Conductivity Of ZnO Research Articles

Preparation of Dye-Adsorbing ZnO Thin Films by Electroless Deposition and Their Photoelectrochemical Properties

Dye-adsorbing ZnO thin films were prepared on ITO films by electroless deposition. The films were formed in an aqueous solution containing zinc nitrate, dimethylamine-borane, and eosin Y at 328 K. The film thickness was 1.2-2.0 μm. Thinner and larger-plane hexagonal columns were produced from the solution containing a higher concentration of eosin Y. A photocurrent was observed in the electrodes containing such ZnO films during light irradiation. The photoelectrochemical performance of the film was improved by increasing the concentration of eosin Y because of increases in the amount of absorbed photons and the electronic conductivity of ZnO.

First-principles study on the effect of high In doping on the conductivity of ZnO

Based on the density functional theory (DFT), using first-principles plane-wave ultrasoft pseudopotential method, the models of the unit cell of pure ZnO and two highly In-doped supercells of Zn0.9375In0.0625O and Zn0.875In0.125O are constructed, and the geometry optimizations of the three models are carried out. The total density of states (DOS) and the band structures (BS) are also calculated. The calculation results show that in the range of high doping concentration, when the doping concentration is hihger than a specific value, the conductivity decreases with the increase of the doping concentration of In in ZnO, which is in consistence with the change trend of the experimental results.

Improving conductivity and texture in ZnO:Al sputtered thin films by sequential chemical and thermal treatments

Smooth, transparent and conductive ZnO:Al (AZO) thin films have been deposited by sputtering at room temperature on glass substrates. The conductivity and texture of the as-grown layers have been enhanced by the application of thermal heating at 350°C and chemical etching in HCl or HF solutions. The structure, morphology, optical and electrical properties of the layers have been analysed before and after the successive thermal and chemical treatments, which were performed in different sequences. The surface texture has been characterized optically by the haze parameter for transmittance, which has been related to the root mean square roughness and the autocorrelation length coefficients for each sample. The highest optical scattering has been achieved with the HF etching and subsequent heating sequence, which gave also the highest root mean square roughness. In this way, an average haze of 36%, together with total visible transmittance about 80% and sheet resistance of 8Ω/sq have been obtained. Otherwise, the application of heating before HCl or HF etching resulted in AZO films with average haze of 11%, visible transmittance about 85% and sheet resistance near 6Ω/sq.

Theoretical and experimental investigation on enhanced thermal behaviour in chunk-shaped nano ZnO

Thermal properties of chunk-shaped ZnO nanostructures are studied for diffusivity, conductivity, and effusivity by photoacoustics (PA) and simulation methods. Thermal conductivity of nano ZnO was determined from simulation in the temperature range of 100–1000 K. Thermal conductivity of ZnO nanostructures at room temperature is approximately 52 and 128 times lower than that of bulk ZnO for PA and simulation, respectively. For simulation, Tersoff potential is used for the interatomic interaction. The velocity autocorrelation function and Green–Kubo relation are used to compute the thermal conductivity.

Effects of Ag-induced acceptor defects on the band gap tuning and conductivity of Li:ZnO films

The effects of Ag-induced acceptor defects on the band gap tuning and conductivity of Li:ZnO film grown by the sol-gel method were investigated. The structural analyses indicate that the Ag-Li:ZnO films possess hexagonal structure with the substitutional Ag defect at the Zn site (AgZn) and the interstitial Li defect (Lii). The decreased film transmittance and band gap with Ag-Li codoping is mainly due to the incorporation of foreign impurity levels by the AgZn and Lii defects. The electrical measurements reveal that doping can obviously improve the film conductivity, which could be attributed to the reduction of the grain boundary scattering and the inter-diffusion of the Ag nanoparticles, as well as the decreased ionization energy of the acceptor owing to the AgZn defects. The electronic structures of Ag-Li:ZnO were further studied by the first-principles calculations and the results show that the AgZn defects may lead to p-type conductivity of ZnO.

