talk delivered by Prof. Paul Chalker, University of Liverpool @ the CDTPV TCO masterclass on Nov 12 2014

1. Atomic Layer Deposition:
a process technology for
transparent conducting
oxides
Paul Chalker
PV-CDT TCO Master class November 2014

2. Outline
• Introduction to atomic layer deposition processes
• In-situ monitoring
• Role of co-reagent - copper oxide based TCO
• Conformal 3D deposition - TiO2 in AAO templates
• Doping in atomic layer deposition - doped ZnO based TCO
• Some conclusions

3. Atomic layer deposition

4. Atomic layer deposition of thin films
ALD is a ‘saturative’ layer-by-layer process

5. Characteristics of ALD processes
a) Flat “ALD window”.
b) Precursor saturation.
c) Linear growth per cycle.
Condensa-on
Incomplete
reac-on
Decomposi-on
ALD
window
Re
evapora-on
Temperature
Growth/cycle
(nm/cycle)
Pulse
dura8on
(s)
Growth/cycle
(nm/cycle)
Cycles
thickness
Film
a)
b)
c)
Saturation
Induction
Linear regime

6. By-products must be
removed here
It has to be
volatile here
Selection of ALD precursors
Plasma
Platen
ALD
valves
Precursors
Pump
Carrier gas
It has to decompose
here when exposed to
the ‘oxidant’

7. Schematic of precursor delivery
Heated delivery line
Heated jacket
Ar purge mfc
Ar bubbler mfc
bubbler
Ch amber
bubbler bubbler
• Precursor delivery
– Heated bubbler.
– Ar carrier gas bubbled
through precursor to aid
transport.
– Allows the use of lower
volatility precursors.

8. Precursor selection - transport
(m.p. -75°C) La(mmp)3
8
• (iPrCp)3La compatible with Cp-Zr
precursor
• n La/2s purge /0.5s H2O /3.5s purge /–
m Zr /2s purge/0.5s H2O /3.5s purge
La(mmp)3 + tetraglyme
( mmp’ = OCMe2CH2OMe )
100
90
80
70
60
50
40
30
20
10
0
0 100 200 300 400 500 600
Weight (%)
Temperature (°C)
(CH3Cp)2ZrCH3 (OCH3)
Bis(methylcyclopentadienyl)methoxymethylzirconium(IV)
(iPrCp)3 La

9. In-situ monitoring methods
Quartz crystal microbalance (QCM) - measures a
mass per unit area by measuring the change in
frequency of a quartz (or GaPO4) crystal
resonator. Sauerbrey equation
9
f0 – crystal natural resonant frequency (Hz),
ρc - crystal density (g/cm3),
μc is the crystal shear modulus (g/cm s2).
!" #!" $!!" $#!" %!!" %#!"
0" 50" 100" 150" 200" 250"
QCM$mass$gain$(rel.)$
Time$(s)$
!"#$%&'"(%)'*")+,-'
./0)'*1-'
1s"
5s"
rate$(rel.)$
0" 50" Growth$1"cycle"
UV"on"
Δm

10. In-situ monitoring methods
In-situ spectroscopic ellipsometry (SE) - measures
a change in polarization as light reflects or
transmits from a material structure. The
polarization change is represented as an
amplitude ratio, Ψ, and the phase difference, Δ.
10
Sr
Hf
SHfO3

11. Role of co-reagent – copper oxide TCO’s
11
Cu2O cuprous oxide:
• p-type conduction via presence of holes
in VB due to doping/annealing.
• Top of the VB from Cu 3d10 states and
are more mobile when converted into
holes.
• MCu2O2 (M=Ca, Sr, Ba) and CuMO2
(M=Al, Ga, In, delafossite) are well
known alloyed Cu2O systems
Structure of the (2×2×2) Cu2O cell with one Cu
vacancy. (A) Simple Cu vacancy structure, (B) split-vacancy
structure. The V in (A) indicates the site
from which the Cu atom is removed. In (B), the large
sphere indicates the displaced copper atom.
M. Nolan, S.D. Elliott / Thin Solid Films 516 (2008)
1468–1472

12. CpCutBuCN - characteristics
Log10P(mTorr) = -4772.5/T(Kelvin) + 15.863
100°C, giving a vapour pressure of 1.183T
Ref:
S.
L.
Hindley,
PhD
Thesis,
Liverpool,
2013.
cyclopentadienyl copper tertiarybutyl isocyanide,

13. CpCutBuCN + O2 plasma è CuO cupric oxide
a) b)
c) d)
O2 plasma
RT Raman λ = 514nm
XPS hν = 1486.6eV
Ref:
S.
L.
Hindley,
PhD
Thesis,
Liverpool,
2013.

