Influence of substrate temperature and post-treatment on the properties of ZnO:Al thin films prepared by pulsed laser deposition

Highly transparent conductive Al₂O₃ doped zinc oxide (AZO) thin films have been deposited on the glass substrate by pulsed laser deposition technique. The effects of substrate temperature and post-deposition annealing treatment on structural, electrical and optical properties of AZO thin films were investigated. The experimental results show that the electrical resistivity of films deposited at 240 °C is 6.1 × 10⁻⁴ Ω cm, which can be further reduced to as low as 4.7 × 10⁻⁴ Ω cm by post-deposition annealing at 400 °C for 2 h in argon. The average transmission of AZO films in the visible range is 90%. The optical direct band gap of films was dependent on the substrate temperature and the annealing treatment in argon. The optical direct band gap value of AZO films increased with increasing annealing temperature.     

Effects of substrate temperature and Zn addition on the properties of Al-doped ZnO films prepared by magnetron sputtering

Al-doped ZnO (AZO) films prepared at different substrate temperature and AZO films with intentional Zn addition (ZAZO) during deposition at elevated substrate temperature were fabricated by radio frequency magnetron sputtering on glass substrate, and the resulting structural, electrical, optical properties together with the etching characteristics and annealing behavior were comparatively examined. AZO films deposited at 150 °C showed the optimum electrical properties and the largest grain size. XPS analysis revealed that AZO films deposited at elevated temperature of 450 °C contained large amount of Al content due to Zn deficiency, and that intentional Zn addition during deposition could compensate the deficiency of Zn to some extent. It was shown that the electrical, optical and structural properties of ZAZO films were almost comparable to those of AZO film deposited at 150 °C, and that ZAZO films had much smaller etching rate together with better stability in severe annealing conditions than AZO films due possibly to formation of dense structure.     

Characteristics of ZnO:Al thin films co-doped with hydrogen and fluorine

Fluorine and hydrogen co-doped ZnO:Al (AZO) films were prepared by radio frequency (rf) magnetron sputtering of ZnO targets containing 1 wt.% Al₂O₃ on Corning glass at substrate temperature of 150 °C with Ar/CF₄/H₂ gas mixtures, and the structural, electrical and optical properties of the as-deposited and the vacuum-annealed films were investigated. In as-deposited state, films with fairly low resistivity of 3.9-4 × 10⁻⁴ Ω cm and very low absorption coefficient below 900 cm⁻¹ when averaged in 400-800 nm could be fabricated. After vacuum-heating at 300 °C, the minimum resistivity of 2.9 × 10⁻⁴ Ω cm combined with low absorption loss in visible region, which enabled the figure of merit to uplift as high as 4 Ω⁻¹, could be obtained for vacuum-annealed film. It was shown that, unlike hydrogenated ZnO films which resulted in degradation upon heating in vacuum at moderately high temperature, films with fluorine addition could yield improved electrical properties mostly due to enhanced Hall mobility while preserving carrier concentration level. Furthermore, stability in oxidizing environment could be improved by fluorine addition, which was ascribed to the filling effect of dangling bonds at the grain boundaries. These results showed that co-doping of hydrogen and fluorine into AZO films with low Al concentration could be remarkably compatible with thin film solar cell applications.     

Structural and optical properties of sputtered ZnO thin films

Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode sputtering at varying deposition conditions. The effects of negative bias voltage and RF power on structural and optical properties were investigated. X-ray diffraction measurements (XRD) confirmed that both un-doped and Al-doped ZnO films are polycrystalline and have hexagonal wurtzite structure. The preferential 〈0 0 1〉 orientation and surface roughness evaluated by AFM measurements showed dependence on applied bias voltage and RF power. The sputtered ZnO and ZnO:Al films had high optical transmittance (>90%) in the wavelength range of 400-800 nm, which was not influenced by bias voltage and RF power. ZnO:Al were conductive and highly transparent. Optical band gap of un-doped and Al-doped ZnO thin films depended on negative bias and RF power and in both cases showed tendency to narrowing.     

Structural, optical and electrical properties of Al-N codoped ZnO films by RF-assisted MOCVD method

N-doped ZnO films were produced using N₂ as N source by metal-organic chemical vapor deposition (MOCVD) system which has been improved with radio-frequency (RF)-assisted equipments. The data of secondary ion mass spectroscopy (SIMS) indicate that the concentration of N in N-doped ZnO films is around 5 × 10²⁰ cm⁻³, implying that sufficient incorporation of N into ZnO can be obtained by RF-assisted equipment. On this basis, the structural, optical and electrical properties of Al-N codoped ZnO films were studied. Then, the effect of RF power on crystal quality, surface morphologies, optical properties was analyzed using X-ray diffraction, atomic force microscopy and photo-luminescence methods. The results illustrate that the RF plasma is the key factor for the improvement of crystal quality. Then the observation of A⁰X recombination associated with N_O acceptor in low-temperature PL spectrum proved that some N atoms have occupied the positions of O atoms in ZnO films. Hall measurements shown that p-type ZnO film deposited on quartz glasses was obtained when RF power was 150 W for the Al-N codoped ZnO films, while the resistivity of N-doped ZnO films was rather high. Compared with the Al-doped ZnO film, the obviously increased resistivity of codoped films indicates that the formation of N_O acceptors compensate some donors in ZnO films effectively.     

