Time and place: 13:00, bldg. 303A, aud. 44, DTU Lyngby Campus.
Principal supervisor: Professor Kristian S. Mølhave
Co-supervisor: Professor Kimberly D. Thelander, Lund University
Co-supervisor: Associate Professor Nika Akopian, DTU Electro
Examiners:
Professor Jakob Birkedal Wagner, Chair. DTU Nanolab
Professor Hannah Jane Joyce (University of Cambridge)
Professor Reine Wallenberg (Lund University)
Chairperson at defence:
Professor Thomas Willum Hansen
Abstract
A new industrial revolution started at the beginning of this century. Modern technology has started to implement quantum mechanical properties requiring new components designed down to the atomic level. Some of these components are able to interact with photons at certain energies using so-called quantum dots, which can be used for e.g. quantum computing. This can be achieved with so-called nanowires, which consist of semiconductor compounds from group III and group V in the periodic table.
This thesis investigates the mechanisms that form quantum dots through different crystal phases in the nanowires. The crystal phases can consist of different ways the atoms are arranged in the crystal structures. Hence, this thesis has a practical contribution to the development of new components for future technology, but it also challenges well-established theories about crystal phase changes for nanowires growing with a liquid catalyst droplet in a vapour atmosphere. More concretely, the change in the contact angle between the surface of the catalyst droplet and the cross-sectional area of the nanowire has often been understood as the parameter that leads to a change in the crystal phases forming. However, this thesis shows that this does not seem to be the case. This is done by decoupling the geometry of the drop from the rest of the growth conditions with an applied external electric field. In addition, this thesis finds an empirical value for the surface tension of the gold catalyst droplet for nanowires consisting of gallium and arsenic. This value is commonly used in theories modelling crystal growth, but it has so far only been estimated from theoretical models, which suggests different values depending on the estimation models used in the literature.
The experiments in this thesis have been carried out by implementing silicon microheaters on so-called MEMS chips in a gas environment inside an electron microscope, where the nanowire growth can be observed directly. Two neighbouring microheaters can be used to implement an electric field between the microheaters affecting the nanowire and their catalyst droplets. The field changes the shape of the droplet by an electric pressure and the effect of this electric field is examined in detail in this thesis.
The microheater chips have been optimised and microfabricated as part of the work with this thesis. Correct temperature calibration is crucial to create the right growth conditions for the III-V nanowires on the silicon microheater surface. However, a precise estimate for temperature has been difficult to achieve, as the microheaters have variations in geometry and properties, and they change behaviour when used in the microscope.
Nucleating of the gold-catalysed GaAs nanowires have successfully been achieved directly at the silicon surface, which so far has not been documented and described by imaging the early steps of the nucleation in the literature. It has also been possible to change the crystal phases by changing both gas and temperature conditions. For the growth in the microscope with the microheaters, it has been shown that rapid temperature changes are a better way to make quantum dots in the nanowire than changes in the gas conditions that are otherwise preferable in traditional growth chambers.
