Impurity-doped ZnO Thin Films Research Articles

Optoelectronic properties of doped hydrothermal ZnO thin films

Group III impurity doped ZnO thin films were deposited on MgAl2O3 substrates using a simple low temperature two‐step deposition method involving atomic layer deposition and hydrothermal epitaxy. Films with varying concentrations of either Al, Ga, or In were evaluated for their optoelectronic properties. Inductively coupled plasma atomic emission spectroscopy was used to determine the concentration of dopants within the ZnO films. While Al and Ga‐doped films showed linear incorporation rates with the addition of precursors salts in the hydrothermal growth solution, In‐doped films were shown to saturate at relatively low concentrations. It was found that Ga‐doped films showed the best performance in terms of electrical resistivity and optical absorbance when compared to those doped with In or Al, with a resistivity as low as 1.9 mΩ cm and an optical absorption coefficient of 441 cm−1 at 450 nm.

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.

Effect of dopants on the structural, optical and electrical properties of sol–gel derived ZnO semiconductor thin films

Undoped, Ga-, In-, Zr-, and Sn-doped ZnO transparent semiconductor thin films were deposited on alkali-free glasses by sol–gel method. 2-methoxyethanol (2-ME) and diethanolamine (DEA) were chosen as a solvent and a stabilizer, respectively. The doping concentration was maintained at 2 at.% in the impurity doping precursor solutions. The effects of different dopants on the structural, optical, and electrical properties of ZnO thin films were investigated. XRD results show that all annealed ZnO-based thin films had a hexagonal (wurtzite) structure. ZnO thin films doped with impurity elements obviously improved the surface flatness and enhanced the optical transmittance. All impurity doped ZnO thin films showed high transparency in the visible range (>91%). The Ga- and In- doped ZnO thin films exhibited higher Hall mobility and lower resistivity than did the undoped ZnO thin film.

Optical analysis of doped ZnO thin films using nonparabolic conduction-band parameters

The optical properties of impurity doped ZnO thin films were analyzed by taking into account the nonparabolicity in the conduction-band and the optically determined carrier concentration and mobility were correlated with those measured by Hall measurement. The Drude parameters obtained by applying a simple Drude model combined with the Lorentz oscillator model for the optical transmittance and reflectance spectrum were analyzed by using the carrier density dependent bare band effective mass determined by the first-order nonparabolicity approximation. The squared plasma energy multiplied by the carrier density dependent effective mass yielded fairly linear relationship with respect to the carrier concentration in wide carrier density range of 1019 − 1021 cm−3, verifying the applicability of the nonparabolicity parameter for various types of impurity doped ZnO thin films. The correlation between the optical and Hall analyses was examined by taking the ratios of optical to Hall measurements for carrier density, mobility, and resistivity by introducing a parameter, Rdl, which represents the ratio of the resistances to electron transport from the inside of the lattice and from the crystallographic defects. For both the carrier concentration and mobility, the ratios of optical to Hall measurements were shown to exhibit a monotonically decreasing function of Rdl, indicating that the parameter Rdl could be used as a yardstick in correlating the optically determined carrier density and mobility with those measured by Hall analysis.

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.

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.

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.

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