Conductive ZnO Thin Films Research Articles

Structural insight into nanoscale inhomogeneity of electrical properties in highly conductive polycrystalline ZnO thin films doped using methane

Recently, methane has been demonstrated as an effective n-type dopant for ZnO thin films deposited using the RF-magnetron sputtering method. It was shown that the major electrical doping effect of methane is caused by hydrogen released during methane decomposition. This work investigates the origin of the observed increase in conductivity of methane-doped ZnO films with the increase in thickness. The study is aimed at describing the nature of this thickness-dependent effect through a detailed analysis of the thickness-dependent morphology and crystalline structure. A combination of structural, electrical, and optical characterization revealed a transition from fine-grained films with a random orientation at early stages to partially (002)-textured films with columnar grains at later stages of growth. It is demonstrated that grain/sub-grain boundaries increase the electrical conductivity and that the contribution of such buried inner boundaries increases with increasing thickness. It is proposed that hydrogen diffuses along the grain and sub-grain boundaries during growth, leading to continuous doping of the buried interfaces. This hydrogen diffusion mechanism results in an apparent ‘additional doping’ of thicker films. The results provide new insights into the thickness-dependent conductivity of doped polycrystalline ZnO films mediated by hydrogen diffusion along internal interfaces.

Air and Vacuum Annealing Effect on the Highly Conducting and Transparent Properties of the Undoped Zinc Oxide Thin Films Prepared by DC Magnetron Sputtering

In this study, we aim to investigate the effect of zinc interstitials (Zni) and oxygen vacancies (VO) on the ZnO electrical conductivity. ZnO films were synthesized via DC magnetron sputtering process using pure Zn target in gases mixture of Ar/O2 = 80/17.5 (sccm). In order to improve the optical and electrical prosperities, the obtained films were subjected to air and vacuum annealing treatment. Several techniques such as field emission scanning electron microscopy (FESEM), Grazing Incidence X-ray Diffraction (GIXRD), Raman spectroscopy, photoluminescence spectroscopy (PL) and UV-visible were used to study the influence of heat treatment on structural and physical properties of ZnO films. Electrical conductivity of ZnO thin films was determined by measuring the sheet resistance and thickness of the films. XRD results confirm the synthesis of annealed ZnO films of the hexagonal structure with a preferential orientation along the (002) plane. The average crystallite size is altered between 22.6 to 28.4 nm dependent on the plan orientation of the ZnO film. Morphology and crystallinity of the ZnO structure could efficiently control the transmittance, electrical resistivity and optical band gap. As deposited ZnO film showed a lower electrical resistivity of 2.72×10-3 Ωcm due to the Zn-rich conditions. Under vacuum annealing, a combination of low resistivity (1.17×10-2 Ωcm) and better optical transmittance (87 %) are obtained. ZnO films developed in this study with high transmittance and low resistivity and good electro-optical quality supports their use in transparent and conductive electrode applications. The plan presentation was visualized using Vesta, with the lattice parameter set as follows: a = b = 3.249 Å; c = 5.207 Å; α = β = 90°; γ = 120°. Based on the construction and optimization of primitive cells, the supercells were constructed and then optimized. Finally, (002) and (103) planes were cut and the planar supercell structure was constructed. In order to make a plane representation for the solid bulk with 10 Å of thickness.

Improvement of the uniformity of structural and electrical properties of transparent conductive Al-doped ZnO thin films by inductively coupled plasma-assisted radio frequency magnetron sputtering

