Electrical and optical properties of Al doped cadmium oxide thin films deposited by radio frequency magnetron sputtering

Cadmium oxide thin films with different percentages of aluminum doping have been synthesized via radio frequency magnetron sputtering technique. Thin films were deposited on glass and silicon substrates with different percentages of aluminum at a substrate temperature of 573 K and pressure of 0.1 mbar in Ar+O₂ atmosphere. The deposited films were characterized by studying their structural, electrical and optical properties. The X-ray diffraction pattern revealed good crystallinity with preferred (1 1 1) orientation in the films. Aluminum doping in CdO thin films were confirmed by X-ray photoelectron spectroscopic studies and actual doping percentages were also measured from it. The optical band gap was found to decrease first and then increase with increasing percentages of aluminum concentrations. The electrical conductivity was found to increase with increase of aluminum doping concentration up to 5% but for higher doping concentration (>5%) the conductivity was found to decrease.     

Si doped ZnO thin films for transparent conducting oxides

Abstract

Transparent conductive silicon doped zinc oxide (SZO, 3%Si) thin films are grown by direct current magnetron sputtering on glass substrates at room temperature. Experimental results show that the sputtering time has a significance impact on the growth rate, crystal quality and electrical properties of the films, and have little impact on the optical properties of the films. The growth rate decreases with the sputtering time. The resistivity of ZnO/Si films decreases as the sputtering time increases from 8 to 20 min. However, as the sputtering time increases further, the electrical resistivity increases instead. When other sputtering conditions are kept unchanged, it is found that the optimum sputtering time is 20 min and the achieved lowest resistivity is 4·92×10^?4 Ω cm (sheet resistance?=?11·5 Ω/sq for thickness 427·5 nm). The UV-vis transmission spectrum shows that all film samples present a transmission of above 90·0% in the visible range.     

Properties of fluorine doped ZnO thin films deposited by magnetron sputtering

Fluorine doped ZnO (FZO) films were deposited on Corning glass by radio frequency (rf) magnetron sputtering of pure ZnO target in CF₄ containing gas mixtures, and the compositional, electrical, optical, and structural properties of the as-grown films as well as the vacuum-annealed films were investigated. The fluorine content in FZO films increased with increasing CF₄ content in sputter gas. FZO films deposited at elevated temperature of 150 °C had considerably lower fluorine content and showed a poorer electrical properties than the films deposited at room temperature. Despite high fluorine contents in the films, for all the FZO films, the carrier concentration remained below 2×10²⁰ cm⁻³, leading to fairly low doping efficiency level. Vacuum-annealing of the FZO films deposited at room temperature resulted in substantial increase of Hall mobilities, reaching as high as 43 cm²/Vs. This was attributed partly to the removing of oxygen vacancies and/or the forming chemical bonds with interstitial zinc atoms by fluorine interstitials and partly to the passivation effect of excess fluorine atoms by filling in the dangling bonds at the grain boundaries. For all the films with thickness of around 300 nm, the optical transmissions in visible were higher than 80%, and increased with increasing fluorine content up to 85% for the film with highest fluorine content.     

Effects of deposition temperature on characteristics of Ga-doped ZnO film prepared by highly efficient cylindrical rotating magnetron sputtering for organic solar cells

We report the characteristics of Ga-doped zinc oxide (GZO) films prepared by a highly efficient cylindrical rotating magnetron sputtering (CRMS) system as a function of substrate temperature for use as a transparent conducting electrode in bulk hetero-junction organic solar cells (OSCs). Using a rotating cylindrical GZO target, low sheet resistance (∼11.67 Ω/square) and highly transparent (90%) GZO films were deposited with high usage (∼80%) of the cylindrical GZO target. High usage of the cylindrical GZO target in the CRMS system indicates that CRMS is a promising deposition technique to prepare cost-efficient GZO electrodes for low cost OSCs. Resistivity and optical transmittance of the CRMS-grown GZO film were mainly affected by substrate temperature because the grain size and activation of the Ga dopant were critically dependent on the substrate temperature. In addition, the performance of OSC fabricated on GZO electrode sputtered at 230 °C (11.67 Ω/square) is better than OSC fabricated on as-deposited GZO electrode (29.20 Ω/square). OSCs fabricated on the GZO electrode sputtered at 230 °C showed an open circuit voltage of 0.558 V, short circuit current of 8.987 m A/cm², fill factor of 0.628 and power conversion efficiency of 3.149%.     

