ZnO Transparent Conductive Films Research Articles

Ga, Mg, F ternary-doped ZnO-based transparent conductive film prepared by radio-frequency sputtering with high transmittance

Ga/Mg/F ternary-doped ZnO transparent conductive films were prepared by radio-frequency (RF) magnetron sputtering. The influences of RF power, work pressure, and substrate temperature on the films are studied. The electrical and optical properties, structure and surface morphology of the films are discussed. Compared to other doped systems, the deposited films in this work exhibit excellent electrical and optical properties (ρ: 4.79 × 10−4 Ω·cm, T: 91.4%), which is comparable to commercial ITO. Besides, the films with the best performance show good stability. Our work has verified the advantages of triple-doped ZnO transparent conductive films, which is meaningful for future research and applications.

Effect of Substrate Temperature on Photoelectric Properties of AZO Thin Films

Herein, Al‐doped ZnO (AZO) thin films are prepared on quartz glass substrates by RF magnetron sputtering. The effects of substrate temperature (T s) on the structure, morphology, electrical, and optical properties of AZO films are investigated by X‐ray diffraction, field emission electron microscopy, Hall effect testing instrument, and ultraviolet spectrophotometer, respectively. The results show that the doping of Al element does not change the structure of ZnO films, and all films have typical hexagonal wurtzite structure with preferred c‐axis orientation. The film sample has the strongest (002) diffraction peak and the smallest value of full‐width at half‐maximum at the T s value of 150 °C. With the increase of T s value, the grain size of the film increases first and then decreases. At 150 °C, the film exhibits good electrical conductivity, and its resistivity is 0.6210−2 Ω cm, the carrier concentration is 9.841019 cm−3, and the Hall mobility is 13.66 cm2 V−1 s−1. All samples show high transmittance, and the average transmittance in the stable visible light region is as high as 95% at the T s value of room temperature and 250 °C, and the average transmittance of all samples is above 80%. This provides a method for the application of ZnO transparent conductive films.

Magnetron-sputtered Mg-Ga co-doped ZnO transparent conductive thin films: Microstructural and optoelectrical investigation

The Mg–Ga co-doped ZnO (MGZO) transparent conducting thin films (TCTFs) were fabricated via magnetron-sputtering. The dependence of microstructural, morphological and optoelectrical characteristics on sputtering power was investigated. The findings demonstrate that all the TCTFs present a wurtzite hexagonal crystal structure and (002)-preferred orientation. The sputtering power has a significant impact on the properties of the TCTFs. The sample fabricated at 150 W possesses the highest optoelectrical performance and crystalline quality, with the maximum figure of merit, highest average visible transmittance, minimum resistivity, lowest dislocation density and lattice strain of 1.042×104 Ω−1·cm−1, 92.21%, 1.181×10−3 Ω·cm, 1.041×1011 cm−2 and 3.936×10−3, respectively. Moreover, the optical constants (OCs) of the MGZO TCTFs were extracted by the optical spectrum fitting method (OSFM). The dispersion behavior of refractive index (RI) was assessed. The oscillator parameters, optical bandgaps and nonlinear OCs were realized. This study provides a reference basis for the applications of MGZO TCTFs in photoelectronic devices.

F, Mg and Ga co-doped ZnO transparent conductive thin films by dual-target magnetron sputtering: Fabrication, structure, and characteristics

F, Mg and Ga co-doped ZnO (FMGZO) films are deposited on glass substrates at room temperature by magnetron sputtering using MgF2 and ZnO:Ga targets. The effects of RF sputtering power applied to the MgF2 target on structure, morphology, composition, electrical and optical properties of the films are investigated in detail. The experimental results show that all samples are polycrystalline films with hexagonal wurtzite structure and low surface roughness. The FMGZO film deposited at RF sputtering power of 15 W exhibits the highest figure of merit of 5.66 × 10−2 Ω−1 with the resistivity of 6.5 × 10−4 Ω cm, the carrier concentration of 4.12 × 1020 cm−3 and the Hall mobility of 23.38 cm2/V s, while the average optical transmittance is as high as 95.23% in the visible range. The photoelectric performance of the FMGZO film is significantly higher than that of previously reported co-doped ZnO films, which makes it suitable for various high-efficiency optoelectronic devices.

