Doped Zinc Oxide Films Research Articles

Aerosol Assisted Chemical Vapor Deposition of Transparent Conductive Zinc Oxide Films

Highly conductive and transparent ZnO films were synthesized by the reaction of diethyl zinc (in toluene) with methanol by dual source aerosol assisted chemical vapor deposition. These films displayed a stable sheet resistance of 7.2 Ω/□ and high transmission across the visible region comparable to commercial transparent conducting oxides (TCOs) based on oxides of tin or indium. Doping the zinc oxide structure with fluorine (trifluorotoluene) resulted in dense compact films with improved electrical properties than ZnO films with a sheet resistance of 4.5 Ω/□. These films also displayed idealized surface texturing for photovoltaic applications. Fluorine concentration was 2 at.% determined by wavelength dispersive X-ray analysis (WDX). Aluminum doped zinc oxide films were also synthesized by introducing dopant amounts of trimethylaluminium solution (in toluene) into the system. These films exhibited low sheet resistances of 14 Ω/□. The aluminum concentration in the films was 4 at.% determined by WDX.

Low resistive aluminum doped nanocrystalline zinc oxide for reducing gas sensor application via sol–gel process

Abstract Aluminum doped zinc oxide films were prepared by a sol–gel process using two different precursors. Zinc acetate and zinc nitrate were used as the starting precursor materials for sol–gel mixtures. The effects of the starting materials on the microstructure, surface morphology, electrical conductivity and gas sensor performance of ZnO were investigated as a function of Al concentration. The results indicated that the crystallite size decreased with the increase of Al dopant concentration from 0 to 5.0 at%. The films prepared by zinc acetate exhibited a skeletal wrinkle structure, which influenced the properties of ZnO films significantly. The gas sensing properties of Al-doped ZnO films prepared by two precursors were investigated for different concentrations of hydrogen in the air. The sensor response, as well as the response and recovery times of ZnO sensors, was analyzed to understand the effect of starting materials on the film quality. It was found that 3.0 at% Al-doped zinc oxide films had optimum properties, such as high electrical conductivity, crystallinity, higher response and faster gas response. Also, the films prepared with the zinc acetate-based precursor exhibited a better performance of gas sensor than those prepared with zinc nitrate-based sol–gel mixtures. Advantages of Al-doped ZnO over undoped ZnO in gas sensors appeared to be (i) low resistive devices, which do not require high impedance peripheral circuits and systems, (ii) tunability of selectivity based on response time constant, and (iii) low operation temperature.

Atomic layer deposition of Ga-doped ZnO transparent conducting oxide substrates for CdTe-based photovoltaics

The atomic layer deposition of gallium doped zinc oxide films is investigated as a method of fabricating transparent conducting oxide substrates for cadmium telluride based photovoltaic cells. The growth parameters and properties of gallium-doped ZnO were established for a range of dopant concentrations. 1 at. % gallium-doped films exhibited the lowest electrical sheet resistances and were selected as substrates to deposit Cd1−xZnxS/CdTe photovoltaic cells. The average current density–voltage characteristics of 16 cells under AM1.5 illumination yielded a conversion efficiency of 10.8% and a fill-factor of 65%.

Ag Interlayered Transparent Conducting Electrode for Photovoltaic Cells

We fabricated the organic photovoltaic (OPV) cells on the transparent and low resistive multilayer electrode of a few nm Ag layer embedded Al doped zinc oxide film (AZO/Ag/AZO) on a glass. AZO and Ag consisting of the continuous multilayer were fabricated using rf magnetron at room temperature successively. AZO (45 nm)/Ag (11 nm)/AZO (45 nm) electrode shows an electrical resistivity of 6.56×10-5 Ω cm, optical transmittance of 89.3% at 550 nm and figure of merit (FOM) value of 4.94×10-2 Ω-1. An inverted structure OPV was fabricated on AZO/Ag/AZO transparent electrode in which TiOx and PEDOT:PSS were used as an electron transport and hole transport layer respectively. The OPVs showed power conversion efficiency and fill factor as high as 2.07%, 0.51 respectively.

