Surface Electronic Characterization with SPM

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Surface Electronic Characterization with SPM

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• Sidney Cohen

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Preview

• Introduction to modes of SPM electronic
  characterization -
• current/voltage spectroscopy (I/V), scanning
  spreading resistance microscopy (SSRM), scanning
  capacitance microscopy (SCM), scanning Kelvin
  microscopy (SKPM).
• Examples
• Study of electronic states in Quantum dots
• Study of electron transport in thin organic films
• Investigation of transport at grain boundaries

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Why use these techniques?

• Combination of high resolution imaging with
  electronic characterization
• Possible to identify, characterize, modify, and
  characterize again with same probe.
• BUT !!Need to consider interaction of probe with
  sample.

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Current-Voltage Spectroscopy
I

• dI/dV gives directly local density of electronic
  states.
• Possible influence of measurement (band bending,
  charging)
• Difference between I/V in STM/SFM

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I/V spectroscopy
Fermi Level
DOS
eV Bias voltage
energy
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Contact Resistance
(acontact radius,
electron mean free path
Contact resistance)
1. Spreading resistance,
2. Sharvin (Ballistic) transport,
Note
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For typical experimental values, metals
But measure
(Contaminants, oxidation, etc)
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Modes based on capacitative force

• Scanning Capacitance Microscopy
• Scanning Kelvin Probe Microscopy
• Forces are long-range. Finite size of tip causes
  broadening of features.

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Scanned Probe Measurements of CdSe Quantum dot
Structures

• Want to correlate size of dot with electronic
  properties
• Due to confinement, gap varies inversely with
  size
• bulk Eg
• Alperson, Cohen, Rubinstein, Hodes, Phys. Rev. B
  52

Localization energy
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I/V spectroscopy on CdSe Q. Dot
Gap 1 0.15 eV
Gap 2, 0.2 eV
Eg2.1 V
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Double capacitor configuration
4 nm gaps in parallel gives C 6e-19. This
translates to charging energy of 0.15 eV
Supports premise that each peak corresponds to
addition of electron to quantum dot Coulomb
Charging
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Size Distribution vs. Msd. Energy Gap
TEM
This Exp.,with Calculated Gap
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Work Function Variations on thin film surfaces

• May be expected due to microscopic domain
  structure
• SKPM can be used to detect domains with different
  work function down to 50 nm size.
• Evidence supports domain existence
• Macroscopic Kelvin Msmts. Cannot give the spatial
  resolution
• Cohen, Efimov, Dimitrov, Trakhtenberg, Naaman,
• submitted

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Microscopic Domain Structure in Mixed Film
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Results show NO variation of signal across surface
Topography
Raw SKPM
Contrast lt 5 mV
Corrected SKPM
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Contact Potential Differences on Different
Surfaces
Monolayer types
CN MIX3 MIX2 MIX 1 RL858
OM Mstm. 1 -600 -260 210 20 410
450 (mV) Msmt. 2 -640 -300 190 -40
360 500 (mV)
Monolayer Lewis Acid Lewis Base
CPD is of tip relative to surface. More
negative CPD therefore corresponds to higher work
function because monolayers have extracted
electrons from the gold substrate.
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Electron Transport at grain boundaries in
semiconductors
For polycrystalline semiconductors, the electron
transport properties across grain boundaries play
a significant role in solar cell function, and
particularly in their degradation. Crystallites
can be a fraction of a micron in size, making it
difficult to determine these transport
properties by conventional means. Scanning
Spreading Resistance, I/V spectroscopy, and SKPM
can give this information
I. Visoly-Fisher, D. Cahen, S. Cohen
(samples from C. Farakadis
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Electronic properties of Grain Boundaries can be
measured by
1. Comparing I/V curves across the grain boundary
2. Monitoring change in surface potential
across boundary with SKPM
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Spatially-resolved I/V spectroscopy on CdTe
film Using conducting SFM
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1
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Forward-biased currents are highest near grain
boundary. May be due to lower gap energy or
higher carrier concentration
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SKPM - Contrast in CPD image
CdTe with Molecular Layer contrast 15 meV
Uncoated CdTe contrast 30 meV
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Conclusions

• SPM can give useful information on the nanoscale
  surface electronic properties
• Correlation can be made between topography and
  electronic characteristic
• Knowledge of the effect of measurement on the
  system is required to interpret results
• Many possibilities untouched here (photo-effects,
  direct capacitance msmt., STM UHV work)

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Acknowledgements
Quantum Dot Work - I. Rubinstein, G. Hodes, B.
Alperson
Organic Films - R. Naaman, D. Dimitrov
Photovoltaics - D. Cahen, I. Visoli-Fisher
All work performed at