Nanoporous patterns have been widely used to dramatically reduce the thermal conductivity of thin films to achieve superior thermoelectric performance. Ion milling and reactive ion etching (RIE) are two common approaches to drill nanopores. Although ultra-fine pores are desired to minimize the thermal conductivity by classical phonon size effects and sometimes wave effects, the minimum achievable pore size is largely restricted due to the restriction of the aspect ratio for the employed techniques.

For nanostructured bulk Si, the thermoelectric performance monotonously improves till \~1200 K, beyond which the material can become unstable.\[1-3\] For nanoporous Si, one concern is the thermal stability of nanoporous structures because bulk nanoporous materials may change their porous structures even at 673 K for 45 min annealing.\[4\]  The high thermoelectric performance of nanoporous Si thin films may not reach its full potential with this temperature limit.

In this work, high-temperature annealing above 1073 K has been carried out to study the possible shape change of nanoporous Si thin films. At 1273 K, it is found that the pore size can still be largely maintained when the nanoporous Si thin film is on a SiO2/Si substrate. However, the pore size can significantly shrink when the film is suspended. The contrast suggests that the SiO2/Si substrate can play an important role in maintaining the nanoporous patterns at a high temperature. In comparison, annealing of the suspended nanoporous thin films has also been performed. Our results suggest a much better long-term thermal stability of nanoporous Si films when they are compared with randomly etched bulk nanoporous Si.  Besides the pore shape changes, the influence of annealing on the pore-edge defects (e.g., ion implantation and other damages) is also investigated. It can be shown the pore-edge defects can be largely reduced after annealing for ion-beam-drilled pores.  The thermal conductivity variation is also experimentally measured to reveal the impact of annealing on the transport properties.   

The finding in this work can provide important guidelines for the high-temperature application of nanoporous structures.

 

\[1\] Zhu, G. H., Lee, H., Lan, Y. C., Wang, X. W., Joshi, G., Wang, D. Z., Yang, J., Vashaee, D., Guilbert, H., Pillitteri, A., Dresselhaus, M. S., Chen, G., and Ren, Z. F., 2009, "Increased Phonon Scattering by Nanograins and Point Defects in Nanostructured Silicon with a Low Concentration of Germanium," Physical Review Letters, 102(19), p. 196803.

\[2\] Yusufu, A., Kurosaki, K., Miyazaki, Y., Ishimaru, M., Kosuga, A., Ohishi, Y., Muta, H., and Yamanaka, S., 2014, "Bottom-up nanostructured bulk silicon: a practical high-efficiency thermoelectric material," Nanoscale, 6(22), pp. 13921-13927.

\[3\] Bux, S. K., Blair, R. G., Gogna, P. K., Lee, H., Chen, G., Dresselhaus, M. S., Kaner, R. B., and Fleurial, J.-P., 2009, "Nanostructured Bulk Silicon as an Effective Thermoelectric Material," Advanced Functional Materials, 19(15), pp. 2445-2452.

\[4\] Salonen, J., Mäkilä, E., Riikonen, J., Heikkilä, T., and Lehto, V.-P., 2009, "Controlled enlargement of pores by annealing of porous silicon," physica status solidi (a), 206(6), pp. 1313-1317.

 


High-Temperature Thermal Stability of Nanoporous Si Thin Films

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Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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