Highly transparent and conducting fluorine-doped ZnO thin films prepared by pulsed laser deposition

Highly transparent and conducting fluorine-doped ZnO (FZO) thin films were deposited on glass substrates by pulsed laser deposition. Structural, electrical, and optical properties of the films were investigated as a function of oxygen pressure ranging from 0.01 to 0.5 Pa. All the films had a highly preferential c-axis orientation. The films obtained were dense and very smooth with a typical columnar structure. A minimum resistivity of 4.83×10⁻⁴ Ω cm, with a carrier concentration of 5.43×10²⁰ cm⁻³ and a Hall mobility of 23.8 cm² V⁻¹ s⁻¹, was obtained for FZO film prepared at the optimal oxygen pressure of 0.1 Pa. The average optical transmittance in the entire visible wavelength region was higher than 90%.     

Low temperature Si doped ZnO thin films for transparent conducting oxides

Si doped zinc oxide (SZO, Si 3%) thin films are grown at low substrate temperature (T≤150 °C) under oxygen atmosphere, using pulsed laser deposition (PLD). Si addition leads to film amorphization and higher densification. Hall effect measurements indicate a resistivity of 7.9×10⁻⁴ Ω cm for SZO thin films deposited at 100 °C under optimized 1.0 Pa oxygen pressure. This value is in good agreement with optical resistivity simulated from the transmittance spectra. XPS measurements suggest more than one oxygen environment, and a Si oxidation state lying in between 2 and 3 only. As a matter of fact, the values of both measured and simulated carrier numbers are smaller than the ones expected, assuming that all Si cations in the ZnO matrix are at the 4⁺ oxidation state. Finally, the differences in the electrical and optical properties of SZO thin films deposited both on glass and PET substrates confirm the strong dependency of the electronic properties to the film crystallinity and stoichiometry in relationship with the substrate nature.     

Effect of thermal annealing on the structural and optical properties of nanostructured zinc oxide thin films prepared by pulsed laser ablation

Zinc oxide (ZnO) films are prepared by pulsed laser ablation, on an optically flat quartz substrate for different deposition time. The influence of annealing temperature, on the structural and optical properties of ZnO films is investigated systematically using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectra, UV-vis spectroscopy and photoluminescence spectroscopy (PL). The XRD pattern shows that the as-deposited films are amorphous and the annealed films are polycrystalline. The average size of the crystalline grains varies from 9 to 26 nm in the films. The SEM and AFM images reveal uniform distribution of grains in the films and the grains are in the nanoscale dimension. Raman spectra suggest the hexagonal wurtzite phase for the ZnO films. The UV-visible spectra show an increase in transmittance with annealing temperature. The observation of very intense PL emission from the films annealed at 773 K, suggest the suitability of these films for applications as light emitters in the visible region. The ability to produce the stochiometric ZnO thin films with reproducible structural, morphological and optical characteristics should be useful as a suitable window material for practical industrial solar cell and display devices.     

Tunable electrical resistivity of oxygen-deficient zinc oxide thin films

Oxygen-deficient zinc oxide thin films were deposited by radio frequency magnetron sputtering with a sintered zinc oxide ceramic target under an atmosphere of manipulated sputtering gas pressure (SGP). Under the designed deposition conditions, all the prepared films were of hexagonal würtzite structure with c-axis as the preferential growth orientation. With increasing SGP, the film thickness, deposition rate, grain size and atomic ratio of O to Zn in the films initially increased and then decreased. Interestingly, when the SGP was low, the main defects in the films were oxygen vacancies; when it was high, the dominant defects were interstitial zinc; but when it was in a moderate value, the film composition might be relatively close to the stoichiometric ZnO, possessing the least number of defects. As a result, the electrical resistivity of the films first increased and then dropped down as the SGP increased.     

Highly textured ZnO thin films doped with indium prepared by the pyrosol method

Indium doped ZnO thin films have been prepared on heated Corning 7059 glass by the pyrosol spray method. It was found that indium doping has an important role in grain growth at high substrate temperature. Indium also was used to improve the electrical properties, acting as an N type dopant, and we obtained highly conductive ZnO:In thin films with a resistivity of 3.0 × 10⁻³ Ω cm. At substrate temperatures from 425°C to 475°C, the deposited ZnO:In thin films have clear hexagonal crystallites and, therefore, a highly textured surface showing optical haze phenomena due to the crystallites. The haze ratio of ZnO:ln thin films can be controlled from 10% to 50% at the wavelength of 550 nm by varying the substrate temperature from 375°C to 475°C.     

Sputtered manganese oxide thin film on carbon nanotubes sheet as a flexible and binder‐free electrode for supercapacitors

In this work, flexible carbon nanotubes (CNTs)/manganese oxide (MnO₂) composite electrode was fabricated by direct deposition of MnO₂ nanoparticles on CNTs sheet by RF magnetron sputtering. The surface morphology and microstructure of the CNTs and CNTs/MnO₂ composite electrodes were characterized by X‐ray diffraction (XRD), scanning electron microscope‐energy dispersive spectroscopy (SEM‐EDS), X‐ray photoelectron spectroscopy (XPS), and Raman spectroscopy. It was found that 1‐μm thick MnO₂ film covered the surface of CNTs sheet with MnO₂ mass loading of 0.125 mg/cm². CNTs/MnO₂ composite was tested as electrode materials for supercapacitors in sulfate media (1‐M H₂SO₄ and Na₂SO₄) by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD). The results obtained showed that CNTs/MnO₂ composite electrode displayed good electrochemical performance in 1‐M Na₂SO₄, while the chemical stability of MnO₂ film was highly affected due its dissolution in acidic medium. A specific capacitance of 940 F/g was retained (with a capacitance retention of about 80%) after 3000 GCD cycles. CNTs/MnO₂ all‐solid symmetric supercapacitor using PVA/H₃PO₄ gel electrolyte exhibited an initial specific capacitance 80 F/g and decreased by 25% after 3000 cycles.     

The structural and electrical properties of Ga-doped ZnO and Ga, B-codoped ZnO thin films: The effects of additional boron impurity

Transparent conducting films of Ga-doped ZnO (GZO) and Ga-, B-codoped ZnO (GZOB) were deposited by dc magnetron sputtering. The dependence of the electrical and structural properties on the type of doping (Ga-doping or Ga-, B-codoping) and substrate temperature were investigated. Microstructural analysis suggests that the substrate temperature and the type of doping modify the microstructure and surface morphology of thin films. GZOB films grown at 200 °C showed a dense structure without columns, a low-resistivity value of 4.2×10⁻⁴ Ω cm, and a visible transmission of 90% with a thickness of 200 nm. In addition, the thermal stability of resistivity of GZOB films was greater than one of GZO films.     

TCO and light trapping in silicon thin film solar cells

For thin film silicon solar cells and modules incorporating amorphous (a-Si:H) or microcrystalline (μc-Si:H) silicon as absorber materials, light trapping, i.e. increasing the path length of incoming light, plays a decisive role for device performance. This paper discusses ways to realize efficient light trapping schemes by using textured transparent conductive oxides (TCOs) as light scattering, highly conductive and transparent front contact in silicon p–i–n (superstrate) solar cells. Focus is on the concept of applying aluminum-doped zinc oxide (ZnO:Al) films, which are prepared by magnetron sputtering and subsequently textured by a wet-chemical etching step. The influence of electrical, optical and light scattering properties of the ZnO:Al front contact and the role of the back reflector are studied in experimentally prepared a-Si:H and μc-Si:H solar cells. Furthermore, a model is presented which allows to analyze optical losses in the individual layers of a solar cell structure. The model is applied to develop a roadmap for achieving a stable cell efficiency up to 15% in an amorphous/microcrystalline tandem cell. To realize this, necessary prerequisites are the incorporation of an efficient intermediate reflector between a-Si:H top and μc-Si:H bottom cell, the use of a front TCO with very low absorbance and ideal light scattering properties and a low-loss highly reflective back contact. Finally, the mid-frequency reactive sputtering technique is presented as a promising and potentially cost-effective way to up-scale the ZnO front contact preparation to industrial size substrate areas.     

Preparation of ZnO thin films using MOCVD technique with D2O/H2O gas mixture for use as TCO in silicon-based thin film solar cells

Zinc oxide (ZnO) thin films have been successfully grown by metal organic chemical vapor deposition (MOCVD) technique using deuterium water (D₂O) and water (H₂O) mixtures as oxidants for diethylzinc (DEZ). B₂H₆ was also employed as a dopant gas. It was found that the crystal orientation of ZnO films strongly depends on D₂O/H₂O ratio. As a result, the surface morphology of ZnO changed from textured surface morphology to smooth surface morphology with increase in the ratio of D₂O/H₂O. Moreover, it was also observed that the carrier concentration of ZnO films did not change with the ratio of D₂O/H₂O, while the mobility of these films was strongly dependent on the D₂O/H₂O ratio. Without D₂O addition, the resistivity of films had its lowest value and the minimum sheet resistance was 10 Ω/square. All films showed transmittance higher than 80% in the visible region. Moreover, the haze values of these films could be controlled by the ratio of D₂O/H₂O. These results indicate that the crystal orientation and surface morphology of the low resistivity ZnO films can be modified by using a mixture of D₂O and H₂O without changing the deposition temperature. Thus, the obtained ZnO films are promising for use as a front TCO layer in Si-based thin film solar cells.     

