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Franco-Israel Conference on Nanocharacterization
Surface Electronic Characterization with SPM
Sidney Cohen
Franco-Israel Conference on Nanocharacterization
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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?
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
dI/dV gives directly local density of electronic states.
Possible influence of measurement (band bending, charging)
Difference between I/V in STM/SFM
I
Franco-Israel Conference on Nanocharacterization
I/V spectroscopy
eV
ff eVEdVdIdeVEI
0
:)(
DOS fEFermi Level
eV = Bias voltage = energy
Franco-Israel Conference on Nanocharacterization
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)
Franco-Israel Conference on Nanocharacterization
For typical experimental values, metals:
nmr
GPaE
nNNcm
mJ
cm
10
100
1
400
2
*
2
10R
But measure sM (Contaminants, oxidation, etc)
Franco-Israel Conference on Nanocharacterization
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
Franco-Israel Conference on Nanocharacterization
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
Franco-Israel Conference on Nanocharacterization
I/V spectroscopy on CdSe Q. Dot
Gap 1 0.15 eV Gap 2, 0.2 eV
Eg=2.1 V
Franco-Israel Conference on Nanocharacterization
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
Franco-Israel Conference on Nanocharacterization
Size Distribution vs. Msd. Energy Gap
TEM
This Exp.,with
Calculated Gap
Franco-Israel Conference on Nanocharacterization
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
Franco-Israel Conference on Nanocharacterization
Microscopic Domain Structure in Mixed Film
Franco-Israel Conference on Nanocharacterization
Franco-Israel Conference on Nanocharacterization
Results show NO variation of signal across surface
Topography
Raw SKPM
Corrected
SKPM
Contrast
< 5 mV
Franco-Israel Conference on Nanocharacterization
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
Franco-Israel Conference on Nanocharacterization
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
Franco-Israel Conference on Nanocharacterization
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
Franco-Israel Conference on Nanocharacterization
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-
+-+
+-
+
-
-
+-+
Franco-Israel Conference on Nanocharacterization
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
Franco-Israel Conference on Nanocharacterization
Uncoated CdTecontrast = 30 meV
CdTe with Molecular Layercontrast = 15 meV
SKPM - Contrast in CPD image
Franco-Israel Conference on Nanocharacterization
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
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: