In2O3 ZnO Films Research Articles

Indium oxide-based transparent conductive films deposited by reactive sputtering using alloy targets

High-quality transparent conductive oxide (TCO) films, Sn-doped In2O3 (ITO) and In2O3–ZnO (IZO), were successfully deposited on either synthetic silica or polyethylene terephthalate (PET) substrates in the “transition region” by reactive dc magnetron sputtering using In–Zn and In–Sn alloy targets, respectively, with a specially designed plasma emission feedback system. The composition, crystallinity, surface morphology, and electrical and optical properties of the films were analyzed. All of the IZO films were amorphous, whereas the ITO films were polycrystalline over a wide range of deposition conditions. The minimum resistivities of the IZO and ITO films deposited on the heated PET substrates at 150 °C were 3.3 × 10−4 and 5.4 × 10−4 Ω·cm, respectively. By applying rf bias to unheated PET substrates, ITO films with a resistivity of 4.4 × 10−4 Ω·cm were deposited at a dc self-bias voltage of −60 V.

Crystallization behavior during transparent In2O3‐ZnO film growth

AbstractStructural evolution of transparent conductive In2O3‐ZnO (IZO) films with increasing substrate temperature during sputtering was studied using transmission electron microscopy (TEM) and spectroscopic ellipsometry. Increasing the substrate temperature can induce film crystallization in the initial growth stage, and enhance the crystallization of IZO films. Extensive simulations using ellipsometry data demonstrated a decrease in the crystallization rate for IZO films deposited between 200 and 300 °C, which is attributed to the influence of the interference between nearby growing grains. TEM observations also reveal that the growth competition between different crystallites leads to an increase in the lateral grain size with increasing substrate temperature.

Activation like behaviour on the temperature dependence of the carrier density in In2O3-ZnO films

We study the effect of annealing in high vacuum on the transport properties for In2O3-ZnO films. We prepared indium zinc oxide films by the DC-magnetron sputtering method using an In2O3-ZnO target (89.3 wt % In2O3 and 10.7 wt % ZnO). The annealing temperature is from 373 to 773K. From the XRD analysis, we find that all as deposited films are amorphous. In addition we find that amorphous films are crystallized by annealing at a temperature above 773 K over 2 hours. The temperature dependence of resistivity ρ of all amorphous films shows metallic behaviour. On the other hand, ρ(T) of poly In2O3-ZnO films shows semi-conducting behaviour. We carry out a detailed analysis of the temperature dependence of Hall mobility. The activation energy Ed has been obtained from the slope of the carrier concentration Ne vs. the inverse temperature plot at high temperatures. We found that the Ed takes values between 0.43 and 0.19 meV. Meanwhile, temperature dependence of Ne for poly-In2O3-ZnO films did not show activation-like behaviour. This behaviour is thought to be causally related to impurity conduction band.

Comparative study on the thickness-dependent properties of ITO and GZO thin films grown on glass and PET substrates

The thickness-dependent properties of amorphous Sn-doped In2O3 (ITO) and polycrystalline Ga-doped ZnO (GZO) films grown on polyethylene terephthalate (PET) with a polymeric hard coating were compared with those deposited on Corning glass. The film thickness varied from 20 to 1310 nm. The electrical properties of the ITO films on PET were almost similar to those of the ITO films on glass. On the other hand, GZO films showed slightly poorer electrical properties when deposited on PET, but the difference was marginal. The electrical properties of amorphous ITO films were independent of film thickness, but polycrystalline GZO films exhibited monotonicallyimproving behavior with increasing thickness, mainly due to enhanced crystallinity and increased grain size with increasing film thickness. Although the air-referenced transmittance spectra of films on PET were about 2–3% lower than those on glass due to the lower transmittance of PET, the substrate-referenced optical transmittances of films on PET were higher than those on glass, reflecting the somewhat coarse structure of films on PET. Both the ITO and the GZO films on PET with a polymeric hard coating were shown to yield properties comparable to those oof both films on glass.

Weak localization and percolation effects in annealed In2O3-ZnO thin films

We have investigated the temperature T and magnetic field H dependences of the sheet resistance R□ of thin (In2O3)0.975-(ZnO)0.025 films with different resistivities and carrier densities prepared by postannealing in air at various annealing temperatures Ta. Regarding the magnetoconductance Δσ(H) ≡ 1/R□(H) − 1/R□(0) of films with large values of sheet resistance R□, agreement between weak localization theory and the data cannot be obtained for any value of the localization length $L_{{\rm in}} (T) = \sqrt {D\tau _{{\rm in}} (T)}$L in (T)=Dτ in (T), where D and τin are the diffusion constant and inelastic scattering time, respectively. Taking account of the inhomogeneous morphology confirmed by Scanning Electron Microscopy (SEM) observation, we introduced the effective sheet resistance R□eff given by R□eff = α × R□meas., where the strength of reduction factor α is less than unit, α ⩽ 1. Using a suitable value of α(Ta), we successfully fitted the theory to data for Δσeff(H, T), regarding Lin2(T) as a fitting parameter in the region 2.0 K⩽T ⩽ 50 K. It was confirmed that the rate 1/τin(T) is given by the sum of the electron-electron and electron-phonon inelastic scattering rates.