Conversion mechanism of conductivity of phosphorus-doped ZnO films induced by post-annealing

The effects of post-annealing on conductivity of phosphorus-doped ZnO (PZO) films grown at 500 °C by radio frequency magnetron sputtering are investigated in a temperature ranging from 600 °C to 900 °C. The as-grown PZO exhibits n-type conductivity with an electron concentration of 1.19 × 1020 cm−3, and keeps n-type conductivity as annealed at 600 °C-700 °C but electron concentration decreases with increasing temperature. However, it converts to p-type conductivity as annealed at 800 °C. Further increasing temperature, it still shows p-type conductivity but the hole concentration decreases. It is found that the P occupies mainly Zn site (PZn) in the as-grown PZO, which accounts for good n-type conductivity of the as-grown PZO. The amount of the PZn decreases with increasing temperature, while the amount of Zn vacancy (VZn) increases from 600 °C to 800 °C but decreases greatly at 900 °C, resulting in that the amount of PZn-2VZn complex increases with increasing temperature up to 800 °C but decreases above 800 °C. It is suggested that the PZn-2VZn complex acceptor is responsible for p-type conductivity, and that the conversion of conductivity is due to the change of the amount of the PZn and PZn-2VZn with annealing temperature.

Activation of p-type conduction in ZnO:N films by annealing in atomic oxygen

ZnO:N epitaxial films have been grown by reactive magnetron sputtering. The effect of annealing in atomic oxygen on the structural and electrical properties of the ZnO:N films has been studied by X-ray diffraction, atomic force microscopy, Hall effect measurements, and X-ray photoelectron spectroscopy. By annealing at temperatures from 500 to 700°C, we have obtained p-type ZnO:N films with a resistivity of ∼57 Ω cm, hole mobility of ∼2.7 cm2/(V s), and hole concentration of ∼6.8 × 1017 cm−3. X-ray photoelectron spectroscopy results suggest that the p-type conductivity of the films is due to a decrease in the concentration of (N2)O and VO donors.

Electronic Properties of Zno Doped by Rare-Earth Elements from First-Principles

We perform first-principles calculations to investigate the band structure and density of states of rare-elements doped ZnO. The calculated results show that the shapes of band structures for ZnO by rare-element doping are similar. And the rare-elements incorporation has a little influence on the band gap of the doping system under our considered doping concentration, but after doping, the Fermi level goes into the conduction band, and the electrons from the conduction band minimum to the Fermi level are increasing after rare-earth doping, which means that rare-element doping can change the electrical conductivity of ZnO to a great extent. Meanwhile, it is found that the conduction band near the Fermi level is determined by the rare-element-d states and O-p states, demonstrating as a strong hybridization. This study could provide a theoretical explanation for the factors influencing the properties of ZnO.

Influence of Fe-doping on the optical and electrical properties of ZnO films

We have prepared Fe doped ZnO films, by spin coating, and investigated their structure, morphology, optical and electrical properties by means of the X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, UV–vis–NIR spectrometer and the temperature dependence of electrical conductivity measurements, respectively. XRD results indicates that the films under study posses a polycrystalline structure. A deterioration of the crystallite preferentially orientation, with the increase of Fe doping concentration, was observed. The optical transmittance measurements reveal that the transparency of studied films decreases with the increase of Fe concentration in ZnO. The electrical behavior was analyzed during a heat treatment. The experimental results indicate that the introduction of Fe impurities decreases the conductivity of ZnO with two orders of magnitude. Mott variable range hopping model was proposed to analyze the mechanisms of electronic transport, in low temperature ranges.

Effects of (P, N) dual acceptor doping on band gap and p-type conduction behavior of ZnO films

A reproducible p-type P-N codoped ZnO [ZnO:(P, N)] film with high quality was achieved by magnetron sputtering and post-annealing techniques. It has room-temperature resistivity of 3.98 Ωcm, Hall mobility of 1.35 cm2/Vs, and carrier concentration of 1.16 × 1018 cm−3, which is better than electrical properties of the p-type N-doped ZnO (ZnO:N) and p-type P-doped ZnO (ZnO:P) films. Additionally, the p-ZnO:(P, N)/n-ZnO homojunction showed a clear p-n diode characteristic. The p-type conductivity of ZnO:(P, N) is attributed to the formation of an impurity band above the valance band maximum, resulting in a reduction in the band gap and a decrease in the ionization energy of the acceptor, as well as an improvement in the conductivity and stability of the p-type ZnO:(P, N).

