The effects of carrier gas and substrate temperature on ZnO films prepared by ultrasonic spray pyrolysis

ZnO films were deposited on glass substrates in the temperature range of 350–470 °C under an atmosphere of compressed air or nitrogen (N₂) by using ultrasonic spray pyrolysis technique. Structural, electrical and optical properties of the ZnO films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical two-probe and optical transmittance measurements. The ZnO films deposited in the range of 350–430 °C were polycrystalline with the wurtzite hexagonal structure having preferred orientation depending on the substrate temperature. The ZnO films deposited below 400 °C had a preferred (100) orientation while those deposited above 400 °C mostly had a preferred (002) orientation. The resistivity values of ZnO films depended on the types of carrier gas. The ZnO thin films deposited under N₂ atmosphere in the range of 370–410 °C showed dense surface morphologies and resistivity values of 0.6–1.1 Ω-cm, a few orders of magnitude lower than those deposited under compressed air. Hydrogen substition in ZnO possibly contributed to decreasing resistivity in ZnO thin films deposited under N₂ gas. The Hall measurements showed that the behavior of ZnO films deposited at 410 °C under the N₂ atmosphere was n-type with a carrier density of 8.9–9.2×10¹⁶ cm^-3 and mobility of ～70 cm²/Vs. ZnO thin films showed transmission values at 550 nm wavelength in a range of 70–80%. The values of band gaps extrapolated from the transmission results showed bandgap shrinkage in an order of milli electron volts in ZnO films deposited under N₂ compared to those deposited under compressed air. The calculation showed that the bandgap reduction was possibly a result of carrier–carrier interactions.     

Effect of substrate temperature on the properties of ZnO thin films prepared by spray pyrolysis

Sprayed ZnO films were grown on glass at different substrate temperatures from 200 °C to 500 °C and their structural, optical and electrical properties were investigated. All films are polycrystalline with hexagonal wurtzite structure. ZnO films at substrate temperatures above 400 °C appear to be better crystalized with (002) plane as preferential orientation. Optical transmission spectrum shows that ZnO films have high transmission (above 80%) in visible region for substrate temperatures above 400 °C. Photoluminescence spectra at room temperature show an ultraviolet emission and two visible emissions at 2.82 eV and 2.37 eV. The resistivity of ZnO films increases with increasing substrate temperatures (above 400 °C). The ZnO film deposited at 400 °C shows highest figure of merit.     

Highly transparent and conducting zinc oxide films deposited by activated reactive evaporation

Highly transparent and conducting undoped zinc oxide films have been obtained with a best resistivity of ～1.1 × 10^-3 Ω cm, a carrier density of ～1.5 × 10²⁰ cm^?3 and a mobility of ～38 cm²V^{?1s ?1}. These were produced by activated reactive evaporation at a deposition rate of 2 to 8Å/s with a substrate temperature ≤200° C. The films deposited by this process were found to have resistivities that were thickness independent and also were relatively insensitive to deposition parameters. In terms of conductivity, it was found that films deposited at higher temperatures (T > 300°+ C) were always inferior to the films deposited below 200° C. High temperature vacuum annealing (350° C) significantly degraded the resistivity of the undoped films deposited at low temperature; this was attributable to a drop in both the electron concentration and the mobility. Aluminum doping was found to be able to stabilize the electron concentration while the drop in mobility was found to be related to the choice of substrate.     

