1

Ekaterine Chikoidze

GEMaC, CNRS,Versailles University, France

Transparent Oxides for Electronics:

problem of electrical transport

2

OXIDES: Transparent & Conducting

Heat reflecting windows, electrochromic windows, LCD panel, OLED, solar cells,

touch panels ,….

Advantege of oxides:

Higher voltage tolerances;

More stability at high temperatures;

Oxygen tolerance, opening up gas and air monitoring applications;

The ability to make transparent electronics

Magnetic sensors

Goodbye silicon, hello oxides!!!

Expect market value for TCOs in coming years will be more then 200 billions $

3

Figure of Merit for TCO

/ = -Rsln(T+R)-1

R.Gordon MRS Bull.2000

Electrical Conductivity

Optical absorption

The first TCO by J.M.MOCHEL, U.S. PATENT NO.2,564, (1947) 706

4

n type TCO

The cost of the raw materials

Cd < Zn <Ti < Sn <Ag <In

Toxicity of the elements

Zn< Sn < In< Ag < Cd R.Gordon,MRS Bull.2000

ZnO

F

Cl

I

Br

B

Al

Ga

In

Zr

Ti

P

As

VII group

Important criteria for TCO:

1018

1019

1020

0

5

10

15

20

25

30

35 sapphire

fused silica

µ (

V-1s

-1c

m2)

Carrier concentration (cm-3)

ZnO:Cl

Phys. stat. sol. (a) 203,2007

0 50 100 150 200 250

1018

1019

1020

on sapphire

on fused silica

Ca

rrie

r co

nce

ntr

atio

n (

cm

-3)

n-BuCl pressure, (Pa)

By MOCVD

1% Cl

5

n type TCO

AZO by PLD

Thin Solid Films 516 (2008) 8141–8145

Transmittance

T(VIS)=78-82%

T(NIR)≈ 86%

ZnO:Cl

ρ=1.3x10-3Ohm cm

n=6.1x1020cm-3

µ=7 cm2/Vs

1 2 3 4 5 610

15

20

25

Ele

ctr

on m

obili

ties, cm

2/V

s

x1021

Carrier Concentration (cm-3)

AZO

2%Al ρ=1.2x10-4Ohm cm

n=1.2x1021cm-3

µ=18 cm2/VS

T(VIS)=85%

6

p type TCO

WBG metal-oxydes

Most are Insulators ;Few are naturally n-type; rarely naturally p-type

The monopolarity in existing TCOs results from strong localization of positive holes

at oxygen 2p levels or an upper edge of the valence band due to the strong

electronegative nature of oxygen.

Conductivity type inversion in n-type TCO by cation doping SnO2, CdO, ZnO,In2O3….

Difficulty (solubility limit, killer defects, deep levels)

Delocalize holes by reduction of Columb interaction

delafossites : CuAlO2, (Kawazoe et al, 1997)

CuGaO2;CuInO2;CuCrO2;CuBO2; Problem: very high deposition T, indirect band gap

« Chemical modulation of VB »

Transparent p-n junction: oxide with direct band gap, low deposition T,

high quality epi-layers

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Cu based p type TCO

Cu2O (archetype of Cu based TCO)

V.B.Hybrydised Cu 3d & O2p

localized holes

Polaronic-hopping conduction (R.S.Toth, Phys.Rev. 122,482, 1961;

L.C.Bourne,PRB 40,10973,1989)

D.O.Scanlon, PRL, 103,096405, 2009

K.G.Godhino, J.Matt.Chem.20,1086,2010

SrCu2O2

300 600 900 1200 1500 1800 2100 2400 2700 3000

0

20

40

60

80

100

T [%

]

[nm]

3% Ba doped SCO, Tdep

=450oC, p

O2= 10

-5 mbar

6% Ba doped SCO, Tdep

=350oC, p

O2= 10

-5 mbar

T(VIS)=75-80%

Coll: Tyndall National Institute University College Cork, Ireland;

