Selective-Area Atomic Layer Deposition of Copper Nanostructures for Direct Electro- Optical Solar Energy Conversion

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1 Selective-Area Atomic Layer Deposition of Copper Nanostructures for Direct Electro- Optical Solar Energy Conversion Brian Willis UCONN Cancun, MX 2014

2 Plasmonics Au, Ag, Cu Nanoparticles interact strongly with visible/ near IR radiation, tunable by size/shape. Van Duyne, Nat. Mater E Quasi-Static Approximation α = 4πa 3 ε(ω) ε m ε(ω) + 2ε m Resonance at Re [ε(ω)] = -2ε m plasmons=collective modes that strongly concentrate light at the nanoscale. Plasmonic nanoparticles have applications for localized heating, SERS, nanophotonics, catalysis, sensors and more

3 Plasmonic Dimers hot spots are created by plasmonic dimers Nordlander, Science 2014 Geometric effect concentrates electric fields at nano-gaps leading to large field enhancements. EM field induces a voltage drop across the nanoelectrodes.

4 Plasmonic Enhancement Geometric Asymmetry Mayer, J. Phys. Conds. Mater nm radius tip Ag tip lightning rod effect <i+>/<i-> is rectification ratio Theoretical studies predict large enhancements in rectification ratios for plasmonic resonances. Lightning rod effect concentrates field at tip = asymmetry.

5 Optical Rectification Antenna Diode DC Filter Load I I( V ) I Concept: direct conversion of EM radiation (solar) into DC power by antenna coupled high speed diodes to rectify optical frequency charge waves. Potential Advantages: low cost fabrication, tunable absorbance including IR (waste heat), and device integration. DC photo I( V DC ) 1 V 4 2 photo 2 I V No band-gap limitations! 2

6 Literature Data Ward, Nat. Nano. v. 5, 732 (2010) Ward et al have demonstrated optical rectification in an electromigration junction. - not tuned for plasmonic resonance - not scalable - no control of geometry - isolated nanostructures

7 Rectenna Concept Electrically connected Sized for plasmon resonance in visible Asymmetric geometry Nanoscale tunnel gap Not commercially available!

8 Rectenna Challenges Nanoscale antenna tuned to visible/near-ir Geometric asymmetry, diode response at low voltage Electrically contacted, tunneling devices RC time constant, impedance matching Extremely fast diodes, Hz Diode is most critical need!

9 Rectenna Concept Electrically connected Asymmetric geometry Sized for plasmon resonance in visible Nanoscale tunnel gap ALD used to make tunnel diodes

10 ALD Metals Review Year Reactants GPC T window (nm) ( C) type 1998 Cu(II)thd 2 // H thermal 2003 bis(n,n -diisopropylacetamidinato)dicu(i) // H thermal 1,3-Diisopropyl-imidazolin-2-ylidene Cu(I) hexamethyldisilazide 2013 // H radicals plasma 2014 Cu(II)(OCHMeCH 2 NMe 3 ) 2 // BH 3 (NHMe 2 ) thermal 2007 (2,2-dimethylpropionato) Ag(I)triethylphosphine // H radicals plasma 2010 (hfac)ag(i)(1,5-cod) // propanol thermal 2011 Ag(I)(fod)(PEt 3 ) // H radicals plasma 2014 (hfac)ag(i)(pet 3 ) // HCHO thermal Au?? Choices for ALD Cu, Ag, and Au are limited, non-ideal. Cu(II)thd 2 /H 2 process is well-behaved for selective growth.

11 Selective Area Growth metal seed oxide/nitride Growth No growth Selective area ALD enables deposition on seeded regions and eliminates the need for etching. SA-ALD enables nanostructures.

12 Plasmonic Structures Pd nanostructures 10x10 Rectangular Dipole Rectenna Array, 550 nm x 800 nm spacing 10x10 Large Triangle Rectenna Array 500 nm x 500 nm spacing Electrically isolated structures are used to investigate tuning of plasmonic response by selective area ALD.

13 Resonance Tuning Gap size dependence Nanostructure size dependence bar size: 150 x 50 x 35 nm bar-shape structures FDTD simulations predict that resonance red-shifts for decreasing gap size and for increasing nanostructure size.

14 Tuning Plasmonics with ALD lightening rod effect further intensifies electric fields FDTD simulations Plasmonic dimers have strong electric field enhancement >10 3, with fields > 5x10 8 V/m 1. 1 Ward et al., Nature Nano v. 5 p. 732 (2010)

15 Optical measurements Focus spot is 3-7 um; sized to the test array.

16 Optical Response Red-shift & increased extinction Spectroscopy data show red-shift after ALD.

17 Red-Shifts Red-shift magnitude tracks ALD thickness/cycles. In-situ measurements would provide feedback on nanogap.

18 Nanoscale Tunnel Junctions by ALD geometric asymmetry SiO2 Device arrays with 1000 s of tips converge to tunneling similarly to suspended structures. Geometric asymmetry contributes to desired diode IV character.

