Device engineering of III-Nitride semiconductors based sensors

<p>In the 20th century, technological advancement in semiconductor-based industry has revolutionized the way humans live in this world. Silicon semiconductor has extensively been exploited for this purpose but possesses certain constraints due to the theoretical limits of the material and indirect bandgap nature. This motivates the exploration of new semiconductor materials and novel device designs to fulfill the current demands for deep space communication, ultraviolet (UV) signal detection, broad-spectrum image sensing applications, arctic research, and many more... III-nitrides, being a direct bandgap compound semiconductor and having high thermal chemical and thermal stability, are a suitable candidate to breach the limitation of existing technological aspects and provide a pathway towards futuristic photodetection device technologies.</p> <p>This thesis focuses on the epitaxial growth of III-nitrides semiconductors and their application for optical sensing following three novel photodetection device structures. The biggest challenge with the epitaxial growth of the III-nitrides semiconductor system is the unavailability of native substrates which forces us to utilize foreign substrates such as silicon (Si), sapphire, and silicon-carbide. These foreign substrates are lattice and thermally mismatched with the III-nitrides structures, building strain in the film which leads to defect generations and cracks. We can mitigate this effect by introducing aluminium nitride (AlN) a buffer layer to provide a cushion between the substrate (mainly Si) and III-nitrides heterostructure. We grow high-quality III-nitrides heterostructures under ultra-high vacuum growth conditions using a plasma-assisted molecular beam epitaxy system by optimizing the growth parameters for AlN, gallium nitride (GaN), and aluminium gallium nitride (AlGaN). The high-quality GaN heterostructures are investigated towards efficient photodetection applications via incorporating three novel device structures.</p> <p>The first device structure introduces asymmetric metal semiconductor metal structure to fabricate UV photodetectors on GaN on both Si and sapphire substrates. The asymmetric metal semiconductor metal structure enhances the photoresponsivity of both the devices by many folds in comparison to symmetric metal semiconductor metal structure based GaN photodetectors.</p> <p>Secondly, the surface morphology of a thick GaN film is tweaked to synthesize honeycomb-like nanostructures having more photon absorption sites which can scatter and trap the incident photons, and provides smoother charge transportation for better collection of photogenerated charge carriers. This novel morphology results in a giant responsivity of the fabricated device which is one among the highest reported value for GaN photodetector. The photodetector is further tested for harsh operating temperature conditions where the device display stable operations from low-temperature (-75 °C) to high-temperature (225 °C).</p> <p>Lastly, the enormous possibilities of 2-dimensional (2D) material system is integrated with a technologically viable 3-dimensional GaN platform, and the 2D/3D hybrid heterostructure are exploited for a broadband photodetection system which can be deployed for image sensing applications.</p> <p>This thesis provides a comprehensive understating of III-nitrides material systems, integration with a 2D material system, and exploration of novel device structures for broadband photodetection applications which can be functioned from earth to outer space having stable and reliable operations.</p>