Surfaces and Interfaces of Transparent Conducting Oxides › fahl › FAHL2010 ›...

Surfaces and Interfaces of Transparent Conducting Oxides

Andreas Klein Technische Universität Darmstadt, Institute of Materials Science, Surface Science Division

2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 2

Transparent conducting oxides

• high electrical conductivity (>103 /Ωcm)

• high optical transparency (>80%)

• high infrared reflectivity

dR

T

Realization:

Degenerately doped oxides (d10-systems)

(ZnO, In2O3, SnO2, CdO and mixed oxides)

CB

VB

EF

Ener

gy

Eg>3eV


Thin film solar cells

SEM courtesy ZSW

Cu(In,Ga)(S,Se)2 CdS ZnO

n+i

EVB

ECB

∆ECB

∆EVB

ener

gy

TeITO SnO2 CdS CdTe Te Met

glassMo

CIGS

CdSZnO

TCOCdS

metal

glass

CdTe

EF at interface


The TCO electrode in OLEDs

grain orientation determined by surface energy + ???

Work function determined by surface orientation and

termination

Hole injection determined by work function

Oxygen exchange determined by surface

properties

Defect concentration (e.g. Oi) and mobility determined by material properties and doping

cathode


Chemical gas sensors

semicond.

EF

surface/metal

EVB

ECB change of surface potential by adsorption

Sensor response explained by

surface potential changes

(ionosorption model)

Changes of SnO2 bulk doping,

(e.g. changes in [VO]) neglected

SnO2

contactsensitizer Pt

R


Issues of TCO surfaces and interfaces

Surface properties

Workfunction of TCOs

Oxygen Exchange at TCO surfaces

Interfaces Properties

Reactivity of interfaces

Energy level alignment (barrier heights)

TCO oxide surfaces and interfaces are considerably more

complex than those of conventional semiconductors


defect properties


Defects in semiconductors

defect concentration

• Vacancies (Vcation (acceptor), Vanion (donor) )

• Interstitials (Cati (donor), Ani (acceptor) )

• Antisite defects (e.g. GaAs and AsGa in GaAs )

• Schottky defect pair (Vcat + Van), (Cati + Ani)

• Frenkel defect pair (Vcat + Cati), (Van + Ani)

• F-centers (vacancy + e, h)

• Polarons

• Electrons and holes

• Impurities (substitutional, interstitial)

not stoichiometric,charged

stoichiometric,uncharged

∆−⋅≈

TkhNNB

dd exp

oxygen interstitial in ZnO


ZnO – Defects

compensation of donors by Zinc-vacancies (VZn)

under oxygen rich conditions

∆Hdef

EF

Akzeptor

Donator

EDEA

pinning position


ZnO – Fermi level

EF

ECB

50403020100

Oxygen content in sputter gas [%]

EF-

EVB

[eV]

2.6

2.8

3.0

3.2

3.4

3.6

3.8

VB

CB

i-ZnOZnO:Al

Change of doping with pO2

Pinning by Zn-vacancies

low O2 content

Highly doped film

high O2 content

Low doped film pinning position


Intrinsic defects of TCOs

Conductivity determined by doping and intrinsic defects (Stoichiometry)

Changes of conductivity by changes of stoichiometry require oxygen exchange and diffusion

material defects

ZnO VZn, Zni, VO

ZnO:Al AlZn, VZn

In2O3 VO

In2O3:Sn SnIn, Oi

SnO2 VO

SnO2:Sb SbSn, ??

exchange

diffusion

p(O2)


Assessment of surface and

interface properties


Thin film deposition

plasma

cooling

RF/DC power

magnetic field

substrate

NSNtarget

radiative heater

(halogen lamp)

gas

magnetron sputtering Wide range of materials

low substrate temperatures

epitaxial growth possible

wide range of parameter variation

high deposition rates

large area deposition


Integrated System – DAISY-MAT

electronanalyser

UV-sourceIon-source

monochromatedX-ray source

sample handler

load lock

magnetron sputtering

MBEorganics,metals

MOCVDALD

Preparation

Analysis




Photoemission – Barrier Heights

counts

EF-EVB

binding energy 0=EF

core level valence band

EVB

ECB

BE (CL)

ECL

energy diagram

Schottky barrierInterface experiment

1: substrate 2-N: thin film (0.1-10 nm)

N+1: thick film (>10 nm)

hνe-

∆EVB,CL

EF-EVB

∆EVB,CL

EF-E

VB

thickness0

Eg

0

ΦB,n

ΦB,p

hν-workfunction


work functionsof TCOs


SnO2 – ionization potential

oxidized “stoichiometric“ (110) surface

reduced (110) surfaceE

vac-

EV

B[e

V]