Influence of deposition temperature on the crystallinity of Al-doped ZnO thin films at glass substrates prepared by RF magnetron sputtering method

Al-doped ZnO (AZO) transparent conducting films were successfully prepared on glass substrates by RF magnetron sputtering method under different substrate temperatures. The microstructural, electrical and optical properties of AZO films were investigated in a wide temperature range from room temperature up to 350 °C by X-ray Diffraction (XRD), Field-Emission Scanning Electron Microscopy (FESEM), High-Resolution Transmission Electron Microscopy (HRTEM), Hall measurement, and UV–visible meter. The nature of AZO films is polycrystalline thin films with hexagonal wurtzite structure and a preferred orientation along c-axis. The crystallinity and surface morphologies of the films are strongly dependent on the growth temperature, which in turn exerts a great effect on microstructural, electrical and optical properties of the AZO films. The atomic arrangement of AZO film having an wurtzite structure was indeed identified by the HRTEM as well as the Selected Area Electron Diffraction (SAED). The defect density of AZO film was investigated by HRTEM. The film deposited at 100 °C exhibited the relatively well crystallinity and the lowest resistivity of 3.6 × 10⁻⁴ Ω cm. The average transmission of AZO films in the visible range is all over 85%. More importantly, the low-resistance and high-transmittance AZO film was also prepared at a low temperature of 100 °C.     

Influence of post-annealing on the properties of Sc-doped ZnO transparent conductive films deposited by radio-frequency sputtering

Sc-doped ZnO transparent conductive films are deposited on glass substrates by radio-frequency sputtering. The influence of post-annealing on the structural, morphologic, electrical, and optical properties of the films is investigated by energy dispersion X-ray spectroscopy, X-ray diffraction, Hall measurement, and optical transmission spectroscopy. The experimental results show that these films are polycrystalline with a preferred [0 0 1] orientation. The lowest resistivity of 2.6 × 10⁻⁴ Ω cm is obtained from the film annealed at 500 °C. The average optical transmittance of the films is over 90%. These results suggest that Sc-doped ZnO is a good candidate for fabricating high performance transparent conductive films.     

Deposition of Ni, Ag, and Pt-based Al-doped ZnO double films for the transparent conductive electrodes by RF magnetron sputtering

Ni, Ag, and Pt-based Al-doped ZnO (AZO) films have been deposited as transparent conductivity layers on quartz by RF magnetron sputtering and characterized by X-ray diffraction, Hall measurement, optical transmission spectroscopy, scanning electron microscopy (SEM). The deposition of thicker metal layer in double layers resulted in lowering the effective electrical resistivity with a slight reduction of their optical transmittance. A film consisting of AZO (250 nm)/Ni (2 nm) double structure, exhibits a sheet resistance of 21.0 Ω/sq, a high transmittance of 76.5%, and characterize good adhesion to substrate. These results make the satisfactory for GaN-based light-emitting diodes (LEDs) and solar cells with metal-based AZO double films as current spread layers.     

Deposition of quasi-crystal Al-doped ZnO thin films for photovoltaic device applications

Quasi-crystal aluminum-doped zinc oxide (AZO) films were prepared by in situ radio frequency (RF) magnetron sputtering (sputtering without annealing) on glass substrates. The influence of deposition parameters on the optoelectronic and structural properties of the in situ deposited quasi-crystal AZO films was investigated in order to compare resulting samples. X-ray diffraction (XRD) patterns show that the quasi-crystal AZO thin films have excellent crystallization improved with increase of the RF power and substrate temperature, with an extremely preferential c-axis orientation exhibit sharp and narrow XRD pattern similar to that of single-crystal. Field emission scanning electron microscopy (FESEM) images show that quasi-crystal AZO thin films have uniform grains and the grain size increase with the increase of RF power and substrate temperature. Craters of irregular size with the columnar structure are observed in the quasi-crystal AZO thin films at a lower substrate temperature while many spherical shaped grains appeared at a higher substrate temperature. The average optical transmittance of all the quasi-crystal AZO films was over 85% in the 400-800 nm wavelength range. The resistivity of 4.176 × 10⁻⁴ Ω cm with the grain size of 76.4891 nm was obtained in the quasi-crystal AZO thin film deposited at 300 °C, under sputtering power of 140 W.     