Electrical properties of carbon nitride thin films: Role of morphology and hydrogen content

The influence of hydrogen content and ambient humidity on the electrical properties of carbon nitride (CN_x) films deposited by reactive magnetron sputtering from a graphite target in Ar discharges mixed with N₂ and H₂ at a substrate temperature of 350°C have been investigated. Carbon films deposited in pure Ar exhibit a dark resistivity at room temperature of ∼4 × 10⁻² Ωcm, while the resistivity is one order of magnitude lower for CN_0.25 films deposited in pure N₂, due to their denser morphology. The increasing H₂ fraction in the discharge gas leads to an increased resistivity for all gas mixtures. This is most pronounced for the nitrogen-free films deposited in an Ar/H₂ mixture, where the resistivity increases by over four orders of magnitude. This can be related to a decreased electron mobility as H inhibits the formation of double bonds. After exposure to air, the resistivity increases with time through two different diffusion regimes. The measured electrical properties of the films are related to the apparent film microstructure, bonding nature, and ambient humidity.     

Low temperature Solution-Processed ZnO film on flexible substrate

Transparent conductive ZnO films were directly deposited on unseeded polyethersulfone (PES) substrates with a spin-spray method using aqueous solution at a low substrate temperature of 85 °C. All ZnO films were crystalline with wurtzite hexagonal structure and impurity phases were not detected. ZnO films deposited without citrate ions in the reaction solution had a rod array structure. In contrast, ZnO films deposited with citrate ions in the reaction solution had a continuous, dense structure. The transmittance of the ZnO films was improved from 11.9% to 85.3% as their structure changed from rod-like to continuous. After UV irradiation, the ZnO films with a continuous, dense structure had a low resistivity of 9.1×10⁻³ Ω cm, high carrier concentration of 2.7×10²⁰ cm⁻³ and mobility of 2.5 cm² V⁻¹ s⁻¹.     

High rate (~7 nm/s), atmospheric pressure deposition of ZnO front electrode for Cu(In,Ga)Se2 thin‐film solar cells with efficiency beyond 15%

Undoped zinc oxide (ZnO) films have been grown on a moving glass substrate by plasma‐enhanced chemical vapor deposition at atmospheric pressure. High deposition rates of ~7 nm/s are achieved at low temperature (200 °C) for a substrate speed from 20 to 60 mm/min. ZnO films are highly transparent in the visible range (90%). By a short (~minute) post‐deposition exposure to near‐ultraviolet light, a very low resistivity value of 1.6·10⁻³ Ω cm for undoped ZnO is achieved, which is independent on the film thickness in the range from 180 to 1200 nm. The photo‐enhanced conductivity is stable in time at room temperature when ZnO is coated by an Al₂O₃ barrier film, deposited by the industrially scalable spatial atomic layer deposition technique. ZnO and Al₂O₃ films have been used as front electrode and barrier, respectively, in Cu(In,Ga)Se2 (CIGS) solar cells. An average efficiency of 15.4 ± 0.2% (15 cells) is obtained that is similar to the efficiency of CIGS reference cells in which sputtered ZnO:Al is used as electrode. Copyright © 2013 John Wiley & Sons, Ltd.     

ZnO films deposited on GaAs substrates with a SiO2 thin buffer layer

Sputter deposition of ZnO films on GaAs substrates has been investigated. ZnO films were radio frequency (rf)-magnetron sputter deposited on GaAs substrates with or without SiO₂ thin buffer layers. Deposition parameters such as rf power, substrate-target distance, and gas composition/pressure were optimized to obtain highly c-axis oriented and highly resistive films. Deposited films were characterized by x-ray diffraction, scanning electron microscopy (SEM), capacitance, and resistivity measurements. Thermal stability of sputter-deposited ZnO films (0.5–2.0 μm thick) was tested with a post-deposition heat treatment at 430°C for 10 min, which is similar to a standard ohmic contact alloying condition for GaAs. The ZnO/SiO₂/GaAs films tolerated the heat treatment well while the ZnO/GaAs films disintegrated. The resistivity (10¹¹ Ω-cm) of the ZnO films on SiO₂-buffered GaAs substrates remained high during the heat treatment. The post-deposition anneal treatment also enhances c-axis orientation of the ZnO films dramatically and relieves intrinsic stress almost completely. These improvements are attributed to a reduction of grain boundaries and voids with the anneal treatment as supported by SEM and x-ray diffraction measurement results.     

