Transparent ZnO Thin Films Research Articles

Correlation between intrinsic defect and carrier transport in ZnO thin films by confocal Raman spectroscopy

An electric current through a transparent 80nm ZnO thin film produces relevant changes on its Raman spectrum. The electrical current affects the vibrational modes of ZnO that are associated with intrinsic defects. A reversible Raman intensity decrease of the band centered at ∼578cm−1 with the electrical current in the thin film is measured. The Raman peak decreases without a measurable energy shift, ruling out anharmornic contributions related to temperature increasing. The most probable mechanism is through a barrier height reduction with the applied electric field, producing a decrease of the charge at the crystallites interface or grain boundaries. The reported experiment is a direct proof of the significant role that intrinsic defects play on the electrical transport through ZnO thin films.

Temperature-dependent luminescence dynamics for ZnO thin films

The paper presents the photoluminescence investigation of zinc oxide thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz wafers. The films with different thickness (number of layers) were annealed at different temperatures after the preparation process. It was found that high quality, transparent ZnO thin films could be produced on quartz substrates at relatively low annealing temperature (450–550 \(^{\circ }\mathrm{C}\)). The dependence of the ZnO thin film quality was studied by X-ray diffraction and atomic force microscopy techniques. Optical properties were investigated by classic and time-resolved photoluminescence (TRPL) measurements. Photoluminescence spectra allowed us to estimate energy of the free excitons, bond excitons and their longitudinal optical (LO) phonon replicas as a function of the annealing temperature. An innovative TRPL technique let us precisely measure the decay time of the free- and bond excitons’ in the real time. TRPL measurements as a function of temperature reveal a biexponential decay behavior with typical free/bound exciton decay constants of 970/5310 ps for the as-grown sample and 1380/5980 ps after annealing process. Presented spectra confirm high structural and optical quality of investigated films. We proved that the thermal treatment improve both optical and structural quality and extend the photoluminescence’s lifetimes. The obtained experimental results are important for identification of exciton’s peaks and their LO phonon replicas for the investigated ZnO films.

Exciton and core-level electron confinement effects in transparent ZnO thin films

The excitonic light emission of ZnO films have been investigated by means of photoluminescence measurements in ultraviolet-visible region. Exciton confinement effects have been observed in thin ZnO coatings with thickness below 20 nm. This is enhanced by a rise of the intensity and a blue shift of the photoluminescence peak after extraction of the adsorbed species upon annealing in air. It is found experimentally that the free exciton energy (determined by the photoluminescence peak) is inversely proportional to the square of the thickness while core-level binding energy is inversely proportional to the thickness. These findings correlate very well with the theory of kinetic and potential confinements.

Microstructural, electrical and optical properties of ZnO:Mo thin films with various thickness by spray pyrolysis

Transparent conductive Mo-doped ZnO (MZO) thin films with different thickness are prepared by spray pyrolysis method on glass substrates. The structural, surface morphological, electrical and optical properties of these films have been investigated as a function of the film thickness by a series of characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy-dispersive X-ray (EDX) analysis, Hall effect measurements, UV–vis and luminescence spectroscopy. The structural analysis results show that the films are crystallized in the wurtzite phase type. The structural disorder decreases and the crystalline quality of the films is gradually improved up to an optimum thickness of 1865nm. The SEM–EDX and elemental mapping results show that Mo is incorporated and distributed uniformly in all the MZO films. Hall measurement results indicated that the electrical properties depend considerably on film thickness. The resistivity decreases from 1.04×101 to 5.24×10−3Ωcm with the increase of MZO film thickness from 632nm to 1865nm, and thereafter resistivity increases as thickness increases to 2290nm. UV–vis measurements show that the average transmittance of the MZO thin films is 71.2% and average surface reflectance is about 8.8% in the visible range. The optical band gap and Urbach energy of the MZO films are also significantly influenced by the film thickness. The photoluminescence (PL) measurements indicate that the intensity of deep level emission significantly vary with the film thickness.

