Fabrication Of ZnO Thin Films Research Articles

Investigation on the effect of annealing period on the structural and optical properties of ZnO thin films prepared using a polymer precursor

Numerous synthesis strategies including sputtering,molecular beam epitaxy,chemical vapour deposition, pulsed laser deposition etc., have been reported to grow high quality ZnO thin films[1]. However among these mounted synthesis strategies, simple and price powerful path to develop ZnO thin films by way of usage of cheaper, non poisonous and environmentally benign precursors are nevertheless the important issues. This work advise a simple and novel technique to the fabrication of ZnO thin films with the aid of poly vinyl alcohol (PVA) as precursor. This article also investigate the impact of annealing period on the structural and optical properties of grown films by analyzing XRD, UV absorption and photo luminescence spectra. The XRD pattern reveals poly crystalline hexagonal wurtzite structure. The particle size and stress values varies with changes in annealing period. The values of UV absorption spectra are utilized to plot Tauc plot to get the band gap of the films. The band gap shows significant decrease from 3.35ev to 3.10ev with increase in annealing period. The films shows good transmittance >75% and transmittance increase as the annealing parameter increases. The PL spectra have peaks in UV and visible regions,. the mechanism behind these emission peaks are also discussed.

Fabrication of ZnO thin film by dip-coating technique and study of photocatalytic activity in Rhodamine B degradation reaction

In this study, ZnO thin films were fabricated by dip-coating technique used solution synthesized by sol-gel method. The influence of film area and RhB concentration on the photocatalytic activity of ZnO films was determined. Accordingly, the survey results irradiation with a UV lamp 5W showed that both the efficiency and the rate constant increased when the film’s area used increased, but the mass of RhB decomposed in 1 hour calculated according to 1 m2 films again decreased. In the case of increased RhB concentration, the results show that the efficiency and the reaction rate constant decrease in value. Finally, an important point to mention is the film samples can be reused many times showing high potential for practical applications.

Aerosol assisted chemical vapour deposition of nanostructured ZnO thin films for NO2 and ethanol monitoring

Gas response control through structural and defect engineering in metal oxides are critical approaches to realize high performance sensors. Herein, we present the fabrication of ZnO thin films containing nanosized structures and high concentration of zinc interstitials for NO2 and ethanol monitoring. The target materials were fabricated by aerosol assisted chemical vapour deposition (AACVD) method on ceramic and glass substrates, and were characterized by complementary analytical techniques. The obtained ZnO nanoparticulate and nanopyramidal thin films yielded an outstanding response of ~41 to 1 ppm NO2 and ~236 towards 100 ppm ethanol, respectively. The reasons accounting for such an enhancement were revealed by the surface-depletion model in ZnO and the chemisorption energies of oxygen molecules on Zni-enriched crystal facets.

Comprehensive characterization of ZnO thin films for surface acoustic wave applications

This paper presents a comprehensive characterization of a very smooth, c-axis oriented, highly piezoelectric and electrically resistive 2.5 µm-thick ZnO thin film deposited by a radio frequency (RF) magnetron sputtering system on SiO2/Si substrate. Thin film properties such as surface roughness, crystallography, stoichiometry, and electrical resistivity are measured. Two-port surface acoustic wave (SAW) devices with bidirectional interdigital transducer (IDT) periods of 16 µm, 20 µm and 24 µm are fabricated on top of the ZnO thin film. A detailed finite element analysis (FEA) of the thin film is elaborated by varying ZnO thickness and IDT configuration. FEA results shows that acoustic wave velocities and resonance frequencies of the SAW devices are decreasing together with increasing ZnO thickness. Frequency response of the fabricated SAW devices are measured with a vector network analyzer (VNA) and compared to FEA results. First two wave modes, namely the Rayleigh and Sezawa waves, of the fabricated SAW devices on the ZnO thin films are interrogated. Resonance frequencies of the SAW devices with wavelengths of 16 µm, 20 µm, and 24 µm are measured as 204.8 MHz, 176.3 MHz, and 155.3 MHz for the Rayleigh mode, 335.9 MHz, 275.3 MHz, and 235.1 MHz for the Sezawa mode, respectively. In addition, omnidirectional wave propagation of ZnO thin film is shown with 45° rotated IDTs. Overall, experimental results are in good agreement with simulation results, demonstrating ZnO thin film fabrication is successfully carried out, and FEA is an appropriate method for modeling SAW devices on thin films.

