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(1999)Journal ArticleA survey towards a selection of 35 methanol maser and/or ultracompact (UC) H ii regions, reported in Papers I and II and by Norris et al., has been conducted in the near-infrared (NIR). Out of 25 methanol maser sites surveyed, 12 are associated with a NIR counterpart. Out of 18 UC H ii regions (8 of which overlap with maser emission), 12 are associated with a NIR counterpart. Counterparts can be confidently identified not only by the positional agreements, but also by their unusually red colours. Spectral types for the embedded stars can be unambiguously determined for six sources, all of which imply massive, ionizing stars. One of these infrared sources has methanol maser emission, but no UC H ii region. It is possible that the maser emission associated with this source arises from a pre-UC H ii phase of massive stellar evolution or it could be that nearly all the ultraviolet photons are absorbed by dust within the UC H ii region. We have modelled the spectral energy distributions (SEDs) for some sources and find that a single blackbody can be used to estimate the stellar luminosity, but cannot represent the whole infrared SED. A two-component blackbody model and a radiative transfer model were also used to derive essential parameters of the infrared sources. The radiative transfer model also indicates which infrared sources are relatively young and which are older. Both models show that silicate absorption at 9.7 μm must be a dominant feature of these SEDs.
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(2021)ThesisMultiferroic BiFe0.5Cr0.5O3 (BFCO) in which ferroelectric and magnetic orders coexist has gained research interest owing to its potential applications, e.g., spintronic and resistive random-access memory. Moreover, multiferroics possess a narrower bandgap compared to typical ferroelectrics, extending their application to photovoltaic devices. In contrast to the conventional semiconductors, the polarization-induced electric field facilitates the photoexcited charge separation, leading to an above-bandgap photovoltage in ferroelectrics. Nevertheless, a long-standing issue is the relatively low absorption of visible light. Thus, it is essential but challenging to tune their bandgap without compromising ferroelectricity. This thesis explores structural phase transition in the epitaxial BFCO films grown on SrRuO3 buffered (001) SrTiO3 substrate via Laser Molecular Beam Epitaxy (LMBE). Reciprocal space mapping result shows strain relaxation mechanism is not solely by the formation of misfit dislocation but also by changing the crystal symmetry, transitioning from tetragonal-like to a monoclinically distorted phase as the thickness increases. The crystallographic evolution is also coupled with bandgap modulation, confirming that BFCO structure and its physical properties are strongly intertwined. Using spectroscopic ellipsometry, the slight redshift of the bandgap distinguishes the absorption process of the T-like BFCO layer from that of monoclinically distorted structure, further confirmed by spectral photocurrent measurement via conductive-atomic force microscopy. The preparation of pure phase BFCO film with a robust polarization is of paramount importance for practical application. Yet, similar to the parental bismuth ferrite, BFCO suffers from poor electrical leakage performance. We report a three-order of magnitude suppression in the leakage current for the BFCO film through judicious adjustment of the growth rate. Scanning probe microscopy (PFM, AFM and c-AFM) results reveal that both microstructure and ferroelectric properties can be tuned by lowering the growth rate, ensuing realization of the room-temperature ferroelectric polarization comparable to the ab-initio predicted value. This thesis provides a facile strategy to tailor the structure-property of epitaxial BFCO film and its functional response for emerging optoelectronic devices.