Quality enhancement of AZO thin films at various thicknesses by introducing ITO buffer layer

Due to the simultaneously superior optical transmittance and low electrical resistivity, transparent conductive electrodes play a significant role in semiconductor electronics. To enhance the electrical properties of these films, one approach is thickness increment which degrades the optical properties. However, a preferred way to optimize both electrical and optical properties of these layers is to introduce a buffer layer. In this work, the effects of buffer layer and film thickness on the structural, electrical, optical and morphological properties of AZO thin films are investigated. Al-doped zinc oxide (AZO) is prepared at various thicknesses of 100 to 300 nm on the bare and 100 nm-thick indium tin oxide (ITO) coated glass substrates by radio frequency sputtering. Results demonstrate that by introducing ITO as a buffer layer, the average values of sheet resistance and strain within the film are decreased (about 76 and 3.3 times lower than films deposited on bare glasses), respectively. Furthermore, the average transmittance of ITO/AZO bilayer is improved nearly 10% regarding single AZO thin film. This indicates that bilayer thin films show better physical properties rather than conventional monolayer thin films. As the AZO film thickness increases, the interplanar spacing, d₍₀₀₂₎, strain within the film and compressive stress of the film in the hexagonal lattice, decreases indicating the higher yield of AZO crystal. Moreover, with the growth in film thickness, carrier concentration and optical band gap (E_g) of AZO film are increased from 4.62?×?10¹⁹ to 8.21?×?10¹⁹ cm^?3 and from 3.55 to 3.62 eV, respectively due to the Burstein-Moss (BM) effect. The refractive index of AZO thin film is obtained in the range of 2.24–2.26. With the presence of ITO buffer layer, the AZO thin film exhibits a resistivity as low as 6?×?10^?4 Ω cm, a sheet resistance of 15 Ω/sq and a high figure of merit (FOM) of 1.19?×?10⁴ (Ω cm)^?1 at a film thickness of 300 nm. As a result, the quality of AZO thin films deposited on ITO buffer layer is found to be superior regarding those grown on a bare glass substrate. This study has been performed over these two substrates because of their significant usage in the organic light emitting diodes and photovoltaic applications as an enhanced carrier injecting electrodes.     

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.     

Characterization of aluminum-doped zinc oxide thin films by RF magnetron sputtering at different substrate temperature and sputtering power

In this study, transparent conductive Al doped zinc oxide (ZnO: Al, AZO) thin films with a thickness of 40 nm were prepared on the Corning glass substrate by radio frequency magnetron sputtering. The properties of the AZO thin films are investigated at different substrate temperatures (from 27 to 150 °C) and sputtering power (from 150 to 250 W). The structural, optical and electrical properties of the AZO thin films were investigated. The optical transmittance of about 78 % (at 415 nm)–92.5 % (at 630 nm) in the visible range and the electrical resistivity of 7 × 10^?4 Ω-cm (175.2 Ω/sq) were obtained at sputtering power of 250 W and substrate temperature of 70 °C. The observed property of the AZO thin films is suitable for transparent conductive electrode applications.     

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.     

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.     

Effect of acetic acid and water content in the spray solution on structural,morphological, optical and electrical properties of Al and In co-doped zinc oxide thin films

Aluminium and indium co-doped zinc oxide (AIZO) thin films were deposited using ultrasonic spray pyrolysis. Depositions were performed by varying the acetic acid and water content in the spraying solution which resulted in the formation of different nanostructures like hexagons, flowers, chisels, curved nanostructures, hexagonal pyramids, super grown hexagons, and inter-connected nanostructures. Further, the physical properties such as structural, optical, electrical, and surface texture parameters were examined. The structural studies showed that films were of crystalline nature, with different crystallite sizes and grown with a preferential orientation along (002) plane. The optical transmittance assessments proved that films were highly transparent (>?80%) in the visible region. The electrical sheet resistance was found to be in the range 29–1K Ω/□. Surface parameters like average roughness, root mean square roughness, and peak-valley height values helped to understand the homogeneity of the thin films. Finally, the suitability of AIZO films for transparent conductive oxide applications were tested by estimating the figure of merit (FOM). Among the different solution conditions, films fabricated using a starting solution containing 25 ml of acetic acid and 25 ml of water exhibited the lowest resistivity (2.47?±?0.03?×?10^?3 Ω-cm) along with the highest FOM (5.83?±?0.42?×?10^?3/Ω).     

