Ultra-wide Band-gap Semiconductor Heterostructures for UV Opto-electronics
A plethora of strategic, astronomical, commercial, and biological applications necessitate development of high-performance ultra-violet (UV) photodetectors to enable sensitive detection of low intensity UV signals. While a well-established manufacturing process line, easy circuit integration and cost-effectiveness make silicon (Si)-based photodetectors, the choice for most of the applications today; its narrow band-gap and consequently, a relatively high intrinsic carrier concentration, limit its use for applications that require sensitive room and high temperature UV detection. Photodetectors utilizing wide bandgap (WBG) semiconducting absorber layers such as Aluminium Gallium Nitride (AlGaN) and Gallium Oxide (Ga2O3) offer multiple advantages such as intrinsic solar-blindness, room temperature operation, improved external quantum efficiency (EQE), and radiation hardness. Vertical device architectures such as p-i-n and Schottky detectors are especially interesting since they benefit from an intrinsic field in the absorber region that allows UV sensing without the application of an external bias and a vertical topology that enables integration into focal plane arrays. This circumvents the need of an added voltage supply and a cooling assembly, thereby reducing the system complexity, cost, footprint, and weight significantly. Finally, epitaxial heterojunctions of these WBG semiconductors hold promise towards enabling next-generation multi-spectral/broad-band detectors whose response can be tuned to suit specific applications. The first part of the thesis focuses on development of high efficiency, low noise, vertical, deep-UV (sub-290 nm) AlGaN detectors on sapphire substrates. The presence of a high density of defects in the hetero-epitaxially grown AlGaN epi-layers and the inability to realize low resistance contacts impedes the performance of these detectors. This is especially challenging for Al-rich AlGaN epilayers required for sub-290 nm detection as an increase in the Al-mole fraction leads to an increased bond strength, making the diffusive mechanisms that aid dislocation density (DD) reduction difficult. Also, the dopant activation energies increase in going from GaN to AlN, thus posing difficulties in growth of low resistive Al-rich AlGaN epi-layers. The presented work therefore aims to develop growth strategies to address these challenges, establish a microstructure-defect-detector performance correlation and utilize this understanding for the development of record performance Schottky and p-i-n ultraviolet photodetectors on c-plane sapphire substrates. The second part of the thesis aims at exploring device designs for the realization of broadband UV detectors, whose response can be tuned in any application-specific wavelength range. Such a multi-spectral/broadband response cannot be achieved using a single absorber layer since the response of any detector peaks at a given wavelength and falls on either side of the peak. Energy band engineering at epitaxial heterojunctions of AlGaN and β-Ga2O3 has been explored as a novel, scalable approach for demonstration of UV-A/C and UV-C broadband detectors.