Effect of composition and deposition temperature on the characteristics of Ga doped ZnO thin films

ZnO films doped with Ga (GZO) of varying composition were prepared on Corning glass substrate by radio frequency magnetron sputtering at various deposition temperatures of room temperature, 150, 250 and 400 °C, and their temperature dependent photoelectric and structural properties were correlated with Ga composition. With increasing deposition temperature, the Ga content, at which the lowest electrical resistivity and the best crystallinity were observed, decreased. Films with optimal electrical resistivity of 2-3 × 10⁻⁴ Ω cm and with good crystallinity were obtained in the substrate temperature range from 150 to 250 °C, and the corresponding C_Ga/(C_Ga + C_Zn) atomic ratio was about 0.049. GZO films grown at room temperature had coarse columnar structure and low optical transmittance, while films deposited at 400 °C yielded the highest figure of merit (FOM) due to very low optical absorption despite rather moderate electrical resistivity slightly higher than 4 × 10⁻⁴ Ω cm. The optimum Ga content at which the maximum figure of merit was obtained decreased with increasing deposition temperature.     

Room temperature growth of InxGa1−xN thin films by mixed source modified activated reactive evaporation

Polycrystalline In_xGa_1−xN thin films were prepared by mixed source modified activated reactive evaporation (MARE) technique. The films were deposited at room temperature on glass substrates without any buffer layer. All the films crystallize in the hexagonal wurtzite structure. The indium concentration calculated from XRD peak shift using Vegard's law was found to be varying from 2% to 92%. The band gap varies from 1.72 eV to 3.2 eV for different indium compositions. The indium rich films have higher refractive indices as compared to the gallium rich films. The near infra-red absorption decreases with gallium incorporation into InN lattice which is mainly due to decrease in the free carrier concentration in the alloy system. This fact is further supported from Hall effect measurements. MARE turns out to be a promising technique to grow In_xGa_1−xN films over the entire composition range at room temperature.     

Structural and optical properties of Ga2(1−x)In2xO3 films prepared on α-Al2O3 (0 0 0 1) by MOCVD

Ga_2(1−x)In_2xO₃ thin films with different indium content x [In/(Ga + In) atomic ratio] were prepared on α-Al₂O₃ (0 0 0 1) substrates by the metal organic chemical vapor deposition (MOCVD). The structural and optical properties of the Ga_2(1−x)In_2xO₃ films were investigated in detail. Microstructure analysis revealed that the film deposited with composition x = 0.2 was polycrystalline structure and the sample prepared with x up to 0.8 exhibited single crystalline structure of In₂O₃. The optical band gap of the films varied with increasing Ga content from 3.72 to 4.58 eV. The average transmittance for the films in the visible range was over 90%.     

Growth of Ga-doped ZnO films with ZnO buffer layer by sputtering at room temperature

Gallium-doped zinc oxide films have been grown on glass substrates with and without ZnO buffer layers by r.f. magnetron sputtering at room temperature. In this approach, the grey relational Taguchi method analysis is adopted to solve the coating process with multiple deposition qualities. Optimal coating parameters can then be determined by using the gray relational grade as a performance index. The GZO coating parameters (r.f. power, sputtering pressure, O₂/(Ar+O₂) flow-rate ratios, and deposition time) are optimized, by taking into account the multiple performance characteristics (structural, morphological, deposition rate, electrical resistivity, and optical transmittance). The results indicate that with the grey relational Taguchi method, the electrical resistivity of GZO films is reduced from 9.23×10⁻³ to 5.77×10⁻³ Ω cm and optical transmittance increases from 79.42% to 82.95%, respectively. The ZnO buffer layer can reduce the electrical resistivity of GZO films from 5.77×10⁻³ to 2.38×10⁻³ Ω cm. It can be anticipated that room temperature deposition enables film deposition onto polymeric substrates for flexible optoelectronic devices.     

Substitution mechanism of Ga for Zn site depending on deposition temperature for transparent conducting oxides

High quality transparent conductive gallium-doped zinc oxide (GZO) thin films were deposited on glass substrates using rf-magnetron sputtering system at the temperature ranging from room temperature (RT) to 500 °C. The temperature-dependence of Ga doping effect on the structural, optical and electrical properties in ZnO has been investigated. For the GZO thin films deposited at over 200 °C, (103) orientation was strongly observed by X-ray diffraction analysis, which is attributed to the substitution of Ga elements into Zn site. X-ray photoelectron spectroscopy measurements have confirmed that oxygen vacancies were generated at the temperature higher than 300 °C. This might be due to the effective substitution of Ga³⁺ for Zn site at higher temperature. It was also found that the optical band gap increases with deposition temperature. The optical transmittance of GZO thin films was above 87% in the visible region. The GZO thin films grown at 500 °C showed a low electrical resistivity of 4.50 × 10^?4 Ω cm, a carrier concentration of 6.38 × 10²⁰ cm^?3 and a carrier mobility of 21.69 cm²/V.     

