Recap (so far) Low-Dimensional & Boundary Effects

Transcription

1 Recap (so far) Ohm s & Fourier s Laws Mobility & Thermal Conductivity Heat Capacity Wiedemann-Franz Relationship Size Effects and Breakdown of Classical Laws 1 Low-Dimensional & Boundary Effects Energy Transport in Thin Films, Nanowires, Nanotubes Landauer Transport Quantum of Electrical and Thermal Conductance Electrical and Thermal Contacts Materials Thermometry Guest Lecture: Prof. David Cahill (MSE) 2

2 Sub-Continuum Energy Transport Macroscale (D >> L) T Cs t k T Q Nanoscale (D < L) s Ox Me D t si e e eq e v e Q t phon Size and Non-Equilibrium Effects optical-acoustic small heat source impurity scattering boundary scattering boundary resistance L ~ 200 nm Ox Si Thermal Simulation Hierarchy MFP ~ 200 nm at 300 K in Si D ~ L Continuum Fourier s Law, FE q k " T lattice wave phonon E L defect D Phonon Transport BTE & Monte Carlo nq nq nq vq. nq t q D ~ Wavelength Waves & Atoms Waves & Atoms MD & QMD 4

3 Drift Diffusion BTE Moments Monte Carlo & BTE Monte Carlo with Quantum Models Full Quantum Phonons Thermal and Electrical Simulation Atomistic electrons phonons BTE with Wave models MFP ~5 nm ~100 nm BTE or Monte Carlo ~5 nm ~1 nm Diffusion Isothermal Electrons 5 Nanowire Formation: Bottom-Up Vapor-Liquid-Solid (VLS) growth Need gas reactant as Si source (e.g. silane, SiH 4 ) Generated through Chemical vapor deposition (CVD) Laser ablation or MBE (solid target) Lu & Lieber, J. Phys. D (2006) 6

4 Top-Down and Templated Nanowires Suspended nanowire (Tilke 03) Top-down = through conventional lithography Guided growth = through porous templates (anodic Al 2 O 3 ) Vapor or electrochemical deposition 7 Semimetal-Semiconductor Transition Bi (bismuth) has semimetal-semiconductor transition at wire D ~ 50 nm due to quantum confinement effects Source: M. Dresselhaus (MIT) 8

5 When to Worry About Confinement 2-D Electrons 2-D Phonons d d E n 2 2 n * 2m d 2 n 2 2 n vkn v ky kz d 9 Nanowire Applications Transistors Interconnects Thermoelectrics Heterostructures Single-electron devices 10

6 Nanowire Thermal Conductivity Nanowire diameter Li, Appl. Phys. Lett. 83, 3187 (2003) 11 Interconnects = Top-Down Nanowires SEM of AMD s Hammer microprocessor in 130 nm CMOS with 9 copper layers Cross-section 8 metal levels + ILD Intel 65 nm M1 pitch Transistor 12

7 Cu Resistivity Increase <100 nm Lines Size Matters Why? Remember Matthiessen s Rule 13 Cu Interconnect Delays Increase Too Source: ITRS 14

8 Industry Acknowledged Challenges Source: ITRS 15 Cu Resistivity and Line Width Steinhögl et al., Phys. Rev. B66 (2002) 16

9 Modeling Cu Line Resistivity Steinhögl et al., Phys. Rev. B66 (2002) 17 Model Applications Steinhögl et al., Phys. Rev. B66 (2002) Plombon et al., Appl. Phys. Lett 89 (2006) 18

10 Resistivity Temperature Dependence 19 Other Material Resistivity and MFP Greater MFP (λ) means greater impact of size effects Will Aluminum get a second chance?! 20

11 k (W/m/K) k (W/m/K) Same Effect for Thermal Conductivity! Thin Si Si NW Thin Ge SiGe NW d (nm) Recall: bulk Si k th ~ 150 W/m/K bulk Ge k th ~ 60 W/m/K Approximate bulk MFP s: λ Si ~ 100 nm λ Ge ~ 60 nm (at room temperature) Material with longer (bulk, phonon-limited) MFP λ suffers a stronger % decrease in conductivity in thin films or nanowires (when d λ) Nanowire (NW) data by Li (2003), model Pop (2004) 21 Back-of-Envelope Estimates Thin Si Si NW Thin Ge SiGe NW d (nm) 1 k( d) Cv d D b G C (MJm -3 K -1 ) λ b (nm) v L (m/s) v T (m/s) k b (Wm -1 K -1 ) Si 1.66 ~ Ge 1.73 ~ (at room temperature) 22

