Electrical Properties Of Zinc Oxide Films Research Articles

The semi-conductor of ZnO deposited in reactive HiPIMS

In this paper we report the semi-conductor of zinc oxide (ZnO) thin film deposited by reactive high power impulse magnetron sputtering (R-HiPIMS) on glass substrates without external heating. We focus on the influence of deposition parameter, the ratio of O2/Ar exactly, on the crystalline and electrical properties of ZnO films. It is found that n-type ZnO film with a high carrier concentration can be grown through HiPIMS technique, the ratio of O2/Ar remarkably affects the ZnO crystalline and electrical properties. At O2/Ar = 14/80 sccm, the ZnO film was orientated at c-axis (002) plane. Based on the temporal resolution optical emission spectroscopy (OES), for the proposal of the correlation of film structure with plasma radicals and the reaction process in ZnO growth, the components in plasma are diagnosed. It is concluded that in a higher oxygen partial pressure, owing to the high ionization rate in HiPIMS, the oxide layer formed between pulses is removed, the growth of a new oxide layer during the pulse is hindered, hence the growth rate is increased, and the carrier concentration caused from point defects in ZnO film is high.

Effects of UV irradiation on the electrical and optical properties of solution-processed transparent ZnO films

Abstract Transparent zinc oxide (ZnO) films that were prepared via a spin-spray process using sodium citrate as a surfactant were exposed to UV light of varying wavelengths and irradiation times. The effects of the UV irradiation on the optical and electrical properties of the ZnO films were examined. The band gap of the as-prepared ZnO film (3.46 eV) increased after UV irradiation, even when exposed to relatively long wavelengths (>365 nm) with a low power density (~0.55 W/cm2). The modifications in the band gap and conductivity of the spin-sprayed ZnO films were primarily caused by the persistent photo-conductivity effect and photodecomposition-induced doping. Examination of the ZnO films using FTIR indicated that irradiation with shorter UV wavelengths (≤350 nm) improved the photodecomposition of carbon impurities that were inside the ZnO films, which resulted in enhanced conductivity and electrical stability for >1 days.

Passivation Effect on ZnO Films by SF6 Plasma Treatment

The passivation effects of SF6 plasma on zinc oxide (ZnO) films prepared by magnetron sputtering were researched. After the SF6 plasma passivation of ZnO films, the grain size increases, there is low surface roughness, and a small amount of Zn-F bonds are formed, resulting in the narrowing of band gap. The photoluminescence (PL) intensity of SF6-passivated ZnO films has a 120% increase compared to the untreated samples, and the reduction in defects can increase the resistivity and stability of ZnO films. ZnO films are used in the preparation of ZnO/p-Si heterojunction diodes. The results of the measurement of current voltage (J–V) show that the reverse current is reduced after SF6 plasma passivation, indicating an improvement in the electrical properties of ZnO films.

Influence of Annealing Time on the Structural, Morphological and Optical Properties of ZnO Films

Zinc oxide (ZnO) thin films have been synthesized by thermally oxidizing the direct current magnetron sputtered zinc (Zn) films at 500[Formula: see text]C for different annealing time periods (3–5[Formula: see text]h) in ambient air. Influence of annealing time on the structural, morphological, optical and electrical properties of ZnO films was studied using X-ray diffractometer, field emission-scanning electron microscopy (FE-SEM), UV–Vis spectrophotometer and two-probe method, respectively. ZnO films exhibited hexagonal crystal structure with (1 0 1) as preferential orientation plane, and the crystallite size of the films increased from 30[Formula: see text]nm to 83[Formula: see text]nm on increasing the annealing time. Pebble-like surface morphology was observed from the FE-SEM micrographs. Optical band gap of the films varied between 3[Formula: see text]eV and 3.5[Formula: see text]eV for different annealing times. Electrical resistance of the ZnO films was decreased with increase in the annealing time, which is due to the improvement in the crystallinity.