Analyzing nanotextured transparent conductive oxides for efficient light trapping in silicon thin film solar cells

Nanotextured contact layers are used in silicon thin film solar cells for increasing the short circuit current and conversion efficiency. We developed an approach to analyze random nanotextured surfaces by atomic force microscopy and image segmentation. It was used to investigate sputtered and wet chemically etched aluminum doped zinc oxide films with various morphologies. The information extracted from the surfaces was correlated with optical simulations of periodically textured thin film solar cells. The results from the surface analysis and optical simulations were also compared with the experimental results obtained for amorphous silicon solar cells prepared on the nanotextured substrates.

Highly transparent and conducting boron doped zinc oxide films for window of Dye Sensitized Solar Cell applications

Highly transparent and conducting boron doped zinc oxide (ZnO:B) films grown by sol–gel method are reported. The annealing temperature is varied from 350 to 550°C and doping concentration of boron is kept fixed for 0.6at.% for all the films. At low temperature the stress in the films is compressive, which becomes tensile for the films annealed at higher temperature. A minimum resistivity of 7.9×10−4Ωcm and maximum transmittance of ∼91% are observed for the film annealed at 450°C. This could be attributed to minimum stress of films, which is further evident by the evolution of A1 and defect related Raman modes without any shifting in its position. Such kind of highly transparent and conducting ZnO:B thin film could be used as window material in Dye Sensitized Solar Cell (DSSC).

Improvement of electrical and optical properties of molybdenum doped zinc oxide films by introducing hydrogen

Doped ZnO films are promising candidates as a front electrode in silicon film solar cells. In present work, we report on efforts to obtain highly conductive and transparent hydrogen and molybdenum co-doped ZnO (HMZO) films prepared by pulsed direct current (DC) magnetron sputtering. Investigations were made to see the effect of hydrogen (H2) flow rate on structural, electrical and optical properties. The results indicate that H2 flow rate has a considerable influence on the transparent conductive properties of molybdenum doped ZnO (MZO) films. A reduced resistivity of 69.2% and a broadening optical band gap of 5.7% were obtained with the optimal H2 flow rate of 5sccm. The average transmittance of more than 86% in the range of 400–1100nm was obtained with the optimal H2 flow rate. Crater-like surface morphology with root-mean-square (RMS) roughness of 46.5nm was also obtained after etching by diluted hydrochloric acid (HCl). Such a co-doping growing method present here may be useful for wide spectra absorption thin film solar cells.

Optical and photoluminescence properties of Ga doped ZnO nanostructures by sol-gel method

Zinc oxide (ZnO) nanocrystallites with different Ga-doping levels were successfully prepared by spin coating sol–gel technique. The morphological properties of Ga doped ZnO films were studied by atomic force microscopy (AFM). Alignment of ZnO nanorods with respect to the substrate depends on the amount of Ga dopant content. The dopant content varies from 1 % to 4 %, based on Ga-doping levels. The optical properties of the ZnO nanocrystallites following Ga-doping were also investigated by UV–Visible absorption and Photoluminescence spectra. Our results indicate that Ga-doping can change the energy-band structure and effectively adjust the intensity of the luminescence properties of ZnO nanocrystallites. Transmittance spectra of the films indicate that the films have high transparency. The refractive index dispersion was analyzed by single oscillator model developed by Wemple and DiDomenico. The oscillator energy, dispersion energy, high frequency dielectric constant values for the films were determined were calculated and it is found that the optical parameters are changed with Ga-doping content.

Sol Gel Dip Coated Aluminum Doped Zinc Oxide Films and Their Properties

Aluminium doped ZnO films were deposited by the sol gel dip coating method. The crystalline quality decreased with increase of Al doping. The band gap varies from 3.32 eV to 3.56 eV with increase of Al doping. This blue shift behavior or broadening in the band gap is mainly due to the Moss-Burstein band filling effect. The films exhibited 60 - 70 % transmission in the visible region. The grain size and surface roughness varied from 45 nm to 15 nm and 3.5 nm to 1.3 nm respectively. The intensity of the Raman peaks at 331, 385, 438 and 582 cm-1 modes decreased with increasing the Al concentration.