Transparent conductive Al-doped Zinc oxide thin films were deposited on glass substrates by conventional radio frequency magnetron sputtering (C-RFMS) and inductively coupled plasma-assisted radio frequency magnetron sputtering (ICP-RFMS) methods. The structural and electrical properties of films were studied as a function of the radial position (r) of the substrate in both methods. A good thickness uniformity was obtained in ICP-RFMS, while a significant decrease in film thickness was observed at r=30mm (erosion zone) in C-RFMS. The c-axis orientation was achieved in all positions in ICP-RFMS, while crystallinity was degraded at r≥25mm in C-RFMS. Low resistivity ρ of the order of 10−4Ωcm was obtained at all positions in ICP-RFMS, while ρ was rapidly increased at r≥30mm in C-RFMS. In ICP-RFMS, high-energy ion bombardment is suppressed due to the modified plasma density profile and lower space potential. As for transparency, the sample prepared at r=0mm in ICP-RFMS showed an average visible transmittance of 81%, higher than that in C-RFMS. Throughout the study, it was demonstrated that ICP-RFMS method was promising to improve the radial distribution of structural, electrical, and optical properties of Al-doped ZnO thin films.

Analysis of current-voltage curves of ZnO thin films under dark and optical stimulus

In this work, we are reporting the current conduction mechanism in ZnO thin films of various thicknesses under the dark and optical stimulus. The ZnO thin films of different thicknesses (45, 70, 110, 160, 220 nm) were grown on a glass substrate at 250 °C using the spray pyrolysis method. For photoconductivity measurements, electrodes were made using silver paste by putting a ∼1 mm gap between the electrodes. The photocurrent under UV illumination increases with increasing film thickness. Linear fitting of the data suggests the major role of space charges in the film for the rise in the photocurrent. Therefore the obtained conduction mechanism is found to be space charge-limited conduction in ZnO thin films under UV illumination.

Lithium doping effect on microstructural and electrical properties of zinc oxide thin film grown by metal-organic chemical vapor deposition

In this study, the undoped and Li-doped ZnO thin films were grown on Si(100) substrate using metal-organic chemical vapor deposition (MOCVD). Zinc acetylacetonate hydrate and lithium acetylacetonate solution were used as ZnO thin film precursor and Li dopant source. The effect of lithium doping on microstructural was characterized using a scanning electron microscope (SEM) and X-ray diffractometer (XRD). XRD diffractogram analysis shows that undoped and Li-doped ZnO thin films have polycrystalline hexagonal wurtzite structures with preferred peak crystal orientation (103). Li doping slightly changes the lattice parameters and cell volume of ZnO thin films through the increase of crystallite size and slightly affects the surface morphology of ZnO thin films. Current-voltage (I-V) measurement and four-point probe method were used to measure the electrical properties of lithium doped ZnO thin films. The electrical conductivity of ZnO thin films increases as Li doping is given compared to undoped films. These results are also supported by the I-V curve of Li-doped ZnO thin films by having a higher slope, indicating improvement in electrical properties.

Precursor molarity dependent growth rate, microstructural, optical and electrical properties of spray pyrolytically deposited transparent conducting ZnO thin films

The spray pyrolysis method is employed to deposit ZnO films at different precursor molarity (SC). The X-ray diffraction study reveals the polycrystalline film growth along [0 0 2], c-axis direction. The SC dependent variations of microstructural parameters are considered to propose the film’s growth mechanism. The growth rate increases with SC due to the rising number of Zn species till SC ​= ​0.5 ​M, above which it slightly decreases due to oxygen insufficiency. The rise in SC causes film thickness and crystallite size to increase, whereas bandgap energy decreases from 3.35eV to 3.22eV. At optimized SC ​= ​0.3 ​M, the film shows 96.3% transmittance with maximum electrical conductivity (93.9 ​Ω-1-cm−1), carrier concentration (4.94 ​× ​1019/cm3) and mobility (11.9 ​cm−2V−1s−1). The maximum figure of merit (1.76 ​× ​10−3 ​Ω−1) with the least sheet resistance (388 ​Ω/□) is recorded for films deposited at SC ​= ​0.3 ​M.