Highly Transparent and Low Resistivity Al-Doped ZnO Thin Films Grown by Pulsed Laser Deposition under Different Oxygen Flow Rates

Polycrystalline Al-doped ZnO (AZO) thin films with a thickness of 1300 Å were grown on Corning 1737 glass by pulsed laser deposition (PLD) at a low substrate temperature. The presence of oxygen gas during deposition led to a remarkable enhancement of the (002) c-axis preferential orientation as well as increased crystallite size. Highly transparent films with a transmittance of 85% could be obtained by controlling the oxygen flow rate, while causing a Burstein-Moss shift toward a shorter wavelength as well. The resistivities of the films were found to be functions of both the oxygen flow rate and substrate temperature, with the lowest value being 2.3 x 10-4 Ωcm (18Ω/sq sheet resistance). It was found that both the oxygen flow rate and substrate temperature are crucial in order to grow superior device quality films with an appropriate degree of crystallinity, less surface roughness, high transmittance and low resistivity, which are characteristics of great technological importance.

Highly conducting and optically transparent Si-doped ZnO thin films prepared by spray pyrolysis

We report the fabrication and investigation of highly conducting and optically transparent ZnO thin films prepared by the spray pyrolysis technique. Undoped and Si-doped ZnO thin films were deposited on glass substrates at deposition temperatures between 250 °C and 500 °C from a precursor solution containing zinc acetylacetonate and silicon tetraacetate. X-ray diffraction analysis confirms that the deposited films have the wurtzite ZnO structure with individual crystallite sizes varying between 65 and 122 nm. The optical transparency of the films in the visible range is in the range of 80–85%. The effect of deposition temperature and Si dopant concentration on structural, optical and electrical properties of Si-doped films was investigated. For 3% Si-doped ZnO films the electrical resistivity and carrier concentration reached values of 3.7 × 10−3 Ωcm and 1.7 × 1020 cm−3, respectively. The temperature dependence of the electrical transport properties were measured across the range of 77–350 K. The carrier concentration for all the films is practically temperature independent, illustrating that the samples are degenerate semiconductors. Interestingly, a temperature-activated Hall mobility is observed for the thin films over the whole temperature range of measurements. This is attributed to the thermionic emission of free electrons across potential barriers formed between grains. Si-doped ZnO thin films prepared by a vacuum-free solution-based technique can provide cost-effective transparent conducting layers for optoelectronic and energy applications.

Electrical and Optical Properties of Nanostructured Zinc Oxide Thin Films Influenced by Annealing Temperature

Transparent nanostructured ZnO thin films were successfully deposited using sol-gel spin coating method on a quartz substrate. The 0.4 M ZnO solution gel was prepared using zinc acetate dihydrate (Zn(CH3COO)22H2O) as the precursor, 2-methoxyethanol as the solvent and monoethanolamine (MEA) as the stabilizer. The electrical and optical properties dependencies on the annealing temperature of the nanostructured ZnO thin films were investigated. It was found that as the annealing temperature increased, the particle size, conductivity and the peak of the UV emission also increased.

ANNEALING TREATMENT OF ZnO THIN FILMS DEPOSITED BY SOL–GEL METHOD

Highly transparent ZnO thin films were deposited on glass substrates by using a simple and inexpensive multi-step sol–gel spin coating process. This research investigated the effects of annealing temperature in the range from 350–600°C on the microstructure, surface morphology and optical properties of thin films by using XRD, SEM and transmittance spectra. The XRD results showed that the c-axis orientation of ZnO thin films was improved with the increase of annealing temperature. The grain size increases from 16.6–19.7 nm with the increase in temperature. The transmittance spectra indicated that the transmittance and direct optical band gap Eg of the films showed a decreased trend with annealing temperature. It is found that the tensile stress exist in the films, which decreases with the increase in annealing temperature up to 500°C, on further increasing the annealing temperature up to 600°C, the stress in the film changes from tensile to compressive nature.

ZnO thin films by spray pyrolysis and its doping with Sb


 
 
 Highly transparent ZnO thin films were prepared using spray pyrolysis of a solution containing Zn(OAc)2 and SbC13 on glass substrates kept at 400ºC. Further, these films were doped with Sb by in situ process by dissolving SbCl3 in the spray solution. Crystallographic structural analysis was done using X-ray diffractometer and elemental analysis was done using Auger electron spectrometer. Morphological characterization was done by atomic force microscopy. The UV-Vis transmittance measurements indicated that the films were 90% transparent in the visible region. The value of band gap energy calculated from UV-Vis characterization showed that undoped and Sb doped films have slightly different band-gap energies. The electrical resistivity measurements showed a substantial change in the resistance of the ZnO thin films due to Sb doping.
 