Highly transparent conductive ZnO films prepared by reactive RF sputtering with Zn/ZnO composite target

Highly transparent conductive ZnO films prepared by reactive RF sputtering with Zn/ZnO composite target

Fabrication of Fluorine, Chlorine, and Gallium Co-Doped ZnO Transparent Conductive Films by Sol–Gel Spin Coating

Fabrication of Fluorine, Chlorine, and Gallium Co-Doped ZnO Transparent Conductive Films by Sol–Gel Spin Coating

Infrared Hf-doped ZnO Transparent Conductive Film

Infrared Hf-doped ZnO Transparent Conductive Film

Electrical and Optical Properties of Al-Doped ZnO Transparent Conductive Oxide Films Prepared via Radio Frequency Magnetron Co-Sputtering System.

In this study, the optical and electrical properties of a transparent conductive oxide (TCO) film synthesized via the radio frequency (RF) magnetron co-sputtering of Al-doped ZnO (AZO) and ZnO targets on a glass substrate were investigated. In the visible region, the resistivity, transmittance, and carrier concentration of the TCO film are influenced by the ratio of Al doping. The samples were prepared using two targets with the same deposition condition, except several different power levels on an AZO target to obtain different Al compositions in the film. The power range was 100-160 W in 20 W steps on the AZO target with a constant 50 W power level on the ZnO target. The electrical and optical characteristics of the film were measured using several apparatuses. The cross-section of the films was measured with via field emission scanning electron microscopy (FESEM) to determine the thickness of the film. The electrical and optical properties of the AZO films were measured via Hall measurement and UV-visible spectroscopy. The structural characteristics of the AZO films were confirmed by Raman spectroscopy.

ZnO transparent conducting thin films codoped with anions and cations

In the past two decades, ZnO related materials and devices are widely used for various applications, such as thin-film transistors, photodetectors, light-emitting diodes, and transparent conductors because ZnO has several appealing features like abundant raw materials, environmental friendliness, wide and direct band gap, high electron mobility, and large exciton bonding energy. Especially, ZnO has been recognized as one of the most promising candidates for substituting commercial transparent conductors ITO which is exploited as transparent electrodes in solar cells and flat planar display and transparent heaters etc. For example, aluminum doped ZnO (AZO) transparent conducting films have been commercialized successfully. However, the figure of merits and stability of ZnO transparent conducting films still need to be further improved in order to meet the strict demands of practical large-scale applications. So far, various techniques have been developed to alleviate that contradiction between conductivity and visible light transmittance. In the past few years, a new strategy called anion and cation co-doping in ZnO has been widely implemented to improve the figure of merit, in which IIIA group elements are used as cation dopants and fluorine is used as anion dopants. It is worth noting that F has several advantages as anion dopant in ZnO. First, fluorine has a similar ionic radius with oxygen, which will avoid large lattice distortion. Second, the substitution for oxygen with fluorine only disturbs the valence band of ZnO and leaves the conduction band unaffected, which is beneficial for achieving high electron mobility. In this review, the latest researches of F and boron, or aluminum, or gallium, or indium co-doped ZnO thin films are summarized from both the theoretical and the experimental aspects. Effects of anion and cation co-doping on the optical and electrical properties and thermal stability of ZnO thin films are systematically discussed. Both theoretical and experimental results show that cation and anion co-doping can effectively improve the conductivity and visible light transmittance, no matter ZnO thin films were prepared by physical vapor deposition or synthesized by chemical solution processs. Taking Al and F co-doped ZnO (AFZO) thin films prepared with pulse laser deposition as an example, the mobility of AFZO thin film was apparently higher than that of Al-doped ZnO at the same carrier concentration. The high carrier mobility of AFZO is due to that the F can passivate the surface defects and acceptor-like complex defects in ZnO. In addition, the highly electronegative F makes the AFZO thin films exhibit excellent thermal stability, e.g. maintaining high conductive even after air-annealing at 600°C. These results indicate that anion and cation co-doping can effectively improve the performance of ZnO transparent conductive thin films and broaden their applications. Despite these exciting achievements, challenges still remain in the field of anion and cation co-doped ZnO thin films. For example, the mechanism of the co-doping strategy has not been clarified because the formation energy of intrinsic defects in ZnO is relatively low and the defects configurations for anion and cation co-doping are more complicated than single element doping case. Besides, we are still short of effective techniques and theoretical methods to reveal and simulate the behavior of defect clusters in co-doped ZnO. It is also urgent to establish the industrial standard of ceramic targets for co-doped ZnO to eliminate the discrepancies between different groups. More efforts should also be made in developing new equipment for large-scale manufacturing co-doped ZnO thin films. We hope this paper will provide reference to the readers who are interested in transparent conducting oxides.

