Optoelectronic devices bridge optics to electronics and they are ubiquitous in our society. They include solid state light sources such as light emitting diodes and laser diodes, as well as modulators (that can encode an electrical or radiofrequency signal onto light) and detectors (which can convert light back into an electrical signal). Many modern optoelectronic devices rely on quantum confinement effects, and more in general on mesoscopic quantum structures which can be realized, for instance, by growing a sequence of thin layers of semiconductors (with thicknesses in the order of few nanometers). These structures possess new electrical and optical properties which the bulk semiconductors lack and are widely used for lasers (e.g. in quantum cascade lasers and quantum well laser diodes), modulators (e.g. in electro-absorption modulators) and detectors (such as in quantum dots and quantum well based detectors). New materials and technologies are very actively investigated by the scientific community to create faster, smaller and low-cost optoelectronic devices, and integration on silicon is often a must for telecommunications and imaging applications. 2D and van der Waals materials have attracted a lot of attention for optoelectronics, since they host new physical effects and can often be tuned electrically, by applying a voltage between the 2D material and a substrate via a gate oxide. However, the creation of large-scale quantum confinement in these materials is prohibitive since it is limited by the resolution of the lithographic and etching processes used to pattern either the 2D materials or the electrical gates.
So far, quantum engineering of 2D materials has been achieved mostly using heterostructures, including the realization of Moiré patterns. But these approaches have limits for large scale production and typically show weak effects. This EAGER research project aims to explore a new technique to create large scale quantum confined effects in 2D materials and to demonstrate devices based on this new technology. The approach is based on a new gate oxide fabrication technology. The gate consists of alternating layers of different oxides that are used to create a variable electrical potential on 2D materials when a voltage is applied on the gate. Unlike their bulk 3D counterparts, these quantum confined structures are widely tunable since the depth of the quantum wells is proportional to the applied gate. The PI plans to use these new effects to realize new types of modulators and photodetectors. Modulators can be realized by creating arrays of coupled two-dimensional quantum wells (also known as superlattices), which absorb light in different ways accordingly to the applied voltage. Both interband and intersubband absorptions in the 2D materials can be engineered using this new approach. Because these structures can be gated with highly conductive electrodes, the modulation speed is expected to be at least one order of magnitude greater than today’s modulators based on 2D materials. Furthermore, these devices will benefit from the possibility of quantum-engineering the excitons in 2D materials which appear at room temperature in the visible and near infrared ranges even when no quantum confinement is used. In addition to its obvious technological relevance, this project will advance understanding of 2D materials and associated quantum phenomena and offer opportunities for integrating this new knowledge in several courses on materials and devices at Harvard University.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