High work function of Al-doped zinc-oxide thin films as transparent conductive anodes in organic light-emitting devices

Deposition of Al-doped ZnO (AZO) films with various film thicknesses on glass substrates was performed to investigate the feasibility of using AZO films as anode electrodes in organic light-emitting devices (OLEDs). The electrical resistivity of the AZO films with a 180-nm thickness was 4.085 × 10⁻² Ω cm, and the average optical transmittance in the visible range was 80.2%. The surface work function for the AZO films, determined from the secondary electron emission coefficients obtained with a focused ion beam, was as high as 4.62 eV. These results indicate that AZO films grown on glass substrates hold promise for potential applications as anode electrodes in high-efficiency OLEDs.     

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.     

Effects of post-annealing on structural, optical and electrical properties of Al-doped ZnO thin films

Aluminum-doped zinc oxide (AZO) films were deposited at 400 °C by radio-frequency magnetron sputtering using a compound AZO target. The effects of annealing atmospheres as well as hydrogen annealing temperatures on the structural, optical and electrical properties of the AZO films were investigated. It was found that the electrical resistivity varied depending on the atmospheres while annealing in air, nitrogen and hydrogen at 300 °C, respectively. Comparing with that for the un-annealed films, the resistivity of the films annealed in hydrogen decreased from 9.8 × 10⁻⁴ Ω cm to 3.5 × 10⁻⁴ Ω cm, while that of the films annealed in air and nitrogen increased. The variations in electrical properties are ascribed to both the changes in the concentration of oxygen vacancies and adsorbed oxygen at the grain boundaries. These results were clarified by the comparatively XPS analyzing about the states of oxygen on the surface of the AZO films. There was great increase in electrical resistivity due to the damage of the surfaces, when AZO films were annealed in hydrogen with a temperature higher than 500 °C, but high average optical transmittance of 80-90% in the range of 390-1100 nm were still obtained.     

Effects of substrate temperature on the efficiency of hydrogen incorporation on the properties of Al-doped ZnO films

Al-doped ZnO (AZO) transparent conducting films were successfully prepared on glass substrates by RF magnetron sputtering at different substrate temperatures in Ar and H₂ + Ar sputtering ambient. The effects of substrate temperature on the effectiveness of hydrogen incorporation in Al-doped ZnO films were investigated. The microstructural, electrical and optical properties of AZO films were systematically analyzed by surface profiler, X-ray diffractometry, scanning electron microscope, four-point probe measurement and UV/vis spectrophotometer. The XRD patterns and SEM pictures indicate that the crystallinity of AZO thin films was markedly improved with hydrogen incorporation at low substrate temperature, while the improvement of crystallinity was not an obvious change at high substrate temperature. The results also indicate that hydrogen incorporation has the stronger effectiveness on the transparent conductive properties of AZO films with the substrate temperature decreasing. The resistivity of the films decreases, especially for lower substrate temperatures, due to the incorporation of hydrogen atoms. These results suggest that substrate temperature should be controlled to the lower level to effectively reduce resistivity without detriment to transmittance of AZO thin films when hydrogen is incorporated.     

A simplified reactive thermal evaporation method for indium tin oxide electrodes

Indium tin oxide (ITO) films approximately 120 nm thick were deposited onto unheated glass substrates by using reactive thermal evaporation (RTE) and in situ post-evaporation annealing in oxygen. We show that this simplified method can be used to produce high quality ITO thin films with low electrical resistivity (10⁻³ Ω cm) and high transmittance (approximately 80% at 550 nm). The refractive index is approximately 2.0 and the direct optical band gap of the films (above 3.0 eV) is in good agreement with previously reported values. Since this deposition method does not require heating the substrates or furnace annealing at high temperatures, it can be advantageous when it is necessary to decrease the thermal budget on underlying devices or layers.     

Water electrolysis-induced optical degradation of aluminum-doped zinc oxide films

A type of optical degradation of aluminium-doped zinc oxide (AZO) films due to water electrolysis-induced reduction reaction was reported. An experiment was designed in which AZO films were immersed in a 0.01 M NaOH aqueous solution as cathode to electrolyze water. Significant decreases in the optical transmission of the treated samples were observed. Studies by X-ray diffraction and scanning electron microscope showed that the degradation of AZO films was due to compositional and structural changes with the treatment of water electrolysis, which resulted from the reduction reaction of atomic hydrogen generated in the electrolysis of water. This optical degradation reflects the stability degradation of AZO films under water electrolysis environment.     

