4. Heat Conduction in Silicon Films and Devices
Dr. Uma Srinivasan, Mehdi Asheghi, and Y. Sungtaek Ju
The simulation of compact transistors and photonic devices requires models and data describing thermal conduction in semiconductor structures. Phonon transport in semiconducting microstructures is reduced compared to that in bulk silicon due to the scattering mechanisms depicted in Figure 1. Phonon-boundary scattering is particularly important in silicon-on-insulator (SOI) and multilayer gallium-arsenide and silicon-germanium devices. In doped regions, the impurities and the associated free carriers can further impede heat transport. These scattering processes are complicated by phonon dispersion, which reduces the phonon group velocities.
This project uses thermal conductivity measurements in microfabricated structures and the Peierls Boltzmann transport equation to provide fundamental information about phonon conduction in semiconducting microstructures. Figure 2 shows the large impact of interface scattering on the temperature-dependent thermal conductivities along thin layers. The reduction is extreme at low temperatures and agrees reasonably well with the solution to the transport equation. For transient and high-temperature processes it is important to account for phonon dispersion. This motivates a transport model based on two phonon modes, distinguished by their average group velocities. The propagating mode is analyzed using a transport equation together with the damping effects of a capacitive mode, which accounts for the small group velocities of high-frequency transverse and optical phonons in silicon.
This research yields transport models that can be incorporated into simulations of practical semiconducting devices and are of particular relevance for those subjected to electrical overstress (see Project 1). The data and experiments also provide fundamental information about the interaction of phonons with interfaces in small spatial domains.
Collaboration
Group of Professor S.S. Wong, Electrical Engineering Department, Stanford
University
Recent Publications
Asheghi, M., Touzelbaev, M.N., Goodson, K.E., Leung, Y.K., and Wong,
S.S., 1998, "Temperature-Dependent Thermal Conductivity of Single-Crystal
Silicon Layers in SOI Substrates," ASME Journal of Heat Transfer,
Vol. 120, pp. 31-36.
Asheghi, M., Leung, Y.K., Wong, S.S., and Goodson, K.E., 1997, "Phonon-Boundary Scattering in Thin Silicon Layers," Applied Physics Letters, Vol. 71, pp. 1798-1800.
Ju, Y.S., and Goodson, K.E., 1997, "Size Effect on the Thermal Conductivity of Silicon-on Insulator Devices under Electrostatic Discharge (ESD) Conditions," Japanese Journal of Applied Physics, Part 2, Letters, Vol. 36, pp. L798-L800.
Ju, Y.S., and Goodson, K.E., 1997, "Impact of Phonon Dispersion upon the Size Effect on Thermal Conduction along Thin Semiconductor Films," Proceedings of the 1997 International Mechanical Engineering Congress and Exposition, Dallas, Texas, November 16-21, in Microscale Energy Transport, K.E. Goodson et al., eds., HTD-Vol. 354, pp. 181-190.
Sponsorship
Semiconductor Research Corporation Contract 98-SJ-461
ONR Grant 96-1-0688 (Young Investigator Award, Electronics Division)