franco-israel conference on nanocharacterization surface electronic characterization with spm sidney...

Franco-Israel Conference on Nanocharacterization

Surface Electronic Characterization with SPM

Sidney Cohen


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


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.


Current-Voltage Spectroscopy

dI/dV gives directly local density of electronic states.

Possible influence of measurement (band bending, charging)

Difference between I/V in STM/SFM

I


I/V spectroscopy

eV

ff eVEdVdIdeVEI

0

:)(

DOS fEFermi Level

eV = Bias voltage = energy


Contact Resistance

1. Spreading resistance,

a

aaRM 44

21

2. Sharvin (Ballistic) transport, a

22

2

22 aehR f

s

31

*4)2(3

E

rwrNaDMT

Note:

(a=contact radius, =electron mean free pathContact resistance)


For typical experimental values, metals:

nmr

GPaE

nNNcm

mJ

cm

10

100

1

400

2

*

2

10R

But measure sM (Contaminants, oxidation, etc)


Scanning Capacitance Microscopy

Scanning Kelvin Probe Microscopy

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

Modes based on capacitative force

2CV E

2VdZdC

FdZdE

biasts VVdVdC

F


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

Scanned Probe Measurements of CdSe Quantum dot Structures*

2

22

2)(

RERE g

Localizationenergy


I/V spectroscopy on CdSe Q. Dot

Gap 1 0.15 eV Gap 2, 0.2 eV

Eg=2.1 V


Double capacitor configuration

Supports premise that each peak corresponds to addition of electron to

quantum dot “Coulomb Charging”

4 nm gaps in parallel

gives C = 6e-19.

This translates to charging

energy of 0.15 eV


Size Distribution vs. Msd. Energy Gap

TEM

This Exp.,with

Calculated Gap


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


Microscopic Domain Structure in Mixed Film


Results show NO variation of signal across surface

Topography

Raw SKPM

Corrected

SKPM

Contrast

< 5 mV


Contact Potential Differences on Different Surfaces

CN MIX3 MIX2 MIX 1 RL858 OMMstm. 1 -600 -260 210 20 410 450 (mV)Msmt. 2 -640 -300 190 -40 360 500 (mV)

Monolayer types

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.

Monolayer = Lewis Acid Lewis Base


Electron Transport at grain boundariesin semiconductors*

For polycrystalline semiconductors, the electron transport properties across grainboundaries play a significant role in solar cellfunction, and particularly in their degradation.Crystallites can be a fraction of a micronin size, making it difficult to determine these transport properties by conventional means.Scanning Spreading Resistance, I/Vspectroscopy, and SKPM can givethis information

*I. Visoly-Fisher, D. Cahen, S. Cohen (samples

from C. Farakadis


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


SURFACE TREATMENT OF CdTe

Purpose: Control CdTe grain boundary chemistry and through this, GB electronic behaviour.The organic molecules were introduced via the CdTe back surface prior to contact deposition.

Such molecules give control over the electron affinity and band bending of n-CdTe surfaces1; theiradsorption in Cu(In, Ga)Se2/CdS junctions was shown to affect charge transport properties of that cell2.

1 R. Cohen, et al., Adv. Mater., 9, 746 (1997); JACS 121, 10545 (1999).2 D. Gal, et al., Proc. Indian Acad. Sci. (Chem. Sci.), 109 (6), 487 (1997).

COOH

R

C CO O

HOOCCOOH

OCC

R R

O

dicarboxylic acid derivatives,R = CF3, CN.

benzoic acid derivatives,R = OCH3, N(CH3)2, Br, CN-

+-+

+-

+

-

-

+-+


12

3

Spatially-resolved I/V spectroscopy on CdTe filmUsing conducting SFM

Forward-biased currents are highest near grain boundary.May be due to lower gap energy or higher carrier concentration


Uncoated CdTecontrast = 30 meV

CdTe with Molecular Layercontrast = 15 meV

SKPM - Contrast in CPD image


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)


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 surface electronic characterization with spm sidney...

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