IMAGING DIFFRACTION SPECTROSCOPY

Transcription

1 TEM Techniques TEM/STEM IMAGING DIFFRACTION SPECTROSCOPY Amplitude contrast (diffracion contrast) Phase contrast (highresolution imaging) Selected area diffraction Energy dispersive X-ray spectroscopy Electron energy loss spectroscopy Electron holography Z-contrast imaging Convergent beam diffraction Micro-/ nanodiffraction X-ray mapping Energy-filtered TEM (EFTEM) Tomography

2 Phase contrast imaging (high resolution TEM: HRTEM)

3 Phase shift due to the inner potential of specimen Electron beam Phase shift: dz dz d 2 2 V x, y, ' z z with Total phase shift: E (interaction constant) d V x, y, z dz Vt x, y d! phase change depends on potential V which electrons see, as they pass through sample

4 T(u) E u u HRTEM: contrast transfer function point resolution f E u information limit sin χ(u) u, [nm -1 ]! opposite sign of T(u) - oposite contribution to contrast u < point resolution: images are directly interpretable u > point resolution: no direct interpretation is possible No simple correspondence between the image intensity and the atom column positions! Additional calculations are necessary! u C s 3 u 4 f - defocus - wave length C s - spherical aberration u - spatial frequency

5 Example: HRTEM simulation for GaAs projected potential same thickness, only defocus change by courtesy of Prof. Kerstin Volz

6 HRTEM of an isolated ZnTe nanowire - visualization of crystal structure - analysis of defects

7 HRTEM of an isolated ZnTe nanowire {211} {111} {110}

8 HRTEM of an isolated ZnTe nanowire {211} {111} {110} Twin formation

9 TEM Techniques TEM/STEM IMAGING DIFFRACTION SPECTROSCOPY Amplitude contrast (diffracion contrast) Phase contrast (highresolution imaging) Selected area diffraction Energy dispersive X-ray spectroscopy Electron energy loss spectroscopy Electron holography Z-contrast imaging Convergent beam diffraction Micro-/ nanodiffraction X-ray mapping Energy-filtered TEM (EFTEM) Tomography

10 High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF STEM) Z contrast imaging

11 Parallel incidence of converegent electron probe The electron beam must scan parallel to the optic axis at all times! Magnifications is controlled by scan dimensions on the specimen, not the imaging lenses of the TEM! Williams & Carter

12 HAADF STEM High Angle Annular Dark Field Scanning Transmission Electron Microscopy primary electrons Energy-Dispersive X-ray Spectrometer X-rays thin crystalline specimen diffracted beam direct beam elastically and inelastically scattered electrons High-Angle Annular Dark-Field Detector Electron Energy Loss Spectrometer

13 Z contrast technique y Electron probe x intensity Sample Z position y Z contrast image HAADF detector I ~ Z 3/2 r

14 Z contrast technique y Electron probe x intensity Sample Z position y Z contrast image ZnTe Au HAADF detector I ~ Z 3/2 r

15 Element Atomic number Z Ga 31 As 33 Sb 51 Material Mean Atomic number <Z> GaAs ½ ( ) = 32 GaAs 0.5 Sb 0.5 ½ {31+½(33+51)} =½ {31+42} =36.5 c S -corrected HAADF STEM of (In,Ga)As and Ga(Sb,As) layers embedded in GaAs Humboldt-Universität zu Berlin, Institut für Physik, AG Kristallographie Technische Universität Berlin, Institut für Physik

16 c S -corrected HAADF STEM of (In,Ga)As and Ga(Sb,As) layers embedded in GaAs Humboldt-Universität zu Berlin, Institut für Physik, AG Kristallographie Technische Universität Berlin, Institut für Physik

17 TEM Techniques TEM/STEM IMAGING DIFFRACTION SPECTROSCOPY Amplitude contrast (diffracion contrast) Phase contrast (highresolution imaging) Selected area diffraction Energy dispersive X-ray spectroscopy Electron energy loss spectroscopy Electron holography Z-contrast imaging Convergent beam diffraction Micro-/ nanodiffraction X-ray mapping Energy-filtered TEM (EFTEM) Tomography