The influence of substrate temperature on the properties of aluminum-doped zinc oxide thin films deposited by DC magnetron sputtering

Transparent, conducting, aluminum-doped zinc oxide (AZO) thin films were deposited on Corning 1737 glass by a DC magnetron sputter. The structural, electrical, and optical properties of the films, deposited using various substrate temperatures, were investigated. The AZO thin films were fabricated with an AZO ceramic target (Al₂O₃:2 wt%). The obtained films were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. The lowest resistivity was 6.0 × 10⁻⁴Ω cm, with a carrier concentration of 2.7 × 10²⁰ cm⁻³ and a Hall mobility of 20.4 cm²/Vs. The average transmittance in the visible range was above 90%.     

Optical and hall properties of Al-doped ZnO thin films fabricated by pulsed laser deposition with various substrate temperatures

The 3 wt% Al-doped zinc oxide (AZO) thin films were fabricated on quartz substrates at a fixed oxygen pressure of 200 mTorr with various substrate temperatures (room temp. ～500 °C) by using pulsed laser deposition in order to investigate the microstructure, optical, and electrical properties of AZO thin films. All thin films were shown to be c-axis oriented, exhibiting only a (002) diffraction peak. The AZO thin film, fabricated at 200 mTorr and 400 °C, showed the highest (002) orientation and the full width at half maximum (FWHM) of the (002) diffraction peak was 0.42°. The c-axis lattice constant decreased with increasing substrate temperature. The electrical property indicated that the highest carrier concentration (1.27 × 10²¹ cm^?3) and the lowest resistivity (6.72 × 10^?4 Ωcm) were obtained in the AZO thin film fabricated at 200 mTorr and 400 °C. The optical transmittance in the visible region was higher than 80 %. The Burstein-Moss effect, which shifts to a high photon energy, was observed.     

The properties of Al doped ZnO thin films deposited on various substrate materials by RF magnetron sputtering

Transparent conductive Al-doped ZnO (AZO) thin films were deposited on various substrates including glass, polyimide film (PI) and stainless steel, using radio frequency magnetron sputtering method. The structural, electrical and optical properties of AZO thin films grown on various substrates were systematically investigated. We observe that substrate materials play important roles in film crystallization and resistivity but little on optical transmittance. X-ray diffractometer study shows that all obtained AZO thin films have wurtzite phase with highly c-axis preferred orientation, and films on glass present the strongest (002) diffraction peaks. The presence of compression stress plays critical role in determining the crystalline structure of AZO films, which tends to stretch the lattice constant c and enlarge the (002) diffraction angle. Although the films on the glass present the finest electrical properties and the resistivity reaches 12.52 × 10-4 Ωm, AFM study manifests that films on flexible substrates, especially stainless steel, bestrew similar inverted pyramid structure which are suitable for window material and electrode of solar cells. The average optical transmittance of AZO thin films deposited on glass and PI are both around 85% in the visible light range (400–800 nm).     

On the oxygen content in sputtering InOx film for transparent electronics

Transparent undoped semiconductor indium oxide films were deposited by using a long-throw rf magnetron sputtering at room temperature. It was found that the variation of oxygen content in sputtering gas has a strong influence on the microstructure and electrical properties of the films. The electrical resistivity varying from 3.5 × 10^?2 to 4.7 × 10⁴ Ω-cm for oxygen contents ranging from 0 to 50 % was obtained. The optical band gap decreases as the oxygen content increases, and the average visible transmittance of the indium oxide is ~85 %. To put into practice, the as-sputtered indium oxide was employed as a channel of thin film transistors on glass substrate with a channel length, 6 μm, and a channel width, 20 μm. Its saturation mobility, threshold voltage, and on/off ratio were obtained to be 9.4 V^?1 s^?1, 1.5 V, and 2.2 × 10⁷. To approach a flexible device, a plastic substrate is employed to replace the rigid substrate, glass; and the relative parameters, saturation mobility, threshold voltage, and on/off ratio, are also measured to be 8.2 V^?1 s^?1, 1.8 V, and 1.4 × 10⁶, respectively.     

