Impurity-doped ZnO Research Articles

Electronic Structure of Rh and Ir Single Atom Catalysts Supported on Defective and Doped ZnO: Assessment of Their Activity Towards CO Oxidation.

This study investigated the electronic structure of single-atom Rhodium (Rh) and Iridium (Ir) adsorbed on defective and impurity-doped ZnO(0001) surfaces, and assessed their activity towards the CO oxidation reaction. Our findings reveal that surface impurities significantly influence the binding energies and electronic properties of the metal atoms, with Al and Cr serving as particularly effective promoters. While Rh and Ir acquire a positive charge upon incorporation on the unpromoted Zn(0001) surface, adsorption directly on the promoter results in a net negative charge, thus facilitating the activation of both CO and O2 species. These results highlight the potential of impurity-promoted ZnO surfaces in modulating and tailoring the electronic properties of SACs, which can be used for a rational design of active single-atom catalysts.

Band gap engineering, enhanced morphology and photoluminescence of un-doped, Ga and/or Al-doped ZnO nanoparticles by reflux precipitation method

In this study, un-doped ZnO, Ga/Al-doped and co-doped ZnO nanoparticles (NPs) at 2mol% Ga doping (GZO), 2mol% Al doping and combined Ga and Al doping concentrations summing up to 2mol% (ranging from 1:1 to 1.9:0.1mol% Ga/Al) were synthesized using the reflux precipitation method. The effects of increasing the mole ratios of Ga in the ZnO host material or reducing Al mol%, on the structural, luminescence and optical properties of impurity-doped ZnO were investigated along with the properties of ZnO NPs. The X-ray diffraction patterns for all samples exhibit highly crystalline NPs with a hexagonal wurtzite structure. The crystallite size and average relative intensity of the diffraction peaks of the samples increased with increasing Ga doping indicating improved crystallinity with an increase in the content of gallium in the ratio. Photoluminescence spectra also displayed an increase in exciton peak emission with increasing Ga doping levels and reached a maximum at a doping concentration of 2.0mol% Ga, as well as a corresponding decrease in the deep level emission intensity, indicative that the best optical quality was obtained at the 2mol% Ga level. It was observed a blue shift in the PL exciton emission peak position and the band edge emission of the reflectance spectra with increasing concentration of Ga in the sample. The optical band gap calculated from the reflectance data also increased with increasing Ga concentration. These GZO NPs with improved crystallinity, minimum lattice stress, and enhanced luminescence intensity and optical properties would be suitable for deposition on a glass substrate to form the seed layer to produce a transparent conducting oxide for a dye- sensitized solar cell based thin films.

Influence of the kind and content of doped impurities on impurity-doped ZnO transparent electrode applications in thin-film solar cells

The influence of the kind and content of doped impurity on the electrical properties and their stability as well as on the light scattering characteristics and the surface texture formation obtainable by wet-chemical etching was investigated to develop transparent conducting impurity-doped ZnO thin films suitable for transparent electrode applications in thin-film solar cells. Al-, B- and Ga-doped ZnO (AZO, BZO and GZO) thin films were prepared on glass substrates at 200°C by doping each impurity at an appropriate content to achieve lower resistivity using a pulsed laser deposition and by varying the doped Al or Ga impurity content using r.f. superimposed d.c. magnetron sputtering deposition. The difference in the resulting electrical properties and their stability (air at 85% relative humidity and 85°C) among the AZO, BZO and GZO thin films is attributed to the content of each impurity rather than the kind of impurity. In evaluations of the light scattering characteristics and the surface texture formation in a 0.1% HCl solution at 25°C, the difference in the haze value obtainable among the AZO, BZO and GZO thin films is mainly attributed to the content of each impurity doped into the films. The AZO, BZO and GZO films prepared with doped impurity contents below approximately 2at.% are suitable for transparent electrode applications in thin-film solar cells, irrespective of the kind of impurity as well as the deposition method used.

