Surface electronic characterization with spm
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Surface Electronic Characterization with SPM

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Surface electronic characterization with spm

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Surface Electronic Characterization with SPM

Sidney Cohen

Franco-Israel Conference on Nanocharacterization


Preview

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

Franco-Israel Conference on Nanocharacterization


Why use these techniques

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.

Franco-Israel Conference on Nanocharacterization


Current voltage spectroscopy

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

I/V spectroscopy

Fermi Level

DOS

eV =Bias voltage

= energy

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Surface electronic characterization with spm

Contact Resistance

(a=contact radius,

=electron mean free path

Contact resistance)

1. Spreading resistance,

2. Sharvin (Ballistic) transport,

Note:

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Surface electronic characterization with spm

For typical experimental values, metals:

But measure

(Contaminants, oxidation, etc)

Franco-Israel Conference on Nanocharacterization


Modes based on capacitative force

Modes based on capacitative force

  • Scanning Capacitance Microscopy

  • Scanning Kelvin Probe Microscopy

  • Forces are long-range. Finite size of tip causes broadening of features.

Franco-Israel Conference on Nanocharacterization


Scanned probe measurements of cdse quantum dot structures

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

I/V spectroscopy on CdSe Q. Dot

Gap 1 0.15 eV

Gap 2, 0.2 eV

Eg=2.1 V

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Double capacitor configuration

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

Size Distribution vs. Msd. Energy Gap

TEM

This Exp.,with

Calculated Gap

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Work function variations on thin film surfaces

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

Microscopic Domain Structure in Mixed Film

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Surface electronic characterization with spm

Franco-Israel Conference on Nanocharacterization


Results show no variation of signal across surface

Results show NO variation of signal across surface

Topography

Raw SKPM

Contrast

< 5 mV

Corrected

SKPM

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Contact potential differences on different surfaces

Contact Potential Differences on Different Surfaces

Monolayer types

CN MIX3MIX2 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.

Franco-Israel Conference on Nanocharacterization


Surface electronic characterization with spm

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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Surface electronic characterization with spm

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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Surface electronic characterization with spm

Franco-Israel Conference on Nanocharacterization


Surface electronic characterization with spm

Spatially-resolved I/V spectroscopy on CdTe film

Using conducting SFM

2

1

3

Forward-biased currents are highest near grain boundary.

May be due to lower gap energy or higher carrier concentration

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Surface electronic characterization with spm

SKPM - Contrast in CPD image

CdTe with Molecular Layer

contrast = 15 meV

Uncoated CdTe

contrast = 30 meV

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Conclusions

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)

Franco-Israel Conference on Nanocharacterization


Surface electronic characterization with spm

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:

Franco-Israel Conference on Nanocharacterization


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