Indium-doped Zinc Oxide Research Articles

Nonstoichiometric Zinc Oxide and Indium-Doped Zinc Oxide: Electrical Conductivity and111In-TDPAC Studies

Indium-doped zinc oxide powders have been prepared which show room-temperature electrical conductivities as high as 30 Ω−1cm−1. The indium doping apparently occurs as Zn1−xInxO, Zn1−yInyO1+y/2, or a combination of these. Optimum conductivity occurs for Zn1−xInxO where the maximum value ofxobtained was about 0.5 at%. This substitution results in a lattice volume expansion of 0.4%. The degrees of sample reduction were determined by iodimetric titration. Time differential perturbed angular correlation (TDPAC) spectroscopy on indium doped zinc oxide is consistent with indium substituting at normal zinc sites in the ZnO lattice. TDPAC studies on zinc oxide annealed under zinc vapors show a second environment for the111In probe. In this case, there is an unusually high temperature dependence of the electric field gradient which may be caused by a nearby zinc interstitial. An important conclusion of this work is that zinc interstitials are not ionized and do not therefore contribute significantly to the increased conductivity of reduced zinc oxide.

Transparent conducting undoped and indium-doped zinc oxide films prepared by spray pyrolysis

Undoped and indium-doped ZnO thin films are deposited by the spray pyrolysis process onto glass substrates. The effects of starting solution composition and indium doping on the structural, electrical, and optical properties are investigated. The surface morphology of the deposited films is a strong function of the source compounds. Thus two distinct morphologies with large difference in the average grain size are obtained by using separately zinc chloride and zinc acetate. The polycrystalline nature with no second phases is observed by X-ray diffraction. Indium doping leads to a very low resistivity of about 10−3 Ω cm. High transparency of the films, more than 85% in the visible range, is observed.

Dopant induced modifications in the physical properties of sprayed ZnO:In films

Indium-doped zinc oxide (IZO) films were prepared by the spray pyrolysis technique. The effect of gradual incorporation of indium cations on the structural, electrical, and compositional properties of IZO films was studied in detail. It was observed that even a small addition of indium modifies the preferred growth of IZO film from the [002] direction to the [101] direction. Such a modification in growth pattern is a result of more nucleating centers created by indium doping. Indium dopant improves the electrical properties of the films. The carrier concentration depends mainly on the indium dopant level while the mobility is affected by the changes in crystal orientation that take place due to addition of dopants. X-ray photoelectron spectroscopy results show that indium doping does not lead to any stoichiometric changes in the IZO films and the dopant incorporation in the film is linearly proportional to that in the solution.

Interpretation of the optical absorption by indium ions in zinc oxide

The total optical reflectance, R ∞, of indium-doped zinc oxide powders was measured over the wavelength range from 350 to 750 nm. The indium contents of the powders were from 119 to 255 ppm. Indium, which in solid solution forms a shallow, donor-like state in ZnO, produces a uniform decrease in the optical reflectance of ZnO powder over the visible spectrum. From the behavior of the remission function f( R ∞) with respect to the In ion density N the optical absorption coefficient k is shown to be nonlinear with N, exhibiting an exponential dependence. The data can be interpreted to indicate that the donor electrons on the In are interacting among In ions, as both pair-wise In interactions and interactions of higher orders. It is demonstrated by experiment that the dependence of the In optical cross section upon ion density can be represented by an effective optical cross section for In given by δ eff = δ exp(( 4 3 )π a 3 N) where δ is the photoionization cross section for the In ion, and a is the effective Bohr radius of the donor electron about the defect.

