Karsten Gömann

Sr diffusion in undoped and La-doped SrTiO3 single crystals under oxidizing conditions

Strontium titanate SrTiO3(100), (110), and (111) single crystals, undoped or donor doped with up to 1 at% La, were isothermally equilibrated at temperatures between 1523 and 1773 K in synthetic air followed by two different methods of Sr tracer deposition: ion implantation of 87Sr and chemical solution deposition of a thin 86SrTiO3 layer. Subsequently, the samples were diffusion annealed under the same conditions as before. The initial and final depth profiles were measured by SIMS. For strong La-doping both tracer deposition methods yield similar Sr diffusion coefficients, whereas for weak doping the tracer seems to be immobile in the case of ion implantation. The Sr diffusivity does not depend on the crystal orientation, but shows strong dependency on the dopant concentration supporting the defect chemical model that under oxidizing conditions the donor is compensated by Sr vacancies. A comparison with literature data on Sr vacancy, Ti, and La diffusion in this system confirms the concept that all cations move via Sr vacancies. Cation diffusion is several orders of magnitude slower than oxygen diffusion.

Cation Transport and Surface Reconstruction in Lanthanum Doped Strontium Titanate at High Temperatures

Tracer diffusion experiments were carried out in synthetic air at 1573 K in SrTiO 3 (100) and (110) single crystals, which were either undoped or doped with up to 1 at.% La, respectively. Tracer sources of 139 La and 142 Nd were applied by ion implantation. The resulting depth profiles were measured by SIMS. The reconstruction of the surface was monitored ex-situ using microscopic and spectroscopic methods including SEM, EPMA, and AFM. The measured tracer diffusivities show no dependency on orientation. The tracer diffusion takes place via cation vacancies. Under oxidizing conditions the dopant is compensated by Sr vacancies. Hence the diffusion is increasing strongly with La concentration. The observed time dependency of the diffusivities may be related to a space charge layer postulated by the current defect chemistry model for donor doped SrTiO 3 . At high dopant concentrations annealing leads to segregation of bulk La to the surface. La is not significantly incorporated into the secondary crystallites at the surface which consist almost entirely of Sr and O.