Abstract
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 four 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 was related to the growth of MnSe1-xTex 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 MnSe1-xTex can be achieved. We reported a set of lattice plane spacing of MnSe1-xTex with 0.27 ≤ x ≤ 1 and discussed the lattice distortion issue for these thin films. We also fabricated a lattice-matched double-barrier MnSe0.49Te0.51/ZnTe/MnSe0.49Te0.51 resonant tunneling diode (RTD) and obtained the I-V curve of this device that displays a negative differential resistance (NDR) feature.
The second work was 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 wide-bandgap II-VI 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 was 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, was 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.
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.
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