Conductive ZnO Research Articles

Enhanced Performance in Al-Doped ZnO Based Transparent Flexible Transparent Thin-Film Transistors Due to Oxygen Vacancy in ZnO Film with Zn-Al-O Interfaces Fabricated by Atomic Layer Deposition.

Highly conductive and optical transparent Al-doped ZnO (AZO) thin film composed of ZnO with a Zn-Al-O interface was fabricated by thermal atomic layer deposition (ALD) method. The as-prepared AZO thin film exhibits excellent electrical and optical properties with high stability and compatibility with temperature-sensitive flexible photoelectronic devices; film resistivity is as low as 5.7 × 10-4 Ω·cm, the carrier concentration is high up to 2.2 × 1021 cm-3. optical transparency is greater than 80% in a visible range, and the growth temperature is below 150 °C on the PEN substrate. Compared with the conventional AZO film containing by a ZnO-Al2O3 interface, we propose that the underlying mechanism of the enhanced electrical conductivity for the current AZO thin film is attributed to the oxygen vacancies deficiency derived from the free competitive growth mode of Zn-O and Al-O bonds in the Zn-Al-O interface. The flexible transparent transistor based on this AZO electrode exhibits a favorable threshold voltage and Ion/Ioff ratio, showing promising for use in high-resolution, fully transparent, and flexible display applications.

Realization of highly conductive Ga-doped ZnO film with abnormally wide band-gap using magnetron sputtering by simply lowering working pressure

Sputter growth of highly conductive Ga-doped ZnO films with abnormally wide band-gap (Eg) is discussed in detail, as well as the accompanying defect behavior. It was observed that the optical Eg and the conductivity increased as the working pressure of the Ar plasma decreases. The ZnGaO films with Eg of 3.95 eV and resistivity of 2 × 10−4 Ω cm was obtained by simply lowering the working pressure to 0.3 Pa, which represent the highest Eg value so far reported for the ZnO-based transparent conducting materials with a resistivity lower than 10−3 Ω cm. It was proposed that this phenomenon may be attributed to the improved crystalline quality and the increased amount of the Ga incorporation into the Zn sites and the oxygen vacancy concentration. It was also suggested that the oxygen vacancy acts as the electron donor for the sputtered ZnO and ZnGaO films.

Mechanisms involved in the hydrothermal growth of ultra-thin and high aspect ratio ZnO nanowires

Hydrothermal synthesis of ZnO nanowires (NWs) with tailored dimensions, notably high aspect ratios (AR) and small diameters, is a major concern for a wide range of applications and still represents a challenging and recurring issue. In this work, an additive-free and reproducible hydrothermal procedure has been developed to grow ultra-thin and high AR ZnO NWs on sol-gel deposited ZnO seed layers. Controlling the substrate temperature and using a low reagent concentration (1mM) has been found to be essential for obtaining such NWs. We show that the NW diameter remains constant at about 20–25nm with growth time contrary to the NW length that can be selectively increased leading to NWs with ARs up to 400. On the basis of investigated experimental conditions along with thermodynamic and kinetic considerations, a ZnO NW growth mechanism has been developed which involves the formation and growth of nuclei followed by NW growth when the nuclei reach a critical size of about 20–25nm. The low reagent concentration inhibits NW lateral growth leading to ultra-thin and high AR NWs. These NWs have been assembled into electrically conductive ZnO nanowire networks, which opens attractive perspectives toward the development of highly sensitive low-cost gas- or bio-sensors.

Surface microstructure evolution of highly transparent and conductive Al-doped ZnO thin films and its application in CIGS solar cells

Aluminum-doped zinc oxide (AZO) has attained intensive attention as being a very good transparent conducting oxide for photovoltaic applications. In this work, AZO films have been deposited on glass substrate by radio frequency (RF) magnetron sputtering. The influences of substrate temperatures on morphological, structural, optical and electrical properties of AZO films were systematically investigated. The results indicate that all AZO films have the hexagonal structure with c-axis preferred orientation. Morphological and electrical measurements have revealed that the substrate temperatures have strong influence on the microstructure, optical and electrical properties of AZO films. The AZO film is highly transparent from ultraviolet up to near infrared range with highest average transparency exceeding 83%. The minimum resistivity is as low as 6.1×10−4Ωcm. The carrier concentration and mobility are as high as 3.357×1020cm−3 and 30.48cm2/Vs, respectively. Finally, the performances of the AZO film are evaluated by its practical application in Cu(In1-xGax)Se2 (CIGS) photovoltaic device as a transparent electrode. Benefited from its highly transparent and conductive feature, the most efficient device reveals an efficiency of 7.8% with a short-circuit current density of 28.99mA/cm2, an open-circuit voltage of 430mV, and a fill factor of 62.44 under standard conditions.

