Optimization of growth parameters of TiO2 thin films using a slow positron beam

TiO2 thin films grown on fused silica were investigated using positron Doppler broadening spectroscopy at the slow-positron-beam SPONSOR [1] at the Helmholtz-Zentrum Dresden-Rossendorf. Effects of changes in different parameters like temperature or oxygen flow during film deposition on positron sensitive parameters have been investigated and first results will be presented.


Introduction
The ferromagnetic properties of transition metal doped TiO 2 are intensely investigated in recent years. Such dilute magnetic semiconductors (DMS) have potential applications in spintronics, i.e. due to spin-polarization of mobile charge carriers, or in magneto-optics. Ferromagnetic transition metal doped TiO 2 films can be created in different ways, i.e. from thin film growth or ion implantation. Until now there is an intense debate about the origin of the ferromagnetic properties and whether they are related to highly mobile spin-polarized electron currents or not. Consequently, highly sensitive structural investigations are applied which revealed that ferromagnetic properties may arise from either secondary phases [1], Co enriched anatase clusters [2] or intrinsic defects [3,4]. Moreover, structural point defects such as vacancies can influence the ferromagnetic interaction between the transition metal dopants [5]. It was also found that Co doped TiO 2 can be ferromagnetic and insulating at the same time which can not be explained by the usual ferromagnetic double exchange model describing the coupling in DMS [6]. The major problem, however, is the occurrence of metallic Co secondary phases, especially in ion-implanted TiO 2 [7], which have the potential to mimic all of these effects and thus camouflage an intrinsic ferromagnetic coupling.
In future we will study the origin of ferromagnetic properties of V, Co or Mn doped anatase TiO 2 thin films. The effects of oxygen pressure and substrate temperature on the (defect) structure of the grown films are of importance concerning the film quality (for further treatments and investigations) and can be easily investigated by depth-resolving positron Doppler broadening spectroscopy.

The SPONSOR setup
The Slow Positron System of Rossendorf (SPONSOR) [8] was used for depth-resolving Doppler broadening measurements. There positrons were implanted with energies between 27 eV… 36 keV resulting in an implantation depth of several micrometers (the planned film thickness is about 300 nm). The energy resolution at the annihilation line is (1.09 ± 0.01) keV.

Choice of the substrate material
The substrate material can have a strong influence on the film structure: TiO 2 films are either polycrystalline, if deposited on fused silica (a), or epitaxial, if deposited on SrTiO 3 (100) (b). From (a) it is evident that the TiO 2 structure can be adjusted to be either rutile or anatase depending on growth temperature. The largest electric conductivities have been achieved for TiO 2 anatase films either undoped or doped with Nb. Besides SrTiO 3 , LaAlO 3 may also serve as a possible substrate material. Depth-profiles obtained by Doppler broadening spectroscopy at SPONSOR for as-received material (figure 1) indicate that SrTiO 3 is the better candidate: the diffusion length L + (obtained by the VEPFIT package [9]) is more than double the value of that for LaAlO 3 .

Figure 1.
Depth profiles of substrate candidates for TiO 2 films. The diffusion length L + was calculated using the VEPFIT package [9].
Due to these investigations TiO 2 will be grown on SrTiO 3 for further investigations of ferromagnetic properties.

Effect of substrate temperature during film growth on the structure of TiO 2
Thin films of TiO 2 were grown by sputter deposition. The substrate temperature influences structure and defect situation of deposited films and influences defect induced room-temperature ferromagnetism. To investigate a temperature effect we used a series of thin TiO 2 films grown on fused silica where the films were deposited at constant partial oxygen pressure. The related profiles are shown in figure 2 also containing the profile of the substrate of fused silica (f-SiO 2 ). One can see the effect of the substrate on the S parameter but the layer is not affected by this meaning that the films are not too thin. Again the diffusion length L + and the S parameter of theTiO 2 film layer were calculated ( figure 3). Surprisingly there is a non-monotonous behaviour of the S parameters with a maximum for a substrate temperature of around 400 °C. The small layer thickness leads to larger error bars for L + complicating the interpretation of results.  TiO 2 will be grown at 400 °C due to the optimum of the diffusion length for this temperature.

Influence of oxygen partial pressure during film growth on the structure of TiO 2
The origin of the ferromagnetic properties was explained by molecular orbitals formed from the valence electrons on the three Ti ions surrounding the oxygen vacancy and couple via direct exchange interaction. A direct relationship between the oxygen partial pressure during growth, the electronic conductivity as well as the saturation magnetic moment of anatase TiO2 films was established [3]. We used positron Doppler broadening spectroscopy to investigate the influence of oxygen partial pressure on film structure on a nano-scale. This investigated series consists also of TiO 2 grown on fused silica at a temperature of 400 °C at different oxygen partial pressures. The profiles (figure 4) as well as the extracted S parameters and diffusion lengths L + ( figure 5) show a clear dependence on oxygen partial pressure. A simultaneous increase of S and L + indicates that with increasing pressure smaller defects begin to agglomerate, leading to larger values for S and L + . The jump in L + (within the error bars) indicates a sensitive pressure range of around 4•10 -4 mbar.

Conclusions and Outlook
First pre-investigations for the investigation of thin TiO 2 films deposited on fused silica were done by using positron Doppler broadening spectroscopy. The aim was to improve the conditions during the film growth and to decide for a suitable substrate material within the framework of investigating the ferromagnetic properties of V, Co or Mn doped anatase TiO 2 thin films. SrTiO 3 was chosen as substrate material for the future film deposition. It also turned out that variations in substrate temperature and oxygen partial pressure have great influence on the defect structure of the films. Optimal parameters were found as following: a) the substrate temperature should be in the range of 400 °C b) at an oxygen partial pressure of around 4•10 -4 mbar present defects in the layer start to agglomerate Next step will be the film deposition on SrTiO 3 at different substrate temperatures and oxygen partial pressures to investigate the effects also for this substrate material.