Post on 27-Jun-2020
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 3
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 4
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 5
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 6
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 7
defect properties
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 8
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 9
ZnO – Defects
compensation of donors by Zinc-vacancies (VZn)
under oxygen rich conditions
∆Hdef
EF
Akzeptor
Donator
EDEA
pinning position
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 10
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 11
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)
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 12
Assessment of surface and
interface properties
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 13
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 14
Integrated System – DAISY-MAT
electronanalyser
UV-sourceIon-source
monochromatedX-ray source
sample handler
load lock
magnetron sputtering
MBEorganics,metals
MOCVDALD
Preparation
Analysis
magnetron sputtering
magnetron sputtering
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 15
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 16
work functionsof TCOs
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 17
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 18
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 20
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 21
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 22
Oxygen Exchange
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 23
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)
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 24
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 25
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 26
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 27
Relaxation at low pressure
Relaxation observed when starting from reduced surface
Oxidation of surface faster than oxidation of bulk
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 28
Oxygen exchange of SnO2
Surface termination is important for oxygen exchange
reduced-SnO2 SnO2/In2O3oxidized-SnO2
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 29
Interface Formation
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 30
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 31
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 32
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.
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 33
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 34
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 35
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.
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 36
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 37
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.
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 38
SnO2/Pt – interface chemistry
Reversible oxidation/reduction of Sn
Sn 3d
490 488 486 484binding energy [eV]
coun
t rat
e [a
rb. u
nits
]
oxyg
en p
ress
ure
T=150°C
oxidation @ 150°C reduction @ 150°C
490 488 486 484binding energy [eV]
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 39
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 40
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
2010-09-28 | Andreas Klein | TCO Surfaces and Interfaces | 41
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)