Thermal and Thermoelectric Characterization of Nanostructures Li Shi, PhD Assistant Professor Department of Mechanical Engineering & Center for Nano and Molecular Science and Technology, Texas Materials Institute The University of Texas at Austin Tutorial on Micro and Nano Scale Heat Transfer, 2003 IMECE

Outline  Scanning Thermal Microscopy of Nanoelectronics  Thermoelectric Measurements of Nanostructures

Silicon Nanoelectronics Heat dissipation influences speed and reliability Device scaling is limited by power dissipation IBM Silicon-On-Insulator (SOI) Technology

Carbon Nanoelectronics TubeFET (McEuen et al., Berkeley) Nanotube Logic (Avouris et al., IBM) Current density: 10 9 A/cm 2 Ballistic charge transport - V

Infrared Thermometry 1-10  m* Laser Surface Reflectance 1  m* Raman Spectroscopy 1  m* Liquid Crystals 1  m* Near-Field Optical Thermometry < 100 nm Scanning Thermal Microscopy (SThM) < 100 nm Techniques Spatial Resolution *Diffraction limit for far-field optics Thermometry of Nanoelectronics

X-Y-Z Actuator Scanning Thermal Microscopy Sample Temperature sensor Laser Atomic Force Microscope (AFM) + Thermal Probe Cantilever Deflection Sensing Thermal X T Topographic X Z

Microfabricated Thermal Probes Pt-Cr Junction Shi, Kwon, Miner, Majumdar, J. MicroElectroMechanical Sys., 10, p. 370 (2001) 10  m Pt Line Cr Line Tip Laser Reflector SiN x Cantilever

Thermal Imaging of Nanotubes Multiwall Carbon Nanotube 1  m Topography 1  m 3 V 88  A Distance (nm) Thermal signal (  V) nm Distance (nm) Height (nm) 30 nm Distance (nm) Height (nm) 30 nm Thermal Shi, Plyosunov, Bachtold, McEuen, Majumdar, Appl. Phys. Lett., 77, p (2000) Spatial Resolution

Metallic Single Wall Nanotube TopographicThermal 1  m ABCD Low bias: Ballistic High bias: Dissipative (optical phonon emission)  T tip 2 K 0

Polymer-coated Nanotubes GND -2 V, 4.4  A 2 V, 7.8  A The polymer melted at a ~3V bias Topography Before coating After coating Thermal Asymmetric heating at the two contacts 1  m

SiGe Devices Future Challenge: Temperature Mapping of Nanotransistors SOI Devices Low thermal conductivities of SiO 2 and SiGe Interface thermal resistance Short ( nm) channel effects (ballistic transport, quantum transport) Phonon “bottle neck” (optical-acoustic phonon decay length > channel length) Few thermal measurements are available to verify simulation results

Thermal Transport in Nanostructures Long mean free path l Strong SP 2 bonding: high sound velocity v  high thermal conductivity: k = Cvl/3 ~ 6000 W/m-K Below 30 K, thermal conductance  4G 0 = ( 4 x T) W/m-K, linear T dependence (G 0 :Quantum of thermal conductance) Hot Cold p Heat capacity Carbon Nanotubes

Semiconductor Nanowires Increased phonon-boundary scattering Modified phonon dispersion  Suppressed thermal conductivity Ref: Chen and Shakouri, J. Heat Transfer 124, 242 Hot Cold p VLS-grown Si Nanowires (P. Yang, Berkeley) Source Drain Gate Nanowire Channel Nano-patterned Si Nanotransistor (Berkeley Device group) Hot Spots

Efficient Peltier Cooling using Nanowires Dresselhaus et al., Phys. Rev. B. 62, 4610 Bi Nanowires Low   high COP Thermoelectric figure of merit:

Pt resistance thermometer Suspended SiN x membrane Long SiN x beams Q I Thermal Measurements of Nanostructures Kim, Shi, Majumdar, McEuen, Phys. Rev. Lett. 87, Shi, Li, Yu, Jang, Kim, Yao, Kim, Majumdar, J. Heat Tran 125, 881

Sample Preparation Wet deposition Spin Chip Nanostructure suspension Pipet Direct CVD growth SnO 2 nanobelt Nanotube bundle Individual Nanotube Dielectrophoretic trapping

Thermal Conductance Measurement T 0 T 0 G -1 G b -1 T h T s Q Q h 2Q L G b -1

Measurement Errors and Uncertainties Contact Resistance ~ d ~ d 2 -- Thickness: 1 nm uncertainty in tapping mode AFM  d/d = 10 % for d = 10 nm  d/d = 50 % for d = 2 nm (individual SWCN)  Raman Spectroscopy Size -- G -1 Sample /G -1 Contact decreases with d, and is estimated to larger than 10 for measurements reported here

Carbon Nanotubes CVD SWCN An individual nanotube has a high  ~ W/m-K at 300 K The diameter and chirality of a CN may be probed using Raman spectroscopy  of a CN bundle is reduced by thermal resistance at tube-tube junctions

Collaboration: N. Mingo, NASA Ames Shi, Hao, Yu, Mingo, Kong, Wang, submitted SnO 2 Nanobelts 64 nm 53 nm 39 nm 53 nm,  i -1 =10  -1 i, bulk 64 nm 53 nm Circles: Measurements Lines: Simulation using a Full Dispersion Transmission Function approach  U -1 =  U,bulk -1  i -1 =  i,bulk -1  b -1 = v/FL v: phonon group velocity FL: effective thickness Phonon scattering rate: Umklapp Boundary Impurity Phonon-boundary scattering is the primary effect determining the suppressed thermal conductivities

Si Nanowires Symbols: Measurements Lines: Simulation using a modified Callaway method Li, Wu, Kim, Shi, Yang, Majumdar, Appl. Phys. Lett. 83, 2934 (2003) Phonon-boundary scattering is the primary effect determining the suppressed thermal conductivities except for the 22 nm sample, where boundary scattering alone can not account for the measurement results.

Seebeck Coefficient V TE I ThTh TsTs S = V TE / (T h –T s ) Oxygen doped Quasilinear (metallic) behavior Phonon drag effect at low T

Future Challenge: Nanomanufacturing of Nanowire Arrays as Efficient Peltier Devices Nano- imprint Pattering of Thermoelectric Nanowire Arrays 40 nm Cr nanowire array 10 nm Cr nanowire array Test-bed Peltier devices for cooling IR sensors

Summary Scanning Thermal Microscopy of Nanoelectronics: -- Thermal imaging with 50 nm spatial resolution Thermoelectric (k, , S) Measurements of Nanostructures Using a Microfabricated Device: -- Super-high  of nanotubes -- Suppressed  of nanowires

Acknowledgment Students: Choongho Yu; Jianhua Zhou; Qing Hao; Rehan Farooqi; Sanjoy Saha; Anastassios Marvrokefalos; Anthony Hayes; Carlos Vallalobos Collaborations: UC Berkeley: Arun Majumdar; Deyu Li (now at Vanderbilt); Philip Kim (now at Columbia); Paul McEuen (now at Cornell); Adrian Bachtold (now at Paris); Sergei Plyosunov UT Austin: C. K. Ken Shih & Ho-Ki Lyeo; Zhen Yao; Brian Korgel GaTech: Z. L. Wang NASA: Natalio Mingo UCSC: Ali Shakouri MIT: Rajeev Ram & Kevin Pipe Support: NSF CTS (CAREER; Instrumentation)