Optimization of the optical and electrical properties of electron beam evaporated aluminum-doped zinc oxide films for opto-electronic applications

Aluminum-doped zinc oxide (AZO) thin films have been deposited by electron beam evaporation technique on glass substrates. The structural, electrical and optical properties of AZO films have been investigated as a function of annealing temperature. It was observed that the optical properties such as transmittance, reflectance, optical band gap and refractive index of AZO films were strongly affected by annealing temperature. The transmittance values of 84% in the visible region and 97% in the NIR region were obtained for AZO film annealed at 475 °C. The room temperature electrical resistivity of 4.6×10⁻³ Ω cm has been obtained at the same temperature of annealing. It was found that the calculated refractive index has been affected by the packing density of the thin films, whereas, the high annealing temperature gave rise to improve the homogeneity of the films. The single-oscillator model was used to analyze the optical parameters such as the oscillator and dispersion energies.     

Stabilization in electrical characteristics of hydrogen-annealed ZnO:Al films

Al-doped ZnO (ZnO:Al) films prepared by RF magnetron co-sputtering at room temperature were thermally treated in hydrogen ambient at 300 °C to enhance the films’ characteristics for transparent conductive oxide applications. The electrical properties of the hydrogen-annealed films were improved and preserved in air ambient, even though the crystal structures of the films were not changed by the thermal treatment. The optical and oxygen bonding characteristics of ZnO:Al films manifested that absorbed oxygen species on the films were removed by the hydrogen-annealing process. These results supported that the development of the electrically reliable ZnO:Al films could be realized using the hydrogen-annealing process.     

Structure and electrical transport properties of bismuth thin films prepared by RF magnetron sputtering

Bismuth thin films were prepared on glass substrates with RF magnetron sputtering and the effects of deposition temperature on surface morphology and their electrical transport properties were investigated. Grain growth of bismuth and the coalescence of grains were observed above 393 K with field emission secondary electron microscopy. Continuous thin films could not be obtained above 448 K because of the segregation of grains. Hall effect measurements showed that substrate heating yields the decrease of carrier density and the increase of mobility in exponential ways until 403 K. Resistivity of sputter deposited bismuth films has its minimum (about 0.7 × 10⁻³ Ω cm) in range of 403-433 K. Annealing of bismuth films deposited at room temperature was carried out in a radiation furnace with flowing hydrogen gas. The change of resistivity was not significant due to the cancellation of the decrease of carrier density and the increase of mobility. However, the abrupt change of electrical properties of film annealed above 523 K was observed, which is caused by the oxidation of bismuth layer.     

Near infrared to UV dielectric functions of Al doped ZnO films deposited on c-plane sapphire substrate using pulsed laser deposition

Transparent conducting polycrystalline Al-doped ZnO (AZO) films were deposited on sapphire substrates at substrate temperatures ranging from 200 to 300 °C by pulsed laser deposition (PLD). X-ray diffraction measurement shows that the crystalline quality of AZO films was improved with increased substrate temperature. The electrical and optical properties of the AZO films have been systematically studied via various experimental tools. The room-temperature micro-photoluminescence (µ-PL) spectra show a strong ultraviolet (UV) excitonic emission and weak deep-level emission, which indicate low structural defects in the films. A Raman shift of about 11 cm⁻¹ is observed for the first-order longitudinal-optical (LO) phonon peak for AZO films when compared to the LO phonon peak of bulk ZnO. The Raman spectra obtained with UV resonant excitation at room temperature show multi-phonon LO modes up to third order. Optical response due to free electrons of the AZO films was characterized in the photon energy range from 0.6 to 6.5 eV by spectroscopic ellipsometry (SE). The free electron response was expressed by a simple Drude model combined with the Cauchy model are reported.     

Substrate temperature influence on the properties of nanostructured ZnO transparent ultrathin films grown by PLD

Zinc oxide films of 40 nm thickness have been deposited on glass substrates by pulsed laser deposition using an excimer XeCl laser (308 nm) at different substrate temperatures ranging from room temperature to 650 °C. Surface investigations carried out by using atomic force microscopy have shown a strong influence of temperature on the films surface topography. UV-VIS transmittance measurements have shown that our ZnO films are highly transparent in the visible wavelength region, having an average transmittance of ∼90%. The optical band gap of the films was found to be 3.26 eV, which is lower than the theoretical value of 3.37 eV. Besides the normal absorption edge related to the transition between the valence and the conduction band, an additional absorption band was also recorded in the wavelength region around 364 nm (∼3.4 eV). This additional absorption band may be due to excitonic, impurity, and/or quantum size effects. Photoreduction/oxidation in ozone of the ZnO films lead to larger conductivity changes for higher deposition temperature. In conclusion, the ozone sensing characteristics as well as the optical properties of the ZnO thin films deposited by pulsed laser deposition are strongly influenced by the substrate temperature during growth. The sensitivity of the films towards ozone might be enhanced significantly by the control of the films deposition parameters and surface characteristics.