Active photonic devices based on two-dimensional electron gas in engineered semiconductor heterojunction

Abstract

As electronics-based technologies reach their performance bottlenecks, the research community is shifting its focus from electrons to photons. Photons enjoy various advantages over the electrons, such as wider bandwidth, higher speeds, and almost negligible propagation loss. Silicon emerges as the workhorse for Photonic Integrated Circuits (PIC) owing to its high refractive index, excellent mechanical properties, along with the availability of stable native oxide and already existing mature CMOS industry. While silicon has excellent passive optical properties and has been employed for low-loss optical interconnects, resonators, and directional couplers. Realizing highly efficient active photonic functionalities with silicon has proven to be immensely challenging. In order to fully harness the tantalizing prospects that photonics offer, several electro-optic elements of a typical integrated optical link such as LASERS, optical modulators and photodetectors needs to be realized on a single chip. Thus, recently other best-in-class materials have been actively investigated for their excellent optical functionalities that can complement silicon to exploit the benefits of photonics to its fullest via hybrid integration configuration. Wide bandgap semiconductors like Zinc Oxide (ZnO) have been exploited for their superior tunable optical properties. Besides possessing a direct and wide bandgap, ZnO exhibits a large exciton binding energy (60 meV), amenity to wet chemical etching, high thermal conductivity, and radiation hardness. Additionally, ZnO allows bandgap engineering offering the possibility of realizing ZnO based heterojunction with smaller lattice mismatch, providing ZnO edge over its counterparts. Moreover, several reports have experimentally realized highly dense 2- Dimensional Electron Gas (2-DEG) at the interface of ZnO based heterojunctions. 2-DEG in ZnO based heterojunction exhibits several exquisite properties of 2-D materials due to confined charge carriers, such as high speed, high probability of quantum transition due to high density of charge carriers. Nevertheless, as ZnO has a bandgap of 3.36 eV it exhibits dynamic tunability in UV part of the spectrum, hindering its applications in visible and Infrared region.