Improvement of characteristics of flexible Al-doped ZnO/Ag/Al-doped ZnO transparent conductive film using silver

In recent years, transparent conductive oxide (TCO) films that can be applied to flexible devices have attracted considerable attention. Among TCOs, indium tin oxide (ITO) is frequently used, but ITO contains the rare metal In and is inflexible. Therefore, the authors focused on an Al-doped ZnO (AZO) transparent conductive film as an alternative material for ITO. The authors prepared films on polyethylene naphthalate (PEN) substrates to prepare flexible electrodes. PEN substrates are sensitive to heat and must be deposited at low temperatures. However, there is a limit to improving the conductivity of AZO film formation at low temperatures. Therefore, the authors focused on the AZO/Ag/AZO multilayer transparent conductive films. The AZO layer was deposited by RF magnetron sputtering, and the Ag layer was deposited by electron beam evaporation. The electrical and optical characteristics were evaluated by changing the deposition conditions during the AZO and Ag deposition. Thus, when the thickness of the Ag layer was changed, the maximum transmittance was obtained at a thickness of 10 nm. Next, the upper AZO film was divided into room temperature/50 °C. When the upper AZO is formed at 50 °C, the already deposited Ag is affected by thermal oxidation. Ag was protected from thermal oxidation by first depositing the upper AZO at room temperature and then applying 50 °C, resulting in improved transmittance and resistivity. Furthermore, the resistivity and transmittance were improved by increasing the deposition rate of the Ag layer due to a decrease in the oxygen content in the Ag layer. Among the formed thin films, the best obtained characteristics were the resistivity of 1.24 × 10−4 Ω cm and the transmittance of 84.8%.

Innovative Wide-Spectrum Mg and Ga-Codoped ZnO Transparent Conductive Films Grown via Reactive Plasma Deposition for Si Heterojunction Solar Cells

In this work, wide-spectrum Mg- and Ga co-doped ZnO (MGZO) transparent conductive films are developed via reactive plasma deposition (RPD) technique with soft thin-film growth process. MGZO film with a work function of ~4.36 eV can be achieved within 12 min without any intentional substrate-heating treatment. 480nm-thickness MGZO film exhibits a low resistivity of ~9.9x10-4 Ωcm and a high transmittance of ~82.6% in the UV-VIS-NIR region (λ approximately 400 nm-1200 nm). XRD spectra show that MGZO films exhibit (103) preferred orientation as the film thickness increases. A silicon hetero-junction (SHJ) solar cell based on 480nm-thick MGZO at the front side is completed. Excellent continuity of MGZO film is proven by the cross-sectional SEM images and there are no cracks and pinholes on the top and bottom of the c-Si pyramids. Further efficiency improvements are achieved using an ultra-thin SnOx buffer layer with an ameliorated p-a-Si:H/TCO interface. Also, a silicon hetero-junction (SHJ) solar cell using M...