Chemical speciation at buried interfaces in high-temperature processed polycrystalline silicon thin-film solar cells on ZnO:Al

The combination of polycrystalline silicon (poly-Si) thin films with aluminum doped zinc oxide layers (ZnO:Al) as transparent conductive oxide enables the design of appealing optoelectronic devices at low costs, namely in the field of photovoltaics. The fabrication of both thin-film materials requires high-temperature treatments, which are highly desired for obtaining a high electrical material quality. Annealing procedures are typically applied during crystallization and defect-healing processes for silicon and can boost the carrier mobility and conductivity of ZnO:Al layers. In a combined poly-Si/ZnO:Al layer system, an in-depth knowledge of the interaction of both layers and the control of interface reactions upon thermal treatments is crucial. Therefore, we analyze the influence of rapid thermal treatments up to 1050 °C on solid phase crystallized poly-Si thin-film solar cells on ZnO:Al-coated glass, focusing on chemical interface reactions and modifications of the poly-Si absorber material quality. The presence of a ZnO:Al layer in the solar cell stack was found to limit the poly-Si solar cell performance with open circuit voltages only below 390 mV (compared to 435 mV without ZnO film), even if a silicon nitride (SiN) diffusion barrier was included. A considerable amount of diffused zinc inside the silicon was observed. By grazing-incidence X-ray fluorescence spectrometry, a depth-resolving analysis of the elemental composition close to the poly-Si/(SiN)/ZnO:Al interface was carried out. Temperatures above 1000 °C were found to promote the formation of new chemical compounds within about 10 nm of interface, such as zinc silicates (Zn2SiO4) and aluminium oxide (AlxOy). These results give valuable insights about the temperature-limitations of Si/ZnO thin-film solar cell fabrication and the formation of high-mobility ZnO-layers by thermal anneal.

Stable p-type ZnO films dual-doped with silver and nitrogen

Ag–N dual-acceptors doped p-type ZnO films were grown on glass substrates by the sol–gel method. The influence of dual-doping on the structural, electrical, and optical properties of samples was investigated in detail. The p-type conductivity of ZnO:(Ag,N) film is long-time stable. The resistivity of dual-doped ZnO:(Ag,N) film is much lower than that of mono-doped ZnO:Ag and ZnO:N films. ZnO homostructural p–n junction was fabricated by depositing an n-type ZnO layer on a p-type ZnO:(Ag,N) layer. The current–voltage characteristics show typical rectifying behaviors. Moreover, the ZnO:(Ag,N) film exhibits a good c-axis orientation, a high transmittance in the visible region, and a strong ultraviolet emission at room temperature.

Synthesis, characterization, and electrical conductivity of ZnO with different morphologies

ZnO samples with different morphologies were prepared using a simple direct precipitation method with Zn (NO 3 ) 2 ⋅ 6H 2 O and NH 3 ⋅ H 2 O as raw materials. The morphology and purity of the ZnO powders were studied via scanning electron microscopy, energy dispersive X-ray spectrometry, and X-ray diffraction. Factors influencing the morphology of ZnO, such as NH 3 ⋅ H 2 O concentration and aging temperature, were studied. Photoluminescence spectra and Raman spectra were also used to analyze the native crystal defects of the ZnO samples. The results showed that the ZnO samples prepared in this work possessed a hexagonal wurtzite structure with high purity. In addition, NH 3 ⋅ H 2 O concentration and aging temperature were essential in controlling the morphologies of ZnO. The electrical conductivities of ZnO with different morphologies were different, which mainly depends on the different native crystal defects of the ZnO samples. With adjustment of the experimental parameters, ZnO samples with different morphologies were synthesized, the concentration of the reactant (NH 3 ⋅ H 2 O) and aging temperature played important roles on the morphologies of the ZnO, and the electrical conductivity of ZnO showed that the influence of the crystal morphology on the electrical conductivity of the ZnO mainly depends on the different native crystal defects. ► Different morphological ZnO were prepared by a direct precipitation method. ► Factors Influencing on the morphology of ZnO were studied. ► Influence of crystal morphology on the electrical conductivity of ZnO was discussed. ► The microstructure of ZnO were studied by Raman scattering.

