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.     

Annealing effect on properties of transparent and conducting ZnO thin films

This work presents the effect of postdeposition annealing on the structural, electrical and optical properties of undoped ZnO (zinc oxide) thin films, prepared by radio-frequency sputtering method. Two samples, 0.17 and 0.32 µm-thick, were annealed in vacuum from room temperature to 350 °C while another 0.32 µm-thick sample was annealed in air at 300 °C for 1 h. X-ray diffraction analysis revealed that all the films had a c-axis orientation of the wurtzite structure normal to the substrate. Electrical measurements showed that the resistivity of samples annealed in vacuum decreased gradually with the increase of annealing temperature. For the 0.32 µm-thick sample, the gradual decrease of the resistivity was essentially due to a gradual increase in the mobility. On the other hand, the resistivity of the sample annealed in air increased strongly. The average transmission within the visible wavelength region for all films was higher than 80%. The band gap of samples annealed in vacuum increased whereas the band gap of the one annealed in air decreased. The main changes observed in all samples of this study were explained in terms of the effect of oxygen chemisorption and microstructural properties.     

Effect of alumina doping on structural, electrical, and optical properties of sputtered ZnO thin films

A systematic study of the influence of alumina (Al₂O₃) doping on the optical, electrical, and structural characteristics of sputtered ZnO thin films is reported in this study. The ZnO thin films were prepared on 1737F Corning glass substrates by R.F. magnetron sputtering from a ZnO target mixed with Al₂O₃ of 0-4 wt.%. X-ray diffraction (XRD) analysis demonstrates that the ZnO thin films with Al₂O₃ of 0-4 wt.% have a highly (002) preferred orientation with only one intense diffraction peak with a full width at half maximum (FWHM) less than 0.5°. The electrical properties of the Al₂O₃-doped ZnO thin films appear to be strongly dependent on the Al₂O₃ concentration. The resistivity of the films decreases from 74 Ω·cm to 2.2 × 10^− 3 Ω·cm as the Al₂O₃ content increases from 0 to 4 wt.%. The optical transmittance of the Al₂O₃-doped ZnO thin films is studied as a function of wavelength in the range 200-800 nm. It exhibits high transparency in the visible-NIR wavelength region with some interference fringes and sharp ultraviolet absorption edges. The optical bandgap of the Al₂O₃-doped ZnO thin films show a short-wavelength shift with increasing of Al₂O₃ content.     

Investigation of optical and electrical properties of ZnO thin films

Zinc oxide (ZnO) thin films were deposited on Si substrates using various working pressures by magnetron sputter. The resistivity of the deposited ZnO films decreases with working pressure, and the resistivity of 4.3 × 10⁻³ Ω cm can be obtained without post annealing. According to the optical transmittance measurements, the optical transmittance above 90% in the wavelength longer than 430 nm and about 80% in the wavelength of 380 nm can be found. Using time-resolved photoluminescence measurement, the carrier lifetime increases with working pressure due to the reduction of nonradiative recombination rate. The reduction of nonradiative recombination rate is originated from the decrease of oxygen vacancies in the ZnO films deposited at a higher working pressure. This result is verified by the photoluminescence measurements. Besides, by increasing the working pressure, the absorption coefficient was decreased and the associated optical energy gap of ZnO thin films was increased.     

Growth process observation of homoepitaxial ZnO thin films using optical emission spectra during pulsed laser deposition

Homoepitaxial ZnO thin films were prepared on the Zn-polar or O-polar ZnO substrates by pulsed laser deposition method. Optical emission spectroscopy of the plume was carried out to estimate O/Zn flux ratio under the various deposition conditions such as oxygen pressure, laser fluence, and the distance between target and substrate. It is revealed that the O/Zn flux ratio could be controlled by laser fluence, oxygen pressure, and target-substrate distance. Zn-rich O/Zn flux promotes pit formation and O-rich flux yields the three-dimensional growth. The difference of the growth process on Zn-polar or O-polar substrates is also discussed.     

