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.     

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).     

Effect of deposition parameters and annealing temperature on the structure and properties of Al-doped ZnO thin films

Transparent conductive films of Al-doped ZnO (AZO) were deposited onto inexpensive soda-lime glass substrates by radio frequency (rf) magnetron sputtering using a ZnO target with an Al content of 3 wt%. The Taguchi method with a L₉ orthogonal array, signal-to-noise (S/N) ratio and analysis of variance (ANOVA) were employed to examine the performance characteristics of the coating operations. This study investigated the effect of the deposition parameters (rf power, sputtering pressure, thickness of AZO films, and substrate temperature) on the electrical, structural, morphological and optical properties of AZO films. The grey-based Taguchi method showed the electrical resistivity of AZO films to be about 9.15 × 10⁻³ Ω cm, and the visible range transmittance to be about 89.31%. Additionally, the films were annealed in a vacuum ambient (5.0 × 10⁻⁶ Torr) at temperatures of 400, 450, 500 and 600 °C, for a period of 30 min. It is apparent that the intensity of the X-ray peaks increases with annealing treatment, leading to improved crystallinity of the films. By applying annealing at 500 °C in a vacuum ambient for 30 min, the AZO films show the lowest electrical resistivity of 2.31 × 10⁻³ Ω cm, with about 90% optical transmittance in the visible region and a surface roughness of Ra = 12.25 nm.     

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.     

Effects of substrate on the structural, electrical and optical properties of Al-doped ZnO films prepared by radio frequency magnetron sputtering

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited on substrates including alkali-free glass, quartz glass, Si, and SiO₂ buffer layer on alkali-free glass by using radio frequency magnetron sputtering. The effects of different substrates on the structural, electrical and optical properties of the AZO films were investigated. It was found that the crystal structures were remarkably influenced by the type of the substrates due to their different thermal expansion coefficients, lattice mismatch and flatness. The AZO film (100 nm in thickness) deposited on the quartz glass exhibited the best crystallinity, followed sequentially by those deposited on the Si, the SiO₂ buffer layer, and the alkali-free glass. The film deposited on the quartz glass showed the lowest resistivity of 5.14 × 10^− 4 Ω cm among all the films, a carrier concentration of 1.97 × 10²¹ cm^− 3 and a Hall mobility of 6.14 cm²/v·s. The average transmittance of this film was above 90% in the visible light spectrum range. Investigation into the thickness-dependence of the AZO films revealed that the crystallinity was improved with increasing thickness and decreasing surface roughness, accompanied with a decrease in the film resistivity.     

Poly-silicon thin films by aluminium induced crystallisation of microcrystalline silicon

Al-induced crystallisation of microcrystalline Si thin films prepared by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) on glass and SiO₂ coated Si wafers has been studied. The starting structure was substrate/μc-Si/Al. Annealing this structure in the temperature range 370-520 °C, immediately following deposition of the Al layer, resulted in successful layer exchange and the formation of a substrate/Al+Si layer/poly-Si geometry. The top poly-Si layer exhibited grain sizes generally in the range ∼2-6 μm, although larger grains were also sparsely present. The films did not exhibit any appreciable degree of preferred orientation. The surface roughness was relatively high with a R_a value of ∼20 nm.     

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.     

Preparation of aluminum doped zinc oxide films and the study of their microstructure, electrical and optical properties

Aluminum doped zinc oxide (AZO) polycrystalline thin films were prepared by sol-gel dip-coating process on optical glass substrates. Zinc acetate solutions of 0.5 M in isopropanol stabilized by diethanolamine and doped with a concentrated solution of aluminum nitrate in ethanol were used. The content of aluminum in the sol was varied from 1 to 3 at.%. Crystalline ZnO thin films were obtained following an annealing process at temperatures between 300 °C and 500 °C for 1 h. The coatings have been characterized by X-ray diffraction, UV-Visible spectrophotometry, scanning electron microscopy, and electrical resistance measurement. The ZnO:Al thin films are transparent (∼ 90%) in near ultraviolet and visible regions. With the annealing temperature increasing from 300 °C to 500 °C, the film was oriented more preferentially along the (0 0 2) direction, the grain size of the film increased, the transmittance also became higher and the electrical resistivity decreased. The X-ray diffraction analysis revealed single-phase ZnO hexagonal wurtzite structure. The best conductors were obtained for the AZO films containing 1 at.% of Al, annealed at 500 °C, 780 nm film thickness.     