Effect of different solvents on the key structural,optical and electronic properties of sol–gel dip coated AZO nanostructured thin films for optoelectronic applications

Transparent conducting aluminum (i.e. 2 at.%) doped zinc oxide (AZO) thin films were prepared on glass substrates by sol–gel dip coating technique using different solvents. This inexpensive dip coating method involves dipping of substrate consecutively in zinc solution and tube furnace for required cycles. Prepared films were investigated by XRD, SEM, PL, Raman spectroscopy optical and electrical studies. From the XRD studies, it confirmed the incorporation of aluminum in ZnO lattice. The prepared samples are polycrystalline nature, and these films reveal hexagonal wurtzite arrangement with (002) direction. The structural parameters such as crystallite size, dislocation density, micro strain, texture coefficient and lattice constant were investigated. SEM study showed well defined smooth and uniformed ganglia shaped grains are regularly distributed on to the entire glass substrate without any pinholes and cracks, and the average grain size is 75 nm. From the optical studies, the observed highest transmittance is 93% in the visible range and the band gap (E_g) is 3.26 eV. Room temperature PL spectra exhibited strong UV emission peak located at 386 nm for all the films. The electrical properties of the AZO thin films were studied by Hall-Effect measurements and found as n-type conductivity with high carrier concentrations (n), 2.76?×?10¹⁹ cm^??3 and low resistivity (ρ), 7.56?×?10^??3 Ω cm for the film deposed using methanol as solvent.     

Effect of the substrate temperature on the properties of spray-deposited SnO2:F thin films

The effect of the substrate temperature on the properties of spray-deposited SnO₂:F thin films is investigated. X-ray diffraction patterns show that the crystallinity of the films is enhanced with the increasing of substrate temperature. Comparing the SEM images, both the particle size and density are increased at a higher deposition temperature. The lowest sheet resistance of 8.43 Ω/□ is obtained at the substrate temperature of 350 °C. In addition, the average optical transmittance of the three films reaches up to 85 % in the visible range. The absorption coefficient is the lowest at 350 °C. The band gap increases from 3.36 to 3.61 eV while the electrical resistivity of SnO₂:F thin films decreases from 8.51 × 10^?3 to 9.86 × 10^?4 Ω cm as elevating the substrate temperature from 250 to 350 °C.     

Effects of annealing temperature and thickness on microstructure and properties of sol–gel derived multilayer Al-doped ZnO films

Transparent conductive multilayer Al-doped ZnO (AZO) films were prepared by the spin-on technique with rapid thermal annealing process at low temperature. The effects of annealing temperature and thickness on microstructure, growth behavior, electrical properties and optical properties of AZO films were investigated. It was found that AZO films exhibited stronger preferred c-axis-orientation, the electrical resistivity decreased as it would be expected with the increase of annealing temperature from 400 to 500 °C and the increase of the number of layers in the film from 1 to 6, but the electrical resistivity tended to keep at a certain lowest value of 2.7 × 10⁻⁴ Ω cm when the annealing temperature was above 500 °C and the number of layers did not exceed 6. The average optical transmittance of AZO films was over 90% when number of layers in the film did not exceed 4 and decreased as this number increases, but the annealing temperature had little effect on the average optical transmittance of AZO films.     