Chemical reaction-transport model of diethylzinc hydrolysis in a vertical MOCVD reactor

ZnO thin film has many uses as a semiconductor material. It can be fabricated by chemical vapor deposition with diethylzinc (DEZn) and water vapor. The present study employs density functional theory to examine reaction complexes of DEZn with 1–2 molecules of water, which may occur in the gas phase according to previous studies. The kinetic and thermodynamic data provide a better understanding of the ZnO deposition process. A computational fluid dynamics analysis was carried out using the kinetic parameters to simulate the deposition rate of ZnO in a reaction chamber, showing the dependence of the film growth rate on temperature. The simulation data agreed with the experimental one within 8%, proving the feasibility of the current chemical reaction-transport model of diethylzinc hydrolysis. Furthermore, the fields of the flow, the temperature, the multi-species transport, and the chemical reaction were analyzed. These insights could reveal the reaction kinetics of ZnO thin film fabrication in the metal-organic chemical vapor deposition chamber. The results show that the pathway involving nucleation and growth of oligomers from trimers, and ultimately particle formation, is consistent with the decreased growth rate at increasing temperatures. The maximum growth rate of ZnO film was obtained at 573–773 K when ZnO is grown by DEZn and H2O in a horizontal chamber rotating at high speed. When the temperature is above 773 K, parasitic reactions lead to a rapid decline in the ZnO deposition rate. These insights could reveal the reaction kinetics of ZnO thin film fabrication; and help with the reactor design, optimization, and process parameter adjustment in the metal-organic chemical vapor deposition process.

Low temperature sputter-deposited ZnO films with enhanced Hall mobility using excimer laser post-processing

We report the low temperature (T  <  70 °C) fabrication of ZnO thin films (~140 nm) with Hall mobility of up to 17.3 cm2 V−1 s−1 making them suitable for thin film transistor (TFT) applications. The films were deposited by rf magnetron sputtering at T  <  70 °C and subsequently laser processed in ambient temperature in order to modify the Hall mobility and carrier concentration. Medium-to-low energy laser radiation densities and a high number of pulses were used to avoid damaging the films. Laser annealing of the films after aging in the lab under 25%–35% relative humidity and at an average illuminance of 120 lux resulted in an overall higher mobility and relatively low carrier concentration in comparison to the non-aged films that were laser processed immediately after deposition. A maximum overall measured Hall mobility of 17.3 cm2 V−1 s−1 at a carrier density of 2.3  ×  1018 cm−3 was measured from a 1 GΩ as deposited and aged film after the laser treatment. We suggest that the aging of non-processed films reduces structural defects mainly at grain boundaries by air species chemisorption, with concomitant increase in thermal conductivity so that laser processing can have an enhancing effect. Such a processing combination can act synergistically and produce suitable active layers for TFT applications with low temperature processing requirements.

Fabrication of ZnO Thin Films by Sol–Gel Spin Coating and Their UV and White-Light Emission Properties

ZnO thin films have been fabricated by the sol–gel spin-coating technique and annealed under different conditions, and their ultraviolet (UV) and white-light emission properties investigated. Different ambient conditions including oxygen, nitrogen, zinc-rich nitrogen, and vacuum were used to tune the main properties of the ZnO thin films. The resistivity varied from the conductive to semi-insulating regime, and the luminescence emission from fairly intense UV to polychromatic. The emission intensity was also found to be a function of the annealing conditions. Possible routes to compensate the loss of emission characteristics are discussed. X-ray photoelectron spectroscopy (XPS) analysis was carried out to detect the chemical states of the zinc/oxygen species. The changes in the electrical and emission properties are explained based on annihilation/formation of inherent donor/acceptor-type defects. Such ZnO thin films could have potential applications in solid-state lighting.

