| In this dissertation, we present an experimental study on semiconductor quantum dot spectroscopy by using cryogenic scanning tunneling microscopes. Because a nanometer-scale semiconductor quantum dot is a possible building block for dense, low-power, and high-speed electronics, we study its energy spectrum to gain insights into its fundamental properties. To enhance the size quantization effect in a nanometer-scale quantum dot, the InAs/AlSb heterojunction system has been used here because of its useful material properties, including the Fermi level pinning position at InAs surface, the smaller effective mass of electron in InAs, and the large conduction band offset between InAs and AlSb. However, it is still challenging to fabricate electrodes on a nanometer-scale InAs quantum dot for electrical characterization. In order to avoid this difficulty, cryogenic scanning tunneling microscopes, which have atomic resolution and pico-ampere accuracy, have been built to probe the energy spectra of electrons in the quantum dots.; The scanning tunneling microscopes are equipped with X/Y/Z coarse stages, dual-tube scanners, and damping systems. During the spectroscopic measurement, the STMs are specifically operated in the “contact mode” where the tip is in direct, physical contact with the device under test, such that uncertainties due to the usual tunneling gap are totally removed.; Both InAs and GaSb quantum dots have been fabricated by using electron-beam lithography and either reactive ion etching or selective wet etching. Their current-voltage characteristics have shown the Coulomb gap, the Coulomb staircase, and the size quantization effect. In addition, due to the gating effect produced by the surface states, the appearance and disappearance of the Coulomb gap has been observed in the current-voltage characteristics of InAs quantum dots. This is in contrast to the time-independent Coulomb gap feature observed in GaSb quantum dots. Furthermore, for all quantum dots that we have measured, the tunneling current at a fixed bias shows the random telegraph signal, which is attributed to the charging and discharging processes of the antisite defects in the AlSb tunneling barrier. In other words, mere one electron charge can switch the current between two discrete levels. Through many similar observations, we conclude that the current will have to be affected by one electron charge change in the traps nearby as long as the current path is small enough. |
