An investigation into controlling the growth modes of ferroelectric thin films using pulsed laser deposition and RHEED

Abstract

Thin film ferroelectricss are widely considered for tunable microwave applications, the reduced small dimensions leading to low tuning voltages. The incipient ferroelectric strontium titanate is an ideal solution for tunable microwave devices, particularly in conjunction with high temperature superconductors. It has no spontaneous polarisation yet possesses a large permittivity at low temperatures that is sensitive to an electric field bias with relatively low loss. For such applications it is essential to use a low loss substrate such as magnesium oxide. In general, thin films have less favourable dielectric properties compared with their bulk counterparts due to differences in microstructure. strontium titanate films on MgO substrate prove difficult to grow due to the high lattice mismatch and issues connected with chemical incompatibility at the film/substrate interface. It has been shown here that it is possible to engineer the growth mode of this system, altering the strain and the defect concentration. These are both known factors influencing the dielectric properties of thin films. Reflection high energy electron diffraction (RHEED) in combination with interval pulsed laser deposition (PLD) has been used to achieve a two dimensional, layer-by-layer growth mode. Crucial to this was the deposition of a unit-cell thick titanium dioxide buffer layer on the surface, the deposition of which was also controlled by RHEED. The growth mode and microstructure of films grown using standard PLD with and without the buffer layer and films grown by interval PLD with and without the buffer layer have been compared by analysis of the RHEED data and transmission electron microscopy. This is the first time layer-by-layer growth has been achieved in this highly-mismatched epitaxial systems. The results point the way towards control of defects in oxide thin films from which microstructure-property relationships may be more clearly determined.