Effect of TiO2 nanopatterns on the performance of hydrogenated amorphous silicon thin-film solar cells

We investigate how TiO₂ nanopatterns formed onto ZnO:Al (AZO) films affect the performance of hydrogenated amorphous silicon (a-Si:H) solar cells. Scanning electron microscopy results show that the dome-shaped TiO₂ nanopatterns (300 nm in diameter) having a period of 500 nm are formed onto AZO films and vary from 60 to 180 nm in height. Haze factor increases with an increase in the height of the nanopatterns in the wavelength region below 530 nm. Short circuit current density also increases with an increase in the height of the nanopatterns. As the nanopatterns increases in height, the fill factor of the cells slightly increases, reaches maximum (0.64) at 100 nm, and then decreases. Measurements show that a-Si:H solar cells fabricated with 100 nm-high TiO₂ nanopatterns exhibit the highest conversion efficiency (6.34%) among the solar cells with the nanopatterns and flat AZO sample.     

Dielectric properties of DC reactive magnetron sputtered Al2O3 thin films

Alumina (Al₂O₃) thin films were sputter deposited over well-cleaned glass and Si < 100 > substrates by DC reactive magnetron sputtering under various oxygen gas pressures and sputtering powers. The composition of the films was analyzed by X-ray photoelectron spectroscopy and an optimal O/Al atomic ratio of 1.59 was obtained at a reactive gas pressure of 0.03 Pa and sputtering power of 70 W. X-ray diffraction results revealed that the films were amorphous until 550 °C. The surface morphology of the films was studied using scanning electron microscopy and the as-deposited films were found to be smooth. The topography of the as-deposited and annealed films was analyzed by atomic force microscopy and a progressive increase in the rms roughness of the films from 3.2 nm to 4.53 nm was also observed with increase in the annealing temperature. Al-Al₂O₃-Al thin film capacitors were then fabricated on glass substrates to study the effect of temperature and frequency on the dielectric property of the films. Temperature coefficient of capacitance, AC conductivity and activation energy were determined and the results are discussed.     

Effect of temperature on structural, optical and photoluminescence properties of antimony (Sb) doped polycrystalline CuInS2 thin films prepared by spray pyrolysis

Copper indium disulphide (CuInS₂) is an absorber material for solar cell and photovoltaic applications. By suitably doping CuInS₂ thin films with dopants such as Zn, Cd, Na, Bi, Sn, N, P and As its structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified. In this work, Sb (0.01 mole (M)) doped CuInS₂ thin films are grown in the temperature range 300-400 °C on heated glass substrates. It is observed that the film growth temperature, the ion ratio (Cu/In = 1.25) and Sb-doping affects the structural, optical and photoluminescence properties of sprayed CuInS₂ films.The XRD patterns confirm that the Sb-doping suppresses the growth of CuInS₂ polycrystalline thin films along (1 1 2) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Sb. About 60% of light transmission occurs in the wavelength range 350-1100 nm. The absorption coefficient (α) is found to be in the order of 10⁵ cm⁻¹. The band gap energy increases as the temperature increases from 300-400 °C (1.35-1.40 eV). SEM photographs depict that large sized crystals of Sb-doped CuInS₂ (1 μm) are formed on the surface of the films. Well defined sharp blue and green band emissions are exhibited by Sb-doped CuInS₂ thin films. Defects-related photoluminescence emissions are discussed. These Sb-doped CuInS₂ thin films are prepared by the cost effective method of spray pyrolysis from the aqueous solutions of CuCl₂, InCl₃, SC(NH₂)₂ and SbCl₃ on heated glass substrates.     

Structural, optical, and electrical properties of AZO films by tilted angle sputtering method

We investigate the structural, optical, and electrical properties of aluminum-doped zinc oxide (AZO) films on Si substrate by a tilted angle sputtering method. The substrate holder is tilted by varying the angle from θ_sh = 0° to θ_sh = 80° during the sputtering process. As the tilted angle is increased, the deposition rate is increased due to the decreased distance between the substrate and the target. Without substrate rotation, the deposited AZO films exhibit apparently the inclined nanocolumnar structures, depending on the tilted angle. The refractive index, which is related to the porosity within films, is reduced for the larger inclined nanocolumnar structure while the extinction coefficient remains almost the same in the visible wavelength range. The inclination of nanocolumns disappears when the substrate is rotated. On glass substrate, the electrical properties as well as optical transmittance of AZO films are also dependant on the tilted angle.     

Phase stabilization and microstructural studies of lead lanthanum titanate thin films

Thin ferroelectric films of PLTx (Pb_1−xLa_xTi_1−x/4O₃) have been prepared by a sol-gel spin coating process. As deposited films were thermally treated for crystallization and formation of perovskite structure. Characterization of these films by X-ray diffraction (XRD) have been carried out for various concentrations of La (x = 0.04, 0.08 and 0.12) on ITO coated corning glass substrates. For a better understanding of the crystallization mechanism, the investigations were carried out on films annealed at temperatures (350, 450, 550 and 650 °C). Characterization of these films by X-ray diffraction shows that the films annealed at 650 °C exhibit tetragonal phase with perovskite structure. Atomic force microscope (AFM) images are characterized by slight surface roughness with a uniform crack free, densely packed structure. Fourier transform infrared spectra (FTIR) studies of PLTx thin films (x = 0.08) deposited on Si substrates have been carried out to get more information about the phase stabilization.     