 

Effect of annealing and room temperature sputtering power on optoelectronic properties of pure and Al-doped ZnO thin films

Transparent ZnO and Al-doped ZnO (AZO) thin films have been prepared by radio frequency sputtering deposition at room temperature. The optical, electrical, and structural characteristics of the obtained films have been extensively investigated as a function of sputtering and annealing parameters. Spectrophotometry, X-ray diffraction (XRD), atomic force microscopy (AFM), four-point probe and Hall-effect measurements were employed. The ZnO films generally exhibited excellent crystalline properties, while providing a UV cut-off in the absorption spectrum for optical filtration. AZO thin films exhibited an average transparency (larger than 85%) over the visible region of the spectrum, and resistivity of the order of 10−3Ω cm was obtained. The carrier concentration and electron mobility values proved to be dependent on the deposition parameters and annealing temperature. The obtained results showed that annealing temperatures higher than 400°C were not necessary and potentially degraded the electronic properties of the AZO thin films.

Effect of nickel doping on structural, morphological and optical properties of sol–gel spin coated ZnO films

As a versatile semiconductor material, Ni doped ZnO thin film is studied mainly for its ferromagnetic properties, while the electrical and optical performances are few. It is believed that doping in ZnO thin films can improve the crystalline quality or obtain better optical, electrical or ferromagnetic properties. In this paper, the main research is focused on the preparation process and optical properties of transparent ZnO thin films doped with different Ni concentrations. The proposed research investigates the ZnO thin films doped with different Ni concentrations that are important in understanding the structural and optical performances of Ni doped ZnO thin film. The transparent ZnO thin films doped with different Ni concentrations were deposited on glass substrates via sol–gel technique using zinc acetate dehydrate and nickel chloride as precursor. The morphology and microstructural properties of undoped and Ni doped ZnO thin films were studied by X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy and X-ray photoelectron spectroscopy. The optical properties of the thin films were determined by photoluminescence (PL) and UV–visible spectroscopy. The results indicate that the obtained films have smooth surfaces and are of good crystal quality, which have a pure hexagonal wurtzite ZnO without any Ni related phases. The surface roughness and mean grain size of Ni doped ZnO thin films are sensitive to Ni concentrations. The undoped and Ni doped ZnO thin films with high transmittance in the visible region show room temperature PL. The Ni doped ZnO thin films display room temperature PL at blue and green emissions. Its good crystal quality and optical performances make it a promising transparency and visible luminescence thin film for transparent multispin electronic devices.

Sol–gel synthesis of ZnO transparent and conductive films: A critical approach

The main aspects of the sol–gel synthesis of ZnO thin films were critically examined with particular reference to the optimization of the process parameters that influence specific properties of the films, such as the transparency, the crystallization behavior and the electrical resistivity. Starting from zinc acetate dihydrate, anhydrous ethanol and triethanolamine, a valuable sol–gel route was set. This route allowed the preparation of transparent ZnO thin films by dip-coating. Triethanolamine, thanks to its high chelating efficiency towards Zn2+, permitted to prepare stable zinc acetate dihydrate solutions wholly at room temperature. The structural, optical and morphological characteristics of the films were examined by glancing incidence X-ray diffraction, UV–vis spectroscopy and atomic force microscopy. Single layer films, with a thickness of about 120nm, characterized by a high adhesion force (about 140Kgcm−2), were obtained. The crystallization behavior of dip-coated amorphous films was explored in the 500–600°C range; polycrystalline films, formed by randomly oriented ZnO nanocrystals (about 20nm), with an average transmittance higher than 90% in the visible region, were obtained. Finally, a strong influence of annealing environment on electrical resistivity was observed.