High optoelectronic performance of ZnO films co-doped with ternary functional elements of F, Al and Mg

For use as front electrodes in high-efficiency thin-film solar cells, ZnO transparent conductive films must have both broad spectral transmission and high conductivity. However, these characteristics are difficult to achieve simultaneously. In this work, F, Al, and Mg co-doped ZnO (FAMZO) films with superior electrical and optical properties were fabricated by RF magnetron sputtering and rapid thermal annealing (RTA). F and Al co-doping enables the film to maintain low resistivity and high transmittance in the visible and near-infrared regions, while Mg doping widens the optical band gap. Morphological characterization shows that the FAMZO films undergo recrystallization and growth during RTA, which further improves their electrical and optical properties. Our FAMZO films achieved a resistivity of 4.07 × 10−4 Ω cm, a mobility of 47.85 cm2/V, a carrier concentration of 3.21 × 1020 cm−3, and 90.54% average transmittance between 400 and 1400 nm. The excellent performance of our ZnO films makes them suitable for a broad range of applications in high-efficiency thin-film solar cells.

Effect of Titanium Doping on the Electrical Properties of Conducting ZnO Transparent Thin Films

ZnO transparent conductive thin films are the most prominent substitute materials in the development of Ti-oxide thin films. Zinc oxide is a unique material that exhibits dual properties; semiconducting and piezoelectric. In this paper,a comprehensive study of crystalline size using X-Ray diffraction patterns and TEM images of Ti-doped ZnO with nano TiO2 in rutile phase is made to understand the development and growth of nano devices. ZnO has higher excitation binding energy approximately 60 MeV which is multifunctional and can be utilized in the field of ferroelectric, ferromagnetic and piezoelectric materials.Ti-doped ZnO-based semiconductors and nanowire devices are also of great importance for the integration on a single chip. ZnO nanomaterials can also be applied for biosensors, UV detectors etc. Ti-doped ZnO shows ferromagnetic behavior at room temperature. Also a reviewed is done on the recent activities and progress of ZnO nanomaterial with the brief discussion of applications of ZnO low dimensional structure. Using chemical bath deposition (CBD) technique, Ti-doped ZnO thin films are successfully synthesized. The crystalline size is determined by X-ray diffraction. For the analysis of surface morphology, TEM is used. The dielectric properties and electrical conductivity of the synthesized film are analyzed at different temperature and frequencies

Insight of the doping mechanism of F and Al co-doped ZnO transparent conductive films

Transparent conductive oxide (TCO) films, as transparent electrodes, are widely used in thin-film solar cells. The performance of TCO film has a significant influence on the conversion efficiency of the film solar cell fabricated byusing it. Although the conductivity can be improved by increasing the carrier concentration, the transmittance in the long wave will be sacrificed. Therefore, the only feasible method is to increase the carrier mobility within a certain carrier concentration range, rather than increase the mobility by reducing carrier concentration. In this paper, the F and Al co-doped ZnO (FAZO) films are deposited on glass substrates (Corning XG) by an RF magnetron sputtering technique with using a small amount of ZnF<sub>2</sub> (1 wt.%) and Al<sub>2</sub>O<sub>3</sub> (1 wt.%) dopant. The influences of sputtering pressure on the structure, morphology and photoelectric characteristics of the films are respectively investigated by X-ray diffraction analysis, scanning electron microscope, Hall effect measurement, and ultraviolet–visible–near infrared spectrophotometry. All the thin films show typical wurtzite structure with the <i>c</i> axis preferentially oriented perpendicular to the substrate. With the increase of sputtering pressure, the deposition rate of FAZO film decreases, the crystallization quality is deteriorated, surface topography changes gradually from “crater-like” to co-existent “crater-like” and “granular-like”, and the surface roughness increases. The FAZO film deposited at 0.5 Pa presents the optimal performance with a mobility of 40.03 cm<sup>2</sup>/V·s, carrier concentration of 3.92 × 10<sup>20</sup> cm<sup>–3</sup>, resistivity of 3.98 × 10<sup>–4</sup> Ω·cm, and about 90% average transmittance in a range of 380－1200 nm. The theoretical result shows that the co-doping of F and Al takes the advantages of single F and Al doped ZnO films, and overcomes the shortcoming of metal elements doping, which donates the carriers just from doped metal elements. Furthermore, the co-doping of F and Al not only increases the carriers but also reduces the scatterings caused by the inter-orbital interaction of doped atoms. The doped F 2p electron orbitals repel the O 2p and Zn 4s electron orbitals, making them move down and donate electrons. At the same time, the orbitals of Al 3s and Al 3p also make a contribution to the conductivity. After co-doping of F and Al, both the carrier concentration and conductivity increase significantly.

