The effect of annealing on Al-doped ZnO films deposited by RF magnetron sputtering method for transparent electrodes

In this study, transparent conducting Al-doped zinc oxide (AZO) films with a thickness of 150 nm were prepared on Corning glass substrates by the RF magnetron sputtering with using a ZnO:Al (Al₂O₃: 2 wt.%) target at room temperature. This study investigated the effects of the post-annealing temperature and the annealing ambient on the structural, electrical and optical properties of the AZO films. The films were annealed at temperatures ranging from 300 to 500 °C in steps of 100 °C by using rapid thermal annealing equipment in oxygen. The thicknesses of the films were observed by field emission scanning electron microscopy (FE-SEM); their grain size was calculated from the X-ray diffraction (XRD) spectra using the Scherrer equation. XRD measurements showed the AZO films to be crystallized with strong (002) orientation as substrate temperature increases over 300 °C. Their electrical properties were investigated by using the Hall measurement and their transmittance was measured by UV-vis spectrometry. The AZO film annealed at the 500 °C in oxygen showed an electrical resistivity of 2.24 × 10^− 3 Ω cm and a very high transmittance of 93.5% which were decreased about one order and increased about 9.4%, respectively, compared with as-deposited AZO film.     

Room temperature deposition of Al-doped ZnO films on quartz substrates by radio-frequency magnetron sputtering and effects of thermal annealing

High-quality Al-doped zinc oxide (AZO) thin films have been deposited on quartz substrates by radio-frequency magnetron sputtering at room temperature for thin film solar cell applications as transparent conductive oxide (TCO) electrode layers. Effects of post-deposition annealing treatment in pure nitrogen and nitrogen/hydrogen atmosphere have been investigated. Annealing treatments were carried out from 300 °C to 600 °C for compatibility with typical optoelectronic device fabrication processes. A series of characterization techniques, including X-ray diffraction, scanning electron microscopy, Hall, optical transmission, and X-ray photoelectron spectroscopy has been employed to study these AZO materials. It was found that there were significant changes in crystallinity of the films, resistivity increased from 4.60 × 10^− 4 to 4.66 × 10^− 3 Ω cm and carrier concentration decreased from 8.68 × 10²⁰ to 2.77 × 10²⁰ cm^− 3 when annealing in 400 °C pure nitrogen. Whereas there were no significant changes in electrical and optical properties of the AZO films when annealing in 300-500 °C nitrogen/hydrogen atmosphere, the electrical stability of the AZO films during the hydrogen treatment is attributed to both desorption of adsorbed oxygen from the grain boundaries and production of additional oxygen vacancies that act as donor centers in the films by removal of oxygen from the ZnO matrix. These results demonstrated that the AZO films are stably suited for TCO electrodes in display devices and solar cells.     

Electrical mechanism analysis of Al2O3 doped zinc oxide thin films deposited by rotating cylindrical DC magnetron sputtering

Cost efficient and large area deposition of superior quality Al₂O₃ doped zinc oxide (AZO) films is instrumental in many of its applications, including solar cell fabrication due to its numerous advantages over indium tin oxide (ITO) films. In this study, AZO films were prepared by a highly efficient rotating cylindrical direct current (DC) magnetron sputtering system using an AZO target, which has a target material utilization above 80%, on glass substrates in argon (Ar) ambient. A detailed analysis on the electrical, optical, and structural characteristics of AZO thin films was performed for the solar cell, as well as display applications. The properties of films were found to critically depend on deposition parameters, such as sputtering power, substrate temperature, working pressure, and film thickness. A low resistivity of ~ 5.5 × 10^− 4 Ω cm was obtained for films deposited at 2 kW, keeping the pressure, substrate temperature and thickness constant at 3 mTorr, 230 °C and ~ 1000 nm respectively. This was due to an increase in carrier mobility and large grain size. Mobility is found to be controlled by ionized impurity scattering within the grains, since the mean free path of carriers is much smaller than the grain size of the films. The AZO films showed a high transparency of ~ 90% in the long wavelength region. Our results offer a cost-efficient AZO film deposition method that can fabricate films with significant low resistivity and high transmittance that can be applied in thin-film solar cells, as well as thin film transistor (TFT) and non-volatile memory (NVM).     