Structural, optical and electrical properties of transparent conductive ZnO:Ca films prepared by rf magnetron sputtering from aerogel nanopowders

In the present work, we have deposited calcium doped zinc oxide thin films by magnetron sputtering technique based nanocrystalline particles elaborated by sol-gel method. In the first step, the nanoparticles were synthesized by sol-gel method using supercritical drying in ethyl alcohol. The structural studied by X-ray diffractometry indicates that CZO has a polycrystalline hexagonal wurzite structure with a grain size of about 30nm. Transmission electron microscopy (TEM) measurements have shown that very small particles of CZO are present in the aerogel powder. In the second step, calcium-doped zinc oxide (CZO) films were deposited onto glass substrates by rfmagnetron sputtering at ambient temperature using. The effects of calcium on structural, electrical, morphological and optical properties were investigated. It is found that all the thin films have a preferential c-axis orientation. With increase of Ca doping, the peak position of the (0 0 2) plane is shifted to the high 2θ values. A minimum resistivity of 5.10−3Ωcm is obtained for the film doped with 3% Ca. The films are highly transparent in the visible wavelength region with a transmittance higher than 85% and optical band gap ranging from 3.30 to 3.42eV.

Properties of transparent conductive boron-doped ZnO thin films deposited by pulsed DC magnetron sputtering from Zn1−x B x O targets

Transparent conducting B-doped ZnO thin films were deposited on normal soda-lime glass substrate by pulsed DC magnetron sputtering from homemade Zn1−x B x O ceramic targets. All the Zn1−x B x O targets are single-phase hexagonal wurtzite structure. After introducing B dopant into ZnO, the targets have a slight c-axis orientation and show more compact than the undoped target. The effect of B doping concentration on the crystallization behaviors, morphological, electrical, and optical properties of the Zn1−x B x O films was systematically investigated. XRD patterns reveal that both the B-doped and undoped films exhibit hexagonal wurtzite structure with strong c-axis orientation. With increasing the B doping concentration, the c-axis orientation and the calculated grain size of the Zn1−x B x O films based on the XRD data decrease. The surface morphologies of the films are very flat, and the transmittance spectra of the films show mean values higher than 90% in the visible range. The B-doped ZnO film with the lowest resistivity of 2.1 × 10−3 Ω cm was achieved by sputtering the Zn0.99B0.01O ceramic target.

Sequential PLD in oxygen/argon gas mixture of Al-doped ZnO thin films with improved electrical and optical properties

Highly conductive transparent Al-doped ZnO (AZO) thin films were obtained at room temperature through sequential PLD (SPLD) from Zn and Al metallic targets in an oxygen/argon gas mixture. We have investigated the structural, electrical and optical properties as a function of the oxygen/argon pressure ratio in the chamber. The measured Hall carrier concentration was found to increase with argon injection from 1.3×1020 to 6.7×1020cm−3, while the laser shots ratio for Al/Zn targets ablation was kept constant. This increase was attributed to an enhancement of the substitution doping into the ZnO lattice. The argon injection also leads to an increase of the Hall mobility up to 20cm2V−1s−1, attributed to a reduction of interstitial-type defects. Thus, the approach of using an oxygen/argon gas mixture during SPLD from metallic targets allows obtaining at room temperature AZO samples with high optical transmittance (about 90%) and low electrical resistivity (down to 5.1×10−4Ωcm).

Room-temperature deposition of flexible transparent conductive Ga-doped ZnO thin films by magnetron sputtering on polymer substrates

Flexible Ga doped ZnO transparent conductive films were prepared on polymer substrates at room temperature by RF magnetron sputtering. The effect of the Ar flow rate is more important than that of r.f. power and sputtering pressure. The dependence of the structural, electrical, optical and adhesive properties of flexible Ga doped ZnO transparent conductive thin films on the Ar flow rate was investigated for the first time. When the Ar flow rate is 40 sccm, the residual stress is changed from compressive stress to tensile stress, indicating the crystallinity of the film is best. And the lowest square resistance of 7.9 Ω/sq. is obtained at the Ar flow rate of 40 sccm. Regardless of the Ar flow rate, the optical transmittance in the visible light of all the films is about 90%. The best adhesive behavior is obtained at the Ar flow rate of 40 sccm.

