Public defence in Engineering Physics, M.Sc. Afrina Khanam

Opponent: Professor Lasse Vines, University of Oslo (UiO), Norway
Custos: Professor Peter Liljeroth, Aalto University School of Science, Department of Applied Physics

The doctoral thesis is publicly displayed 10 days before the defence in the publication archive Aaltodoc of Aalto University

Electronic thesis

Public defence announcement:

The twenty-first century has seen an unprecedented need for semiconductor materials. From the smartphones we use daily to the satellites aiding scientific discoveries, semiconductor materials are essential. As the number of transistors on a single circuit board increase, the need for precise control of fabrication processes such as epitaxy and ALD grows. Si is no longer capable of meeting the demands of the modern world, hence, Ge-based microprocessors can provide faster speeds. In addition to that, TiO₂-based VMCO devices are also a viable option for non-volatile memory devices.

The purpose of this research was to identify the source of defects that limit the active donor concentrations in modern semiconductor devices. Positron annihilation spectroscopy (PAS) was utilized to characterize vacancies, vacancy complexes, and their effects on the electrical passivation of n-type dopants in epitaxially grown thin-film Ge and GeSn. Results showed that in n-type Ge, vacancy-donor pairs and vacancy-donor complexes form energy levels in the bandgap which can trap charge carriers. In P-doped epitaxially grown Ge and GeSn, monovacancy defect-donor complexes were identified as the main cause of donor passivation. To achieve higher active donor concentrations, As was used instead of P, and highly As-doped GeSn layers were studied. It was discovered that vacancies surrounded by four As atoms are the main reason behind the massive donor passivation. Additionally, vacancies in atomic layer deposition (ALD) grown ultra-thin anatase TiO₂ layers were studied and differences in positron trapping states were observed due to differences in growth temperatures.

This thesis provides a comprehensive understanding of vacancy-like defects in group IV semiconductors and the mechanisms of donor passivation in modern Ge-based semiconductor devices. The findings can be applied to the design of more efficient transistors and integrated circuits, which would improve the fabrication process of modern transistors and other electronic devices. In addition to that, the results from the second research provide insight into the growth of ultra-thin layers and can be used to optimize fabrication processes. Ultimately, the results of this research demonstrate that PAS is an effective method for studying defects and their impact on the electrical passivation of n-type dopants in Ge-based devices and this technique can characterize technologically relevant ultra-thin heterostructures.

Contact details of the doctoral student: [email protected]