Redeposition and differential sputtering of La in transmission electron microscopy samples of LaAlO<inf>3</inf>/SrTiO<inf>3</inf> multilayers prepared by focused ion beam

E. Montoya, S. Bals, G. Van Tendeloo

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Carefully FIB prepared double-cross-sectional lamellae of a nanostructured LaAlO3/SrTiO3 multilayer show that it is possible to obtain a constant thickness inside the region of interest. Nevertheless, the presence of redeposition, as well as an enrichment of La in the amorphous layer created by Focused Ion Beam (FIB) is detected after a cleaning step with 5 kV Ga ions. The observed enrichment of La is consistent with the results of Monte Carlo simulations. The 5 kV Ga ions produce TEM specimens with a 'bottle-like' shaped thickness profile, exhibiting cleaned thin regions with a thickness of about 45 nm and lateral sizes of several micrometres (width) and about 150 nm (height). LaAlO3/SrTiO3 (LAO/STO) multilayers are important heterostructures because of their electronically coupled interfaces, which can be either high-mobility electron conductors or insulating, depending on the atomic stacking sequences (Ohtomo & Hwang, 2004, Huijben et al., 2006). The determination of the atomic stacking sequences is performed by advanced transmission electron microscopy; in the transmission mode High Resolution Transmission Electron Microscopy (HRTEM) as well as in the scanning transmission mode High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM). Electron energy loss spectroscopy and energy filtered microscopy (EFTEM) are used to provide extra information on the cation valence at the interface and the eventual oxygen non-stoichiometry (Muller et al., 2003; Nakagawa et al., 2006). For all these techniques the quality of the TEM sample preparation is extremely important, particularly for quantitative work where a constant thickness is preferred. This constant thickness is difficult to obtain using conventional preparation techniques such as electro-polishing or broad ion milling. The FIB preparation of high-quality TEM specimens of ceramic multilayers has been described elsewhere (Bals et al., 2007; Montoya et al., 2007), and it had been shown that the use of FIB produces TEM lamellae useful for HRTEM and HAADF-STEM up to atomic resolution. In the present work, we report the results of a double-cross-sectional study of a FIB prepared TEM specimen. The use of double-cross-sectional lamellae ('cross-section of a cross-section') has been reported in the literature (Boxtleitner et al., 2001; Rubanov & Munroe, 2004; Langford, 2006; Montoya et al., 2007). The purpose of our study is to gain information about the thickness profile of the specimen and to investigate the presence of redeposition and other possible preparation artefacts. The LaAlO3/SrTiO3 multilayers are grown by pulsed laser deposition on a (100)-oriented single crystal of SrTiO3 (Huijben et al., 2006). A partial overview of the cross-sectional structure of the studied system is illustrated in Fig. 1. Fifteen layers of LAO and STO have been observed, away from the substrate. The layers have a different thickness, ranging from one or two unit cells (see arrow in Fig. 1) up to approximately 14 unit cells. © 2008 EMAT - University of Antwerp.
Original languageAmerican English
Pages (from-to)359-363
Number of pages5
JournalJournal of Microscopy
StatePublished - 1 Sep 2008
Externally publishedYes

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