Effects of the annealing treatment on electrical and optical properties of ZnO transparent conduction films by ultrasonic spraying pyrolysis

Effects of the annealing treatment on properties of ZnO thin films prepared on silica glass substrates by the ultrasonic spraying pyrolysis process were studied. Zinc acetate dihydrate and methanol were used as a starting material and a solvent, respectively. For ZnO thin films untreated with annealing, the preferred grain growth along the (0 0 2) plane was observed. The electrical resistivity and the direct band gap values of these films decreased with increasing the deposit temperature. By applying the annealing treatment in a reducing atmosphere, while the degree of the preferred (0 0 2) orientation of films decreased, the electrical conductivity of films was improved. When compared with the resistivity values of films without the annealing treatment, the values of films annealed in the reducing atmosphere were decreased by about two orders of magnitude. The lowest resistivity value was 1.62×10⁻¹ Ω cm, which was obtained in the film annealed at 500 °C in nitrogen with 5% hydrogen. The optical transmittances of the films were higher than 80% regardless of the application of the annealing treatment in a reducing atmosphere. The direct band gap values of films annealed in a reducing atmosphere were approximately 3.27 eV.     

Effect of Ga doping on micro/structural, electrical and optical properties of pulsed laser deposited ZnO thin films

Undoped and Ga doped ZnO thin films (1% GZO, 3% GZO and 5% GZO) were grown on c-Al₂O₃ substrates using the 1, 3 and 5 at. wt.% Ga doped ZnO targets by pulsed laser deposition. X-ray diffraction studies revealed that highly c-axis oriented, single phase, undoped and Ga doped ZnO thin films with wurtzite structure were deposited. Micro-Raman scattering analysis showed that Ga doping introduces defects in the host lattice. The E₂^High mode of ZnO in Ga doped ZnO thin film was observed to shift to higher wavenumber indicating the presence of residual compressive stress. Appearance of the normally Raman inactive B₁ modes (B₁^Low, 2B₁^Low and B₁^High) due to breaking of local translational symmetry, also indicated that defects were introduced into the host lattice due to Ga incorporation. Band gap of the Ga doped ZnO thin films was observed to shift to higher energy with the increase in doping concentration and is explicated by the Burstein-Moss effect. Electrical resistivity measurements of the undoped and GZO thin films in the temperature range 50 to 300 K revealed the metal to semiconductor transition for 3 and 5% GZO thin films.     

Effect of doping and substrate temperature on the structural and optical properties of reactive pulsed laser ablated tin oxide doped tantalum oxide thin films

5% SnO₂ doped tantalum oxide (Ta₂O₅) films are deposited on quartz substrates at different substrate temperatures of 300 K, 773 K, 873 K and 973 K using pulsed laser deposition in an oxygen ambient of 0.002 mbar. Undoped Ta₂O₅ films are also deposited on quartz substrates kept at substrate temperature 973 K under the same oxygen ambient using PLD. The films are characterized using GIXRD, AFM, FTIR, micro-Raman and UV-visible spectroscopy. Undoped films show an amorphous nature even at a substrate temperature of 973 K, whereas, SnO₂ doped films show crystalline nature even for deposition at 300 K. As far as our knowledge goes, this is the first report of crystalline Ta₂O₅ films deposited at room temperature. The average size of the crystallites calculated using the Debye-Scherrer formula, shows that the size of the crystallite decreases with increase in substrate temperature. FTIR and micro-Raman spectroscopic analysis reveals the presence of Ta-O-Ta, O-Ta and O-Ta-O vibrational bands in the films. Raman analysis indicates that the addition of SnO₂ suppresses the bond formation and changes the magnitude of bonds in Ta₂O₅. AFM patterns reveal the formation of Ta₂O₅ nanorods of diameter about 100 nm for the doped film deposited at 973 K. Optical transmittance of the films is found to be sensitive to substrate temperature as well as to the presence of SnO₂. A blue shift in the band-gap of the doped films is observed. The decrease of band-gap with decrease of particle size observed for SnO₂ doped films can be due to a band-bending effect. The transmittance of the films is found to depend on SnO₂ doping and substrate temperature.     

Effect of thickness on the structural, electrical and optical properties of ZnO films

A series of ZnO films of different thickness have been deposited on glass substrates using sol-gel technique by varying the number of spin coatings and the effect of film thickness on the structural, electrical and optical properties have been investigated. The XRD results indicate that the full width at half maximum (FWHM) of the (0 0 2) diffraction peak and the strain along c-axis are decreased as the film is grown up to a thickness of 300 nm. Above 300 nm, the strain again becomes appreciable. The surface morphology shows that the grains become more uniform and bigger in size as the film thickness increases. Electrical result shows that although ZnO film with thickness of around 260 nm has the highest resistivity but is better for current conduction. The excitonic nature in the absorption spectrum becomes prominent for a film with thickness of around 260 nm. The band gap increases and then decreases as the film grows thicker.     