18 Selected area diffraction

19 Beam path in image mode object plane (specimen) d 1 f objective lens back focal plane (Brennebene) d 2 image plane (Gaussian image plane) intermediate lens viewing screen

20 Selected area electron diffraction (SAD) using a parallel beam Crystal Structure: amorphous polycrystalline single crystalline Epitaxial Orientation Relations: selected area aperture: d min = nm (100)[001]LiAlO 2 (0001)[11.0]GaN

21 Bragg`s law Bragg 1913 description of diffraction by reflection constructive interference: n: reflection order (integer number) : diffraction angle (Bragg angle) d: interplanar spacing : wave length 2 d sin n At the Bragg angle the electron waves interfere constructively

22 Camera length d: distance of (hkl) reflecting planes r: distance of diffraction spots L: camera length r L tan 2 r 2 L tan 2 2 r L tan 2 2 r d 2sin 2 L d r L tan 2 2

23 Camera length L needs to be calibrated using a known material! [001]Si + a polycrystalline unknown phase d hkl = n L/r 400 Si r Si r phase d 100 (Si) = a = nm n = 4 L calibr = r Si d 100 (Si) / n d phase = L calibr /r phase is compared to the d-values of possible phases

24 TEM Techniques TEM/STEM IMAGING DIFFRACTION SPECTROSCOPY Amplitude contrast (diffracion contrast) Phase contrast (highresolution imaging) Selected area diffraction Energy dispersive X-ray spectroscopy Electron energy loss spectroscopy Electron holography Z-contrast imaging Convergent beam diffraction Micro-/ nanodiffraction X-ray mapping Energy-filtered TEM (EFTEM) Tomography

25 Energy-dispersive X-ray spectroscopy (EDXS)

26 EDXS Energy Dispersive X-ray Spectroscopy primary electrons Energy-Dispersive X-ray Spectrometer X-rays thin crystalline specimen diffracted beam direct beam elastically and inelastically scattered electrons High-Angle Annular Dark-Field Detector Electron Energy Loss Spectrometer

27 Fundamental interaction processes conduction band valence band E E Vac E F Auger electron free electron electron excited into an unoccupied state primary electron L 3 L 2 L 1 characteristic X-rays K energy loss electron

28 EDXS Instrumentation: Silicon Drift Detectors (SDD) Set-up and working principle of a state-of-the-art EDX detector Parameters: Energy resolution: 129 ev (MnK) Semiconductor-based drift technology Peltier cooling (-25 C, no need of LN 2 ) BRUKER AXS

29 Scattering volume in thin specimens Bulk material electron beam TEM specimen 50 nm 1 nm 50 µm 200 nm Monte-Carlo Simulation of the paths of electrons through bulk silicon as used for scanning electron microscopy (SEM); acceleration voltage: 100 kv material: Si 12 nm Full lateral width at half resolution maximum

30 EDXS Quantitative analysis I EDXS spectrum of GaAs Preparation of spectrum for analysis: Removal of Escape peak which is due to detector material Modelling and subtraction of background Deconvolution of peaks basing on Gauss distribution functions Quantification of chemical composition

31 bulk specimen (SEM) - infinite specimen thickness t EDXS Quantitative analysis II t max < t << thin specimen (TEM) t < t max thin foil approximation ZAF-Method: takes into account Absorption (A), Fluorescence (F), atomic number (Z) C C A B ( Z ( Z A A F ) F ) A B I I A B Modified ZAF-Method - Specimen thickness t - Geometry of object - High primary electron beam energy t max = f (mass absorption coefficient, detector angle, mean sample density) C A C B k A k B I A I B Cliff-Lorimer factor (CLF) CLF have to be calibrated for each element (especially light one!) at the same specimen thickness

32 intensity composition position EDXS Experimental modes C A B 1. Point analysis spectrum 2. Line scan composition profile 3. Elemental map 2d elemental distr. A B A C B energy position position

33 Example A: III-V-based overgrown structures STEM HAADF image : Z-contrast Structure I AlGaAs InGaP AlGaAs line scan GaAs InGaP 3 nm AlGaAs 20 nm GaAs Question: segregation of P?