Impurity-doped ZnO Thin Films Prepared by Physical Deposition Methods Appropriate for Transparent Electrode Applications in Thin-film Solar Cells

This paper describes the development of transparent conducting impurity-doped ZnO thin films that would be appropriate for applications as transparent electrodes in thin-film solar cells. Transparent conducting Al-, B- and Ga-doped ZnO (AZO, BZO and GZO) thin films were prepared in a thickness range from 500 to 2000 nm on glass substrates at 200°C using various physical deposition methods: BZO films with vacuum arc plasma evaporation, AZO and GZO films with different types of magnetron sputtering depositions (MSDs) and all films with pulsed laser deposition (PLD). The suitability and stability of the electrical properties and, in addition, the suitability of the light scattering characteristics and surface texture formation were investigated in the prepared thin films. In particular, the suitability and stability evaluation was focused on the use of AZO, BZO and GZO thin films prepared by doping each impurity at an appropriate content to attain the lowest resistivity. The higher Hall mobility obtained in impurity-doped ZnO thin films with a resistivity on the order of 10−4 Ωcm was related more to the content, i.e., the obtained carrier concentration, rather than the kind of impurity doped into the films. The stability of resistivity of the BZO thin films in long-term moisture-resistance tests (in air at 85% relative humidity and 85°C) was found to be lower than that of the AZO and GZO thin films. The surface texture formation was carried out by wet-chemical etching (in a 0.1% HCl solution at 25°C) conducted either before or after being heat-treated either with rapid thermal annealing (RTA) or without RTA. The suitability of the light scattering characteristics and the surface texture formation obtainable by wet-chemical etching (for use in transparent electrode applications) was considerably dependent on the deposition method used as well as whether the wet-chemical etching was conducted with or without RTA. A significant improvement of both transmittance and haze value at wavelengths up to about 1200 nm in the near-infrared region was attained in surface-textured AZO films that were prepared by r.f. power superimposed d.c. MSD as well as etched after being heat treated with RTA at 500°C for 5 min in air. The obtained suitability and stability in impurity-doped ZnO thin films were related more to the content rather than the kind of impurity doped into the films as well as to the deposition method used.

Non-vacuum, single-step conductive transparent ZnO patterning by ultra-short pulsed laser annealing of solution-deposited nanoparticles

A solution-processable, high-concentration transparent ZnO nanoparticle (NP) solution was successfully synthesized in a new process. A highly transparent ZnO thin film was fabricated by spin coating without vacuum deposition. Subsequent ultra-short-pulsed laser annealing at room temperature was performed to change the film properties without using a blanket high temperature heating process. Although the as-deposited NP thin film was not electrically conductive, laser annealing imparted a large conductivity increase and furthermore enabled selective annealing to write conductive patterns directly on the NP thin film without a photolithographic process. Conductivity enhancement could be obtained by altering the laser annealing parameters. Parametric studies including the sheet resistance and optical transmittance of the annealed ZnO NP thin film were conducted for various laser powers, scanning speeds and background gas conditions. The lowest resistivity from laser-annealed ZnO thin film was about 4.75×10−2 Ω cm, exhibiting a factor of 105 higher conductivity than the previously reported furnace-annealed ZnO NP film and is even comparable to that of vacuum-deposited, impurity-doped ZnO films within a factor of 10. The process developed in this work was applied to the fabrication of a thin film transistor (TFT) device that showed enhanced performance compared with furnace-annealed devices. A ZnO TFT performance test revealed that by just changing the laser parameters, the solution-deposited ZnO thin film can also perform as a semiconductor, demonstrating that laser annealing offers tunability of ZnO thin film properties for both transparent conductors and semiconductors.

Polarity of heavily doped ZnO films grown on sapphire and SiO 2 glass substrates by pulsed laser deposition

The crystalline polarity of undoped and impurity-doped ZnO films grown on SiO 2 glass substrates was investigated with the goal of achieving polarity-selective growth of ZnO films on non-crystalline substrates. We first demonstrated that hard X-ray photoelectron spectroscopy (HX-PES) is an appropriate method for determining the crystalline polarity of ZnO. We then characterized the ZnO films grown by pulsed laser deposition using HX-PES. The resulting films deposited with a 1 mol% Al-doped ZnO target had the (0001) surface, whereas films grown with nominally undoped, 0.1 mol% Al-doped, 1 mol% Ga-doped, and 1 mol% In-doped ZnO targets had the (000 1 ¯ ) surface. Since a clear polarity change due to Al-doping was seen at the ZnO/glass structure, we conclude that the essential parameter governing the polarity of the ZnO films is unlikely lattice matching (alignment of the lattice on the atomic scale) at the heterointerface between the ZnO films and substrates.

Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared on glass substrates by a dc magnetron sputtering (dc-MS) deposition, the obtainable lowest resistivity and the spatial resistivity distribution on the substrate surface were improved by a newly developed MS deposition method. The decrease of obtainable lowest resistivity as well as the improvement of spatial resistivity distribution on the substrate surface in Al- or Ga-doped ZnO (AZO or GZO) thin films were successfully achieved by inserting a very thin buffer layer, prepared using the same MS apparatus with the same target, between the thin film and the glass substrate. The deposition of the buffer layer required a more strongly oxidized target surface than possible to attain during a conventional dc-MS deposition. The optimal thickness of the buffer layer was found to be about 10 nm for both GZO and AZO thin films. The resistivity decrease is mainly attributed to an increase of Hall mobility rather than carrier concentration, resulting from an improvement of crystallinity coming from insertion of the buffer layer. Resistivities of 3 × 10 − 4 and 4 × 10 − 4 Ω cm were obtained in 100 nm-thick-GZO and AZO thin films, respectively, incorporating a 10 nm-thick-buffer layer prepared at a substrate temperature around 200 °C.

Effect of target properties on transparent conducting impurity-doped ZnO thin films deposited by DC magnetron sputtering

For the purpose of using transparent conducting impurity-doped ZnO thin films in liquid crystal display (LCD) applications, the relationship between the properties of dc magnetron sputtering (dc-MS) deposited thin films and the properties of the oxide targets used to produce them is investigated. Both Al-doped and Ga-doped ZnO (AZO and GZO) thin films were deposited on glass substrates using a dc-MS apparatus with various high-density sintered AZO or GZO disk targets (diameter of about 150 mm); the target and substrate were both fixed during the depositions. Using targets with a lower resistivity results in attaining more highly stable dc-MS depositions with higher deposition rates and lower arcing. In addition, dc-MS depositions using targets with a lower resistivity produced improvements in resistivity distribution on the substrate surface. It was found that the oxygen content in deposited thin films decreased as the oxygen content of the target used in the deposition was decreased. As a result, the dc-MS deposition of transparent conducting impurity-doped ZnO thin films suitable for LCD applications requires the preparation of significantly reduced AZO and GZO targets with low oxygen content.

Spatial resistivity distribution of transparent conducting impurity-doped ZnO thin films deposited on substrates by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared by a conventional dc magnetron sputtering deposition (dc-MSD), the key factors in the deposition conditions that are necessary for practical use in transparent electrode applications were investigated. It was found that impurity-doped ZnO targets with a resistivity higher than approximately 3 mΩ cm are unsuitable for practical use in the preparation of transparent conducting Al-doped ZnO and Ga-doped ZnO thin films by conventional dc-MSD. Improvements of both the resulting resistivity distribution and resistivity can be sufficiently obtained only by using targets with a resistivity lower than about 0.5 mΩ cm. Using a low oxygen content target having a lower resistivity was found to reduce both the amount of oxygen in the chamber and the amount of oxygen reaching the substrate surface. As a result, it was demonstrated that sintered impurity-doped ZnO targets optimized for the preparation of thin films with lower resistivity as well as more uniform resistivity distribution on the substrate surface tended to exhibit a resistivity lower than about 0.5 mΩ cm.

Resistivity characteristics of transparent conducting impurity-doped ZnO films for use in oxidizing environments at high temperatures

The behavior of resistivity in transparent conducting Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films, prepared with the same thickness under the same deposition conditions by dc magnetron sputtering, was investigated for use in oxidizing environments at high temperatures. These thin films with thicknesses in the range from 100 to 3000 nm were prepared on glass substrates at a temperature of 200 °C. The increase in resistivity of GZO thin films over long-term testing at 85% relative humidity and 85 °C was lower than that of AZO thin films, while, in contrast, that of AZO thin films over long-term testing at 90% relative humidity and 60 °C was lower than that exhibited by GZO. The moist-resistant tests showed that both AZO and GZO thin films prepared with a thickness above 500 nm were suitable for transparent electrode applications used in thin-film solar cells. The resistivity increase of GZO thin films after heat treatment for 30 min in air at a temperature up to 400 °C was lower than that of AZO thin films. The resistivity of both GZO and AZO thin films prepared with a thickness above 200 nm was stable for heat tests at 300 °C. The heat-resistant tests demonstrated that both GZO and AZO thin films were suitable for use as a transparent electrode for thin-film solar cells. The resistivity increase after testing was found to exhibit two different mechanisms: a decrease of both carrier concentration and Hall mobility mainly attributable to grain boundary scattering as well as a decrease of carrier concentration mainly attributable to ionized impurity scattering. This difference in resistivity behavior is mainly attributed to the grain size associated with the crystallinity of deposited films.