The characteristics of chemical and heat stability properties of chromium-vanadium-aluminum co-doped zinc oxide films for dye-sensitized solar cells

Transparent conducting oxide thin films are used as front contact material for dye-sensitized solar cells. This study investigated the effects of chromium (Cr) and vanadium (V) contents on the chemical and heat stability properties of aluminum-doped zinc oxide (AZO) thin films using pulsed direct current magnetic sputtering on Corning 1737F glass substrates. The experimental results show that Cr and V doping is useful for improving the chemical and thermal stability of AZO films. The energy gap for AZO thin films is between 3.65 and 3.69 eV. The resistivity of the AZO:Cr:V thin film was 4.23 × 10^-4 Ω cm at a Cr/V ratio of 0.30/0.23 wt.%, deposition power of 150 W, working distance of 5.5 cm, substrate temperature of 473 K, working pressure of 0.4 Pa, and frequency of 10 kHz. This value is lesser than (and therefore superior to) the resistivity of SnO₂:F (FTO) films (6.5 × 10^-4 Ω cm), but greater than that of SnO₂:In (ITO) thin films (1.2 × 10^-4 Ω cm). The resistivity increased by about 0.27% after electrolyte etching, which is similar to the 0.16% increase observed for the ITO thin film. After a thermal cycle test at 673 K, the resistivity of the AZO:Cr:V film increased to 5.42 × 10^-4 Ω cm, which is better than the resistivity of the ITO and FTO films after the same thermal cycle.     

Synthesis of c-axis preferred orientation ZnO:Al transparent conductive thin films using a novel solvent method

Transparent aluminum doped zinc oxide (ZnO:Al, AZO) conducting thin films with a high-preferential c-axis orientation were synthesized using a new sol-gel formula. The films were deposited using a spin-coating route onto borosilicate glass substrates. We used propylene glycol methyl ether (PGME) as the solvent in place of ethylene glycol monomethyl ether (EGME), which is commonly used because it is easier to deposit onto the substrates. PGME is also superior in terms of health and safety. PGME solvent does not need to settle for several days before use and can be spin-coated as soon as the raw material and solvent are mixed. The effects of this novel solvent on the structural, morphological, electrical and optical properties are discussed using XRD, SEM, a four-point probe and UV-VIS spectrophotometry. It was found that the films produced with PGME showed a high-preferential c-axis orientation and compact microstructure in comparison films produced using EGME. The electrical resistivity of AZO thin films produced with PGME solvent was lowered to 3.474 × 10^− 3Ω cm after annealing in 95 N₂/5H₂ atmosphere. In addition, the optical transmittances of AZO thin films on glass plates were higher than 90% in the visible wavelength region.     

Characteristics of sputtered Al-doped ZnO films for transparent electrodes of organic thin-film transistor

Aluminum-doped ZnO (AZO) thin-films were deposited with various RF powers at room temperature by radio frequency (RF) magnetron sputtering method. The electrical properties of the AZO film were improved with the increasing RF power. These results can be explained by the improvement of the crystallinity in the AZO film. We fabricated the organic thin-film transistor (OTFT) of the bottom gate structure using pentacene active and poly-4-vinyl phenol gate dielectric layers on the indium tin oxide gate electrode, and estimated the device properties of the OTFTs including drain current-drain voltage (I_D-V_D), drain current-gate voltage (I_D-V_G), threshold voltage (V_T), on/off ratio and field effect mobility. The AZO film that grown at 160 W RF power exhibited low resistivity (1.54 × 10^− 3 Ω·cm), high crystallinity and uniform surface morphology. The pentacene thin-film transistor using the AZO film that's fabricated at 160 W RF power exhibited good device performance such as the mobility of 0.94 cm²/V s and the on/off ratio of ~ 10⁵. Consequently, the performance of the OTFT such as larger field-effect carrier mobility was determined the conductivity of the AZO source/drain (S/D) electrode. AZO films prepared at room temperature by the sputtering method are suitable for the S/D electrodes in the OTFTs.     