Room-temperature ferromagnetic ordering in Mn-doped ZnO thin films grown by pulsed laser deposition

The ferromagnetic ordering in Mn-doped ZnO thin films grown by pulsed laser deposition (PLD) as a function of oxygen pressure and substrate temperature has been investigated. Room-temperature ferromagnetic behaviors in the Mn-doped ZnO films grown at 700°C and 800°C under 10⁻¹ torr in oxygen pressure were found, whereas ferromagnetic ordering in the films grown under 10⁻³ torr disappeared at 300 K. The large positive magnetoresistance (MR), ∼10%, was observed at 5 K at low fields and small negative MR was observed at high fields, irrespective of oxygen pressure. In particular, anomalous Hall effect (AHE) in the Mn-doped ZnO film grown at 700°C under 10⁻¹ Torr has been observed up to 210 K. In this work, the observed AHE is believed to be further direct evidence demonstrating that the Mn-doped ZnO thin films are ferromagnetic.     

Low-Temperature Preparation of Undoped ZnO Films with High Transparency and Conductivity by Ion Beam Deposition

Low-temperature growth of undoped ZnO films with high transparency and low electrical resistance was performed by ion beam sputtering. After systematic testing, resistivity as low as 2.95 × 10⁻³ Ω cm was obtained at a substrate temperature of 150°C, ion source voltage of 850 V, and ion beam current of 30 mA. The transmittance of the ZnO films was in the range of 85% to 90%. Hall measurements showed that a high mobility of 21.41 cm²/Vs was obtained for films less than 200 nm thick. The related microstructures and physical properties were measured and are discussed.     

Effect of NaOH solution on surface textured ZnO: Al films prepared by pulsed direct current magnetron sputtering

Transparent conducting Al-doped ZnO (ZnO:Al, AZO) thin films were prepared at substrate temperature of 270 °C by pulsed direct current magnetron sputtering. NaOH solution (5 wt%) was employed to etch the AZO films at room temperature, and the surface textured AZO films were obtained successfully. The relationship between the surface textured structures and the etching process controlled by etching time was discussed. The textured morphology of the etched AZO films became clear as increasing the etching time, and the AZO film etched for 30 min exhibited uniformly and distinctly crater-like surface textured structure. Correspondingly, the haze and the resistivity increased with the increasing etching time. And the resistivity of the AZO film etched for 30 min was 3.2×10⁻³ Ω cm.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.     

Indium doped zinc oxide thin films deposited by ultrasonic spray pyrolysis technique: Effect of the substrate temperature on the physical properties

Indium doped zinc oxide (ZnO:In) thin solid films were deposited on soda-lime glass substrates by the ultrasonic spray pyrolysis technique. The effect of the substrate temperature on the electrical, morphology, and optical characteristics of ZnO:In thin films was studied. It was found that, as the substrate temperature increases, the electrical resistivity decreases, reaching a minimum value in the order of 7.3×10⁻³ Ω cm, at 415 °C. Further increase in the substrate temperature results on an increment on the electrical resistivity of the thin solid films. All the samples were polycrystalline with a well-defined wurtzite structure. The preferred growth shows a switching from a random orientation at low substrates temperatures to (0 0 2) in the case of films deposited at the highest substrate temperature used. As the substrate temperature increases, the corresponding surface morphology changes from an almost faceted pyramidal to round-shaped form. The optical transmittance of the films in a interval of 400 to 700 nm is around 70%, with a band gap value in the order of 3.45 eV.     

