Novel optoelectronic devices for mid-infrared applications: from intersubband thermophotovoltaic detectors to Germanium nanomembrane light emitters
Optoelectronic devices operating in the mid-infrared spectral region are attracting increasing attention due to potential applications in a wide range of disciplines. For example, mid-infrared photodetectors play a key role in thermophotovoltaic (TPV) energy conversion, whereby a photovoltaic device is used to extract electrical power from heat radiation. This technology is attractive for waste heat harvesting and clean energy production in several different environments. Similarly, mid-infrared light sources are particularly useful for biochemical sensing and spectroscopy, where the distinctive absorption features of many molecular species of interest can be exploited for their sensitive identification and detection. Both devices are investigated in this thesis work. In the area of TPV energy conversion, I have studied the use of intersubband transitions in semiconductor quantum cascade structures as a means to overcome the fundamental limitations of existing TPV devices using mature InP-based technology. Very efficient coverage of the incident radiation spectrum and optimal current matching can be achieved using multiple quantum-cascade structures monolithically integrated with a p-n junction, by taking advantage of their intrinsic cascading scheme, spectral agility, and design flexibility. Numerical simulations indicate that this approach can effectively double the present state-of-the-art in TPV output electrical power. In the area of mid-infrared light sources, my work has focused on developing efficient light emitters based on tensilely strained Germanium nanomembranes (Ge NMs). These ultrathin (a few ten nanometers) single-crystal membranes are good candidates for the development of CMOS-compatible Group-IV light sources, by virtue of their ability to sustain large strain levels and in the process become direct-bandgap materials. My research efforts have concentrated on the development of optical cavities based on Ge NMs that can satisfy the mechanical flexibility requirement of this materials platform. In particular, photonic-crystal (PhC) cavities in the form of disconnected dielectric-column arrays have been designed and fabricated based on a novel membrane assembly method, producing clear cavity-mode features in NM photoluminescence spectra.