Robust and Electrically Conductive ZnO Thin Films and Nanostructures: Their Applications in Thermally and Chemically Harsh Environments

Robust and Electrically Conductive ZnO Thin Films and Nanostructures: Their Applications in Thermally and Chemically Harsh Environments

Electrodeposition of Zn-Co-Terephthalate MOF and Its Conversion to Co-Doped ZnO Thin Films

Energy security and sustainability are the most important challenges of mankind for their survival towards the 22nd century. While renewable electricity by solar and wind has already become so cheap, technologies for its conversion to easily storable chemical energies are urgently needed. Electrocatalysts not dependent on noble metals therefore need to be developed.We have recently succeeded in microwave (MW)-assisted hydrothermal synthesis of Co-doped ZnO nanoparticles, which exhibit catalytic activity towards water oxidation superior to Co3O4, although the electrode prepared by paste coating faced problems of its stability due to imperfect connection of the particles [1]. Electrodeposition can overcome this problem since well crystallized, stable and perfectly conducting ZnO thin films can be obtained. However, it is not at all straightforward to dope cobalt because of the large different of solubility (K sp = 5.92 × 10-15 and 3 × 10-17 for Co(OH)2 and Zn(OH)2, respectively), so that almost pure ZnO is solely electrodeposited even in the presence of Co2+ ions in the bath.Taking advantage of structural flexibility of metal-organic framework (MOF) could be a possible solution to achieve co-precipitation of Zn and Co. We have recently achieved cathodic electrodeposition of Zn-terephthalate (TPA) MOF thin films by addition a TPA into the ZnO electrodeposition bath [2]. Layered MOF structure was self-assembled during precipitation by bridging of double-layered Zn hydroxide with TPA.Herein, we report successful electrodeposition Zn-Co-TPA MOF precursor thin films and its conversion to oxides to yield Co-doped ZnO. Compositional and structural characterization as well as their electrocatalytic activities for oxygen evolution reaction (OER) are discussed.Potentiostatic electrolysis at an F-doped SnO2 (FTO) coated glass rotating disk electrode (RDE, ω = 500 rpm) was carried out at -1.0 V (vs. Ag/AgCl) in an O2-saturated aqueous electrolyte (70ºC) containing 5 mM ZnCl2, 200 μM TPA, and 0,5 mM CoCl2, for 30min. The film was annealed at 450℃ for 1 hour, under air.A white film electrodeposited in the presence of CoCl2 turned into green film after annealing, as expected for the formation of Co-doped ZnO. (Fig. 1a). X-ray diffraction pattern (XRD, Fig. 1b) indicates electrodeposition of Zn-TPA MOF and their conversion of ZnO after annealing both in the presence and absence of Co. The absorption spectra of the films on the other hand show clear difference between the samples, for which the one with Co and after annealing exhibit clear red-shift of onset for bandgap absorption and characteristic triple peaks at about 560, 610 and 650 nm arising from d-d transition of doped Co(II) ions (Fig. 1c). These features were commonly found for the Co-doped ZnO nanoparticles by MW reaction, so that similar or superior OER catalysis can be expected for the sample prepared by electrodeposition in this study. Figure 1

Pulsed laser deposited transparent and conductive V-doped ZnO thin films

ZnO and vanadium-doped ZnO (0.7–4.1 at.%) thin films were deposited onto corning glass substrates by the pulsed laser deposition technique using a KrF excimer laser (λ = 248 nm). The films were deposited at 500 °C under an oxygen pressure of 1 Pa with a laser fluence of 2 J/cm2. The structural, morphological, optical and electrical properties as a function of the dopant atomic concentration were investigated by means of X-ray diffraction, Scanning Electron Microscopy, spectrophotometry, conductivity and Hall measurements. All the doped and undoped films show a preferential orientation along the c-axis with a deterioration at higher doping levels (>4 at. %). Besides, as the doping amount increases the in-plane stress leads to an increase of the c-axis lattice parameter. The films are transparent within the wavelength range 400–1200 nm. The electrical resistivity of the films drops from 8.2 10−3 to 1.3 10−3 Ω cm with an increase in the dopant concentration up to 0.9 at. % and then rises as the dopant level is increased further.