Influence of Sn doping on structural, optical and electrical properties of ZnO thin films prepared by cost effective sol–gel process

Tin (Sn) doped zinc oxide (ZnO) thin films were synthesized by sol–gel spin coating method using zinc acetate di-hydrate and tin chloride di-hydrate as the precursor materials. The films were deposited on glass and silicon substrates and annealed at different temperatures in air ambient. The agglomeration of grains was observed by the addition of Sn in ZnO film with an average grain size of 60nm. The optical properties of the films were studied using UV–VIS–NIR spectrophotometer. The optical band gap energies were estimated at different concentrations of Sn. The MOS capacitors were fabricated using Sn doped ZnO films. The capacitance–voltage (C–V), dissipation vs. voltage (D–V) and current–voltage (I–V) characteristics were studied and the electrical resistivity and dielectric constant were estimated. The porosity and surface area of the films were increased with the doping of Sn which makes these films suitable for opto-electronic applications.

Growth of ZnOx:Al by high-throughput CVD at atmospheric pressure

Aluminum doped zinc oxide films (ZnOx:Al) have been deposited on a moving glass substrate by a metalorganic CVD process at atmospheric pressure in an in-line industrial type reactor. Tertiary-butanol has been used as an oxidant for diethylzinc and trimethylaluminium as the dopant gas. The effect of the deposition temperature (from 380 to 540°C) on the deposition rate has been investigated by a numerical code, where a gas phase reaction among tertiary-butanol and diethylzinc is assumed to occur. The structural (crystallinity and morphology) properties of the films as a function of the deposition temperature have been analyzed by using X-ray diffraction and Scanning Electron Microscopy. A maximum growth rate of ∼11nm/s was found at a deposition temperature of 480°C, for which ZnOx:Al films show (002) preferential orientation, good crystalline quality and a naturally rough surface. ZnOx:Al films deposited at 480°C are also highly conductive (R<10Ω/□ for film thicknesses above 1050nm) and transparent (>85% in the visible range). These films have been used as front transparent conductive oxide layers in pin a-Si:H solar cells, achieving an initial efficiency approaching 8%.

Influence of aging time of the starting solution on the physical properties of fluorine doped zinc oxide films deposited by a simplified spray pyrolysis technique

Fluorine doped zinc oxide (FZO) films were fabricated from fresh and aged (4, 8, 12 and 16days) starting solutions using a simplified and low cost spray pyrolysis technique. The X-ray diffraction study showed that the preferential orientation is along the (002) plane for all the films irrespective of the age of the solution. The crystallite size calculated using the Scherrer’s formula is comparatively smaller only for the film prepared from the starting solution having aging time 4days which may be due to the efficient incorporation of fluorine atoms into the ZnO lattice. This phenomenon is confirmed by the minimum resistivity value (3.14×10−2Ωcm) obtained in this particular case. The visible transmittance and the optical band gap values are found to be in the range of 63–83% and 3.20–3.31eV, respectively. The optical transmittance is found to decrease gradually as the aging time of the solution increases and the optical band gap is found to be slightly higher in the case of the film prepared from the fourth day solution. The scanning electron microscopy results depicted that the microstructure of ZnO:F films are largely influenced by the aging of the starting solution.

GZO 타겟 결정성에 따른 박막의 전기적 광학적 특성

In this research, we prepared Ga doped zinc oxide(ZnO:Ga, GZO) targets each difference sintering temperature <TEX>$700^{\circ}C$</TEX>, <TEX>$800^{\circ}C$</TEX>, and doping rate 1 wt.%, 2 wt.%, 3 wt.%. The characteristics of thin film on glass substrates which deposited by facing target sputtering in pure Ar atmosphere are reported. Ga doped zinc oxide film is attracted material through low resistivity, high transmittance, etc. When prepared target powder's structure was investigated by scanning electron microscope, densification and coarsening by driving force was observed. For each ZnO:Ga films with a <TEX>$Ga_2O_3$</TEX> content of 3 wt.% at input power of 45W, the lowest resistivity of <TEX>$9.967{\times}10^{-4}{\Omega}{\cdot}cm$</TEX> (<TEX>$700^{\circ}C$</TEX>) and <TEX>$9.846{\times}10^{-4}{\Omega}{\cdot}cm$</TEX> (<TEX>$800^{\circ}C$</TEX>) was obtained. the carrier concentration and mobility were <TEX>$4.09{\times}10^{20}cm^{-3}$</TEX>(<TEX>$700^{\circ}C$</TEX>), <TEX>$4.12{\times}10^{20}cm^{-3}$</TEX>(<TEX>$800^{\circ}C$</TEX>) and <TEX>$15.31cm^2/V{\cdot}s(700^{\circ}C)$</TEX>, <TEX>$12.51cm^2/V{\cdot}s(800^{\circ}C)$</TEX>, respectively. And except 1 wt.% Ga doped ZnO thin film, average transmittance of these samples in the range 350-800 nm was over 80%.

