The crystalline structure,conductivity and optical properties of Co-doped ZnO thin films

Transparent conductive Co-doped ZnO thin films were deposited by ultrasonic spray technique. Conditions of preparation have been optimized to get good quality. A set of cobalt (Co)-doped ZnO (between 0 and 3 wt%) thin films were grown on glass substrate at 350 °C. The thin films were annealed at 500 °C for improvement of the physical properties. Nanocrystalline films with hexagonal wurtzite structure and a strong (0 0 2) preferred orientation were obtained. The maximum value of grain size G = 63.99 nm is attained with undoped ZnO film. The optical transmissions spectra showed that both the undoped and doped ZnO films have transparency within the visible wavelength region. The band gap energy decreased after doping from 3.367 to 3.319 eV when Co concentration increased from 0 to 2 wt% with slight increase of electrical conductivity of the films from 7.71 to 8.33 (Ω cm)⁻¹. The best estimated structure, optical and electrical results are achieved in Co-doped ZnO film with 2 wt%.     

The effects of solvent nature on spray-deposited ZnO thin film prepared from Zn (CH3COO)2, 2H2O

Transparent conducting zinc oxide was deposited on glass substrate by ultrasonic spray method. The ZnO samples with concentration of 0.1 M were deposited at 300, 350 and 400 °C with 2 min of deposition time. The effects of substrate temperature, ethanol and methanol solution on the structural, electrical and optical properties were examined. The DRX analyses indicated that ZnO films have polycrystalline nature and hexagonal wurtzite structure with (1 0 0) and (0 0 2) preferential orientation corresponding to ZnO films resulting from methanol and ethanol, respectively. The crystallinity of the thin films improved with ethanol solution. All films exhibit an average optical transparency about 80%, in the visible range. The band gap energy of ZnO films obtained with methanol solution higher than of ethanol solution for all the films. The electrical resistivity decrease with ZnO obtained from ethanol indicated; due to the maximum crystallite size retched at this point.     

Influence of substrate temperature and Cobalt concentration on structural and optical properties of ZnO thin films prepared by Ultrasonic spray technique

Pure and Cobalt doped zinc oxide were deposited on glass substrate by Ultrasonic spray method. Zinc acetate dehydrate, Cobalt chloride, 4-methoxyethanol and monoethanolamine were used as a starting materials, dopant source, solvent and stabilizer, respectively. The ZnO samples and ZnO:Co with Cobalt concentration of 2 wt.% were deposited at 300, 350 and 400 °C. The effects of substrate temperature and presence of Co as doping element on the structural, electrical and optical properties were examined. Both pure and Co doped ZnO samples are (0 0 2) preferentially oriented. The X-ray diffraction results indicate that the samples have polycrystalline nature and hexagonal wurtzite structure with the maximum average crystallite size of ZnO and ZnO:Co were 33.28 and 55.46 nm. An increase in the substrate temperature and presence doping the crystallinity of the thin films increased. The optical transmittance spectra showed transmittance higher than 80% within the visible wavelength region. The band gap energy of the thin films increased after doping from 3.25 to 3.36 eV at 350 °C.     

Effect of doping on structural,optical and electrical properties of nanostructure ZnO films deposited onto a-Si:H/Si heterojunction

We investigated the structural; optical and electrical properties of ZnO thin films as the n-type semiconductor for silicon a-Si:H/Si heterojunction photodiodes. The ZnO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of doping on device performance. The results show that the X-ray diffraction (XRD) spectra revealed a preferred orientation of the crystallites along c-axis. SEM images show that all films display a granular, polycrystalline morphology and the ZnO:Al exhibits a better grain uniformity. The transmittance of the doped films was found to be higher when compared to undoped ZnO. A low resistivity of the order of 2.8 × 10⁻⁴ Ω cm is obtained for ZnO:Al using 0.4 M concentration of zinc acetate. The photoluminescence (PL) spectra exhibit a blue band with two peaks centered at 442 nm (2.80 eV) and 490 nm (2.53 eV). It is noted that after doping the ZnO films a shift of the band by 22 nm (0.15 eV) is recorded and a high luminescence occurs when using Al as a dopant. Dark I–V curves of ZnO/a-Si:H/Si structure showed large difference, which means there is a kind of barrier to current flow between ZnO and a-Si:H layer. Doping films was applied and the turn-on voltages are around 0.6 V. Under reverse bias, the current of the ZnO/a-Si:H/Si heterojunction is larger than that of ZnO:Al/a-Si:H/Si. The improvement with ZnO:Al is attributed to a higher number of generated carriers in the nanostructure (due to the higher transmittance and a higher luminescence) that increases the probability of collisions.     

