Textured ZnO Films Research Articles

Controlling Surface Wettability and Plasmonic Resonance of Au/ZnO Heterostructured Films

This work investigated the (0002) textured ZnO films without and with the addition of an Au continuous top layer and its effects on their surface wettability and plasmonic resonance characteristics. The ZnO films were directly fabricated onto glass substrates at the synthesized temperature of 300 °C via a plasma-enhanced chemical vapor deposition (PECVD) system, and the as-synthesized ZnO film exhibited an average optical transmittance value of 85%. The ultraviolet (UV) light irradiation can be applied to enhance the hydrophilicity, changing it from a hydrophobic status to hydrophilic status due to the existing and adjustable characteristics of the photocatalytic activity. On the other hand, the surface wetting/contact angle (CA) value of the ZnO film with a controllable surface wettability switched from 94° (hydrophobicity) to 44° (hydrophilicity), after it was exposed to UV light irradiation for 5 min, and stably reversed back to hydrophobicity (92°) via a post-annealed treatment using rapid thermal annealing (RTA) at 350 °C for 5 min in air. A fast, simple, and reversible method for switching between hydrophilic and hydrophobic status is claimed in this present work. The improved surface plasmonic resonance is owning to the coupled electron and photon oscillations that can be obtained and produced at the interface between the flat Au layer and ZnO (metal/metallic oxide) heterostructured films for future applications of various wide-bandgap compound semiconductors.

Structural Features of Textured Zinc-Oxide Films Obtained by the Ion-Beam Sputtering Method

Textured ZnO films obtained on amorphous substrates using the ion-beam sputtering method are studied. X-ray diffraction and atomic-force microscopy methods show that the resulting films have a polycrystalline structure immediately after deposition. It is established that further annealing of the samples in the temperature range from 200°C to 500°C results in recrystallization, which leads to changes in the grain size and surface roughness. A dependence of the crystallization intensity on the deposition conditions is found, which is related to the number of defects in the unannealed films. In films with an initially more perfect structure, heat treatment at 500°C results in the growth of grains by more than 2 times and a decrease in the roughness by ∼40%.

The effect of small addition of copper on the growth process, structure, surface charge and adsorption properties of ZnO films in the pyrolysis of dithiocarbamates

Highly textured ZnO films are fabricated, the outer polar surface of which is decorated with copper complexes. The segregation of copper on the surface during growth is attributed to the manifestation of the Jahn–Teller effect.

Структурные особенности текстурированных пленок оксида цинка, полученных методом ионного распыления

In this work, we studied textured ZnO films obtained by ion-beam deposition. X-ray diffraction patterns and micrographs of the surface revealed that asdeposited films have a polycrystalline structure. It was found that, after annealing of the samples in the temperature range from 200 ° C to 500 °, recrystallization occurs, leading to a change in the grain size and surface roughness. The dependence of the initial state of the film on the recrystallization intensity is also demonstrated. In films with an initially more perfect structure, temperature treatment at 500 ° C led to grain growth by more than 2 times and a decrease in roughness by ~ 40%.

Characterizations and growth of textured well-faceted ZnO films by low-pressure chemical vapor deposition on ITO glass substrates

In this work, textured well-faceted ZnO films with 0.5 and 2.0 μm film thickness grown on indium tin oxide (ITO) glass substrates by low-pressure chemical vapor deposition (LPCVD) are analyzed by x-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), cathode-luminescence (CL), and transmittance measurements. The XRD results show that the 0.5 μm ZnO film grown on the ITO substrate almost equally presents ZnO(110) ridge-like and ZnO(100) polygon-like grain structures, while the 2.0 μm ZnO film particularly favors the growth of ZnO(110) ridge-like grain structures. These results could be explained by a possible model of the constraints of lattice-match between the ZnO and ITO unit cells. The XRD results also show that the average grain sizes of the 2.0 μm ZnO film are larger than those of the 0.5 μm ZnO film, while the strain of the 2.0 μm ZnO film is smaller than that of the 0.5 μm ZnO film. These XRD results are shown to strongly agree with the measurements from the SEM, AFM, CL, and transmittance as well. The results of this work provide information for the growth of textured well-faceted ZnO films on ITO glass substrates that could be potentially utilized for high-performance and low-cost transparent conductive oxides.

