Conductive Zinc Oxide Thin Films Research Articles

Effect of solutions acidity on Haacke’s Figure of Merit of ZnO and ZnO:F thin films deposited by ultrasonic spray pyrolysis

Fluorine-doped zinc oxide (ZnO:F) thin films are valued for their potential as transparent conductive materials, particularly in optoelectronic applications. In this work, the deposition of highly conductive and transparent fluorine-doped ZnO thin films, deposited by chemical spray on glass substrates is reported. The effect of acetic (AcAc) in the initial fresh solution on Haacke’s Figure of Merit (Φ) of both zinc oxide (ZnO) and fluorine-doped zinc oxide (ZnO:F) thin films was studied. The substrate temperature was fixed to 450°C, and two deposition times (8 and 14 min) were tested. Accordingly, the optical and transport properties, as well as the structural and morphological characteristics of the films were measured. The results indicate that the samples are polycrystalline in all cases and exhibit a wurtzite-type structure of ZnO. The variation of AcAc in the starting solution causes a switch in preferential growth from (002) to (001). As the AcAc content increases, the surface morphology of the films reveals the formation of well-defined hexagonal grains, and the grain size increases. Conversely, the transmittance decreases as the acetic acid increases, which can attributed to carbon incorporation into the film. In addition, the band gap values of the films varied from 3.2 to 3.4 eV. Also, as the acetic acid concentration increased, a drop in the electrical resistivity of ZnO thin films on the order of 0.016 Ω⋅ cm was found for the ZnO:F films deposited with fresh solution for 14 min. This can be ascribed to a dense acetic cloud formed during the synthesis process, trapping volatile F species that incorporate into the ZnO lattice. This enhances Haacke’s Figure of Merit of ZnO:F films deposited with a fresh solution, demonstrating their potential use for application as transparent conductive films. This contrasts with other synthesis methods that require longer aging time in the precursor solution, making these findings useful for large-scale and more efficient production of transparent conductive films.

Ethyl Zinc β-Ketoiminates and β-Amidoenoates: Influence of Precursor Design on the Properties of Highly Conductive Zinc Oxide Thin Films from Aerosol-Assisted Chemical Vapour Deposition.

Highly transparent (>85 %) and conductive (1.086×10-3 Ω cm) zinc oxide thin films have been deposited from specifically selected precursors allowing us to establish a direct correlation between their molecular structure and the optoelectronic properties of the deposited films. Mono-ligated ethyl zinc compounds of varying steric bulk: [EtZn(OC(Me)CH(Me)N(i Pr))]2 (1), [EtZn(OC(OEt)CH(Me)N(i Pr))]2 (2) and [EtZn(OC(OEt)CH(CH3 )N(Dipp))]2 (3) were compared with the related bis-ligated zinc complexes [Zn(OC(Me)CH(Me)N(i Pr))2 ] (4), [Zn(OC(OEt)CH(Me)N(i Pr))2 ] (5) and [Zn(OC(OEt)CH(Me)N(Dipp))2 ] (6). In all cases bulkier ligands resulted in poorer electronic properties of deposited films, whilst all mono-ligated compounds were shown as superior precursors. All complexes were characterised by 1 H and 13 C{1 H} NMR and elemental analysis, with the structure of 6 determined by single crystal X-ray diffraction. Zinc oxide films were deposited from single and dual source (with methanol) reactions of these precursors, and analysed via XRD, XPS and EDX. Optoelectronic properties were investigated through UV/vis spectroscopy and Hall effect measurements, and morphology was examined via SEM. Tauc plots from UV/vis data indicated that Film A showed the lowest band gap of 3.31 eV. Varying the elemental composition of the precursors led to changes in the elemental composition of the resultant films, as well as changes in their structural and optoelectronic properties. Using this approach of precursor design, we have been able to tune single source precursors towards zinc oxide to deposit films with specific properties.