14. CpCutBuCN + H2O è Cu2O cuprous oxide
H2O pulse
Ref:
S.
L.
Hindley,
PhD
Thesis,
Liverpool,
2013.

15. Summary – role of co-reagent
• Choice of co-reagent is not arbitrary
• Water, ozone, O2 plasma play a role in surface
chemistry
• Co-reagent influences growth temperature (enthalpy
of reaction)
• Other co-reagent can produce metal (H2, plasma),
nitride (N2/H2 plasma , NH3, hydrazine) or carbide etc.
15

16. Conformal 3D deposition - AAO and NW’s
•
Choice
of
electrolyte
and
anodisa-on
voltage
determine
template
parameters.
•
Pore
diameters
from
<10nm
to
>250nm
can
be
achieved.
Pore
depths
>100μm
Ref:
J.
Roberts,
PhD
Thesis,
Liverpool,
2014.

17. Conformal 3D deposition – control of exposure
Theory by Gordon and Elam, use of ‘stop-valve’ and the exposure was
increased to increase the likelihood of achieving a conformal coating.
Ref:
J.
Roberts,
PhD
Thesis,
Liverpool,
2014.

18. 100
80
60
40
20
0
Depth
profile
for
A007
0
10
20
30
40
50
60
%
of
element
by
weight
Distance
from
template
surface
(μm)
3s Ti dose, 5s Ti hold, 4s Ti purge, 0.05s H2O draw, 1s H2O hold, 10s H2O purge, 300
cycles
60
50
40
30
20
10
Depth
profile
for
A018
TiO2 ALD on AAO – No stop-flow valve – surface sealing
wt%
Ti
wt%
O
wt%
Al
0
0
10
20
30
40
50
60
%
of
element
bt
weight
Distance
from
template
surface
(μm)
wt%
Ti
wt%
O
wt%
Al
Conformal 3D deposition - AAO and TiO2
Ref:
J.
Roberts,
PhD
Thesis,
Liverpool,
2014.

19. 60
50
40
30
20
10
0
EDX
profile
for
sample
PS6
0
10
20
30
40
50
60
%
of
element
by
weight
3s pulse, 260s purge, 58s stop flow, 200s stop flow emptying, 150 cycles
Max Ti = 10%, min = 3.5%
Distance
from
surface
(μm)
wt%
Ti
wt%
O
wt%
Al
Conformal 3D deposition - AAO and TiO2
TiO2 ALD on AAO – With stop-flow valve – deposition throughout
Ref:
J.
Roberts,
PhD
Thesis,
Liverpool,
2014.

20. Experimental & Results:
Conformal 3D deposition – CuO NWs
ALD ZnO coating of CuO Nanowires
CuO nanowires grown by
oxidation of strained high purity
copper metal
150mins in normal atmosphere at
500°C
followed
by
slow
cooling
Ref:
J.
Roberts,
PhD
Thesis,
Liverpool,
2014.

21. Conformal ZnO coating applied
using ALD
Pt supporting strap deposited
over structure using FIB then
cross sectioned using ion beam
ZnO
CuO
Pt
Ref:
J.
Roberts,
PhD
Thesis,
Liverpool,
2014.
Experimental & Results:
Conformal 3D deposition – CuO NWs and ZnO

22. Summary
–
Conformal
3D
deposi5on
22
• ALD is a surface chemical process
• Adsorbate mobility is determined by temperature,
dose and saturation times
• Using a ‘stop-valve’ can add control to dosing regime
• Highly conformal deposition is possible into porous
structures (AAO, zeolites etc) and high surface area
substrates (nanowires, catalysts etc)

23. Atomic layer deposition - doping
P1
P2
2nd monolayer
P2
1st monolayer
P1 ALD
H2O

24. ALD of Transparent Conducting Oxides
- for CdTe based photovoltaics
First solar is leading the way with high volume thin film PV
manufacture and breaking the $1 per watt barrier
Thin film PV (a-Si, CdTe and CIGS) will be a quarter of the
Commissioned:
Oct
2010;
Sarnia;
80
MW
market by 2013
- Transparent electronics
Courtesy of Steve Hall and Ivona Mitrovic, EEE, University of Liverpool