Structural and optoelectronic properties of Al-doped zinc oxide films deposited on flexible substrates by radio frequency magnetron sputtering

Al-doped zinc oxide (AZO) thin films were deposited onto flexible polyethylene terephthalate substrates, using the radio frequency (RF) magnetron sputtering process, with an AZO ceramic target (The Al₂O₃ content was about 2 wt.%). The effects of the argon sputtering pressure (in the range from 0.66 to 2.0 Pa), thickness of the Al buffer layer (thickness of 2, 5, and 10 nm) and annealing in a vacuum (6.6 × 10^− 4 Pa), for 30 min at 120 °C, on the morphology and optoelectronic performances of AZO films were investigated. The resistivity was 9.22 × 10^− 3 Ω cm, carrier concentration was 4.64 × 10²¹ cm^− 3, Hall mobility was 2.68 cm²/V s and visible range transmittance was about 80%, at an argon sputtering pressure of 2.0 Pa and an RF power of 100 W. Using an Al buffer decreases the resistivity and optical transmittance of the AZO films. The crystalline and microstructure characteristics of the AZO films are improved by annealing.     

Improvements of spatial resistivity distribution in transparent conducting Al-doped ZnO thin films deposited by DC magnetron sputtering

The relationship between two techniques developed for improving the resistivity distribution on the substrate surface in transparent conducting Al-doped ZnO (AZO) thin films prepared at a temperature of 200 °C by dc magnetron sputtering depositions (dc-MSD) using various sintered AZO targets has been investigated. One improvement method superimposes an rf component onto the dc-MSD (rf + dc-MSD). The other improvement method uses conventional dc-MSD with a low resistivity AZO target prepared under optimized conditions. An improvement of resistivity distribution resulted from a decrease in the resistivity of targets used in the preparation of AZO thin films by dc-MSD either with or without superimposing rf power. However, the resistivity distribution of AZO thin films resulting from depositions using rf-superimposed dc-MSD with lower-resistivity targets was not significantly improved over that of AZO thin films prepared by conventional dc-MSD using targets with the same low resistivities. The use of rf superimposition only resulted in improved resistivity distribution in thin films when the AZO targets had a resistivity higher than around 1 × 10^− 3 Ω cm. It should be noted that sintered AZO targets optimized for the preparation of AZO thin films with lower resistivity as well as more uniform resistivity distribution on the substrate surface tended to exhibit a lower resistivity.     

Deterioration and recovery in the resistivity of Al-doped ZnO films prepared by the plasma sputtering

A hot-cathode plasma sputtering technique was used for fabricating the highly transparent and conducting aluminum-doped zinc oxide (AZO) films on glass substrates from a disk-shaped AZO (Al₂O₃: 2 wt.%) target. Under particular conditions where the target voltage was V_T = −200 V and the plasma excitation pressure was P_S = 1.5 × 10⁻³ Torr, the lowest resistivity of 4.2 × 10⁻⁴ Ω cm was obtained at 400 nm, and this was associated with a carrier density of 8.7 × 10²⁰ cm⁻³ and a Hall mobility of 17 cm²/V s. From the annealing experiment of the AZO films in the oxygen and nitrogen gases of the atmospheric pressure it was revealed that both the oxygen vacancies and the grain boundaries in the polycrystalline AZO film played an important role in the electrical properties of the film.     