Methane as a novel doping precursor for deposition of highly conductive ZnO thin films by magnetron sputtering

ZnO thin films were deposited by RF-magnetron sputtering of ZnO powder target using pure argon and argon with methane as reactive gas. It is found that growth morphology and electronic properties of the films are strongly affected by adding of methane to argon during the deposition process. Adding of methane resulted in a high energy shift of near band edge ultraviolet photoluminescence band and quenching of deep level emission in the visible spectral range. The strongest effect of methane has been found for electrical resistivity that reduced by 3 orders of magnitude in comparison with films deposited in pure argon. Unexpectedly, the analysis of the chemical composition showed no carbon incorporated from methane. Therefore, modification effects were assigned to hydrogen incorporation. However, the direct comparison of resistivity of the films deposited using methane and molecular hydrogen as doping precursors has demonstrated that doping efficiency of the methane is about an order of magnitude larger than that of molecular hydrogen under similar deposition conditions. This advantage of the methane is discussed and assigned to specific surface chemistry of Zn–O–C–H system that enhances the formation of shallow donor defects during plasma assisted deposition process.

High-rate reactive high-power impulse magnetron sputtering of transparent conductive Al-doped ZnO thin films prepared at ambient temperature

Reactive high-power impulse magnetron sputtering was used for high-rate (deposition rate of 60 nm/min) deposition of conductive (resistivity of 3 × 10−3 Ωcm) and optically transparent (extinction coefficient at the wavelength of 550 nm of 0.01) ZnO:Al thin films at ambient temperature (< 40 °C). We used planar Zn:Al target (3.09 at.% of Al) with diameter of 100 mm and thickness of 6 mm mounted on strongly unbalanced magnetron. The films were deposited in argon‑oxygen atmosphere at constant argon and oxygen partial pressure of 2.0 Pa and 0.1 Pa, respectively. An average target power and voltage pulse length were kept constant to 1.9 Wcm−2 and 200 μs, respectively. A pulse-averaged target power density was varied in the range of 190 Wcm−2–940 Wcm−2. Optical emission spectroscopy was used for better understanding of correlations between plasma discharge properties and structural, electrical and optical properties of prepared films.

Production of Transparent Conducting ZnO Thin Films with Preheated Temperature by Sol-Gel Spin Coating Technique

Production of Transparent Conducting ZnO Thin Films with Preheated Temperature by Sol-Gel Spin Coating Technique

Manipulation of optical and electrical properties of ZnO thin films via embedded nano structure

The semiconductor ZnO has gained great interest in the past decades because of its wide bandgap and large exciton binding energy. Especially, the doping and embedded metal nano-particles methods could improve the conductivity of ZnO thin films, which made it possible to fabricate transparent conducting oxides based on ZnO. In the present study, Al-doped ZnO thin layer was employed to manipulate the electrical and optical properties. The band engineering in ZnO makes it possible to manipulate the absorption, emission, reflection spectral of ZnO via the embedded quantum well layer. The ZnO thin films with excellent performance in electrical conductivity and optical transmittance are promising to be applied in the devices including the lager flat screen, touch panel, portable digital device and smart windows. In the present study, we summarized the recent progress in ZnO with embedded metal nanoparticles and application in TCOs.

High transparent and conductive undoped ZnO thin films deposited by reactive ion-beam sputtering

High transparent and conductive undoped ZnO thin films deposited by reactive ion-beam sputtering on Si and glass substrates were studied. Our films were found to be polycrystalline ones having a hexagonal wurtzite structure with c-axis preferred orientation. In the course of experiments employing X-ray diffraction, energy dispersive X-ray analysis, multi-angle spectral ellipsometry, atomic force microscopy, optical transmission/reflection and electrical measurements, the influences of substrate temperature and accelerating voltage on the structural, optical and electrical properties of ZnO thin films were revealed and discussed. It was shown that the beneficial conditions for the growth of high-quality film are as follows: substrate temperature ranging in 200–250 °C and acceleration voltage ranging in 5–6 keV. O-rich condition of deposition helped to obtain p-type conductivity, which, however, was not stable over time. The best films with thickness nearly 100 nm demonstrated resistivities of ∼2 mOhm∙cm, being 86% transparent in visible light. Also, free electron concentration of ∼2 × 1020 cm−3 and mobility of ∼17 cm2 V−1s−1 were achieved. All this implies that the as-grown undoped ZnO films are promising for the application as high conductive transparent electrodes.

