Abstract
We report about an in situ study of crystalline structural changes during thermal treatment of a Ba0.5Sr0.5TiO3 (BSTO) film grown on MgO. The study covers the complete cycle of heating, annealing and cooling and reveals simultaneous phenomena of phase transitions and strain evolution, which have been characterized by in situ 2D reciprocal space mapping (2D-RSM) using high-resolution synchrotron x-ray diffraction in coplanar and grazing incidence geometries. In this way, temperature induced phase transformation from the BSTO2 to the BSTO1 phase has been monitored and the appearance of a further crystalline phase was detected. Moreover, for both BSTO phases, transitions between in-plane compressive and tensile states have been determined during thermal treatment. Furthermore, a contraction of the out-of-plane lattice components has been observed during the annealing phase while the in-plane lattice components remain leading to the change of the residual in-plane strain towards tensile state. The in situ 2D-RSM findings provide valuable and versatile insights into strain engineering and structure modification upon thermal treatment.
Highlights
The dielectric properties depend strongly on the microstructure which can be influenced by the growth conditions and by the possible post-growth processing
We report on the post-growth thermal treatment in oxygen background with a pressure of 450 mTorr to a Ba0.5Sr0.5TiO3 BSTO film of about 760 nm thickness grown on [001] oriented MgO substrates by pulsed laser deposition
We studied the changes in the crystalline structure by in situ by XRD during the treatment which was characterized by a heating rate of 150 °C/min, an annealing phase of 180 min at T = 900 °C higher than the growth temperature Tg = 850 °C and a cooling rate of 150 °C/min
Summary
The dielectric properties depend strongly on the microstructure which can be influenced by the growth conditions and by the possible post-growth processing. Knauss et al. have demonstrated that the dielectric losses in the annealed films are lower than in the as-deposited films in the high temperature range and the annealed film had greater tunability with reduced loss This has been achieved through different ex situ investigations carried out on grown films in the as deposited state and in the ex situ post-annealed state in terms of microwave dielectric properties. By measuring the XRD pattern, they found out that the annealing of BSTO film at Tan = 750 °C at a temperature higher than that of the substrate (Ts = 700 °C) may introduce changes in the chemical and phase composition They characterise the dielectric properties of the films annealed at Tan below Ts and they did not detect any variation in the capacitance and the tunability. The highest dielectric constant and dielectric tunability was obtained in a film with a slight tensile strain where the distortion ratio comprises between 0.9992 and 1.0002316
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