Effect of the [Al/Zn] ratio in the starting solution and deposition temperature on the physical properties of sprayed ZnO:Al thin films

Aluminum-doped zinc oxide thin films (ZnO:Al) were deposited on sodocalcic glass substrates by the chemical spray technique, using zinc acetate and aluminum pentanedionate as precursors. The effect of the [Al/Zn] ratio in the starting solution, as well as the substrate temperature, on the physical characteristic of ZnO:Al thin films was analyzed. We have found that the addition of Al to the starting solution decreases the electrical resistivity of the films until a minimum value, located between 2 and 3 at.%; a further increase in the [Al/Zn] ratio leads to an increase in the resistivity. A similar resistivity tendency with the substrate temperature was encountered, namely, as the substrate temperature is increased, a minimum value of around 475 °C in almost all the cases, was obtained. At higher deposition temperatures the film resistivity suffers an increase. After a vacuum-thermal treatment, performed at 400 °C for 1 h, the films showed a resistivity decrease about one order of magnitude, reaching a minimum value, for the films deposited at 475 °C, of 4.3 × 10^− 3 Ω cm.The film morphology is strongly affected by the [Al/Zn] ratio in the starting solution. X-ray analysis shows a (002) preferential growth in all the films. As the substrate temperature increases it is observed a slight increase in the transmittance as well as a shift in the band gap of the ZnO:Al thin films.     

Annealing and oxidation mechanism of evaporated zinc thin films from zinc oxide powder

Zinc oxide (ZnO) thin films were deposited by thermal evaporation of a high quality ZnO powder; the obtained films were then oxidized in the air. We have systematically investigated the influence of annealing temperature ranged from 100 to 400 °C on the films composition and structural and optical properties by using Rutherford Back Scattering (RBS) analysis, X-ray Diffraction (XRD) and UV-Visible transmission respectively. The as grown films exhibit a hexagonal single phase of Zn with no preferential orientation and contain 28% oxygen. With an increase in the annealing temperature the oxygen content is enhanced to the detriment of Zn; samples were totally oxidized at 300 °C and the films are converted to stoichiometric ZnO material. However, in situ XRD pattern analysis shows that the oxidation starts at 250 °C. From the XRD results of annealed Zn samples under an electrical field we inferred that the oxidation mechanism is achieved by the ionization of oxygen atom at the film surface and subsequently followed by the diffusion of the produced ions in the film network.     

Transparent conducting Al and Y codoped ZnO thin film deposited by DC sputtering

Transparent conducting Al and Y codoped zinc oxide (AZOY) thin films with high transparency and low resistivity were deposited by DC magnetron sputtering. The effects of substrate temperature on the structural, electrical and optical properties of AZOY thin films deposited on glass substrates have been investigated. X-ray diffraction spectra indicate that no diffraction peak of Al₂O₃ or Y₂O₃ except that of ZnO (0 0 2) is observed. The AZOY thin film prepared at substrate temperature of 250 °C has the optimal crystal quality inferring from FWHM of ZnO (0 0 2) diffraction peak, but the AZOY thin film deposited at 300 °C has the lowest resistivity of 3.6 × 10⁻⁴ Ω-cm, the highest mobility of 30.7 cm² V⁻¹ s⁻¹ and the highest carrier concentration of 5.6 × 10²⁰ cm⁻³. The films obtained have disorderly polyhedral surface morphology indicating possible application in thin film solar cell with good quality and high haze factor without the need of post-deposition etching.     

Influence of Ga2O3 addition on transparent conductive oxide films of In2O3-ZnO

Influence of incorporation of Ga in amorphous In-Zn-O transparent conductive oxide films was investigated as a function of Zn/(Zn + In). For In-Zn-O films with no Ga₂O₃, the range of Zn/(Zn + In) ratio where the amorphous phase appears became narrow at a substrate temperature of 250 °C. With increasing Ga₂O₃ quantity, amorphous films were obtained even at a high substrate temperature of 250 °C in a wider range of Zn/(Zn + In) than that of In-Zn-O films with no Ga₂O₃. This means that the trend of crystallization at higher substrate temperature was disturbed with additional Ga incorporation. For the film deposited from ZnO:Ga (Ga₂O₃: 4.5-7.5 wt%) and In₂O₃ targets, we obtained a resistivity of 2.8 × 10⁻⁴ Ω cm, nearly the same value as that for an In-Zn-O film with no Ga₂O₃. The addition of more than 7.5 wt% Ga₂O₃ induced a widening of the optical band gap.     

