Electron Transport in GaAs Quantum Dots under High Frequencies

Abstract

This thesis explores transport properties of lateral, gate defined quantum dots in GaAs/AlGaAs heterostructures. The term "quantum dot" as defined in this thesis refers to small regions of charge carriers within a 2-dimensional electron gas (2DEG), established via electrically biased surface gates used to isolate the charge carriers from the rest of the 2DEG, which are confined to lengths scales on the order of nanometers. Several other forms of quantum dots exist in the research community, including colloidal and self-assembled dots. In this thesis, however, we consider only gate defined quantum dots and nanostructures. Recent advancements in the research areas of quantum dot (QD) and single electron transistors (SET) have opened up an exciting opportunity for the development of nanostructure devices. Of the various devices, our attention is drawn in particular to detectors, which can respond to a single photon over a broad frequency spectrum, namely, microwave to infrared (IR) frequencies. Here, we report in chapter 5 transport measurements of parallel quantum dots, fabricated on a GaAs/AlGaAs 2-dimensional electron gas material, under the influence of external fields associated with 110GHz signals. In this experiment, transport measurements are presented for coupled quantum dots in parallel in the strong-tunneling Coulomb blockade (CB) regime. From this experiment we present experimental results and discuss the dependence on quantum dot size, fabrication techniques, as well as the limitations in developing a QD photon detector for microwave and IR frequencies, whose noise equivalent power (NEP) can be as sensitive as 10-22 W/Hz1/2. The charging energy EC of a quantum dot is the dominant term in the Hamiltonian and is inversely related to the self capacitance of the dot Cdot according to EC = e2/Cdot. The temperature of the charge carriers within the 2DEG must be kept below a certain value, namely KBT, so that the thermal energy of the electrons does not exceed the charging energy EC of the dot. Keeping the temperature below the KBT limit prevents electrons from entering or leaving the dot at random, thereby allowing one to precisely control the number of electrons in the dot. In order to raise the operating temperature T of the single photon detector we must also raise the charging energy EC, which is accomplished by decreasing Cdot. Since Cdot is directly related to the dimensions of the quantum dot our focus was directed at decreasing the overall size of the quantum dots. For smaller gate defined quantum dots the inclusion of shallower 2DEG's is necessary. However, the experiments that we carried out to determine the effect of 2DEG depth on lateral gate geometries, described in Chapter 6, indicate that leakage currents within a GaAs/AlGaAs heterostructure increased dramatically as the 2DEG depth became shallower. At this moment the leakage current in shallower 2DEG materials is one of the most significant technical challenges in achieving higher operating temperatures of the single photon detector.