Electronic Characterization of Materials Using
Conductive AFM
Amir Moshar
2011 Purdue Workshop
Electrical Measurements
• SKPM
• EFM
• CAFM
2011 Purdue Workshop
• PFM
• SCM
Non-Contact Electrical Techniques
• Scanning Kelvin Probe Microscopy
• Electric Force Microscopy
Both use ‘nap’ mode
2011 Purdue Workshop
What is Nap Mode?
A two pass data
collection scheme
2
∆z
1
1 – Topography -- trace out the topography
2 – Nap -- non contact, above the surface. EFM, MFM, or SKPM
2011 Purdue Workshop
EFM
Photodetector Signal
AC mode
+nap
+tip-sample bias
=EFM
ADC
DAC
Sample
Tip bias
voltage
i
X
Sine Wave Synthesizer
Vd cos ωt
Vd sin ωt
2011 Purdue Workshop
R
LPF
Amp = i + q
2
X
φ = tan−1
LPF
q
i
2
φ
q
Model the Tip-Sample as a Capacitor
To bring charge element dq to the positive
electrode at potential V you need energy
dU
dU = Vdq
+
−
q
V=
C
q
dU = dq,
C
2
Q q
1Q
1 2
= CV
U =∫
dq =
0 C
2 C 2
2011 Purdue Workshop
+
+
−
+ +
−
+
− −
−
U total
20nm
15
10
5
Attractive
Repulsive
150°
Phase
U total
1 2
= kx + U sample
2
1 2 1
= kx + Ctip − sampleVtip2 − sample
2
2
Amplitude
EFM Phase
Contrast
100
Attractive
Repulsive
50
0
72
Repulsive
Attractive
73
74
75
Drive Frequency
76kHz
dU
dz
1 dC 2
F=
V
2 dz
F=
Attractive – Softens the potential well – Positive Phase shift
2011 Purdue Workshop
Repulsive – Sharpens the potential well – Negative Phase Shift
Conductors in an
Insulating/Semiconducting Matrix
Field Lines
Insulating Sample, no
conductors
Ground Plane
Introduce a conductive particle and the
field lines, and therefore the force
gradient change. This results in a
phase shift.
2011 Purdue Workshop
EFM on Embedded Conductive
Nanoparticles
Height
EFM
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Amplitude
Phase
Conductive Organic Nanorods in a
Polymer
Height
Sample courtesy Sergei Magonov, MI
2011 Purdue Workshop
Phase
SKPM
• Cantilever driven electrically
Photodetector
Signal
• Potential difference between tip
and sample drives cantilever
ADC
• Potential feedback loop cancels
cantilever oscillations
Sample
DAC
i
AC bias plus
DC from FB
loop to tip
X
Sine Wave Synthesizer
Potential
feedback
loop
2011 Purdue Workshop
Vd cos ωt
Vd sin ωt
R
LPF
Amp = i 2 + q 2
X
φ = tan−1
LPF
q
i
φ
q
A Bit of Math
1 dC 2
V
F=
2 dz
V = VSurfacePotential + VDC + Vac sin ωt
Ftotal = F0 + F1 sin(ωt + φ1 ) + F2 cos(2ωt + φ2 )
0
dC
(VSurfacePotential − VDC )Vac
dz
2011 Purdue Workshop
Potential Feedback loop
What Does SKPM Tell You?
•
•
•
•
Trapped Charge
Spontaneous Polarization
Work function variation
Potential difference
Q = CV
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Q
and V =
C
Samples for SKPM
• Insulators with trapped charge
• Work function differences on
metals/semiconductors
• Ferroelectric materials
• Materials with a spontaneous polarization
• Layered materials
• Integrated circuits/Micro circuits
• Solar/photoelectric materials
• Anything with a work function, charge or
potential difference
2011 Purdue Workshop
Electronic Traces
Broken Circuit
Height
Surface Potential
No voltage after break!
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MicroGels
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•
Three layers of microgels
•
SKPM can detect the three
different layers
Images
Se nanowires, courtesy of Byron Gates at SFU
2011 Purdue Workshop
Contact Mode Electrical Techniques
• CAFM – this includes all current/resistive
probing
• PFM
• SCM
2011 Purdue Workshop
CAFM
Rgain
Creates a current map of the surface
Concuctive probe
Vout
Vin
=
Rgain Rsample
Vout
 Vin 

= Rgain 
R

 sample 
but
Vin
= I in
Rsample
Gain is just Rgain
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so
Vout = I in Rgain
CAFM Issues
•
•
•
•
•
•
Contact resistance
Oxidation/reduction
Tip wear/contamination
BW limitations
Repeatability (statistics)
Percolation network
1
1
1
1
= +
++
Rtot R1 R2
Rn
2011 Purdue Workshop
Current
Point and Click IV curves
Topography, 10nm
full scale
Current, 10pA
full scale
Applied Voltage
12.5nm thick Europium-doped ZnO film imaged at a bias of 1.5 volts.
2011 Purdue Workshop
A
B
Point I-V Curves
A
B
•Channels A and B have
different dopant
concentrations
•Material on the
surface is a
contaminant that
penetrates the oxide
2011 Purdue Workshop
Single Molecule CAFM on Polymers
Data Courtesy
Gilbert Walker, University
of Toronto
2011 Purdue Workshop
Photocurrent and Statistics
Courtesy Adam Lazareck and Jimmy Xu, Brown Univ.
2011 Purdue Workshop
Polymer Blend Solar Cells
Sample Setup
2011 Purdue Workshop
Data Courtesy David Ginger,
University of Washington
A Probing Station
Design based on one by Landon
Prisbrey and Ethan Minot,
Oregon State
2011 Purdue Workshop
IV Curves on CNTs
EFM overlay on a grounded CNT Right
side plate is floating, so there is no
signal. Connected CNT shows
electrical signal, indicating they are
conductive
Courtesy Landon Prisbrey
and Ethan Minot, Oregon
State
2011 Purdue Workshop
Single Frequency PFM (Piezo Force Microscopy)
Strain = Bias • d33
•In crystals, d33 ranges
from ~2 pm/V (quartz) to
~500 pm/V (PZT)
•In PFM, we have
Displacements of order of ~pm
•MFP3D AFM measures ~30pm
(Cypher ~10pm)
•Reaching Noise limits!
2011 Purdue Workshop
Piezo Response
Photodetector Signal
DC deflection
ADC
AFM Feedback
HPF
ADC
AC deflection
DAC
Contact Mode
+ AC bias to tip
+ Lock-in on AC deflection
= Piezo Response
Sample
i
Vd cos ωt
X
LPF
Amp = i 2 + q 2
Sine Wave
Synthesizer
Vd sin ωt
X
φ = tan−1
LPF
q
i
R
φ
q
Applied
Voltage
“Down” Polarization
Phase Lead
2011 Purdue Workshop
“Up” Polarization
Phase Lag
piezo-response force microscopy - pfm
Amplitude: VPFM
Phase
PFM
Topography
Amplitude
Phase
Courtesy of S. Jesse. Oak Ridge National Laboratory
2011 Purdue Workshop
THE END
2011 Purdue Workshop