Some physical properties of ZnO thin films prepared by RF sputtering technique

ZnO thin films were deposited with RF sputtering using pure Zn target. In order to generate oxidation process of Zn, Ar:O₂ gas mixing in (9:1), (7:3) and (5:5) ratios of Ar:O₂ was used. To characterize ZnO thin films thickness and transparency were measured using optical method, and refractive index and band gap energies were calculated. Electrical conductivity of the ZnO thin films was also determined. AFM images were used to determine surface morphology of produced ZnO thin films.     

Growth and characterization of ZnO thin films prepared by electrodeposition technique

ZnO thin films were deposited on either indium tin oxide-coated glass or copper substrate by the electrodeposition process, using zinc chloride and flowing air as precursors. The effect of pH on the structural and morphological ZnO films was studied and the optimum deposition conditions have been outlined. The kinetics of the growth of the films have been investigated. We note that the rate of deposition of ZnO in an acidic solution was larger than in a basic solution. The structure of the films was studied using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The surface morphology and thickness of the films were determined using scanning electron microscopy. The X-ray diffraction analysis shows that the films are polycrystalline with hexagonal crystal structure (zincite) at pH 4. The optical transmittance of ZnO decreases with varying film thickness. The optical energy bandgap was found to be 3.26 eV.     

Optical properties of tungsten and titanium oxide thin films prepared by plasma sputter deposition

Tungsten oxide and titanium oxide thin films were prepared by RF reactive magnetron sputter deposition. The stationary and rotating substrate holders were applied to analyze the rotating effect. The optical properties and thicknesses of oxide films were determined by a proposed optical model and the measured transmittance spectra. The dispersed refractive indices of thin films have a wide range distribution in different sputtering conditions. In the situation of rotating substrate holder, the refractive index was lower than that of the stationary substrate holder. Also, amorphous TiO₂ structure can be prepared by using rotating substrate holder. The transmittance spectrum of crystalline TiO₂ reveals that the textured structure on the film surface affects the transmittance characteristic.     

Plasma-enhanced chemical vapor deposition of zinc oxide at atmospheric pressure and low temperature

The plasma-enhanced chemical vapor deposition of aluminum-doped zinc oxide has been demonstrated for the first time at 800 Torr and under 250 °C. A film resistivity of and a transparency of 95% from 375 to 2500 nm was obtained for deposition at 20-mTorr diethylzinc, 1.0 Torr CO₂, 799 Torr He, a TMAl/DEZn ratio of 1:100, RF power, and 225 °C. The average aluminum concentration in the ZnO film was . It was found that, while the growth rate did not change with substrate temperature, both the resistivity and optical absorption coefficient declined with increasing temperature.     

Electrical and optical properties of F-doped textured SnO2 films deposited by APCVD

This paper presents the structural, electrical and optical properties of transparent conducting F-doped textured SnO₂ films prepared by atmosphere pressure chemical vapour deposition (APCVD). Polycrystalline SnO₂:F films having a variable preferred orientation have been obtained with resistivity as low as 5 × 10⁻⁴ Ωcm, with carrier concentrations between 3.5 × 10²⁰ and 7 × 10²⁰ cm⁻³, and Hall mobilities from 15.7 to 20.1 cm²/V/s. The average transmittance (including diffusion transmittance) is as high as 94% in the wavelength range of the visible spectrum and the maximum infrared reflectance reaches 92% for a film 655 nm thick. The figure of merit ƒ_TC = T10/_sh, (7.12 × 10⁻² S) of these films is the highest amongst the results reported on doped SnO₂ films.     

High quality textured ZnO:Al surfaces obtained by a two-step wet-chemical etching method for applications in thin film silicon solar cells

ZnO:Al films deposited at 250 °C on Corning glass by radio frequency magnetron sputtering were studied for their use as front contact for thin film silicon solar cells. For this purpose, a two-step etching method combining different concentrations of diluted hydrochloric acid (from 0.1% to 3%) with different etching times was developed. Its influence on morphological, electrical and optical properties of the etched films was evaluated. This new etching method led to more uniform textured surfaces, where the electrical properties remained unchangeable after the etching process, and with adapted light scattering properties similar to those exhibited by commercial substrates.     

Electrodeposition of ZnO thin films by using molecular oxygen and hydrogen peroxide as oxygen precursors: Structural and optical properties

Zinc oxide thin films were potentiostatically electrodeposited from a ZnCl₂+LiCl bath using two different oxygen precursors: molecular oxygen and hydrogen peroxide. X-ray diffraction (XRD) studies confirmed the presence of the ZnO wurtzite structure with marked preferential orientation along the (0 0 2) axis. The optical transmittance shows a clear absorption edge in the ultraviolet (UV) region which corresponds to an energy band gap of 3.41±0.03 eV. As a general rule the higher band gap energies are related to the more transparent films.