:Ga thin films were all over 90% that of glass while the optical band gap varied in accordance with the carrier concentrations due to changes in Fermi level. Experimental observation in this study suggests that transparent conductive oxide (TCO) films based on Ga doped ZnO with good electrical and optical properties can be realized via simple low-cost process.     

Microstructures, optical and electrical properties of In-doped ZnO thin films prepared by sol-gel method

ZnO and indium-doped ZnO (I_xZO) thin films were prepared on silica-glass substrates by the sol-gel method. The thin films were crystallized at 600 °C and 700 °C for 1 h in 6.9 × 10⁻¹ Torr under pure O₂ atmosphere. The analyzed results were compared to investigate the structural characteristics and optical properties. The surface morphology of the I_xZO films was different from that of the ZnO films, and showed a thin overlay structure. In addition, the crystallization of I_xZO film was depleted at higher crystallized temperatures. From XRD analysis, the ZnO and I_xZO thin films possessed hexagonal structures. Notably, micro-In₂O₃ phases were observed in the I_xZO thin films using EDS. Both of In₂O₃ phases and the crystallization mechanism not only improved the peeling of structure, but also improved the electrical conductivity of I_xZO thin films. For the PL spectrum, the optical property of the I_xZO film was raised at a higher crystallization temperature. Although the In₂O₃ phases reduced the structural defects of I_xZO thin film, the optical effect of the residual In³⁺ was not enhanced completely at higher crystallized temperatures.     

Effect of different dopant elements (Al,Mg and Ni) on microstructural,optical and electrochemical properties of ZnO thin films deposited by spray pyrolysis (SP)

In the present work we studied the influence of the dopant elements and concentration on the microstructural and electrochemical properties of ZnO thin films deposited by spray pyrolysis. Transparent conductive thin films of zinc oxide (ZnO) were prepared by the spray pyrolysis process using an aqueous solution of zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O] on soda glass substrate heated at 400 ± 5 °C. AlCl₃, MgCl₂ and NiCl₂ were used as dopant. The effect of doping percentage (2–4%) has been investigated. Afterwards the samples were thermally annealed in an ambient air during one hour at 500 °C. X-ray diffraction showed that films have a wurtzite structure with a preferential orientation along the (0 0 2) direction for doped ZnO. The lattice parameters a and c are estimated to be 3.24 and 5.20 ?, respectively. Transmission allowed to estimate the band gaps of ZnO layers. The electrochemical studies revealed that the corrosion resistance of the films depended on the concentration of dopants.     

The influence of titanium on the properties of zinc oxide films deposited by radio frequency magnetron sputtering

TiO₂-doped zinc oxide thin films were deposited on glass substrates by radio frequency (RF) magnetron sputtering with TiO₂-doped ZnO targets in an argon atmosphere. The structural properties of TiO₂-doped ZnO films doped with different TiO₂ contents were investigated. The experimental results show that polycrystalline TiO₂-doped ZnO films had the (0 0 2) preferred orientation. The deposition parameters such as the working pressure and substrate temperature of TiO₂-doped ZnO films were also investigated. The crystalline structure of the TiO₂-doped ZnO films gradually improved as the working pressure was lowered and the substrate temperature was raised. The lowest electrical resistivity for the TiO₂-doped ZnO films was obtained when the Ti addition was 1.34 wt%; its value was 2.50 × 10⁻³ Ω cm, smaller than that found in previous studies. The transmittance of the TiO₂-doped ZnO films in the visible wavelength range was more than 80%. The optical energy gap was related to the carrier concentration, and was in the range of 3.30-3.48 eV.     

Highly conductive and transparent laser ablated nanostructured Al: ZnO thin films

Al doped ZnO thin films are prepared by pulsed laser deposition on quartz substrate at substrate temperature 873 K under a background oxygen pressure of 0.02 mbar. The films are systematically analyzed using X-ray diffraction, atomic force microscopy, micro-Raman spectroscopy, UV-vis spectroscopy, photoluminescence spectroscopy, z-scan and temperature-dependent electrical resistivity measurements in the temperature range 70-300 K. XRD patterns show that all the films are well crystallized with hexagonal wurtzite structure with preferred orientation along (0 0 2) plane. Particle size calculations based on XRD analysis show that all the films are nanocrystalline in nature with the size of the quantum dots ranging from 8 to 17 nm. The presence of high frequency E₂ mode and longitudinal optical A₁ (LO) modes in the Raman spectra suggest a hexagonal wurtzite structure for the films. AFM analysis reveals the agglomerated growth mode in the doped films and it reduces the nucleation barrier of ZnO by Al doping. The 1% Al doped ZnO film presents high transmittance of ∼75% in the visible and near infrared region and low dc electrical resistivity of 5.94 × 10⁻⁶ Ω m. PL spectra show emissions corresponding to the near band edge (NBE) ultra violet emission and deep level emission in the visible region. Nonlinear optical measurements using the z-scan technique shows optical limiting behavior for the 5% Al doped ZnO film.     