Structural, optical and magnetic properties of (ZnO)1−x(MnO2)x thin films deposited at room temperature

The structural, magnetic and optical properties of (ZnO)_1−x(MnO₂)_x (with x = 0.03 and 0.05) thin films deposited by pulsed laser deposition (PLD) were studied. The pellets used as target, sintered at different temperatures ranging from 500 °C to 900 °C, were prepared by conventional solid state method using ZnO and MnO₂ powders. The observation of non-monotonic shift in peak position of most preferred (1 0 1) ZnO diffraction plane in XRD spectra of pellets confirmed the substitution of Mn ions in ZnO lattice of the sintered targets. The as-deposited thin film samples are found to be polycrystalline with the preferred orientation mostly along (1 1 0) diffraction plane. The UV-vis spectroscopy of the thin films revealed that the energy band gap exhibit blue shift with increasing Mn content which could be attributed to Burstein-Moss shift caused by Mn doping of the ZnO. The deposited thin films exhibit room temperature ferromagnetism having effective magnetic moment per Mn atom in the range of 0.9-1.4μ_B for both compositions.     

Growth characteristics and properties of ZnO:Ga thin films prepared by pulsed DC magnetron sputtering

Transparent conductive ZnO:Ga thin films were deposited on Corning 1737 glass substrate by pulsed direct current (DC) magnetron sputtering. The effects of process parameters, namely pulse frequency and film thickness on the structural and optoelectronic properties of ZnO:Ga thin films are evaluated. It shows that highly c-axis (0 0 2) oriented polycrystalline films with good visible transparency and electrical conductivity were prepared at a pulsed frequency of 10 kHz. Increasing the film thickness also enlarged the grain size and carrier mobility which will subsequently lead to the decrease in resistivity. In summary, ZnO:Ga thin film with the lowest electrical resistivity of 2.01 × 10⁻⁴ Ω cm was obtained at a pulse frequency of 10 kHz with 500 nm in thickness. The surface RMS (root mean square) roughness of the film is 2.9 nm with visible transmittance around 86% and optical band gap of 3.83 eV.     

Sputter deposition of high transparent TiO2−xNx/TiO2/ZnO layers on glass for development of photocatalytic self-cleaning application

In this study, TiO_2−xN_x/TiO₂ double layers thin film was deposited on ZnO (80 nm thickness)/soda-lime glass substrate by a dc reactive magnetron sputtering. The TiO₂ film was deposited under different total gas pressures of 1 Pa, 2 Pa, and 4 Pa with constant oxygen flow rate of 0.8 sccm. Then, the deposition was continued with various nitrogen flow rates of 0.4, 0.8, and 1.2 sccm in constant total gas pressure of 4 Pa. Post annealing was performed on as-deposited films at various annealing temperatures of 400, 500, and 600 °C in air atmosphere to achieve films crystallinity. The structure and morphology of deposited films were evaluated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). The chemical composition of top layer doped by nitrogen was evaluated by X-ray photoelectron spectroscopy (XPS). Photocatalytic activity of samples was measured by degradation of Methylene Blue (MB) dye. The optical transmittance of the multilayer film was also measured using ultraviolet-visible light (UV-vis) spectrophotometer. The results showed that by nitrogen doping of a fraction (∼1/5) of TiO₂ film thickness, the optical transmittance of TiO_2−xN_x/TiO₂ film was compared with TiO₂ thin film. Deposited films showed also good photocatalytic and hydrophilicity activity at visible light.     

Influence of the grain boundary barrier height on the electrical properties of Gallium doped ZnO thin films

The pulsed laser deposition (PLD) technique is used to deposit Gallium doped zinc oxide (GZO) thin films on glass substrates at 250 with different Gallium (Ga) doping concentration of 0, 1.0, 3.0 and 5.0%. The influence of Ga doping concentration on structure, chemical atomic compositions, electrical and optical properties was investigated by XRD, XPS, Hall measurement and UV spectrophotometer, respectively. The relationship between electrical properties and Ga doping concentration was clarified by analyzing the chemical element compositions and the chemical states on the GZO films. It is found that the carrier concentrations and oxygen vacancies in the GZO films increase with increasing Ga doping concentration. The lowest resistivity (3.63 × 10⁻⁴ Ω cm) and barrier height of grain boundaries (14 mV) were obtained with 3% Ga doping. In particular, we suppose the band gap of 5% Ga doping sample larger than that of 3% Ga doping sample is due to the quantum size effect from the amorphous structure rather than Moss-Burstein shift.     

Properties of Dy-doped ZnO nanocrystalline thin films prepared by pulsed laser deposition

Highly transparent conductive Dy₂O₃ doped zinc oxide (ZnO)_1-x(Dy₂O₃)_x nanocrystalline thin films with x from 0.5% to 5% have been deposited on glass substrate by pulsed laser deposition technique. The structural, electrical and optical properties of Dy₂O₃ doped thin films were investigated as a function of the x value. The experimental results show that the Dy concentration in Dy-doped ZnO thin films has a strong influence on the material properties especially electrical properties. The resistivity decreased to a minimum value of 5.02 × 10⁻⁴ Ω cm with x increasing from 0.5% to 1.0%, then significantly increased with the further increasing of x value. On the contrary, the optical direct band gap of the (ZnO)_1-x(Dy₂O₃)_x films first increased, then decreased with x increasing. The average transmission of Dy₂O₃ doped zinc oxide films in the visible range is above 90%.