In-plane gradient permittivity materials by shadow mask molecular beam epitaxy
Date
2025
Authors
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Publisher
University of Delaware
Abstract
Infrared spectroscopy currently relies on the use of expensive, bulky, and/or fragile spectrometers. For environmental monitoring, gas sensing and additional applications, an inexpensive, compact, sturdy on-chip spectrometer is a much-required technology. One way to attain this is through in-plane gradient permittivity materials (GPMs), in which the material permittivity varies as a function of position in the lateral direction. In this dissertation, I demonstrate the synthesis of infrared GPMs in silicon doped indium arsenide (Si:InAs) thin film samples using shadow mask molecular beam epitaxy (SMMBE) technique. Each of my resulting samples via SMMBE develops in-plane permittivity gradients on two opposite sides: on the flat mesa on one side, and on the film slope on the other side, leading to confining varying wavelengths of infrared light at varying horizontal locations. My first demonstration of in-plane GPM synthesis in Si:InAs using SMMBE exhibited an electric field enhancement corresponding to wavenumbers :650 cm-1 to 900 cm-1 over an in-plane width of 13 μm on the flat mesa; 900 cm-1 to 1250 cm-1 over an in-plane width of 13 μm on the film slope. ☐ In order to be useful, the permittivity gradient requires to be of high crystalline quality and its properties need to be tunable. With this in mind, I have dived further and demonstrated that the permittivity gradient length and steepness can be controlled by varying the shadow mask thickness. Samples synthesized with similar growth parameters but with mask thicknesses of 200 μm and 500 μm show permittivity gradient lengths of 18 μm and 39 μm on the flat mesa on one side and 11 μm and 23 μm on the film slope on the other side, respectively. The resulting gradient steepnesses are 23.3 cm-1/μm and 11.3 cm-1/μm on the flat mesa and 21.8 cm-1/μm and 9.1 cm- 1/μm on the film slope, for samples developed with the 200 μm and 500 μm masks, respectively. This work clearly displays the ability to control the in-plane permittivity gradient in Si:InAs thin films, setting the platform for the creation of a variety of miniature infrared devices. ☐ Overall, this dissertation presents the detailed information on how in-plane permittivity gradients in Si:InAs thin films can be created using SMMBE, which overcomes the shortcomings of the existing GPM synthesis methods that result in film damage and contamination. It also presents a thorough description on how the in-plane permittivity gradients can be controlled utilizing SMMBE and explains how the GPMs can work as the building blocks of optoelectronic infrared devices.
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Keywords
Gradient permittivity materials, Infrared, Molecular beam epitaxy, Plasmonics, Semiconductors, Shadow masks