High performance thin film transistor (flex-TFT) with textured nanostructure ZnO film channel fabricated by exploiting electric double layer gate insulator

We report a flexible thin film transistor (flex-TFT) fabricated on a commonly available polyimide (Kapton®) tape with a channel of highly textured nanocrystalline ZnO film grown by pulsed laser deposition. The flex-TFT with an electric double layer (EDL) gate insulator shows a low threshold for operation (Vth ≤ 1 V), an ON/OFF ratio reaching ≈107 and a subthreshold swing ≈75 mV/dec. The superior performance is enabled by a high saturation mobility (μs ≈ 70 cm2/V s) of the highly textured nanocrystalline channel. The low Vth arises from large charge density (≈1014/cm2) induced into the channel by EDL gate insulator. The large charge density induced by the EDL gate dielectric also enhances the Hall mobility in the film and brings down the sheet resistance by nearly 2 orders, which leads to large ON/OFF ratio. The flex-TFT operation can be sustained with reproducibility when the TFT is bent down to a radius of curvature ≈2 cm.

Textured ZnO films from evaporation-triggered aggregation of nanocrystal dispersions and their use in solar cells

Due to its high electron mobility, good stability and potential for low-temperature synthesis ZnO has received considerable attention for use in solar cells, photodetectors and sensors. Whilst there have been reports involving the formation ZnO films with porous morphologies the majority of those have involved elaborate or time-consuming preparation methods. In this study we investigate a simple new method for preparing textured porous ZnO (tp-ZnO) films. We used colloidal instability triggered by the evaporation of a volatile stabilising ligand during spin-coating of a ZnO nanocrystal (NC) dispersion to deposit crack-free tp-ZnO films. The porosity of the tp-ZnO films was 56% and they could be prepared using amine-based ligands with boiling points less than or equal to 78 °C. To demonstrate the ability to use the tp-ZnO films as electron acceptors they were infiltrated with poly(3-hexylthiophene) (P3HT) and solar cells prepared. The power conversion efficiencies of the tp-ZnO/P3HT devices reached values that were three times higher than a control bilayer ZnO/P3HT device prepared using a sol-gel derived ZnO film. Because our method used a low temperature treatment and ZnO films are used in a wide variety of third-generation solar cells, the new tp-ZnO films introduced here may offer a low cost method for improving the efficiency of other solar cells.

The effects of post-growth thermal annealing on the structural and electrical properties of RF-magnetron sputtered ZnO

The effects of two thermal annealing steps on the crystallographic orientation, film stress and crystal domain size in RF-magnetron sputtered ZnO thin films for ultrasonic transducers were investigated. As-grown films contained high levels of compressive stress introduced by the sputtering process, as well as low (002) crystal orientation selectivity. It was shown that residual stress could be eliminated by post-growth annealing at temperatures of 400°C or higher, however, that annealing process also lead to a reduction in the desired (002) crystal orientation selectivity. The use of the relatively low temperature (250°C) in-situ anneal in the sputtering chamber in an oxygen-rich environment was found essential for prompting grain growth and recrystallisation, resulting in (002) textured ZnO films that have high electrical resistivity.

Dielectric enhancement with low dielectric loss in textured ZnO films inserted with NiFe

A magnetic field dependence can be clearly observed in the dielectric constant of ZnO/NiFe/ZnO heterostructures (typically non-ferroelectric system) at room temperature controlled by a low external magnetic field.

Photoluminescence from porous textured ZnO films grown by chemical bath deposition

Porous textured ZnO films have been fabricated using the low temperature bath chemical deposition method. The photoluminescence (PL) spectra of the ZnO films show a broad emission band peaked at ~400nm. The PL exhibits sensitivity to the emission recording conditions. A decrease in temperature leads to separation of the PL into the UV and violet emission bands; at 10K they are located at 368.4 and 412nm, respectively. The observed violet emission is attributed to zinc vacancy related defects. The observed sensitivity of the emission to the recording conditions is due to oxygen desorption processes and variations in the width of a depletion region on the porous textured ZnO film surface.

Water-assisted nitrogen mediated crystallisation of ZnO films

Nitrogen mediated crystallisation (NMC) being performed in oxygen atmosphere at T ≥600°C is an effective approach to obtain very well (00l)-textured ZnO films. A use of NMC-seed layers remarkably improves electrical transport properties of subsequently deposited ZnO:Al contacts. In this work, crystallisation of quasi-amorphous, nitrogen doped ZnO seed layers has been performed using water vapours at overpressure and temperatures around 100°C. This approach allows employment of soda-lime float-glass or temperature sensitive film stacks as a substrate. We propose here possible mechanism of water-assisted NMC and grope for optimised crystallisation conditions on the basis of optical, microscopic, and textural investigation. Low temperature water-assisted crystallisation of 20nm thick ZnO layers was compared with high temperature annealing methods in terms of composition, microstructure and crystallinity. Electrical properties such as electron Hall mobility (μe), concentration of free electrons (Ne) and sheet resistance (Rsh) have been evaluated and compared for functional ZnO:Al films obtained on glass and on differently crystallised NMC-seed layers. It was found that the crystallised with water assistance at low temperature ZnO seed layers provide comparable improvement in crystallinity and electrical properties of subsequently grown functional ZnO:Al films with respect to the ones crystallised at high temperature. Use of optimised water-assisted crystallisation of seed layers has allowed decreasing Rsh of thin (130–270nm) functional ZnO:Al films twice compared to the glass substrate. Both provide this effect: increase in μe and increase of Ne.