Room temperature transparent conducting oxides based on zinc oxide thin films

Doped zinc oxide thin films are grown on glass substrate at room temperature under oxygen atmosphere, using pulsed laser deposition (PLD). O₂ pressure below 1 Pa leads to conductive films. A careful characterization of the film stoichiometry and microstructure using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) concludes on a decrease in crystallinity with Al and Ga additions (≤3%). The progressive loss of the (0 0 2) orientation is associated with a variation of the c parameter value as a function of the film thickness and substrate nature. ZnO:Al and ZnO:Ga thin films show a high optical transmittance (>80%) with an increase in band gap from 3.27 eV (pure ZnO) to 3.88 eV and 3.61 eV for Al and Ga doping, respectively. Optical carrier concentration, optical mobility and optical resistivity are deduced from simulation of the optical data.     

Influence of annealing temperature on structural and optical properties of ZnO: In thin films prepared by ultrasonic spray technique

Transparent conducting indium doped zinc oxide was deposited on glass substrate by ultrasonic spray method. The In doped ZnO samples with indium concentration of 3 wt.% were deposited at 300, 350 and 400 °C with 2 min of deposition time. The effects of substrate temperature and annealing temperature on the structural, electrical and optical properties were examined. The DRX analyses indicated that In doped ZnO films have polycrystalline nature and hexagonal wurtzite structure with (0 0 2) preferential orientation and the maximum average crystallite size of ZnO: In before and annealed at 500 °C were 45.78 and 55.47 nm at a substrate temperature of 350 °C. The crystallinity of the thin films increased by increasing the substrate temperature up 350 °C, the crystallinity improved after annealing temperature at 500 °C. The film annealed at 500 °C and deposited at 350 °C show lower absorption within the visible wavelength region. The band gap energy increased from E_g = 3.25 to 3.36 eV for without annealing and annealed films at 500 °C, respectively, indicating that the increase in the transition tail width. This is due to the increase in the electrical conductivity of the films after annealing temperature.     

Influence of Co doping content on its valence state in Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films

Zn_1−xCo_xO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol-gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn_1−xCo_xO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet-visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn_1−xCo_xO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn_1−xCo_xO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn_1−xCo_xO (0 ≤ x ≤ 0.05), Zn_0.90Co_0.10O and Zn_0.85Co_0.15O are CoO, Co₃O₄ and Co₂O₃, with an increase of Co content, respectively.     

Mg substitutions in ZnO-Al2O3 thin films and its effect on the optical absorption spectra of the nanocomposite

ZnO-Al₂O₃ nanocomposite thin films were prepared by sol-gel technique. The room temperature synthesis was mainly based on the successful peptization of boehmite (AlO(OH)) and Al(OH)₃ compounds, so as to use it as matrix to confine ZnO nanoparticles. The relative molar concentrations of xZnO to (1 − x) Al₂O₃ were varied as x = 0.1, 0.2 and 0.5. The optical absorption spectra of the thin films showed intense UV absorption peaks with long tails of variable absorption in the visible region of the spectra. The ZnO-Al₂O₃ nanocomposites thin films were doped with MgO by varying its molar concentrations as y = 0.05, 0.75, 0.1, 0.125, 0.15 and 0.2 with respect to the ZnO present in the composite. The MgO doped thin films showed suppression of the intense absorption peaks that was previously attained for undoped samples. The disappearance of the absorption peaks was analyzed in terms of the crystalline features and lattice defects in the nanocomposite system. The bulk absorption edge, which is reportedly found at 3.37 eV, was shifted to 5.44 eV (for y = 0.05), 5.63 eV (for y = 0.075) and maximum to 5.77 eV (for y = 0.1). In contrast, beyond the concentration, y = 0.1 the absorption edges were moved to 5.67 eV (for y = 0.125), 5.61 eV (for y = 0.15) and to 5.49 eV (for y = 0.2). This trend was explained in terms of the Burstein-Moss shift of the absorption edges.