34 Intensity (a.u.) Example 3: III-V-based overgrown structures Structure I STEM probe size: 0.7 nm, spot distance: 0.5 nm dark region As depletion P enrichment In enrichment position (nm) HAADF (Al,Ga)As (In,Ga)P GaAs (Al,Ga)As

35 Example 3: III-V-based overgrown structures HAADF AlGaAs InGaP GaAs In As Elemental map: probe size 0.7nm InGaP III: Ga-In-Al V: P-As Ga P Al Structure I

36 Example B: III-V-based overgrown structures STEM HAADF image : Z-contrast Structure II AlGaAs AlGaAs InGaP InGaP GaAsP ~4 nm 100 nm InGaP GaAsP 3 nm InGaP Question: segregation of In?

37 Intensity (a.u.) 2: III-V-based overgrown structures Structure II STEM probe size: 0.7 nm, spot distance: 0.5 nm bright region As enrichment In depletion P depletion HAADF position (nm) InGaP Ga(In)As(P) AlGaAs

38 EDXS- Energy-Dispersive X-ray Spectroscopy Advantages: all elements are visible at once fast simple qualitative analysis elements down to Be probe sizes used: down to nm Difficulties in TEM: small exitation volume low peak intensity small detector collection angle specimen drift at high magnifications reduced accusition time calibration is necessary for quantitative analysis

39 EDXS mapping on the subnanometer scale HAADF (Z-contrast): EDXS map: 1.47 Å 3 ms per pixel, totally 13 s probe size of ~ 0.1 nm probe current of ~ 33 pa M.-W. Chu et al., National Taiwan University Phys. Rev. Lett. 104 (2010) JEOL-2100FS with a probe C s -corrector

40 Electron energy loss spectroscopy (EELS) + Eenergy Filtered Transmission Electron Microscopy (EFTEM)

41 Fundamental interaction processes conduction band valence band E E Vac E F Auger electron free electron electron excited into an unoccupied state primary electron L 3 L 2 L 1 characteristic X-rays K energy-loss electron

42 Experimental setup for EELS and EFTEM Energy dispersive plane with slit In-column Filter (e.g., LEO EM 922 Omega and JEOL JEM 2200 FS) Magnetic Prism Energy selecting slit Post-column Filter (GATAN Imaging Filter) for any TEM

43 Magnetic prism: a spectrometer and a lens Williams & Carter

44 Electron energy loss spectroscopy (EELS) unoccupied states valence band energy levels of inner shells 283 ev 99 ev L 3 L 2 L 1 K C neighboring atoms Si E F Si-L 23 edge intensity in counts x zero-loss peak plasmon excitation x 100 C-K edge energy loss in ev

45 Imaging of the element distribution in the interface region / phase by PEELS: A) series of spectra, B) STEM-BF image, C) concentration profiles. Humboldt-Universität zu Berlin, Institut für Physik, AG Kristallographie Hahn-Meitner-Institut Berlin

46 Energy-filtered TEM - Three-window technique phase phase 200 nm Pre-edge 1 image Pre-edge 2 image Post-edge image Cr-L 23 map Cr-L 23 edge Cr-L 23 edge 1 2 Post edge Energy loss in ev Net signal Energy loss in ev Series of single energy-filtered images (above), procedure of background extrapolation and subtraction (below)

47 phase phase [010] [100] TEM-BF 200 nm RGB image Al-L 23 Cr-L 23 Ti-L 23 Energy-filtered TEM imaging of the element distribution in SC16 after creep ( = 0.5 %) at 950 C Humboldt-Universität zu Berlin, Institut für Physik, AG Kristallographie Hahn-Meitner-Institut Berlin

