Magnetotransport, structural and optical characterization of p-type modulation doped heterostructures with high Ge content Si1-xGex channel grown by SS-MBE on Si1-yGey/Si(001) virtual substrates

This thesis is a report on experimental investigations of magnetotransport, structural and optical properties of p-type modulation doped (MOD) heterostructures with Si1-xGex channel of high Ge content (0.6<x<1) grown on Si1-yGey/Si(001) virtual substrate (VS). The active layers of MOD heterostructures were grown by solid source molecular beam epitaxy (SS-MBE). The VSs were grown either by SS-MBE or low-pressure chemical vapour deposition (LP-CVD). The influence of thermal annealing on magnetotransport, structural and optical properties of Si1-xGex/Si1-yGey heterostructures was studied by performing the post growth furnace thermal annealing (FTA) treatments in the temperature range of 600-900C for 30min and rapid thermal annealing (RTA) treatments at temperature 750C for 30sec. Structural and optical analysis of p-type MOD Si1-xGex/Si1-yGey heterostructures involved the techniques of cross-sectional transmission electron microscopy, ultra low energy secondary ion mass spectrometry, photoluminescence spectroscopy, micro-Raman spectroscopy and scanning white-light interferometry. From the combinations of experimental results obtained by these techniques the Ge composition in the SiGe heteroepilayers, their thicknesses, state of strain in the heteroepilayers and dislocations microstructure in VSs were obtained. After post growth thermal annealing treatments were observed broadening of the Si1-xGex channel accompanied with the reduction of Ge content in the channel and smearing of Si1-xGex/Si1-yGey interfaces. The Si0.7Ge0.3 on low-temperature Si buffer VSs with very good structural properties were designed and grown by SS-MBE. These include: relatively thin 850nm total thickness of VS, 4-6nm Peak-to-Valley values of surface roughness, less than 105cm-2 threading dislocations density and more than 95% degree of relaxation in the top layers of VS. The Hall mobility and sheet carrier density of as-grown and annealed p-type MOD Si1-xGex/Si1-yGey heterostructures were obtained by a combination of resistivity and Hall effect measurements in the temperature range of 9-300K. The FTA at 600C for 30min was seen to have a negligible effect on the Hall mobility and sheet carrier density. Increasing the annealing temperature resulted in pronounced successive increases of Hall mobility accompanied by the opposite behaviour of sheet carrier density. Each sample had the optimum FTA temperature corresponded to the maximum Hall mobility. After RTA at 750C for 30sec the increase of Hall mobility for researched samples was observed as well. The highest mobility (at sheet carrier density) of 2DHG measured at 9K was observed for sample containing Ge channel grown on thick Si0.4Ge0.6 linearly graded VS and corresponds to 14855cm2·V-1·s-1 (2.87·1012cm-2). The highest Hall mobility (at sheet carrier density) measured at 293K was observed for Si0.2Ge0.8/Si0.65Ge0.35 heterostructure after FTA at 750C for 30min and corresponds to 1776cm2·V-1·s-1 (2.37·1013cm-2). To extract the drift mobility and sheet carrier density of 2DHG at temperatures up to 300K, magnetotransport measurements in magnetic fields up to 11T were performed on several heterostructures. The data were analyzed by technique of Maximum-Entropy Mobility Spectrum Analysis. The highest drift mobility (at sheet carrier density) of 2DHG at 290K was obtained for the Si0.2Ge0.8/Si0.65Ge0.35 heterostructure after FTA at 750C and corresponds to 3607 cm2·V-1·s-1 (4.94·1012cm-2. Low temperature magnetotransport measurements down to 350 mK and at magnetic fields up to 11T were carried out on several heterostructures. From the temperature dependence of the Shubnikov-de Haas oscillations observed at temperatures below 20K were extracted followed parameters of 2DHG, -- effective mass, sheet carrier density, transport and quantum scattering times, and related parameters. For the Si0.05Ge0.95/Si0.37Ge0.63 heterostructure was obtained the lowest hole effective mass m*=0.15·m0 and the highest transport to quantum scattering times ratio α=2.18. (author)