MSE 321 Structural Characterization

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1 Auger Spectroscopy Auger Electron Spectroscopy (AES) Scanning Auger Microscopy (SAM) Incident Electron Ejected Electron Auger Electron Initial State Intermediate State Final State Physical Electronics PHI 660 UIUC o Operates like an SEM at about kv o Auger electron energies (a few kev) determined with spectrometer (serial, electrostatic) o Requires high beam currents (~10 na) and therefore large spot sizes, so resolution is limited to ~50 nm. o Requires ultra-high vacuum o The Auger electron characteristic energy depends upon: The chemical element involved The initial energy level of the electron which eventually becomes the Auger electron (allows chemical state of element to be determined) Pierre Auger c Scanning TEM (STEM) o Resolution limited by probe size (1 nm in JEOL 2100) o Annular dark field (ADF) detector allows Z contrast scattering proportional to Z ½ inelastic scattering from atomic columns can be imaged o Dedicated STEMs use FEG emitters to achieve high resolution (small probe size) o Require ultra-high vacuums enables long count times without contamination o Excellent spatial resolution possible o No lenses below specimen Ge-rich Si-rich Ti O W O Si 200 nm Semiconductor device ASI ADF 10 Å x = Si 21-x Ge x Top interface of a Ge 30 Si 70 /Si/Ge 30 Si 70 quantum well, IBM

2 Tomography Another way of representing 3D data Many images are obtained at incremental tilts up to ±70 at 60, specimen twice as thick at 70 nearly three times as thick (can t go up to ±90 ) Series of images knitted together into a tomogram software uses either fiducial (marker) or cross-correlation (non-marker) technique Diesel soot, Arizona State University, synthetic magnetite crystals grown by Hua Xin, Arizona State University, Interplanetary dust particle, Ilke Arslan and John P. Bradley, RHEED & LEED Reflection High-Energy Electron Diffraction (RHEED) Low Energy Electron Diffraction (LEED) Reduce the depth from which diffraction information is obtained. Used to obtain extremely detailed surface structural information. RHEED: impact angle typically < 5, typically kv characterizing thin films during growth sensitive to surface roughness down to monolayer intensity oscillations correspond to single monolayers LEED: Normal incidence, ev qualitative identification of surface symmetries and 2D periodicities quantitative structure determination of ordered surfaces.

3 Fluorescence Absorption of light at one wavelength and its re-emission in any direction at a longer wavelength Phosphorescence relaxation occurs via an intermediate state and so is delayed The electron knocked out in EDS is not a photoelectron, but in XPS it is hv hv 2 1 XPS X-Ray Photoelectron Spectroscopy (XPS) Electron Spectroscopy for Chemical Analysis (ESCA) PHI 5000 Versaprobe Soft x-ray photons excite (outer-shell) photoelectrons in specimen E = E 0 E binding φ, can detect Z 3 (Li) Energy resolution of 0.50 ev for Ag 3d 5/2 Sensitive to state of bonding Specific to top few atomic layers (0 < E 1500 ev) Poor spatial resolution (100 µm 2 ) as x-rays cannot easily be focused Requires ultra-high vacuum system to avoid surface contamination

4 Scanning Probe Microscopy Scanning Tunneling Microscopy (STM) 71 Å quantum corral of Fe atoms on Cu Gerd Binnig and Heinrich Rohrer won the Nobel Prize in 1986 for the invention of the STM IBM Electron quantum distributions of two conductors brought very close together overlap tunneling current possible when V applied Current very sensitive to spacing As sample height varies, tip height is adjusted to maintain current, thus height can be measured Vibrations in lab must be minimised STM is the highest-resolution imaging and nanofabrication technique available No primary beam No secondary effects No lenses laser Atomic Force Microscopy (AFM) Probe mounted on cantilever and kept at constant distance from surface by interatomic forces Vertical displacement (< 1 Å) of cantilever measured by laser Atomic resolution in 3D tip surface photodiode cantilever Magnetic Force Microscopy (MFM) is a variant in which magnetic forces, rather than interatomic ones, dominate (tip is coated with a magnetic film) Magnetic domain walls on a BaFe 12 O 19 single crystal Si(111) Field Ion Microscopy Allows individual atoms and point defects to be imaged in conducting/semiconducting samples Sample is prepared as a fine point with radius r (typically r < 50 nm) and cooled to K Low pressure of inert image gas maintained in chamber (e.g., He or Ne) Atom probe field ion microscopy (APFIM) Up to 10 kv applied between sample and screen a distance R away Gas atoms adsorbed onto sample are field-ionised and accelerated towards screen Curvature causes natural magnification, M = R/r Many images combined to improve resolution Invented by Erwin E. Mueller in 1951 at the Pennsylvania State University (FIM) Combines ToF spectroscopy with FIM to produce 3D reconstructions of hundreds of millions of atoms High-frequency voltage (or pulsed laser) ionises whole surface layer and strips it off of sample Ions travel through aperture (probe hole) to detector Fast timing circuit measures time between pulse and impact and calculates mass-to-charge ratio

5 Ion Beam Analysis Secondary Ion Mass Spectroscopy (SIMS) primary ion beam (1 30 kev) produces monatomic and polyatomic particles of sample material, re-sputtered primary ions, electrons, and photons only useful signal is from secondary ions (charge-to-mass 0) A 20 nm beam of ions (usually Ar + ) is scanned across sample surface Secondary ions are ejected (similar to ion-beam thinning) Charge-to-mass ratio can be measured by mass spectrometer Sensitive to light elements and isotope-specific More surface sensitive than using electrons (ions penetrate less deeply) Better signal-to-noise ratio for secondary effects excited by ions than by electrons, so better sensitivity most sensitive surface analysis technique (ppm or ppb) Poor spatial resolution (~ 1 µm) compared to electrons Static SIMS: low beam currents less than a full monolayer can be detected Dynamic SIMS: high beam currents successive layers of atoms are removed and analysed (depth profiling) Ion Beam Analysis Rutherford Backscattering (RBS) Backscattering of electrons is nearly elastic not true for ions (conservation of momentum & energy) He 2+ ions of MeV directed at specimen, some are backscattered (θ > 90 ). Ions lose well-defined amount of energy on BS, which depends on mass of ion, mass of target, and scattering angle (k = 1 as m 0), plus more on traveling through the specimen (scattering). = The energy spectrum represents a convolution of target mass and depth which is deconvoluted by computer. Spectrum of energies reveals atom species (ppm) and depth distribution to within ~10 nm. RBS is well suited to the analysis of medium and heavy contaminants in light matrices (eg, optical coatings, glasses, metals, and complex compound semiconductors). It is often used to establish graded compositions and film thicknesses in multilayer structures or simply to accurately establish the homogeneous bulk composition. Ion beam can be directed (channeled) along a particular crystalline axis more sensitive to crystal defects.

6 SAXS (Small-Angle X-Ray Scattering)