Transport properties and microstructures of polycrystalline In2O3–ZnO thin films

We prepared polycrystalline In2O3–ZnO films by post annealing the amorphous films (1.0 wt % ZnO) at 200 °C with various annealing times ta 0≤ta≤20 h. We have measured the electric resistivity and Hall mobility and also observed film structures by not only the x-ray diffraction but also scanning transmission electron microscopy (STEM) with electron energy-loss spectroscopy (EELS). We have found the following: (1) Hall mobility takes the maximum with respect to the carrier density and the annealed films clearly show the superconductivity of which transition temperature increases with increase in ta. (2) The data on EELS spectra mapping of indium plasmon indicate that droplets of the pure indium phase exist on grain boundaries and near the interface between the film and the glass substrate. However, it seems that these droplets do not form an electrical conducting path but contribute to the scattering centers for carrier electrons, from the dispersed distribution of these droplets in STEM-EELS spectra mapping and the relationship between the electron mean free path and the grain size.

Characteristics of indium zinc oxide thin films prepared by direct current magnetron sputtering for flexible solar cells

The In 2O 3–ZnO (IZO) thin films were prepared on polyethylene terephthalate substrate at room temperature by direct current (dc) magnetron sputtering. The properties of IZO thin films were studied in terms of O 2 concentration and deposition parameters. As the O 2 concentration in O 2/Ar gas increased, the transmittances of the films were increased up to 90% and the resistivities were decreased. The systematic variation of process parameters including dc power, gas pressure and target-to-substrate distance was performed to examine the properties of the deposited films. It was disclosed that there was an optimum O 2 concentration for high transmittance and low resistivity. With decrease in dc power and gas pressure and increase in target-to-substrate distance, the IZO films with high transmittance and low resistivity were obtained. The observation of the IZO films by atomic force microscopy indicated that the microstructure and surface morphology of the films were responsible for the transmittance. It was demonstrated that IZO films with a resistivity of 5.1×10 −4 Ω cm and an optical transmission of 90% in the visible spectrum could be prepared at room temperature on flexible substrates.

Superconductivity in In2O3-ZnO crystalline films

Thin In2O3-ZnO (IZO) films were prepared by DC-magnetron sputtering with changing the weight concentration x of ZnO, 0 ≤ x ≤ 0.07. As-grown amorphous IZO films show the metallic characteristic but do not show superconductivity. We have investigated the T and H dependence of the resistivity ρ and Hall coefficient for annealed films with thickness d = 350 nm in air. It is found that the films with 0 ≤ x ≤ 0.03 show superconductivity. Transition temperature Tc and the carrier density n of these films are ≈ 2.0 K and 1025 -1026 m-3, respectively. The annealed In2O3-ZnO films were examined by a high resolution transparent electron microscopy. This investigation reveals that films annealed at 300 °C are sufficient crystalline. There is a difference of crystal structures between the superconducting film and normal film. We studied the upper critical magnetic field HC2 (T) for the film with x = 0.005. From the slope of dHc2/dT, we obtain the coherent length ζ(0) ≈ 100 A at T = 0 K and the coefficient of the electronic heat capacity γ ≈ 3.2 (J/m3K2) which is small compared with that of other oxide materials.

Superconductivity in transparent zinc-doped In2O3 films having low carrier density

Thin polycrystalline zinc-doped indium oxide (In2O3–ZnO) films were prepared by post-annealing amorphous films with various weight concentrations x of ZnO in the range 0x 0.06. We have studied the dependences of the resistivity ρ and Hall coefficient on temperature T and magnetic field H in the range 0.5T 300 K, H6 Tfor 350 nm films annealed in air. Films with 0x0.03 show the superconducting resistive transition. The transition temperature Tc is below 3.3 K and the carrier density n is about 1025–1026 m−3. The annealed In2O3–ZnO films were examined by transmission electron microscopy and x-ray diffraction analysis revealing that the crystallinity of the films depends on the annealing time. We studied the upper critical magnetic field Hc2 (T) for the film with x = 0.01. From the slope of dHc2 /dT, we obtain the coherence length ξ (0) ≈ 10 nm at T = 0 K and a coefficient of electronic heat capacity that is small compared with those of other oxide materials.