Preparation of transparent ZnO thin films and their application in UV sensor devices

We report a simple and inexpensive way for the preparation of highly transparent ZnO thin films and their application as active layer in UV ray sensor devices. ZnO thin films were deposited on glass substrates by thermal evaporation of pure ZnO powder. The as-deposited films were then annealed at different temperatures (100, 200, 300 and 400°C) for various time durations (5, 15, 25 and 35min) to make optically transparent in the visible region. The films annealed at 300°C for 15min show very good visible transparency and other material properties. These films were used as the active material for Ag/ZnO/Ag UV sensor devices. The sensor devices are photo conductive type and only sensitive in the UV region of the electromagnetic spectrum. Maximum photo-current gain of the UV sensor device is ∼2. Possible sensing mechanism has been discussed.

Quantum Effects of Indium/Ytterbium Doping on ZnO-Like Nano-Condensed Matter in terms of Urbach-Martienssen and Wemple-DiDomenico Single-Oscillator Models Parameters

Conducting and transparent optical ZnO thin films were deposited on glass substrates by a simple mini spray technique. Alternatively, some of the obtained films were doped with indium and ytterbium at the molar rates of: 1, 2, and 3% (In) and 100, 200, and 300 ppm (Yb). In addition to the classical structural investigations including XRD, microhardness vickers (Hv), and optothermal techniques, thorough optical measurements have been carried out for comparison purposes. The refractive indices and the extinction coefficients of the differently doped layers have been deduced from their transmission-reflection spectra over an extended wavelength range. Analysis of the refractive index data through Wemple-DiDomenico single oscillator model yielded quantum characteristics along with the values of long-wavelength dielectric constant, average oscillator wavelength, average oscillator strength, average oscillator energy and dispersion energy. Real and imaginary parts of dielectric constant have also been used to calculate free carrier plasma resonance frequency, optical relaxation time, and free carriers concentration-to-effective mass ratio. Finally, analysis of Urbach-Martienssen model parameters allowed proposing nanoscale explanations to the divergence about doping-related evolution of Urbach tails, this intriguing item having been intensively discussed in the literature in the last decades.

Influence of thickness and annealing temperature on the electrical, optical and structural properties of AZO thin films

Transparent conductive Al-doped ZnO (AZO) thin films with various thicknesses between 520 and 1420 nm were deposited on quartz substrates by radio frequency (RF) magnetron sputtering at room temperature for thin film solar cells as transparent conductive oxide (TCO) electrode layers. After deposition, the samples were annealed in a vacuum ambient at temperatures between 250 and 550 °C for a period of 30 min. The structural, electrical, and optical properties of these films have been analyzed as a function of the thickness and the annealing temperature by a series of characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Hall effect measurements and spectrophotometry. All of these samples exhibited strong (002) diffraction peaks and the visible range transmittance was over 80%. In addition, with the increase of thickness, the Hall mobility increased from 4.88 to 7.86 cm2/V, the resistivity decreased from 1.2 × 10−2 Ω cm to 4.2 × 10−3 Ω cm. Annealing in vacuum improved the crystallinity together with some changes of the electrical resistance that depended on the annealing temperature. The best characteristics have been obtained at 450 °C, where the lowest resistivity was 2.7 × 10−3 Ω cm for the thickest films.

Spray-deposited Al-doped ZnO transparent contacts for CdTe solar cells

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited by ultrasonic spray pyrolysis of alcoholic solutions in ambient atmosphere without any post-growth treatment. Spraying parameters, composition and acidity of the sprayed solution, and doping level were systematically investigated in order to obtain AZO films suitable for applications in solar cells. It was confirmed that the amount of aluminum chloride as the dopant and acetic acid as the stabilizing agent are decisive for film morphology and opto-electrical properties. Films of 1.1μm in thickness deposited under optimized conditions on borosilicate substrates exhibited resistivity of 5×10−3Ωcm and an average transmittance of 82% in the wavelength range 400–900nm. A resistivity of 3×10−3Ωcm, one of the lowest values reported in literature for sprayed AZO, was obtained for a film thickness of 3μm maintaining a high optical transmittance of >74%. The latter film was applied as a transparent electrical contact in a CdTe solar cell and an efficiency of 12.1% without antireflection coating was achieved. The average efficiency value of >11% measured for 22 fabricated solar cells demonstrates the feasibility of using spray-pyrolysis technique for the deposition of large area AZO contacts for solar cell applications.