Propriétés électriques et optiques de couches minces de ZnO et ZnO dopé à l'indium, obtenues par le procédé Pyrosol

Undoped and indium-doped ZnO thin films deposited by the Pyrosol process onto soda-lime glass substrates were electrically and optically characterized. Resistivities as low as 2×10 -3 Ω.cm. Hall mobilities as high as 21 cm 2 V -1 s -1 and effective carrier concentrations as high as 1 × 10 20 cm -3 have been obtained. Electron concentrations are always lowe than indium contents on the films. Average otpical transmissions on the whole visible range as high as 85% for the best conductive films have been obtained. Refractive index of layers is modified by the growth temperature and indium doping, been less dependent on doping for high deposition temperatures (better crystallinity). Haacke's figure of merit up to 5 × 10 -3 Ω -1 in a 500 nm thick films were obtained. Indium doping improves the time-dependent stability of the electrical properties of ZnO conducting films

Voltage-controlled DC reactive magnetron sputtering of indium-doped zinc oxide films

Transparent and conducting indium-doped zinc oxide (IZO) films were deposited by DC reactive magnetron sputtering from Zn+2.5 at.% In alloy target. Stabilization of the operating point in the transition region was achieved by means of controlling the cathode voltage. The discharge current and the argon flow were kept constant. The drift of the cathode voltage was compensated for by induced changes in the oxygen flow. Different values of cathode voltage were chosen within the range corresponding to the transition between metallic and reactive modes. It was found that there exists a correlation between the cathode voltage and the film composition. The changes in structural, electrical and optical parameters of IZO films with cathode voltage were also studied.

Photoluminescent properties of indium-doped zinc oxide films prepared by spray pyrolysis

Photoluminescence from indium-doped zinc oxide is reported on films prepared by spray pyrolysis as a function of the substrate temperature and solution flow rate. All deposited films are polycrystalline and have a hexagonal crystalline structure, but high solution flow rates result in larger disorder on the orientation of the polycrystallites. The optical absorption edge is independent of the preparation conditions. The photoluminescent spectra depend on both the substrate temperature and on the solution flow rate. Spectra from films deposited at low substrate temperature or with high solution flow rate show a broad peak centered at λ⋍530 nm which could be associated with a (In Zn V Zn) - luminescent center. Luminescence measurements were also carried out on films with similar thickness to rule out the effects due to variations of thickness with the different deposition conditions investigated.

Development of transparent and conductive ZnO films by spray pyrolysis

Indium-doped zinc oxide (IZO) films were deposited on Corning 7059 substrates by the spray pyrolysis technique. To achieve higher electrical conductivity both the zinc acetate concentration and indium concentration in the solution were varied. The films were characterized for their structural and electrical properties. Film stability in H2 plasma was also checked for possible use in amorphous and microcrystalline silicon related fields. It was observed that the films can be sustained in a hydrogen plasma, and hence IZO films of high conductivity can be used for the development of amorphous and microcrystalline silicon solar cells.

Electrical and Optical Properties of Indium Doped Zinc Oxide Films Prepared by Atmospheric Pressure Chemical Vapor Deposition

ABSTRACTIndium doped zinc oxide films have been deposited from diethyl zinc, ethanol and trimethyl indium in the temperature range between 225°C and 450°C in a laminar flow atmospheric chemical vapor deposition reactor. Both doped and undoped films were crystalline. The doped films have electron density up to 9×1020 cm-3, conductivity up to 850 Ω-1cm-1, and mobility up to 8 cm2/Vs. The indium doping increases the average visible absorption from less than 1% to above 20%. The transparency and conductivity of indium doped zinc oxide films are lower than those of fluorine doped films.

Studies on electron transport properties and the Burstein-Moss shift in indium-doped ZnO films

Electrical conductivity, Hall mobility, thermoelectric power and optical properties were studied for indium-doped zinc oxide films produced by the magnetron sputtering technique. The data were analysed in the light of the existing theories and it was observed that scattering due to neutral impurities together with optical phonons is operative in these films. The optical data indicated a distinct Burstein-Moss shift. The values of carrier concentration, plasma frequency and optical band gap were correlated with each other, indicating the effective mass of carriers to be equal to 0.35 m e.

