AbstractsEngineering

In this Thesis different aspects of band degree of freedom are explored in 2D electron transport and electron-phonon (e-ph) energy relaxation in 2D and 3D electron systems. Here the bands of interest are the conduction band valleys of many-valley semiconductors and spatial sub-bands of two-dimensional-electron gas in a quantum well. The experimental studies of electronic transport focus on double-gate SiO2-Si-SiO2 quantum well field-effect-transistors (FETs), which are fabricated utilizing silicon-on-insulator structures and wafer bonding. Double-gate FETs are intensively explored at the moment due to their prospects in microelectronics. The inclusion of a back gate electrode provides means to adjust the electron wave functions and the occupancy of the spatial 2D sub-bands. The contrast between single and two-sub-band transport is studied in low temperature conductivity/mobility and magneto transport. For example, the conductivity shows significant drop at the threshold of the second spatial sub-band due to inter-sub-band coupling and sub-band delocalization effect is observed at symmetric well potential. At room temperature several sub-bands are inevitably populated and the most relevant observed effect is the mobility enhancement towards symmetric quantum well potential. This mobility enhancement is one of the benefits of double-gate FETs in comparison to similar single-gate FETs. In the studies of e-ph energy relaxation we focus on the case where the phonons cannot directly couple the bands of the electron system. If the e-ph matrix elements depend on the band index then the band degree of freedom plays an important role. We developed a mean field theory, which allows elastic inter and intra-band scattering and also Coulomb interaction. Our model reproduces the long wavelength single-band energy loss rate results found in the literature. In the multi-band regime we find a set of new results, which suggest that the energy loss rate is strongly enhanced if the phonons couple asymmetrically to different bands and the single-band interaction is strongly screened. The effect is tested experimentally in heavily doped n-type Si samples by low temperature heating experiments. We find good agreement between the theory and experiment. Our findings enable a design of a novel electron-phonon heat switch. Työssä tutkitaan elektronien energiavöistä aiheutuvan vapausasteen vaikutuksia 2D-kuljetusilmiöissä ja elektroni-fononi (e-ph) -energiarelaksaatiossa 2D- ja 3D-elektronikaasuissa. Tutkimukset keskittyvät lähinnä johtovyön laaksoihin monen vyön puolijohteissa sekä kvanttikaivojen 2D-alivöihin. Kuljetusilmiöiden kokeelliset tutkimukset keskittyvät erityisesti SiO2-Si-SiO2-kvanttikaivoon perustuvaan kenttäefektitransistoriin (FET), jossa on kaksi hilaelektrodia ja joka on valmistettu SOI-kiekolle suoraliittämistekniikalla. Kaksoishilalliset FET-komponentit ovat tällä hetkellä suuren mielenkiinnon kohteena johtuen niiden potentiaalisista sovelluksista mikroelektroniikassa. Nämä komponentit ovat myös erinomaisia…