Experimental and theoretical study of polarized photoluminescence caused by anisotropic strain relaxation in nonpolar a-plane textured ZnO grown by a low-pressure chemical vapor deposition

Anisotropic strain relaxation and the resulting degree of polarization of photoluminescence (PL) in nonpolar a-plane textured ZnO are experimentally and theoretically studied. A thicker nonpolar a-plane textured ZnO film enhances the anisotropic in-plane strain relaxation, resulting in a larger degree of polarization of PL and better sample quality. Anisotropic in-plane strains, sample quality, and degree of polarization of PL in nonpolar a-plane ZnO are consequences of the degree of anisotropic in-plane strain relaxation. By the k·p perturbation approach, simulation results of the variation of the degree of polarization for the electronic transition upon anisotropic in-plane strain relaxation agree with experimental results.

Highly textured conductive and transparent ZnO films for HIT solar cell applications

In this paper, aluminium-doped zinc oxide (AZO) thin-films were fabricated on both glass and silicon substrates by radio-frequency magnetron sputtering at various working pressures of 0.15–0.46 Pa. The effect of working pressure on the structural, electrical, and optical properties of the deposited AZO films was carefully studied. The lower working pressure is more favourable to large grain size, smooth surface, low electrical resistivity, and moderate optical transparency. For the AZO films deposited at lower working pressures, the larger grain size can be ascribed to the higher kinetic energy of the sputtered particles while the lower electrical resistivity is strongly related to both the presence of fewer grain boundaries due to the larger grain size and more activated amount of dopants in the films. We then applied AZO films deposited at 0.15 Pa to Al/AZO/n-a-Si : H/i-a-Si : H/p-c-Si/Al heterojunction Si solar cells with intrinsic thin layer (HIT) solar cells and achieved highly textured surfaces via acid etching. The HIT solar cells after acid texturing showed a higher external quantum efficiency (EQE) value than those with smooth AZO films mainly in the wavelength region from 300 to 500 nm, leading to an obvious increase in the conversion efficiency from 14.1% to 14.7%.

Topology and texture controlled ZnO thin film electrodeposition for superior solar cell efficiency

It is known that the properly textured ZnO film can increase light-scattering and therefore improves light-trapping inside the solar cells, resulting in increased Jsc value and consequently solar cell efficiency. Using a low cost solution-based electrochemical deposition, we have developed a ZnO thin film deposition process that offers fine-control of crystal orientation, surface morphology and texture by tuning an additive concentration. Using the optimized textured ZnO films, we have achieved superior light trapping performance especially in long wavelength spectrum (λ>610nm) comparing to the vacuum sputtered ZnO. Using single junction a-SiGe:H and multi-junction a-Si:H/a-SiGe:H/a-SiGe:H thin film solar cells, high quantum efficiency (current density) and solar cell efficiencies are attained. It demonstrates that the low cost electrochemical ZnO deposition is an effective method to prepare high quality ZnO film for solar cells.

Preparation method of double-textured ZnO:B films deposited by MOCVD on plasma etched polymer buffer

With the aim of enhancing light trapping phenomenon in PV thin film silicon devices, a method has been developed to produce highly textured TCO surfaces by the growth of ZnO:B on a buffer layer consisting of UV-cured highly transparent acrylic polymer (IBA) whose surface had been modified by RIE process. Different RIE etching conditions, particularly RF-power and process duration, have been used to obtain different polymer surface modifications. The effect of different treatment conditions on polymer surface has been systematically investigated and subsequently correlated to the properties of ZnO:B films deposited by MOCVD on these modified polymer surfaces.The surface morphology of zinc oxide film, growth on IBA buffer layer, changed from conventional pyramid-like single-texture to cauliflower-like double-texture. The morphology of double-textured ZnO strongly depended on the surface properties of the etched polymer. Excellent scattering properties were obtained and at the wavelength of 800nm the haze value increased from 7.5% (pyramidal-like single-texture ZnO layer on glass) to 68% (double textured ZnO film obtained on etched IBA, treated for 20min at RF-power of 80W). The RMS roughness showed similar behaviour: it increased at the increasing of the etching time and the highest value of 160nm was obtained for cauliflower-like ZnO:B film. Good electrical properties were maintained for double-textured ZnO:B films with excellent scattering properties.

