Electron Transport in Quantum Dots Defined in Low-Dimensional Semiconductor Structures

Sammanfattning: This thesis focuses on electron transport in single and double quantum dots defined in low-dimensional, narrow-band-gap III-V semiconductor materials. Fabrication schemes are presented for defining single and double quantum dots in lateral InGaAs/InP heterostructures, either by a combination of etching and local gating or solely by local top gating. The quantum dots are here electrostatically confined in at least one dimension. This allows for insitu control of the tunnel coupling of the quantum dots using gate voltages. Nanowire-defined quantum dots have also been studied. Here, the quantum dots are formed in an InSb nanowire segment through a metal electrode Schottky barrier. Electron transport properties have been investigated in these systems at low temperatures and single-electron charging behavior consistent with both many- and few-electron single quantum dots was observed. Magneto-transport measurements of InGaAs defined many- and few-electron single quantum dots show level-dependent effective electron g-factors of ~ 2 - 4. For the InSb nanowire quantum dot, giant and level-depended g-factors of up to ~ 70 were observed. The level-to-level fluctuation in the g-factor is attributed to the presence of strong spin-orbit interaction in these systems. The magnitude of the spin-orbit interaction was investigated in an InSb nanowire quantum dot by finite bias magneto-spectroscopy. Spin-orbit mixing of a ground state and a first excited state with opposite spins induced an avoided level crossing in the quantum dot dominated by Zeeman energy. The avoided level crossing allowed a spin-orbit energy of ~ 280 μeV to be directly extracted. The spin filling sequence of a few-electron InGaAs single quantum dot was also investigated using ground state magneto-spectroscopy and parallel spin filling configurations were identified. From these configurations the lower bound of the exchange energy in the dot was estimated to be ~ 210 μeV. Single quantum dots were also studied in the strong coupling regime where correlated electron transport processes become important. Here, co-tunneling, the spin-1/2 Kondo effect as well as an gate-induced splitting of the Kondo effect of a few-electron InGaAs quantum dot were investigated and the characteristic energy scales of the system were extracted. In addition, the degeneracy point of two quantum levels of equal spin was studied in the strong coupling regime of an InSb nanowire quantum dot. Strong suppression of the co-tunneling background current was observed at the level degeneracy. This current suppression is attributed to the destructive interference of two spin-correlated conduction paths. Pauli spin blockade at the (3,5) → (2,6) charge state transition in an InGaAs defined double quantum dot was identified, where (N1,N2) refers to the number of electrons in dot 1 and dot 2, respectively. An integrated quantum point contact charge state read-out sensor was used to determine the exact charge state of the double quantum dot. Leakage current was observed through the spin blockade, caused by triplet-to-singlet relaxation. The main contribution to the mixing of the singlet and triplet states is attributed to the hyperfine interaction between the electron spin and nuclear spin in the material. An effective nuclear magnetic field of ~ 2.7 mT was determined from the magnetic field dependence of the leakage current for detuned energy levels.