Structural and functional properties of Al:ZnO thin films grown by Pulsed Laser Deposition at room temperature

Current research on transparent conductive oxides (TCOs) is focusing on indium-free TCOs, such as Al-doped ZnO (AZO), as an alternative to indium-tin oxide. In this work, AZO thin films were grown by Pulsed Laser Deposition at room temperature in oxygen atmosphere. The O₂ pressure was varied from 0.01 Pa to 10 Pa, highlighting the effects of defect formation and oxygen vacancies on the film properties. Structural properties were characterized by X-ray diffraction and Scanning Electron Microscopy, while functional properties were characterized by measurement of electrical conductivity, Hall mobility, carrier density and optical transmission. At an optimal deposition pressure of 2 Pa, optical transparency in the visible range and minimum resistivity (4.5 ? 10^− 4 Ω cm) were found, comparable to state-of-the-art TCOs. Mean value of visible transparency was shown to increase with increasing pressure, up to 88% at a deposition pressure of 10 Pa.     

Transparent and conductive ZnO:Al thin films grown by pulsed magnetron sputtering in current or voltage regulation modes

Aluminium-doped zinc oxide (AZO) thin films have been prepared by pulsed magnetron sputtering from a ceramic oxide target in pure argon atmosphere. Sputtering processes were performed in current or voltage regulation modes at different pulsing frequencies up to 200 kHz. Several growth parameters (discharge power, substrate temperature and growth rate) as well as AZO film properties (crystalline structure, optical transmittance and electrical characteristics) have been measured and analysed as a function of the current or voltage applied, the pulsing frequency and the operation time. By adjusting these deposition parameters, AZO layers with resistivity as low as 1.0×10⁻³ Ωcm and average visible transmittance above 85% have been obtained at growth rate in the range 70-80 nm/min, at substrate temperatures below 150 °C.     

Structural, optical and electrical properties of AZO/Cu/AZO tri-layer films prepared by radio frequency magnetron sputtering and ion-beam sputtering

Highly conducting tri-layer films consisting of a Cu layer sandwiched between Al-doped ZnO (AZO) layers (AZO/Cu/AZO) were prepared on glass substrates at room temperature by radio frequency (RF) magnetron sputtering of AZO and ion-beam sputtering of Cu. The tri-layer films have superior photoelectric properties compared with the bi-layer films (Cu/AZO, AZO/Cu) and single AZO films. The effect of AZO thickness on the properties of the tri-layer films was discussed. The X-ray diffraction spectra show that all films are polycrystalline consisting of a Cu layer with the cubic structure and two AZO layers with the ZnO hexagonal structure having a preferred orientation of (0 0 2) along the c-axis, and the crystallite size and the surface roughness increase simultaneously with the increase of AZO thickness. When the AZO thickness increases from 20 to 100 nm, the average transmittance increases initially and then decreases. When the fixed Cu thickness is 8 nm and the optimum AZO thickness of 40 nm was found, a resistivity of 7.92 × 10⁻⁵ Ω cm and an average transmittance of 84% in the wavelength range of visible spectrum of tri-layer films have been obtained. The merit figure (F_TC) for revaluing transparent electrodes can reach to 1.94 × 10⁻² Ω⁻¹.     

High rate direct current magnetron sputtered and texture-etched zinc oxide films for silicon thin film solar cells

Aluminum-doped zinc oxide (AZO) films were prepared by in-line direct current (dc) magnetron sputtering on glass substrates. Four types of ceramic targets with 0.5 wt.% or 1 wt.% of aluminum oxide and different preparation methods, namely normal sintered, soft sintered and hot pressed, were employed. The influence of different target manufacturing processes, aluminum concentration and sputtering conditions on AZO films were investigated. Depending on the type of targets and deposition conditions, highly transparent films with low resistivity values in the range of 3.6-11 × 10^− 4 Ω cm were obtained. The etching behaviour in hydrochloric acid and the resulting light scattering properties of the AZO films were strongly influenced by the choice of the target and the deposition conditions. The most favourable films have been successfully applied in thin film solar cells with 1.1-μm microcrystalline silicon absorber layer leading to an initial efficiency of 7.8%.     