Microstructure and properties of Al-doped ZnO thin films by nonreactive DC magnetron sputtering at room temperature following rapid thermal annealing

A series of Al-doped ZnO (AZO) thin films deposited by nonreactive DC magnetron sputtering at room temperature following rapid thermal annealing was studied to examine the influence of these Al doping concentration, sputtering power and annealing temperature on their microstructure, electrical and optical transport properties. AZO thin films with Al dopant of 3 wt% were oriented more preferentially along the (002) direction, bigger grain size and lower electrical resistivity The resistivity of AZO films decreases with the increase of Al content from 1 to 3 wt%, sputtering power from 60 to 100 W and the annealing temperature from 50 to 250 °C. Sputtering power and annealing had some effect on the average transmittance of AZO thin films. For AZO thin films with Al doping level of 3 wt%, the lowest electrical resistivity of 5.3 × 10⁻⁴ Ω cm and the highest optical transmittance of 88.7% could gain when the sputtering power was 100 W and the annealing temperature was 200 °C or above.     

Influence of hydrogen on the properties of Al and Ga-doped ZnO films at room temperature

Low resistivity and high transmittance of Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) transparent conductive thin films have been achieved by use of pulsed dc magnetron sputtering in a hydrogen environment at room temperature. The addition of hydrogen to the sputtering gas can reduce the resistivity of the films and improve their electrical properties compared to those prepared without H2, because the hydrogen acts a shallow donor. The average transmittance was over 85% in the visible region, and the lowest resistivity of the AZO and GZO films was 4.01×10(-4) (Ω-cm) and 4.39×10(-4) (Ω-cm), respectively.     

Microstructural and optoelectronic properties of rf magnetron sputtered ZnO:(Ga,Ti) semiconductor thin films

Applying radio-frequency (rf) magnetron sputtering technique, Ga–Ti co-doped ZnO [ZnO:(Ga,Ti)] transparent conductive oxide films were deposited onto glass substrates. The films were characterized by X-ray diffraction, four-point probe and UV–visible spectrophotometer. The influence of sputtering pressure on microstructure and optoelectronic properties of the films was investigated. The results show that all the films are polycrystalline with a hexagonal wurtzite structure and grow preferentially in the (002) direction. The ZnO:(Ga,Ti) films deposited at sputtering pressure of 0.4 Pa exhibit the maximum grain size of 86.6 nm, the highest transmittance of 85.9 %, the lowest resistivity of 1.67 × 10^?3 Ω cm, and the highest figure of merit of 1.38 × 10^?2 Ω^?1. The optical constants such as refractive index, extinction coefficient, dielectric constant and dissipation factor were determined using the method of whole optical spectrum fitting. Meanwhile, the dispersion behaviour of the films was studied by the single electronic oscillator model. The oscillator parameters and optical energy gaps were achieved. The results demonstrate that the microstructure and optoelectronic properties of the films are closely related to the sputtering pressure.     

Low temperature synthesis of sol–gel derived Al-doped ZnO thin films with rapid thermal annealing process

A series of sol–gel derived Al-doped ZnO (AZO) thin films with rapid thermal annealing process at low temperature were studied to examine the influence of annealing temperature and the Al doping concentration on their microstructure, electrical and optical transport properties. Crystalline AZO thin films were obtained following an annealing process at temperatures between 400 and 600 °C for 10 min in argon gas ambient. AZO thin films with Al doping of 1 at% were oriented more preferentially along the (002) direction, and have larger grain size and lower electrical resistivity, while the highest average optical transmittances of 92% were observed in AZO films with Al doping of 2 at%. With the annealing temperature increasing from 400 to 600 °C, the grain size of AZO films increased, the optical transmittance became higher, and the electrical resistivity decreased to a lowest value of 1.2 × 10⁻⁴ Ω cm resulting from the increase of the carrier concentration and the mobility.     