Preparation of highly conducting Al-doped ZnO target by vacuum heat-treatment for thin film solar cell applications

In thin film solar cell technologies, room temperature deposition of highly conductive Al-doped ZnO thin films as transparent conductive oxide is one of the most important factors that prevents the distortion of the bottom layers. In this work, we prepared Al-doped ZnO targets at different heat-treatment pressures including 103 mbar (air), 10−2 mbar (low-vacuum) and 10−5 mbar (high-vacuum). The results indicated that by performing the heat-treatment in vacuum under a pressure of 10−2 mbar (low-vacuum heat-treatment), the electrical resistivity of the AZO target reached the lowest value compared to the other heat-treatment conditions. Subsequently, the AZO thin films with a thickness of 1150 nm were deposited on soda-lime glass substrates by RF magnetron sputtering at room temperature. The XRD patterns of all the as-deposited AZO thin films clearly showed that the thin films had a wurtzite structure with a preferred orientation along the c-axis. Furthermore, the Hall effect measurement results confirmed that the AZO thin film prepared from the low-vacuum heat-treated target had the excellent electrical properties in comparison with the other ones. The optical band gap of the as-deposited AZO thin films was about 3.15 eV and the minimum value of the electrical resistivity was measured about 1.8 × 10−4 Ω cm.

Characterization of conductive Al-doped ZnO thin films for plasmonic applications

Highly conductive and transparent Al-doped zinc oxide films were produced by RF magnetron sputtering for plasmonic applications in the infrared region of the spectrum. These films were characterized using Fourier transform infrared spectroscopy, the Hall effect, Rutherford backscattering spectroscopy and spectral data analysis. Analysis of the results shows a carrier concentration of up to 2.6 × 1020 cm−3, as well as transmission over 80% near the plasma frequency where plasmonic properties are expected. The plasma frequency was calculated from the spectroscopy measurements and subsequent data analysis, and was in agreement with the results from the Hall effect measurements and the free electron gas (Drude) model. Based on these results, the Al-doped zinc oxide thin films are well-suited for plasmonic applications in the infrared region.

Production of acceptor complexes in sol-gel ZnO thin films by Sb doping

Sb doped ZnO nanofilms were successfully deposited on glass substrates via economical sol-gel method. X-ray diffraction (XRD) patterns revealed the hexagonal wurtzite structure of the elaborated films and showed that the crystal growth of the doped samples is (002) preferentially oriented. X-ray photoelectron analysis suggested the successful substitution of antimony in Zn sites. According to XPS analysis, the presence of an important antimony ions states in fivefold coordination suggested the formation of the complex (SbZn−2VZn) level. It was found that low Sb doping (less than 1%) leads to slight broadening of the band gap and blue shift of the near band edge (NBE) emission of ZnO. It indicates the reduction of the intrinsic defects density for this doping regime. However, for Sb doping more than 1%, the optical results revealed the creation of an important defects density such as stacking faults and zinc vacancies (VZn) that lead to the formation of acceptors levels. From photoluminescence (PL) at low temperature, the presence of acceptor-bound exciton A0X (152meV) has been demonstrated. Stacking faults in ZnO seems to contribute particularly to the PL signal through the 3.316eV emission attributed to a free electron-to-neutral acceptor (e, A°) transition with binding energy of about 137meV. The p-type conductivity of ZnO thin films is confirmed by Hall measurement.

