PhD Oral Examination - Zijian Li, Tuesday May 15, 9:30 AM, CIS-X 101

University Ph.D. Dissertation Defense

Thermal Conduction Phenomena in Nanostructured Semiconductor Devices and Materials

Zijian Li

Advisor: Prof. Kenneth E. Goodson

Department of Mechanical Engineering

 

Tuesday, May 15^th, 2012

9:30 AM (Refreshments at 9:15 AM)

Paul G. Allen Auditorium (CIS-X 101)

http://campus-map.stanford.edu/index.cfm?ID=04-055

 

Abstract

Thermal phenomena have become increasingly important in a variety of nanostructured semiconductor devices and materials. The reduced dimensions and large interface densities lead to unique heat transfer behaviors which are not available in bulk materials. Successful design of high-performance semiconductor devices (such as phase change memory and high electron mobility transistors) relies on the accurate thermal characterization of thin film materials and the understanding of nanoscale energy transport physics. 

The first part of this work investigates the thermal conduction phenomena in phase change memory (PCM). A combination of the frequency-domain electrical thermometry and the suspended microstructure measure the in- and out-of-plane thermal conductivities of the thin-film phase change chalcogenide Ge₂Sb₂Te₅ in the amorphous, fcc, and hcp phases. The preferential grain orientation and mixed phase distribution lead to a reduced in-plane thermal conductivity that is only 60%-80% of the out-of-plane value. The anisotropic heat conduction benefits PCM devices by reducing the programming current and mitigating the thermal disturbance to adjacent cells. A fully coupled electrothermal simulation algorithm unveils the transient phase distribution during a programming operation.  

The second half of the talk discusses the thermal transport across interfaces of drastically different materials with dimensions of only a few nanometers, such as the Mo/Si multilayers (2.8nm/4.1nm) used in the mirror systems for extreme ultraviolet lithography. Strong anisotropy in the thermal conductivities exists in the multilayers, where the in-plane conductivity is 13 times higher than the out-of-plane value, owing to the frequent metal-semiconductor interfaces. Thermal conduction in such periodic multilayer composites can be strongly influenced by nonequilibrium electron-phonon scattering for periods shorter than the relevant free paths. This work demonstrates that two additional mechanisms – quasi-ballistic phonon transport normal to the metal film and inelastic electron-interface scattering – can also impact conduction in metal-dielectric multilayers with period below 10 nm.