48 Example: EELS mapping on subnanometer scale La-Mn-containing film on SrTiO 3 (La 0.7 Sr 0.3 MnO 3 ) Atom column EEL - Spectrum Imaging: Data courtesy: D. Muller et al. Cornell University From Lit.: P. Hawkes, new book: Advances in Imaging and Electron Physics

49 Fine structures of the ionisation edges unoccupied states valence band energy levels of inner shells ELNES Electron Loss Near Edge Structure (bonding information) 283 ev 99 ev K C neighboring atoms Si L 3 L 2 L 1 EXELFS Extended Energy Loss Fine Structure (information on shortrange order) intensity in counts x E F zero-loss peak plasmon excitation Si-L 23 edge x 100 C-K edge energy loss in ev

50 Electron Energy Loss Spectrometry (EELS) & Energy Loss Near-Edge fine Structure (ELNES) Carbon: Diamond structure ELNES fingerprints of carbon Carbon: Graphite structure

51 Energy resolution of EDXS/EELS EDXS EELS Energy resolution ev down to 0.3 ev

52 Comparison between EDXS and EELS EDXS EELS Energy scale up to 40 kev up to 3 kev Energy resolution ev down to 0.3 ev Lateral resolution down to 1 nm down to 1 nm Element mapping line profile, elemental map series of EEL spectra, EFTEM Detectable elements Z > 4 (Be) 2 < Z < 40 Detection limit 1 at% 1 at% Quantitative analysis of chemical comp. Analysis of chemical bonding yes - yes by ELNES and chemical shift of edges Analysis of structure - EXELFS

53 TEM Techniques TEM/STEM IMAGING DIFFRACTION SPECTROSCOPY Amplitude contrast (diffracion contrast) Phase contrast (highresolution imaging) Selected area diffraction Energy dispersive X-ray spectroscopy Electron energy loss spectroscopy Electron holography Z-contrast imaging Convergent beam diffraction Micro-/ nanodiffraction X-ray mapping Energy-filtered TEM (EFTEM) Tomography

54 Electron tomography reconstruction of 3D structure

55 Electron tomography: from 2D to 3D imaging

56 Electron tomography: from 2D to 3D imaging Please note that in TEM you would also see the rabbit s internal features (organs, bones, etc.)

57 Electron tomography: from 2D to 3D imaging Tomography: reconstruction of the interior of an object from its projections Tilt angles of 90 are required to cover the whole range! - conventional TEM specimen holder: tilt - special tomography holder: 75 tilt Figure from J. Frank, Electron Tomography. Methods for Three-Dimensional Visualization of Structures in the Cell, Springer Verlag

58 Electron tomography x-ray tomography in medicine electron tomography in science

59 Resolution, sources of artifacts reduction of missing edge (from wedge to pyramid) for a dual axis tilt series Sources of arrows: missing edge tilting angle is limited by shadowing of the specimen by holder edge and limited space between the objective lens pole pieces signal-to noise ratio of original projection images original resolution of images misalignment of the tilt axis Resolution: to the tilt axis: d x is original resolution of projections to the tilt axis (if the images are equaly distributed over ±90 ) : N - number of images D object size e yz elongation factor d y d z D N In practice: d depends on maximum tilt angle z for a 100 nm object 140 images to get a 2.2 nm resolution Figure from: Jenna Tong et al., IMC16, Sapporo 2006 d y e yz

60 Conventional EM sample structure defect structure High resolution EM atomic arrangement defect structure strain analysis Analytical EM Electron microscopy (EM) in material science chemical composition bonding magnetic properties Diffraction in EM crystallography crystalline structure strain dependent temperature dependent current dependent properties in-situ EM

61 pdf-dateien der Vorlesungen unter: Teaching Inorganic Materials" Vorlesungen zur Elektronenmikroskopie: Teil 1, Teil 2