Electrical Characterisation of TCO

thin films (method of four coefficients).

Eric Don, SemiMetrics Ltd.

Functional Thin Films

4th Vacuum Symposium

Thursday 17th October 2013

Agenda

• TCO Basics

• TCO Applications

• TCO in-line quick comment

• TCO off-line characterisation

• Results correlation with NREL

• Questions

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

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

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

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).

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.

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

• 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

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

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)

Transport phenomena measurement on thin-films

V

Conductivity (van der Pauw)

V

Bz

Hall effect

Seebeck effect Nernst effect

V Bz

V

T

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

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

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

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

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)