Low-temperature solution growth of textured zinc oxide films for light trapping enhancement in thin film silicon solar cells

Solution based textured ZnO film significantly improves efficiency of thin film silicon solar cells due to its light trapping effect.

Characteristics of surface acoustic waves excited by (1120) ZnO films deposited on R-sapphire substrates.

(1120)-textured ZnO films are deposited on Rsapphire substrate by RF magnetron sputtering and the effects of deposition conditions on ZnO films are investigated. The chemical compositions of the ZnO films are characterized by X-ray photoelectron spectroscopy and the results indicate that the ratio of latticed oxygen to zinc increases with increasing of oxygen concentration in the sputtering gas, which demonstrates the improvement of crystal structures in ZnO films. To investigate the characteristics of surface acoustic waves excited by the (1120)-textured ZnO films, SAW delay lines based on layered structures of (1120) ZnO film/R-sapphire substrate are fabricated, in which Rayleigh and Love modes are excited along the (0001)- and (1100)-directions of the ZnO films, respectively. The phase velocities and electromechanical coupling factors of both wave modes are characterized as functions of the film-thickness-to-wavelength ratio. The acoustic properties of the layered structures are calculated using the transfer matrix method. The experimental and theoretical results are in good agreement with each other.

Improving electrical performance of ultrasonic spray pyrolysis grown textured ZnO films by introducing In2O3:W interface layer

The main focus of this paper was the improvement in the Hall mobility of ZnO films prepared by ultrasonic spray pyrolysis. In order to achieve low resistivity and high transparency simultaneously, we used the tungsten doped In2O3 (IWO) deposited by electron beam evaporation as interface layer to prepare IWO/ZnO films. Hall mobility can be improved to close to 50cm2V−1s−1, and the resistivity kept as low as 1.48×10−3Ωcm, while the transparency of the IWO/ZnO was about 80% in the range of 400–2000nm (including glass) at the same time. X-ray diffraction and scanning electron microscope measurements were used to investigate the structure properties and surface morphologies of the ZnO and IWO/ZnO films. Secondary ion mass spectrometry measurement revealed that the indium and tungsten can diffuse from the IWO layer into the following deposited ZnO layer. The oxygen interstitial of the ZnO layer can be decreased by the tungsten diffusing from the IWO layer, and this was the main reason of the higher Hall mobility. Atom force microscopy measurement indicated the IWO/ZnO films with high value texture.

Nanometer-Thick Textured ZnO Films: Preparation, Characterization and Interaction with Ethanol Vapor

A study was carried out on the effect of the reaction temperature in atomic layer deposition on the structural and piezoelectric properties of thin ZnO films (5–50 nm) by scanning electron microscopy, X-ray diffraction, piezoresponse force microscopy, and surface acoustic wave (SAW) spectroscopy. The piezoresponse was shown to be mainly a function of ZnO grain size and the monocrystalline structure perpendicular to the substrate plane. The results demonstrate promise for the use of textured ZnO films as sensitive coatings for acoustic gas sensors in instruments based on the SAW transducer amplification-frequency response.

Photoelectrochemical Properties of Cadmium Chalcogenide-Sensitized Textured Porous Zinc Oxide Plate Electrodes

We report the photoelectrochemical (PEC) performance of textured porous ZnO and CdX-coated ZnO films (X = S, Se). Porous ZnO films were grown with a platelike morphology on F-doped SnO(2) (FTO) substrates. The growth of ZnO films involves a two-step procedure. In the first step, we electrochemically grew simonkolleite (Zn(5)(OH)(8)Cl(2)·H(2)O) plate films. Annealing of the simonkolleite at 450 °C results in textured porous ZnO films. The as-obtained porous ZnO electrodes were then used in PEC studies. To increase the light-harvesting efficiency, we sensitized these ZnO electrodes with CdS and CdSe quantum dots, using the so-called "successive ion layer adsorption and reaction (SILAR) method". As expected, the photocurrent density systematically increases when going from ZnO to ZnO/CdS to ZnO/CdSe. The highest photocurrent, ∼3.1 mA/cm(2) at 1.2 V vs RHE, was obtained in the CdSe-sensitized ZnO electrodes, because of their enhanced absorption in the visible range. Additionally, quantum efficiency values as high as 90% were achieved with the textured porous ZnO films. These results demonstrate that both CdS and CdSe-sensitized textured porous ZnO electrodes could be potentially useful materials in light-harvesting applications.