ZnO Transparent Conductive Films Research Articles

Improving the optoelectronic properties of gallium ZnO transparent conductive thin films through titanium doping

The optoelectronic characteristics of gallium-doped ZnO ternary alloy transparent conductive oxide were improved through titanium (Ti) doping, and showed enhanced optoelectronic properties, and crystallite quality by radio frequency magnetron sputtering. The room-temperature photoluminescence measurement showed improved near band edge emission for ZnO peaks, and suppressed deep defect level emission at the green light band. The study results show that the lowest thin film resistivity was 4.95×10−4Ω-cm, with a mobility and carrier concentration of 4.8cm2/V-s and 2.64×1021cm−3, respectively. The superior carrier concentration of Ti-doped GZO alloys (>1021cm−3) with high figure of merit (35.3×10−3Ω−1), demonstrating the pronounced contribution made by Ti doping, and is highly suitable for the optoelectronic device applications.

Study on the thermal stability of Ga-doped ZnO thin film: A transparent conductive layer for dye-sensitized TiO2 nanoparticles based solar cells

Generally, optoelectronic devices are fabricated at a high temperature. So the stability of properties for transparent conductive oxide (TCO) films at such a high temperature must be excellent. In the paper, we investigated the thermal stability of Ga-doped ZnO (GZO) transparent conductive films which were heated in air at a high temperature up to 500°C for 30min. After heating in air at 500°C for 30min, the lowest sheet resistance value for the GZO film grown at 300°C increased from 5.5Ω/sq to 8.3Ω/sq, which is lower than 10Ω/sq. The average transmittance in the visible light of all the GZO films is over 90%, and the highest transmittance is as high as 96%, which is not influenced by heating. However, the transmittance in the near-infrared (NIR) region for the GZO film grown at 350°C increases significantly after heating. And the grain size of the GZO film grown at 350°C after annealing at 500°C for 30min is the biggest. Then dye-sensitized TiO2 NPs based solar cells were fabricated on the GZO film grown at 350°C (which exhibits the highest transmittance in NIR region after heating at 500°C for 30min) and 300°C (which exhibits the lowest sheet resistance after heating at 500°C for 30min). The dye-sensitized solar cell (DSSC) fabricated on the GZO film grown at 350°C exhibits superior conversion efficiency. Therefore, transparent conductive glass applying in DSSCs must have a low sheet resistance, a high transmittance in the ultraviolet–visible–infrared region and an excellent surface microstructure.

Highly moisture resistant Ga-doped ZnO films with textured surface for thin-film solar cells with indium codoping fabricated by ion plating with direct-current arc discharge

Highly transparent conductive Ga-doped ZnO (GZO) films and GZO films with indium codoping (GZO:In) were deposited on glass substrates at 200 °C by ion plating using direct-current arc discharge. Etching in 0.1 wt % HCl solution made it possible to form a craterlike textured surface with a high haze value. For a textured GZO:In film with a remaining thickness of 388 nm, the sheet resistance was 8.1 Ω/□. Damp-heat test results showed that highly moisture-resistant GZO films with a textured surface could be achieved after indium codoping. For the textured GZO:In film, the relative change in sheet resistance was as low as 5.8% while maintaining a high transparency. The moisture-resistant GZO:In films with a textured surface have great potential for use as window layers in thin-film solar cells.

Influence of film thickness on the properties of sprayed ZnO thin films for gas sensor applications

Transparent conducting ZnO films were prepared at substrate temperature 400°C with different film thicknesses by nebulizer spray pyrolysis method on glass substrates. XRD studies reveal that the films are polycrystalline in nature having hexagonal crystal structure with preferred grain orientations along (002) and (101) directions. The crystallite size increases along (002) plane with the thickness increase and attains a maximum 109nm for 913nm film thickness. Analysis of structural parameters indicates that the films having thickness 913nm are found to have minimum dislocation density and strain values. The HRSEM measurements show that the surface morphology of the films also changes with film thickness. EDAX estimates the average atomic percentage ratio of Zn and O in the ZnO films. Optical studies reveal the band gap energy decrease from 3.27 to 3.14eV with increase of film thickness. Room temperature PL spectra show the near-band-edge emission and deep-level emission due to the presence of defects in the ZnO thin films. Impedance spectroscopy analysis indicates that grain boundary resistance decreases with the increasing ammonia concentration up to 500ppm and the maximum sensitivity is found to be 1.7 for 500ppm of ammonia.

