Experimental Research Of Quantum Transport Properties Of Graphene Quantum Dot

Quantum information area keeps attracting scientists'research interest since it comes out. It reveals a cornucopia of new physics and potential applications and has always maintained a very active research environment, many striking research efforts and advanced technologies emerge. New ideas and issues are continuously proposed. Spin-based semiconductor quantum dot for the solid-state quantum information process has been considered as a very promising direction. A lot of excellent efforts have been made in this area, but there are also many difficulties to deal with, such as how to extend the spin coherence time. Scientists have tried many methods to solve this problem, one is to use new material to substitute currently used traditional gallium arsenide semiconductor material.In this thesis, we fabricate all kind of graphene-based quantum dot devices, including single dot, single dot with integrated SET charge detector, double dot in series, double dot in parallel; We investigate devices'properties by doing the low temperature quantum transport measurement.Main Content of the thesis:1. We give an introduction on the conception of quantum information, quantum computer, and semiconductor quantum chip. We also describe the crystal and electronical properties of the newly discoveried material graphene.2. We give a detailed description on the fabrication method of graphene quantum dot devices, and also introduce the relevant facilities that have been used in the fabrication process.3. We fabricate graphene single dot device and present transport measurements in low temperature. We extract the information of the ground states and excited states and also the relevant energy scales and capacitances of the graphene quantum dot, as denoted by the presence of characteristic Coulomb blockade diamond diagrams.4. We design and fabricate a twin-dot structure in which the larger dot serves as a single electron transistor (SET) to read out the charge state of the nearby gated controlled small dot. A high SET sensitivity of 10^-3e(?) allowed us to probe Coulomb charging as well as excited state spectra of the QD, even in the regime where the current through the QD is too small to be measured by conventional transport means. 5. We fabricate and measure graphene double quantum dot devices with multiple electrostatic gates. Low temperature transport measurements reveal honeycomb charge stability diagrams which can be tuned from weak to strong interdot tunnel coupling regimes. We precisely extract a large interdot tunnel coupling strength for this system allowing for the observation of tunnel-coupled molecular states extending over the whole double dot.6. We report the fabrication and transport measurements of gates controlled parallel-coupled bilayer graphene double quantum dot. It is shown that the interdot coupling strength of the parallel double dots can be effectively tuned from weak to strong regime by both the in-plane plunger gates and back gate. All the relevant energy scales and parameters of the double dot can be extracted from the honeycomb charge stability diagrams revealed through the transport measurements.7. We summarize the work in the thesis. Based on what we have done, we give a proposal on the future plans and aims. We make new design of double dot with integrated charge detector devices to perform our upcoming experiment.Main innovations of the thesis:1. We present a systematical research on the graphene quantum dot system, including single dot, single dot with SET charge detector, double dot in series, double dot in parallel. We have clear ideas and research plans towards the goal of good performance and functionality graphene quantum chip.2. We see clear excited states in grpahene quantum dot device by using transport measurement and charge detection technology.3. We use quantum mechanical tunnel principal to investigate the molecular state of the in series coupled double dot system, and precisely extract the interdot tunnel coupling strength of the two dots.4. We design and fabricate parallel coupled graphene double dot devices. Low temperature measurements present well honeycomb charge stability diagrams. The interdot coupling strength can be effectively tuned from weak to strong by bake gate and in-plane plunger gates.5. We further propose and make the double dot with integrated charge detector devices. Primary measurements show good performance of such devices.