Optimization of RF Magnetron Sputtering Process Parameters Based on Genetic Algorithm and Neural Network

A procedure for RF magnetron sputtering process parameter optimization is proposed in this paper. This study has been focusing on determining the optimal parameter combination for producing the desirable optical band gap. In this proposed procedure, Genetic Algorithm (GA) has been adapted as the optimization tool, while Artificial Neural Network (ANN) has been implemented as the prediction model. GA was adapted to search for the optimal parameter combination from the set of parameters, while later the ANN modeling had been utilized to predict the optical band gap energies for each of the parameter combinations. The result from the GA optimization is expected to produce the highest band gap value. The computational results from the proposed procedure were then compared with the actual laboratory experimental results from the ZnO thin film fabrication. Based on the comparison result, the performance of the proposed procedure had proven to be promising in determining the most optimized process parameter combination from the set of parameters.

Determination of the optimal parameters for the fabrication of ZnO thin films prepared by spray pyrolysis method

In this work, ZnO thin films have been prepared by spray pyrolysis deposition method on the glass substrates. The effect of deposition parameters, such as deposition rate, substrate temperature and solution volume has been studied by X-ray diffraction (XRD) method, UV–Vis–NIR spectroscopy, scanning electron microscopy (SEM), and electrical measurements. The XRD patterns indicate polycrystalline wurtzite structure with preferred direction along (0 0 2) planes. Thin films have transparency around 90% in the visible range. The optical band gap was determined at 3.27 eV which did not change significantly. Evolution of electrical results containing the carriers’ density, sheet resistance and resistivity are in agreement with structural results. All the results suggest the best deposition parameters are: deposition rate, R = 3 ml/min, substrate temperature, T s = 450°C and thickness of the thin films t = 110–130 nm.

ZnO Thin Films Deposited on Sapphire by High Vacuum High Temperature Sputtering

Zinc oxide is widely recognised as a material suitable for applications in transparent electronics, photonics, sensors, and high power electronics due to its semiconductor and oxide properties and today is one of the most studied semiconductors worldwide. One of the challenges for ZnO thin film fabrication is obtaining layers of high crystalline quality. Due to high prices of free standing high-quality ZnO substrates, most of the scientific work on ZnO deposition is performed on sapphire substrates, which however has an 18% lattice mismatch to ZnO. To cope with this problem, thin MgO buffer layers are widely used in MBE ZnO growth. The closer fit of the atom positions on [111] MgO and ZnO [0002] surfaces than on sapphire [0001] and ZnO [0001] surfaces leads to less strain and defects in the ZnO layers. Thus MgO layers can promote the correct crystalline growth of the ZnO thin films [1–4]. They can also be used to control the polarity of ZnO films grown on them [5, 6]. Ease of implementation and low costs in comparison to MBE or metal-organic chemical vapor deposition (MOCVD) make sputtering one of the methods of choice for commercial ZnO thin film fabrication. Furthermore, recent improvements in the technology of sputtering systems allow it to perform better than widely expected in the areas of layer purity, homogeneity and physical properties’ control, which can lead to obtaining very high quality crystals. These reasons motivated us to undertake the attempt at transferring the MgO buffer layer technology from MBE to sputtering, the results of which are reported herein.

Effect of substrate temperature on ZnO thin film fabrication by using an atmospheric pressure cold plasma generator

AbstractUnder atmospheric pressure, homogeneous non‐equilibrium cold plasma was generated stably by high voltage pulsed power (1 kV, 20 kHz) excitation of He and O2 gases. By feeding Zn‐MOPD (C18H30O6Zn) into this plasma with He carrier gas, transparent flat ZnO films about 240 nm thick were successfully fabricated on glass substrates directly under a slit made into the cathode. Transmittance of the film is about 80% in the wavelength range from 400 to 600 nm. An XRD measurement revealed that these ZnO films had a c‐axis oriented polycrystalline structure. By increasing the substrate temperature from 250 to 400 °C, electrical resistivity of the film decreased from 0.42 Ωm to 9.4 × 10–4 Ωm. A high temperature of the substrate may change the growth pattern of ZnO film and its microstructure. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Fabrication of ZnO Thin Films from Nanocrystal Inks