(Invited) Photoluminescence Properties of ZnO Nanostructures Growth Vertically on Transparent and Conductive Al-Doped ZnO Substrates

ZnO nanostructures were fabricated on transparent conductive Al-doped ZnO substrates by a novel multi-annealing process in which the reducing annealing and oxidization annealing were carried out in turn. As the result, the fabricated ZnO nanostructures were aligned vertically on the Al-doped ZnO film under the optimizing annealing parameters during both reduction and oxidization reactions. The photoluminescence properties of ZnO nanostructures were influenced significantly by the annealing processes. An intense blue-green emission while a suppressed UV emission was observed from obtained ZnO nanostructures, as well as the high transmittance over 65%. The growth mechanism of the ZnO nanostructures was revealed in detail.

Effect of aging under ambient conditions on the optical properties of Al-doped ZnO thin films deposited by direct current sputtering

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited on a glass substrate by direct current sputtering. In a previous study, we noted the influence of time on structural and electrical characteristics of films. In the present paper, the effect of a two-year aging under ambient conditions on the optical properties was investigated. A global improvement of the optical properties of AZO thin films was observed. The optical transmittance spectra revealed a high transmittance more than 90% in the Vis-NIR regions and a high absorption in the ultraviolet range. It is assumed that the crystallinity segregation leads to the decrease of optical scattering. The results from the optical measurements showed a reorganization of the structure leading to the degradation of the structural homogeneity. Nevertheless, the evolution of the figure of merit shows that Al-doped ZnO is a good candidate for the manufacturing and the commercialization of transparent conducting oxide devices.

Sol-gel derived Al-doped zinc oxide – Reduced graphene oxide nanocomposite thin films

Transparent conductive ZnO films doped with 0–6 mol% of Al3+ (or AZO films) and AZO- rGO (rGO - reduced graphene oxide, ∼0–0.5 g/L) nanocomposite films were deposited on silica glass substrates by sol-gel multilayer spin-coating, with annealing at 250 °C in vacuum. The effects of the processing conditions on the structure, optical and electrical properties of the AZO-rGO films, as well as the AZO thin films for comparison, were investigated by XRD, Raman spectroscopy, TEM and SEM, UV-Vis-NIR spectroscopy, and with the four-point prober and Hall effect measurements. Al3+ ions and rGO sheets existed in the composite films, and both of more Al3+ amount and more rGO could result in worse crystallinity of ZnO. More amount of rGO led to more rGO flakers existed in the surface of the AZO-rGO films. Increasing Al-doping amount could not obviously affect the optical transmittances of the AZO-rGO films, but lead to the UV absorption edge of the films slightly blue-shifted. Optical transmittances of the AZO-rGO films decreased obviously with increasing the rGO ratio, but slightly decreased with increasing the annealing time. The minimum sheet resistance R□ could be obtained when Al-doping amount was ∼2 mol% and rGO ∼0.25 g/L: 50.1 kΩ/□ for the AZO film and 15.2 kΩ/□ for the AZO-rGO film. Addition of rGO could improve the electric conductivity of AZO films. Longer time of vacuum-annealing could also improve the electric conductivity of the AZO-rGO films. For the AZO-rGO film doped with 2 mol% Al3+ and 10 mg rGO after vacuum-annealing for 12 h, Hall mobility was ∼17.7 cm2/Vs and the carrier concentration was ∼10.6 × 1019/cm3.

Biaxial stress and optoelectronic properties of Al-doped ZnO thin films deposited on flexible substrates by radio frequency magnetron sputtering.

