Defect chemical Investigations of the all-oxide SrRuO$_3$|SrTiO$_3$ heterostructure

Henning Schraknepper, M.Sc. The behaviour of point defects in thin, epitaxial films of the oxide electrode SrRuO3 was probed by means of diffusion measurements. Thin-film SrRuO3 was deposited by means of pulsed laser deposition (PLD) on (100) oriented, undoped single crystal SrTiO3 substrates. 16O/18O exchange anneals were employed to probe the behavior of oxygen vacancies. Anneals were performed in the temperature range 850 ≤ T/K ≤ 1100 at an oxygen partial pressure of pO2 = 500 mbar. Samples were subsequently analyzed by means of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The investigation yields that the measured oxygen isotope penetration profiles as well as the observed diffusion of Ti from the SrTiO3 substrates into the SrRuO3 thin films comprised, surprisingly, two features. A model is proposed — cation sublattice equilibration — that accounts for the appearance of two features in both anion and cation diffusion profiles. Further it is suggested that the observed complex behavior arises from the metastable defect structure of PLD thin films and the unusual defect structure of SrRuO3. To further investigate the point-defect chemistry in SrRuO3, the metastable state of the thin film’s defect structure was deliberately varied. This variation was achieved by depositing SrRuO3 films at different frequencies, ranging from f = 2 Hz–18 Hz. Our results reveal that the deposition kinetics is decisively determining the film stoichiometry and the deposition frequency is the key parameter to vary the Sr:Ru ratio. Here the choice of a low deposition frequency results in a suppression of adatom interaction and thus a Sr:Ru ratio of one. The influence of Sr:Ru ratio on several physical properties is discussed. The measured tracer diffusion coefficients show a clear dependence with the Sr:Ru ratio, whereas the corresponding activation-enthalpies neither exhibit a dependence on the cation stoichiometry nor on the strain-state of the film. Conductivity measurements in the temperature range 2 ≤ T/K ≤ 300 on the deposition frequency and thus the Sr:Ru ratio. Further investigation of the conductivity data revealed a decrease in Curie-Temperature and an increase in disorder parameters such as Tmin with increasing Ru deficiency. In the here measured tracer diffusion profiles as well as in comparable studies two or sometimes three features can be discerned. In these cases the intriguing questions are: How are the point defects that mediate the diffusion process are distributed and is point defect migration uniform or do fast-diffusion pathways exist? To answer these questions, tracer diffusion profiles for two different cases — namely diffusion through a space-charge layer and fast diffusion along a grain-boundary with coupled bulk diffusion — were simulated and quantitative criteria to distinguish between these two cases were derived. In this context the impact of space-charge layers around dislocation loops was also investigated. Application of the theoretical considerations to experimental data revealed that it is these space-charge layers around dislocation loops depleted of oxygen vacancies that have a blocking effect on oxygen tracer diffusion in the near surface area of SrTiO3 single crystals. By applying the gained knowledge about the diffusion behaviour in SrRuO3 as well as in SrTiO3, interface potentials can be determined by interpreting oxygen tracer diffusion profiles measured on SrRuO3|SrTiO3 heterostructure samples. The derived interface potentials are in the order of φi ≈ 0.25 V. A temperature dependent analysis in the range of of the obtained data yields a temperature independent Schottky barrier height of ΦpSB = 1 eV as a characteristic quantity describing the SrRuO3|SrTiO3 interface. This behaviour suggests that the Fermi level Ef at the interface is pinned relative to the valence band edge Ev. At low temperatures, however, the situation is different. Here interface states are of negligible importance and the Schottky barrier height is stipulated by by the difference in work functions of SrRuO3 and electron affinity of SrTiO3.