Zinc oxide nanocrystals were prepared in ethanol and spin-cast to form semiconductor nanocrystal thin films that were thermally annealed at temperatures between 100 and 800 °C. Particle size, monodispersity, and film porosity were determined by X-ray diffraction, ultraviolet−visible absorption spectroscopy, and spectroscopic ellipsometry, respectively. Film porosity rapidly decreased above 400 °C, from 32% to 26%, which coincided with a change in electronic properties. Above 400 °C, the ZnO electron mobility, determined from FET transfer characteristics, increased from 10−3 to 10−1 cm2 V s−1, while the surface resistivity, determined from electrical impedance, decreased from 107 to 103 Ω m over the same temperature range. Below the densification point, nanoparticle core resistivity was found to increase from 104 to 106 Ω m, which is caused by the increasing polydispersity in the quantized energy levels of the nanocrystals. From 100 to 800 °C, crystallite size was found to increase from 5 to 18 nm in diame...

Fabrication of ZnO thin films by femtosecond pulsed laser deposition

The growth of ZnO thin films on sapphire substrate using the femtosecond PLD technique is reported. The effect of substrate temperature and oxygen pressure on the structural properties of the films was studied. Highly c-axis oriented ZnO films can be grown on sapphire substrates under vacuum conditions using the femtosecond PLD process. There is an optimum substrate temperature for the pulsed laser deposition of ZnO film that enhances the thermodynamic stability and allows the formation of well-crystallized thin films. The crystal quality of the films can be further improved by increasing the deposition time and introducing oxygen during the pulsed laser deposition process.

Effect of annealing temperature on the optical and electrical properties of aluminum doped ZnO films

abstract Aluminum doped ZnO thin films were successfully deposited on the silicon substrates by spin coatingmethod. The effects of an annealing temperature on electrical and optical properties were investigatedfor 1.5 at.% of aluminum. Refractive index profile has been obtained for the film annealed at 350 C usingellipsometry and it has shown minimum refractive index of 1.95 and maximum value of 2.1. Thicknessprofile shows quite good uniformity of the film having minimum thickness value of 30.1 nm and maxi-mum value of 34.5 nm. Maximum conductivity value obtained was 4.63 X 1 -cm 1 for the film annealedat 350 3C. Maximum carrier density of 2.20 10 17 cm was deduced from the Hall measurement andFourier transform infrared spectroscopy clearly reveals major peak at 407 cm 1 in the spectra associatedwith the ZnO bond. 2009 Elsevier B.V. All rights reserved. 1. IntroductionTransparent conducting oxide (TCO) films are of prominentinterest due to their potential in optoelectronic devices such astouch panels, flat panel displays (FPD), and thin film solar cells[1,2]. For display applications, TCO films of high optical transmit-tance in the visible region and high electrical conductivity are re-quired. Indium tin oxide (ITO) has been commercially used forthe flat panel displays [3,4]. However, impurity-doped zinc oxides(ZnOs) serves as an alternative for ITO-based TCO because of itsadvantages of low cost, resource availability and nontoxicity. Zincoxide is accepted as wide band gap semiconductors as an alterna-tive to GaN because it has band gap of 3.37 eV at room tempera-ture, large free exciton binding energy (60 meV), and highmechanical and thermal stabilities. With enhancement in the qual-ity andcontrolof conductivityinbulkZnO [5] foundits potentialasa material for short wavelength light emitters and transparentelectronics [6,7].Generally, the undoped ZnO is showing a n-type conductivity.Particularly, an Al doped ZnO transparent conductive oxide (TCO)filmhashightransmittanceinthevisibleregionwithalowresistiv-ity and by doping it with aluminum its electrical properties can beimproved[8,9].Asaresult,theAldopedZnOshowsexcellenttrans-parency over the entire visible spectrum and has better transportproperties than tin oxide due to higher electron mobility. Thereare several methods used for the fabrication of ZnO thin films suchas pulsed laser deposition [10–12], sputtering [13,14], molecularbeam epitaxy [15], and sol–gel process [16], etc. Among thesemethods, the sol–gel technique has several advantages over theothers such as low cost, simplicity; homogeneity and large areafilms can be prepared at lower temperature.Many factors can affect the properties of aluminum doped ZnOfilms prepared by sol–gel method. Among them, Aluminum com-position and annealing temperature have significant effect on theoptical and electrical properties of the film. Present work reportsaluminum-doped zinc oxide films deposited on silicon substrateusing sol–gel spin coating process. We have already reported effectof aluminum doping on the properties of the films [9]. Here, wehave investigated effect of annealing temperature on the Alumi-num doped ZnO films prepared by sol–gel method on the siliconsubstrate by considering 1.5 at.% of aluminum. The main goal ofthe paper is to investigate effect of annealing temperature on theelectrical and optical properties of the aluminum doped ZnO filmsby keeping aluminum concentration fixed.

