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)
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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. |