Pulsed Laser Deposition Caught by Synchrotron X-rays
With the rapid development of nano-science and nano-technology, it is increasingly important to control crystal growth at the atomic level. Pulsed Laser Deposition (PLD) is a technique that allows the growth of thin films with a controlled thickness down to the nanometre scale and under well-defined conditions.
In PLD a powerful pulsed laser beam is focussed onto a target material, creating a plasma, which subsequently gets deposited onto a single crystalline substrate. Each laser pulse deposits only a fraction of a monolayer, which allows accurate control of the layer thickness. This PLD technique is particularly suited for growing layers of complex oxides, like high-Tc superconductors, because one can deposit in relatively high oxygen background pressure [1,2]. The fundamentals of this growth process are still badly understood, the main reason being that up to now people have used Reflection High Energy Electron Diffraction (RHEED) to study the growth in situ. Although RHEED is well suited for qualitative studies, obtaining quantitative information is practically impossible due to multiple scattering effects. The most direct way to obtain structural information of the growth process of the thin films is by in situ surface X-ray diffraction. For this a miniature PLD chamber has been constructed and tested.
In situ surface X-ray diffraction was performed during the growth of a thin layer of YBa2Cu3O7-x on a SrTiO3 single crystal substrate. Figure 1 shows the intensity oscillations of a specular reflection. The oscillations show a rich and complicated pattern where every maximum corresponds to the growth of one more monolayer. Figure 2 shows the Kiessig fringes of the 001 Bragg reflection after a 20 nm thin film had been deposited. The measurements were done at the deposition temperature of 780 degrees C. The figure shows the film to be extremely smooth, even at these high temperatures.
Fig. 1: Intensity oscillations of the (0 0 0.2) specular reflection as function of time during deposition.
Fig. 2: Kiessig fringes of the 001 Bragg reflection of a 20 nm thin film at 780°C.
V. Vonk (a), K. Driessen (a), B. Gorges (a), L. Barthe (a), M. Huijben (b), S. Harkema (b) and H. Graafsma (a)
(a) ESRF (France)
(b) Low Temperature Division, University of Twente (The Netherlands)