![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.](https://image.slidesharecdn.com/aldtcomasterclass3-141124053857-conversion-gate02/85/Atomic-Layer-Deposition-a-process-technology-for-transparent-conducting-oxides-26-320.jpg)
![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](https://image.slidesharecdn.com/aldtcomasterclass3-141124053857-conversion-gate02/85/Atomic-Layer-Deposition-a-process-technology-for-transparent-conducting-oxides-28-320.jpg)
Atomic Layer Deposition: a process technology for transparent conducting oxides
- 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
- 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