Doping induced structural and compositional changes in ZnO spray pyrolysed films and the effects on optical and electrical properties

Highly transparent, conductive ZnO films have been deposited by spray pyrolysis of a zinc acetate based solution. Quality films are yielded as our process is analogous to an aerosol assisted chemical vapour deposition rather than a droplet deposition spray pyrolysis technique. The properties of the films are governed by the additives to the base precursor solution. When aluminium acetylacetonate is added to the precursor solution, ZnO:Al films are grown with free charge carrier concentrations of more than 10²⁰ cm⁻³. The carrier density and mobility are measured by both Hall probe and near infrared spectroscopy. Film growth and grain size, morphology and orientation are altered using an increased percentage of ZnCl₂ in the precursor, which results in a 10 fold increase in charge carrier mobility. An investigation is presented correlating the composition of the precursor solution with the chemical, structural, electrical and optical properties of the grown films.     

Preparation and electrical/optical bistable property of potassium tetracyanoquinodimethane thin films

Potassium tetracyanoquinodimethane (K(TCNQ)) thin films were prepared using physical vapor deposition combined with solid state chemical replacement reaction. Reversible electrical bistable behavior at or even above room temperature was observed; and, the optical bistable property of K(TCNQ) film was observed.     

Properties of ZnO:Al films deposited on polycarbonate substrate

Transparent conducting aluminum-doped zinc oxide (ZnO:Al) films have been prepared on polycarbonate (PC) substrates by pulsed laser deposition technique at low substrate temperature (room-100 °C); Nd-YAG laser with wavelength of 1064 nm was used as laser source. The experiments were performed at various oxygen pressures (3 pa, 5 pa, and 7 Pa). In order to study the influence of the process parameters on the deposited (ZnO:Al) films, X-ray diffraction and atomic force microscopy were applied to characterize the structure and surface morphology of the deposited (ZnO:Al) films. Polycrystalline ZnO:Al films having a preferred orientation with the c-axis perpendicular to the substrate were deposited with a strong single violet emission centering about 377–379 nm without any accompanying deep level emission. The average transmittances exceed 85% in the visible spectrum for 300 nm thick films deposited on polycarbonate.     

Morphological, optical and electrical properties of samarium oxide thin films

We present here results on samarium oxide thin films, obtained by pulsed laser deposition and by radio frequency assisted pulsed laser deposition. Three different substrate types were used: silicon, platinum covered silicon and titanium covered silicon. The influence of the deposition parameters (oxygen pressure and laser fluence) on the structure and morphology of the thin films was studied. The substrate-thin film interface zone was investigated; the optical and electrical properties (the losses, dielectric constant and leakage currents) were also determined.     

Influence of lithium doping on the structural and electrical characteristics of ZnO thin films

Thin films of undoped and lithium-doped Zinc oxide, (Zn_1 − xLi_x)O; x = 0, 0.05, 0.10 and 0.20 were prepared by sol-gel method using spin-coating technique on silicon substrates [(111)Pt/Ti/SiO₂/Si)]. The influence of lithium doping on the structural, electrical and microstructural characteristics have been investigated by means of X-ray diffraction, leakage current, piezoelectric measurements and scanning electron microscopy. The resistivity of the ZnO film is found to increase markedly with low levels (x ≤ 0.05) of lithium doping thereby enhancing their piezoelectric applications. The transverse piezoelectric coefficient, e₃₁^? has been determined for the thin films having the composition (Zn_0.95Li_0.05)O, to study their suitability for piezoelectric applications.     

Optical and electrical properties of ZnO nanoparticle thin films deposited on quartz by sparking process

ZnO nanoparticle thin films were deposited on quartz substrates by a novel sparking deposition which is a simple and cost-effective technique. The sparking off two zinc tips above the substrate was done repeatedly 50-200 times through a high voltage of 10 kV in air at atmospheric pressure. The film deposition rate by sparking process was approximately 1.0 nm/spark. The ZnO thin films were characterized by X-ray diffraction, Raman spectroscopy, UV-vis spectrophotometry, and ionoluminescence at room temperature. The two broad emission peaks centered at 483 nm (green emission) and 650 nm (orange-red emission) were varied after two-step annealing treatments at 400-800 °C. Moreover, the electrical resistivity of the films was likely to be proportional to the peak intensity of the orange-red emission.     