Deposition and characterization of transparent and conductive sprayed ZnO:B thin films

Highly transparent and conductive Boron doped zinc oxide (ZnO:B) thin films were deposited using chemical spray pyrolysis (CSP) technique on glass substrate. The effect of variation of boron doping concentration in reducing solution on film properties was investigated. Low angle X-ray analysis showed that the films were polycrystalline fitting well with a hexagonal wurtzite structure and have preferred orientation in [002] direction. The films with resistivity 2.54×10⁻³ Ω-cm and optical transmittance >90% were obtained at optimized boron doping concentration. The optical band gap of ZnO:B films was found ∼3.27 eV from the optical transmittance spectra for the as-deposited films. Due to their excellent optical and electrical properties, ZnO:B films are promising contender for their potential use as transparent window layer and electrodes in solar cells.     

Influence of annealing temperature on structural and optical properties of ZnO: In thin films prepared by ultrasonic spray technique

Transparent conducting indium doped zinc oxide was deposited on glass substrate by ultrasonic spray method. The In doped ZnO samples with indium concentration of 3 wt.% were deposited at 300, 350 and 400 °C with 2 min of deposition time. The effects of substrate temperature and annealing temperature on the structural, electrical and optical properties were examined. The DRX analyses indicated that In doped ZnO films have polycrystalline nature and hexagonal wurtzite structure with (0 0 2) preferential orientation and the maximum average crystallite size of ZnO: In before and annealed at 500 °C were 45.78 and 55.47 nm at a substrate temperature of 350 °C. The crystallinity of the thin films increased by increasing the substrate temperature up 350 °C, the crystallinity improved after annealing temperature at 500 °C. The film annealed at 500 °C and deposited at 350 °C show lower absorption within the visible wavelength region. The band gap energy increased from E_g = 3.25 to 3.36 eV for without annealing and annealed films at 500 °C, respectively, indicating that the increase in the transition tail width. This is due to the increase in the electrical conductivity of the films after annealing temperature.     

Correlation between the structural and optical properties of Co doped ZnO thin films prepared at different film thickness

Cobalt doped zinc oxide (ZnO:Co) thin films were deposited on glass substrates by ultrasonic spray technique decomposition of Zinc acetate dihydrate and cobalt acetate tetrahydrate in an ethanol solution with film thickness. All films are polycrystalline with a hexagonal wurtzite-type structure with a preferential orientation according to the direction (0 0 2), with the maximum crystallite size was found of 59.42 nm at 569 nm. The average transmittance of all films is about 65–95% measured by UV–vis analyzer. The band gap energy increased from 3.08 to 3.32 eV with increasing the film thickness from 192 to 569 nm. The increase of the electrical conductivity with increases in the film thickness to maximum value of 9.27 (Ω cm)⁻¹ can be explained by the increase in carrier concentration and displacement of the electrons of the films. The correlation between the band gap and crystal structure suggests that the band gap energy of Co doped ZnO is influenced by the crystallite size and the mean strain.     

