Photoexcitation of valley-orbit currents in (111)-oriented silicon metal-oxide-semiconductor field-effect transistors

We demonstrate the injection of pure valley-orbit currents in multivalley semiconductors and present the phenomenological theory of this effect. We studied photoinduced transport in (111)-oriented silicon metal-oxide-semiconductor field effect transistors at room temperature. By shining circularly polarized light on exact oriented structures with six equivalent valleys, nonzero electron fluxes within each valley are generated, which compensate each other and do not yield a net electric current. By disturbing the balance between the valley fluxes, we demonstrate that the pure valley-orbit currents can be converted into a measurable electric current.

Figure 1

(a) Excitation of (111)-oriented Si-MOS structures with, e.g., circularly polarized light. (b) Orientation of an individual valley. (c) Distribution of helicity-dependent fluxes over valleys induced in the exact (111) Si-MOSFET by normally incident light. The fluxes ${\mathit{i}}_{\nu }^c$ are calculated after Eq. (4) based on symmetry arguments and determined by the coefficient $A$. They are directed perpendicularly to the valley principal axes and compensate each other to nullify the net electric current. The in-plane coordinates $y_{\nu }$ attached to the valleys $X$, $Y$, and $Z$ are shown. (d) Intravalley fluxes under excitation with linearly polarized light, the total current $\mathit{j}=e{\sum }_{\nu }{\mathit{i}}_{\nu }\ne 0$. The directions and magnitudes of fluxes in valleys are calculated after Eq. (4) for a particular case of $C=D=0$. However, we emphasize that the choice of parameters does not affect the polarization dependence of the total current $\mathit{j}$ [see Eq. (7)].

Figure 2

(a) Photocurrent as function of the azimuth angle $\alpha$ measured in the precisely (111)-oriented MOSFET. Lines are fits to Eqs. (1) with one fitting parameter $\chi$. The upper inset shows the experimental geometry and the lower the definition of the angle $\alpha$ with respect to the sample orientation. (b) Comparison of the wavelength dependencies of the linear photocurrent $J_1=[J_y(\alpha =0^{°})-J_y(\alpha ={90}^{°})]/2\propto \chi$ (triangles) and the calculated coefficient of free carrier absorption (solid curve). (c) Helicity dependence of the photocurrent, full line is a fit to Eq. (2).

Figure 3

Photocurrent as a function of the angle $\phi$ measured in the transistors prepared on the miscut surface. Lines are fits to Eqs. (3). The inset shows the experimental geometry. On top, the polarization ellipses corresponding to various angles $\phi$ are illustrated.