Atomic Elemental and Chemical Analysis of SrTiO 3 /LaMnO 3 Multilayers Using Fast Simultaneous EELS and EDS Analysis in DigitalMicrograph
Paolo Longo , 1 * Paul J Thomas , 1 Aziz Aitouchen , 1 Phil Rice , 2 Teya Topuria , 2 and
Ray D Twesten 1 1 Gatan Inc. , 5794 W Las Positas Blvd. , Pleasanton , CA , 94588 2 IBM Research Division , Almaden Research Center , San Jose CA , 95120
*
plongo@gatan.com Introduction
In an age of atomic-scale control of materials and interfaces, the need for high spatial resolution characterization of composition and local bonding is accelerating. Scanning transmission electron microscopy (STEM) based techniques continue to be the gold standard for such analysis at interfaces, and aberration correction of the STEM probe has extended this ability to atomic levels [ 1 , 2 ]. T e utility of this technique, however, is limited by the ability to detect the signals generated in the STEM. To confront this challenge, STEM-based electron energy loss spectrometers (EELS) and energy-dispersive (EDS) spectrometers have undergone improvements in effi ciency and speed, enabling atomic-scale composition maps using EELS [ 3 , 4 ] and in some cases EDS [ 4 , 5 , 6 , 7 ] for a range of materials. In addition, atomic-scale bonding maps using EELS fi ne structure are possible in certain selected systems. In this article, we show how jointly acquired EELS and EDS data can extend the capability of each technique over the case when data are acquired alone. We use a system that links the acquisition of the otherwise independent EELS, EDS, and probe positioning systems. T ese disparate types of data are acquired in exact synchrony and with the fi delity of their native applications [ 8 ]. In our results here we show the level of SrTiO 3 /LaMnO 3 interface data that can be captured with GIF Quantum atomic-scale mapping of compositional details over hundreds of unit cells at the interface. Zooming in at a particular imperfection at the interface, we have used the ability of EELS to detect changes in local bonding, as well as detection of interfacial diff usion at steps and the associated change in the bonding state of the metal atoms in the perovskite lattice.
Materials and Methods Instrumentation . Data for this article were acquired at IBM, San Jose, CA, using a probe-corrected 200 kV STEM equipped with a cold FEG high-brightness source (JEM-ARM200F), including a large solid-angle SDD-EDS system. EELS data were acquired with a GIF Quantum® ER system, and spectrum imaging acquisition was with the high-speed STEMPack® for EELS and EDS [ 9 ]. The GIF Quantum ER is configured with the following features: (a) DualEELS capability, which allows the nearly simultaneous acquisition of two different regions of the EELS spectrum [ 3 , 9 , 10 , 11 ]; (b) a low-dispersion mode that allows the acquisition of EELS data with an energy range up to 2,000 eV, which can be extended up to 4,000 eV when the data are
44
acquired in DualEELS mode [ 9 ]; (c) a fast 2k×2k CCD, which allows the acquisition of EELS spectra at over 1,000 spectra per second [ 9 ]; and (d) a dodecapole-based lens system capable of correcting spectral aberrations up to the 5th order, allowing collection angles over 100 mrad and yet maintaining sub-eV energy resolution [ 9 ]. Test Specimen . T e sample analyzed was a SrTiO 3 (STO)/
LaMnO 3 (LMO) bilayer structure grown on an STO substrate. T e bilayer was capped with STO, creating a buried STO/ LMO/STO/LMO structure. T e sample was prepared for TEM by conventional cross-sectioning, which involves cutting, grinding, dimpling, and Ar ion milling using a Gatan PIPS. T is procedure ensures high-quality, relatively thin TEM specimens, which are free of the amorphous layer present in samples prepared using FIB as a result of the interaction of the Ga ion beam with the sample. T is amorphous layer, even at the 1–2 nm level, oſt en limits the quality of atomic-level analysis unless the lamella is re-polished with a lower-damage technique beforehand [ 12 ].
Acquisition Parameters . Data shown here were acquired using a STEM probe of about 200 pA measured using the pico-ammeter built in the driſt tube in the GIF Quantum [ 9 ]. A probe convergence angle of 28 mrad and a collection angle of approximately 100 mrad were employed. Using a large collection angle for the EELS experiment provides a larger signal and also has some advantages in compositional mapping [ 13 ]. T e collection solid angle of the detector used to acquire the EDS data was reported to be 0.98 sr, but this was not independently measured in the present experiments. T e sample was tilted to the [001] zone axis, which caused the plane of the specimen to be tilted about 1° toward the EDS detector. T ree diff erent experiments were carried out with this setup: large-fi eld-of-view mapping, high-speed simultaneous EELS/ EDS acquisition, and detection of atomic-level bonding eff ects at an interphase interface. T ese are described below. For the fast mapping and compositional analysis experiments, the spectrometer was set up with an energy dispersion of 1 eV/ channel. Such low dispersion has the advantage of giving a large fi eld of view (2,000 eV) in the EELS spectrum and also the maximum amount of intensity in the CCD. T e main disadvantage is the poor energy resolution that can be 3 eV at the best.
Results
Large fi eld-of-view mapping at atomic resolution . Figure 1 shows a large fi eld-of-view annular dark fi eld (ADF)
doi: 10.1017/S1551929515000589
www.microscopy-today.com • 2015 July
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84