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The Study of H Doping in ZAO:H Transparent Conductive Thin Films

Author: WangJun
Tutor: ZhuBaiLin
School: Wuhan University of Science and Technology
Course: Materials Engineering
Keywords: ZAO thin films radio frequency magnetron sputtering H doping vacuum annealing stability
CLC: TB43
Type: Master's thesis
Year: 2011
Downloads: 31
Quote: 0
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Abstract


Transparent conducting oxides （TCO） are widely used in flat panel displays, solar cells, touch screens, light emitting diodes, and low-emissivity windows due to its low resistivity and high transparency. Al-doped ZnO （ZAO） thin films exhibit considerable electrical and optical properties with low material cost, non-toxicity and stability in hydrogen plasma, and thus they are regarded as an alternative material to In2O3:SnO2 （ITO）. Theoretical and experimental studies have confirmed that hydrogen not only affect the structure of the ZAO film, but also act as a shallow donor to achieve high performance films. However, there was much controversy over the behavior of H doping, which is significant for the development of ZAO films.In this paper, ZAO:H thin films were prepared on ordinary glass by radio frequency magnetron sputtering in Ar + H2 ambient, and the annealing treatment of as deposited films was performed in vacuum ambient. The structure, electrical and optical properties of as deposited and annealed ZAO:H thin films were investigated by X-ray diffraction measurement （XRD）, scanning electron microscope （SEM）, Hall effects measurement, and UV-visible spectrophotometer. The behavior of H doping was discussed by the experiment results. In addition, the stability of electrical properties was tested in conditions of atmosphere and high damp environment （similar to the IEC1646 Standard）, respectively. Then the influence of deposition temperature and H2 fluxes on the conductive stability were analysed.The result shows that the effects of H2 flux on the structure and properties of films are related to deposite temperature tightly. When the film was prepared at 100℃, （001） orientation of the film became more obvious with the increasing H2 flux. As the substrate temperature increase to 150℃and 200℃, the films are highly （002） orientation and becomes more compact with appropriate addition of H2, while the grain size increases with the increase of H2 amount. However, the addition of H2 has no obvious effect on the crystallinity of ZAO films at higher substrate temperature. The resistivity of the film decreases with the increase of H2 flux, accompanying with the increase of carrier concentration and Hall mobility. The transmittance in visible region of obtained film is all over 80%. The resistivity of ZAO film decreases after annealing treatment, which is contrary to the ZAO:H film. It was found that the resistivity of all transparent conducting ZAO and ZAO:H thin films increased with exposure time up to720 hours when tested in a high damp environment （air at 80% relative humidity and 80℃）, and their rising rates were much higher than those tested in atmosphere. The increase of substrate temperature and H2 flux can effectively improve the conductive stability of ZAO thin films in damp heat condition.Though analysis of the experiment result, the incorporated H not only in favor of the effective substitution of the Zn2+ sites by Al3+, but also can act as a shallow donor in thin films. The incorporated hydrogen atoms most likely situated in the Zn-O center with the BC∥configuration （bond center in a bond that is parallel to the c axis） at low temperature, which will diffuse out from the thin films during the annealing process to decrease carrier concentration of the films. With deposite temperature increasing, a fraction of Hi reacts with oxygen vacancies to form HO, which is relatively stable at high temperature.

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