Electron Spin Physics in Silicon-Germanium Quantum Dots

My previous work focused on the spin properties, in particular on the effects of spin-orbit interaction (SOI) of electrons confined in low-dimensional SiGe structures. Si is the most important material in the classical semiconductor technology, and it also has very promising properties for future applications in the field of spin- electronics (spintronics) and quantum computation. In particular, the strength of the SOI is weak in Si compared to other semiconductor materials. The properties of Si/SiGe heterostructures grown by molecular beam epitaxy on Si substrates were investigated in electron spin resonance (ESR) experiments. The layer sequence of these structures is chosen in such a way that a two-dimensional electron gas (2DEG) forms, in which the electron motion is confined within the Si quantum well layer. The electron mobility in such structures is generally very high. The structural asymmetry of the quantum well, imposed by the layer structure of the device changes the SOI acting on the 2D electrons dramatically, and has a strong influence on the ESR signal. In particular, both the ESR line width and the electron g-factor depend strongly on the relative orientation of the external magnetic field and the 2DEG plane. This so-called Bychkov- Rashba (BR) effect depends on the device geometry and on the electron momentum. The spin relaxation times in these structures are on the order of microseconds, limited by the BR effect. Quantum dot systems are attractive candidates for electron-spin-based quantum computation, as the spins of electrons in the dots is decoupled from its environment. The spin relaxation times in quantum dot systems are considerably longer than in two-dimensional structures since the SOI-related spin relaxation mechanisms are quenched. On the other hand, spin relaxation via hyperfine interaction with lattice nuclei is enhanced compared to the 2D case. In this project I plan to investigate the spin properties, in particular the spin relaxation mechanisms of conduction electrons confined in large ensembles of quantum dots created from Si/SiGe heterostructures. There are several ways to fabricate ensembles of quantum dots. Firstly, quantum dot arrays will be defined using either electron beam or nanoimprint lithography followed by etching. This way, only pillars remain of the original 2DEG, which confines the electron motion in all three spatial dimensions. Furthermore, the etched quantum dots will be furnished with electrostatic top-gates in order to directly control the dot occupancy. Secondly, metallic gate structures deposited on top of the 2DEG structures will be patterned using either electron beam or nanoimprint lithography followed by etching. This creates arrays of holes in the top-gate. A negative voltage applied to these gates locally depletes the 2DEG below, and thus creates a large ensemble of quantum dots. In such structures the strength of the confining potential can be directly controlled by adjusting the gate voltage. This method also allows for asymmetric quantum dots, in which the strength of the coupling between electrons inside a dot can be controlled. These devices will be analyzed using ESR at various temperatures (down to 300 mK). ESR has turned out to be a versatile and sensitive method for the investigation of the spin properties of low dimensional SiGe structures. This work - carried out at the Department of Electrical Engineering at Princeton University will lead to a deeper insight into the magnetic properties and the spin relaxation mechanisms in SiGe quantum dot systems. This will give us some important information on the choice of materials and device structures for quantum dot based spin-electronics and quantum computation applications.