Experimental methods in physics. Local probe microscopies I

Transcription

1 Experimental methods in physics Local probe microscopies I Scanning tunnelling microscopy (STM) Jean-Marc Bonard Academic year Scanning Tunneling Microscopy 1.1. Introduction Image of surface reconstruction on a clean Gold (Au(100)) surface

2 usual!scopies! optical!scopy! transmission electron!scopy! projection!scopy! Object probed by a source " Integral observation of the image " Magnification determined by # Focal length of the lenses # Distances between source, object and observation screen " Resolution limited by # Wavelength of probe particles # Lens aberrations # Source coherence Schmid and Fink, APL 70, 2679 (1997) Local probe!scopies! Interaction between a probe and the sample " Tunnelling current (STM) " Atomic force (AFM), magnetic, electrostatic, " Luminescence (SNOM) " Secondary electrons (SEM)! Image formed by scanning the probe on the sample " Magnification determined by size of scanned surface " Resolution limited by # Probe size # Sensitivity of the detection

3 1. Scanning Tunneling Microscopy 1.2. Principle of STM STM, first experiments: Young s topografiner (1971)! Young s goal: detecting atomic steps " Strong dependence of field emission current with tip-sample distance " Topografiner # x-y-z scanner to raster the sample # Profiles at constant current: topography " Resolution too low to detect atomic steps " Project abandoned Young, Physics Today 11, 42 (1971) Young et al., PRL 27, 922 (1971) Optical grating, 180 lines/mm

4 The first STM Binnig and Rohrer! 1982 " Binnig and Rohrer take Young s work one step further " Goal: local spectroscopy of electronic properties " First constant current scans on a CaIrSn 4 surface Binnig et al., PRL 49, 57 (1982) " Mono-, bi-, triatomic steps! Key differences " Tunnelling regime (field emission for Young) " Mechanical isolation " Vibration damping! The first STM Binnig and Rohrer II! 1983 " Binnig and Rohrer study the Si (111)(7x7) surface # At that time, structure not known with certitude # Unit cell with 49 atoms " First observation in direct space! Atomic resolution! " Greeted with great caution " Nobel prize in 1986 Binnig et al., PRL 50, 120 (1983)

5 The tunnelling effect! Transmission probability of electrons between two materials > 0 " Low barrier width # High electric field: field emission, I! exp(v) # Small distance between electrodes: STM, I! V! Quantum effect " Overlap between wavefunctions of electrons at tip and sample " Applied field " 0 # Tunnelling current: I! exp(-z), I! V " Contact # Atomic chain (quantum contact, one conduction path) # Ohmic contact Tunnelling regime Ohmic contact Quantum contact Lang, PRB 36, 8173 (1987) Tunnelling current " First order perturbation I t! $ µ," f(e µ )[1-f(E " +ev)] M µ," 2 %(E µ -(E " +ev)) #!: initial state; ": final state # M: matrix element # Elastic tunneling effect between an occupied state and an unoccupied state " Spherical tip, with only states! "( r,e f ) #T (E f,v )! "( $2 kz r,e f ) # e I t # #: local density of states (LDOS) # T: transmission coefficient # k: depends on V and workfunctions of both materials; k # 1Å -1 E µ "$I t depends strongly on z! #$Very high z resolution #$Typical tunnelling currents between 10pA and 1nA $E = ev E "

6 1. Scanning Tunneling Microscopy 1.3. Instrumentation Instrumentation! An STM is composed of " Probe tip " x-y-z scanners " Electronics " Vibration damping " Vacuum chamber # STM is not necessarily done under UHV Electrochemical STM (tunnelling in a liquid) STM in ambient atmosphere (surface cleaneliness?) " Options # Cryostat # Magnet # Surface preparation tools (ion gun, electron diffraction setup, )

7 Probe preparation! Au, W, Pt-Ir wire! Coarse sharpening " Fracture, cutting " Electrochemical sharpening # Tip with ~0.1!m radius of curvature! Fine sharpening " High voltage pulse (~5V) # Transfer of atoms between probe and sample # Probe ends with one (a few) atom(s) Scanning I! Specifications " Resolution of 0.05Å " Full course of 10nm - 1!m (10!m?) " Linear behavior of displacement as a function of voltage! Piezoceramics " Dilatation/contraction under applied voltage " Typ. 2Å/V! x-y-z scanner " Tube tripod " Sticks tripod " x-y-z tube

8 Scanning II! Binnig and Rohrer design " Coarse approach: piezo slab and electrostatic clamps " Scanning: sticks tripod Control electronics! Constraints " Tunnelling current between 10pA and 1nA # Low noise electronics # No ground loops " Scanning # Resolution in x,y of 1Å " Approach and measurement # Resolution in z of 0.05Å # Feedback: ln(i t )! z