Highly selective gas sensors from photo-activated ZnO/PANI thin films synthesized by mSILAR

Zinc oxide (ZnO) nano-polycrystalline thin films has been prepared by cost-effective microwave assisted successive ionic layer adsorption and reaction (mSILAR) technique. ZnO/PANI prepared by in situ polymerization technique and thin films were fabricated using spin coating. X-ray Diffraction analysis confirms the presence of hexagonal wurtzite ZnO structure in the ZnO/PANI composite. The field emission scanning electron microscope revealed the porous nature of ZnO/PANI films with nanosized grains. We observed PANI intensively affected the structural and electrical properties of ZnO films. The examination of sensors was carried out in the liquefied petroleum gas (LPG) concentration range of 30 to 450ppm. It was noticed that ZnO/PANI nanocomposite film possesses excellent LPG sensing properties at a room temperature compared with other volatile organic compounds, at an applied voltage of 1.5V. The composite films also exhibited significant sensing response of ∼6.11×102 towards temperature and light with recovery and response time of ∼3.5min and 2.16min, respectively. Finally, the fabricated sensor showed good repeatability and sensitivity upon cyclic exposure to gas, light, and temperature. The ZnO/PANI nanocomposite film demonstrated overall sensing behavior in terms of sensor recovery time and response as well as repeatability.

UV and visible photoluminescence emission intensity of undoped and In-doped ZnO thin film and photoresponsivity of ZnO:In/Si hetero-junction

Undoped zinc oxide (ZnO) and indium-doped (ZnO:In) thin films were grown at different temperatures (250–400°C) on alkali-free borosilicate glass and n-Si (100) substrates by Ultrasonic Spray Pyrolysis method. The structural, compositional, optical and electrical properties of ZnO films were investigated by X-ray diffraction, Scanning Electron Microscopy, Rutherford Back Scattering Spectroscopy, Fourier Transform Infrared spectroscopy, photoluminescence (PL) and the four-point probe technique. The predominance of ultraviolet (UV) and blue emission intensities was found to be closely dependent on the resistivity of the film. The visible emission band (peaking at 432nm) prevails for low film resistivity, ranging from 10−2 to 1Ω·cm. By contrast, for higher resistivity (>1Ω·cm), there is a predominance of the UV band (382nm). The PL and photoresponsivity results of fabricated ZnO:In/n-Si(100) heterojunctions prepared at different temperatures are discussed. The maximum spectral response of the ZnO:8%In/Si heterojunction diode fabricated at 250°C was about 80mA/W at zero bias. The highlighted results are attractive for the optoelectronic applications.

Be-Doped ZnO Thin-Film Transistors and Circuits Fabricated by Spray Pyrolysis in Air

We report the fabrication of zinc oxide (ZnO) thin-film transistors (TFTs) and simple integrated circuits by spray pyrolysis, and examine the role of beryllium (Be) as the chemical dopant. Doping is achieved through addition of Be-acetylacetonate into the parent Zn-acetate precursor solution followed by film deposition through spray pyrolysis. The microstructural properties of as-grown Be-ZnO films with different dopant concentrations are investigated using a combination of atomic force microscopy and x-ray diffraction techniques, which show the formation of polycrystalline films. Introduction of Be is found to impact the degree of crystallinity of ZnO films where a dramatic decrease in the average grain size is observed with increasing Be concentration. To assess the effects of Be-doping on the electrical properties of ZnO films we have fabricated Be-ZnO based TFTs using different doping concentrations. The average electron mobility calculated from these transistors is on the order of ~ 2 cm2·V-1·s-1 with the threshold voltage (VTH) exhibiting a strong dependence on Be concentration. The ability to control VTH through the introduction of Be has been exploited for the fabrication of unipolar inverters with symmetric trip-voltages and good noise margins.

Hydrogen mediated self-textured zinc oxide films for silicon thin film solar cells

In the present study, we introduce a method for preparing textured zinc oxide (ZnO) thin films by hydrogen associated magnetron sputtering. The improved surface morphology and structure characteristic indicate that hydrogen could efficiently mediate the crystal orientation by playing an important role in influencing the surface diffusion and sticking processes. This textured surface morphology leads to a high haze factor which provides effective light trapping efficiency. In addition, the electrical properties of ZnO films are also enhanced with high carrier concentration and mobility. Layers with comparable electrical (sheet resistance <5Ω/sq), optical (average total transmittance >80% from 400nm to 1200nm) and morphological (RMS>50nm) properties to Asahi U-type fluorine doped tin dioxide (FTO) have been obtained. Preliminary microcrystalline silicon solar cells deposited on self-textured ZnO show 13.2% and 20% enhancement in short-circuit current density and efficiency, respectively, illustrating a good potential application in mass production for large area expansibility.