Doping optimization and surface modification of aluminum doped zinc oxide films as transparent conductive coating

Aluminum doped zinc oxide (ZnO:Al) films were grown using spray pyrolysis technique. Effect of doping on structural, electrical, optical and morphological properties was studied. Aluminum doping improved the prominence of [002] growth while maintaining the grain size ~48nm. Using an intermediate Al/Zn atomic ratio in precursor (1.5:100), we could achieve a low resistivity ρ ~7×10−4Ωcm. These films possessed an average visible transmittance ~88%, an optical gap ~3.7eV and plasma wavelength at 1.87μm. A simultaneous use of methanol and iso-propanol in the precursor lead to a moderate surface roughness ~12nm. The films were surface modified using wet chemical etching in diluted hydrochloric acid, for varied time intervals (5s–15s) and etchant concentrations (0.125%–1%). The etching experiments could be used to know the building of the film as also to modify the surface for desired optical and morphological properties.

Ellipsometric studies of optical properties of copper doped zinc oxide films on glass substrates

Copper-doped ZnO films with thickness about 1200nm were prepared by radio-frequency (RF)-magnetron sputtering on glass substrates. The influence of dopant on the optical properties was investigated using ellipsometry technique. It has been found that only three layer model of surface can be satisfactory fitted with experimental data of ellipsometric measurements obtained in wide range of incident angles. Presence of additional layers within the model can be explained by the interface roughness. The atomic force microscopy (AFM) analysis indicates that thickness of additional layers and value of interface roughness are commensurable. The optical parameters of copper-doped ZnO structures with different concentration of copper dopant were obtained using the fitting procedure within the three-layer model. Absence of copper clusters in the ZnO:Cu layer microstructure was confirmed using X-ray diffraction and the effective media approximation (EMA) analysis. Refractive indices of doped ZnO:Cu layers (n=2.002…2.04) are very close to the index of ZnO film (n=1.98) at 632.8nm. Extinction coefficient increase gradually to (k=0.002) with the dopant concentration increasing.

Preparation and Characterization of ZnO: In Transparent Conductor by Low Cost Dip Coating Technique

Transparent conducting oxide (TCO) based on indium doped zinc oxide films in the nano scale were successfully prepared using combination between dip coating and thermal decomposition process. Structural investigations confirm the polycrystalline ZnO hexagonal wurtzite phase grown along the c-axis with nano crystallite size about 10 nm. Morphology investigation shows that ZnO films consist of fine grains of average size 40 nm. This indicates that each grain contains several crystallites with different orientations. Cross sectional image presents good adhesion of the films with the substrate and the film thickness has been determined. Compositional analysis detects the indium content in the host ZnO matrix, the In/Zn ratio is close to the calculated concentration ratios of the precursor. The optical transmittance shows that the films are transparent in the UV and VIS-IR spectral region and interference fringes were observed to be thickness dependent. Preparation parameters were investigated and optimized such as dipping rate, number of deposition cycles, precursor concentration, annealing process and In/Zn ratio. Optimization process was investigated for low resistivity, high optical spectral window transmission and easy preparation process. Dipping rate in the range 2 - 38 mm/s is the most suitable range for good film quality while dipping rate range 30 - 38 mm/s produces thicker films in lower deposition cycles. The higher dipping rate produces films with lower transparency (milky films) while the small deposition rate rate requires large number of deposition cycles in order to increase the thickness. Besides, the higher dipping rate reflects lower resistivity of the deposited films. Precursor molar concentration was observed to have an essential effect on the film thickness, film quality and transparency. Lower precursor concentration requires also large number of deposition cycles for thickening the films. The higher concentration results also milky films (high scattering process by powder film). Precursor concentrations in the range 0.7 - 0.9 mol/liter were found to be the optimal for better quality and for faster deposition process. The resistivity of the films has been reduced from the range 1.5 - 2.5 kW?cm to the range 100 - 400 W.cm as the molar concentration reaches the range 0.07 - 0.09 mol/liter. The resistivity of films increases from 330 to 1686 .cm as the decomposition temperature increases from 200C to 350C. Annealing at 450C process after completing the decomposition at 200?C results the lowest resistivity with annealing time in the range 1.5 - 2 h. In/Zn percentage in the range 1.5% - 5% produces the lowest electrical resistivity. The absorption edge of the deposited films was observed to be critical affected by the preparation parameters. The band gap change was discussed through the degenerate semiconductors as well as nanostructured semiconducting materials of the energy gap confinement effect. Deposition of TCO based on ZnO:In was optimized depending on all deposition parameters forwide area, the lower cost and good performance TCO films.

