REPOSITORY > RESULTS

Abstract

SrTiO3 (STO) thin films are used as buffer layers that enable epitaxial integration of various functional oxides with silicon. When pulsed laser deposition (PLD) is used to grow STO on Si, a thin amorphous layer is formed at the STO-Si interface that limits the use of such heterostructures in certain applications. This thesis deals with the structural and chemical properties of surfaces, interfaces, and films formed during a multi-stage procedure used for the growth of ultra-thin STO films on Si(001) surfaces using the PLD technique, in order to understand the mechanisms that lead to epitaxial integration with an atomically abrupt interface. In this procedure, the deoxidation of Si substrates is followed by a deposition of ½ monolayer (ML) of Sr, which is used to passivate the Si surface, and the STO film growth, where the deposition, oxidation, and crystallization steps are carried out cyclically until a thickness of ~ 10 ML is achieved.
In the first part, the quality of deoxidized Si(001)(2×1) surfaces was inspected and it was confirmed that the contamination of the surface due to exposure to ~1×10-8 mbar of residual atmosphere for 5 min (which is difficult to avoid in PLD systems) is low enough to preserve a crystalline surface until it is passivated with Sr.
In the second part, PLD was used to prepare Sr/Si(001)(1×2) surfaces, followed by structural and chemical analysis performed without breaking vacuum. Scanning tunneling microscopy (STM) images showed an atomically ordered surface with terraces composed of corrugated one-dimensional chains running perpendicularly on neighboring terraces forming a double domain (2×1)+(1×2) surface reconstruction revealing a surface unit cell with 0.78 nm × 0.39 nm dimensions along with different types of surface defects. The interpretation of experimental STM images is supported by simulated images based on density functional theory.
In the final part, PLD was used to grow epitaxial ultra-thin (3–4 nm) STO films on Sr-Si surfaces. The optimization of STO deposition, oxidation, and crystallization parameters led to a significant improvement of the crystalline quality of the STO films and reduction of the interlayer thickness. It has been found that the minimization of the thermal budget during crystallization increases the interface sharpness, however, it can limit the densification of the STO film. The STO films prepared by the optimised procedure are single-phase and exhibit STO(001)||Si(001) out-of-plane and STO[110]||Si[100] in-plane orientation. Most of the STO film has a cubic unit cell 2-5% larger compared to bulk STO which is related to a slight non-stoichiometry of the film. A large part of the STO film is relaxed due to an amorphous silicate layer present at the STO-Si interface which starts forming during the deposition of the first 2 ML of STO and expands by every STO deposition-oxidation-crystallization cycle reaching a thickness of 1.2–1.9 nm after deposition of ~10 ML of STO.
Although the amorphization of the STO-Si interface cannot be avoided completely, ultra-thin STO films on Si produced by PLD can still be used as an excellent template for the growth of thicker STO films and the epitaxial integration of various functional oxides with Si.