Preparation of Sb:SnO2 thin films and its effect on opto-electrical properties

The present study focuses on pure and antimony (Sb)-doped tin oxide thin film and its influence on their structural, optical, and electrical properties. Both undoped and Sb-doped SnO₂ thin films have been grown by using simple, inexpensive pyrolysis spray technique. The deposition temperature was optimized to 450 °C. X-ray diffractions pattern have revealed that the films are polycrystalline and have tetragonal rutile-type crystal structure. Undoped SnO₂ films grow along (110) preferred orientation, while the Sb-doped SnO₂ films grow along (200) direction. The size of Sb-doped tin oxide crystals changes from 26.3 to 58.0 nm when dopant concentration is changed from 5 to 25 wt%. The transmission spectra revealed that all the samples are transparent in the visible region, and the optical bandgap varies between 3.92 and 3.98 eV. SEM analysis shows that the surface morphology and grain size are affected by the doping rate. All the films exhibit a high transmittance in the visible region and show a sharp fundamental absorption edge at about 0.38–0.40 nm. The maximum electrical conductivity of 362.5 S/cm was obtained for the film doped with 5 wt% Sb. However, the carrier concentration is increased from 0.708?×?10¹⁸ to 4.058?×?10²⁰ cm³. The electrical study reveals that the films have n-type electrical conductivity and depend on Sb concentration. We observed a decrease in sheet resistance and resistivity with the increase in Sb dopant concentration. For the dopant concentration of 5 wt% of Sb in SnO₂, the Rs and ρ were found minimum with the values of 88.55 (Ω cm^?2) and 2.75 (Ω cm), respectively. We observed an increase in carrier concentration and a decrease in mobility with the addition of Sb up to 25 wt%. The highest figure of merit values 2.5?×?10^–3 Ω^?1 is obtained for the 5wt% Sb, which may be considered potential materials for solar cells' transparent windows.

     

Improved characteristics of sprayed CdO films by rapid thermal annealing

We report the influence of rapid thermal annealing (RTA) on optical, electrical and structural properties of sprayed undoped CdO thin films. XRD investigation revealed that as-grown films are polycrystalline in nature with cubic structure; and pronounced improvement in crystallinity of the films have been noticed after annealing. RTA-treated films showed preferred orientation in the (2 0 0) direction. The optical band gap was deduced from transmittance data for as-grown and annealed films and found to be 2.51 eV for as-grown film and ranged between 2.25 and 2.48 eV for annealed films. Figure of merit was calculated and found its maximum value (1.2 × 10^?2 Ω^?1) was for film annealed with of 6 min. Thermoelectric power (TEP) measurements revealed that the CdO films are having n-type conductivity. Furthermore, the activation energy was calculated for films from TEP data. The lowest electrical resistivity was found to be 6 × 10^?4 Ω cm for CdO film annealed with time ≥6 min. The observed changes demonstrated that RTA is a viable technique for improving characteristics of sprayed CdO films.     

Structural,optical, and electrical properties of ZnTe:Cu thin films by PLD

In this work, ZnTe and ZnTe:Cu films were obtained by pulsed laser deposition using the co-deposition method. ZnTe and Cu₂Te were used as targets and the shots ratio were varied to obtain 0.61, 1.47, 1.72, and 3.46% Cu concentration. Doping of ZnTe films with Cu was performed with the purpose of increasing the p-type carrier concentration and establishing the effect of concentration of Cu on structural, optical, and electrical properties of ZnTe thin films to consider their potential application in electronic devices. According to X-ray diffraction, X-ray photoelectron spectroscopy, UV–visible spectroscopy, and Hall effect results, ZnTe and ZnTe:Cu films correspond to polycrystalline zinc–blende phase with preferential orientation in (111) plane. Optical characterization results indicate that as-deposited films (band gap?=?2.16 eV) exhibit a band gap decrease as function of the increase of Cu concentration (2.09–1.64 eV), while, annealed films exhibit a decrease from 1.75 to 1.46 eV, as the Cu concentration increases. Lastly, Hall effect results show that ZnTe films correspond to a p-type semiconductor with a carrier concentration of 3?×?10¹³ cm^?3 and a resistivity of 1.64?×?10⁵ Ω?cm. ZnTe:Cu films remain like a p-type material and present an increasing carrier concentration (from 3.8?×?10¹⁵ to 1.26?×?10¹⁹ cm^?3) as function of Cu concentration and a decreasing resistivity (from 7.01?×?103 to 2.6?×?10^?1 Ω cm). ZnTe and ZnTe:Cu thin films, with the aforementioned characteristics, can find potential application in electronic devices, such as, solar cells and photodetectors.