Silicon and silicon-germanium epitaxy for quantum dot device fabrications towards an electron spin-based quantum computer

Semiconductor quantum dots are promising candidates as qubits for spin-based quantum computation as they provide highly tunable structures for trapping and manipulating individual electrons. It is the objective of this doctorate thesis to study on the development of silicon and silicon-germanium epitaxy and nanofabrication techniques for quantum dot devices, and the performance level achieved in the silicon/silicon-germanium material heterosystem.;We describe the growth of two-dimensional electron gas structures in strained Si on high-quality SiGe relaxed buffers with low temperature mobility exceeding 10,000 cm2/Vs, currently limited by the background impurities in our RTCVD system. The modulation of the electron gases using atomic layer deposited Al2O3 is also demonstrated. We have developed a wide range of fabrication methods of the electron gases for quantum dot applications including nanolithography and etching techniques optimized for etch selectivity and anisotropy. Feature sizes well under 100 nm can be reliably obtained.;To achieve precise control of exchange coupling of qubits, we present a new concept of parallel 2-D electron gases in a double quantum wells as interaction dimers. A typical value of 0.1 meV for symmetric-anti-symmetric splitting of subbands is predicted by modeling. The signature of inter-well scattering is proved by a negative transconductance effect measured in such structures. The physical realization of such qubit dimers can also enable a novel "flying qubit" scalable architecture for semiconductor-based quantum computers.;The robustness of quantum dot devices is often strongly affected by defect states on the surface arising from the Si/SiO2 interface. We demonstrate the use of epitaxial regrowth of SiGe for surface passivation, done with thermal cleaning temperatures less than 800°C and negligible degradation of device performance. Side-gated multiple quantum point contacts are fabricated. They can be used to completely deplete electrons on a quantum dot with gate leakage less than a few nA. We have also observed periodic single electron tunneling conductance peaks in a single quantum dot transistor with a side-gate-to-dot capacitance of 4.4 aF.