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The Role of Electrons in Nanostructured Si1-xGex/Si for Energy Conversion
Md Hossain
University of Illinois at Urbana-Champaign
1206 W Green St, MC 244, Mechanical Engineering Bldg, Urbana, 61801, US
Phone: 12173778474, Email: hossain2@illinois.edu
Harley T Johnson
University of Illinois at Urbana Champaign, Urbana, IL
Abstract:
Nanostructured materials such as Si1-xGex/Si have attracted significant attention in the recent literature
because of their potential for on-chip heat management or energy conversion utilizing waste heat.
Maximizing the efficiency of energy conversion involves optimizing several interdependent transport
properties of electrons as well as phonons. Nonetheless, because of the limitations in experimental
measurements of conductivities on the nanoscale, especially for lengths on the order of a few
Angstroms, ascertaining the influence of physical dimension on energy conversion is a challenging task.
Consequently, efforts on improving ZT have mainly been focused on phonons, disregarding the
prospects of enhancing thermopower that originate solely from electrons.
In this work, using a combination of first-principles calculations and a semi-classical Boltzmann
transport formalism, the role of electrons on energy conversion is studied for Si1-xGex/Si
heterostructures. In addition to providing quantitative measures for the role of electrons on nanoscale
energy conversion in Si1-xGex/Si, a rigorous computational framework is developed for computing
electronic contributions to thermopower and electron thermal conductivity, taking into account the
influences of various relaxation time functions or scattering events, temperature, doping, and layer
thickness ratio. It is found that alloying can substantially improve thermopower while nanoscale lengths
can lead to degradation and adversely affect ZT values. At room temperature, the maximum
thermopower for a 3.3 nm Si0.5Ge0.5/Si heterostructure with layer thickness ratio of 1:1 is obtained as
336 V/K, while for a similar size Si0.0Ge1.0/Si heterostructure with the same layer thickness ratio the
maximum thermopower is only 129 V/K. Furthermore, the maximum values occur at different
chemical potentials or carrier densities: 0.0025 Ry (6x1018 cm-3) for Si0.5Ge0.5:Si = 1:1 and 0.0058
Ry (3x1019 cm-3) for Ge:Si = 1:1. Thus, the results indicate that nano-structuring alone does not offer
beneficial effects for maximizing the efficiency of energy conversion in nanostructured thermoelectrics.
Rather, alloying and a proper choice of carrier density and type of carriers - which can create a sharp
increase in the density of states around the Fermi energy of the system - are also essential for enhancing
thermoelectric efficiency. Moreover, because of a difference in mean free path for electrons and
phonons, it is important to identify length scales that can amplify the combined effects of phonons and
electrons on ZT. It has already been reported in the literature that a reduction in phonon thermal
conductivity results in improved energy conversion in nanoscale materials; nonetheless, the limiting
values of lengths at which the thermal conductivity can be a minimum and transport becomes ballistic
are still under investigation. Our study on the role of electrons on thermopower and electron thermal
conductivity at different lengths can help identify the length scales that play the biggest role in
achieving optimum ZT values in nanoscale materials.
389 ABSTRACTS