Preparation and characterization of Al doped ZnO thin films by PLD

This paper describes the effect of doping on the composition, surface morphology and optical, structural and electrical properties of Al doped ZnO thin films by pulsed laser deposition. SEM analysis shows that the crystalline nature of the deposited films decreases with an increase of Al doping concentration from 1% to 6%. In the AFM analysis, the surface roughness of the deposited films increases by increasing the doping concentration of Al. Al doping strongly influences the optical properties of the ZnO thin films. Optical transmittance spectra show a very good transmittance in the visible region (450–700 nm). The calculated optical band gap was found to be in the range from 3.405 to 3.464 eV. Structural analysis confirms that the increases of Al concentration decrease the crystallinity of the ZnO films and the particle size decreases from 45.7±0.09 to 28.0±0.02 nm. In the Raman analysis, the active mode of Al(=1%) doped ZnO films were observed at 434.81 cm⁻¹. The shifts of the active mode (_E2)$\left(E_2\right)$ show the presence of tensile stress in the deposited films. The electrical properties of the deposited films showed that the values of the Hall mobility was in the range between 2.51 and 10.64 cm²/V s and the carrier concentration between 15.7 and 0.78×10¹⁷ and the resistivity values between 1.59 and 10.97 Ωcm, depending on the doping concentration.     

Microstructure and optical properties of nanocrystalline ZnO and ZnO:(Li or Al) thin films

Zinc oxide thin films (ZnO, ZnO:Li, ZnO:Al) were deposited on glass substrates by a sol-gel technique. Zinc acetate, lithium acetate, and aluminum chloride were used as metal ion sources in the precursor solutions. XRD analysis revealed that Li doped and undoped ZnO films formed single phase zincite structure in contrast to Al:ZnO films which did not fully crystallize at the annealing temperature of 550 °C. Crystallized films had a grain size under 50 nm and showed c-axis grain orientation. All films had a very smooth surface with RMS surface roughness values between 0.23 and 0.35 nm. Surface roughness and optical band tail values increased by Al doping. Compared to undoped ZnO films, Li doping slightly increased the optical band gap of the films.     

Optical and electrical properties of aluminum-doped ZnO thin films grown by pulsed laser deposition

Transparent aluminum-doped zinc oxide (AZO) thin films were deposited on quartz glass substrates by pulsed laser deposition (PLD) from ablating Zn-Al metallic targets. The structural, electrical and optical properties of these films were characterized as a function of Al concentration (0-8 wt.%) in the target. Films were deposited at a low substrate temperature of 150 °C under 11 Pa of oxygen pressure. It was observed that 2 wt.% of Al in the target (or 1.37 wt.% of Al doped in the AZO film) is the optimum concentration to achieve the minimum film resistivity and strong ultraviolet emission. The presence of Al in the ZnO film changes the carrier concentration and the intrinsic defects.     

Highly conductive and transparent laser ablated nanostructured Al: ZnO thin films

Al doped ZnO thin films are prepared by pulsed laser deposition on quartz substrate at substrate temperature 873 K under a background oxygen pressure of 0.02 mbar. The films are systematically analyzed using X-ray diffraction, atomic force microscopy, micro-Raman spectroscopy, UV-vis spectroscopy, photoluminescence spectroscopy, z-scan and temperature-dependent electrical resistivity measurements in the temperature range 70-300 K. XRD patterns show that all the films are well crystallized with hexagonal wurtzite structure with preferred orientation along (0 0 2) plane. Particle size calculations based on XRD analysis show that all the films are nanocrystalline in nature with the size of the quantum dots ranging from 8 to 17 nm. The presence of high frequency E₂ mode and longitudinal optical A₁ (LO) modes in the Raman spectra suggest a hexagonal wurtzite structure for the films. AFM analysis reveals the agglomerated growth mode in the doped films and it reduces the nucleation barrier of ZnO by Al doping. The 1% Al doped ZnO film presents high transmittance of ∼75% in the visible and near infrared region and low dc electrical resistivity of 5.94 × 10⁻⁶ Ω m. PL spectra show emissions corresponding to the near band edge (NBE) ultra violet emission and deep level emission in the visible region. Nonlinear optical measurements using the z-scan technique shows optical limiting behavior for the 5% Al doped ZnO film.     