Transparent conductive Al-doped ZnO (AZO) thin films were deposited on polyethylene terephthalate (PET) and polycarbonate (PC) substrates using radio frequency (RF) magnetron sputtering. The biaxial stress was measured with a double beam shadow moiré interferometer, and x-ray diffraction (XRD) was used to investigate the crystal orientation of ZnO. The substrate temperature was varied from room temperature to 150°C in steps of 25°C. The experimental results showed that the residual and shearing stresses increased with the increase in substrate temperature. The residual stress can be separated into principle and shearing stresses by Mohr's circle rule, and the shearing stress (tensile stress) was different from the compressive stress of the residual stress. However, the optimal substrate temperatures for PET and PC were 75°C and 100°C, and the shearing stresses were 424.82 and 543.68 MPa, respectively. AZO/PET and AZO/PC thin films cracked at substrate temperatures of 75°C and 100°C, respectively. AZO/PET thin film at a substrate temperature of 100°C had a resistivity low to the order of 10-3 Ω-cm.

Deposition temperature dependence and long-term stability of the conductivity of undoped ZnO grown by atomic layer deposition

Understanding the stability and deposition parameter dependence of intrinsically conductive undoped ZnO prepared by thermal atomic layer deposition is mandatory for future applications. The authors investigate the conductivity of ZnO films deposited at temperatures between 100 and 200 °C as well as its evolution over a period of 160 days under different storing conditions. Most importantly, the conductivity increases by about 1 order of magnitude when the deposition temperature is increased from 100 to 150 °C. Highest conductivities of up to 170 S/cm are reached for ≥175 °C, and these samples do not show any aging effects of the conductivity under ambient storing conditions. In contrast, for deposition temperatures ≤150 °C, accelerated aging led to a significant decrease in conductivity. The best trade-off between the low deposition temperature and good long-term stable conductivity is found to be at 175 °C. A correlation between the intensity of the well-known defect photoluminescence peak (∼1.9 eV) and the conductivity was observed, which indicates that both are related to the same physical origin.

Characterization of dopant density dependence on transmittance and resistance of ZnO films fabricated using spin-coating method

ABSTRACTWe fabricated aluminum (Al) and gallium (Ga) co-doped ZnO films by wet-process and evaluated their sheet resistances and transmission spectra. It was found that the sheet resistance becomes minimum when the content ratios of Al and Ga are 0.02 at the annealing temperature of 550 degree. Moreover, we found that the sheet resistance can be improved by performing the spin-coating and annealing twice. In this case, the transmission probability in the visible light regime is more than 90%. This result is a simple method to fabricate fine transparence conductive ZnO films.

Lithium Cobalt Oxides Functionalized by Conductive Al-doped ZnO Coating as Cathode for High-performance Lithium Ion Batteries

LiCoO2 (LCO) has been functionally modified with electronic conductive Al-doped ZnO (AZO) via a sol-gel method. The physicochemical characterization results demonstrate that the AZO layer is coated onto the surface of LiCoO2 particles successfully, and the AZO-coated LCO material shows a higher electronic conductivity than pristine sample. The LCO sample coated by 2wt.% AZO shows the excellent cyclability with a capacity retention of 98.2% after 50 cycles in the voltage range of 2.75–4.5V under room temperature. More importantly, this material exhibits outstanding rate capability and delivers a quite high capacity of 156mAhg−1 at the rate of 8C (1600mAg−1), corresponding to 86.6% of the capacity at the rate of 0.1C. Such superior electrochemical performance results from the effective coating of AZO layer which not only acts as a physical protection layer to stabilize the surface structure, but also supplies the conductive networks to decrease charge transfer impedance. The conducting functionalized AZO coating is a facile and efficient approach to improve the electrochemical performance of LCO at a high cut-off voltage of 4.5V vs. Li+/Li for large-scale application of high energy density lithium ion batteries.

Effect of N2 flow during deposition on p-type ZnO film

In this study, the influence of a nitrogen source on p-type conductive ZnO films was studied. Rapid thermal oxidation was conducted to oxidize ZnN films and convert them to ZnO films. When an as-deposited ZnN film was prepared at a high nitrogen gas flow rate, the converted ZnO film possessed many acceptors and showed stable p-type conduction. This p-type conduction was attributed to the nitrogen gas flow providing many “No” states, which act as acceptors within the processed ZnO film. It was found that the as-deposited ZnN film prepared at a high nitrogen gas flow rate is oxidized slightly so that only a few nitrogen atoms were replaced by oxygen. The carrier concentration and mobility of the optimized oxidized ZnN film were 9.76 × 1017 cm−3 and 62.78 cm2 V−1 s−1, respectively. A good rectified current–voltage characteristic with a turn-on voltage of 3.65 V was achieved for the optimized ZnO:N/ZnO junction.