Microstructural and optical properties of Ga-doped ZnO semiconductor thin films prepared by sol-gel process

Transparent thin films of Ga-doped ZnO (GZO), with Ga dopant levels that varied from 0 to 7 at.%, were deposited onto alkali-free glass substrates by a sol-gel process. Each spin-coated film was preheated at 300 °C for 10 min, and then annealed at 500 °C for 1 h under air ambiance. The effects of Ga dopant concentrations on crystallinity levels, microstructures, optical properties, and electrical resistivities of these ZnO thin films were systematically investigated. Photoluminescence spectra of GZO thin films were examined at room temperature. XRD results revealed that the undoped ZnO thin films exhibited a preferred orientation along the (002) plane and that the ZnO thin films doped with Ga showed degraded crystallinity. Experimental results also showed that Ga doping of ZnO thin films could markedly decrease surface roughness, improve transparency in the visible range, and produce finer microstructures than those of undoped ZnO thin films. The most promising films for transparent thin film transistor (TTFT) application produced in this study, were the 3 and 5 at.% Ga-doped ZnO thin films, both of which exhibited an average transmittance of 90.6% and an RMS roughness value of about 2.0 nm.     

Novel properties of AZO film sputtered in Ar+H2 ambient at high temperature

AZO (ZnO:Al) transparent conductive thin film was prepared by RF magnetron sputtering with a AZO (98 wt% ZnO 2 wt% Al₂O₃) ceramic target in the same Ar+H₂ ambient at different substrate temperatures ranging from 100 to 300 °C. The minimum resistivity of AZO films was 7.9×10⁻⁴ Ω cm at the substrate temperature of 200 °C. The average transmission in the visible rang was more than 90%. Scanning electron microscopy and XRD analyses showed that the surface morphology of the AZO samples altered with the increasing of the substrate temperature. AZO film prepared at 200 °C in the pure Ar ambient was also made as comparison about the resistivity, carrier concentration and the average crystallite size. The resistivity became about 3 times higher. The carrier concentration became lower and the average crystallite size was smaller.     

Improvement in moisture durability of ZnO transparent conductive films with Ga heavy doping process

Moisture durability of ZnO transparent conductive films was achieved with Ga heavy doping by off-axis type rf magnetron sputtering. The resistivity of 10.9 at.% Ga-doped ZnO was 1.3 × 10⁻³ Ωcm and changed less than 5% of resistivity over a 9400-h reliability test at a temperature of 85 °C and humidity of 85%. The crystal structural analysis of the heavily Ga-doped ZnO films indicated that the c-axis was oriented in various directions as well as the perpendicular direction to the substrate surface. The heavily doped Ga disorders the crystal growth of ZnO films and forms a different crystal structure from conventional ZnO.     

Porous nanostructured ZnO films deposited by picosecond laser ablation

Porous nanostructured polycrystalline ZnO films, free of large particulates, were deposited by picosecond laser ablation. Using a Zn target, zinc oxide films were deposited on indium tin oxide (ITO) substrates using a picosecond Nd:YVO₄ laser (8 ps, 50 kHz, 532 nm, 0.17 J/cm²) in an oxygen atmosphere at room temperature (RT). The morpho-structural characteristics of ZnO films deposited at different oxygen pressures (150–900 mTorr) and gas flow rates (0.25 and 10 sccm) were studied. The post-deposition influence of annealing (250–550 °C) in oxygen on the film characteristics was also investigated. At RT, a mixture of Zn and ZnO formed. At substrate temperatures above 350 °C, the films were completely oxidized, containing a ZnO wurtzite phase with crystallite sizes of 12.2–40.1 nm. At pressures of up to 450 mTorr, the porous films consisted of well-distinguished primary nanoparticles with average sizes of 45–58 nm, while at higher pressures, larger clusters (3.1–14.7 μm) were dominant, leading to thicker films; higher flow rates favored clustering.     

Effects of phosphorus doping source temperatures on fabrication and properties of p-type ZnO thin films

Phosphorus-doped p-type ZnO thin films have been deposited by metalorganic chemical vapor deposition using P₂O₅ as the dopant source. The conductivity types of the as-grown thin films were strongly temperature-dependent. When the substrate temperature maintains at the optimal one of 420 °C, the evaporating temperature of the phosphorus source plays significant roles in controlling the phosphorus content doping into films, then influences the films' performance. Optimizing the growth parameters, the optimal results were obtained with a resistivity of 6.49 Ω cm, a Hall mobility of 0.40 cm²/V s and a hole concentration of 2.42 × 10¹⁸ cm^− 3. The optical property of the optimal film was characterized by PL measurements, which indicated the film is of high optical quality.     

