Preparation And Characterization Of Transparent Conducting Zirconium-doped Zinc Oxide Films Deposited By Sputtering

Transparent conducting films are extensively used for a variety of applications due to high transparency in the visible region and low resistivity. Presently, indium tin oxide (ITO) film is the most widely used transparent conducting film. Fluorine-doped tin oxide (FTO) and aluminum-doped zinc oxide (AZO) are also widely used and AZO is considered as an appropriate substitute of ITO in many applications.In applications involving transparent heaters and chemical sensors, it is required that the films be chemically stable as they cycle through high temperatures. However, ITO films are not stable and show degradation at temperatures above 700 K, and AZO can only work stably below 800 K. Meanwhile, for ITO, FTO and AZO, only one free electron can be supplied to the conduction band for every substitutional incorporation, thus a relatively high doping content is needed to obtain high carrier concentration, which decreases the mobility due to the increase of ionized impurity scattering.It is reported that doping is an efficient way to improve the thermal stability of ZnO films. ZrO₂-ZnO films have been prepared and proved to be stable after multiple cycles over a wide range of temperatures. Thus, ZZO is a promising substitute of ITO to work at high temperatures stably. And in the case of zirconium-doped zinc oxide (ZZO), two free electrons are produced for every Zn²⁺ replaced by Zr⁴⁺. Therefore, it is possible that high carrier concentration can be achieved with a relatively low doping content, which consequently results in high mobility. And the radii of Zn²⁺ and Zr⁴⁺ are almost the same in ZnO structure, which is beneficial to the substitution and makes the lattice vary little after doping. Transparent conducting ZZO films have been prepared by some groups.Moreover, ZnO and ZrO₂ are nontoxic, inexpensive, and abundant compared with ITO and FTO. To our knowledge, the study of ZZO films is limited and they have been rarely prepared by RF magnetron sputtering.In this dissertation, we report the preparation of transparent and conducting ZZO films using RF magnetron sputtering and analyze the effects of doping content and deposition parameters (film thickness, deposition pressure and RF power) on the properties (structural, chemical, electrical, and optical properties) of ZZO film in detail. All the films prepared are polycrystalline with the hexagonal structure and have a preferred orientation with the c axis perpendicular to the substrates.Five samples with ZrO₂ content of 0, 3, 5, 7 and 10 wt.% are used for the analysis of doping content effect, which are prepared under 0.6 Pa and 100 W with a thickness of about 300 nm for all the samples. The as-prepared surface is contaminated by organic carbon and the peaks of Zr 3d can not be distinguished. After Ar⁺ etching, carbon is mostly removed and the peaks of Zr 3d appear. About half of O atoms exit as chemisorberd oxygen ions in the forms of -OH or OH.. .O for the as-prepared film. For all the samples after etching, most zinc atoms and oxygen atoms exit as Zn²⁺ ions and O^2- respectively in the stoichiometric wurtzite structure of ZnO, and a little amount of oxygen atoms exit in oxygen deficient regions. Most of zirconium atoms are in the sate of Zr⁴⁺, but they either replace zinc atoms or in the form of ZrO₂ for different samples. Both atomic ratios of O/Zn and Zr/Zn in the film are lower than those in the target for each sample. The film without doping is in a state of high oxygen deficiency and the ratio of O/Zn is above 80% and shows little change with the variation of ZrO₂ content for the doped films. For all the samples, the c axis lattice parameter values are larger than that of bulk ZnO. With the increase of the ZrO₂ content from 0 to 5 wt.%, the lattice parameter increases from 5.24 to 5.27 A. When the ZrO₂ content increases from 5 to 10 wt.%, the lattice parameter shows a significant increase to 5.40 A. As the ZrO₂ content increases from 0 to 5 wt.%, the crystallinity of the film is improved and the crystallite size increases from 16.6 to 20 nm. As the ZrO₂ content increases further to 10 wt.%., the crystallite size decreases to 11.1 nm. The 5 wt.% ZrO₂ film has a compact and relatively smooth surface. For the undoped sample, when T<179 K, ionized impurity scattering is dominant, and grain boundary scattering becomes a main scattering mechanism when T> 179 K. For 5 wt.% ZrO₂ sample, ionized impurity scattering is dominant in the measuring temperature range. For 7 wt.