Structural and Electrical Characterization of ZnO Films Grown by Spray Pyrolysis and Their Application in Thin‐Film Transistors

The role of the substrate temperature on the structural, optical, and electronic properties of ZnO thin films deposited by spray pyrolysis using a zinc acetate precursor solution is reported. Analysis of the precursor compound using thermogravimentry and differential scanning calorimetry indicates complete decomposition of the precursor at around 350 °C. Film characterization using Fourier Transform Infrared Spectroscopy (FTIR), photoluminescence spectroscopy (PL), and ultraviolet–visible (UV–Vis) optical transmission spectroscopy suggests the onset of ZnO growth at temperatures as low as 100 °C as well as the transformation to a polycrystalline phase at deposition temperatures >200 °C. Atomic force microscopy (AFM) and X‐ray diffraction (XRD) reveal that as‐deposited films exhibit low surface roughness (rms ≈ 2.9 nm at 500 °C) and a crystal size that is monotonously increasing from 8 to 32 nm for deposition temperatures in the range of 200–500 °C. The latter appears to have a direct impact on the field‐effect electron mobility, which is found to increase with increasing ZnO crystal size. The maximum mobility and current on/off ratio is obtained from thin‐film transistors fabricated using ZnO films deposited at >400 °C yielding values on the order of 25 cm² V^?1s^?1 and 10⁶, respectively.     

Zinc telluride films by photoenhanced metalorganic chemical vapor deposition

Polycrystalline films of zinc telluride (ZnTe) have been deposited on glass and conducting semiconductor coated glass substrates at 270°-350° C by photoenhanced metalorganic chemical vapor deposition (PECVD) using the reaction of dimethylzinc (DMZn) or diethylzinc (DEZn) and diisopropyltellurium (DIPTe) in hydrogen under atmospheric pressure. The deposited films are always ofp-type conductivity. Their properties are affected by the DMZn/DIPTe or DEZn/DIPTe molar ratio in the reaction mixture. The optimum DMZn/DIPTe ratio has been found to be approximately 0.9 on the basis of the open-circuit voltage of ZnTe/CdS heterojunctions and photoconductivity measurements. Without intentional doping, the deposited films are of high resistivity (>10⁷ ohm-cm) at room temperature, and the resistivity of these films has been controlled by using arsine as a dopant. The structural, optical, and electrical properties of ZnTe films have been characterized. Supported by the Solar Energy Institute under Subcontract XL-8-18091-1.     

Influence of Substrate Temperature on Structural and Thermoelectric Properties of Antimony Telluride Thin Films Fabricated by RF and DC Cosputtering

Antimony and tellurium were deposited on BK7 glass using direct-current magnetron and radiofrequency magnetron cosputtering. Antimony telluride thermoelectric thin films were synthesized with a heated substrate. The effects of substrate temperature on the structure, surface morphology, and thermoelectric properties of the thin films were investigated. X-ray diffraction patterns revealed that the thin films were well crystallized. c-Axis preferred orientation was observed in thin films deposited above 250°C. Scanning electron microscopy images showed hexagonal crystallites and crystal grains of around 500 nm in thin film fabricated at 250°C. Energy-dispersive spectroscopy indicated that a temperature of 250°C resulted in stoichiometric Sb₂Te₃. Sb₂Te₃ thin film deposited at room temperature exhibited the maximum Seebeck coefficient of 190 μV/K and the lowest power factor (PF), S ² σ, of 8.75 × 10⁻⁵ W/mK². When the substrate temperature was 250°C, the PF increased to its highest value of 3.26 × 10⁻³ W/mK². The electrical conductivity and Seebeck coefficient of the thin film were 2.66 × 10⁵ S/m and 113 μV/K, respectively.     