Electron-phonon scattering in amorphous In2O3–ZnO films

For amorphous transparent conductive oxide In2O3–ZnO films over a wide range of resistivities ρ, the temperature dependences of ρ and Hall coefficient RH have been measured in the temperature range of 2.0–300 K. The low-resistivity films show a metallic characteristic (dρ/dT>0), although high-resistivity films show an insulating behavior (dρ/dT<0). Even in metallic films, however, the resistivity slightly increases with decreasing temperature below 20 K because of the term ρquanta(T) due to quantum effects. Through a careful analysis, we have found that the ρ(T) of metallic films changes in the form of ρ(T)−ρ0−ρquanta∝ρ0T2 at temperatures below ∼100 K. This temperature dependence can be explained by the interference term ρel-imp-ph between the impurity scattering and the electron-phonon scattering. At temperatures of 20–300 K, it has been found that ρ(T) agrees well with the sum of the Grüneisen–Bloch term ρel-ph(T)=βel-phF(T,Θd) and the term ρel-imp-ph(T)=Bel-imp-phG(T,Θd). From analyses, with the coefficients βel-ph and Bel-imp-ph and the Debye temperature Θd as fitting parameters, we have obtained Θd≃1013±13 K and the longitudinal sound velocity uℓ≃13 500 m/s under some assumptions.

The Electronic and Optical Properties of IZO Thin Films Prepared by Pulsed DC Magnetron Sputtering

The electrical and the optical properties of In2O3-ZnO (IZO) films grown by using the pulsed dc magnetron sputtering method are investigated. We find that the average transmittance of the films in the visible range becomes over 85 % and that the optical band gap is about 3.3 eV, irrespective of the repetition frequency. The X-ray diffraction patterns for the film with a Zn content of 22 at.% show the crystalline structure to be amorphous. The growth rate and the sheet resistance of the film are shown to increase with increasing repetition frequency. Also, in a range of relatively thin thickness, the films grown with higher repetition frequency have higher refractive indices while, in the range of relatively thick thicknesses, the refractive index is nearly independent of the frequency.

Amorphous transparent conductive oxide films of In2O3-ZnO with additional Al2O3 impurities

Al-doped In2O3-ZnO films were developed and the influence of Al doping in electrical and optical properties of In2O3-ZnO films was investigated. Amorphous and homologous phases appeared with increasing δ=Zn∕(Zn+In) ratios. The carrier generation mechanism is discussed for these films. Native oxygen defects are the primary donors for the In2O3-ZnO films. Then the low resistivities of 2–4×10−4Ωcm were attained within a narrow range of δ=Zn∕(Zn+In) in the amorphous phase film. Two wt % (percent by weight) Al2O3-doping decreased the resistivity to 1.5–2.1×10−4Ωcm. At doping of 3 and 4 wt % Al2O3, film resistivities of 2–4×10−4Ωcm were attained for a relatively wide range of δ in the amorphous phase due to an increase in carrier concentration at δ=0.3–0.6. However, Al2O3 doping in homologous phase In2O3-ZnO films decreased carrier concentration independently of the levels of Al2O3 doping. Al2O3 doped in the homologous ZnkIn2Ok+3 crystalline films did not act as a donor impurity. The optical band-gap energy for the films deposited above 2 wt% Al2O3 for amorphous film was proportional to Al2O3 content. This is ascribed to lattice distortion caused by Al2O3.

Enhanced properties of In 2O 3–ZnO thin films deposited on soda lime glass due to barrier layers of SiO 2 and TiO 2

Electrical resistivity, resistance to moist heat, transmittance and surface uniformity of In 2O 3–ZnO (IZO) films prepared on soda lime glass (SG) substrate by rf-magnetron sputtering are investigated. These films show a poorer resistance to moist heat, a very rough uniformity and a high electrical resistivity as compared with those deposited on the corning glass substrates. These poor properties are originated by the diffusion of alkali metal from the soda lime glass and its rough surface. However, these properties can be improved by introducing a barrier layer of SiO 2 or TiO 2 between IZO film and the soda lime glass substrate. There is no actual decrease of the average transmittance by introducing the barrier layers. Especially, IZO films deposited on the TiO 2 barrier layer have higher resistance to moist heat and better surface uniformity than those deposited on the SiO 2 barrier layer.