Epitaxial growth of Sb-doped nonpolar a-plane ZnO thin films on r-plane sapphire substrates by RF magnetron sputtering

► Sb-doped nonpolar a -plane ZnO layers were epitaxially grown on sapphire substrates. ► Crystallinity and electrical properties were studied upon growth condition and doping concentration. ► The out-of-plane lattice spacing of ZnO films reduces monotonically with increasing Sb doping level. ► The p-type conductivity of ZnO:Sb film is closely correlated with annealing condition and Sb doping level. In this study, the epitaxial growth of Sb-doped nonpolar a -plane ( 1 1 2 ¯ 0 ) ZnO thin films on r -plane ( 1 1 ¯ 0 2 ) sapphire substrates was performed by radio-frequency magnetron sputtering. The influence of the sputter deposition conditions and Sb doping concentration on the microstructural and electrical properties of Sb-doped ZnO epitaxial films was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and the Hall-effect measurement. The measurement of the XRD phi-scan indicated that the epitaxial relationship between the ZnO:Sb layer and sapphire substrate was 1 1 2 ¯ 0 ZnO / / ( 1 1 ¯ 0 2 ) Al 2 O 3 and 1 1 ¯ 0 0 ZnO / / 1 1 2 ¯ 0 Al 2 O 3 . The out-of-plane a -axis lattice parameter of ZnO films was reduced monotonically with the increasing Sb doping level. The cross-sectional transmission electron microscopy (XTEM) observation confirmed the absence of any significant antimony oxide phase segregation across the thickness of the Sb-doped ZnO epitaxial film. However, the epitaxial quality of the films deteriorated as the level of Sb dopant increased. The electrical properties of ZnO:Sb film are closely correlated with post-annealing conditions and Sb doping concentrations.

Growth and conduction mechanism of As-doped p-type ZnO thin films deposited by MOCVD

As-doped p-type ZnO thin films were fabricated by metal organic chemical vapor deposition (MOCVD) after in situ annealing in a vacuum. The p-type conduction mechanism was suggested by the analysis of X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy. It was found that most of the As dopants in p-ZnO thin films formed AsZn–2VZn shallow acceptor complex, simultaneously, carbon impurities also played an important role in realizing p-type conductivity in ZnO. Substitutional carbon on oxygen site created passivated defect bands by combining with Ga atoms due to the donor-acceptor pair Coulomb binding, which shifted the valence-band maximum upwards for ZnO and thus increased the hole concentration.

Low resistive aluminum doped nanocrystalline zinc oxide for reducing gas sensor application via sol–gel process

Abstract Aluminum doped zinc oxide films were prepared by a sol–gel process using two different precursors. Zinc acetate and zinc nitrate were used as the starting precursor materials for sol–gel mixtures. The effects of the starting materials on the microstructure, surface morphology, electrical conductivity and gas sensor performance of ZnO were investigated as a function of Al concentration. The results indicated that the crystallite size decreased with the increase of Al dopant concentration from 0 to 5.0 at%. The films prepared by zinc acetate exhibited a skeletal wrinkle structure, which influenced the properties of ZnO films significantly. The gas sensing properties of Al-doped ZnO films prepared by two precursors were investigated for different concentrations of hydrogen in the air. The sensor response, as well as the response and recovery times of ZnO sensors, was analyzed to understand the effect of starting materials on the film quality. It was found that 3.0 at% Al-doped zinc oxide films had optimum properties, such as high electrical conductivity, crystallinity, higher response and faster gas response. Also, the films prepared with the zinc acetate-based precursor exhibited a better performance of gas sensor than those prepared with zinc nitrate-based sol–gel mixtures. Advantages of Al-doped ZnO over undoped ZnO in gas sensors appeared to be (i) low resistive devices, which do not require high impedance peripheral circuits and systems, (ii) tunability of selectivity based on response time constant, and (iii) low operation temperature.

