Strained germanium photonic crystal membranes for scalable and efficient silicon-based photonic devices

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Silicon is ubiquitous for electronics and the most widely exploited semiconductor in the world available in plentiful and cheap supply. In spite of its success in electronics, silicon is fundamentally limited in terms of its ability to produce light. This is due to its so-called indirect band gap which means that electrons cannot easily lose energy by producing photons. In contrast, in direct band gap compound semiconductors such as GaAs and InP electrons can very easily lose energy resulting in the production of photons. Consequently, such semiconductors are widely exploited in light emitters including lasers and light emitting diodes. However, compound semiconductors are much more expensive to produce. Hence there is a strong desire to be able to produce optically-efficient direct band gap semiconductors on a silicon-based platform. This project aims to resolve this fundamental constraint by develop an entirely new approach to fabricating direct band gap germanium layers on silicon. Germanium can be readily grown on silicon and has a band gap that is much closer to being direct. It has been theoretically predicted that by straining the germanium crystal by >2% (tensile), it will become a direct band gap semiconductor. Producing stable highly strained germanium layers has proven to be technologically challenging. We will overcome this challenge using our recently discovered ion-implantation method to generate stable high tensile strained germanium layers. Such layers offer the potential to achieve record optical efficiencies in germanium. Using these layers we will demonstrate optical gain and lasing in photonic crystal nanocavities in the mid-infrared using an all group-IV based system. This combination of electronic- and photonic band structure and strain engineering offers a step-change in developing lasers on silicon with strong exploitation potential to scale-up and transform sensors for medical, environmental and industrial applications.