Characteristics of Al-doped, Ga-doped and In-doped zinc-oxide films as transparent conducting electrodes in organic light-emitting diodes

The AZO, GZO and ZIO films were deposited on glass substrates by DC magnetron sputtering at substrate temperatures ranging from RT to 300 °C using impurity-doped ZnO targets containing 2.0 wt.% Al 2O 3, 5.56 wt.% Ga 2O 3 and 9.54 wt.% In 2O 3, respectively. The lowest resistivity of 6.41 × 10 –4 Ω cm was obtained for the GZO films deposited at 300 °C. The AZO, GZO and ZIO films were used as an anode contact to fabricate organic emitting diodes. The turn-on voltage at current density of 0.01 mA/cm 2 is 6.42, 9.33 and 12.19 V for the devices with AZO, ZIO and GZO anodes, respectively. These results can be explained by carrier density and resistivity distribution of AZO, ZIO and GZO films.

Large-sized-mismatched group-V element doped ZnO films fabricated on silicon substrates by pulsed laser deposition

Pulsed laser deposition has been utilized to synthesize impurity-doped ZnO thin films on silicon substrate. Large-sized-mismatched group-V elements (A V) including P, As, Sb and Bi were used as dopants. Hall effect measurements show that hole concentration in the order of 10 16–10 18 cm −3, resistivity in the range of 10–100 Ω cm, Hall mobility in the range of 10–100 cm 2/Vs were obtained only for ZnO:As and ZnO:Bi thin films. X-ray diffraction measurements reveal that the films possess polycrystallinity or nanocrystallinity with ZnO (002) preferred orientation. Guided by X-ray photoemission spectroscopy analyses and theoretical calculations for large-sized-mismatched group-V dopant in ZnO, the A Zn V–2V Zn complexes are believed to be the most possible acceptors in the p-type A V-doped ZnO thin films.

Transparent conducting impurity-doped ZnO thin films prepared using oxide targets sintered by millimeter-wave heating

The preparation of transparent conducting impurity-doped ZnO thin films by both pulsed laser deposition (PLD) and magnetron sputtering deposition (MSD) using impurity-doped ZnO targets sintered with a newly developed energy saving millimeter-wave (28GHz) heating technique is described. Al-doped ZnO (AZO) and V-co-doped AZO (AZO:V) targets were prepared by sintering with various impurity contents for 30min at a temperature of approximately 1250°C in an air or Ar gas atmosphere using the millimeter-wave heating technique. The resulting resistivity and its thickness dependence obtainable in thin films prepared by PLD using millimeter-wave-sintered AZO targets were comparable to those obtained in thin films prepared by PLD using conventional furnace-sintered AZO targets; a low resistivity on the order of 3×10−4Ωcm was obtained in AZO thin films prepared with an Al content [Al∕(Al+Zn) atomic ratio] of 3.2at.% and a thickness of 100nm. In addition, the resulting resistivity and its spatial distribution on the substrate surface obtainable in thin films prepared by rf-MSD using a millimeter-wave-sintered AZO target were almost the same as those obtained in thin films prepared by rf-MSD using a conventional powder AZO target. Thin films prepared by PLD using millimeter-wave-sintered AZO:V targets exhibited an improved resistivity stability in a high humidity environment. Thin films deposited with a thickness of approximately 100nm using an AZO:V target codoped with an Al content of 4at.% and a V content [V∕(V+Zn) atomic ratio] of 0.2at.% were sufficiently stable when long-term tested in air at 90% relative humidity and 60°C.

Present status and future prospects for development of non- or reduced-indium transparent conducting oxide thin films

This paper surveys the recent progress and prospects for further development of indium–tin-oxide (ITO) substitute materials for practical use as thin-film transparent electrodes. The best, and only practical, indium-free candidate for an alternative material to ITO is impurity-doped ZnO such as Al- or Ga-doped ZnO (AZO or GZO). Presented here are newly developed impurity-doped ZnO thin-film deposition techniques, suitable for practical use, that reduce resistivity as well as improve the uniformity of the resistivity distribution using oxidization-suppressing magnetron sputtering deposition methods. Also presented are descriptions concerning the observed increase in resistivity of impurity-doped ZnO thin films resulting from exposure to long-term testing in a high humidity environment (air at 90% relative humidity and 60 °C) as well as the increase in resistivity associated with a decrease of film thickness. The resistivity stability of AZO thin films could be considerably improved by co-doping another impurity.

