Thermo-electric properties of two-dimensional silicon based heterostructures

The thermoelectric properties of a material are strongly sensitive to the details of heat charge transport within. Throughout the literature, measurements of thermoelectric power have consistently provided new or improved information for a large variety of materials. For two-dimensional electronic systems the thermopower thus offers high quality data, if not unique, for use in the optimisation of device structure and material growth of these increasingly important devices. The work of this thesis has significantly included the development of experimental apparatus and technique appropriate to successfully obtaining good quality data on the thermoelectric power and thermal conductivity of the heterostructure samples studied. The equipment and procedures developed were very successful and have yielded results of a high quality. In particular, the specialised cryostat design is the subject of a paper in preparation. The newly developed apparatus and techniques have been fully characterised and 'standard' samples measured in order to provide confirmation that the results obtained are reliable, accurate and traceable. The phenomena of parallel conduction in Si based heterostructure systems has been investigated for the first time via the thermopower. Results from both Si/Si_1_-xGe_x samples and Sb and B delta doped samples have been used to demonstrate the scope of this effect which is dominant above 80K and contributes approximately a 5% error at Helium temperatures for most samples of this type. The persistent photoconductivity effect has been studied for the first time in Si/Si_1_-_xGe_x two-dimensional hole gases (2DHG's) and initial results provided. Evidence for an enhancement of the 2DHG sheet carrier density by photogenerated carriers is presented and a clear effect on the thermopower is shown. Extension of the work of previous authors on the phonon mean free path in heterostructure samples has been accomplished by the introduction of a novel technique for directly influencing the mean free path for phonons in these systems. This work contributes strong evidence for the dominance of the 'phonon drag' thermopower in these samples at low temperatures and a novel confirmation of the theory of phonon drag in 2D heterostructure systems. The dominance of phonon drag in Si/Si_1_-_xGe_x 2DHG's has been shown for the first time over a range of sheet carrier densities from 0.39 x 10"1"1 cm"-"2 to 10.5 x 10"1"1 cm"-"2. This work is extended by theoretical analysis from other workers to provide a value for the important effective deformation potential constant of a strained Si/Si_0_._8Ge_0_._2 channels in these heterostructures. The value of 4.5eV for this constant is supported by independent work on hot electron properties within the author's group. Thermopower results for delta layer systems have been provided for the first time. Sb and B layer samples have been studied over a range of sheet carrier density from 1 x 10"1"3 cm"-"2 to 1 x 10"1"4 cm"-"2. The results clearly demonstrate the dominance of phonon drag in these systems at low temperatures. Clear evidence for a metal insulator transition in both systems is given and supported by previous independent experimental investigations. (author)Available from British Library Document Supply Centre-DSC:DXN043744 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo