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Electrical Characterisation of TCO
thin films (method of four coefficients).
Eric Don, SemiMetrics Ltd.
Functional Thin Films
4th Vacuum Symposium
Thursday 17th October 2013
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Agenda
• TCO Basics
• TCO Applications
• TCO in-line quick comment
• TCO off-line characterisation
• Results correlation with NREL
• Questions
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Transparent Conducting Films - Basics
• SnO is a good example
• ITO electrically even better…
Except for the indium cost
• Hence the search for new
Transparent conducting films
Photon energy (eV)
TCO window of transmission
Ewp Egap
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
0 11
Transmission
Reflection
IR
Absorption
UV
σ = neμ
Currently TCO’s rule
• Wide band gap ~ 3eV
• Degenerate semiconductor
• Reasonable mobility
Impact on the
transmission window
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TCO’s many consumer applications .
LCD
> 80% total power is
for the display
(Imax = 1.5 A,4” x 3”)
OLED
Defrost Windows FET backplane
now IGZO
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Cold Climate
Warm Climate
TCO low-emissive windows – Electrical Properties
100x10 6
80
60
40
20
0
Black
bod
y R
adian
cy (W
/m
3 )
3 4 5 6 7 8 9
10 3
2 3 4 5 6 7 8 9
10 4
2 3
Wavelength (nm)
100
80
60
40
20
0
Per
cen
t T
ran
smis
sion
/ R
efle
ctio
n
AM1.5
Ideal Cold Climate Window
Blackbody Radiation
350 K
(µ = 10)
100
80
60
40
20
0
Per
cen
t
3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 10
2 3 Wavelength (m m)
100x10 6
80
60
40
20
0
Black
bo
dy R
adian
cy (W
/m 3 )
Blackbody Radiation
350 K
Ideal Warm CIimate Window,
AM1.5
(µ = 100)
1.0
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First Solar Inc.
1GWatt peak
in one year
Glass TCO CdS CdTe
Back Contact
100
80
60
40
20
0 P
erce
nt
Ref
lect
ion
3000 2500 2000 1500 1000 500
Wavelength (nm)
AM 1.5
Ideal Single Bandgap cell = 1.5 eV
Ideal TCO µ=100,
n = 1.9 x 1021
CTO µ = 62, n = 6.8 x 1020
(CdTe cell)
ZnO µ = 22,
n = 4.7 x 1020
(1.0 eV CIS cell)
Thin Film Solar Cells
In context:
28 miles2 windows were installed,
in one year (U.S. & Europe).
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7
Option eddy current
µ
In-line Electrical resistivity measurement
Wafer
Eddy current (non-contact) An AC current flows in a coil. The magnetic field of the
coil induces circulating (eddy) currents in the sample.
The eddy current measurement is actually the
measurement of the electrical loss in the material,
which depends on the resistivity.
Four Point Probe (Contact) Measuring resistivity and sheet resistance
according to SEMI MF84-0307 and
SEMI MF374-0307 standards.
Probe wear film damage.
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In-line resistivity & mobility control.
Optical method-IR reflection (non-contact)
Hyperspectral camera
Line light source
Roll-to-roll material
Additional opaque layers or other further processing
Computer
Calculation processor
Manufacturing equipment
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• t , relaxation time for carriers - “Extrinsic Parameter”
Crystal defects
Impurity scattering
Grain size
Grain barrier.
Off-line Electrical Characterization
• m*, effective mass “Intrinsic Parameter”
Determined by Band/Crystal structure
m qt
m*
• We need to go even deeper than mobility
• m = Velocity per unit electric field
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Relaxation Time Approximation
• Boltzmann’s equation may be solved by approximation
• t is the mean time between redistribution of carriers
• t can be a function of temperature and energy
Degenerate,
parabolic band
materials.
t a( T )Es
Then by plotting temp dependence of ; m
Acoustic phonon scattering:
Neutral impurity scattering:
Ionized impurity scattering:
t T - 1
E - 1 2
t TbE
0
t T0E
3 2
b > 0
m
m
m
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Method of four coefficients
• Measurement of Conductivity (s), Hall (Rh), Seebeck (α ) and Nernst (Q) coefficients gives m*,
M.K. Zhitinskaya et al.’66
m d *
3 n
p
2 3 q h 2
k B
2 T a -
Q
Rh s
m q m* t
Rh s m •Four coefficients (α , Rh, s, Q) give m*d and s.
• m*d measurement independent of l
(the non-parabolicity)
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Transport phenomena measurement on thin-films
V
Conductivity (van der Pauw)
V
Bz
Hall effect
Seebeck effect Nernst effect
V Bz
V
T
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Figure 3, D. L. Young, Review of Scientific Instruments
Schematic of 4C System
Computer GPIB Bus Line
Temperature Controllers (2)
Electromagnet
Sample
Cryostat
Switch Matrix
WindowsTM 4C Application Software
Bipolar DC
Power Supply
Current Source
Voltmeter
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Four coefficients – Next
• Measurement of Conductivity, Hall, Seebeck and Nernst
coefficients gives s and band non-parabolicity
s 3
2
QRs
a -QRs
ll
n
md*
dmd*
dn
Band non-parabolicity term
M.K. Zhitinskaya et al.’66
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Correlation with NREL
Sample ID Temp Thickness Resistivity Hall Coeff Seebeck Coeff Nernst Coeff
(K) (nm) (Ohm.cm) (m^3/C) (uV/K) (uV/K)
H3522 UNN A 308 1.50E+03 2.81E-03 7.60E-08 -1.12E+02 -9.16E-03
H3522 NREL 295 1.50E+03 2.81E-03 7.14E-08 -1.34E+02 -3.70E-02
System= 22(uV/K)
Sample ID Temp Thickness Doping Mobility DOS mass Tau (s)
(K) (nm) (cm^-3) (cm^2/Vs) (m/me)
H3522 UNN A 308 1.50E+03 -8.22E+19 2.71E+01 7.02E-01 1.08E-14
H3522 NREL 295 1.50E+03 -8.74E+19 2.51E+01 9.76E-01 1.41E-14
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Conclusion • The method of four coefficients is an ideal experimental
technique for transparent conductive oxides (TCO) and
other “low” mobility samples such as semiconductor or
metal thin films but equally can be used for single crystal
or epitaxial thin film semiconductors.
Four transport phenomena measurements:
Conductivity
Hall
Seebeck
Nernst
n, µ m*, s, EFermi
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SemiMetrics 4C System
is based upon work done at NREL
and Colorado School of Mines (CSM)
Colorado USA.
We specifically acknowledge:
D. L. Young, NREL, CSM
V. I. Kaydanov, (CSM)