Effect of Cu layer thickness on the structural, optical and electrical properties of AZO/Cu/AZO tri-layer films

Highly conducting AZO/Cu/AZO tri-layer films were successfully deposited on glass substrates by RF magnetron sputtering of Al-doped ZnO (AZO) and ion-beam sputtering of Cu at room temperature. The microstructures of the AZO/Cu/AZO multilayer films were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and atomic force microscope (AFM). X-Ray diffraction measurements indicate that the AZO layers in the tri-layer films are polycrystalline with the ZnO hexagonal structure and have a preferred orientation with the c-axis perpendicular to the substrates. With the increase of Cu thickness, the crystallinity of AZO and Cu layers is simultaneously improved. When the Cu thickness increases from 3 to 13 nm, the resistivity decreases initially and then varies little, and the average transmittance shows a first increase and then decreases. The maximum figure of merit achieved is 1.94 × 10⁻² Ω⁻¹ for a Cu thickness of 8 nm with a resistivity of 7.92 × 10⁻⁵ Ω cm and an average transmittance of 84%.     

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.     

Study of optoelectronics and microstructures on the AZO/nano-layer metals/AZO sandwich structures

In this study, growth nano-layer metals (Al, Cu, Ag) and Al-doped ZnO (AZO) thin films are deposited on glass substrates as the transparent conducting oxides (TCOs) to form AZO/nano-layer metals/AZO sandwich structures. The conductivity properties of thin films are enhanced when the average transmittance over the wavelengths 400–800 nm is maintained at higher than 80 %. A radio frequency magnetron sputtering system is used to deposit the metal layers and AZO thin films of different thickness, to form AZO/Al/AZO (ALA), AZO/Cu/AZO (ACA) and AZO/Ag/AZO (AGA) structures. X-ray diffraction and field emission scanning electron microscopy are used to analyze the crystal orientation and structural characteristic. The optical transmission and resistivity are measured by UV–VIS–NIR spectroscopy and Hall effect measurement system, respectively. The results show that when the Ag thickness is maintained at approximately 9 nm, the TCOs thin film has the lowest resistivity of 8.9 × 10^?5 Ω-cm and the highest average transmittance of 81 % over the wavelengths 400–800 nm. The crystalline Ag nano-crystal structures are observed by high-resolution transmission electron microscopy. In addition, the best figure of merit for the AZO/Ag/AZO tri-layer film is 2.7 × 10^?2 (Ω^?1), which is much larger than that for other structures.     

Tungsten-doped ZnO transparent conducting films deposited by direct current magnetron sputtering

Highly conducting and transparent thin films of tungsten-doped ZnO (ZnO:W) were prepared on glass substrates by direct current (DC) magnetron sputtering at low temperature. The effect of film thickness on the structural, electrical and optical properties of ZnO:W films was investigated. All the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity first decreases with film thickness, and then increases with further increase in film thickness. The lowest resistivity achieved was 6.97 × 10⁻⁴ Ω cm for a thickness of 332 nm with a Hall mobility of 6.7 cm² V⁻¹ s⁻¹ and a carrier concentration of 1.35 × 10²¹ cm⁻³. However, the average transmittance of the films does not change much with an increase in film thickness, and all the deposited films show a high transmittance of approximately 90% in the visible range.     

透明导电AZO/Cu双层薄膜制备及其性能

采用直流磁控溅射方法在玻璃衬底上室温生长了AZO/Cu双层薄膜,Cu层厚度控制在9nm,研究了AZO层厚度对薄膜电学和光学性能的影响。当AZO层厚度为20～80nm时,AZO/Cu双层薄膜具有良好的综合光电性能,方块电阻为12～14Ω/sq,可见光平均透过率为70～75%,品质因子为2×10-3～5×10-3Ω-1。AZO/Cu双层薄膜可以观察到Cu（111）和ZnO（002）的XRD衍射峰。通过退火研究表明,AZO/Cu双层薄膜的光电性能可在400℃下保持稳定,具有良好的热稳定性。本研究制备的透明导电AZO/Cu双层薄膜具有室温制程、综合光电性能良好、结晶性能较好、稳定性高的优点,可以广泛应用于光电器件透明电极及镀膜玻璃等领域。     

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.     

