Chalcogenide compounds describe the compounds consisting of S, Se and Te chalcogen anion, and owning a wide variety of optical, electronic, thermal and mechanical properties that could be used in electronics and optoelectronic devices. The growth and some interesting physical properties of chalcogenide compound thin films and heterostructures involving II-VI semiconductor compounds, manganese monochalcogenides, 2D transition metal dichalcogenides, chalcogenide topological insulators and iron monochalcogenides are demonstrated in this dissertation. This dissertation presents five research works carried out on chalcogenide compound thin films and heterostructures fabricated either in a molecular beam epitaxy system or an ultra high vacumn system.

The first work is related to the growth of...[ Read more ]

Chalcogenide compounds describe the compounds consisting of S, Se and Te chalcogen anion, and owning a wide variety of optical, electronic, thermal and mechanical properties that could be used in electronics and optoelectronic devices. The growth and some interesting physical properties of chalcogenide compound thin films and heterostructures involving II-VI semiconductor compounds, manganese monochalcogenides, 2D transition metal dichalcogenides, chalcogenide topological insulators and iron monochalcogenides are demonstrated in this dissertation. This dissertation presents five research works carried out on chalcogenide compound thin films and heterostructures fabricated either in a molecular beam epitaxy system or an ultra high vacumn system.

The first work is related to the growth of MnSe_1-xTe_x thin films and heterostructures. The stable phase of MnSe (rocksalt) and MnTe (NiAs type hexagonal) can be grown directly on GaAs (001) substrate. We discovered, with the help from a ZnTe buffer layer, the growth of a ZB manganese chalcogenide compound MnSe_1-xTe_x can be achieved. We reported a set of lattice plane spacing of MnSe_1-xTe_x with 0.27 ≤ x ≤ 1 and discussed the lattice distortion issue for these thin films. We also fabricated a lattice-matched double-barrier MnSe_0.49Te_0.51/ZnTe/MnSe_0.49Te_0.51 resonant tunneling diode (RTD) and obtained the I-V curve of this device, which displays a negative differential resistance (NDR) feature.

The second work is about the development of a compact solid-state UV flame sensing system based on wide-gap II–VI thin film materials. ZnSSe was selected to be the wide-gap II–VI thin film active layer. We have addressed an approach for obtaining additional six orders of long-wavelength rejection power on top of that of the built-in UV sensor based on a Schottky-barrier structure. An op-amp-based amplification circuit with an average gain of 16,600 was used for signal amplification. It was demonstrated that the developed sensing system could detect a standard butane-air flame with excellent solar-blind characteristics, potentially capable for monitoring industrial boilers. Several feasible approaches were discussed for further improvement in the performance of the developed sensing system targeting its general usage in fire-safety applications.

In the third work, the development of an incandescent Mo source to fabricate large-area single-crystalline MoSe₂ thin films, together with their various characterizations, is presented. The as-grown MoSe₂ thin films were characterized using transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), Raman spectroscopy, photoluminescence spectroscopy (PL), reflection high-energy electron diffraction (RHEED) and angle-resolved photoemission spectroscopy (ARPES). An unreported Raman characteristic peak at 1591 cm⁻¹ was identified. Results from Raman spectroscopy, PL, RHEED and ARPES studies consistently reveal that large-area single crystalline mono-layer of MoSe₂ could be achieved by this technique. This technique enjoys several advantages over conventional approaches and could be extended to the growth of other two-dimensional layered materials containing a low-vapor-pressure element.

In the fourth work, a simple and low-cost experimental setup for thermoelectric effect measurements of thin film materials near room temperature, which can be used to determine their conductivity types, is presented. Bi₂Te₃ and Sb₂Te₃ thin films grown by the MBE technique were used as the tested samples. Their Seebeck coefficients were determined to be (-141 ± 1) μV/K and (39 ± 2) μV/K, respectively, confirming that the former is an n-type material and the latter is a p-type material. An MBE-grown heterostructure composed of Sb₂Te₃ and Bi₂Te₃ was characterized by electrical transport measurements. Data fitting was carried out for its current-voltage characteristics with the Shockley diode model and a real diode model. Some physical parameters of the heterostructure were extracted, including its ideality factor and saturation current. Based on its rectifying current-voltage behavior, we confirm that the aforementioned heterostructure is a p-n junction, which echoes the contrast in the conductivity types of Sb₂Te₃ and Bi₂Te₃ as determined by the thermoelectric effect measurements.

In the fifth work, an ongoing project that aims to trap and detect Majorana fermions using a Bi₂Te₃/Fe_1+yTe nanowire was addressed. The fabrication process of the Bi₂Te₃/Fe_1+yTe nanowire inside a FIB/SEM dual system was described. Two ends of the Bi₂Te₃/Fe_1+yTe nanowire were connected to two electrodes for point contact measurements. The surface morphology of the Bi₂Te₃/Fe_1+yTe nanowire was measured by AFM, showing the heterostructure still exists in the fabricated nanowire. Two-point transport and differential conductance measurements were performed with the help from Prof. W. Lortz's group in HKUST. Superconductivity and zero-bias conductance (ZBC) peak were observed in our Bi₂Te₃/Fe_1+yTe nanowire device in its R-T curve and dI/dV curve, respectively. A quasi-1D nanoribbon device using the same heterostructure was also fabricated in order to reveal the transport properties of the proximity-effect induced topological superconductivity of the top Bi₂Te₃ layer in the nanoribbon. Its magnetic field dependence of the ZBC peak seems to show characteristics as expected from Majorana zero modes. At the moment, the connection between our observed ZBC peak and Majorana fermions in the Bi₂Te₃/Fe_1+yTe quasi-1D structure is not conclusive and further efforts are required to broaden the understanding of the nature of the ZBC peak.

The findings of this thesis study provide some novel approaches for fabricating chalcogenide compound thin films and heterostructures and reveal some interesting physical properties they possess, which could find applications in telecommnunication, optoelectronics and semiconductor characterization.