Ultrathin Al-doped transparent conducting zinc oxide films fabricated by pulsed laser deposition

Al-doped transparent conducting zinc oxide (AZO) films, approximately 20-110 nm-thick, were deposited on glass substrates at substrate temperatures between 200 and 300 °C by pulsed laser deposition (PLD) using an ArF excimer laser (λ = 193 nm). When fabricated at a substrate temperature of 260 °C, a 40-nm-thick AZO film showed a low resistivity of 2.61 × 10^− 4 Ω·cm, carrier concentration of 8.64 × 10²⁰ cm^− 3, and Hall mobility of 27.7 cm²/V·s. Furthermore, for an ultrathin 20-nm-thick film, a resistivity of 3.91 × 10^− 4 Ω·cm, carrier concentration of 7.14 × 10²⁰ cm^− 3, and Hall mobility of 22.4 cm²/V·s were obtained. X-ray diffraction (XRD) spectra, obtained by the θ-2θ method, of the AZO films grown at a substrate temperature of 260 °C showed that the diffraction peak of the ZnO (0002) plane increased as the film thickness increased from 20 to 110 nm. The full-width-at-half-maximum (FWHM) values were 0.5500°, 0.3845°, and 0.2979° for film thicknesses of 20, 40, and 110 nm, respectively. For these films, the values of the average transmittance in visible light wavelengths (400-700 nm) were 95.1%, 94.2%, and 96.6%, respectively. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) observations showed that even the 20-nm-thick films did not show island structures. In addition, exfoliated areas or vacant and void spaces were not observed for any of the films.     

Sol-gel derived aluminum doped zinc oxide for application as anti-reflection coating in terrestrial silicon solar cells

Sol-gel grown polycrystalline Al doped zinc oxide (AZO) thin films have been deposited on Si wafers, microscopy slide glass and fluorine doped tin oxide coated glass substrates using the spin coating technique. The atomic ratio of Al:Zn in the films is 0.2. From the X-ray diffraction investigations it is found that the preferential growth of (100) reflection peak has taken place in the 450, 550 and 600 °C annealed films. Scanning electron microscopic study has shown that the films contain well-defined grains arranged in a closely packed array. The resistivity of the 500 °C annealed film is measured to be 5 × 10^− 1 Ω cm. The films have exhibited excellent optical transmittance (~ 90%) in the 400-1100 nm wavelength range. Refractive indices (n = 1.9-1.95) of the films on Si wafer are independent of the annealing temperature. Thickness of the films produced at 4000 rpm is in the range of 58-62 nm. The refractive index and thickness of these films are nearly appropriate to cause destructive interference after reflection from front emitters of solar cells. These films have demonstrated a reflectivity value of about 3% at a wavelength of 700 nm. The AZO coated silicon solar cells possess V_oc and I_sc values of 573 mV and 237 mA, respectively.     

Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared on glass substrates by a dc magnetron sputtering (dc-MS) deposition, the obtainable lowest resistivity and the spatial resistivity distribution on the substrate surface were improved by a newly developed MS deposition method. The decrease of obtainable lowest resistivity as well as the improvement of spatial resistivity distribution on the substrate surface in Al- or Ga-doped ZnO (AZO or GZO) thin films were successfully achieved by inserting a very thin buffer layer, prepared using the same MS apparatus with the same target, between the thin film and the glass substrate. The deposition of the buffer layer required a more strongly oxidized target surface than possible to attain during a conventional dc-MS deposition. The optimal thickness of the buffer layer was found to be about 10 nm for both GZO and AZO thin films. The resistivity decrease is mainly attributed to an increase of Hall mobility rather than carrier concentration, resulting from an improvement of crystallinity coming from insertion of the buffer layer. Resistivities of 3 × 10^− 4 and 4 × 10^− 4Ω cm were obtained in 100 nm-thick-GZO and AZO thin films, respectively, incorporating a 10 nm-thick-buffer layer prepared at a substrate temperature around 200 °C.     

