Shin, Dong Hoon; (1999) Magnetotransport phenomena in modulation doped n-channel Si/Si0.7Ge0.3 quantum well structures. Doctoral thesis (Ph.D), UCL (University College London).

Text Magnetotransport phenomena in modulation doped n-channel SiSi0.7Ge0.3 quantum well structure.pdf Download (7MB)

Abstract

The work described in this thesis involves magnetotransport measurements performed on n-channel Si0.7Ge0.3/Si modulation doped quantum well structures. These systems are grown by gas source molecular beam epitaxy, and are found to be quasi two-dimensional. Most of the work is carried out at low temperatures between 0.05K and 4.2K using magnetoresistivity measurements on Hall bar devices, in order to understand the physics of the two-dimensional electron gas (2DEG). Shubnikov-de Haas measurements between 0.05 and 1.6K have been performed on three modulation-doped n-type Si/Si0.7Ge0.3 heterostructures, and the results analysed to extract the quantum relaxation time. Use of the conventional Dingle formula resulted in deviations from the expected theoretical behaviour above 0.3K. The corresponding quantum relaxation time appeared to increase with temperature in the same range. The data are then re-analysed using a modified expression, in which the thermal damping term is neglected. This gave plots with the correct characteristics, and as expected for ionised impurity scattering in a degenerate electron gas, the quantum relaxation time is found to be approximately constant with temperature. SdH oscillations corresponding to the spin-Landau levels of a 2DEG at 100mK are also studied. The method used to obtain the effective Lande g factor can be described as the method of coincidences, where the tilt angle that causes adjacent SdH minima to be equal, is used to determine the g* factor. The effective g factor for v=6 – 8 is 3.34 ± 0.05. The results demonstrated that the effective g factor oscillates as a function of the Landau level. The temperature dependence of the σxx in their ρxx minima in SdH oscillation at integer filling factor is analysed. Above 1K, the resistivity is thermally activated. However, the activation energy obtained from the experiments is much lower than the cyclotron energy. This has been attributed to the fact that the Lande g factor should be larger than the bulk value of g=2 and the Landau levels are broadened due to impurity scattering. The temperature dependence of the resistivity deviated from the expected behaviour below 1K, as also found in measurements on disordered systems, and this is interpreted as being due to hopping conduction.

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