Layer Deposition. (Sn,Al)O x Films Grown by Atomic. Jaeyeong Heo, Yiqun Liu, Prasert Sinsermsuksakul, Zhefeng Li, Jaeyeong Heo.

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1 1/14 (Sn,Al)O x Films Grown by Atomic Layer Deposition June 29 th 2011, Yiqun Liu, Prasert Sinsermsuksakul, Zhefeng Li, Leizhi Sun, Wontae Noh, and Roy G. Gordon Harvard University, Cambridge, MA, USA

2 Transparent Conducting Oxide (TCO) -SnO 2 2/14 O 2- Sn 4+ - Transparency and conductivity with high stability - Crystal structure: Rutile (tetragonal) - n-type semiconductor with E g 3.6 ev FTO-energy conserving window Gas Sensors Photovoltaics H 2 or CO

3 Higher resistivity oxide 3/14 (1) Hole blocking layer for solar cells 10-2 ~10 0 ohm cm (2) AOS TTFT for displays 10 0 ~10 6 ohm cm (3) Microchannel electron multiplier plates 10 6 ~10 8 ohm cm SONY (SID 2010) The possibility of controlling film s resistivity over a wide range forming (Sn,Al)O x composite materials by ALD Chemisorption behaviors Properties of materials

4 (Sn, Al)O x supercycle -exp. set - 4/14 m supercycle n SnO 2 subcycle l Al 2 O 3 subcycle.... m (n, l) 300(1,0) (SnO 2 ) 3(99,1) 12(24,1) 30(9,1) 33(8,1) 38(7,1) 75(3,1) 60(3,2) Sn feeding purge H2O2 feeding purge Al feeding purge H2O2 feeding purge - Temperature: 120 o C - Sn precursor: CAT(II) - Al precursor: TMA - Oxidant: 50 wt.% H 2 O 2 - Purge: 30-60s Time 60(2,3) 400(0,1) (Al 2 O 3 ) t: ~40-50 nm Notation: m(n,l) ex. 30(9,1): 30 supercycles [9 SnO Al 2 O 3 ]

5 Cyclic amide tin (II) t-bu N Sn N Sn t-bu 13bi 1,3-bis(1,1-dimethylethyl)-4,5-dimethyl-(4R,5R)- 1 l) 45di l (4R 5R) 5/14 Picture o C 95.0 % o C 50.4 % ght (%) 1,3,2-diazastannolidin-2-ylidene (CAS # ) 25 0 TGA o C 1.7 % Wei - Sn-N distance is 2.02 Å - N-Sn-N angle is 82.7 o - Planar five-member ring *Dr. Adam S. Hock & Dr. Sang Bok Kim Temperature ( o C) - Clean evaporation w/o residue - Vapor pressure: 0.42 o C

6 ALD SnO 2 results 6/ x x10-2 Transmittance 4x10-2 3x10-2 le) Resistivity ( cm) Reflectance 0.2 Deposition Rate (nm/cyc T & R Temperature ( o C) 2x Temperature ( o C) Wavelength (nm) - G/R: o C - Resistivity: ohm cm - Mobility: 7 cm 2 /V s - CC C.C: ~10 20 #/cm 3 - Transmittance: 87.8% - No C and N impurities J. Heo et al. Chem. Mater. 22, 4964 (2010).

7 Basic composite films properties 7/14 Growth rate Density Refractive index 0.18 data 0.17 rule of mixture Refractivee Index Density (g/cm 3 ) Growth rate (nm/cycle) Percentage of Al 2 O 3 subcycle Percentage of Al 2 O 3 subcycle Percentage of Al 2 O 3 Subcycle - G/R, film density, and refractive index deviates from the expected plot (rule of mixture) - Percentage of Al 2 O 3 subcycle ~20% ~50 at.% Al J. Heo et al. J. Phys. Chem. C 115, (2011).

8 Composition analysis (Sn,Al)O x - RBS 8/14 Energy (MeV) Al 300(1,0) 12(24,1) 30(9,1) 75(3,1) 60(3,2) 100 Rule of Mixture Measured (RBS) unit) concentration (at.%) (2,3) 400(0,1) Yi eld (arb. 0 Al C sub. O Al Sn Channel Percentage of Al 2 O 3 subcycle Rule of Mixture % Al 2 O 3 Al content (%) [ Al ] % Al O (100 % Al O ) ( Al 2 3 Sn : area density of cations per cycle, %( ): percentage of subcycle Higher Al concentration than expected over the entire range

9 Chemisorption analysis by RBS 9/ Al Al2 O SnO2 2.2 Average chemisorp ption rate TMA on CAT on Sn Al/(Sn+Al) from RBS (at.%) Al 2 O 3 subcycle: 1 Al 2 O 3 subcycle:2 60(3,2) Al 2 O 3 subcycle:3 60(2,3) Cycles 0.2 1) TMA -1 st : ~2.2 times enhancement on SnO 2-2 nd : Return to its own chemisorption 2) CAT -~5 cycles for full recovery J. Heo et al. J. Phys. Chem. C 115, (2011).

10 What s happening? - TMA on SnO 2 10/ SnO 2 t: 100 nm 1) Surface area: Smaller R rms 2) # of OH: SnO 2 :33 o Al 2 O 3 :47 o 3) Monolayer diffusion of Al? hness (nm) Al 2 O (99,1) - ~2% Al Zalar rotation SnO 2 17nm r..m.s. roug Al/(Sn+Al) from RBS (at.%) O Concentration (%) nm μm μm μm μm m Sn Al ( 3) nm SnO 2 (Sn,Al)O x 7.9% Al Sputter time (min) - Tailing minor diffusion - Nanolaminate Al distribution

11 Si n 5 nm Polycrystalline SnO2 Si Polycrystalline Nanolaminate n 12(24,1) - 7.9% Al n Less crystallized Si 30(9,1) % Al Microstructure of composite films Si n Amorphous n 75(3,1) % Al 11/14

12 Electrical properties of composite films 12/ Vertical direction Lateral direction Low Al % anisotropic High Al % isotropic Vertical direction Vertical direction Lateral direction Lateral direction 10 0 Resistivity ( cm) Al/(Sn+Al) (at.%) - Electrical anisotropy - Less Al: nanolaminate distribution of resistive Al 2 O 3 - More Al: well mixed/ amorphous - Resistivity change over 15 orders of magnitude

13 Summary 13/14 # Macroscopic properties of (Sn,Al)O x composite films were studied. # Chemisorption behaviors govern films microstructures and electrical properties. # Nanolaminated Al distribution for small Al concentrations. ti # Controlled adjustment of film s electrical resistivity over more than 15 orders of magnitude was achieved.

14 Acknowledgements 14/14 (1)Camille and Henry Dreyfus Postdoctoral Program (2)Air Force Office of Scientific Research (FA C-0075) (3) Center for Nanoscale Systems (NSF award no. ECS ) - Group members - Dr. Harish Bhandari - Dr. Sang Bok Kim