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STMTunneling

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In STM-no use of external particles. Principle-Electrons tunneling between an ... Two dimensional 'Inchworm'='louse' (????) STM Scanners (cont.) 'Inchworm' ... – PowerPoint PPT presentation

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Title: STMTunneling


1
STM-Tunneling
2
STM-Tunneling
3
STM-Introduction
  • Why STM ?
  • The electronic microscopes gives volume images
    (penetration depth)
  • In STM-no use of external particles
  • Principle-Electrons tunneling between an
    atomically sharp tip and a surface
  • (animation-program files/netscape/communicat
    or/program/stmanimation)

4
STM-Introduction
  • The STM combines three main concepts
  • Scanning
  • Tunneling
  • Tip-point probing
  • Uniqueness

5
STM-History
  • In March 1981, Gerd Binning, H. Rohrer, Ch.
    Gerber and E. Weibel observed electrons tunneling
    in vacuum between W tip and Pt this in
    combination with scanning marked the birth of
    STM.
  • The breakthrough-atomic imaging in real space
  • The development of STM paved the way for a new
    family of techniques called scanning probe
    microscopy.
  • 1986-Nobel prize to G. Binnig and H. Rohrer.

6
Comparison of Characterization Techniques
7
Comparison of Characterization Techniques
8
STM Instrumentation
  • Constant height vs constant current imaging

9
Constant Current Imaging (CCI)
10
Si (111) Surface
STM Si(7x7)
11
STM Images1. GaSb/InAs Only every-other lattice
plane is exposed on the (110) surface, where only
the Sb (reddish) and As (blueish) atoms can

This color-enhanced 3-D rendered STM image shows
the atomic-scale structure of the interfaces
between GaSb and InAs in cross-section. A
superlattice of alternating GaSb (12 monolayers)
and InAs (14 monolayers) was grown by molecular
beam epitaxy. A piece of the wafer was cleaved in
vacuum to expose the (110) surface, and then the
tip was positioned over the superlattice about 1
µm from the edge. Due to the structure of the
crystal, only every-other lattice plane is
exposed on the (110) surface, where only the Sb
(reddish) and As (blueish) atoms can be seen. The
atoms are 4.3 ? apart
along the rows, with a corrugation of lt0.5 ?.
From work of W. Barvosa-Carter, B. R. Bennett,
and L. J. Whitman.
12
The first STM image
13
Nanolithography
14
Nanolithography STM
  • Here, the artist, shortly after discovering how
    to move atoms with the STM,
  • found a way to give something back to the
    corporation which gave him a job when he needed
    one and provided him with the tools he needed in
    order to be successful. (Xe on Nickel, Nature
    344, 524 (1990).
  • Here they have positioned 48 iron atoms into a
    circular ring in order to "corral" some surface
    state electrons and force them into "quantum"
    states of the circular structure. The ripples in
    the ring of atoms are the density distribution of
    a particular set of quantum states of the corral.
    The artists were delighted to discover that they
    could predict what goes on in the corral by
    solving the classic eigenvalue problem in quantum
    mechanics -- a particle in a hard-wall box.
    Crommie, Lutz Eigler, Science 262, 218 (1993)

15
STM Instrumentation
  • Pohl, D. W. IBM J. Res. Dev. 30, 417 (1986),
  • Kuk and silverman, Rev. Sci. Instrum. 60, 165
  • 1. Mechanical Construction
  • 2. Electronics
  • 3. Data Acquisition
  • 1.1 Vibration Isolation
  • To obtain vertical resolution of 0.01? ,
  • a stability of tip-sample 0.001 ? is required
  • Typical floor vibration 0.1-1.0 ?m, 0.1-50 Hz,
  • ? Vibration Isolation

16
STM Instrumentation(cont.)
  • Damping low frequency (lt20 Hz, building) ?
    tension wires or springs (resonance frequency
    1-5 Hz) , air table(res freq 1 Hz). Medium
    frequency (20-200 Hz,motors, acoustic noise) ?
    mounting on plates.
  • External vibration isolation systemrigid STM
    design can reduce external vibrations by 10-6
    10-7 (Interference filter).

17
STM Instrumentation (cont)
  • 1.2 Positioning Devices
  • Three dimensional movement of tip and sample
  • Tip movement -piezoelectric drives.
  • Sample-piezoelectric, magnetic (magnet inside a
    coil), mechanical.

18
STM Scanners
  • Piezoelectric bars

h
V
l
19
STM-Principle
20
Tube Scanners

21
STM Scanners (cont.)
  • Design Considerations
  • Tube scanner higher sensitivity due to thin
    walls, symmetrical vs. unsymmetrical voltage.
  • Single tube scanner higher resonance frequency?
    scan rate, stability
  • voltage signals to produce a scan.
  • Large piezoresponse.
  • Low cross-talk between x,y and z piezodrives.
  • Low nonlinearity, hysteresis, creep, and thermal
    drift.
  • Sample-Tip Approach Mechanisms
  • Step size has to be smaller then the total range
    of z piezodrive Step resolution of 50 ? ,and
    dynamic range of cm is reuired!
  • Inchworm (electrostrictive actuator), motion
    not limited in distance. Two dimensional
    Inchwormlouse (????)

22
Inchworm
23
STM Electronics (general)
24
STM Electronics (cont.)
  • Carefull design because of low currents, stable
    feedback circuits.
  • Sample and hold amplifier?local I-V
    characteristics
  • Computer 4-5 DACs, 2 ADCs, real-time plane fit
    etc.

25
STM Tips
  • 1.5 Tip preparation and characterization
  • Atomic resolution single atom termination,
    macroscopic shape of little importance 1?
    difference? 1 order of magnitude in tunneling
    current.
  • Large scale structure macroscopic tip shape of
    importance

26
STM Tips (cont.)
  • Chemical composition of tip oxide layer (jump to
    contact), atomic resolution - type of atom,
    higher resolution has been obtained by tunneling
    into or from d-orbitals (W).
  • Tip material does not dictate atom at
    the tip !
  • Tip material hard , UHV-W, Mo, Ir Air-Pt, Au
    (soft) ? Pt-Ir.
  • Tip preparation Electrolytical etching (drop-off
    technique),

27
STM Tips (cont.)
  • Disadvantage of electrolytical etching (drop-off
    technique), formation of oxide ? ion milling tip
    radii of 4 nm, cone angle 100.
  • In air cutting of Pt-Ir wire (only for smooth
    surfaces)

28
STM Tips (cont.)
  • Tips are produced by electron beam
  • deposition (EBD) inside SEM.
  • Dissociation of chamber residual gases (H2, O2,
    CO, H2O)
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