Atomic Layer Deposition: a process technology for transparent conducting oxides

Paul Chalker

PV-CDT TCO Master class November 2014

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

Atomic layer deposition

Atomic layer deposition of thin films

ALD is a ‘saturative’ layer-by-layer process

Characteristics of ALD processes

a) Flat “ALD window”. b) Precursor saturation. c) Linear growth per cycle.

Incomplete  reac-on  

Condensa-on  

ALD  window  

Decomposi-on  

Re  evapora-on  

Temperature  

Growth/cycle    (nm

/cycle)  

Pulse  dura8on  (s)  

Growth/cycle  (nm/cycle)  

Cycles  

Film

 thickn

ess    

a)  

b)  

c)  

Saturation

Induction

Linear regime

It has to be volatile here

By-products must be removed here

Selection of ALD precursors

Plasma

Platen

Precursors

ALD valves

Pump

Carrier gas

It has to decompose here when exposed to the ‘oxidant’

Schematic of precursor delivery

Heated delivery line

Heated jacket

Ar purge mfc Ar bubbler mfc

bubbler

Chamber

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

•  (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

( mmp’ = OCMe2CH2OMe )

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600

We

igh

t (%

)

Temperature (°C)

(CH3Cp)2ZrCH3 (OCH3) Bis(methylcyclopentadienyl)methoxymethylzirconium(IV)

(iPrCp)3 La

(m.p. -75°C) La(mmp)3

La(mmp)3 + tetraglyme

In-situ monitoring methods

9

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

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"

0" 50" 100" 150" 200" 250"

Growth$rate$(rel.)$

Time$(s)$

TMA"with"UV"

TMA"without"UV"

UV"without"TMA"

1"cycle"

UV"on"

Δm

In-situ monitoring methods

10

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.

Sr

Hf

SHfO3

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

Role of co-reagent – copper oxide TCO’s

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

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,

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.    

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.

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.    

Theory by Gordon and Elam, use of ‘stop-valve’ and the exposure was increased to increase the likelihood of achieving a conformal coating.

Conformal 3D deposition – control of exposure

 Ref:  J.  Roberts,  PhD  Thesis,  Liverpool,  2014.    

TiO2 ALD on AAO – No stop-flow valve – surface sealing

0  

20  

40  

60  

80  

100  

0   10   20   30   40   50   60  

%  of  e

lemen

t  by  weight  

Distance  from  template  surface  (μm)  

Depth  profile  for  A007  

wt%  Ti  

wt%  O  

wt%  Al  

0  10  20  30  40  50  60  

0   10   20   30   40   50   60  

%  of  e

lemen

t  bt  w

eight  

Distance  from  template  surface  (μm)  

Depth  profile  for  A018  

wt%  Ti  

wt%  O  

wt%  Al  

3s Ti dose, 5s Ti hold, 4s Ti purge, 0.05s H2O draw, 1s H2O hold, 10s H2O purge, 300 cycles

Conformal 3D deposition - AAO and TiO2

 Ref:  J.  Roberts,  PhD  Thesis,  Liverpool,  2014.    

3s pulse, 260s purge, 58s stop flow, 200s stop flow emptying, 150 cycles Max Ti = 10%, min = 3.5%

0  

10  

20  

30  

40  

50  

60  

0   10   20   30   40   50   60  

%  of  e

lemen

t  by  weight  

Distance  from  surface  (μm)  

EDX  profile  for  sample  PS6  

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.    

Experimental & Results: 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

Conformal 3D deposition – CuO NWs

 Ref:  J.  Roberts,  PhD  Thesis,  Liverpool,  2014.    

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    

•  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)

Atomic layer deposition - doping

P1

P2

ALD P1 P2

1st monolayer

2nd monolayer H2O

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 market by 2013 Commissioned:  Oct  2010;  Sarnia;  80  MW  

ALD of Transparent Conducting Oxides - for CdTe based photovoltaics

- Transparent electronics

Courtesy of Steve Hall and Ivona Mitrovic, EEE, University of Liverpool

Gallium, germanium and aluminium doped ZnO

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200 250 300 350

Temperature (˚C)

Gro

wth

rate

(nm

per

cyc

le)

DEZn

DEZn Diethyl zinc

TMA

TMA Trimethyl aluminium

TEGa TEGa Triethyl gallium GEME

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

Dopant incorporation rates

[Al]

[Ga]

[Ge]

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

5

10

15

20 D

opan

t ALD

cyc

le fr

actio

n (%

)

Dopant / (Dopant + Zn) content in film (%)

•  The proportion of [dopant], measured by EDX spectroscopy is proportional to the dopant precursor ALD cycle fraction.

As grown microstructures

Ga – doping 300˚C Al – doping 150˚C Ge – doping 250˚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

Sheet resistance versus doping fraction

•  Minimum sheet resistance between 4 – 6% [dopant] incorporation

•  Gallium doping produces lowest sheet resistances

106

105

104

103

102

Dopant ALD cycle fraction (dopant / (dopant + DEZn), %)

[Al] [Ge] [Ga]

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

0 500 1000 1500 2000 2500 Wavelength (nm)

100

80

60

40

20

0

Tran

smis

sion

(%)

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

8.4 x 1020

9.4 x 1020

Reflectivity

AP- MOCVD of the Cd(1-x)Zn(x)S/CdTe device

•  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)

Heated substrate: 200 – 450 oC

Reactor cell @ 1 atm H2

Metal-organic precursors

Courtesy of Stuart J C Irvine, Daniel A. Lamb, Andrew J. Clayton

GZO TCO Device properties

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  

•  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

Courtesy of Stuart J C Irvine, Daniel A. Lamb, Andrew J. Clayton

Summary of doping

33

•  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 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.

Conclusions

34

•  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.)

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