Much of the ground work into the physics of 2-dimensional electron systems was carried out using the inversion layer of silicon MOSFETs as the experimental system more than two decades ago [1]. Since that time, most physicists have turned to III-V based structures for their 2D (and lower D) electrons, attracted by their higher mobilities and new possibilities opened up to them through band engineering.

In the mean time, of course, silicon technology continued to develop and has firmly established itself as a cornerstone of modern society.

Among the relatively recent developments are key technologies which we believe will allow new experimental systems for new physics. Physical insights into new systems, in turn, can be expected to yield not only advances in basic physics and material characterization required for modeling and design of smaller devices, but new device concepts inspired by the new physics. Silicon also possesses a number of special properties, some of which allow all this technolgy in the first place, and others which attract more scientific interest.

Specific projects:

Silicon based two-dimensional electron-hole systems:

Electrons and holes are charge carriers with opposite charge. Excitons are their bound pairs. In two dimensions, electrons and holes can be spatially separated as parallel sheets. If the two layers are far apart, they will behave as separate and independent layers of electrons and holes. If, on the other hand, the electrons and holes are closer together than electrons or holes are among themselves, the system should behave more like a sheet of excitons with diople moments. Various states of matter can be anticipated, including exotic ones, depending on the densities, electron-hole separation, temperature etc. The silicon / silicon dioxide material system provides unique opportunities for probing the electron-hole system and we are intent on taking them [2]. Please contact me for more details.

FIGURE: Schematic diagram of a two-dimensional electron-hole system [2].

The silicon-silicon dioxide interface and band structure:

The interface between silicon and its thermal oxide is one of the most thoroughly investigated interfaces between any two materials, yet there is a basic issue that remains unresolved concerning the band structure and its valley degeneracy and splitting [3]. We aim to perfom experiments to clarify this issue. Please contact me for more details.

FIGURE: Double derrivative of the conductance of a silicon 2DEG generated in a silicon MOSFET. Features A, B and C are due to the onset of occupation of subbands. A arises from the first excited confinement subband as predicted by self-consistent calculations (squares) while C and B are due to valley splitting. [3]

Silicon / silicon-dioxide multilayer structures:

Compared to III-V based systems where epitaxial techniques allow complicated heterostructures to be built, the silicon / silicon-dioxide system is at a fundamental disadvantage in that, a monocrystalline layer of silicon cannot be epitaxially grown on amorphous oxide. We aim to make steps to overcome this by use of direct bonding. This can be expected to provide access to new physical regimes inaccessible to GaAs based systems. It may also yield steps towards real three-dimensional freedom in semiconductor fabrication technology. [Figure shows a cross-sectional TEM image of a double layer structure of monocrystalline silicon (dark) with amorphous thermal oxide (light) barriers [4]. Presented at NMDC (Kyoto, 2008).] Please contact me for more details.

FIGURE: A cross-sectional TEM image of a double layer structure of monocrystalline silicon (dark) with amorphous thermal oxide (light) barriers [4].

References:

[1] T. Ando, A. Fowler and F. Stern, Rev. Mod. Phys. 54, 437 (1982) [LINK]
[2] K. Takashina, K. Nishiguchi, Y. Ono, A. Fujiwara, T. Fujisawa, Y. Hirayama, and K. Muraki, Appl. Phys. Lett. 94, 142104 (2009) [LINK]
[3] K. Takashina, Y. Ono, A. Fujiwara, Y. Takahashi, and Y. Hirayama, Phys. Rev. Lett. 96, 236801 (2006) [LINK]
[4] K Takashina, M Nagase, K Nishiguchi, Y Ono, H Omi, A Fujiwara, T Fujisawa and K Muraki, Semicond. Sci. Technol. 25, 125001 (2010) [LINK]