Hypothesis on the Influence of the Magnetic Behaviour of Hydrogen Doped Zinc Oxide during Its Plasma Sputtering Process

We investigate the complexity of the reactive sputtering of highly conductive zinc oxide thin films in the presence of hydrogen at room temperature. We report on the importance of precise geometric positioning of the substrate with respect to the magnetron to achieve maximum conductivity. We examine the electrical properties of the deposited thins films based on their position on the substrate holder relative to the magnetron. By considering early reports by other researchers on the angular dependency of plasma parameters and the effect of hydrogen doping on electric and magnetic properties of hydrogen-doped zinc oxide, we propose a hypothesis on the possibility of such properties resulting in the observations presented in this report pending further tests to verify this hypothesis. Overall, in this report we present the guide by which highly conductive zinc oxide thin film coatings can be prepared via RF sputtering with hydrogen presence along with argon as the sputtering gas.

Highly Conductive Zinc Oxide Based Transparent Conductive Oxide Films Prepared Using RF Plasma Sputtering Under Reducing Atmosphere

The spectral properties and colour functions of a radio frequency (RF)-based sputtering plasma source was monitored during consecutive sputter deposition of zinc doped indium oxide (IZO) thin films under argon and argon/hydrogen mix. The effect of target exposure to the hydrogen gas on charge density/mobility and spectral transmittance of the deposited films was investigated. We demonstrate that consecutive exposure to the hydrogen gas during the deposition process progressively affects the properties of thin films with a certain degree of continuous improvement in electrical conductivity while demonstrating that reverting to only argon from argon/ hydrogen mix follows a complex pathway, which has not been reported previously in such detail to our knowledge. We then demonstrate that this effect can be used to prepare highly conductive zinc oxide thin films without indium presence and as such eliminating the need for the expensive indium addition. We shall demonstrate that complexity observed in emission spectra can be simply identified by monitoring the colour of the plasma through its colour functions, making this technique a simple real-time monitoring method for the deposition process.

Resistivity transients in solution‐processed transparent ZnO thin films as a function of UV illumination wavelength

Transparent conductive zinc oxide thin films are deposited by chemical bath deposition (CBD) followed by exposure to UV‐visible light to increase their electrical conductivity. Selected spectral bandwidths are filtered from a Hg(Xe) lamp in the range of 280–546 nm to investigate resistivity transients as a function of the illumination wavelength. The conductivity increases by a factor of ∼103 for illumination wavelengths shorter than the fundamental absorption edge, as predicted by the ionized oxygen species adsorption model. If the thin film thickness exceeds the UV radiation penetration depth significantly (≳1 µm), the conductivity can most effectively be increased by exposing the films to wavelengths situated in the Urbach tail at ∼365 nm as both, absorptance and transmittance lie above 20%, enabling the radiation to penetrate the whole layer, while still locally generating enough electron‐hole pairs for the desorption of chemisorbed species.

Deposition of high-electron-mobility transparent conducting zinc oxide thin films by DC magnetron sputtering in gas mixture of argon and hydrogen

Transparent conducting zinc oxide thin films having high electron-mobility are deposited on glass substrates by DC magnetron sputtering in gas mixture of argon and hydrogen at room temperature. Introducing a little amount of hydrogen gas into sputtering Ar gas can remarkedly improve the electron mobility in ZnO thin films. With gas flow rate ratios of hydrogen to argon range from 7.2 % to 19.2 %, ZnO films with hydrogen (ZnO:H) have stable and low resistivity of 6.6×10-4 Ω.cm. At H2/Ar flow rate ratio 7.2 %, electron Hall mobility in ZnO:H film reaches a maximum value of 61 cm2.V-1.s-1. This value is much higher than the one of 23 cm2.V-1.s-1 in pure ZnO films under the same deposition condition. Morever, electron density of 1.51020 cm-3 in ZnO:H films is also higher than the one of 6×1019 cm-3 in pure ZnO films. XRD and FESEM show that the average crystallinegrain size in ZnO:H films are larger than the one in pure ZnO films. The 600-nm-thick ZnO:H films (substrate included) have average transmission of 83 % in the wide wavelength range of 380-1100 nm and low sheet resistance of 11 /square.