12 More Sophisticated Analytic Models δ = d/λ < 1 S = (1 δ 2 ) 1/2 Flik and Tien, J. Heat Transfer (1990) Goodson, Annu. Rev. Mater. Sci. (1999) 23 A Few Other Scenarios anisotropy Goodson, Annu. Rev. Mater. Sci. (1999) 24

13 Onto Nanotubes Nanowires: Shrunk-down 3D cylinders of a larger solid (large surface area to volume ratio) Diameter d typically < {electron, phonon} bulk MFP Λ: surface roughness and grain boundary scattering important Quantum confinement does not play a role unless d < {electron, phonon} wavelength λ ~ 1-5 nm (rarely!) Nanotubes: Rolled-up sheets of a 2D atomic plane There is no volume, everything is a surface* Diameter 1-3 nm (single-wall) comparable to wavelength λ so nanotubes do have 1D characteristics b * people usually define thickness b ~ 0.34 nm 25 Single-Wall Carbon Nanotubes Carbon nanotube = rolled up graphene sheet Great electrical properties Semiconducting Transistors Metallic Interconnects Electrical Conductivity σ 100 x σ Cu d ~ 1-3 nm Thermal Conductivity k k diamond 5 x k Cu Nanotube challenges: HfO 2 top gate (Al) CNT Reproducible growth S (Pd) D (Pd) Control of electrical and thermal properties SiO 2 Going from one to a billion 26

14 CVD Growth at ~900 o C 27 Fe Nanoparticle-Assisted Nanotube Growth Particle size corresponds to nanotube diameter Catalytic particles ( active end ) remain stuck to substrate The other end is dome-closed Base growth 28

15 Water-Assisted CVD and Breakdown People can also grow macroscopic nanotubebased structures Nanotubes break down at ~600 o C in O 2, 1000 o C in N 2 in O 2 in N 2 Hata et al., Science (2004) 29 Graphite Electronic Structure b ~ 3.4 Å a CC ~ 1.42 Å a 1 = a 2 = 3a CC 30

16 Collins and Avouris, Scientific American (2000) Nanotube Electronic Structure E G = 0 E G > 0 E G = 0 E G > 0 31 Band Gap Variation with Diameter Red: metallic Black: semiconducting Charlier, Rev. Mod. Phys. (2007) E 11,M E 22,M E 22,S E 11,M E 11,S = E G 0.8/d Kataura plot 32

17 Nanotube Current Density ~ 10 9 A/cm 2 Nanotubes are nearly ballistic conductors up to room temperature Electron mean free path ~ nm CNT L = 60 nm V DS = 1 mv S (Pd) G (Si) SiO 2 D (Pd) Javey et al., Phys. Rev. Lett. (2004) 33 Transport in Suspended Nanotubes E. Pop et al., Phys. Rev. Lett. 95, (2005) nanotube on substrate 2 μm suspended over trench nanotube Pt Si 3 N 4 Pt gate SiO 2 Observation: significant current degradation and negative differential conductance at high bias in suspended tubes Question: Why? Answer: Tube gets HOT (how?) 34

18 Phonon Temperature (K) Transport Model Including Hot Phonons I 2 (R-R c ) E. Pop et al., Phys. Rev. Lett. 95, (2005) R OP T OP Non-equilibrium OP: T T ( T T ) OP AC AC 0 R TH T AC = T L T 0 Heat transfer via AC: A kt I R R L 2 ( ) ( C ) / I 2 (R-R C ) T OP T AC = T L oxidation T Optical T OP Acoustic T AC V (V) Landauer electrical resistance h R( V, T) RC 2 4q Include OP absorption: eff AC OP, ems OP, abs L eff ( V, T) eff ( V, T) 1 35 Extracting SWNT Thermal Conductivity E. Pop et al., Nano Letters 6, 96 (2006) Yu et al. (NL 05) This work ~T ~1/T Ask the inverse question: Can I extract thermal properties from electrical data? Numerical extraction of k from the high bias (V > 0.3 V) tail of I-V data Compare to data from K of UT Austin group (C. Yu, NL Sep 05) Result: first complete picture of SWNT thermal conductivity from K 36