Thales TRT, France

Delocalization of holes

VCu 3d10Cu(I) 3d9Cu(II)

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Introducing holes in SrCu2O2 by Ba doping

Hybrid DFT

New localized state in band

gap 1.1 eV above VB

Structure Eg / HSE06

SrCu2O2 3.20 eV

Ba-SrCu2O2 3.24 eV

Ba changes band gap but not the polaronic nature of the material

O

Sr

Cu

SrCu2O2

Holes Predominantly localised on Oxygen atoms

near vacancy site – Polaron

Ba:SrCu2O2

M.Nolan

Tyndall National Institute ,University College Cork, Ireland

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Conduction Mechanism of Ba:SCO

2 4 6 8 10 12 14

-14

-12

-10

-8

-6

-4

-2

0

ln[

(Oh

m.c

m) -1

]

1000/T(K-1)

Ba:SrCu2O

2

3%Ba

Ea=0.12eV = 1+2

NNH

1= 0 exp(-WH/kBT)

Constant Activation energy

Ea=[Ea(p(T));Ea(µ(T)]

Ea=Ea(µ(T)]=WH polaron hopping energy WH=120 meV

polaron binding energy Ep=240eV

0.15 0.20 0.25 0.30 0.35

-14

-12

-10

-8

-6

-4

-2

0

2

ln[

(Oh

m.c

m)-1

]

1/T1/4

(K)-1/4

VRH

2= 0 exp(T0/T)-1/4

Activation energy

T0 ~β/kNEf r p

T0=9x104K

Mobility is thermally activated

1

2

Polaron conductivity

@300K ρ=79 Ω.cm

σ=1.2x10-2Scm-1

p= ~1017cm-3

µ<1 V-1s-1cm2

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Influence of Ba doping on Conductivity of SCO

%Ba

0 (Scm-1)

1x10-3

µH(cm2/VS) Ep eV

Polaron

binding energy

3 3x10-2 <1 240

6 1.2x10-2 <1 180

Ba:SCO keeps good transparancy in VIS (with small open of Gap)

Ba doping induces less localised holes

Problem remains: low mobility of carriers

Delocalization of carriers by doping another dopant?

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Ilmenite FeTiO3

Eg=3.5 eV

Antiferromagnet TN =55K

By PLD

Growth PO2=10-7 – 2 mTorr

vacuum 0.5 1.0 1.5 2.0 2.5-8

-6

-4

-2

0

2

ln(C

onductivity)

Po2(mtorr)

Conductivity increases

with oxygen partial pressure

200 250 300 350 400

102

103

104

Re

sis

tivity(O

hm

.cm

)

T(K)

Semiconducting

behaviour

FTO- IR transparent conducting magnetic Oxide

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Thermodynamical analyses

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2

10

11

12

13

14

15

16

17

18

Ppa

B2

[VFe

']= p[VFe

']= [VO2

.][V

O2

.],=n

[VFe

']

[VO2

.], n

p

log

co

ncen

trati

on

(c

m-3

)

log PO2

(atm)

p

n

[VFe']

[VO2.][V

Fe'], [V

O2

.]

[VFe

'], p

[VO2

.]

n

Pna

B2

n type p type insulator

T=4400C Experiment:

Treatment in oxygen atmosphere

PO2=5.6x10-4atm

p

PO2=2x10-13atm

n

Coll: T.Tchelidze, TSU, Georgia

Conductivity type inversion

Conductivity type inversion in FTO

Hole conductivity PO2>6x10-5atm

Electron conductivity PO2<3x10-13atm

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Conclusion:

Present challenge is to get low temperature deposition of

high quality p type type TCO…..

Future belongs to

‘‘Invisible Electronics’’

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0.050 0.055 0.060 0.065 0.070 0.075-12

-10

-8

-6

-4

ln(c

ond

uctivity)

T-1/2

(K-1/2

)

T0=5.3x10

4K

ES VRH

Hopping conductivity