9.0

8.5

8.0

2520151050

SnO2:Sb

SnO2

Oxygen content in sputter gas [%]

Evac

ECB

EVB

EF

IPφ


SnO2 – work function

Identification of surface termination by ionization potential

Evac

ECB

EVB

EF

Thin Solid Films 518, 1197 (2009)

5.5

5.0

4.5

wo

rk f

un

ctio

n [

eV

]

4.03.83.63.43.23.0

ECB

SnO

2:F

EF-EVB [eV]

SnO2

SnO2:Sb

pO2

2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 19 J. Phys. D 43, 055301 (2010)

• Change of stable surface orientation with oxygen pressure

norm

ierte

Inte

nsitä

t [w

illk. E

inh.

]

908070605040302010

2Θ [°]

110

101

111

211

112

222

220

002

321

100% Ar

1% O2

2% O2

5% O2

10% O2

25% O2

(110)

(101) xO

2[%

]

XRD

norm

aliz

ed inte

nsi

ty

SnO2 – preferred orientation


Work function of In2O3 and ITO

Almost no change of surface termination with oxygen

Work function depends on surface orientation

Differences between In2O3 and ITO explained by texture of films

Surface oxidation (e.g. via ozone) only possible for (100) orientation

(100)

(111)

DFT


TCO work functions

5.5

5.0

4.5

4.03.83.63.43.23.0

SnO

2:F

EF-EVB [eV]

SnO2:Sb

SnO2

3.63.22.82.42.0

EF-EVB [eV]

5.5

5.0

4.5

4.0

ITO5.0

4.5

4.0

3.5

3.63.22.82.4

work

funct

ion [

eV]

EF-EVB [eV]

ZnO

ZnO:Al

Thin Solid Films 518, 1197 (2009)

Large variation of work function

but ∆EVB does not depend on work function


Oxygen Exchange


ITO/organic interface

OOOO

O OOO O

O

ITO ITO + O2

EVB

ECB

EF

HOMO

ZnPC

ener

gy

292 288 284

0.4Å

536 532 528

binding energy [eV]-11

O1s C1s UPS

0.8Å

0Å

536 532 528 1 -1292 288 284

C-O

binding energy [eV]

O1s C1s UPS

ΦB

J. Phys. Chem. B 110, 4793 (2006)


Conductivity relaxation of ITO (1 bar)

6 ppm16 ppm

1010 ppm

0.97 %

9.94 %

97.6 %

Conductivity depends on oxygen pressure

Slope related to dominant defect species

Log (pO2)

Log

(con

duct

ivity

)

-1/8

T=590°C

d=400nm


Conductivity relaxation of SnO2 (1 bar)

Almost no change of σ with pO2 at 400°C

Equilibrium carrier concentration not achieved

time [h]

con

du

ctiv

ity [

S/

cm]

oxyg

en

pre

ssu

re [

Pa]400°C8

6

4

2

03024181260

10-1

100

101

102

103

104

105

time [h]co

nd

uct

ivit

y [

S/

cm]

oxyg

en

pre

ssu

re [

Pa]600°C

3.0

2.5

2.0

1.5

1.0

0.5

0.0363024181260

10-1

100

101

102

103

104

105


Surface modification

Exchange at SnO2 possible with 1nm In2O3 on surface

144012009607204802400time [min]

400

300

200

100

0

tem

per

ature

[°C

]

0.01

0.1

1

10

conduct

ivity

[S/c

m]

9607204802400

time [min]

σT

0.6Pa O2 0.6Pa O2

SnO2 SnO2/In2O3P = 1 bar


Relaxation at low pressure

Relaxation observed when starting from reduced surface

Oxidation of surface faster than oxidation of bulk


Oxygen exchange of SnO2

Surface termination is important for oxygen exchange

reduced-SnO2 SnO2/In2O3oxidized-SnO2


Interface Formation


CdS/ZnO – substrate oxidation

CdS/ZnO CIGS/ZnO In2S3/ZnO

no oxidation of substrate oxidation of substrate

425 415166 160 1115 1100 1085 1075binding energy [eV]

inte

nsi

ty [

arb.

units]

S 2p Cd MNN Ga LMM In MNN

binding energy [eV]

Zn

O f

ilm

th

ickn

ess


CdS/ZnO – initial growth

534 530 534 530binding energy [eV]

inte

nsi

ty [

arb.

units]

CdS/ZnO CIGS/ZnO

surface species

Zn

O f

ilm

th

ickn

ess CIGS or In2S3ZnO or CdS

Zn

O2

dissociation of O2 not possible

dissociation of O2 possible

catalytic activity of surface responsible forsubstrate oxidation

non-reactive reactive

O 1s


CdS/ZnO – interface properties

band alignment

polycrystalline,preferred

orientationχ = 3,6 eV

polycrystalline,statisticalorientationχ = 4,5 eV

amorphous

polycrystallineχ = 4,45 eV

CdS

ZnO

microstructure

amorphous

deposition time [s]

EF

-E

VBM

[eV]

1,2 01 eV

200010000

2.0

2.4

2.8

3.2

3.6 ~2 nm

Venkata Rao et al., Appl. Phys. Lett. 87 (2005), 032101.


