THESIS
2018
xxii, 107 pages : illustrations ; 30 cm
Abstract
Rapidly increasing integration density in modern miniaturized microelectronic and optical
devices stimulates the demands for low-power and fast-response on-chip refrigeration solutions.
Thermoelectric microrefrigerators offer an attractive all-solid-state solution with many prominent
merits such as high reliability, quick response, capability of cooling below ambient and good
scalability. Conventional thermoelectric microrefrigerators widely adopt integrated-circuit (IC)-incompatible toxic heavy metal compounds such as Bi
2Te
3 as thermoelectric elements, making
them unsuitable for on-chip integration. Here we developed single- and two-stage free-standing
planar thermoelectric microrefrigerators based on nanograined SiGe thin films. By combining
theoretical modeling, numerical simu...[
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Rapidly increasing integration density in modern miniaturized microelectronic and optical
devices stimulates the demands for low-power and fast-response on-chip refrigeration solutions.
Thermoelectric microrefrigerators offer an attractive all-solid-state solution with many prominent
merits such as high reliability, quick response, capability of cooling below ambient and good
scalability. Conventional thermoelectric microrefrigerators widely adopt integrated-circuit (IC)-incompatible toxic heavy metal compounds such as Bi
2Te
3 as thermoelectric elements, making
them unsuitable for on-chip integration. Here we developed single- and two-stage free-standing
planar thermoelectric microrefrigerators based on nanograined SiGe thin films. By combining
theoretical modeling, numerical simulations and experiments, we conducted a comprehensive
investigation of the steady-state and transient performances of the proposed microrefrigerators
and various factors that might influence their performance, such as contact resistances, element
geometries, convection and radiation, have been explored. Both thermal and contact resistances
are found to be important for the cooling performance of the proposed microrefrigerators while
they play different roles on the cold and hot sides of a refrigerator. The influence of contact
resistances on the design strategies of a microrefrigerator is also discussed. It is demonstrated that
microrefrigerators based on IC-compatible low-cost SiGe thin films can potentially achieve a
cooling temperature more than 20 K with a response time shorter than 40 ms near room
temperature, rendering them competitive against the state-of-the-art microfrigerators based on
toxic conventional heavy metal thermoelectrics such as Bi
2Te
3 and Sb
2Te
3.
According to our model, poor electrical contact between metal and semiconductor materials
such as SiGe will lead to a significant deterioration in the performance of thermoelectric devices
especially in micro or nano regime. Multiple metals have been used to form contacts with n-type
and p-type SiGe separately and the contact resistivities were characterized using the Kelvin
structure. Ti/Al and Cr/Pt were chosen in particular for detailed study considering the contact area
and annealing temperature effects. Ti/Al is found to be a best choice of forming ohmic contact to
heavy doped SiGe thin film with a contact resistivity around 10
-6Ωcm
2.
Based on the modeling and optimized process recipes, single-stage and two-stage
thermoelectric coolers were designed and fabricated. A maximum cooling temperature of 10.3 K
together with a response time of 16 ms has been achieved in the single-stage microrefrigerator
with a power consumption of 56 μW near room temperature while a maximum cooling
temperature of 11.2 K can be achieved in the two-stage refrigerator with 0.41 mW input power.
The cooling temperatures of the SiGe microrefrigerators also improve with the increasing
ambient temperature, reaching up to 15 K and 17 K for the single and two-stage
microrefrigerators respectively at 340 K. To our best knowledge, this is the highest cooling
temperature achieved in Si-based thermoelectric microrefrigerators so far. These SiGe thin-film
devices can deliver cooling performances comparable to the records for traditional heavy-metal-based
microrefrigerators while providing excellent IC compatibility and almost one to two orders
of magnitude higher cooling temperature to power consumption ratios (184 K/mW), making
them ideal candidates for low-power on-chip refrigeration.
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