Studies on structural and electrical properties of copper-doped zinc oxide powders prepared by a solid state method at high temperatures

The synthesis, crystal structure and electrical conductivity properties of Cu-doped ZnO powders (in the range of 0.25 – 15 mole %) is reported. I-phase samples, which were indexed as single phase with a hexagonal (wurtzite) structure in the Cu-doped ZnO binary system, were determined by X-ray diffraction. The limit solubility of Cu in the ZnO lattice at this temperature is 5 mole % at 1000°C. The impurity phase was determined as CuO when compared with standard XRD data using the PDF program. We focused on single I-phase ZnO samples which synthesised at 1000°C because the limit solubility range is widest at this temperature. It was observed that the lattice parameters a increased and c decreased with Cu doping concentration. The morphology of the I-phase samples was analysed with a scanning electron microscope. The electrical conductivity of the pure ZnO and single I-phase samples were studied using the four-probe dc method at temperatures between 100 and 950°C in an air atmosphere. The electrical conductivity values of pure ZnO and 5 mole % Cu-doped ZnO samples at 100°C were 2 × 10−6 and 1.4 × 10−4 ohm−1 cm−1, and at 950°C they were 1.8 and 3.4 ohm−1 cm−1, respectively. In other words, the electrical conductivity slightly increased with Cu doping concentration. Also, it was observed that the activation energy of the I-phase samples was decreased with Cu doping concentration.

Electrical and structural properties of antimony-doped p-type ZnO nanorods with self-corrugated surfaces

We report on p-type conductivity in antimony (Sb)-doped ZnO (ZnO:Sb) nanorods which have self-corrugated surfaces. The p-ZnO:Sb/n-ZnO nanorod diode shows good rectification characteristics, confirming that a p–n homojunction is formed in the ZnO nanorod diode. The low-temperature photoluminescence (PL) spectra of the ZnO:Sb nanorods reveal that the p-type conductivity in p-ZnO:Sb is related to the SbZn–2VZn complex acceptors. Transmission electron microscopy (TEM) analysis of the ZnO:Sb nanorods also shows that the p-type conductivity is attributed to the SbZn–2VZn complex acceptors which can be easily formed near the self-corrugated surface regions of ZnO:Sb nanorods. These results suggest that the SbZn–2VZn complex acceptors are mainly responsible for the p-type conductivity in ZnO:Sb nanorods which have corrugated surfaces.

Silicon-on-insulator based ZnO nanowire photodetector

The authors present here a technologically easy process to electrically contact ensembles of ZnO nanowires on silicon-on-insulator substrates for multiple sensor applications. Microtrenches defined by standard photolithography and etching processes were bridged by ZnO nanowires. The ZnO nanowires grown by a high temperature process show excellent crystalline quality as confirmed by photoluminescence. These wires connecting the opposite sides of the microtrenches were investigated here as solar-blind photosensitive detectors, but may equally well serve for any other sensing application, where the ZnO conductivity is altered by the attachment of specific gases, proteins, etc. The straightforward design allows for an easy integration into CMOS processes.

New composites of ZnO–P2O5/Ni having PTC transition and high Seebeck coefficient

In this article, we report the electrical conductivity (σ) and Seebeck coefficient (S) of ZnO–P2O5 matrix filled with conductive powder of nickel (Ni). The variation of σ versus volume fraction of Ni showed a non-conducting to conducting phase transition at percolation threshold (28vol. %). The change of S from high positive to negative values exhibits that this transition is accompanied by the passing of carrier charge from p to n type. On the other hand, the measurements of σ and S as function of temperature, above the percolation threshold, showed a positive temperature coefficient (PTC) phase transition at Tc≥400K, linked with a high S=−5000μV/K, giving highest power factor PF=σ.S2≈2.10−4Wm−1K−2. The temperature dependence of the volume expansion enabled to confirm that this transition is associated to the thermal volume variation in matrix. However, the temperature dependence of σ below the percolation threshold showed two different mechanisms: thermally activated hopping behavior at high temperatures and Mott's variable range hopping (VRH) at low temperatures.