In situ annealing of nanoporous silicon thin films with transmission electron microscopy

• Published in: Applied physics letters 2023-12, Vol.123 (24)
• Main Authors: Li, Qin-Yi, Medina, Fabian Javier, Kokura, Kosuke, Jin, Zheyu, Takahashi, Koji, Hao, Qing
• Format: Article
• Language: English
• Subjects: Amorphization; Applied physics; Coherent scattering; Defect annealing; Desalination; High vacuum; Nanofabrication; Phase coherence; Phonons; Photovoltaic cells; Silicon films; Solar cells; Thermoelectric cooling; Thin films; Transmission electron microscopy
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/5.0181143

Nanoporous films have potential applications in thermoelectric cooling on a chip, sensors, solar cells, and desalination. For phonon transport, amorphization and other pore-edge defects introduced by the nanofabrication processes can eliminate wave effects by diffusively scattering short-wavelength phonons and thus destroying the phonon phase coherence. As a result, phononic effects can only be observed at 10 K or below, when long-wavelength phonons become dominant for thermal transport. In this work, a 70-nm-thick silicon thin film with approximately 100-nm-diameter nanopores was annealed under a high vacuum, and the change of pore-edge defects was observed with in situ transmission electron microscopy. It was found that the pore-edge defects can be minimized to a sub-1-nm layer by annealing between 773 and 873 K for 30 min, without changing the pore sizes. The largely reduced pore-edge defects are critical to the desired phonon wave effects within a periodic nanoporous structure.