9 Vibration damping! Binnig and Rohrer " Damping springs (3 stages) " Damping through Foucault currents! Pocket-size STM " Copper slabs isolated with elastomer half-rings! In general " Pocket-size damping " Suspension of UHV chamber on springs or on a damping table Other challenges! Cryostat " He cryostat: %4K (typ. 20K) " 3 He cryostat: ~250mK " Vibrations, thermal shifts?! Environment " UHV chamber " STM under air (adsorbates?) " Electrochemical STM (tunnelling in a liquid)! Other useful add-ons " Auger spectrometer, ion gun, electron diffraction (surface preparation) " Sputtering setup (deposition/growth) " Magnetic field Röder et al., Thin Solid Films 264, 230 (1995)

10 1. Scanning Tunneling Microscopy 1.4. Imaging basics Point defects on Cu(111), possibly impurity atoms, and scattered surface state electrons (Crommie et al., IBM) STM imaging I! Tunnelling current " Proportional to local electronic density of states at the Fermi level " Constant current images # Constant electronic LDOS # Defects, steps: topography! Example: Al(111) and adsorbed C " Steps: 2.34Å " C atoms: apparent height of ~0.2Å

11 STM imaging II! Image depends on z! C on Al(111) " High apparent height or "$Transparent atoms! #$Destructive interference between probe and sample wavefunctions #$Redistribution of electronic charge and modification of LDOS Brune et al., Europhys. Lett. 13, 123 (1990) STM imaging III! Image depends on V " Electronic structure of surface " Tunnelling current initiates from occupied states (or goes to empty states) # Density of states of corresponding electronic levels " Si(111)(7x7) # Empty states: dangling bonds of upper atoms # Filled states: bonds between first and second layers " GaAs(110) # Empty states: on Ga atoms # Filled states: on As atoms Si(111)(7x7) GaAs(110)

12 STM imaging IV! Cu(111) " Steps: ~2Å " Adsorbates: ~0.2Å " Lines, circles around adsorbates: ~0.05Å! What are these structures? " Atomic arrangements? " Defects? " Surface electrons! # Quasi-2D electron sea # Interference between incident and reflected electrons at atomic steps at defects Crommie et al., Surf. Rev. Lett. 1, 127 (1995) 1. Scanning Tunneling Microscopy Annex 1 Nomenclature of surface structures and reconstructions

13 Preparing a surface face-centered cubic crystal (fcc)! Metal/semiconductor: well-defined crystalline structure " In theory, one should be able to form a surface of arbitrary orientation " In practice: only a few orientations are energetically favourable 1st layer 2nd layer! Preparation " Cleavage " Machining " Chemical etching " Ion bombardment " Vacuum annealing 25 High symmetry surfaces! Most common surfaces " High density of atoms " High number of neighbouring atoms! Notation: Miller indices (100) fcc structures (Cu, Pt, Si ) (110) STM image of a Cu(111) surface (111) 26

14 Surface reconstruction! Lowering of the free surface energy " Relaxation " Reconstruction!$Notation (Wood): Size and orientation of unit cell of the reconstruction with respect to the 2D unit cell (2 x 2) c: centred R: rotation c(2 x 2) or (&2 x &2)R45 27 Surface reconstruction GaAs c(2 x 4) Si(111)(7 x 7) 28

15 Vicinal surface! Surface with an orientation that is very close to a high symmetry surface " Low indices surface with periodic terraces " Diffusion/segregation studies Pt(997) Scanning Tunneling Microscopy Annex 2 Tip electron microscopies

16 Electron field emission! Emission of cold electrons " First observed in 1897 " Applying an electric field on a sample renders the surface potential barrier triangular, with a slope that depends on the applied field " Significant probability of crossing of surface barrier by tunnelling effect for F % 2 V/nm F = 0 V/nm D(E) E F D(E F ) D(E) F! 2 V/nm E " Observed on sharp tips (field amplification) " Non-linear behavior between local applied field and emitted current I + V - 2 ln(i/v ) FV Field emission microscopy! Observation of a metallic tip during field emission " Sharp tip with low radius of curvature: high field enhancement " High sensitivity to local protuberances and work function " Adsorption/desorbtion studies " Diffusion studies e + V

17 Field emission microscopy! Example: NO+H 2 on Ir " Clean tip: emission from (110) " With NO+H 2 partial pressure # Change of emission on (110) # Short lowering of work function on (100) planes: probable presence of NH x or O # Oscillatory local chemical reactions Cobden et al, Surf. Sci. 402, 155 (1998) Field ion microscopy (1956)! Ionisation of He atoms (F%2V/Å) " Sharp tip: field enhancement " High electric field at atoms located at edges of terraces " He atoms ionized near these atoms " Ions follow the electric field lines to the observation screen + First images with atomic resolution

18 Field evaporation microscopy: Atom Probe! Field evaporation " High electric field can lead to atom evaporation # Time-of-flight measurement: determination of mass of atom # Comparison of images before and after evaporation: position of atom " 3D probe of atomic composition Superalloy 708 precipitate and grain boundary 12 nm Miller, Mater. Charact. 44, 11 (2000)