Probing and electron tunneling of quantum dot systems.

摘要

This thesis is on the electronic transport properties of quantum dot systems. We investigate three issues in this thesis. The first one is how to probe the electronic structures of a single quantum dot. The second one is the mechanism of negative differential capacitance (NDC) of a single quantum dot due to the bias dependence of tunneling rates. The last one is the negative differential resistance (NDR) due to the resonance coupling in a parallel coupled quantum dot dimer. We propose a new method of probing the electronic structures of a single quantum dot system and new mechanisms for NDC and NDR in single and double quantum dot systems.; Our knowledge on the electronic properties of a quantum dot system is mainly obtained from two experimental methods—optical and transport measurements. Each of the two methods is suitable to measure some particular properties of quantum dot systems. By combining these two methods, that is, the photon-assisted tunneling measurement, we show that we can obtain some important quantities, which may be difficult or even impossible to obtain by using either of them. For example, we can obtain the spontaneous emission rate of an electron level of a quantum dot from the photon-assisted tunneling measurement.; We numerically study the external voltage bias dependence of charges accumulated on a quantum dot. We show that charges are sensitive to the change of the number of both filled and conducting levels (channels). Furthermore, we clarify that there are many possible outcomes of applying a bias. For example, the number of conducting channels increases, but the number of filled levels decreases. Or the number of filled levels does not change while the number of conducting channels increases with the bias. In the second case, charges are generally expected to increase monotonically with the applied bias. However, we numerically show that charges may decrease with the applied bias if the electron transmission coefficients depend on bias. We also provide a theoretical explanation of this new mechanism of the NDC.; At last, we show a new mechanism of NDR in a parallel coupled quantum-dot dimer. We investigate the electron tunneling through a coupled quantum-dot dimer under a dc-bias. We find that a peak in the I-V curve appears at low temperature when two discrete electronic states in each quantum dots are aligned, which we call the resonance coupling. This leads to a NDR. We also study the dependence of peak height and width on the interdot coupling and temperature. We expect that this new NDR may be useful to understand results of STM experiments.