Becker, Christian E.; (2003) Transport properties of modulation-doped Si/SiGe quantum well structures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Magnetotransport measurements have been performed at low temperatures and high magnetic fields on a range of high-quality n- and p-type modulation doped Si/SiGe heterostructures grown by gas source molecular beam epitaxy in order to study the transport properties of the respective two-dimensional electron and hole gas systems. The analysis of the Shubnikov-de Haas effect provided vital information that allowed a structural and electrical characterization of the investigated samples. The observed anomalies in the low-temperature magnetotransport phenomena could be explained by giving a full description of the quantum mechanical states of the system. A self-consistent Poisson-Schroedinger solver has been used to model the band structure of the samples and to engineer the wavefunction. As a result, novel p-type structures were suggested in which the wavefunction is pulled away from the inferior upper interface by employing a step-graded Ge profile. In agreement with theory, the electron effective mass is unaffected by variations in strain or magnetic field, whereas the hole effective mass displays a dependence on both the strain and the magnetic field due to nonparabolicity effects in the valence band. The high-mobility n-type samples are dominated by long-range scattering from remote ionized impurities. However, the contribution of short-range scattering from the interface and especially from threading dislocations in the virtual substrate is still believed to be significant, and a substantial increase in scattering from threading dislocations could be seen in the low-mobility n-type structures. In the p-type samples, the mobility is predominantly limited by interface charge scattering with a growing contribution from interface roughness scattering as the Ge content and thus the sheet density increases. Representing the dominant scattering mechanism at room temperature, phonon processes have also been studied at low temperatures, whereby the energy relaxation of hot electrons and hot holes occurs by acoustic phonon interactions via unscreened deformation-potential coupling. Values for the deformation potential have been extracted from the temperature dependence of the mobility in the equipartition regime and from the energy-loss rate in the Bloch-Gruneisen regime. In both types of structures, the deformation potential has been found to decrease with increasing strain.

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