Study of resonant tunneling through single quantum dots by atomic force microscopy

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1. Resonant tunneling through a single InAs quantum dot probed via novel over layered quantum dot electrodeElectrons tunneling through the quantum levels of a single self-assembled InAs quantum dot (QD) have been observed using an atomic force microscope (AFM) with a conductive tip. The unique structure employed here consists of two layers of InAs QDs, which were separated by thin undoped GaAs barrier layer. When the conductive tip is in contact, the naked surface InAs QD functions as a nanoscale electrode to where electrons from the n^+-GaAs substrate flow via a buried QD. In the current-voltage characteristics data taken at 〜120 K, resonance peaks are observed at different bias voltages depending on the size of the top InAs nano-electrodes. This is due to the size dependence of the surface potential on the InAs QDs.2. Control of the size and density of InAs quantum dots by molecular beam epitaxyFor the purpose of the resonant tunneling measurement, the InAs QD growth technique by mol … More ecular beam epitaxy was highly developed to control their size and density. Namely, we demonstrated the size and density of InAs QDs are changed from 15 to 45 nm and from 〜10^8 to 〜10^<11> cm^<-2>, respectively by precise control of the growth conditions.3. Resonant tunneling through a single InAs quantum dot embedded in a micro RTDThis growthcontrol is not only essential for the fabrication of the above-mentioned double layered InAs QD structure but also have realized a micro resonant tunneling diode which contains only one QD in it. We also studied resonant tunneling transport of electrons through a single InAs QD by forming a micron-size Schottky diode in a GaAs/n+-GaAs wafer with very low density (〜4x10^8 cm^<-2>)InAs QDs embedded in the top GaAs layer. As the diode defined by electron beam lithography and chemical etching has a size of 〜1μm^2 and the edges of each diode are depleted, the number of active Ods contained each diode can be reduced to less than unity in average. Current-voltage measurements were performed using a conductive-tip AFM at 〜130 K, and current peaks caused by electrons resonantly tunneling through the energy levels of single QDs were successfully observed. Less