Property enhancement of transparent conducting zinc oxide thin films—Effect of simultaneous (Sn+F) doping

Undoped and simultaneously (Sn+F) doped ZnO thin films were fabricated using a simplified spray pyrolysis technique and the effects of Sn doping level on their electrical, structural, optical and surface morphological properties were studied. The XRD patterns confirmed the hexagonal wurtzite structure of ZnO. The minimum electrical resistivity of 0.45×10−2Ωcm was obtained for ZnO films having Sn+F doping levels of 8+20at%. All the films exhibited average optical transmittance of 85% in the visible region, suitable for transparent electrode applications. The overall quality of the fabricated films was confirmed from photoluminescence (PL) studies. The PL and surface morphological studies along with the elemental analysis showed the increase of Sn diffusion into the ZnO lattice which was consistent with the concentration of Sn in the starting solution. The results of the analysis of physical properties of simultaneously doped ZnO films proved that these films might be considered as promising candidates for solar cells and other opto-electronic applications.

Atmospheric pressure based electrostatic spray deposition of transparent conductive ZnO and Al-doped ZnO (AZO) thin films: Effects of Al doping and annealing treatment

Zinc oxide (ZnO) and Al-doped ZnO (AZO) thin films were prepared by electrostatic spray deposition method at atmospheric pressure followed by annealing. The effects of annealing and Al doping on the structural, electrical and optical properties of the films were investigated. The results show that films have random orientation with compact hexagonal wurtzite structure. It also implies that the annealing and the Al doping help to improve the electrical conductivity and optical properties as well. The minimum value of resistivity was 1.10 × 10−4 Ω cm for 0.5 at. % AZO film and transmittance was greater than 96% in the visible region. The present value of resistivity is comparable to the lowest values for AZO films reported in open literatures. All the films prepared by this method have a good crystalline structure and homogenous surface. We proposed that the substitution of Al in the ZnO lattice has positive effects in terms of increasing the free electron concentration. At atmospheric pressure, the electrospraying method was confirmed to be suitable for the preparation of AZO films with low resistivity and high transmittance.

Highly conducting zinc oxide thin films achieved without postgrowth annealing

Heavily doped zinc oxide thin films were grown by radio frequency magnetron sputtering. High level of Al doping has been achieved at high growth temperatures (TG), thereby avoiding the need for postgrowth annealing. It is further shown that due to ionized impurity scattering, mobility decreases with increasing TG. Optical transmission spectra show that the bandgap varies from 3.22 to 3.68 eV with increasing Al content due to the combined effect of Burstien–Moss effect and bandgap renormalization.

Influences in high quality zinc oxide films and their photoelectrochemical performance

Transparent conductive zinc oxide thin films have been synthesized by spray pyrolytic decomposition of aqueous solution of zinc acetate onto the corning glasses with different pre-heating temperatures. The influence of pre-heating temperature on the structural, morphological, optoelectronic and electrical properties is investigated. The X-ray diffraction patterns revealed the polycrystalline nature with hexagonal (wurtzite) crystal structure. Films show preferred c-axis orientated with (0 0 2) growth. Films deposited at various deposition temperatures exhibit different surface morphologies. It is observed that the temperature plays an important role in modifying surface morphology and size of crystallites. Compact hexagonal platelets like structure is observed for lower temperature samples. The films exhibit nanoparticles of size 60–100 nm for 250–500 °C temperatures. Films are highly transparent and conducting, we achieved 95% transmittance at 550 nm with electrical resistivity 0.062 Ω cm and figure of merit 7.88 × 10 −4 □/Ω. Moreover samples are photoelectrochemically active and exhibit the highest photocurrent of 212 μA, a photovoltage of 353 mV and 0.32 fill factor for the 450 °C films.

Effect of thickness on structural, optical and electrical properties of nanostructured ZnO thin films by spray pyrolysis

Transparent conducting zinc oxide thin films were prepared by spray pyrolytic decomposition of zinc acetate onto glass substrates with different thickness. The crystallographic structure of the films was studied by X-ray diffraction (XRD). XRD measurement showed that the films were crystallized in the wurtzite phase type. The grain size, lattice constants and strain in films were calculated. The grain size increases with thickness. The studies on the optical properties show that the direct band gap value increases from 3.15 to 3.24 eV when the thickness varies from 600 to 2350 nm. The temperature dependence of the electrical conductivity during the heat treatment was studied. It was observed that heat treatment improve the electrical conductivity of the ZnO thin films. The conductivity was found to increase with film thickness.

