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
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
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
(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 2 + 1 Al 2 O 3 ]
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 100 94.1 o C 95.0 % 75 50 14 o C 50.4 % ght (%) 1,3,2-diazastannolidin-2-ylidene (CAS # 1268357-44-3) 25 0 TGA 215.2 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 50 100 150 200 Temperature ( o C) - Clean evaporation w/o residue - Vapor pressure: 0.42 Torr @40 o C
ALD SnO 2 results 6/14 0.20 6x10-2 08 0.8 5x10-2 Transmittance 4x10-2 3x10-2 le) Resistivity ( cm) 0.18 0.16 014 0.14 0.12 04 0.4 Reflectance 0.2 Deposition Rate (nm/cyc T & R 010 0.10 50 100 150 200 250 Temperature ( o C) 2x10-2 50 100 150 200 250 Temperature ( o C) 00 0.0 400 500 600 700 800 Wavelength (nm) - G/R: 0.175 nm/cycle @ 50 o C - Resistivity: 2 10-2 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).
Basic composite films properties 7/14 Growth rate Density Refractive index 0.18 data 0.17 rule of mixture 60 6.0 5.5 2.00 1.95 1.90 1.85 Refractivee Index 0.16 0.15 5.0 4.5 Density (g/cm 3 ) 0.14 0.13 0.12 0.11 0.10 4.0 3.5 3.0 1.80 1.75 1.70 1.65 Growth rate (nm/cycle) 0.09 0 20 40 60 80 100 Percentage of Al 2 O 3 subcycle 2.5 0 20 40 60 80 100 Percentage of Al 2 O 3 subcycle 1.60 0 20 40 60 80 100 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, 10277 (2011).
Composition analysis (Sn,Al)O x - RBS 8/14 Energy (MeV) 0.5 1.5 Al 300(1,0) 12(24,1) 30(9,1) 75(3,1) 60(3,2) 100 Rule of Mixture Measured (RBS) 80 60 unit) concentration (at.%) 740 760 780 800 60(2,3) 400(0,1) 40 20 Yi eld (arb. 0 Al C sub. O Al Sn 0 200 400 600 800 1000 1200 1400 Channel 0 20 40 60 80 100 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 2 3 100 : area density of cations per cycle, %( ): percentage of subcycle Higher Al concentration than expected over the entire range
Chemisorption analysis by RBS 9/14 26 2.6 Al 22 2.2 22 2.2 Al2 O 3 1.8 14 1.4 1.8 14 1.4 SnO2 2.2 Average chemisorp ption rate TMA on CAT on 1.8 1.4 Sn 0 20 40 60 80 100 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) 0.2 0 1 2 3 4 5 6 7 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, 10277 (2011).
What s happening? - TMA on SnO 2 10/14 3.5 3.0 2.5 2.0 1.5 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) 0.5 0.0 Al 2 O 3 80 70 3(99,1) - ~2% Al Zalar rotation SnO 2 17nm r..m.s. roug 0 10 20 100 Al/(Sn+Al) from RBS (at.%) 60 50 40 O Concentration (%) 0.8 6 4 2 nm μm μm 0.4 0.2 0.0 0.0 0.2 0.4 μm 0.8-2 -4-6 0 0.0 0.0 0.2 0.4 μm 0.8 30 0.8 1.5 20 m10 0.5 0.4 00 0.0-0.5 0 0.2 - -1.5 Sn Al ( 3) nm SnO 2 (Sn,Al)O x 7.9% Al 0 5 10 15 Sputter time (min) - Tailing minor diffusion - Nanolaminate Al distribution
Si n 5 nm Polycrystalline SnO2 Si Polycrystalline Nanolaminate n 12(24,1) - 7.9% Al n Less crystallized Si 30(9,1) - 20.4% Al Microstructure of composite films Si n Amorphous n 75(3,1) - 52.3% Al 11/14
Electrical properties of composite films 12/14 10 15 Vertical direction Lateral direction 10 12 Low Al % anisotropic High Al % isotropic Vertical direction Vertical direction 10 9 10 6 10 3 Lateral direction Lateral direction 10 0 Resistivity ( cm) 10-3 0 20 40 60 80 100 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
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.
Acknowledgements 14/14 (1)Camille and Henry Dreyfus Postdoctoral Program (2)Air Force Office of Scientific Research (FA9550-09-C-0075) (3) Center for Nanoscale Systems (NSF award no. ECS-0335765) - Group members - Dr. Harish Bhandari - Dr. Sang Bok Kim