% ZrO₂ sample, grain boundary scattering plays a main role in the scattering of carriers. The carrier sources are oxygen vacancies and zirconium doping for our films. The resistivity initially decreases with increasing the ZrO₂ content from 0 to 5 wt.%, and then shows a sharp increase with the further increase of the ZrO₂ content to 10 wt.%. The lowest resistivity obtained is 1.61×10^-3Ωcm with a Hall mobility of 10.3 cm² V^-1s^-1 and a carrier concentration of 3.78×10²⁰cm^-3. The average transmittance in the visible range is above 90% for all the films. The optical band gap value initially increases from 3.28 to 3.35 eV with the ZrO₂ content increasing from 0 to 5 wt.%, and then shows a slight decrease to 3.32 eV with further increase of the ZrO₂ content to 10 wt.%.Six samples with thicknesses of 100, 170, 225, 345, 475 and 600 nm are used for the analysis of film thickness effect, which are prepared under 125 W and 0.6 Pa with the 5 wt.% ZrO₂ target. For samples with different thicknesses, the growth rates are about 30 nm/min with little variation. As the thickness increases from 100 to 345 nm, the crystallinity of the film is improved significantly and the crystallite size increases from 9.39 to 14 nm. As the thickness increases further, the crystallite size varies little and then shows a little decrease when the thickness is more than 475 nm. The surface roughness is increased simultaneously with the increase of crystallite size. The resistivity decreases continuously when the thickness increases from 100 to 475 nm and then increases a little when the thickness increases further. The lowest resistivity obtained is 2.93×10^-3 D. cm with a Hall mobility of 13 cm² V^-1 s^-1 and a carrier concentration of 1.71×10²⁰cm^-3. The average transmittance in the visible range is approximately 90% for all the films, and it shows a little decrease with the increase of thickness. The optical band gap value decreases from 3.42 to 3.27 eV with the increase of thickness.Five samples with deposition pressure of 0.6, 1.2,1.6, 2.1 and 2.5 Pa are used for the analysis of deposition pressure effect, which are prepared under 100W with the 5 wt.% ZrO₂ target with a thickness of about 300 nm for all the samples. The growth rate decreases clearly from 20 to 9 nm/min when the pressure increases from 0.6 to 2.5 Pa. The crystallite size increase shows a little change with the variation of deposition pressure. The crystallite size increases from 20 to 23.5 nm when the increases from 0.6 to 1.6 Pa, and then decreases to 20.5 nm when the pressure increases further to 3.0 Pa. As the pressure increases, the surface morphology transforms from a relatively compact structure with some rugged structures on the surface to individually grains surrounded by voids. As the pressure increases from 0.6 to 2.5 Pa, the resistivity increases sharply with the simultaneous decrease of Hall mobility and carrier concentration. The lowest resistivity achieved is 2.07×10^-3Ωcm with a Hall mobility of 16 cm² V^-1s^-1 and a carrier concentration of 1.95×10²⁰ cm^-3. The average transmittance in the visible range is above 90% for all the films, and it shows a little decrease with the increase of pressure. The optical band gap value decreases from 3.35 to 3.20 eV with the increase of pressure.Four samples with RF power of 75, 100, 125, 150 W are used for the analysis of RF power effect, which are prepared under 0.6 Pa with the 5 wt.% ZrO-2 target with a thickness of about 300 nm for all the samples. The growth rate increases apparently from 14 to 33 nm/min with the increase of the RF power from 75 to 150 W. For all the samples, the c axis lattice parameter values are between 5.25 and 5.28 A, which are larger than 5.207 A for bulk ZnO. When the RF power increases from 75 to 100 W, the crystallinity is improved and the crystallite size increases from 15.1 to 20 nm. However, as the RF power increases further to 150 W, the crystallite size decreases clearly from 20 to 10.3 nm. The surface morphology transforms from small and individually distributed grains to a much more compact structure when the RF power increases from 75 to 100 W, and the surface roughness increases obviously and the surface is composed of grains surrounded by voids with the further increase of RF power to 150 W. The resistivity decreases sharply when the RF power increases from 75 to 100 W, and shows a continuous increase when the RF power increases further to 150 W. The lowest resistivity achieved is 2.07×10^-3Ωcm with a Hall mobility of 16 cm² V^-1s^-1 and a carrier concentration of 1.95×10²⁰cm^-3. For all the films, the average transmittance in the visible range is approximately 92% and the optical band gap value is about 3.33 eV.