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Exploiting the material-specific current-voltage-pressure relationship of the reactive magnetron discharge for the growth of transparent conductive (Al,Ga):ZnO and Nb:TiO2

Cornelius, S.; Vinnichenko, M.; Möller, W.

Reactive magnetron sputtering is an attractive technique for the fabrication of transparent conductive oxide thin films, due to several advantages compared to other PVD methods. These include the scalability to large substrate areas and the use of cost efficient metallic alloy targets as well as
(pulsed) DC technology for plasma excitation. Besides these technological benefits reactive magnetron sputtering (MS) inherently offers great flexibility for the control of the film stoichiometry. In particular for transparent conductive oxide (TCO) materials, the precise control of the oxygen deficiency of the layers is of utmost importance to achieve the desired electrical and optical properties.
The present work demonstrates that the material-specific shape of the current-voltage-pressure relationship of the reactive magnetron discharge may be used to tailor the electrical and optical properties of transpar-ent conductors by controlling the metal to oxygen flux ratio towards the growing film. It is shown that two groups of metals with distinctly different reactive behavior in Ar/O2 magnetron plasmas exist. Consequently, the reactive process control must be adapted in a material-specific way in order to stabilize the reactive discharge in the transition mode. This enables both control of oxygen deficiency as well as high film growth rates. In contrast to conventional reactive MS operation schemes like optical plasma emission to oxygen flow feedback, in this work an alternative technique is explained which allows controlling the oxygen partial pressure without changing the oxygen gas flow. The effect is based on the interplay of changes in secondary electron emission and sputter yield at the (partially) oxidized sputter target surface in conjunction with the internal reactive gas gettering effect of the MS setup itself.
Model experiments for the reactive magnetron sputter deposition of transparent conductive (Al,Ga) doped ZnO and Nb doped TiO2 layers are presented in detail. The effect of oxygen deficiency induced by exploiting the current-voltage-pressure relationship on the electrical and optical film properties will be discussed. The results demonstrate that the proposed method of reactive MS control is suitable to prepare high quality transparent conductive oxide thin films. Crucial parameters for TCO performance like free electron mobility and dopant activation are found to be highly dependent on oxygen to metal flux ratio during growth.

Keywords: physical vapor deposition; reactive magnetron sputtering; transparent conductive oxide; ZnO; TiO2

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Publ.-Id: 21072