Effects of sputtering pressure and Al buffer layer thickness on properties of AZO films grown by rf magnetron sputtering

Transparent and conductive Al-doped (2 wt.%) zinc oxide (AZO) films were deposited on inexpensive soda-lime glass substrates by using rf magnetron sputtering at room temperature. This study analyzed the effects of argon sputtering pressure, which varied in the range from 0.46 to 2.0 Pa, on the morphological, electrical and optical properties of AZO films. The only (0 0 2) diffraction peak of the film were observed at 2θ～34.45°, exhibiting that the AZO films had hexagonal ZnO wurtzite structure, and a preferred orientation with the c-axis perpendicular to the substrate. By applying a very thin aluminum buffer layer with the thickness of 2 nm, findings show that the electrical resistivity was 9.46 × 10⁻⁴ Ω-cm, and the average optical transmittance in the visible part of the spectra was approximately 81%. Furthermore, as for 10 nm thick buffer layer, the electrical resistivity was lower, but the transmittance was decreased.     

Deposition of Al-doped ZnO films on polyethylene naphthalate substrate with radio frequency magnetron sputtering

100 nm Al-doped ZnO (AZO) thin films were deposited on polyethylene naphthalate (PEN) substrates with radio frequency magnetron sputtering using 2 wt.% Al-doped ZnO target at various deposition conditions including sputtering power, target to substrate distance, working pressure and substrate temperature. When the sputtering power, target to substrate distance and working pressure were decreased, the resistivity was decreased due to the improvement of crystallinity with larger grain size. As the substrate temperature was increased from 25 to 120 °C, AZO films showed lower electrical resistivity and better optical transmittance due to the significant improvement of the crystallinity. 2 wt.% Al-doped ZnO films deposited on glass and PEN substrates at sputtering power of 25 W, target to substrate distance of 6.8 cm, working pressure of 0.4 Pa and substrate temperature of 120 °C showed the lowest resistivity (5.12 × 10^− 3 Ω cm on PEN substrate, 3.85 × 10^− 3 Ω cm on glass substrate) and high average transmittance (> 90% in both substrates). AZO films deposited on PEN substrate showed similar electrical and optical properties like AZO films deposited on glass substrates.     

Al-doped ZnO thin films deposited by reactive frequency magnetron sputtering: H2-induced property changes

Al-doped ZnO (AZO) transparent conductive thin films have been prepared by radio-frequency magnetron sputtering with a ceramic target (98 wt.% ZnO, 2 wt.% Al₂O₃) in different Ar + H₂ ambient at a substrate temperature of 200 °C. To investigate the influence of H₂-flow on the properties of AZO films, H₂-flow was changed during the growth process with a fixed Ar-flow of 60 sccm. The results indicate that H₂-flow has a considerable influence on the transparent conductive properties of AZO films. The low resistivity in the order of 10^− 4 Ω cm and the high average transmittance more than 92% in the visible range were obtained for the samples prepared in the optimal H₂-flow range from 0.4 sccm to 1.0 sccm. In addition, the influence of H₂-flow on the structure and composition of AZO films have also been studied.     

Al-doped ZnO (AZO) films deposited by reactive sputtering with unipolar-pulsing and plasma-emission control systems

Al-doped ZnO (AZO) films were deposited on fused silica glass substrates unheated or heated at 200 °C by reactive dc sputtering using a Zn-Al alloy target with mid-frequency pulsing (50 kHz) and the plasma control unit with a feedback system of the optical emission intensity of the atomic O* line at 777 nm to control oxygen gas flow. The stable and reproducible depositions were successfully carried out in the transition region. The deposition rates attained in this study were about 10-20 times higher than the one by conventional sputtering using oxide targets. The AZO films with the lowest resistivity of 3.8 × 10^− 4 Ω cm was deposited on the substrate heated at 200 °C with a sputter power of 4 kW, where the deposition rate was 385 nm/min.