Aluminium- and indium-doped zinc oxide thin films prepared by DC magnetron reactive sputtering

Electrically conductive, optically transparent aluminium- and indium-doped zinc oxide films have been produced by DC magnetron reactive sputtering of ZnAl and ZnIn composite metal targets. The deposition parameters investigated include target design (zinc/dopant ratio), substrate temperature and the effect on film growth of ion-bombardment resulting from the use of an unbalanced DC magnetron. The process windows determined by the range of oxygen flows which produce films of low resistivity and high transparency were investigated for various target designs and deposition conditions. Ion bombardment of films deposited at low substrate temperatures leads to improved film transparency by partly re-sputtering unoxidised zinc. However, optimum quality films are produced only at those temperatures high enough to re-evaporate unoxidised zinc completely. Optimum indium-doped films of resistivity ∼1.4×10 −3 Ω cm , were produced on substrates held at ≥220°C. Aluminium-doped zinc oxide films were superior to indium-doped films in terms of both resistivity and transparency. Optimum aluminium-doped films of resistivity ∼5×10 −4 Ω cm and luminous transmittance ∼0.82 for a 750 nm thick film were produced on substrates held at ≥180°C. Film composition, deposition rates and degradation in air and water are also discussed.

Études sur la croissance, la structure et la composition de couches minces de ZnO et ZnO dopé a l'indium, obtenues par procédé pyrosol

Undoped and indium-doped ZnO thin films were deposited by the Pyrosol process onto soda-lime glass substrates. Several precursors and solvents were tested. Only a solution of dihydrated zinc acetate diluted in methanol allowed us to obtain good quality undoped ZnO thin films; indium doping was achieved by adding a saturated solution of indium acetylacetonate diluted in acetylacetone. A strong preferred orientation of grains which changed with deposition temperature and indium doping was observed. The average grain size varied linearly with substrate temperature for undoped ZnO films. The indium content of the films was only lower than that of the solution for high deposition temperatures. Alkali diffusion from the substrate which diminished with substrate temperature and/or indium doping was observed.

Indium-doped zinc oxide films as transparent electrodes for solar cells

Indium-doped zinc oxide films possess high conductivity and transparency with negligible absorption in the wavelength range of 0.4–0.8 μm which is the useful region for hydrogenated amorphous silicon solar cells. These films are thermally stable in both oxidizing and reducing ambients up to ∼800 K. These films do not degrade when exposed to hydrogen plasma. Pure ZnO films are rough, while In-doped ZnO films are very smooth. By making a double layer structure of In-doped ZnO/ZnO, the film surface has been texturized, which results in a large haze factor ( ∼16%) at a wavelength of 0.7 μm.

Lack of chemical interaction of hydrogenated amorphous silicon with indium-doped zinc oxide transparent conductive films

Indium-doped zinc oxide films have been prepared by the spray pyrolysis technique using air as a carrier gas at atmospheric pressure. These films show the excellent optical and electrical characteristics of a transparent conductive coating, with a sheet resistance of 15 Ω/square and an average optical transmission of about 88%. All the studied films show the wurtzite hexagonal crystalline structure. The results of Auger electron spectroscopy show that there was no reduction of the oxide film when an a-Si : H film was deposited by the plasma decomposition of silane at substrate temperatures in the range 150–300°C. A steep interface between the transparent conductive film and the hydrogenated amorphous silicon film was observed. Also, it was found that the depth distribution of the constituent atoms zinc, oxygen and indium in the ZnO : In film was homogeneous.

Indium-doped zinc oxide films prepared by d.c. magnetron sputtering

Transparent and conductive films of indium-doped zinc oxide have been prepared by reactive sputtering of ZnIn alloy targets with a conventional d.c. magnetron technique in controlled Ar + O 2 gas mixtures. The films were deposited onto glass substrates at a constant discharge current of 50 mA. Indium-doped films were polycrystalline with small crystal grains (5–15 nm) exhibiting low Hall mobility of 1–4 x 10 -4m 2Vs -1, high carrier concentration of 1–3 x 10 26m -3 and optical transmittance of 70–80% in the wavelength range from 400 to 800 nm. A correlation between the electrical, optical and structural properties of these films was also investigated.