Structure, morphology and electrical characterizations of direct current sputtered ZnO thin films

ZnO thin films were deposited on glass substrates by direct current (DC) sputtering technique at room temperature (RT) to 400 °C with a 99.999% pure ZnO target. Then the samples deposited at RT were annealed in air from the RT to 400 °C. The effects of substrate temperature (T_s) and annealing treatment (T_a) on the crystallization behavior and the morphology have been studied by X-ray diffraction and atomic force microscopy. We also compared the structural properties of samples deposited at 400 °C on glass to those deposited on Pt/silicon substrate. The resistivity, surface roughness and size of the grains have also been studied and correlated to the thickness of ZnO films deposited on Pt/Si substrates. The experimental results reveal that the substrate has a major influence on the structural and morphological properties. For the films deposited on glass, below 400 °C, T_s and T_a have a similar influence on the structure of the films. Moreover, the ZnO samples deposited at RT and annealed in air have poor electrical properties.     

Highly transparent conductive and near-infrared reflective ZnO:Al thin films

Al-doped ZnO (AZO) thin films have been prepared on glass substrates by pulsed laser deposition. The structural, optical, and electrical properties were strongly dependent on the growth temperatures. The lowest resistivity of 4.5 × 10⁻⁴ Ωcm was obtained at an optimized temperature of 350 °C. The AZO films deposited at 350 °C also had the high optical transmittance above 87% in the visible range and the low transmittance (<15% at 1500 nm) and high reflectance (∼50% at 2000 nm) in the near-IR region. The good IR-reflective properties of ZnO:Al films show that they are promising for near-IR reflecting mirrors and heat reflectors.     

Optical and electrical properties of 2 wt.% Al2O3-doped ZnO films and characteristics of Al-doped ZnO thin-film transistors with ultra-thin gate insulators

Al-doped ZnO (AZO) thin films have been prepared on the c-Si oriented direction of (100) and glass substrates, by radio frequency magnetron sputtering from ZnO-2 wt.% Al₂O₃ ceramic targets. The effects of the working pressure on the optical and electrical properties of the films have been studied. The optical properties, measured by the ultraviolet-visible system, show that the transmittance and optical bandgap energy are influenced by the working pressure. The Hall resistivity, mobility, and carrier concentration were obtained by a Hall measurement system and these parameters were also influenced by the working pressure. The AZO thin-film transistors (TFTs) were fabricated on highly doped c-Si substrates. The TFT structures were made up AZO as the active layer and SiO_xN_y/SiN_x/SiO_x as the gate layer with 20 nm and 35 nm thickness, respectively. The ultra-thin TFTs had an on/off current ratio of 10⁴ and a field-effect mobility of 0.17 cm²/V·s. These results show that it is possible to fabricate an AZO TFT that can be operated with an ultra-thin gate dielectric.     

Growth and characterization of epitaxial anatase TiO2(001) on SrTiO3-buffered Si(001) using atomic layer deposition

Epitaxial anatase titanium dioxide (TiO₂) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO₂ was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225-250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10^− 7 Pa) for 1-2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO₂ by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO₂ growth. X-ray diffraction revealed that the TiO₂ films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO₂ films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.     

The use of aluminium doped ZnO as transparent conductive oxide for CdS/CdTe solar cells

CdTe/CdS and CdTe/ZnO thin film solar cells were grown with a high vacuum evaporation based low temperature process (≤ 420 °C). Aluminium doped zinc oxide (AZO) was used as transparent conducting oxide (TCO) material. AZO exhibited excellent stability during the solar cell processing, and no significant change in electrical conductivity or transparency was observed. The current density loss due to absorption in the 1 μm thick AZO layer with 5 Ω per square sheet resistance was found to be 1.2 mA/cm². We investigated the influence of an intrinsic ZnO layer (i:ZnO) in combination with various CdS thicknesses. The i:ZnO layer was found to significantly increase the open circuit voltage of the solar cells with very thin CdS layer. Increasing thickness of the i:ZnO layer leads to UV absorption losses, narrowing of the depletion layer width and hence reduced collection efficiency in the long wavelength (685-830 nm) part. With AZO/i:ZnO bi-layer TCO we could achieve cell efficiencies of 15.6% on glass and 12.4% on the flexible polyimide film.     

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.     

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.