Influence of strain/stress on the nonlinear-optical properties of sprayed deposited ZnO:Al thin films

Nanocrystalline ZnO:Al thin films were deposited by reactive chemical pulverization spray pyrolysis technique on heated glass substrates at 450 °C to study their crystalline structure, composition, strain, stress, roughness characteristics and nonlinear optical susceptibility as a function of Al concentration (0, 2, 3, 5 at.%). The films were characterized by X-ray diffractometer (XRD), EDAX 9100 analyser, atomic force microscopy (AFM) and third harmonic generation (THG). The Al (3 at.%) doped ZnO thin films exhibited the lower strain/stress than undoped films. The nonlinear properties of the ZnO:Al thin films have been found to be influenced by the films strain/stress.     

Boron doped ZnO thin films fabricated by RF-magnetron sputtering

By using the radio frequency-magnetron sputtering (RF-MS) method, both pure ZnO and boron doped ZnO (ZnO:B) thin films were deposited on glass substrates at ambient temperature and then annealed at 450 °C for 2 h in air. It is found that both ZnO and ZnO:B thin films have wurtzite structure of ZnO with (0 0 2) preferred orientation and high average optical transmission (≥80%). Compared with the resistivity of 6.3 × 10² Ω cm for ZnO film, both as-deposited and annealed ZnO:B films exhibit much lower resistivity of 9.2 × 10⁻³ Ω cm and 7.5 × 10⁻³ Ω cm, respectively, due to increase in the carrier concentration. Furthermore, the optical band gaps of 3.38 eV and 3.42 eV for as-deposited and annealed ZnO:B films are broader than that of 3.35 eV for ZnO film. The first-principles calculations show that in ZnO:B thin films not only the band gap becomes narrower but also the Fermi level shifts up into the conduction band with respect to the pure ZnO film. These are consistent with their lower resistivities and suggest that in the process of annealing some substituted B in the lattice change into interstitial B because of its smaller ion radius and this transformation widens the optical band gap of ZnO:B thin film.     

Low temperature synthesis wide optical band gap Al and (Al, Na) co-doped ZnO thin films

Al-doped ZnO (AZO) and (Al, Na) co-doped ZnO (ANZO) thin films were prepared via sol-gel technique with an annealing process at temperatures between 450 and 550 °C for 60 min in air ambient, and their structural and optical properties have been investigated. The deposited films exhibited hexagonal zinc oxide structure except annealing at 450 °C. For the 500 °C-annealed samples, the surface morphology was analyzed via scanning electron microscopy, Photoluminescence (PL) of different Na content ANZO thin films showed that there were very obvious violet and blue emission bands between 400 and 500 nm, and intensity of which were enhanced with Na content increasing. Transparency of the films was improved along with increasing Na content. The result of UV indicated the absorb bands appeared obviously red shift with Na doping into ZnO, the optical gaps of all films far beyond 3.37 eV of pure ZnO, and gradually decreased with Na content increasing, this is very virtual for improving photoelectricity performance of transparent conduct oxide (TCO) film. The possible origins responsible for structure and optical properties also had been discussed.     

Growth characteristics and properties of ZnO:Ga thin films prepared by pulsed DC magnetron sputtering

Transparent conductive ZnO:Ga thin films were deposited on Corning 1737 glass substrate by pulsed direct current (DC) magnetron sputtering. The effects of process parameters, namely pulse frequency and film thickness on the structural and optoelectronic properties of ZnO:Ga thin films are evaluated. It shows that highly c-axis (0 0 2) oriented polycrystalline films with good visible transparency and electrical conductivity were prepared at a pulsed frequency of 10 kHz. Increasing the film thickness also enlarged the grain size and carrier mobility which will subsequently lead to the decrease in resistivity. In summary, ZnO:Ga thin film with the lowest electrical resistivity of 2.01 × 10⁻⁴ Ω cm was obtained at a pulse frequency of 10 kHz with 500 nm in thickness. The surface RMS (root mean square) roughness of the film is 2.9 nm with visible transmittance around 86% and optical band gap of 3.83 eV.     

