Sputtered tin oxide and titanium oxide thin films as alternative transparent conductive oxides

  • Gesputterte Zinnoxid- und Titanoxid-Dünnschichten als alternative transparent leitfähige Oxide

Boltz, Janika; Wuttig, Matthias (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2011, 2012)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2011

Abstract

Alternative transparent conductive oxides to tin doped indium oxide have been investigated. In this work, antimony doped tin oxide and niobium doped titanium oxide have been studied with the aim to prepare transparent and conductive films. Antimony doped tin oxide and niobium doped titanium oxide belong to different groups of oxides; tin oxide is a soft oxide, while titanium oxide is a hard oxide. Both oxides are isolating materials, in case the stoichiometry is SnO2 and TiO2. In order to achieve transparent and conductive films free carriers have to be generated by oxygen vacancies, by metal ions at interstitial positions in the crystal lattice or by cation doping with Sb or Nb, respectively. Antimony doped tin oxide and niobium doped titanium oxide films have been prepared by reactive direct current magnetron sputtering (dc MS) from metallic targets. The process parameters and the doping concentration in the films have been varied. The films have been electrically, optically and structurally analysed in order to analyse the influence of the process parameters and the doping concentration on the film properties. Post-deposition treatments of the films have been performed in order to improve the film properties. For the deposition of transparent and conductive tin oxide, the dominant parameter during the deposition is the oxygen content in the sputtering gas. The Sb incorporation as doping atoms has a minor influence on the electrical, optical and structural properties. Within a narrow oxygen content in the sputtering gas highly transparent and conductive tin oxide films have been prepared. In this study, the lowest resistivity in the as deposited state is 2.9 mOhm cm for undoped tin oxide without any post-deposition treatment. The minimum resistivity is related to a transition to crystalline films with the stoichiometry of SnO2. At higher oxygen content the films turn out to have a higher resistivity due to an oxygen excess. After post-deposition annealing the suitable range of the process parameters for the preparation of transparent and conductive oxides is enlarged, the resistivity strongly decreased upon annealing at 300°C. Best results are obtained for an undoped SnO2 film with a minimum resistivity of 1.15 mOhm cm after annealing at 300°C. At higher temperatures the resistivity increased due to stress and microstrain in the films. For the preparation of niobium doped titanium oxide as a transparent conductive oxide, the reducing atmosphere during the post-deposition annealing is necessary to obtain a low resistivity. Best results have been obtained by the deposition of amorphous films at higher sputtering pressure (1.5 Pa) that crystallise during the post-deposition annealing in the anatase TiO2 phase. After post-deposition annealing in vacuum the lowest resistivity obtained here was 671 mOhm cm. The Nb incorporation as doping atoms is important for the formation of free charge carriers; a higher Nb doping concentration leads to a higher conductivity.

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