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Title: Optoelectronic properties of two-dimensional molybdenum ditelluride
Author: Octon, T.
ISNI:       0000 0004 7654 6237
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2019
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In this thesis the layered, two-dimensional material MoTe2 is examined experimentally for its optoelectronic properties, using a field effect transistor device configuration. MoTe2 experiences a strong light matter interaction, which is highly dependent on the conditions of the measurement, and the wavelength of light used. Light is able to: produce a photocurrent in MoTe2, desorb adsorbates from the surface, and even controllably thin by a single layer at a time. A theoretical study on MoTe2 also provides insights on the source of some of these interesting light matter interactions. MoTe2 is found to be a fast and responsive photodetector when illuminated with red laser light in ambient conditions, with increases in current stemming from the photovoltaic effect. Due to the generated charge carriers from the photovoltaic effect, conductivity can increase by increasing the Fermi energy of the material, or by a photogating effect where excited charges are trapped and behave as an artificial gate for the field effect transistor. The mechanisms of charge trapping are experimentally investigated due to their prevalence in the photodetection mechanisms. A theoretical study points towards the existence of two types of trap states, in not just MoTe2 but all transition metal dichalcogenides, with shallow traps closer to the valence band edge (τ ~ 500 s) and deeper traps (τ ~ 1000 s), further away from the valence band edge. MoTe2, under the effects of higher energy photons from blue and green lasers, showed different photocurrent mechanisms to red light. From the increased energy of the photons, photo-desorption of adsorbates on the surface of MoTe2 occurred causing a decrease in the overall current, in a rarely seen photocurrent mechanism. Again, both shallow and deep traps are evident from the experimental measurements, with the shallow traps being removed when illuminated by higher energy photons. Finally, a humidity assisted photochemical layer-by-layer etching process was developed with an in-situ Raman spectroscopy system, able to thin MoTe2 by a single layer at a time with 200 nm spatial resolution. MoTe2 FETs were created with thinned channels to examine the effect of the thinning technique on optoelectronic properties. Some improvement in optoelectronic performance (higher responsivity, higher mobility) was seen for the thinned channel devices, with great improvement observed for monolayer MoTe2.
Supervisor: Wright, C. D. ; Craciun, M. Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: MoTe2 ; 2D materials ; Optoelectronic