Structural, electrical and optical properties of Gd doped and undoped ZnO:Al (ZAO) thin films prepared by RF magnetron sputtering

The influence of the gadolinium doping on the structural features and opto-electrical properties of ZnO:Al (ZAO) films deposited by radio frequency (RF) magnetron sputtering method onto glass substrates was investigated. X-ray analysis showed that the films were polycrystalline fitting well with a hexagonal wurtzite structure and have preferred orientation in [0 0 2] direction. The Gd doped ZAO film with a thickness of 140 nm showed a high visible region transmittance of 90%. The optical band gap was found to be 3.38 eV for pure ZnO film and 3.58 eV for ZAO films while a drop in optical band gap of ZAO film was observed by Gd doping. The lowest resistivities of 8.4 × 10⁻³ and 10.6 × 10⁻³ Ω cm were observed for Gd doped and undoped ZAO films, respectively, which were deposited at room temperature and annealed at 150 °C.     

Structural,electrical and optical studies on spray-deposited aluminium-doped ZnO thin films

Thin films of zinc oxide (ZnO) were deposited on cleaned glass substrates by chemical spray pyrolysis technique using Zn(CH₃COO)₂ as precursor solution. Also, aluminium-doped thin films of ZnO were prepared by using AlCl₃ as doping solution for aluminium. The dopant concentration [Al/Zn atomic percentage (at%)] was varied from 0 to 1.5 at% in thin films of ZnO prepared in different depositions. Structural characterization of the deposited films was performed with X-ray diffraction (XRD) studies. It confirmed that all the films were of zinc oxide having polycrystalline nature and possessing typical hexagonal wurtzite structure with crystallite size varying between 100.7 and 268.6 nm. The films exhibited changes in relative intensities and crystallite size with changes in the doping concentration of Al. The electrical studies established that 1 at% of Al-doping was the optimum for enhancing electrical conduction in ZnO thin films and beyond that the distortion caused in the lattice lowered the conductivity. The films also exhibited distinct changes in their optical properties at different doping concentrations, including a blue shift and slight widening of bandgap with increasing Al dopant concentration.     

Microstructural, optical and electrical studies on sol gel derived ZnO and ZnO:Al films

ZnO and ZnO:Al films were deposited onto glass substrates by the sol gel method using spin coating technique. The effects of aluminum dopant on the crystalline structure and orientation of the ZnO films have been investigated by X-ray diffraction (XRD) study. Surface morphology of the films has also been analyzed by a field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The average optical transmittance values of all the films is over >83% in the visible region. The optical band gap and Urbach energy values of these films were determined. The absorption edge shifted to the lower energy depending on the Al doping level. The shift of absorption edge is associated with shrinkage effect. The electrical conductivity of the ZnO film enhanced with the Al dopant. From the temperature dependence of conductivity measurements, the activation energy of the films was also calculated.     

Optical and electrical characteristics of Al-doped ZnO thin films prepared by aqueous phase deposition

Transparent conducting Al-doped ZnO (AZO) thin films have been deposited by sol-gel route. Starting from an aqueous solution of zinc acetate by adding aluminum chloride as dopant, a c-axis oriented polycrystalline ZnO thin film 100 nm in thickness could be spin-coated on glass substrates via a two-step annealing process under reducing atmosphere. The effects of thermal annealing and dopant concentration on the structural, electrical and optical properties of AZO thin films were investigated. The post-treated AZO films exhibited a homogenous dense microstructure with grain sizes less than 10 nm as characterized by SEM photographs. The annealing atmosphere has prominent impact on the crystallinity of the films which will in turn influence the electrical conductivity. By varying the doping concentrations, the optical and electrical properties could be further adjusted. An optimal doping concentration of Al/Zn = 2.25 at.% was obtained with minimum resistivity of 9.90 × 10⁻³ Ω-cm whereas the carrier concentration and mobility was 1.25 × 10²⁰ cm⁻³ and 5.04 cm² V⁻¹ s⁻¹, respectively. In this case, the optical transmittance in the visible region is over 90%.