Annealing effect on properties of transparent and conducting ZnO thin films

This work presents the effect of postdeposition annealing on the structural, electrical and optical properties of undoped ZnO (zinc oxide) thin films, prepared by radio-frequency sputtering method. Two samples, 0.17 and 0.32 µm-thick, were annealed in vacuum from room temperature to 350 °C while another 0.32 µm-thick sample was annealed in air at 300 °C for 1 h. X-ray diffraction analysis revealed that all the films had a c-axis orientation of the wurtzite structure normal to the substrate. Electrical measurements showed that the resistivity of samples annealed in vacuum decreased gradually with the increase of annealing temperature. For the 0.32 µm-thick sample, the gradual decrease of the resistivity was essentially due to a gradual increase in the mobility. On the other hand, the resistivity of the sample annealed in air increased strongly. The average transmission within the visible wavelength region for all films was higher than 80%. The band gap of samples annealed in vacuum increased whereas the band gap of the one annealed in air decreased. The main changes observed in all samples of this study were explained in terms of the effect of oxygen chemisorption and microstructural properties.

Gallium doping in transparent conductive ZnO thin films prepared by chemical spray pyrolysis

Zinc oxide (ZnO) and ZnO : Ga films have been deposited by the spray pyrolysis method onto preheated glass substrates using zinc acetate and gallium nitrate as precursors for Zn and Ga ions, respectively. The effect of Ga doping on the structural, morphological, optical and electrical properties of sprayed ZnO thin films were investigated using x-ray diffraction (XRD), scanning electron microscopy, optical absorption, photoluminescence (PL) and Hall effect techniques. XRD studies reveal that films are polycrystalline with hexagonal (wurtzite) crystal structure. The thin films were oriented along the (0 0 2) plane. Room temperature PL measurements indicate that the deposited films exhibit proper doping of Ga in ZnO lattice. The average transparency in the visible range was around ∼85–95% for typical thin film deposited using 2 at% gallium doping. The optical band gap increased from 3.31 to 3.34 eV with Ga doping of 2 at%. The addition of gallium induces a decrease in electrical resistivity of the ZnO : Ga films up to 2 at% gallium doping. The highest figure of merit observed in this present study was 3.09 × 10−3 cm2 Ω−1.

Transparent conducting zinc oxide thin films doped with aluminum and molybdenum

Undoped ZnO, ZnO:Al (0.5, 1, and 2wt% Al2O3), and ZnO:Mo (2wt% Mo) films were deposited by radio-frequency magnetron sputtering. Optimal deposition temperature was found to be ∼200°C for all films. Electron mobilities of 48cm2V−1s−1 were achieved for undoped ZnO films using a sputtering gas with H2∕Ar ratio of 0.3%; corresponding carrier concentrations were ∼3×1019cm−3. A target incorporating 0.5wt% Al2O3 in ZnO yielded films with mobility of 36cm2V−1s−1 and carrier concentration of 3.4×1020cm−3. These films present comparable conductivity and lower free-carrier absorption than films grown from a target containing 2wt% Al2O3. Mo was found to be an n-type dopant of ZnO, though electrical and optical properties were inferior to those of ZnO:Al. Temperature-dependent Hall measurements of ZnO:Al films show evidence of a different scattering mechanism than ZnO:Mo films.

