The Role of Interfacial Microstructure of Perovskite Thin FIlms - A High Resolution and In Situ Study

Doktorsavhandling, 2009

Perovskite based materials have recently shown to have many unique properties that make them suitable for future electronic devices. In this thesis, the role of the interfacial microstructure on the properties of perovskite thin film heterostructures has been investigated. Thin films of SrRuO3, LaAlO3 and KTaO3 on SrTiO3 substrates have been studied using a number of experimental techniques with the focus on high resolution transmission electron microscopy (TEM). The techniques include high resolution scanning TEM, electron diffraction, x-ray diffraction, atomic force microscopy, Raman spectroscopy and in-situ electric transport measurements. The electrical and chemical properties of SrRuO3 make it a good candidate for electrode material in such devices. However, the properties of thin film SrRuO3 are dependent on the microstructure. An extensive investigation of the microstructure of SrRuO3 thin films grown on (001) SrTiO3 with pulsed laser deposition (PLD) at various growth temperatures is reported. It showed that the presence on nanoscale orientation twinning is related to non-misfit dislocations in the films. Furthermore, a difference in the crystallographic structure and dislocation density is linked to Ru deficiency. A new unique phenomenon has recently been reported for ultrathin films of LaAlO3 on (001) SrTiO3. Both materials are wide band gap insulators but show a conductive interface region. One possible explanation for this unexpected conductivity is that a charge transfer of electrons to the interface is present in order for polar discontinuities at the interface. According to this model, for an interface between KTaO3 and SrTiO3, a charge transfer of holes should be expected. The interfacial microstructure of thin films of LaAlO3 and KTaO3 on (001) SrTiO3 substrates has been investigated using high angle annular dark field (HAADF) STEM imaging. Evidence on material diffusion across the interface is shown and is related to the conductivity of the samples.To extract more information about the interfacial conductivity on these samples, we report on electric transport measurements performed with in situ TEM. This technique offers conductivity measurements to be performed with a lateral precision in the nanometer regime. Furthermore, a new method for sample preparation was developed for these measurements and is described in detail.