Reversible Electrical and Optical Behaviors due to Excess Oxygen in RF-Sputtered In2O3–ZnO Films

Thin films are prepared by rf-magnetron sputtering using a In2O3–ZnO powder target with a Zn content [Zn/(Zn+In)] of 33%. Their electrical and optical properties are strongly dependent on the oxygen concentration on the grain boundary surface of the films during heat treatment. The oxygen adsorption begins to occur around 400°C in air and the excess oxygen is almost desorbed around 400°C in vacuum. Therefore, the electrical and optical behaviors can be reversibly controlled by the desorption of the oxygen corresponding to the heat treatment environment.

Electrical and Optical Properties of In2O3–ZnO Films Deposited on Polyethylene Terephthalate Substrates by Radio Frequency Magnetron Sputtering

Using a In2O3 and ZnO powder mixture with a ratio of 95:5 in wt% as a target, highly transparent conducting oxide (TCO) films are prepared on polyethylene terephthalate (PET) substrates by rf-magnetron sputtering. These films have amorphous structures with excellent electrical stability, surface uniformity and adhesion to the PET substrate and can be prepared at room temperature. In particular, In2O3–ZnO (IZO) films directly deposited on PET without a hard-coated layer have nearly similar electrical properties, higher transmittance and a more uniform surface in comparison with those deposited on a hard-coated layer of PET. When a TCO material is deposited on PET, IZO is more suitable than indium tin oxide (ITO) in several aspects such as the optical, electrical, and etching properties and its surface uniformity.

Heat treatment effects on electrical and optical properties of ternary compound In2O3–ZnO films

Heat treatment effects on the electrical and optical properties are investigated for In2O3–ZnO film with a Zn content [Zn/(In+Zn)] of 33 at. %. Thin films deposited in a mixed gas atmosphere (Ar+20% O2) by a magnetron sputtering method show high resistivity of about 1.7×10−1 Ω cm. A mixture of In2O3 (99.999%) and ZnO (99.999%) powders calcined at 1000 °C in an Ar atmosphere for 2 h was used as the target. On the other hand, the films deposited in a pure Ar gas environment show low resistivity of about 3×10−4 Ω cm, comparable to that of indium tin oxide films (10−4 Ω cm). After heat treatment at 650 °C in vacuum, it was found that the films deposited in a mixed gas atmosphere revealed low resistivity similar to that of films deposited in a pure Ar gas. For these annealed films, their electrical properties are very stable when treated at temperatures up to 500 °C in air or 650 °C in vacuum. Also, it is found that the optical band gap of In2O3–ZnO films increase (or decrease) after the heat treatment in vacuum (or O2 atmosphere). These effects are mainly due to the change in carrier concentration before and after heat treatment.

Effects of Oxygen Concentration on Characteristics of RF-Sputtered In2O3–ZnO Thin Films

The effects of oxygen concentration on the electrical and optical properties of In2O3–ZnO films are investigated. In2O3–ZnO films deposited under the pure Ar gas show a resistivity of about 3.8 ×10-4 Ω·cm, comparable to that of indium-tin-oxide (ITO) films (∼10-4 Ω·cm), and an average transmittance of over 90% in the visible range. Thin films deposited at higher oxygen concentration also show larger absorption at about 400–500 nm compared to those deposited at the oxygen concentration of 0.0%. We propose that it is desirable to use pure Ar gas to obtain films with lower resistivity and higher transmittance in the visible range. Moreover, in the case of films deposited with a higher oxygen concentration, it is found that the resistivity significantly decreases with increasing heat treatment temperature.

Polytypoid structures in annealed In2O3–ZnO films

Atomic-resolution Z-contrast images demonstrate unambiguously that the annealed, metalorganic chemical vapor deposition derived transparent In2O3–ZnO films have a polytypoid microstructure, consisting of ZnO slabs of variable width separated by single In–O octahedral layers. These In–O layers induce a polarity inversion in the two adjacent ZnO layers, which is reversed again by a mirror domain boundary inside each ZnO slab.

Preparation of transparent and conductive In2O3–ZnO films by radio frequency magnetron sputtering

Highly transparent and conductive In2O3–ZnO films have been prepared by rf magnetron sputtering using targets composed of In2O3 and ZnO. Polycrystalline Zn2In2O5 films were deposited at a substrate temperature of 350 °C using targets with a Zn content [Zn/(Zn+In)] of about 10–60 at. %. A uniform spatial resistivity distribution on the substrate surface was obtained even in Zn2In2O5 films deposited at room temperature. A resistivity of 3.4×10−4 Ω cm was obtained in Sn-doped Zn2In2O5 films deposited at 350 °C. A sheet resistance of 400 Ω/sq and an average transmittance above 90% in the visible range were obtained for an undoped Zn2In2O5 film with a thickness of about 20 nm. The etching rate of In2O3–ZnO films when using HCl as the etchant could be controlled by the Zn content in the films.