Highly c-axis oriented ZnO film on silica glass using zinc naphthenate precursor

High transmittance and good electrical conductivity thin conducting oxides (TCOs) have potential use in a variety of technological applications including plasma display panels (PDPs), electroluminescent devices, flat cathode ray tube (CRTs) and solar cells [1–4]. Recently, zinc oxide (ZnO) thin films have emerged as one of the most promising oxide materials owing to their optical and electrical properties, together with their high chemical and mechanical stability. ZnO is a wide band gap (3.3 eV) semiconducting, piezoelectric, and photoconducting material. In particular ZnO films with preferential orientation along the c-axis have been shown to work as SAW devices because of large piezoelectric constant. Ohyama et al. [5] prepared ZnO films on silica glass substrates from zinc acetate sol containing 2-methoxyethanol and monoethanolamine (MEA) with extremely preferred orientation along (002) plane. Ohya et al. [6] reported transparent, electrically conductive ZnO thin films with polycrystalline structure from zinc acetate and alkoxide. Recently Wessler et al. [7] reported textured ZnO thin films on (0001) sapphire by spin-coating a sol of acetate and ethanolamine. In this letter, the preparing of highly c-axis oriented ZnO thin film on amorphous silica substrates from a zinc naphthenate precursor is reported. Crystallinity, surface morphology, surface roughness, and optical behavior were studied. A coating solution was prepared using zinc naphthenate (Nihon Kagaku Sangyo Co., Ltd., Japan) and by diluting the solution with toluene (concentration: 4 wt% metal/100 mL sol). Metal naphthenates are more advantageous than metal alkoxides as starting materials, in terms of cost, stability in air, and ease of handling. Preparation of the coating solution using zinc naphthenate was easily performed by the addition of toluene, while complicated procedures were needed for the preparation of sol from zinc alkoxide [6]. The starting solution was spin-coated onto the cleaned substrate at 1500 rpm for 10 s in air. The asdeposited film was prefired at 500 ◦C for 10 min in air. The coating process was repeated five times to prepare

Characteristics of transparent and conductive undoped ZnO thin films obtained by chemical spray using zinc pentanedionate

Zinc oxide (ZnO) thin films without intentionally doping were prepared from 2,4-zinc pentanedionate by the chemical spray technique. Electrical, structural and morphological properties of these films as a function of the substrate temperature were studied. A resistivity of 2×10 −1 Ω cm, a mobility of 11 cm 2/V s, a carrier concentration of 5×10 19/cm 3 and an average transmittance of 85% in the visible range were obtained for undoped ZnO films prepared at 500 °C, with no post-annealing process. Also, the morphology varies with the substrate temperature. It was found that the surface shows a texture suitable for light trapping, as well.

Transparent conducting ZnO thin films prepared on low temperature substrates by chemical vapour deposition using Zn(C 5H 7O 2) 2

Transparent and conducting zinc oxide (ZnO) films have been prepared on glass substrates by atmospheric and low pressure chemical vapour deposition (CVD) using zinc acetylacetonate (Zn(C 5H 7O 2) 2) and various oxygen sources such as air, water (H 2O) and hydrogen peroxide (H 2O 2). A resistivity of (4–6) × 10 −3 Ω cm and an average transmittance above 80% in the visible range were obtained for undoped ZnO thin films prepared at a substrate temperature of 550°C using H 2O or H 2O 2. It was found that H 2O and H 2O 2, which include hydrogen, were better oxygen sources than an oxygen-containing gas such as air. The deposition rate of ZnO films was controlled by the Zn(C 5H 7O 2) 2 temperature. Al-doped ZnO films with a resistivity as low as 4.6 × 10 −3 Ω cm were prepared at a substrate temperature of 350°C by CVD at a low pressure of 60 Torr using aluminium acetylacetonate (Al(C 5H 7O 2) 3) as the dopant source.

Temperature-dependent studies on the electrical properties of pyrolytic ZnO thin film prepared from Zn(C2H3O2)2

Pyrolytically deposited transparent conducting zinc oxide thin films on glass substrates were successively subjected to a post-deposition heat treatment in air and in vacuum. The effects of heat treatment on the electrical transport properties were studied in detail. The films were polycrystalline in structure and the oxygen chemisorption-desorption process was found to play an important role in controlling the electronic properties. Various grain-boundary and energy-band parameters were calculated by taking conventional extrinsic semiconductor theory and grain-boundary trapping models into account. The samples were non-degenerative at room temperature and Hall mobility was found to be modulated by the grain-boundary potential barrier height via sample temperature.

Temperature effects on the electrical properties of ZnO thin films prepared by the pyrosol process

Abstract Pyrolytically deposited transparent conducting zinc oxide thin films on glass substrates have been subjected to successive post-deposition heat treatments in air and in vacuum. The effects of heat treatment on the electrical transport properties have been studied in detail. The films were polycrystalline, and oxygen chemisorption-desorption processes were found to play an important role for controlling the electronic properties. Various grain boundary and energy band parameters have been calculated by taking conventional extrinsic semiconductor theory and grain boundary trapping models into account. The samples were nondegenerate at room temperature, and the Hall mobility was found to be modulated by the grain boundary potential barrier height via the sample temperature.