The intrinsic spin-orbit interaction of electrons in semiconductor nanostructures and the Zeeman interaction between the electrons and the fringe field of a ferromagnetic stripe placed on top of the nanostructures have recently been employed extensively in the development of semiconductor-based spintronic device concepts. In a III-V semiconductor quantum-dot structure, large spin-orbit interaction can be present. Thus the system can show rich electrical properties. In this work, we report a study of a few-electron quantum dot made from an asymmetric InGaAs/InP heterostructure by transport spectroscopy measurements.
The sample was fabricated by two steps of electron beam lithography on a high-mobility InGaAs/InP quantum well structure grown by metal organic vapor phase exitaxy. First, the quantum dot and six in-plane control gates defined by etching trenches were made. Then a local metal gate was fabricated on top of the dot. Here an isolation layer made of cross linked PMMA was used to minimize current leakage between the top gate and the device. In the fabricated device, the number of electrons, the spin-orbit interaction strength and the shape of the quantum dot can all be tuned independently, which allows us to extract the physical parameters of the quantum dot in the few-electron regime by transport measurements.
All the measurements were performed at 300 mK using lock-in technique. At zero magnetic field, the measured zero-bias conductance as a function of the top gate voltage shows up in pairs, indicating the formation of a shell structure in the dot. This interpretation of the results was confirmed by the measurements under finite bias, in which a sequence of large and small diamond structures in the charge stability diagram was observed. When in-plane magnetic field was applied to the dot, Zeeman shifts of the conductance peaks were observed. The measured conductance peaks also show an anti-cross behavior with increasing magnetic field at certain gate voltages, indicating a lift of spin degeneracy by the spin-orbit interaction in the dot. Further measurements and analyses are needed to extract the effective g factor of the few-electron states and the strength of spin-orbital interaction in the dot.
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