Chlorine-doped CdS thin films from CdCl<Subscript>2</Subscript>-mixed CdS powder

The CdS:Cl thin films have been prepared using thermally evaporated, CdCl₂-mixed CdS powder at 200°C substrate temperature. The percentage of CdCl₂ in the mixture varied from 0% to 0.20%. The electrical properties and the grain size of the deposited films were investigated. The results show that light doping, resistivity, carrier concentration, and mobility follow Seto’s model for polycrystalline material. However, with heavy doping, these properties undergo a saturation trend. The saturation behavior can be understood in terms of the rapid formation of the A-center complexes in the films. The deposited films were annealed at 250°C and 300°C. The resistivity of pure and lightly doped CdS films increased with annealing temperature, whereas carrier concentration and mobility in these films decreased. However, for the higher doping concentrations, the resistivity decreased, whereas carrier concentration and mobility showed improvement. These changes in electrical properties of the deposited films with annealing and doping concentration are attributed to a reduction in the lattice defect sites in CdS upon annealing. The experimental results are interpreted in terms of a modified version of Seto’s model for polycrystalline materials.     

Structure and optoelectrical properties of transparent conductive MGZO films deposited by magnetron sputtering

The transparent conductive Mg-Ga co-doped ZnO (MGZO) films were prepared by radio-frequency (RF) magnetron sputtering. The influence of substrate temperature on the structural and optoelectrical properties of the films is studied. The results show that all the films possess a preferential orientation along the (002) plane. With the increase of substrate temperature, the structure and optoelectrical properties of the films can be changed. When substrate temperature is 300 °C, the deposited film exhibits the best crystalline quality and optoelectrical properties, with the minimum micro strain of 1.09´10-3, the highest average visible transmittance of 82.42%, the lowest resistivity of 1.62×10-3 W?cm and the highest figure of merit of 3.18×103 W-1?cm-1. The optical bandgaps of the films are observed to be in the range of 3.342—3.545 eV. The refractive index dispersion curves obey the Sellmeier’s dispersion model. #$TABThis work has been supported by the National Natural Science Foundation of China (No.11504436), and the Fundamental Research Funds for the Central Universities (Nos.CZP17002 and CZW14019).#$TABE-mail:zyzhongzy@163.com      

Properties of antimony doped ZnO thin films deposited by spray pyrolysis technique

Antimony (Sb) doped zinc oxide (ZnO) thin films were deposited on the glass substrate at 450°C using spray pyrolysis technique. Effect of Sb doping on surface morphology structural, optical and electrical properties were studied. X-ray diffraction (XRD) analysis showed that both the undoped and doped ZnO thin films are polycrystalline in nature with (101) preferred orientation. SEM analysis showed a change in surface morphology of Sb doped ZnO thin films. Doping results in a marked increase in conductivity without affecting the transmittance of the films. ZnO films prepared with 3 at % Sb shows the lowest resistivity of 0.185 Ohm cm with a Hall mobility of 54.05 cm² V^–1 s^–1, and a hole concentration of 6.25 × 10¹⁷ cm^–3.     

Flash evaporated cadmium sulfide films

Cadmium sulfide (CdS) thin films were deposited by the flash evaporation technique onto glass substrates kept at temperatures in the range 30–300 °C. The source material was CdS powder synthesized in the laboratory. The films exhibited hexagonal structure with dislocation density and the stress decreased as the substrate temperature increased. An optical band gap of 2.39 eV was obtained for the films deposited at 300 °C. Raman spectra exhibited peaks corresponding to Longitudinal Optical phonons of CdS with the full width at half maximum decreasing with increase of substrate temperature. Room temperature resistivity values are lower than earlier reports on chemically deposited CdS films.     

Spin-coated ZnO thin films using ZnO nano-colloid

Using ZnO nano-colloids, thin films of ZnO have been deposited and characterized. The nano-colloid has been synthesized using a top-down wet chemistry method starting from submicron ZnO particles. The steric stabilization technique is implemented using stearic acid as a capping agent to prevent agglomeration of the ZnO nanoparticles. A stable suspension is achieved, exhibiting strong optical absorbance and a band edge at 376 nm. The simple chemistry enables a uniform film on a variety of substrates using spin coating. Current-voltage (I–V) measurements of ZnO films deposited on quartz after annealing treatment show a resistivity of 1.89×10⁵ Ω · cm.