Cyclotron resonance of electrons in surface space-charge layers on silicon

Abstract

We study the high-frequency magnetoconductivity of the quasi-two-dimensional electron gas in a surface space-charge layer on Si. The experiments are carried out at low temperatures and in high magnetic field, using a far-infrared laser transmission spectrometer. Surface cyclotron resonance of electrons in inversion and accumulation layers on the (100) surface of Si has been investigated. The two dimensionality of the electron system is demonstrated by experiments in magnetic fields tilted with respect to the surface normal. Quantum oscillations are observed in the resonance amplitude, and are explained by a recent theory of Ando and Uemura. The resonance line shape is compared with the theoretical predictions. Scattering times, as extracted from the line shape, are related to those obtained from low-frequency conductivity experiments. The influence of a substrate bias voltage on the resonance is investigated. Subharmonic structure of the cyclotron resonance and its dependence on the surface charge density $n_s$ are explored in the experiments. From the position of the fundamental resonance the cyclotron effective mass $m_c^{*}$ is obtained. For $n_s\gtrsim 1\times {10}^{12}$ ${\mathrm{cm}}^{-2\mathrm{}}$ and temperatures in the liquid-helium range we find $m_c^{*}=(0.197\mathrm{}\pm 0.005)m_0$ independent of $n_s$. For $n_s<1\mathrm{}\times {10}^{12}$ ${\mathrm{cm}}^{-2\mathrm{}}$ a marked increase of $m_c^{*}$ with decreasing $n_s$ is observed. In some samples, and at $n_s\lesssim 0.5\times {10}^{12}$ ${\mathrm{cm}}^{-2\mathrm{}}$, a sample- and $n_s$-dependent decrease of the resonance field is found and interpreted as resulting from effects of localization. Results for electrons on (110) and (111) surfaces are also presented. The effect of multiple reflections in the semiconductor substrate on the transmission line shape of the surface cyclotron resonance is discussed.