Electron Transport in Semiconductor Quantum Dots

Abstract

Recent advances in epitaxial growth and processing technologies have enabled us to fabricate semiconductor nanostructures whose dimensions are comparable to the de Broglie wavelength of electrons. Transport measurements on these nanostructures have revealed a rich variety of phenomena associated with the effects of quantum mechanical confinement1). Conductance quantization in one- dimensional quantum point contacts, and resonant tunneling through quantum wires and quantum boxes are such examples. These properties directly reflect the quantization of energy. In addition, charge quantization is observed for tunneling through a small dot, which acts as an island for electrons. When tunneling occurs, the charge on the island suddenly changes by a quantized amount namely “e”. This leads to the change in the electrostatic potential of the dot by the charging energy, Ec=e2/C, where C is the typical capacitance of the island. The one-by-one change in the number of electrons on the island gives rise to oscillations in the tunneling conductance (Coulomb oscillations) when the gate voltage is swept. These oscillations are usually periodic when the number of electrons is “large”. However, in a small dot holding just a few electrons, the charging energy can no longer be parameterized in terms of a constant capacitance, and the Coulomb oscillations are significantly modified by electron-electron interactions and quantum confinement effects. Both the quantized energy level spacing, and the interaction energy become large when the dot size is decreased, and can be similar when the dot size is comparable to the electron wavelength. Thus, the addition energy needed to put an extra electron on the dot becomes strongly dependent on the number of electrons in the dot. Such a system can be regarded as an artificial atom.2) We show in 2.2 that the addition energy spectrum of an artificial atom reflects atom-like features such as a shell filling and the obeyance of Hund’s first rule when the dot has a high degree of cylindrical symmetry. Such a dot has only recently been developed by using a sub-micorn diameter double barrier heterostructure in which a circular disk-shaped dot is located. 3,4) The shell structure arises from the single-particle level degeneracy imposed by the two-dimensional (2D