Oxygen effect in Magnetron Sputtered Aluminum doped Zinc oxide films.

In this work, polycrystalline transparent conductive aluminum doped zinc oxide (ZnO:Al) films, have been successfully grown on glass and silicon substrates by rf magnetron sputtering technique at room temperature. The effect of oxygen content in plasma on the structural, optical and electrical properties of the films was systematically studied. The growth rate was fond to decrease with the increase in O2 content. The crystal structure of ZnO:Al films deposited on glass is hexagonal with C-axis preferential orientation, while for film deposited on silicon substrate, the preferred orientation of crystallite shifts from (002) to (100) direction with the increase in O2 content. Intrinsic stress increases with an increase of oxygen content, and near stress-free film was obtained at 0 % O2 content. Low resistivity (ρ= 1.25x10 -3 Ωcm) associated to high transmittance (T>92 %) in the visible regions, were obtained for ZnO:Al film deposited at room temperature without oxygen content in the deposition chamber. From the optical characterization, we deduced that the band gap shifts towards lower energy with an increase of oxygen content.

Structural and optical properties of copper doped ZnO films derived by sol–gel

Undoped and copper doped ZnO (ZnO:Cu) films were prepared by sol–gel spin coating technique. The effect of Cu incorporation on the structural, morphological and optical properties of the Zinc oxide (ZnO) film was investigated. The chemical composition of the Cu doped ZnO films was confirmed by XPS and XRD measurements. The structural disorder was observed in XRD spectra depending on the Cu incorporation and these results were confirmed by the increasing Urbach energy values. The refractive index dispersion was analyzed by single oscillator model developed by Wemple and DiDomenico. The oscillator energy and dispersion energy values for the films were determined. The oscillator energy value Eo for the ZnO film decreases with Cu incorporation due to increase in the probability of electronic transitions between electronic bands. It is evaluated that the structural and optical properties of ZnO films can be controlled with Cu doping.

Properties of Al doped zinc oxide films prepared by electron beam-PVD

Al doped ZnO (AZO) films were prepared on quartz substrates by the co-evaporation of ZnO and Al2O3 ingots by EB-PVD. EB power applied to the Al2O3 source was ranged from 3kW to 10kW, while EB power to the ZnO source was fixed at 3kW, at a substrate temperature of 400°C. X-ray diffraction measurement showed that the AZO films were weakly c-axis oriented. Transmittance of All the AZO films was over 80% in the visible range. Highest reflectance in the near IR range was obtained at the EB power on Al2O3 of 5kW. The lowest resistivity of 3.05 × 10−4 Ωcm was obtained for the film deposited with the EB power on Al2O3 of 5kW with the deposition time of 300s.

Effect of boron doping on optical properties of sol–gel based nanostructured zinc oxide films on glass

Boron doped zinc oxide thin films (∼80 nm) were deposited onto pure silica glass by sol–gel dip coating technique from the precursor sol/solution of 4.0 wt.% equivalent oxide content. The boron concentration was varied from 0 to 2 at.% w.r.t. Zn using crystalline boric acid. The nanostructured feature of the films was visualized by FESEM images and the largest cluster size of ZnO was found in 1 at.% boron doped film (B1ZO). The presence of mixed crystal phases with hexagonal as major phase was identified from XRD reflections of the films. Particle size, optical band gap, visible specular reflection, room temperature photoluminescence (PL) emissions (3.24–2.28 eV), infra-red (IR) and Raman active longitudinal optical (LO) phonon vibration were found to be dependent on dopant concentration. For the first time, we report the room temperature fine structured PL emissions as phonon replicas originated from the LO phonon (both IR and Raman active) in 1 at.% boron doped zinc oxide film.