Assistant Professor, Department of Materials Science and Engineering Kyushu University, Japan

Biography: Yoshifumi Ikoma received the B. Eng., M. Eng., and Dr. Eng. degrees in materials science and engineering from Kyushu University, Fukuoka, Japan, in 1994, 1996, and 1999. He joined Department of Materials Science and Engineering, Kyushu University, as a research associate in 1999, and an assistant professor in 2007. His research interests are semiconductor materials science including heteroepitaxial growth and multilayer formation of SiC/Si on Si substrates by pulsed jet CVD, the formation of semiconductor nanopores by utilizing SiC/Si heteroepitaxial growth, and bulk nanostructured semiconductor materials such as Si, Ge and GaAs. He is currently interested in severe plastic deformation of semiconductor materials by high-pressure torsion.

Speech Title: Development of novel semiconductor materials using process of high-pressure torsion

Abstract: Semiconductor materials having novel properties are essential for developing future high-performance electronic devices. Among various approaches to improve material fuctionalities, semiconductor nanocrystals and allotropes are attractive, because they exhibit different optical, electrical and electronic properties from the bulk crystal and stable phases. Severe plastic deformation (SPD) has been widely studied to obtaine ultrafine grained metallic materials [1]. High-pressure torsion (HPT) [2] is one of the SPD technique, and is applicable to various brittle materials including ceramics [3] and semiconductors [4]. We have investigated HPT processing to various semiconductor materials such as Si and Ge, and found that the grain size was reduced to nanometer scale [4]. We also found that the HPT-processed Si consisted of diamond-cubic Si-I and metastable phases such as semimetallic Si-III and narrow-bandgap Si-XII [5]. No appreciable photoluminescence (PL) was observed from as-HPT-processed samples while a broad PL peak in the visible light region appeared after annealing [5]. The resistivity increased by 10–100 after compression, but it decreased after HPT processing due to the formation of metastable phases [6]. The resistivity significantly inceased after annealing because metastable phases transformed to Si-I retaining nanograins. The similar grain refinement and the appearance of PL after annealing were also observed in the Ge case, but the formation of metastable phases was depedent on the process temperture during HPT processing [7]. These results indicate that HPT processing is promising for exploring novel functional properties of existing semiconductor materials.

[1] R.Z. Valiev, Y. Estrin, Z. Horita, T.G. Langdon, M.J. Zehetbauer and Y.T. Zhu: JOM 58 (2006) 33–39.
[2] A.P. Zhilyaev and T.G. Langdon: Prog. Mater. Sci. 53 (2008) 893–979.
[3] K. Edalati, S. Toh, Y. Ikoma and Z. Horita: Scr. Mater. 65 (2011) 974–977.
[4] Y. Ikoma: Mater. Trans. 60 (2019) 1168–1176.
[5] Y. Ikoma, K. Hayano, K. Edalati, K. Saito, Q. Guo and Z. Horita: Appl. Phys. Lett. 101 (2012) 121908.
[6] B. Chon, Y. Ikoma, M. Kohno, J. Shiomi, M.R. McCartney, D.J. Smith and Z. Horita: Scr. Mater. 157 (2018) 120–123.
[7] Y. Ikoma, K. Kumano, K. Edalati, K. Saito, Q. Guo and Z. Horita: Philos. Mag. Lett. 97 (2017) 2734.

Keywords: Si, Ge, nanograins, phase transformation, metastable phases, severe plastic deformation