Silicon Quantum Dot Superla7ces for High Efficiency Thermoelectrics Bingyuan Huang, Xiao Guo, Duck Hyun Lee, Peter F. Green and Akram I. Boukai Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI
Project Overview Ø Advantages of silicon as a thermoelectric material. • High abundance on the Earth. • Less toxic than other thermoelectric materials, e.g., Pb, Bi, Te. • BeKer integraLon with electric devices. Ø Thermoelectric figure of merit
ZT =
S 2σ
T , Power Factor = S 2σ
κ S − thermopower , σ − electrical conductivity , κ − thermal conductivity , T − temperature • At room temperature, ZT of bulk silicon is only ~0.01. • Control S, σ and κ independently. Ø Decrease the thermal conducLvity using nanostructures. • As feature dimensions shrink to the phonon mean free path (~300nm), strong scaKering of phonons should occur and κ decreases. In the meanLme, the electrical conducLvity is not heavily affected.
Thermopower CharacterizaLon
Device FabricaLon
Ø All thermopower measurements were conducted in a cryostat using a LabVIEW interface.
Ø Nanomesh is made on the strained silicon thin film on SiO2 through steps as shown in the right schematics. • PS-PMMA block copolymer layer is first spincoated on top of the strained silicon. • After annealing and UV treatment, PMMA is removed by acetic acid. • Nanomesh is imprinted into strained silicon thin film by DRIE through the PS mask. • PS is finally removed by Piranha cleaning.
A schematic of the experimental setup.
A screenshot of the LabVIEW program
Ø SEM images show almost perfect hexagonal nanomesh on the strained silicon.
Top view. nearly 20nm with pitches about 40 nm.
Ø Increase the electrical conducLvity using crystal la7ce strain. • La7ce strain affect the curvature of the energy band, resulLng in lower effecLve mass of charge carriers and consequently higher mobility and conducLvity. Ø In this work, nanomesh structure was made on strained silicon thin films. • Thermopower values were enhanced for both strained silicon and normal silicon by adding nanomesh . ZT≈0.4 at 300K • Strained silicon showed higher electrical conducLvity than normal silicon.
Ø Thermopower is improved by adding nanomesh for both strained silicon and normal silicon. Raw data Linear fitting
0 Strained Silicon 1.2E19 doping w/o nanomesh Silicon 1.45E19 doping w/o nanomesh
Electrical ConducLvity CharacterizaLon
Ø Raman spectroscopy was conducted to verify that nanomesh didn’t release all la7ce strain in the strained silicon. The green curve indicates that the nanomesh did release some la3ce strain. However, most strain is s8ll retained.
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Temperature (K)
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Strained Silicon 3.3E19 doping no nanomesh Strained Silicon 2.5E19 doping nanomesh Silicon 8.14E19 doping nanomesh
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Temperature (K)
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Thermopower (µV/K)
Thermopower (µV/K)
Ø Strained silicon shows much higher electrical conductivity than normal silicon, both with and without nanomesh.
Electric Conductivity (Ω−1cm-1)
Raman Spectroscopy
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Ø Electrical conductivity were measured in a cryostat using a four-point setup.
Electric Conductivity (Ω−1cm-1)
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Temperature (K)
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Silicon w/ nanomesh Silicon w/o nanomesh
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Temperature (K)