Structure, optical and electrical properties of Nb-doped ZnO transparent conductive thin films prepared by co-sputtering method

Nb-doped ZnO thin films were fabricated on glass substrates by using co-sputtering with direct-current and radio frequency magnetron sputtering. The structures, optical and electrical performances of Nb-doped ZnO thin films were investigated. The results showed that all thin films have (0 0 2) [Formula: see text]-axis preferential orientation. The minimum resistivity of [Formula: see text][Formula: see text]cm and the maximum carrier concentration of [Formula: see text][Formula: see text][Formula: see text] were obtained at the direct-current sputtering power of 10[Formula: see text]W, respectively. Nb-doped ZnO thin films have also shown high average transmittance of 89.6%, and lower surface roughness of 2.74[Formula: see text]nm. Meanwhile, a distinct absorption edge in the ultraviolet range of 300–400[Formula: see text]nm was observed in absorbance, the optical band gap of Nb-doped ZnO thin films illustrates an increased tendency with increasing Nb concentration.

Effect of Electron Beam Annealing Duration on the Properties of Zno Thin Films

The effect of electron beam (EB) annealing duration on the properties of pure ZnO thin films was studied. Pure ZnO precursor films prepared by sol-gel spin coating method were directly annealed by EB for different duration from 20 s to 480 s. The structural, optical and electrical properties were investigated. XRD patterns show (002) preferential growth for all the ZnO films. SEM results reveal that the average grain size changes from ∼10 nm to ∼70 nm with the increase of annealing duration and AFM results show the surface RMS roughness is less than 3 nm. The optical transmittance spectra present the absorption edge of the films is at about 380 nm and the average transmittance is more than 85% in visible region. It’s exciting to find that the ZnO film annealed for only 120 s exhibits resistivity 2×10−2 Ωcm and carrier concentration as high as 5.5×1019 cm−3, which shows an advantage of EB annealing in the preparation of ZnO transparent conductive films.

Photoelectric properties of F-doped ZnO thin films prepared by sol-combustion

ABSTRACTIn this study, fluorine-doped ZnO (FZO) transparent conductive film was prepared using sol-combustion method at a low temperature. The optimized synthesis conditions were evaluated based on the analysis of crystal structure, morphology, and photoelectric properties. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–visible absorption spectroscopy (UV–vis), and Hall test (HALL) show that under the optimal ratio of precursor solution and optimal doping concentration (10%), the sol-combustion method can produce FZO transparent conductive film with a higher transmittance (80%), higher conductivity (36.9 Ω cm), and better crystalline quality and surface morphology at a relatively low temperature of 250°C. The FZO films produced using sol-combustion method have good application prospect.