19 Current (A) d 2 I/dV 2 Intensity ALD Grown Tunnel Junctions IETS Volts (V) adsorption Time (seconds) Trapping and IETS detection of Acetic Acid molecules in nanojunctions

20 HIM Images Hang Dong, Rutgers < 9 nm < 7 nm HIM provides some estimates for gap size.

21 Nano-Nucleation Challenges Without UVO With UVO Growth is sensitive to surface pre-treatment, growth inhibition is observed without UVO. ALD metal films are not layer-by-layer, Nanograins form with inherent surface roughness. How smooth can we achieve? Could be shape dependence, crystal structure effects.

22 UV/Ozone Sample Pre-treatment XPS acetate-like planar films ALD Cu roughness values are similar to other reports 2, scales with thickness. UVO enhances/enables growth, removes most C 1s, but residual C remains even after long time (60 min). 2 Winter et al. Chem. Mater. v. 26, p (2014)

23 Selectivity High T growth, 230 C Annealed in He at 600 C prior to growth at 220 C Selectivity is lost at higher temperatures, inverted selectivity? Selectivity was a mystery; how does Cu grow on oxide?

24 ALD Fundamentals with in-situ SE note O content of precursor Cu(thd) 2 is a solid source precursor. H 2 is coreactant. In-situ, real time SE provides saturation curves.

25 SE Growth Trajectory ~15/466 = 0.03 delta/cycle in-situ SE provides growth finger-print, sensitive to substrate preparation and growth conditions. extracted GPC ~ 0.04 nm/cycle matches SEM data.

26 Original Mechanism Martensson s mechanism Cu(thd) 2 adsorbs to saturation during precursor dose. Martensson et al, JES, v. 145, p (1998)

27 SE Growth Signature Cu ALD SrO ALD SrO ALD shows characteristic signature for stable precursors adsorption. Cu ALD has opposite signature with reversible adsorption.

28 Original mechanism is NOT consistent with Cu I CVD by disproportionation at C, and can t explain observed GPC. Martensson et al, JES, v. 145, p (1998) Original Mechanism Martensson s mechanism Selectivity follows from H 2 dissociation requirement.

29 New Mechanism New mechanism explains SE signature and is consistent with reversible adsorption. Also explains high GPC. Roll of Pd-H 2 is now more complex, strong chemisorption.

30 SE Growth vs. Temperature Real-time SE vs. Temp Coverage Simulations CuL 2 + 2* CuL* + L* Mechanism not sensitive to temperature. H* must be stable. Dissociative reversible adsorption is consistent with thermodynamics if H ads ~ 34 kcal/mol.

31 QCM Studies QCM measures mass and thermal effects. Reaction signal (at 150 C) is consistent with RTSE data, GPC ~ 0.04 nm/cycle.

32 Role of Pd XRD XPS Temperature and time dependence of Cu XRD and XPS signals indicate extensive Cu/Pd mixing. Lower temperatures = higher Cu/Pd ratios. ARXPS shows evidence for Pd enrichment at surface. Pd explains H chemisorption. What happens if the Pd runs out? Pd segregation

33 Cu/Pd vs. Cu/Pt Both Pt & Pd act as seed layers, but Pd mixes more extensively with Cu.

34 GPC on Pd/H* Cu ALD on Pd 150C GPC decreases as Cu/Pd becomes more Cu rich. Cu/Pd ratio measured as 45:1 for this sample.

35 Selectivity No adsorption/ desorption cycles for SiO 2, Si 3 N 4 Martensson et al, JES, v. 145, p (1998) Selectivity is evident from in-situ SE data, but how to explain non-selective growth on SiO 2?

36 Non-Selective Growth Mechanism? C 1s Growth on SiO 2 at 205 C O1s Cu 2p C 1s signal is removed by sputtering, but O 1s signal remains. Cu 2p signal could be Cu 0 or Cu I. Cu/O ratios are ~1.3. AES peaks indicates Cu I. CuO/Cu 2 O growth explains non-selective growth mechanism. Mechanism requires self-decomposition. H 2 not required, but may affect thickness. For thick Cu films, self-decomposition may contribute to growth.

37 ph2 (Torr) H 2 effect on Non-selective Growth C 1.5 Torr Excellent Good Poor 160 C 3 Torr Temperature (C) H 2 pressure plays a significant role for non-selective growth. Why?

38 H 2 effect on Non-Selective Growth 180 C 180 C 1.5 torr H2 3 torr H C 0.5 torr H2

39 GPC(nm/cycle) H 2 Effect on Selective Growth Cu ALD on Pd Seed Layers C 160C H 2 partial pressure (torr) GPC Measured by QCM in selective window shows no effect of H 2 partial pressure. No growth without H 2.

40 Summary ALD Cu is useful for tuning the optical response of plasmonic materials. ALD of Ag, Au would be nice. Selective Area Cu ALD works well and is useful for tunnel junctions with many potential applications. Cu ALD mechanism is partially understood, Pd, H effects are important.

41 Acknowledgements Co-Pi s (Physics, Penn State): Darin Zimmerman, Gary Weisel, James Chen Students: X. Jiang & J. Qi (UCONN) R. Wamboldt (Penn State) Funding: NSF-EECS