Optical study of electron and acoustic phonon confinement in ultrathin-body germanium-on-insulator nanolayers

Abstract

Nanoelectronics require semiconductor nanomaterials with high electron mobility like Ge nanolayers. Phonon and electron states in nanolayers undergo size-dependent changes induced by confinement and surface effects. Confined electrons and acoustic phonons determine layer optical, electric and thermal properties. Despite scientific and practical significance, their experimental studies in individual nanolayers are still lacking. Thanks to recent progress in the fabrication of high-quality nanolayers, here, we report the thickness dependencies of Raman spectra of acoustic phonons and optical spectra of electrons confined in germanium-on-insulator (GeOI) nanolayers with thicknesses T_GeOI = 1–20 nm. We show that for T_GeOI > 5 nm, both GeOI acoustic phonon Raman spectra and the E₁ electron energy gap display dependencies on T_GeOI which are reasonably described by the corresponding phonon and electron confinement theories. Accordingly, T_GeOI can be probed using acoustic phonon Raman spectra at T_GeOI > 5 nm. However, both confinement theories fail to describe GeOI thickness dependencies at T_GeOI < 5 nm. We attribute this discrepancy to an increased influence of the Ge–GeO₂ interface disorder with T_GeOI reduction. The acoustic phonon data suggest a decrease of Ge normal-to-the-layer longitudinal sound velocity. Generation of interface-disorder-induced dispersionless phonons might contribute to this. The change in GeOI phonon properties at T_GeOI < 5 nm might influence E₁(T_GeOI) dependence via a change in the GeOI electron–phonon interaction. We demonstrate that the Al₂O₃ coating improves the agreement between experimental and confinement theories, probably, via reduction of disorder at the Ge–GeO_x–Al₂O₃-interface. Our results are important for control of nanolayer-confined electrons and phonons with benefits for modern and future nanoelectronic devices.