Controlled Valley Splitting in SiO2/Si/SiO2 Quantum Wells

Kei Takashina, Akira Fujiwara and Yoshiro Hirayama
Physical Science Laboratory

@Besides being of immense technological importance, electrons in silicon offer a number of unique possibilities for exploring new physical conditions and new phenomena. One of these arises due to their bulk dispersion relation where there are six, energetically degenerate conduction band valleys. In (100) Si-MOSFETs where electrons are two-dimensionally confined, this six-fold degeneracy is lifted, due to anisotropic effective mass, to leave only two low lying valleys available for occupation. 2-D electrons in such structures consequently have freedom as to how they occupy these degenerate valleys giving them a valley degree of freedom on top of in-plane motion and spin.
@In the present study, we have been able to show that valley-splitting, which lifts this remaining two-fold valley degeneracy can be controlled over an unprecedented extent using SOI (Silicon-On-Insulator) MOSFETs. These structures consist of silicon sandwitched between layers of SiO2 forming SiO2/Si/SiO2 quantum wells. Since there are two Si-SiO2 interfaces present in such structures, by using interfaces with different properties, electronic properties can be readily tuned by shifting the wavefunction between them. Here, we have used SIMOX (Separation by IMplantation of OXygen) MOSFETs where we find the valley-splitting to be strongly enhanced at the Si-buried oxide interface. The valley splitting can therefore be tuned, simply by adjusting front and back gate voltages to shift the wavefunction between the interfaces with small and large valley splitting [1,2].

[1] T. Ouisse, D.K. Maude, S. Horiguchi, Y. Ono, Y. Takahashi, K. Murase and S. Christoleanu, Physica B, 249-251, 731 (1998)
[2] K. Takashina, A. Fujiwara, S. Horiguchi, Y. Takahashi, Y. Hirayama, Phys. Rev. B, 69, 161304(R), (2004), K. Takashina, Y. Hirayama, A. Fujiwara, S. Horiguchi, Y. Takahashi,
Physica E, Vol 22, 72 (2004)


Left: A schematic diagram of Landau levels with increased valley-splitting.
Right: Longitudinal resistance at 11T. Numbers indicate filling factors associated with quantized Hall states.