FeTe, a simple Fe-based layered compound, does not exhibit bulk superconductivity even under a high pressure, in contrast to its superconducting cousin materials such as FeSe and FeS. However, our research group has made an intriguing discovery: induced two-dimensional (2D) superconductivity at the interface of topological insulator (TI)/FeTe heterostructures, including the Bi2Te3/FeTe and Sb2Te3/FeTe systems. The successful fabrication of these heterostructures can be attributed to their layered nature, despite the mismatch in crystal structure symmetry between the topological insulators (TIs) and the FeTe. A further investigation of the interface configurations between TIs and FeTe could yield valuable insights into the underlying mechanisms of the emergent 2D superconductivity in these systems. In this thesis study, three layered structures containing FeTe were fabricated. The first is a tetragonal FeTe phase comprising nanocrystals with their c-axes aligned (CAT-FeTe), with the in-plane orientations of these crystals occurring randomly; the second is a heterostructure comprising a FeTe/Bi-Te system, with the Bi-Te components including Bi4Te3, Bi6Te3, and Bi2Te3. The third structure is a heterostructure of FeTe/MnTe. MnTe refers to different phases of MnTe, including newly discovered layered MnTe (l-MnTe) and layered Mn4Te3 (l-Mn4Te3), distorted zinc-blende MnTe (dZB-MnTe), and wurtzite MnTe (WZ-MnTe).
 

The first structure, designated c-axis-aligned nanocrystals tetragonal FeTe (CAT-FeTe), was directly grown on c-plane sapphire via molecular beam epitaxy, resulting in the emergence of a novel structural phase comprising c-axis-aligned nanocrystals. The reflection high-energy electron diffraction patterns (RHEED) display two sets of streaks simultaneously at all rotation angles of the sample. High-resolution X-ray diffraction (HRXRD) studies have confirmed that the nanocrystals are tetragonal FeTe, with their c-axes aligned to the growth direction. Atomic force microscopy (AFM) imaging reveals that further growth of these nanocrystals involves a cannibalism process, resulting in nanocrystal pillars with sizes of approximately 0.5 to 1 µm. The temperature-dependent resistance of these thin films displays an overall semiconducting behaviour, although there are instances where the resistance is non-measurable or exhibits jumps and falls, which can be attributed to the thermal responses of the nanocrystals during cooling and heating processes. This discovery offers a method for the formation of 17 inhomogeneous heterostructures, such as those comprising Bi2Te3/FeTe and Sb2Te3/FeTe systems, with all possible twisted angles between the two components.

The second structure is an FeTe/Bi-Te heterostructure system. The Bi-Te binary system, characterised by the homologous series of the (Bi2)m(Bi2Te3)n, has consistently attracted research interest due to its layered structures and potential in advanced materials applications. Despite extensive studies of Bi2Te3, exploration of other compounds has been constrained by synthesis challenges. The findings of our study demonstrate that the MBE growth of FeTe on Bi2Te3 can transform the Bi2Te3 layer into distinct Bi-Te phases, thereby forming corresponding FeTe/Bi-Te heterostructures through the manipulation of growth conditions. The combined analysis, utilising RHEED, HRXRD and cross-sectional high-resolution scanning transmission electron microscopy (HR-STEM), indicates that specific growth conditions employed for the growth of the FeTe layer can facilitate the extraction of Te from Bi2Te3, resulting in the formation of Bi4Te3 and Bi6Te3. Furthermore, by reducing the growth temperature of the FeTe layer to 230°C, the extraction of Te from the Bi2Te3 layer could be prevented, thus preserving the Bi2Te3 structure. Notably, all three FeTe/Bi-Te structures exhibit superconductivity, with the FeTe/Bi2Te3 heterostructure exhibiting the highest quality of superconductivity. These findings introduce a novel method for realizing Bi4Te3 and Bi6Te3 through Te extraction by growing FeTe on Bi2Te3, driven by the high reactivity between Fe and Te. This approach holds promise for synthesizing other members of the Bi-Te series, potentially extending the applications of the Bi-Te family.

The third structure we investigated in this thesis study is the FeTe/MnTe heterostructure system. Manganese telluride (MnTe) has recently attracted considerable interest due to its antiferromagnetic semiconductor properties, which have the potential to be exploited in a range of applications, including spintronics, data storage and quantum computing. In the study in this FeTe/MnTe heterostructure system, we discovered that the deposition of FeTe at 300℃ onto zinc-blende MnTe (ZB-MnTe) via MBE results in a phase transition from ZB[1]MnTe to a l-MnTe phase with van der Waals (vdW) gaps, which is a previously unreported phase of MnTe. The l-MnTe phase was characterised using cross-sectional high-resolution scanning transmission electron microscopy (HR-STEM) imaging, energy-dispersive X-ray spectroscopy (EDS) mapping, and X-ray photoelectron spectroscopy (XPS). The Fe/Te flux ratio during FeTe deposition was found to be critical to the phase transition, an increased Fe/Te flux ratio used for the FeTe growth leads to localized formation of layered Mn4Te3 (l- 18 Mn4Te3), while a decreased Fe/Te flux ratio only generates a single monolayer of l-MnTe at the interface and the rest turns into a dZB-MnTe phase. Additionally, it was observed that FT-MT heterostructures grown at a reduced substrate temperature of 250°C exhibited a transformation of the ZB-MnTe layer as the Fe/Te flux ratio decreased. Initially, the ZB[1]MnTe layer underwent a transition to dZB-MnTe, and subsequently, to WZ-MnTe. The FeTe/l-MnTe heterostructure displays high-quality superconducting characteristics with a three-dimensional nature, as evidenced by its magneto-transport properties. There is compelling evidence that l-MnTe plays a pivotal role in inducing the observed superconductivity. It is of particular significance that this study reports the realisation of layered structures of MnTe by an in-situ approach via chemical interactions. This finding may be further applied to generating unprecedented phases of materials under certain conditions