The properties of Al doped ZnO thin films deposited on various substrate materials by RF magnetron sputtering

Transparent conductive Al-doped ZnO (AZO) thin films were deposited on various substrates including glass, polyimide film (PI) and stainless steel, using radio frequency magnetron sputtering method. The structural, electrical and optical properties of AZO thin films grown on various substrates were systematically investigated. We observe that substrate materials play important roles in film crystallization and resistivity but little on optical transmittance. X-ray diffractometer study shows that all obtained AZO thin films have wurtzite phase with highly c-axis preferred orientation, and films on glass present the strongest (002) diffraction peaks. The presence of compression stress plays critical role in determining the crystalline structure of AZO films, which tends to stretch the lattice constant c and enlarge the (002) diffraction angle. Although the films on the glass present the finest electrical properties and the resistivity reaches 12.52 × 10-4 Ωm, AFM study manifests that films on flexible substrates, especially stainless steel, bestrew similar inverted pyramid structure which are suitable for window material and electrode of solar cells. The average optical transmittance of AZO thin films deposited on glass and PI are both around 85% in the visible light range (400–800 nm).

Non-vacuum, single-step conductive transparent ZnO patterning by ultra-short pulsed laser annealing of solution-deposited nanoparticles

A solution-processable, high-concentration transparent ZnO nanoparticle (NP) solution was successfully synthesized in a new process. A highly transparent ZnO thin film was fabricated by spin coating without vacuum deposition. Subsequent ultra-short-pulsed laser annealing at room temperature was performed to change the film properties without using a blanket high temperature heating process. Although the as-deposited NP thin film was not electrically conductive, laser annealing imparted a large conductivity increase and furthermore enabled selective annealing to write conductive patterns directly on the NP thin film without a photolithographic process. Conductivity enhancement could be obtained by altering the laser annealing parameters. Parametric studies including the sheet resistance and optical transmittance of the annealed ZnO NP thin film were conducted for various laser powers, scanning speeds and background gas conditions. The lowest resistivity from laser-annealed ZnO thin film was about 4.75×10−2 Ω cm, exhibiting a factor of 105 higher conductivity than the previously reported furnace-annealed ZnO NP film and is even comparable to that of vacuum-deposited, impurity-doped ZnO films within a factor of 10. The process developed in this work was applied to the fabrication of a thin film transistor (TFT) device that showed enhanced performance compared with furnace-annealed devices. A ZnO TFT performance test revealed that by just changing the laser parameters, the solution-deposited ZnO thin film can also perform as a semiconductor, demonstrating that laser annealing offers tunability of ZnO thin film properties for both transparent conductors and semiconductors.

Room-temperature deposition of transparent conductive Al-doped ZnO thin films using low energy ion bombardment

Al-doped zinc oxide (AZO) films are prepared on quartz substrates by dual-ion-beam sputtering deposition at room temperature (∼25°C). An assisting argon ion beam (ion energy E i =0–300 eV) directly bombards the substrate surface to modify the properties of AZO films. The effects of assisted-ion beam energy on the characteristics of AZO films were investigated in terms of X-ray diffraction, atomic force microscopy, Raman spectra, Hall measurement and optical transmittance. With increasing assisting-ion beam bombardment, AZO films have a strong improved crystalline quality and increased radiation damage such as oxygen vacancies and zinc interstitials. The lowest resistivity of 4.9×10−3Ω cm and highest transmittance of above 85% in the visible region were obtained under the assisting-ion beam energy 200 eV. It was found that the bandgap of AZO films increased from 3.37 to 3.59 eV when the assisting-ion beam energy increased from 0 to 300 eV.

Highly Transparent p-Type ZnO Thin Films Prepared by Non-Toxic Sol-Gel Process

In-N co-doped p-type ZnO thin films were demonstrated using a non-toxic sol-gel spin coating process. The resistivity of the p-type ZnO films is 4.43 Ω cm with the carrier concentration of 1.36 × 1018 cm−3 at room temperature. The X-ray photoelectron spectra of N 1s core level measured with an Al Kα photon line identify the existence of nitrogen in the p-type ZnO. The secondary ion mass spectrometry depth profile also confirms the nitrogen contents in the In-N co-doped films. In addition, the In-N co-doped film with a thickness of 200 nm has a high transparency about 90% in the visible region.