M.Sc Thesis


M.Sc StudentIrina Dumchin
SubjectGrowth and Characterization of Nb Doped SrTiO3 Epilayers
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Rothschild Avner


Abstract

Strontium titanate is widely used as a model system for solid state electrochemical systems and oxide electronic devices because it can be doped by donors (e.g., Nb) or acceptors (e.g., Fe) in order to tailor the electrical conductivity n-type or p-type, respectively. Epitaxial growth of high quality coherent films with precisely controlled compositions is an essential step for further investigation of their functional properties towards possible device applications.

In this work we investigate epitaxial growth of Nb-doped SrTiO3 thin films by Pulsed Laser Deposition (PLD). The films were deposited from a 2 site% Nb-doped SrTiO3 ceramic target on SrTiO3 (001) single crystal substrates using a PVD Products PLD/MBE-2100 system equipped with KrF excimer laser (λ = 248 nm). The laser projection optics was aligned to achieve a uniform footprint scanning across the target, with controlled energy density (fluence) between 0.83 and 0.95 J cm-2. The effect of deposition conditions such as gas (oxygen) pressure, laser fluence, specimen position and laser scanning geometry on the growth mode and crystalline quality of the films was examined using HRXRD, RSM, HRTEM/STEM and AFM.

AFM examination of films that were grown under suitable conditions for epitaxial 2D growth displayed smooth surface with stepped terraces similar to the substrate. HRXRD and RSM examinations of these films revealed fully strained coherent films of high crystalline order. The lattice parameter was found to expand with reducing gas pressure during deposition in the range 1-60 mTorr, indicating non-stoichiometric cationic ratio in the epilayers. Films grown in high vacuum (0.003 mTorr) or in high oxygen pressure (60 mTorr) displayed strain relaxation by forming mosaic structure with small angle twist boundaries. We observed no difference in the microstructure of vacuum-grown films as a function of the target-to-substrate distance in the range of 60-75 mm; all films were of high crystalline order. But since RSM gave indication of non-stoichiometric films, fine tuning of the laser fluence was essential in order to obtain stoichiometric films. Deposition in high oxygen pressure (60 mTorr) yielded films of high crystalline quality at large (75 mm) target-to-substrate distance. At lower distance the films display 3D growth with columnar grains. HRXRD and RSM measurements revealed that stoichiometric growth which was very stable with no observable effect of the substrate-to-target distance (in the range of 60-75 mm) and small variations in the laser fluence during film deposition on the lattice parameter of the films.