Deposition of Al-doped ZnO films on polyethylene naphthalate substrate with radio frequency magnetron sputtering

100 nm Al-doped ZnO (AZO) thin films were deposited on polyethylene naphthalate (PEN) substrates with radio frequency magnetron sputtering using 2 wt.% Al-doped ZnO target at various deposition conditions including sputtering power, target to substrate distance, working pressure and substrate temperature. When the sputtering power, target to substrate distance and working pressure were decreased, the resistivity was decreased due to the improvement of crystallinity with larger grain size. As the substrate temperature was increased from 25 to 120 °C, AZO films showed lower electrical resistivity and better optical transmittance due to the significant improvement of the crystallinity. 2 wt.% Al-doped ZnO films deposited on glass and PEN substrates at sputtering power of 25 W, target to substrate distance of 6.8 cm, working pressure of 0.4 Pa and substrate temperature of 120 °C showed the lowest resistivity (5.12 × 10^− 3 Ω cm on PEN substrate, 3.85 × 10^− 3 Ω cm on glass substrate) and high average transmittance (> 90% in both substrates). AZO films deposited on PEN substrate showed similar electrical and optical properties like AZO films deposited on glass substrates.     

Electrical and optical ethanol sensitivity of thermally oxidized zinc oxide films

Zinc oxide (ZnO) films have been prepared by thermal oxidation of vacuum deposited zinc (Zn) films onto glass substrate kept at room temperature (35 °C). The structural, electrical, optical and gas sensing properties of films annealed at 350 and 500 °C have been investigated. X-ray diffraction measurements indicate that the ZnO films oxidized at these temperatures are polycrystalline in nature with (101) as preferential crystallographic orientation. Practically no change in lattice parameters of ZnO films is observed when oxidation temperature is increased from 350 to 500 °C. Field emission scanning electron microscopy shows nanoparticles and nanowires at the surface of the ZnO films oxidized at 350 and 500 °C, respectively. At room temperature (35 °C), the film oxidized at 350 °C shows a gradual increase of response up to 96% for 2000 ppm exposure of ethanol, while film oxidized at 500 °C could detect a response of 99% for 500 ppm beyond which it saturates. An increase in the optical absorbance of the film has also been observed when ethanol concentration increases from 50 to 200 ppm beyond which no significant change is noticed even up to 2000 ppm.     

Effect of precursor on epitaxially grown of ZnO thin film on p-GaN/sapphire (0 0 0 1) substrate by hydrothermal technique

Single crystalline ZnO thin film on p-GaN/sapphire (0 0 0 1) substrate, using two different precursors by hydrothermal route at a temperature of 90 °C were successfully grown. The effect of starting precursor on crystalline nature, surface morphology and optical emission of the films were studied. ZnO thin films were grown in aqueous solution of zinc acetate and zinc nitrate. X-ray diffraction analysis revealed that all the thin films were single crystalline in nature and exhibited wurtzite symmetry and c-axis orientation. The thin films obtained with zinc nitrate had a more pitted rough surface morphology compared to the film grown in zinc acetate. However the thickness of the films remained unaffected by the nature of the starting precursor. Sharp luminescence peaks were observed from the thin films almost at identical energies but deep level emission was slightly prominent for the thin film grown in zinc nitrate.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Basal plane-oriented gallium nitride films on fused silica via acetate dip coating

c-axis-oriented gallium nitride (wurtzite GaN) thin films were fabricated by nitridation of acetate derived precursor films deposited on fused silica substrates without any buffer layer on the top of the substrate. The acetate derived precursors were obtained by (i) preparing a gallium-acetate sol by reacting Ga metal with acetic acid, (ii) coating cleared fused silica substrate with the sol and (iii) after drying the coated films at 100 °C, annealing them in air at 300°, 500° and 900 °C. Only films showing crystallization of α-GaO(OH) (300 °C) and (α + β)-Ga₂O₃ (500 °C) were selected for nitridation. In spite of the amorphous nature of the substrate, the GaN films showed a strong preferred orientation for the basal plane (002) under selected conditions of precursor annealing (300°, 500 °C) and subsequent nitridation (under flowing NH₃) temperature and time. In other cases formation of an additional plane, i.e. (101) was indicated as a weaker peak in X-ray diffraction (XRD) patterns. The precursors and nitride films were characterized by Fourier transform infrared spectroscopy, UV-Visible spectroscopy, XRD, high resolution transmission electron microscopy and atomic force microscopy analyses.     

