The source oxygen incorporated during the growth of oxide thin films, by pulsed laser deposition (PLD), often comes in to question - is it target, background or substrate. This question is essential, as the optimization of the deposition conditions depend on it. 18O exchanged SrTiO3 and LaA1O3 were used as substrates, and a strong oxygen substrate-to-film transfer was demonstrated for epitaxially grown SrTiO3 and LaAlO3 thin films, by SIMS depth profiling. The thin film properties can be influenced through this oxygen transfer effect.
In recent years, significant developments have been made in the fabrication of high quality, complex oxides thin films, and heterostructures with atomic precision, providing a crystalline quality comparable with that of semiconductors and oxide semiconductors structures. Providing and controlling the amount of oxygen during growth of oxides, in order to gain the desired oxygen stoichiometry and appropriate physical properties, is where the challenge lies. When using pulsed laser deposition, it appears to be necessary to produce the correct quantity of plasma species within an oxygen background, to generate a thin film of the selected compound on a suitable substrate, and to provide the missing oxygen through a subsequent annealing process.
Dynamic secondary ion mass spectrometry (D-SIMS) provides elemental depth profiles. Using this process the oxygen diffusion properties of SrTiO3 and LaAlO3 thin films grown on 18O isotope exchanged SrTi18O3 and LaAl18O3 substrates were investigated to analyze the role of oxygen provided by the substrate during a deposition. SrTiO3 and LaAlO3 thin films were prepared at three separate deposition temperatures - 650°C, 750°C and nominal room temperature - at a background pressure p=1.5x10-5 mbar and a laser fluence F=4J cm-2.
A Hiden Analytical EQS quadrupole mass spectrometer, operated with a 2.5 keV Ar ion beam focused to 150 µm diameter rastering over a square of 1 x 1 mm with an effective sampling area of 500 x 500 µm, was employed to record SIMS spectra. The etched area measurement was carried out using a Dektak 8 profilometer to convert etching time into depth (Figure 1). Species with the same mass were separated through a kinetic energy selection scheme.
Figure 1. Schematic representation of the raster crater (left) and a real crater measured with a profilometer (right). The different colors correspond to different depth levels
The dependence of 18O diffusion from SrTi18O3 into SrTiO3 on the deposition temperature was observed (Figure 2a). However no traceable 18O diffusion into the film was evident at a room temperature deposition. For elevated deposition temperatures, the scenario is completely different. Significant 18O diffusion from the substrate into the film was observed at 650°C. No considerable difference was shown at TS=750°C, with respect to the amount of 18O measured in the film and substrate.
When SrTiO3 was grown on LaAl18O3, a significant and homogeneous oxygen contribution was measured for the SrTiO3 film prepared at TS=750°C (Figure. 2b). A significant 18O intake was observed even at 650°C, which can be detected up to the film surface. The elemental composition of film and substrate was found to be similar while measuring Sr and Ti species from the substrate and film along with 16O and 18O. This shows that the substrate supplies oxygen in the SrTiO3 thin film, whereas the oxygen supply by the target has a lower contribution in this system. It is also evident that the initial film formed has an oxygen deficiency, and the oxygen supply through the substrate is favored by a chemical gradient.
Figure 2. a) 18O SIMS depth profile of SrTiO3 on SrTi18O3 grown at Ts=750°C, 650°C and room temperature. The sharp drop of the 18O signal near the SrTiO3 surface for the film grown at TS=750°C could be related to a back-exchange of 16O at room temperature. b) 18O SIMS depth profile of SrTiO3 on LaAl 18O3 grown at Ts=750°C, 650°C and room temperature.
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
For more information on this source, please visit Hiden Analytical.