On the Dynamics of Single-Electron Tunneling in Semiconductor Quantum Dots under Microwave Radiation

Efforts are made in this thesis to reveal the dynamics of single-electron tunneling and to realize quantum bits (qubits) in semiconductor quantum dots. At low temperatures, confined single quantum dots and double quantum dots are realized in the twodimensional electron gas (2DEG) of AlGaAs/GaAs heterostructures. For transport studies, quantum dots are coupled to the drain and source contacts via tunnel barriers. Electron-electron interaction in such closed quantum dots leads to Coulomb-blockade (CB) effect and single-electron tunneling (SET) through discrete quantum states. SET and its dynamics in single and double quantum dots are studied using both transport and microwave spectroscopy. In transport spectroscopy, SET is monitored by measuring the direct tunnel current through the quantum dots in both the linear and nonlinear transport regimes, where ground states and excited states of the quantum dots are resolved. In a double quantum dot, bonding and anti-bonding molecular states are formed. Quantum dots proved to be well controlled quantum mechanical systems. In analogy to real atoms and molecules, single quantum dots and double quantum dots are termed artificial atoms and artificial molecules, respectively. In microwave spectroscopy, continuous microwave radiation is applied to quantum dots. Photon-assisted tunneling (PAT) through the ground state and excited states is observed in single quantum dots. In a double quantum dot, the molecular states can be coherently superimposed by microwave photons, inducing the Rabi oscillations and a net direct tunnel current which is experimentally measurable. A qubit is formed in a double quantum dot. Two new microwave spectroscopy techniques are developed in this thesis to explore the dynamics of PAT (SET) in quantum dots. Both techniques are called heterodyne detection of photon-induced tunnel current (photocurrent). In one method, two coherent continuous microwave sources with a slight frequency offset are combined to generate a flux of microwave photons. The photon intensity varies in time at the offset frequency. The induced alternating photocurrent at the offset frequency is detected by a lock-in amplifier. The in-phase component of the photocurrent reflects the tunneling strength, and the out-of-phase component reveals the dynamics of electron tunneling. In the other method, two coherent pulsed, i.e., broadband, microwaves are applied to irradiate the quantum dots, where the dynamic charge relaxation and the pumping by microwave pulses are studied. Both techniques allow to resolve PAT in the nonlinear transport regime. A long charge relaxation time of single quantum dots is found by using both techniques. No superposition of the ground state and the excited state is achieved.