Tailoring of optical and electrical properties of transparent and conductive Al-doped ZnO films by adjustment of Al concentration

Highly optically transparent and electrically conductive Al-doped ZnO (AZO) thin films with varied Al concentrations were synthesized by means of reactive deposition of ZnO host films and in-situ doping of Al by pulsed laser co-ablation of a Zn target and an Al target with the assistance of an oxygen plasma. The optical and electrical properties of AZO films were tailored by the adjustment of Al concentration which was realized by varying the laser fluence on the Al target. The morphology, composition and structure as well as the optical and electrical properties were characterized and the effects of Al doping and post-deposition annealing were investigated. Similar with undoped ZnO, the synthesized AZO films have a hexagonal wurtzite structure with the crystallinity deteriorated, present high visible transparency with the absorption edge blue shifted and show good electrical properties with the electrical conductivity increased. The structural, optical and electrical properties are strongly dependent on Al concentration and are significantly improved after annealing in H2/N2 mixed gas. Annealed AZO films containing 3.0at% Al have optical properties including an absorption edge near 325nm and an optical band gap of 3.67eV and electrical properties covering an electrical resistivity of 5.27 × 10−4Ωcm and a carrier concentration of 1.11 × 1021cm−3 with a Hall mobility of 10.7cm2/V s.

A comparative study on physical properties of Al-doped zinc oxide thin films deposited from zinc acetate and zinc acetylacetonate by spray pyrolysis

Herein we present a comparative study on highly transparent, conductive aluminium-doped ZnO (AZO) thin films deposited by pneumatic spray pyrolysis (PSP) of a zinc acetate based solution and a zinc acetylacetonate based solution on soda lime glass. The structural, optical and electrical properties of the films were studied depending on aluminium content in the precursor solution and substrate temperature during deposition (Ts). The solution used to prepare AZO thin films contained 0.2 M zinc acetate or 0.2 M zinc acetylacetonate and aluminium acetylacetonate between 0–15 at.% [Al]/[Zn]. Substrate temperature was varied from 275 to 450 °C. Transmittance and reflectance spectra were recorded by UV-VIS-NIR spectroscopy. Resistivity, mobility and charge carrier density were determined by four point probe measurements in ambient conditions. According to X-ray diffraction analysis, AZO films are highly c-axis oriented until 2–3 at.% [Al]/[Zn] in the spray solution, depending on zinc precursor. Crystallite size is 25–30 nm for AZO films deposited at Ts 400 °C. All AZO films exhibited high average optical transmittance of 76–94% in the visible spectrum 400–800 nm wavelength range. Eg values ranged from 3.32 in undoped ZnO to 3.58 eV in in highly doped AZO films deposited from zinc acetylacetonate due to the formation of secondary AlOx phases. The lowest resistivities were obtained at Ts 400 °C for films deposited from zinc acetate, 3–7.5 at.% [Al]/[Zn] in solution (0.40 Ωcm) and at Ts 400 °C, 3–7.5 at.% [Al]/[Zn] in solution for films deposited from zinc acetylacetonate (0.47 Ωcm).

High mobility transparent conductive Al-doped ZnO thin films by atomic layer deposition

The effects of growth temperature on the microstructure, transport and optoelectronic properties of a series of Al-doped ZnO (AZO) films with thickness of ∼30 nm deposited on polished silicon-(100) and glass substrates by the atomic layer deposition (ALD) were investigated. By adopting an in-situ doping growth scheme the critical length effect associated with adjacent Al2O3 layers commonly encountered in previous ALD growth schemes was avoided and effective Al-doping was achieved with the growth temperature ranging from 100 °C to 300 °C. Experimental results showed that, in general, increasing the growth temperature would result in much improved film crystallinity and carrier mobility, with the average transmittance in the visible wavelength range being exceeding 95% in all cases. In particular, for AZO films grown at 300 °C, an unprecedented mobility of 136 cm2V−1s−1 was obtained, comparing to the typical values of 50–60 cm2V−1s−1 reported previously. The resistivity of these 300 °C films (ρ ≈ 6 × 10−4 Ω-cm), nevertheless, is slightly higher than that of some highly-doped ZnO (ρ ≈ 2–4 × 10−4 Ω-cm) prepared by sputtering methods. The secondary ion mass spectroscopy (SIMS) analyses revealed that hydrogen incorporation is the key in reducing the charge trap density and, hence, resulting in much enhanced carrier mobility. The present results promise a keen competitiveness of AZO with the indium tin oxide (ITO) film for thin-film-transistor (TFT) as well as in photovoltaic device applications.