Nanoarchitectured Semiconducting Photoelectrodes for Enhanced Stability and Photon Conversion Efficiency

The characteristics of two dimensional (graphene), one dimensional (single walled carbon nanotube incorporated poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, composite) and zero dimensional (ZnO quantum dots) nanostructures have been systematically analyzed to elucidate the possibility of employing such nanostructures (transparent conducting corrosion resistant (TCCR)) for the passivation of the surface of a photoelectrode. Thin layer of graphene, carbon nanotubes embedded on a conductive polymer matrix (PEDOT:PSS) and ZnO quantum dots are incorporated on the surface of photoelectrode namely ‘In’ rich CuInS2 (ICIS) to act as corrosion resistant layers. The TCCR layers are found to be effective in inhibiting the surface recombination of charge carriers. Graphene exhibits a higher tendency towards the suppression of surface recombination across electrode-electrolyte interface, where an enhancement by a factor of 104 s is observed in the electron recombination life time. SWCNT/PEDOT:PSS composite films when employed as transparent conducting corrosion resistant layers enabled an enhancement of the cell stability by 95% when compared to unprotected ‘In’ rich CuInS2 electrode. The TCCR layers are found have a positive impact on the conversion efficiency of the photoelectrode and the maximum enhancement in the conversion efficiency achieved is 71% with ZnO quantum dot sensitized electrodes.

(Invited) Photoluminescence Properties of ZnO Nanostructures Growth Vertically on Transparent and Conductive Al-Doped ZnO Substrates

ZnO nanostructures were fabricated on transparent conductive Al-doped ZnO films by a novel multi-annealing process in which the reducing annealing and oxidization annealing were carried out in turn to achieve the re-crystallization of ZnO film on AZO film. As the result, ZnO nanostructures with good alignment were fabricated vertically on the AZO film with the high transmittance over 80%. The photoluminescence properties of ZnO nanostructures were influenced significantly by the annealing processes. An intense blue-green emission centered at 491nm in the visible range was observed from obtained ZnO nanostructures. The growth mechanism of the ZnO nanostructures was investigated in detail.

High carrier concentration ZnO nanowire arrays for binder-free conductive support of supercapacitors electrodes by Al doping

Doping semiconductor nanowires (NWs) for altering their electrical and optical properties is a critical strategy for tailoring the performance of nanodevices. Here, we prepared in situ Al-doped ZnO nanowire arrays by using continuous flow injection (CFI) hydrothermal method to promote the conductivity. This reasonable method offers highly stable precursor concentration for doping that effectively avoid the appearance of the low conductivity ZnO nanosheets. Benefit from this, three orders of magnitude rise of the carrier concentration from 1016cm−3 to 1019cm−3 can be achieved compared with the common hydrothermal (CH) mothed in Mott-Schottky measurement. Possible effect of Al-doping was discussed by first-principle theory. On this basis, Al-doped ZnO nanowire arrays was developed as a binder-free conductive support for supercapacitor electrodes and high capacitance was triggered. It is owing to the dramatically decreased transfer resistance induced by the growing free-moving electrons and holes. Our results have a profound significance not merely in the controlled synthesis of other doping nanomaterials by co-precipitation method but also in the application of binder-free energy materials or other materials.

Correlation between carrier transport and orientation evolution of polycrystalline transparent conductive Al-doped ZnO films

We have been developing a deposition technology to achieve Al-doped ZnO (AZO) polycrystalline films with a well-defined (0001) orientation. We propose the use of very thin critical layers (CLs) made from Ga-doped ZnO (GZO) films deposited by ion plating with direct-current arc discharge, which strongly affect the orientation of AZO films deposited by direct-current magnetron sputtering. The Al2O3 content in the sputtering target was 0.5wt.%. 500-, 200- and 100-nm-thick AZO films with CLs exhibited high-Hall-mobility values of 50.1cm2/Vs, 40.2cm2/Vs and 32.5cm2/Vs, respectively, compared with those of CL-free AZO films. The presence of CLs with a preferential c-axis orientation as interfaces between AZO films and glass substrates plays a critical role in producing AZO films having a textured polycrystalline structure with a well-defined (0001) orientation.