Sol–gel synthesis of ZnO transparent conductive films: The role of pH

Abstract The sol–gel synthesis of undoped and B- or Al-doped ZnO thin films were critically examined with particular reference to the influence of the pH of the reaction medium on some of their specific characteristics, such as thickness, morphology, doping level and optical properties, in view of their application in the photovoltaic field. Using triethanolamine (TEA) as chelating agent, a range of basic pH from 7.66 to 8.76 was explored starting from a very concentrated zinc acetate dehydrate (ZAD) solution in ethanol, [Zn2+] = 1.0 M, and keeping the ZAD/TEA = 1. A more basic environment gives more porous films whose thickness and crystallinity are higher than those achieved at lower pH. It was found that the morphology, as well as the sheet resistance (Rs) of films, depends on both pH and doping. Increasing the pH the Rs decreases for both undoped and doped films. At a certain pH undoped films exhibit a granular microstructure and lower Rs than B- or Al-doped films which exhibit a finer texture, characterized by a lower porosity. Optical properties strongly depend on the pH as well. Increasing the pH, a noticeable blue shift effect was observed, that was attributed mainly to structural changes and to a lesser extent to the Burnstein–Moss effect.

Surface Morphology of Transparent Conductive ZnO Film Grown by DC Sputtering Method

The growth of (0002) orientated polycrystalline undoped and Ga-doped ZnO films by DC sputtering under Ar is described. The (0002) peak intensity decreases with increasing substrate temperature in both doped and undoped samples. The average grain sizes are very small. This indicates that ZnO films with low crystallinity are obtained at high substrate temperatures. It is deduced that surface damage can be increased by high energy plasmas of neutral Ar particles at high substrate temperatures. The average surface roughness for both undoped and Ga-doped ZnO films decreases with increasing substrate temperatures. It is deduced that energies of sputter particles decrease with increasing substrate temperatures due to collisions with Ar particles. The surface roughness corresponds well to the structure model.

Optimization of process parameters for the electrical properties in Ga-doped ZnO thin films prepared by r.f. magnetron sputtering

Ga-doped ZnO (GZO) transparent conductive thin films have been deposited on quartz substrates by r.f. magnetron sputtering. The optimization of four process parameters (i.e., vacuum annealing temperature, r.f. power, sputtering pressure, and Ar flow rate) based on Taguchi method has been systematically studied in order to obtain the minimum resistivity. Compared to the optimal parameter set selected from orthogonal array by Taguchi method, the optimal prediction design can receive an improvement of 22.3% in electrical resistivity, and the corresponding resistivity is 8.08×10−4Ω cm. The analysis of variance shows that vacuum annealing temperature is the most significant influencing parameter on the electrical properties in GZO films. X-ray photoelectron spectroscopy and photoluminescence results exhibit that the enhancement in electrical conductivity after vacuum annealing is ascribed to the variation of the chemical states of oxygen in GZO films. With the increase in annealing temperature, the content of absorbed oxygen and interstitial oxygen as acceptors will decrease.

Influence of H2 introduction on wide-spectrum Mg and Ga co-doped ZnO transparent conductive thin films

To meet the demands of high efficient silicon thin film solar cells, transparent conductive hydrogenated Mg and Ga co-doped ZnO (HMGZO) thin films were deposited via pulsed direct current (DC) magnetron sputtering on glass substrates at a substrate temperature of 553 K. The micro-structural, morphological, electrical, and optical properties of HMGZO thin films were investigated at various H2 flow rates. Experimental results show that all the HMGZO thin films are polycrystalline with a hexagonal wurtzite structure exhibiting a preferred (002) crystal plane orientation. Appropriate H2 flow rate increases grain size and also enhances the RMS roughness. The deposition rate of HMGZO films decreases with the increase of H2 flow rate due to the decrease of sputtering yield. Resistivity of HMGZO thin films decreases rapidly from 117 to 7.2×10-3 Ω·cm with increasing H2 flow rate from 0 to 4.0 sccm. With further increasing H2 flow rate (4.0–16.0 sccm), the resistivity increases slightly due to the reduced carrier concentration and excessive H atoms as impurity. Optical transmittance of all the HMGZO thin films is higher than 87.7% in the wavelength range from 320 to 1100 nm. Burstein-Moss band-filling determined by carrier concentrations and the incorporation of Mg atoms together contribute to the band-gap (Eg) widening phenomenon. The band gap Eg varies from ～ 3.49–3.70 eV and the maximum Eg of 3.70 eV is obtained at a H2 flow rate of 16.0 sccm.