25. Gallium, germanium and aluminium doped ZnO
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 50 100 150 200 250 300 350
Temperature (°C)
Growth rate (nm per cycle)
DEZn
DEZn
Diethyl zinc
TMA
Trimethyl aluminium
TMA
TEGa
TEGa GEME Triethyl gallium
GEME
Tetramethoxy germanium
Co-reagent H2O
• No growth of Ga2O3 or GeO2 with TEGa or GEME
• BUT they can be incorporated as dopants in ZnO

26. Dopant incorporation rates
[Al]
[Ga]
[Ge]
0 2 4 6 8 10 12 14 16 18 20
20
15
10
5
0
Dopant ALD cycle fraction (%)
Dopant / (Dopant + Zn) content in film (%)
• The proportion of [dopant], measured by EDX spectroscopy is proportional to
the dopant precursor ALD cycle fraction.

27. As grown microstructures
Al – doping 150°C Ge – doping 250°C Ga – doping 300°C
5 nm 5 nm 5 nm
• Doped ZnO films are all polycrystalline as deposited across the temperature range
• All have similar microstructures e.g. average grain sizes

28. Sheet resistance versus doping fraction
106
105
104
103
102
[Al] [Ge]
[Ga]
Dopant ALD cycle fraction (dopant / (dopant + DEZn), %)
• Minimum sheet resistance between 4 – 6% [dopant] incorporation
• Gallium doping produces lowest sheet resistances

29. GZO – carrier concentrations and mobility
• Carrier concentrations and
mobilities assessed by Hall
Effect
• Comparable mobilities arise
from similar microstructures
(e.g. TEM’s)
• Higher carrier concentrations
achievable with gallium
compared to germanium
dopants

30. Reflectivity
8.4 x 1020
0 500 1000 1500 2000 2500
Wavelength (nm)
100
80
60
40
20
0
Transmission (%)
Ga:ZnO – optical properties
• IR ‘cut-off’ extended by
reducing carrier the
concentration
• Potential trade-off between
thermal management and
electrical conductivity
• High performance optical
properties
6.9 x 1020
9.4 x 1020

31. AP- MOCVD of the Cd(1-x)Zn(x)S/CdTe device
Reactor cell @ 1 atm
Heated substrate: 200 – 450 oC
H2
Metal-organic
precursors
• GZO coated float glass substrate (front electrical contact)
• Cd(1-x)Zn(x)S n-type window layer (240 nm)
• CdTe p-type absorber layer (2250 nm)
• Cl treatment - in situ CdCl2 deposition & anneal
• Deionized water rinsing of excess CdCl2
• Revealing of TCO and contact with metal paste
• Thermal evaporation of Au onto CdTe (back electrical contact)
Courtesy of Stuart J C Irvine, Daniel A. Lamb, Andrew J. Clayton

32. GZO TCO Device properties
• Best GZO TCO efficiency 12 % directly comparable to NSG TEC C15 SnO2:F TCO
• Average current density/voltage of 16 devices under AM1.5 and a typical J/V curve
for a GZO TCO device
η (%)
10.8
Jsc (mA cm-2)
23.9
Voc (mV)
0.69
FF (%)
65.0
Rs (Ω.cm2)
4.0
Rsh (Ω.cm2)
288
Courtesy of Stuart J C Irvine, Daniel A. Lamb, Andrew J. Clayton

33. Summary of doping
• Al, Ge and Ga doped – ZnO TCO’s are achievable with ALD doping
cycle approach
• Ga-doped ZnO has superior electrical properties to the Al- and Ge-doped
33
ZnO
• ALD TCO’s have ‘comparable’ performance to SnO2:F materials
• Scope to tailor the ZnO – based TCO’s for PV and energy-saving
glass applications
Acknowledgements: Funding through U.K. Technology Strategy Board under project TP11/LIB/6/I/AM092J.

34. Conclusions
• ALD can be used to deposit material with atomic scale
precision uniformly over 3D structures
• Complex compositions are achievable
• Dielectrics, transparent conductors and metallic materials
are feasible
• Application areas span IT, power devices, renewable
energy (and others e.g. optics, displays, energy storage,
catalysts etc.)
34

35. Acknowledgements
Dr Helen Aspinall
Dr Kate Black
Dr Ziwen Fang
Dr Jeff Gaskell
Professor Tony Jones
Dr Peter King
Dr Paul Marshall
Dr Richard Potter
Dr Joe Roberts
Dr Simon Romani
Dr Sarah Hindly
Dr Matt Werner
Dr Jacqueline Wrench