CdS/ZnO band alignment

details explained in: Ellmer, Klein, Rech Transparent Conductive Zinc Oxide (Springer, 2008), chap 4

Large variation of band alignment

∆EVB depends on

Deposition sequence

ZnO dep. temperature

ZnO doping


CdS/ZnO Fermi level pinning

4.0

3.5

3.0

2.5

2.0

1.5

250200150100500

deposition time [sec]

1.2eV

2.2eV

1.8eV

CdS/ZnO:Al

EF-E

VB

[eV

]

1.6eV

Ellmer, Klein, Rech Transparent Conductive Zinc Oxide (Springer, 2008), chap 4

Fermi level in CdS substrate restricted to 2.0 ± 0.2 eV

50403020100

deposition time [min]

ZnO:Al/CdS

ZnO

:Al

CdS

high doping

low doping

1.4eV


In2S3/ZnO Fermi level pinning

ZnO deposition time [sec]2001000

4

3

2

1

2001000

In 3dS 2pZn 2pO 1s

EF-E

VB

[eV

]

2.8 eV1.9 eV

ZnO:AlIn2S3

250°C

ZnO:Al

+O2

In2S3

250°C

3.9

1.1

2.7

2.8

1.1

1.8

EVB

ECB

EF

(a) (b) (a) (b)

No band bending in In2S3 and ZnO

Band alignment defined by doping levels

Klein et al., J. Mater. Sci. 42 (2007), 1890.


TCO / CdS – band alignment

VB

CB

1.5

SnO2 CdS

2.4

3.5

1.4

ZnO CdS

3.3

EF

0.8

In2O3 CdS

2.7

1.2

ITO CdS

2.7

Valence band maxima of TCOs at comparable energy

but: Band alignment influenced by TCO gap and doping

due to Fermi level pinning in CdS

In2O3 CdS

1.5

2.7

Fermi level not within “acceptable“ range


SnO2/Pt – interface formation

10 5 0

VB

78 74 70

Pt 4f

490 486

Sn 3d5/2

0s

1s

2s

4s

8s

16s

32s64s128s1050s

Inte

nsity

534 530

O 1s

Binding energy [eV]

32s

Reduction of SnO2 during deposition

Surf. Sci. 602 (2008), 3246.


SnO2/Pt – interface chemistry

Reversible oxidation/reduction of Sn

Sn 3d


coun

t rat

e [a

rb. u

nits

]

oxyg

en p

ress

ure

T=150°C

oxidation @ 150°C reduction @ 150°C


Sn4+ Sn0

• 150°C: Sn0 <-> Sn4+

with intermediate Sn2+ state

• 100°C: Sn0 <-> Sn2+

• Oxidation/reduction not observable for- large Pt islands- bare SnO2 surface


Chemistry at buried interface

SnO2

EF

Pt

EVB

ECB

SnO2

EF

Pt

EVB

ECB

Reducing environment Oxidizing environment

H

OH

O

OO

SnO2 Pt SnO2 Pt

Sn0 Sn4+

Oxygen is reversibly transported to/from the interface


Summary

Work functions depend on doping, surface orientation (ZnO, In2O3, SnO2?) and surface termination (In2O3, SnO2)

Oxygen exchange generally possible for In2O3 but can be suppressed at stoichiometric SnO2 surfaces

Reactivity of interfaces determined by catalytic activity of surface for dissociation of oxygen

Energy band alignment characterized by similar valence band energies but may be affected by Fermi level pinning

Schottky barrier heights can depend on oxygen pressure


Thanks

Frank Säuberlich, Yvonne Gassenbauer, ChristophKörber, André Wachau, Mareike Hohmann, KarstenRachut (Surface Science)

Paul Erhart, Péter Ágoston, Karsten Albe (Materials Modelling)

S.P Harvey, T.O. Mason (Northwestern University)

BMBF (ZnO), DFG (SFB 595), DFG-NSF (Materials World Network)

Surfaces and Interfaces of Transparent Conducting Oxides › fahl › FAHL2010 ›...

Documents

Transcript of Surfaces and Interfaces of Transparent Conducting Oxides › fahl › FAHL2010 ›...