Influence of external magnetic field on properties of aluminum-doped zinc oxide films prepared by RF magnetron sputtering

Al-doped ZnO (AZO) transparent conductive oxide films were prepared by RF magnetic sputtering. An external magnetic field was applied to the traditional magnetron sputtering system. The influence of the external magnetic field on the crystalline structure, surface topography and photoelectric properties of the AZO transparent conductive film have been studied. XRD diffraction patterns show that under the same processing condition, the intensity of (002) diffraction peak is significantly increased with the external magnetic field, suggesting a higher degree of c-axis preferred orientation. Scanning electron microscope shows that the external magnetic field can enlarge the grain size and density of films; the surface topography of the AZO films deposited without an external magnetic field is wormlike. Deposition rate and square resistance test results show that in an external magnetic field, the deposition rate will increase from 13.04 nm/min to 19.93 nm/min, and the sheet resistance reduce to 12.88 Ω /□ from 30.74 Ω /□ at a sputtering time of 90 min. Optical transmittance spectra shows that the average transmittance of all the films in visible light spectrum is over 85% when the sputtering time is not more than 60 min, while the external magnetic field has little effect on the transmittance of the films, but making a larger blue shift of the absorption edge. Ansys software is used to simulate the two-dimensional magnetic field distribution above the target. Results show that the intensity of the horizontal magnetic field and the uniformity of it are improved by the external magnetic field, the secondary electrons near the target are tightly bound, leading to a much larger target current intensity. So the deposition rate, surface topography and photoelectric properties of the AZO films are improved.

The Effects of Magnetic Field Size on the Electronic Structure of Al‐Doped ZnO Thin Films Studied by X‐ray Absorption and Emission Spectroscopy

This study examines Al‐doped ZnO (AZO) transparent conductive thin films prepared on glass substrate using the pulsed laser deposition method with an expanding magnetic field perpendicular to the sample surface. O K‐, Zn L3‐, and Al K‐edge X‐ray absorption near‐edge structure (XANES) and X‐ray emission spectroscopy (XES) were used to investigate the relationship between the effects of magnetic field size and the electronic structure of AZO thin films. Analysis of the XANES spectra showed increasing O 2p states as film resistance decreases, suggesting that the enrichment of the O 2p‐dangling bond along the c‐axis is the main factor affecting the electric performance of AZO thin films. Magnetic field size affects electrons itinerating from Zn atoms to Al sites through O 2p–Zn 3d and O 2p–Al 3sp hybridization and consequently weakens O 2p–Zn 3d hybridization with the downsizing of the crystallite size. XES and XANES spectra of O 2p states at the O K‐edge exhibit that the conduction‐band minimum affects Eg and the valence‐band maximum is nearly unaffected by changes in magnetic field size. Factors affecting the resistance/electric conductivity of AZO thin films are the preferential orientation of (002) along the c‐axis, the thermal/annealing effect accompanied by the changes of magnetic field size, the surface effect, energy bandgap, and the density of unoccupied O 2p‐derived states.

Mechanical and transparent conductive properties of ZnO and Ga-doped ZnO films sputtered using electron-cyclotron-resonance plasma on polyethylene naphtalate substrates

Transparent conductive ZnO and Ga-doped ZnO (GZO) films were deposited on polyethylene naphtalate (PEN) sheet substrates using electron cyclotron resonance plasma sputtering. Both ZnO and GZO films were highly adhesive to the PEN substrates without inserting an intermediate layer in the interface. When compared at the same thickness, the transparent conductive properties of GZO films on PEN substrates were only slightly inferior to those on glass substrates. However, the carrier concentration of ZnO films on PEN substrates was 1.5 times that of those on glass substrates, whereas their Hall mobility was only 60% at a thickness of 300 nm. The depth profile of elements measured by secondary ion mass spectroscopy revealed the diffusion of hydrocarbons out of the PEN substrate into the ZnO film. Hence, doped carbons may act as donors to enhance carrier concentration, and the intermixing of elements at the interface may deteriorate the crystallinity, resulting in the lower Hall mobility. When the ZnO films were thicker than 400 nm, cracks became prevalent because of the lattice mismatch strain between the film and the substrate, whereas GZO films were free of cracks. The authors investigated how rolling the films around a cylindrical pipe surface affected their conductive properties. Degraded conductivity occurred at a threshold pipe radius of 10 mm when tensile stress was applied to the film, but it occurred at a pipe radius of 5 mm when compressive stress was applied. These values are guidelines for bending actual devices fabricated on PEN substrates.