Mn-doped ZnO transparent conducting films deposited by DC magnetron sputtering

Mn-doped zinc oxide (ZnO:Mn) thin films with low resistivity and relatively high transparency were firstly prepared on glass substrate by direct current (DC) magnetron sputtering at room temperature. Influence of film thickness on the properties of ZnO:Mn films was investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that all the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. As the thickness increases from 144 to 479 nm, the crystallite size increases while the electrical resistivity decreases. However, as the thickness increases from 479 to 783 nm, the crystallite size decreases and the electrical resistivity increases. When film thickness is 479 nm, the deposited films have the lowest resistivity of 2.1 × 10^− 4 Ω cm and a relatively high transmittance of above 84% in the visible range.     

Transparent conductive and infrared reflective AZO/Cu/AZO multilayer film prepared by RF magnetron sputtering

AZO/Cu/AZO multilayer films were prepared on glass substrate by radio frequency magnetron sputtering technology. The prepared films were investigated by a four-point probe system, X-ray diffraction, optical transmittance spectra, scanning electron microscope, atomic force microscopy and Fourier transform infrared spectroscopy. The results showed that Cu inner layer started forming a continuous film at the thickness around 11 nm. The prepared AZO/Cu/AZO samples exhibited the visible transmittance of 60–80 % and sample with 15 nm Cu inner layer showed the highest infrared reflection rate of 67 % in FIR region and the lowest sheet resistance of 16.6 Ω/sq. The proper visible transmittance and infrared reflection property of the AZO/Cu/AZO multilayer film make it a promising candidate for future energy conservation materials.     

Influence of Ag thickness of aluminum-doped ZnO/Ag/aluminum-doped ZnO thin films

Highly conducting aluminum-doped ZnO (30 nm)/Ag (5-15 nm)/aluminum-doped ZnO (30 nm) multilayer thin films were deposited on glass substrate by rf magnetron sputtering (for top/bottom aluminum-doped ZnO films) and e-beam evaporation (for Ag film). The transmittance is more than 70% for wavelengths above 400 nm with the Ag layer thickness of 10 nm. The resistivity is 3.71 × 10^− 4 Ω-cm, which can be decreased to 3.8 × 10^− 5 Ω-cm with the increase of the Ag layer thickness to 15 nm. The Haacke figure of merit has been calculated for the films with the best value being 8 × 10^− 3 Ω^− 1. It was shown that the multilayer thin films have potential for applications in optoelectronics.     

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.     

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.     

Fabrication of transparent conductive films with a sandwich structure composed of ITO/Cu/ITO

New transparent conductive films that had a sandwich structure composed of ITO/Cu/ITO multilayer films were prepared by a conventional RF and DC magnetron sputtering process on a polycarbonate substrate without intentional substrate heating. The thickness of each layer in the ITO/Cu/ITO films was kept constant at 50 nm/5 nm/45 nm. The optoelectrical and structural properties of the films were compared with conventional ITO single-layer films and ITO/Cu/ITO multilayered films. Although both films had identical thickness, 100 nm, the ITO/Cu/ITO films showed a lower resistivity, 3.5 × 10⁻⁴ Ω cm. In optical transmittance measurements, however, the ITO single-layer films showed a higher transmittance of 74% in the wavelength range of 300-800 nm. XRD spectra showed that both the ITO and ITO/Cu/ITO films were amorphous. The figure of merit, φ_TC, reached a maximum of 5.2 × 10⁻⁴ Ω⁻¹ for the ITO/Cu/ITO films, which was higher than the φ_TC of the ITO films (1.6 × 10⁻⁴ Ω⁻¹). The φ_TC results suggested that ITO/Cu/ITO films had better optoelectrical properties than conventional ITO single-layer films.     

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.     