Structure, luminescence and electrical properties of ZnO thin films annealed in H2 and H2O ambient: A comparative study

Undoped ZnO films were grown on a c-plane sapphire by plasma-assisted molecular-beam epitaxy technique, and subsequently annealed at 200-500 °C with steps of 100 °C in water vapour and hydrogen ambient, respectively. It is found that the c-axis lattice constant of the ZnO films annealed in hydrogen or water vapour at 200 °C increases sharply, thereafter decreases slowly with increasing annealing temperature ranging from 300 °C to 500 °C. The stress in the as-grown ZnO films was more easily relaxed in water vapour than in hydrogen ambient. Interestingly, the controversial luminescence band at 3.310 eV, which is often observed in photoluminescence (PL) spectra of the ZnO films doped by p-type dopants, was observed in the PL spectra of the annealed undoped ZnO films and the PL intensity increases with increasing annealing temperature, indicating that the 3.310 eV band is not related to p-type doping of ZnO films. The electron concentration of the ZnO films increases sharply with increasing annealing temperature when annealed in hydrogen ambient but decreases slowly when annealed in water vapour. The mechanisms of the effects of annealing ambient on the properties of the ZnO films are discussed.     

Electronic transport and optical properties of thin oxide films

In this work, we present optical characterization of films of two transparent conductive oxides (ITO: indium tin oxide and ZnO: zinc oxide) including absorption coefficient and optical gap energy. We have also investigated the transport properties of ITO and ZnO films through measurements of electrical conductivity and thermoelectric power versus temperature. These measurements enabled us to investigate conduction mechanisms for metal-nonmetal transitions. Undoped ZnO thin films show a metal-semiconductor transition at temperatures beyond 350 K. We have conducted a similar study on ITO films where we demonstrated, for the first time, the existence of a conductivity transition below 400 K, which indicates a high absolute thermoelectric power at temperatures above the transition temperature.     

Influence of thermal annealing on electrical and optical properties of Ga-doped ZnO thin films

Influence of thermal annealing on electrical properties of GZO films has been studied by means of Hall effect measurements and optical characterization based on Drude model analysis for transmission and reflection spectra. Electrical resistivity increased with increasing annealing temperature. Changes of electrical properties were compared between air and N₂ gas atmosphere. Thermal stability in the air was worse compared to the N₂ gas atmosphere. Annealing at rather high temperature caused decrease in the Hall mobility and increase in optical mobility. The difference between the Hall mobility and the optical mobility was attributed to carrier scattering at grain boundaries. Three kinds of deposition method, ion plating using DC arc discharge, DC magnetron sputtering, and RF power superimposed DC magnetron sputtering were compared in terms of the thermal stability.     

Correlations between optical properties, microstructure, and processing conditions of Aluminum nitride thin films fabricated by pulsed laser deposition

Aluminum nitride (AlN) films were deposited using pulsed laser deposition (PLD) onto sapphire (0001) substrates with varying processing conditions (temperature, pressure, and laser fluence). We have studied the dependence of optical properties, structural properties and their correlations for these AlN films. The optical transmission spectra of the produced films were measured, and a numerical procedure was applied to accurately determine the optical constants for films of non-uniform thickness. The microstructure and texture of the films were studied using various X-ray diffraction techniques. The real part of the refractive index was found to not vary significantly with processing parameters, but absorption was found to be strongly dependent on the deposition temperature and the nitrogen pressure in the deposition chamber. We report that low optical absorption, textured polycrystalline AlN films can be produced by PLD on sapphire substrates at both low and high laser fluence using a background nitrogen pressure of 6.0 × 10^− 2 Pa (4.5 × 10^− 4 Torr) of 99.9% purity.     

Thermally stable, highly conductive, and transparent Ga-doped ZnO thin films

Highly conductive and transparent films of Ga-doped ZnO (GZO) have been prepared by pulsed laser deposition using a ZnO target with Ga₂O₃ dopant of 3 wt.% in content added. Films with resistivity as low as 3.3 × 10^− 4 Ω cm and transmittance above 80% at the wavelength between 400 and 800 nm can be produced on glass substrate at room temperature. It is shown that a stable resistivity for use in oxidation ambient at high temperature can be attained for the films. The electrical and optical properties, as well as the thermal stability of resistivity, of GZO films were comparable to those of undoped ZnO films.