Preparation and characterization of Mg, Al and Ga co-doped ZnO transparent conductive films deposited by magnetron sputtering

Mg, Al and Ga co-doped ZnO (AGMZO) thin films were deposited on flexible substrates by rf magnetron sputtering using ceramic targets at room temperature. The effect of Mg content on structural, morphological, electrical and optical properties of the films was investigated in detail. X-ray photoelectron spectroscopy measurements indicated that the Mg content in the films was substantially consistent with that of corresponding targets. X-ray diffraction (XRD) results revealed that the films had the hexagonal wurtzite structure. The (0 0 2) preferred orientation of the crystallites detected by XRD suggested that the grains grew perpendicular to the surface of the film, which was confirmed by the cross-section image. The average transmittance in the visible range was above 82% for all the films. The lowest resistivity of 2.8 × 10−3 Ω cm with carrier concentration of 1.54 × 1020 cm−3 and Hall mobility of 14.6 cm2/Vs can be obtained. The optical transmission spectra revealed that when the Mg concentration increased, the UV absorption edge moved toward the shorter wavelength direction and the optical band gap increased from 3.4 to 3.5 eV.

Photoluminescence studies of transparent conductive ZnO films to identify their donor species

We studied the correlation between defect species, as probed by using photoluminescence (PL), and the transparent conductive properties of undoped ZnO films sputter-deposited on glass substrates at room temperature. The near-stoichiometric but slightly oxygen-deficient ZnO films had resistivities of 3×10−3 Ωcm and optical transmittances of 85% at visible wavelengths. The PL spectra exhibited only a band-edge emission peaking at 380 nm, suggesting that intrinsic defects were not the origin of the n-type conduction. Post annealing at 500°C in a vacuum reduced the carrier concentration by five orders of magnitude. However, the spectral features of the PL remained intact; i.e., the change was only attenuated band-edge emission. These observations can be consistently explained if we suppose that the donors are hydrogen impurities. Colored ZnO films deposited under a reducing condition had resistivities of 2−4×10−3 Ωcm, and their optical transmittances were 50−70% because of Zni atoms. Post annealing at 500°C desorbed some Zni atoms, and consequently transparency increased. The resultant PL spectra exhibited an emission at 396 nm accompanied with a deep-level emission at 400−500 nm, each corresponding to transitions from the conduction band to VZn and from Zni to the valence band. Thus, Zni was the primary donor in the Zn-rich films. Deposition under a flow of O2 gas produced resistive ZnO films. Incorporating excess oxygen atoms disordered the crystal lattice, as indicated by the broad deep-level emissions from Zni, Oi, VZn, and VO. The high resistivity was due to charge compensation between donors (Zni) and acceptors (Oi and VZn).

Silicon surface passivation by transparent conductive zinc oxide

Surface passivation is essential for high-efficiency crystalline silicon (c-Si) solar cells. Despite the common use of transparent conductive oxides (TCOs) in the field of solar cells, obtaining surface passivation by TCOs has thus far proven to be particularly challenging. In this work, we demonstrate outstanding passivation of c-Si surfaces by highly transparent conductive ZnO films prepared by atomic layer deposition. Effective surface recombination velocities as low as 4.8 cm/s and 11 cm/s are obtained on 3 Ω cm n- and p-type (100) c-Si, respectively. The high levels of surface passivation are achieved by a novel approach by using (i) an ultrathin SiO2 interface layer between ZnO and c-Si, (ii) a sacrificial Al2O3 capping layer on top of the ZnO film during forming gas annealing, and (iii) the extrinsic doping of the ZnO film by Al, B, or H. A combination of isotope labeling, secondary-ion mass spectrometry, and thermal effusion measurements showed that the sacrificial Al2O3 capping layer prevents the effusion of hydrogen from the crystalline ZnO and the underlying Si/SiO2 interface during annealing, which is critical in achieving surface passivation. After annealing, the Al2O3 capping layer can be removed from the ZnO film without impairing the high levels of surface passivation. The surface passivation levels increase with increased doping levels in ZnO, which can be attributed to field-effect passivation by a reduction in the surface hole concentration. The ZnO films of this work are suitable as a transparent conductor, an anti-reflection coating, and a surface passivation layer, which makes them particularly promising for simplifications in future solar cell manufacturing.