Publication No.6



A. Stashans, J. Ortiz.
Quantum-chemical modeling of the SrTiO3 (001) surface and point defects therein
- 5th Int. Conf. Comput. Phys., Kanazawa (Jaapan), 1999, p1-23.

Abstract:
Strontium titanate (SrTiO3) is one of the most widely used electronic ceramic materials. 
The surface of SrTiO3 is important because of its catalytic properties and application 
as substrates for thin film growth of oxide superconductors as well as dielectric materials. 
It is well recognized that the surface defects such as the oxygen vacancies play an important 
role in many processes.
    We have used modified for crystals INDO method and the periodic large unit cell (LUC) 
model in order to study (001) surface of SrTiO3 cubic and tetragonal phases as well as oxygen 
vacancies, F+ and F centers therein. A 40-, 80- and 160-atom two-dimensional LUCs (slabs 
consisting of four plains) were utilized in the computations. The electronic band structure 
of non-polar (001) surface differs from the one of the bulk crystal, in particular, we 
observed the reduction of the bandgap width and more considerable admixture of Ti 3d and 
4s states in the upper valence band. The band structure properties were obtained after 
careful geometry optimization procedure which showed (i) the inward relaxation of the surface 
planes by 0.06 � in the cubic phase and (ii) the inward relaxation of the plane Sr-O by 0.14 
� and the outward displacements of Ti atoms situated in the plane Ti-O by 0.1 � in the 
tetragonal phase.
    The vacancy was inserted and the atomic displacements were calculated within the surface 
planes. The four vacancy-nearest Sr atoms situated in the Sr-O plane moved outwards the 
vacancy by 0.12 � while the two vacancy-nearest Ti atoms situated in the Ti-O plane moved 
outwards the vacancy by 0.09 �. The outward atomic movements can be explained by the Coulomb 
repulsion force since oxygen vacancy is positively charged with respect to the perfect 
crystalline lattice. It was found that oxygen vacancy doping in the tetragonal Ti-O surface 
plane makes this surface highly unstable.
    F+ and F centers (one and two electrons trapped by the positive oxygen vacancy) have 
considerable influence upon the electronic band structure properties of this material. In 
particular, it was found that the F+ center is responsible for the occurence of the local 
one-electron energy level within the forbidden energy gap. This level is composed mainly 
of O2p states and is situated just above the upper valence band. The studies of F center 
shows even more interesting effect, i.e., the electron transfer from the local energy level 
to the conduction band. It has to be noted that it happens without any excitation; the 
system remains in the fundamental state.




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