Effects of Thermal Treatment on Crystallographic and Electrical Properties of Transparent Conductive ZnO Films Deposited by Spin-Spray Method

ZnO films were deposited by a novel aqueous solution process named the spin-spray method and thermally treated at different temperatures from 100 to 300 °C. All films had a high transmittance of 80% in the visible range and exhibited conductivity after UV illumination for 24 h. The dependence of thermal treatment temperature before UV illumination on their crystallographic and electrical properties was investigated in this study. The XRD peak intensity increased and the full width at half maximum of the (101) peak decreased from 0.35 to 0.26° for the sample thermally treated at 100 °C. From the Fourier transform infrared (FT-IR) spectra, decreases in Vas (COO-) and Vs (COO-) intensities with increasing thermal treatment temperature were observed. The resistivity after the UV illumination of the sample without thermal treatment was 4.1×10-2 Ω cm, and it decreased to 1.6×10-2 Ω cm in the sample thermally treated at 100 °C. The sample with the lowest resistivity exhibited a relatively high mobility of 3.3 cm2 V-1 s-1 and a very high carrier concentration of 1.1×1020 cm-3 as the solution-processed ZnO film.

Development of high‐quality titanium‐doped gallium ZnO transparent conductive films by RF magnetron sputtering techniques

AbstractIn this paper, Ti‐doped GaZnO (GTZO) transparent conductive oxide (TCO) was deposited by radio‐frequency magnetron sputtering on glass substrates. X‐ray diffraction (XRD) of the crystalline quality of the resulting GTZO TCO showed a (002) peak with increased intensity for films deposited with annealing temperature from as‐grown to 450 °C. The low thin film resistivity of 1.57 × 10‐4 Ω‐cm, and average optical transmittance of 91% in the range between 350 and 800 nm, are also demonstrated to show the potential applications of this work. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of dual ion beam sputtered transparent conductive Ga-doped ZnO films

Ga-doped ZnO (GZO) transparent conducting films were deposited on sapphire (0001) substrates using dual ion beam sputtering deposition system. The impact of growth temperature on the structural, morphological, elemental, optical, and electrical properties was thoroughly investigated and reported. X-ray diffraction measurements explicitly confirmed that all GZO films had (002) preferred crystal orientation. The film deposited at 400 °C exhibited the narrowest full-width at half-maximum value of 0.24° for (002) crystalline plane and the lowest room temperature electrical resistivity of 4.11 × 10−3 Ω cm. The Raman spectra demonstrated the vibrational modes at 576 and 650–670 cm−1, associated with native oxygen vacancies and elemental Ga doping in ZnO lattice, respectively. All doped films showed an overall transmittance of above 95 % in the visible spectra. A correlation between structural, optical, elemental, and electrical properties with GZO growth temperature was established.

Wide band gap characteristic of quaternary and flexible Mg and Ga co-doped ZnO transparent conductive thin films

Transparent conductive and flexible Mg and Ga co-doped ZnO (MGZO) thin films were prepared on poly-ethylene terephthalate (PET) by RF magnetron sputtering technique at room temperature. The effects of different working pressures on the structural, chemical, morphological, optical and electrical properties of MGZO thin films were investigated. X-ray diffraction results indicate that all the MGZO thin films were grown as polycrystalline wurtzite structures without secondary phases such as MgO, Ga2O3, MgGa2O4, or ZnGa2O4. The MGZO thin film prepared at 6 mTorr has the lowest value of full width at half maximum. A typical survey spectrum of all the MGZO thin films confirmed the presence of Mg, Ga, Zn and O. The MGZO thin film prepared at 6 mTorr showed the widest optical band gap energy of 3.91 eV and lowest electrical resistivity of 5.3 × 10−3 Ω cm.

