PNNL

Abstract

We have investigated the heteroepitaxial growth and materials properties of pure and Co-doped TiO2 anatase on SrTiO3(001) and LaAlO3(001), grown by oxygen plasma assisted molecular beam epitaxy. This material is a promising new diluted magnetic semiconductor that shows large magnetization and a Curie temperature well above room temperature. We have found that epitaxial films with the highest crystalline quality and most uniform distribution of Co result when a rather slow growth rate (~0.01 nm/sec) is used over a substrate temperature range of 550 degrees Celcius to 600 degrees celcius. These conditions result in layer-by-layer growth of single-crystal films, and a very low density of extremely small nanocrystalline inclusions. In contrast, growth at a higher rate (~0.04 nm/sec) leads to extensive formation of secondary phase rutile nanocrystals to which Co diffuses and segregates. The rutile nanocrystals nucleate on the evolving anatase film surface in such a way that lattice strain between the two phases is minimized. Co appears to substitute for Ti in the lattice and exhibits a ?² formal oxidation state. Both pure and Co-doped films are typically n-type semiconductors despite the lack of intentional n-type doping, although a wide range of conductivities is observed.