Al- and Al:In-Doped ZnO Thin Films Deposited by RF Magnetron Sputtering for Spacecraft Charge Mitigation

We investigate the utility of impurity-doped ZnO as charge-mitigating thin films for spacecraft in geosynchronous orbit. Satellite solar panel cover glass and thermal control blankets are commonly coated in tin-doped indium oxide which serves as a leakage path for accumulated surface charge. The increase in demand for indium has generated interest in alternative space-stable satellite coatings. We demonstrate the orbital stability of Al- and Al:In-doped ZnO exposed to a high-energy electron flux and simulated solar ultraviolet (UV) radiation.

Present status of transparent conducting oxide thin-film development for Indium-Tin-Oxide (ITO) substitutes

This paper describes the present status and prospects for further development of transparent conducting oxide materials for use as Indium-Tin-Oxide (ITO) substitutes in the thin-film transparent electrodes of liquid crystal displays (LCDs), currently the largest use of ITO, and, thus, of indium. The best substitute material for the ITO transparent electrodes used in LCDs is impurity-doped ZnO, e.g., Al- and Ga-doped ZnO (AZO and GZO). From resource and environmental points of view, AZO is the best candidate. The most important problems associated with substituting impurity-doped ZnO for the ITO used in LCDs have already been resolved in laboratory trials. Under the present circumstances, (rf and dc)-magnetron sputtering (rf + dc-MS) deposition, both with and without H 2 gas introduction, has been found to be the best deposition method to prepare impurity-doped ZnO thin films for practical use; AZO thin films with a resistivity on the order of 10 − 4 Ω cm were prepared on glass substrates with an approximately uniform resistivity spatial distribution and a thickness above 100 nm. In order to improve the resistivity stability, AZO and GZO thin films co-doped with another impurity have been newly developed. A 50 nm-thick V-co-doped AZO (AZO:V) thin film was stable enough to be acceptable for use in practical transparent electrode applications. However, it seems likely that obtaining a stability comparable to that of ITO using impurity-doped ZnO will be difficult for thin films with a thickness below approximately 30 nm.

Stability of nano-thick transparent conducting oxide films for use in a moist environment

The stability of nano-thick transparent conducting oxide thin films in a high humidity environment was investigated. The stability of ITO and impurity-doped ZnO thin films prepared with a thickness in the range from approximately 20 to 100 nm on glass substrates at a temperature below 200 °C by a pulsed laser deposition was evaluated in air at a relative humidity of 90% and a temperature of 60 °C. The resistivity of all Al- and Ga-doped ZnO thin films tested was found to increase markedly with test time, whereas that of ITO remained relatively stable; the stability (resistivity increase) of the doped ZnO thin films was considerably affected by film thickness but was relatively independent of the deposition substrate temperature. In particular, doped ZnO thin films with a thickness below approximately 50 nm were very unstable under the test conditions. The resistivity increase of doped ZnO films is mainly attributed to the grain boundary scattering resulting from the adsorption of oxygen on the grain boundary.

Present status and prospects of transparent conducting impurity-doped ZnO thin films

Present status and prospects of transparent conducting impurity-doped ZnO thin films

Transparent conducting oxide semiconductors for transparent electrodes

The present status and prospects for further development of polycrystalline or amorphous transparent conducting oxide (TCO) semiconductors used for practical thin-film transparent electrode applications are presented in this paper. The important TCO semiconductors are impurity-doped ZnO, In2O3 and SnO2 as well as multicomponent oxides consisting of combinations of ZnO, In2O3 and SnO2, including some ternary compounds existing in their systems. Development of these and other TCO semiconductors is important because the expanding need for transparent electrodes for optoelectronic device applications is jeopardizing the availability of indium-tin-oxide (ITO), whose main constituent, indium, is a very expensive and scarce material. Al- and Ga-doped ZnO (AZO and GZO) semiconductors are promising as alternatives to ITO for thin-film transparent electrode applications. In particular, AZO thin films, with a low resistivity of the order of 10−5 Ω cm and source materials that are inexpensive and non-toxic, are the best candidates. However, further development of the deposition techniques, such as magnetron sputtering or vacuum arc plasma evaporation, as well as of the targets is required to enable the preparation of AZO and GZO films on large area substrates with a high deposition rate.