Characterization of ZnO:Al/Au/ZnO:Al trilayers for high performance transparent conducting electrodes

Symmetric ZnO:Al/Au/ZnO:Al trilayers were sputter-deposited and characterized for transparent conducting electrodes, varying the thickness of the ZnO:Al (AZO) and Au layers. The optical transmission for normal light incidence is optimum for an AZO thickness of 50 nm, due to the suppression of reflection. In this case, the transmittance is more than 0.7 for wavelengths above 400 nm and for a Au thickness of 5 nm. At the same time, the sheet resistance is approx. 30 Ω, which can be decreased to 12 Ω with the increase of the Au thickness to 9 nm. This is achieved with a moderate loss in the optical transmission. The figure of merit for transparent conducting electrodes, as introduced by G. Haacke (J. Appl. Phys. 47 (1976) 4086) yields values from 29.4 × 10^− 3 to 6.9 × 10^− 3 Ω^− 1, depending on the Au thickness and the considered wavelength range.     

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.     

Fluorine-doped ZnO transparent conducting thin films prepared by radio frequency magnetron sputtering

Fluorine-doped ZnO transparent conducting thin films were prepared by radio frequency magnetron sputtering at 150 °C on glass substrate. Thermal annealing in vacuum was used to improve the optical and electrical properties of the films. X-ray patterns indicated that (002) preferential growth was observed. The grain size of F-doped ZnO thin films calculated from the full-width at half-maximum of the (002) diffraction lines is in the range of 18-24 nm. The average transmittance in visible region is over 90% for all specimens. The specimen annealed at 400 °C has the lowest resistivity of 1.86 × 10^− 3 Ω cm, the highest mobility of 8.9 cm² V^− 1 s^− 1, the highest carrier concentration of 3.78 × 10²⁰ cm^− 3, and the highest energy band gap of 3.40 eV. The resistivity of F-doped ZnO thin films increases gradually to 4.58 × 10^− 3 Ω cm after annealed at 400 °C for 4 h. The variation of the resistivity is slight.     

High quality ZnO:Al transparent conducting oxide films synthesized by pulsed filtered cathodic arc deposition

Aluminum-doped zinc oxide, ZnO:Al or AZO, is a well-known n-type transparent conducting oxide with great potential in a number of applications currently dominated by indium tin oxide. In this study, the optical and electrical properties of AZO thin films deposited on glass and silicon by pulsed filtered cathodic arc deposition are systematically studied. In contrast to magnetron sputtering, this technique does not produce energetic negative ions, and therefore ion damage can be minimized. The quality of the AZO films strongly depends on growth temperature while only marginal improvements are obtained with post-deposition annealing. The best films, grown at a temperature of about 200 °C, have resistivities in the low to mid 10^− 4 Ω cm range with a transmittance better than 85% in the visible part of the spectrum. It is remarkable that relatively good films of small thickness (60 nm) can be fabricated using this method.     

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.     

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).     

Transparent conducting Sc-codoped AZO film prepared from ZnO:Al-Sc by RF-DC sputtering

Al and Sc-codoped zinc oxide (also expressed as Sc-codoped AZO or ZnO:Al-Sc) films were sputtered on STN glass using RF power sources on ZnO and DC power sources on Al-1.7wt.% Sc alloy. X-ray diffraction (XRD) of the codoped films displayed that they are crystalline and textured at (002) and (103). Examination through transmission electron microscopy (TEM) depicted that these films consists of columnar grains. X-ray photoelectron spectroscopy (XPS) analysis of the films indicated that the O1s comprises O(I), O(II), O(III), and O(IV). The component O(I) centered at 530.00 ± 0.15eV was attributable to Sc₂O₃; the O(III) at 531.25 ± 0.20eV was to the oxygen deficient regions within the matrix of ZnO. The transmittance of visible light (i.e., wavelength in the range from 400 to 800nm) for the film was higher than 80%. The electrical resistivity is lower (1.76 < 2.81Ω-cm), the corrosion-resistance in 3.5% NaCl solution is better for the codoped film in comparison with the usual AZO. Heat treatment of the films (at 200-400°C for 1h) improved the optical transmittance, electrical conductivity, and corrosion-resistance in saline solution.