Quantum Dot Low Temperature Measurement and Analysis Thesis

Abstract

In this bachelor project, the electron transport within quantum dot transistor devices, whose barriers are electrostatically induced, was measured and analysed. The properties of devices with this particular architecture are not yet fully understood. We successfully synthesised several 150x50x50 nm of these devices, where the potential barrier gates are defined next to the nanowire to form the quantum dot island. These devices were trialled in a low temperature environment by the use of liquid helium and a dip rig in order to study the electron transport through the quantum dot of each device at a temperature of about 4.2 K.
These electrostatically induced dots were able to show the Coulomb diamonds produced only within the range of n to n+4 number of electrons. The lever arm αɢ, ΔE between energy levels Eₙ, the self capacitance CΣ and the energy Eᴀᴅᴅ required to add another electron per diamond of each device were also determined. In general, each device showed the artificial atomic shell structure shown directly by the even-odd electron coupling principle determining the energy required to add additional electrons to the island.
Due to time constraints, only two devices out of a total of 11 successfully synthesised were used in measurements. Measurements on the first device were used to explore the quantum dot behaviour induced by the potential barriers. The results from the second device showed that a dot could be induced without the barriers, prompting attempts to prove exactly how the second dot had been induced. It was found that the 350 nm distance between source and drain was sufficient to induce quantum dot behaviour.

Popular Abstract

Movement of Electrons in Low Temperature Systems

In the modern world of computers, one of the most revolutionary electrical devices is known as a transistor. These semiconductor devices allow, or stop, large groups of electrons from moving through it to create a current. There are hundreds of thousands, if not millions, of these tiny transistors inside all computers and smartphones, so if they never existed there would be no computers or smartphones to speak of. However, a limit is being reached in how small these devices can be made under normal conditions.
Normally, if transistors are made too small, they will stop working. This is because electrons will tunnel through barriers that would previously stop them, meaning the device can no longer control current like it should. Yet, it possible to overcome this by making a transistor device that instead controls the movement of a single electron at a time, but this cannot be done at room temperature.
At low temperatures, as cold as the vacuum of space, electrons begin to behave differently in nanoscale electric devices. There exists a specific criterion in this state where electrons will be trapped between two barriers only a few atoms thick, a coulomb blockade. Researching why, and how, these electrons move in the way they do is incredibly important if we are to understand how to make even smaller electronic devices.

How can we achieve this?
Using a single electron transistor, there exists an island known as a quantum dot which is separated from the rest by two barriers of a specific energy level. These barriers are spaced at such a small distance that electrons can get trapped, or confined, on this island, elevated to an energy level that is less than the barrier level, any higher and the electrons would escape. This means that electrons trying to tunnel through the barriers, cannot do so unless they have an energy level that is about the same as the levels inside. When they cannot find an energy that matches, they cannot cross and are blocked by the barriers. This is the coulomb blockade, and it can be used to the same effect as the larger transistor described earlier.
This bachelor thesis focuses on building these kinds of devices using side gates to define the barriers, and measuring the conditions that are required to see this coulomb blockade. A total of 11 of these devices, as seen on the right, were successfully built, and a diamond-like shape was found to define this coulomb blockade, something known as coulomb diamonds.

• author: Flodgren, Vidar
• supervisor: Adam Burke
• organization: Department of Physics; Solid State Physics
• course: FYSK02 20171
• year: 2017
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: nanoscience, physics, quantum dots, electron transport
• language: English
• id: 8920539
• date added to LUP: 2017-07-02 17:14:01
• date last changed: 2017-07-02 17:14:01