Effects of growth variables on the properties of single crystalline ZnO thin film grown by inductively coupled plasma metal organic chemical vapor deposition

Single crystalline undoped and Ga-doped n-type zinc oxide (ZnO) films were grown on sapphire (Al 2O 3) substrates by inductively coupled plasma (ICP) metal organic chemical vapor deposition. Effects of growth variables on the structural, optical, and electrical properties of ZnO films have been studied in detail. Single crystal films with flat and smooth surfaces were reproducibly obtained, with application of sample bias and O 2 ICP. The best film properties were obtained at the growth condition of 650 °C, 400 W ICP power, − 94 V bias voltage, O/Zn (VI/II) ratio of 75. Single crystalline Ga doped n-ZnO films were also obtained, with free carrier concentration of about 1.5 × 10 19/cm 3 at 1 at.% Ga concentration.

Growth condition dependence of morphology and electric properties of ZnO films on sapphire substrates prepared by molecular beam epitaxy

Zinc oxide (ZnO) films were grown on sapphire (112̄0) substrates by molecular beam epitaxy under oxygen radical irradiation. The effect of the growth conditions, including the Zn/O ratio supplied to the film surface, on the electrical properties of ZnO films was studied in relation to the film morphology. We found that the growth rate strongly depended on the Zn flux from the Knudsen cell and the optimum condition for high growth rate was very narrow. The grain size in the lateral direction increased with increasing growth rate in the thickness direction. The increase in growth rate, especially in the lateral direction, resulted in the carrier mobility increasing up to 42 cm2 V−1 s−1. The carrier concentration N was sensitive to the substrate temperature, while the value of N was not sensitive to the source supplying ratio Zn/O. We discuss the decrease of the carrier concentration with increasing substrate temperature in regard to the formation of nonequilibrium defects.

In situ measurement of mechanical stress in polycrystalline zinc-oxide thin films prepared by magnetron sputtering

The generation of stress in magnetron-sputtered undoped and aluminum-doped zinc-oxide (ZnO:Al) films has been measured by an in situ technique. Radio-frequency magnetron sputtering from ceramic targets in an argon atmosphere was used for film deposition. The stress was measured by optically detecting the deflection of a thinned silicon cantilever on which the films were deposited. This in situ stress measurement setup was tested by investigating the sputtering pressure dependence of stress in molybdenum films. For zinc-oxide films it was found that by increasing the argon-sputtering pressure during deposition the intrinsic compressive stress decreased by about a factor of 2, both for undoped and doped zinc-oxide films. The undoped films exhibit significantly higher compressive stresses (1–2.5 GPa) compared to the aluminum-doped films (0.7–1.5 GPa). This fact was ascribed to a surfactant-like effect of the dopant aluminum, which improved the crystallinity of the doped films. In contrast to magnetron-sputtered metallic films (like molybdenum) the zinc-oxide films did not show a transition from compressive to tensile stress when the argon-sputtering pressure was increased. It is assumed that this behavior of the zinc-oxide films is due to an additional incorporation of oxygen into the films. By adjusting the oxygen partial pressure during the deposition at a fixed argon pressure, a minimum of the compressive stress in ZnO: Al films was found which coincided with a minimum of the film resistivity. This observation stresses the role of even small changes in deposition parameters on the mechanical and electrical properties of zinc-oxide films. The possibilities of the in situ stress measurement are demonstrated during the deposition of zinc-oxide multilayers.

Effect of In concentration in the starting solution on the structural and electrical properties of ZnO films prepared by the pyrosol process at 450°C

Undoped and indium-doped Zinc oxide (ZnO) solid films were deposited by the pyrosol process at 450°C on glass substrates from solutions where In/Zn ratio was 2, 5, and 10 at.%. Electrical measurements performed at room temperature show that the addition of indium changes the resistance of the films. The resistivities of doped films are less than non-doped ZnO films by one to two orders of magnitude depending on the dopant concentration in the solution. Preferential orientation of the films with the c-axis perpendicular to the substrate was detected by X-ray diffraction and polarized extended X-ray absorption fine structures measurements at the Zn K edge. This orientation depends on the indium concentration in the starting solution. The most textured films were obtained for solutions where In/Zn ratio was 2 and 5 at.%. When In/Zn=10 at.%, the films had a nearly random orientation of crystallites. Evidence of the incorporation of indium in the ZnO lattice was obtained from extended X-ray absorption fine structures at the In and Zn K edges. The structural analysis of the least resistive film (Zn/In=5 at.%) shows that In substitutes Zn in the wurtzite structure.