Epitaxial growth of Nb-doped SrTiO ₃ films by pulsed laser deposition

Nb-doped SrTiO ₃ thin films are potential candidates for transparent conducting oxide electrodes and other applications in oxide electronics and optoelectronics. In this work we grew epitaxial films of Nb-doped SrTiO ₃ on (0 0 1)-oriented SrTiO ₃ single crystal substrates and examined their crystal structure, defects and strain state by means of high-resolution X-ray diffraction (mapping in reciprocal space) and complementary methods. The films were deposited by pulsed laser deposition (PLD) under oxygen flow (60 mTorr) or in vacuum (5 × 10 ^-3 mTorr). The substrate temperature was 700°C and the substrate-to-target distance was 55 or 70 mm, respectively. Strained epitaxial films with homogeneous concentration of point defects and dopants were obtained in both cases as long as the film thickness did not exceed 200 nm. Films deposited in oxygen displayed considerably higher concentration of point defects (presumably cation vacancies) than their vacuum-deposited counterparts. Films of intermediate thickness (between 200 and 1000 nm) exhibited significant structural inhomogeneity accompanied by strain relaxation via redistribution of point defects across the film. Thicker films (>1000 nm) grown in oxygen exhibited changes in the film growth mode and the resultant microstructure, providing a complementary mechanism for strain relaxation.