The effects of substrate temperature on the structure and properties of ZnO films prepared by pulsed laser deposition

ZnO thin films were prepared by pulsed laser deposition (PLD) on glass substrates with growth temperature from room temperature (RT) to 500 °C. The effects of substrate temperature on the structural and optical properties of ZnO films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission spectra, and RT photoluminescence (PL) measurements. The results showed that crystalline and (0 0 2)-oriented ZnO films were obtained at all substrate temperatures. As the substrate temperature increased from RT to 500 °C, the ratio of grain size in height direction to that in the lateral direction gradually decreased. The same grain size in two directions was obtained at 200 °C, and the size was smallest in all samples, which may result in maximum E_g and E₀ of the films. UV emission was observed only in the films grown at 200 °C, which is probably because the stoichiometry of ZnO films was improved at a suitable substrate temperature. It was suggested that the UV emission might be related to the stoichiometry in the ZnO film rather than the grain size of the thin film.     

Fabrication of high frequency ZnO thin film SAW devices on silicon substrate with a diamond-like carbon buffer layer using RF magnetron sputtering

Diamond-like carbon (DLC) film is a promising candidate for surface acoustic wave (SAW) device applications because of its higher acoustic velocity. A zinc oxide (ZnO) thin film has been deposited on DLC film/Si substrate by RF magnetron sputtering; the optimized parameters for the ZnO sputtering are RF power density of 0.55 W/cm², substrate temperature of 380 °C, gas flow ratio (Ar/O₂) of 5/1 and total sputter pressure of 1.33 Pa. The results showed that when the thickness of the ZnO thin films was decreased, the phase velocity of the SAW devices increased significantly.     

ZnO thin films prepared by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on soda lime glass substrates by pulsed laser deposition (PLD) in an oxygen-reactive atmosphere. The structural, optical, and electrical properties of the as-prepared thin films were studied in dependence of substrate temperature and oxygen pressure. High quality polycrystalline ZnO films with hexagonal wurtzite structure were deposited at substrate temperatures of 100 and 300 °C. The RMS roughness of the deposited oxide films was found to be in the range 2-9 nm and was only slightly dependent on substrate temperature and oxygen pressure. Electrical measurements indicated a decrease of film resistivity with the increase of substrate temperature and the decrease of oxygen pressure. The ZnO films exhibited high transmittance of 90% and their energy band gap and thickness were in the range 3.26-3.30 eV and 256-627 nm, respectively.     

Structure, morphology and electrical characterizations of direct current sputtered ZnO thin films

ZnO thin films were deposited on glass substrates by direct current (DC) sputtering technique at room temperature (RT) to 400 °C with a 99.999% pure ZnO target. Then the samples deposited at RT were annealed in air from the RT to 400 °C. The effects of substrate temperature (T_s) and annealing treatment (T_a) on the crystallization behavior and the morphology have been studied by X-ray diffraction and atomic force microscopy. We also compared the structural properties of samples deposited at 400 °C on glass to those deposited on Pt/silicon substrate. The resistivity, surface roughness and size of the grains have also been studied and correlated to the thickness of ZnO films deposited on Pt/Si substrates. The experimental results reveal that the substrate has a major influence on the structural and morphological properties. For the films deposited on glass, below 400 °C, T_s and T_a have a similar influence on the structure of the films. Moreover, the ZnO samples deposited at RT and annealed in air have poor electrical properties.     

Effect of annealing temperature on ZnO:Al/p-Si heterojunctions

Al-doped, zinc oxide (ZnO:Al) films with a 1.2 at.% Al concentration were deposited on p-type silicon wafers using a sol-gel dip coating technique to produce a ZnO:Al/p-Si heterojunction. Following deposition and subsequent drying processes, the films were annealed in vacuum at five different temperatures between 550 and 900 °C for 1 h. The resistivity of the films decreased with increasing annealing temperature, and an annealing temperature of 700 °C provided controlled current flow through the ZnO:Al/p-Si heterojunction up to 20 V. The ZnO:Al film deposited on a p-type silicon wafer with 1.2 at.% Al concentration was concluded to have the potential for use in electronic devices as a diode after annealing at 700 °C.     

Deposition of hydroxyapatite thin films by Nd:YAG laser ablation: a microstructural study

Hydroxyapatite (HA) thin films has been successfully deposited by Nd:YAG laser ablation at λ = 532 nm. The morphology and microstructure of the deposited layers was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). Polycrystalline HA films were directly obtained with the substrate at 300 °C and without introducing water vapors in the deposition chamber. Electron paramagnetic resonance (EPR) measurements show that the oxygen stoichiometry in the HA films is also maintained. Depositions performed at λ = 335 nm laser wavelength and 300 °C substrate temperature resulted in polycrystalline layers of mixed composition of HA and tricalciumphosphate (TCP).