Anodic Aluminum Oxide Templated Channel Electrodes via Atomic Layer Deposition

Dye-sensitized solar cells (DSSCs) utilize high surface area metal oxide sintered particle networks to absorb molecular dyes and transport injected charge carriers. While this sintered particle architecture allows liquid electrolyte DSSCs to achieve efficiencies up to 11%, slow charge transport through the semiconductor network limits the amount of modification that can be made to the electrolyte and dye without adversely affecting the efficiency. The functionalization of anodic aluminum oxide membranes with thin films of transparent, semiconducting oxides via atomic layer deposition may allow for significantly faster charge transport. We demonstrate the fabrication of ZnO thin films within and upon commercially available anodic aluminum oxide membranes via atomic layer deposition.

Fabrication of ZnO thin films by the photochemical deposition method

ZnO thin films were fabricated by the photochemical deposition (PCD) method. The deposition solution contains ZnSO 4, Na 2SO 3, Na 2S 2O 3 and a small amount of NH 4OH for pH adjustment. We blew oxygen or oxygen + ozone (O 3) gas into the solution to increase the dissolved oxygen content and enhance the oxidation reaction. The films were characterized by Auger electron and optical spectroscopy, and a photoelectrochemical (PEC) measurement. On an indium-tin-oxide (ITO) substrate, the films showed high optical transmission in the visible range. In a current–voltage measurement for films on a p-Si substrate, the O 3 bubbling sample showed rectification properties and photovoltaic effects.

Effect of Oxygen Flow Rate on Fabrication of ZnO Thin Films Using Cold He Plasma at Atmospheric Pressure

Homogeneous nonequilibrium low temperature plasma was generated under atmospheric pressure by high voltage pulsed power (1 kV, 20 kHz) excitation of He gas or mixture of He and O2 gases. Using this cold plasma, ZnO thin films were fabricated on glass substrate exposed to air by feeding Zn-MOPD into the plasma with He carrier gas. Dependence of O2 gas flow rate on optical transmittance of the ZnO films and resistivity were investigated. In addition, crystallinity and microstructure of the films was studied by XRD measurement and FE-SEM observation.

Fabrication of ZnO thin films using charged liquid cluster beam technique

A thin-film deposition technique utilizing a charged liquid cluster beam was applied in the fabrication of ZnO thin films on silicon substrates with a thin layer of native oxide. The liquid precursor was prepared by dissolving Zn-trifluoroacetate in methanol. The film was very uniform, densely packed, and predominantly c-axis oriented. The initially resistive ZnO thin film became conductive after annealing in a hydrogen atmosphere for 5 min. The electrical resistivities of the annealed films ranged from 7.99×10−3 to 1.60×10−2 Ω cm. The electron carrier concentrations were in the range of 3.33–5.35×1019 cm−3 and the mobilities were 7.30–19.07 cm2 V−1 s−1. The refractive index varied from 1.89 to 1.96 as the growth temperature increased from 340 to 440 °C.