Fabrication and analysis of all-sprayed CuInS 2/ZnO solar cells

CuInS 2/ZnO solar cells of 2% efficiency with V oc = 280 mV, I sc = 13.3 mA cm −2 and FF = 0.38 were prepared by spray pyrolysis. The role of CuInS 2 deposition parameters and the resistivity of CuInS 2 and ZnO films in the formation of efficient junctions was studied. Maximum efficiency was obtained for indium-doped ZnO films with an [In]/[Zn] atomic concentration ratio of 0.03 and CuInS 2 films deposited with 12% excess copper in spray solution. Deposition of an interlayer of CuInS 2 and post-deposition annealing of junctions are essential for obtaining good quality solar cells. Multistep tunnelling and recombination is the most likely mode of carrier transport in CuInS 2/ZnO heterojunctions.

Air heat treatment of In-doped ZnO thin films

Transparent ZnO thin films with a resistivity of about 1−3 × 10 −3 ω cm have been prepared on glass substrates held in a horizontal position by an rf sputtering technique. The deposition experiments were made using a ZnO target containing 2 wt.% In 2O 3. For the films deposited on glass substrates held in a vertical position, a resistivity of 3 × 10 −4 ω cm has been obtained. Air heat treatment experiments have been performed on the indium-doped ZnO films at temperatures between 200 and 350 °C. It was found that electrical resistance of the films deposited on horizontal substrates was stable for the treatment at temperatures below 250 °C. For treatment experiments at 350 °C, an increase in resistance by about 2 orders of magnitude has been observed after a period of 2 h. Experiments further showed that the low resistivity films deposited on vertical substrates were relatively unstable in air even at 200 °C.

Thickness-dependent properties of indium-doped ZnO films

The thickness dependence of microstructural, electrical and optical properties of transparent conducting indium-doped ZnO films was studied. The density of trap states due to chemisorbed oxygen at the grain boundaries was found to depend on the orientation of grains which, in turn, depended on the film thickness. The thickness dependence of the electrical parameters of oxygen-annealed films was explained on the basis of changes in the trap state density. The changes in the electrical parameters of vacuum-annealed films were attributed to a decrease in grain size. The shifts in optical gap on vacuum and oxygen annealing were attributed to changes in carrier concentration, considering two simultaneous effects on band gap widening and narrowing.

Effect of hydrogen plasma treatment on transparent conducting oxides

The effect of hydrogen plasma treatment on indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and indium-doped zinc oxide (IZO) films has been studied. X-ray photoelectron spectroscopy analysis shows that ITO and FTO surfaces get reduced to yield elemental indium and tin, respectively. Annealing of the plasma treated films in air leads to re-oxidation of the reduced surface and the electro-optical properties are recovered. In contrast, IZO films are not reduced by plasma treatment and show no changes in the electrical and optical properties. The surface of plasma treated IZO films shows a higher binding energy O(1s) peak probably due to OH or OH...O species which appear to form a protective layer against plasma degradation.

Electrical and optical transport in undoped and indium-doped zinc oxide films

Electrical conduction in undoped and indium-doped ZnO films in as-deposited, vacuum-annealed and oxygen-annealed states has been studied. The as-deposited and oxygen-annealed films contain a large density (≥ 1017 m−2) of trap states due to chemisorbed oxygen at the grain boundaries. The role of these trap states has been analyzed in terms of the grain boundary carrier trapping model. The vacuum-annealed films are free of chemisorbed oxygen, and the conduction in these films is controlled by scattering due to ionized impurities and grain boundary barriers. In the case of undoped ZnO films, intrinsic trap states at the grain boundaries also play a significant role. The optical behavior of all films in the UV and visible regions is dielectric-like and the optical bandgap shows a dependence on free carrier concentration that is controlled by a bandgap narrowing effect due to electron-electron and electron-impurity interactions as well as the Moss-Burstein effect of bandgap widening. In the IR region the optical behavior is metal-like due to free-electron effects and follows the Drude model.