Dr. DI Thomas Berer
"Lateral Quantum Dots in Strained Silicon/Silicon-Germanium Heterostructures realized by a Schottky Split-Gate Technique"
In this thesis lateral quantum dots were realized with a Schottky split-gate technique on high mobility silicon/silicon-germanium modulation-doped samples. As the size of the lateral quantum dots is comparable with the electron wavefunction, the wave nature of the electrons plays a significant role, and the energy levels in the dots are quantized.
Starting point for the fabrication were strained Si/SiGe heterostructures, which were modulation-doped to achieve high mobilities. Hall and Shubnikov-de Haas measurements were performed to determine the carrier densities and electron mobilities of the heterostructures. The two-dimensional electron gas (2DEG) of the heterostructure used for the fabrication of the devices showed an electron mobility of about 1.5×105 cm2/Vs at carrier densities between 3.2×1011 cm-2 and 3.6×1011 cm-2. The split-gate arrangements were prepared on Hall bar structures. Different kinds of Ohmic contacts were tested and the parameters for ion etching of the mesa were optimized. The splitgate structures were prepared by e-beam lithography into polymethyl methacrylate (PMMA) resist with a Raith Elphy control unit in combination with two different scanning electron microscopes. Most of the devices were fabricated with a newly commissioned scanning electron microscope (LEO Supra 35 FE-SEM). It was installed and the appropriate e-beam lithography system was brought to working conditions. Also, a new optical mask suited for various types of split-gate based quantum devices was developed.
Fabrication of the split-gates was done by lifting-off a palladium metalization layer in the unexposed areas of the resist. Finally, connections from the split-gates to the bond pads were made by optical lithography, using palladium or chromium/gold lift-off.
Electrical measurements were performed at low temperatures in a 3He cryostat and in a dilution refrigerator. Measurements at low temperatures showed well-controlled leakage currents of the split-gates. Coulomb-blockade and stability diamonds were recorded, showing that the dots contain only a few electrons. The experiments demonstrated that, in contrast to reports in literature, Schottky gates are a feasible approach for the integration of lateral quantum dots in the strained Si/SiGe heterosystem.