Electron effective mass in ultrathin oxide silicon MOSFET inversion layers

The effective mass (m*) of two-dimensional electrons in silicon metal-oxide-semiconductor field-effect transistors (MOSFETs), obtained from measurements of the thermal damping of Shubnikov-de Haas oscillations, has been studied as a function of electron density (n(s)) for samples with physical gate oxide thicknesses (d(ox)) of 4.7 nm and 3.1 nm. For the latter at a low electron density, the ratio (a(B)r(S))/d(OX) (where r(s) is the interaction parameter and a(B) is the Bohr radius in the semiconductor) exceeded 2 and the modification of the electron-electron interaction potential by the presence of the metallic gate was expected to be manifested as a change in the interaction-driven enhancement of the effective mass with increasing r(S). The deduced mass enhancement in both thin-oxide samples is well described by m*/M-b = 0.96 + yr(s), where m(b) is the bare band mass within the plane of confinement, and gamma is a constant. Although the results from both samples are in good quantitative agreement with previous experiments on thicker-oxide MOSFETs, a small but significant difference in the extracted value of gamma between the thin-oxide samples was observed. This difference cannot, however, be unambiguously interpreted as a true renormalization of m* caused by the screening effect of the gate.