Interplay of phase formation, oxygen deficiency, dopant activation and charge transport in Nb doped TiO2 prepared by reactive pulsed magnetron sputtering

Intense research on TiO2 as a transparent conductor material was triggered less than ten years ago by a series of breakthrough publications demonstrating a combination of resistivity below
5x10-4 Ωcm and average visible transmittance above 80% for Nb doped anatase phase TiO2 thin films [1]. Scientific as well as commercial interest in transparent conducting TiO2 is further driven by the prospective additional functionality and low production costs due to the exceptionally high refractive index, the chemical inertness and the very high abundance of TiO2 compared to the conventional transparent conductive oxides (TCOs). Early experiments were focused on pulsed laser deposition (PLD) on single crystalline substrates resulting in record free electron mobilities of up to ~25 cm²/Vs in epitaxial anatase layers [1,2]. However, neither the PLD technique nor epitaxial substrates are suitable for typical large area TCO applications. Consequently, the investigations were extended to magnetron sputter (MS) deposition and low-cost glass substrates. Here, the main focus was put on radio-frequency MS [3] and more recently on direct current MS [4] using (electrically conductive) oxygen deficient ceramic sputter targets. After initial reports on poor electron mobilities of ~1cm²/Vs in sputtered TiO2 films directly grown onto heated amorphous (glass) sub-strates, it was soon realized that controlling the crystallization from the amorphous state, the suppression of the rutile phase formation and the extrinsic doping level in conjunction with the oxygen deficiency are crucial to obtain high quality TiO2 based TCO layers. However, there are several drawbacks associated with sputtering from ceramic targets such as limited variability of the oxygen deficiency, low growth rates and higher material costs compared to metallic targets.
Therefore, this contribution is aimed at understanding the influence of the oxygen deficiency and the phase composition on the electrical and optical film properties of Nb:TiO2 prepared by pulsed direct current MS of TiNb alloy targets in Ar/O2 atmosphere. For this purpose, three routes for film preparation, including direct growth at elevated substrate temperatures, post deposition annealing of amorphous layers and epitaxial growth on SrTiO3(100), are investigated. A non-conventional process stabilization method based on the material specific current-voltage-pressure characteristics of the reactive MS discharge is employed to obtain high growth rates as well as precisely tunable oxygen partial pressures in the so-called transition mode. This approach enables the investigation of the effect of the oxygen deficiency on the crystallization of amorphous films during annealing, the Nb dopant activation (Rutherford Backscattering), the phase composition (X-ray diffraction), the charge transport (Hall Effect) and the optical properties of the Nb:TiO2 films. Using spectroscopic ellipsometry together with spectral photometry, an optical model of the dielectric function of Nb:TiO2 with different crystalline structure and conductivity is established. Moreover, charge transport in degenerately doped anatase TiO2 films will be discussed in the framework of a unified charge transport model including optical phonon scattering, ionized impurity scattering and grain boundary effects. Implications for the maximum achievable electron mobility in polycrystalline anatase TiO2 films are derived from a comparison of transport data of epitaxial and polycrystalline thin films, revealing the crucial role of the highly anisotropic effective electron mass.