Study of electronic states in 10nm-scale semiconductor quantum dots and exploration of their memory functions

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Semiconductors (e.g., InAs) deposited on a lattice-mismatched substrate (e.g., GaAs) yields 10 nm-scale Quantum dots (QDs) spontaneously. We have exploited them to make memory devices, in which the presence and absence of a single electron in InAs QDs correspond to "1" and "0". We have also investigated electronic properties of such QDs.First, we prepared an inverted high electron mobility transistors (HEMT), in which InAs QDs are embedded between the surface gate electrode and a two dimensional electron channel at the GaAs/n-AlGaAs heterojunction. By applying gate voltage (Vg) over 0.9 V, the threshold voltage of the transistor shifted to the positive side by DELTAV, resulting from the supply of electrons from the channel to QDs. This proves the feasibility of new memory effect. Analytical studies of transport characteristics have shown that each QD stores one electron each, since the Coulombic repulsion originating from the first trapped electron prevents the trapping of second electron. If the second electron is stored in the QD, it easily tunnels out from the QD.In addition, we studied the "write" and "erase" (i.e., charge and discharge) processes controlled by the light illumination. Further, we prepared and studied a prototype device of a quantum point contact, in which the charge trapping in QDs near the point contact leads to a clear hysteresis characteristics in the quantized conductance.We have studied electronic levels of InAs QDs in AlGaAs layer by photoluminescence and capacitance-voltage spectroscopy. The quantized state in QDs was found to increase by ^- 200 meV when the Al content in AlGaAs barrier layer is raised. We clarified also the shape of wavefunctions of electrons confined in QDs by magneto-tunneling spectroscopy.