Characteristics of Ga-doped ZnO films deposited by pulsed DC magnetron sputtering at low temperature

Characteristics of Ga-doped ZnO (GZO) transparent conductive oxide films have been investigated based on the absorption behavior and chemical states of dopant Ga in the film. GZO samples were prepared by pulsed DC magnetron sputtering at 423K by varying the sputtering power from 0.6 to 2.4kW and the Ga2O3 concentration in the targets from 0.6 to 5.7wt%. Absorption spectra of the GZO films in the visible to ultraviolet range were characterized by long absorption tails and shoulders near the absorption edges indicating the presence of impurity states or bands that overlap with the conduction band. X-ray photoelectron spectroscopy and X-ray diffraction revealed that substantial portion of the dopant exists as finely dispersed or amorphous metallic Ga and oxide of Ga, which would be related to the formation of the impurity bands or states, especially in the samples with lower Ga content. Presence of these species is correlated to the limited doping efficiency observed in the GZO films.

Growth and optical properties of ZnO nanorod arrays on Al-doped ZnO transparent conductive film

ZnO nanorod arrays (NRAs) on transparent conductive oxide (TCO) films have been grown by a solution-free, catalyst-free, vapor-phase synthesis method at 600°C. TCO films, Al-doped ZnO films, were deposited on quartz substrates by magnetron sputtering. In order to study the effect of the growth duration on the morphological and optical properties of NRAs, the growth duration was changed from 3 to 12 min. The results show that the electrical performance of the TCO films does not degrade after the growth of NRAs and the nanorods are highly crystalline. As the growth duration increases from 3 to 8 min, the diffuse transmittance of the samples decreases, while the total transmittance and UV emission enhance. Two possible nanorod self-attraction models were proposed to interpret the phenomena in the sample with 9-min growth duration. The sample with 8-min growth duration has the highest total transmittance of 87.0%, proper density about 75 μm−2, diameter about 26 nm, and length about 500 nm, indicating that it can be used in hybrid solar cells.

The Effect of Sputtering Power on the Properties of Zr-Ga Co-Doped ZnO Films by DC Magnetron Sputtering

Zr-Ga co-doped ZnO transparent conductive films were prepared on glass substrates by DC magnetron sputtering at room temperature. The influence of sputtering power on the structural, electrical and optical properties of Zr-Ga co-doped ZnO films was investgated by X-ray diffraction, scanning electron microscopy (SEM), digital four-point probe and optical transmission spectroscopy. The lowest resistivity of the Zr-Ga co-doped ZnO films is 3.02×10-4Ω﹒cm and the average transmittance of the films is over 90% in the visible range. The obtained optical band gap of these films is much larger than of pure ZnO (3.34 eV).

Electrical and optical properties of Al-doped ZnO and ZnAl2O4 films prepared by atomic layer deposition

ZnO/Al2O3 multilayers were prepared by alternating atomic layer deposition (ALD) at 150°C using diethylzinc, trimethylaluminum, and water. The growth process, crystallinity, and electrical and optical properties of the multilayers were studied with a variety of the cycle ratios of ZnO and Al2O3 sublayers. Transparent conductive Al-doped ZnO films were prepared with the minimum resistivity of 2.4 × 10−3 Ω·cm at a low Al doping concentration of 2.26%. Photoluminescence spectroscopy in conjunction with X-ray diffraction analysis revealed that the thickness of ZnO sublayers plays an important role on the priority for selective crystallization of ZnAl2O4 and ZnO phases during high-temperature annealing ZnO/Al2O3 multilayers. It was found that pure ZnAl2O4 film was synthesized by annealing the specific composite film containing alternative monocycle of ZnO and Al2O3 sublayers, which could only be deposited precisely by utilizing ALD technology.

Effects of oxygen concentration on the properties of Al-doped ZnO transparent conductive films deposited by pulsed DC magnetron sputtering

The effect of oxygen concentration on the properties of Al-doped ZnO (AZO) transparent conductive films has been investigated on the films deposited by pulsed DC magnetron sputtering using a cylindrical ZnO target containing 2wt% Al. AZO films were deposited at 230°C to the thickness of about 1000nm and the oxygen concentration was controlled by varying the O2/Ar supply ratio from 0 to 0.167. With the increasing O2/Ar ratio, crystallinity of the AZO films deteriorated while the film surface became smooth. Accompanying this, electrical properties also deteriorated significantly. When the O2/Ar ratios were 0 and 0.033, the AZO films showed metallic conduction behavior with the electrical resistivity in the mid 10−4Ωcm range. However, when the ratios were 0.100 and 0.167, the films showed poor electrical conduction behavior similar to semiconductors as deduced from the transmittance behavior. Spectroscopic analysis showed that such deteriorating properties are due to the formation of condensed oxide group through the reaction between excess oxygen and dopant aluminum.