research review
Breaking conventional concepts in SiGe thermoelectric materials
Using a modulation-doping strategy in 3D bulk silicon germanium thermoelectric materials enhances their carrier mobility and hence electrical conductivity by over 50 percent
In the last couple of decades
thermoelectrics have been drawing more and more research interest due to the limited availability and the negative environmental impact of conventional energy strategies.
In the past, as a measuring stick of the conversion efficiency, the term ‘dimensionless figure-of-merit,’ also referred to as ZT, has been widely used. A high ZT value usually promises high thermoelectric performance. Typically, good thermoelectric materials should combine low thermal conductivity and good electrical conductivity.
Striving to enhance the performance of thermoelectric materials, researchers from Boston College and MIT have recently reported a novel materials design to achieve a 30 to 40 percent enhancement in the peak ZT value for n-type SiGe semiconducting alloys.
Bo Yu from Boston college,who is lead author of a paper describing the recent work, says that SiGe has been almost the exclusive choice for high temperature thermoelectric applications. This material has been used in the radioisotope thermoelectric generators employed by US NASA ever since 1976. However, the broader application of SiGe has been limited by the fact that germanium, which is used to reduce the thermal conductivity in such alloys, is extremely expensive and the cost has to justify the performance.
The scientists reported that the modulation- doping strategy, conventionally used in the thin-film semiconductor industry, could also be utilised in the 3D bulk thermoelectric materials to enhance their carrier mobility and therefore the electrical conductivity, by over 50 percent in this case.
By improvising materials design, the team also achieved a simultaneous reduction in the thermal conductivity, which combines to provide a high ZT value of about 1.3 at 900 °C.
“To improve materials ZT is extremely challenging because all the internal
June 2012
www.compoundsemiconductor.net 45
parameters are closely related to each other. Once you change one of them, the others may most likely change accordingly to the other extreme, leading to no net improvement. As a result, a more popular trend in this field of study is to look into new opportunities, or say a new material system. However, our study proved that opportunities are still there for the existing materials, if one could work smartly enough to find some alternative material designs,” explains Bo Yu.
Zhifeng Ren also points out that this reported ZT peak value competes well with the state-of-art n-type SiGe alloy materials while the new material design requires over 30 percent less of germanium.
“That is a significant advantage to cut down the fabrication cost. We want all the materials we studied in the group to be really used by peopl ,” adds Ren.
Bo Yu et al. Nano Letters 12 2077 (2012)
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