Progress in Light Emission from Silicon and Germanium Nanostructures
Authors: Lockwood David J 
Published: February 2020

Light emission from Si and Ge nanostructures (NSs) has been of great interest in recent years (1) owing to the need for silicon-based light sources for applications in silicon optoelectronics and photonics (2). Optical interconnects are required these days for on-chip technology as an alternative to metal wires, because of data transmission bottlenecks introduced by their unavoidable delay times, significant signal degradation, problems with power dissipation, and electromagnetic interference. Two major avenues toward optical interconnects on a chip include a hybrid approach with III-V densely packaged optoelectronic components and the all-group-IV approach (mainly Si, Ge and SiGe), where the all major components, e.g., light emitters, modulators, waveguides and photodetectors, are monolithically integrated into the CMOS environment. Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters (1). Band gap engineering employing quantum wells, quantum wires or quantum dots has been proposed as one way to overcome this limitation and Si/Ge or Si/SiGe-alloy thin multilayer quantum well structures grown on Si have been produced on this principle, and although light emission with greatly improved efficiency has been obtained at low temperatures the emission at room temperature is still very weak, because of exciton dissociation (1–3). Recently, through employing novel band-gap engineering strategies, we have prepared several different entirely new bright light-emitting Si/Ge NSs including one possessing a direct gap. The latter structure is based on constructing a new super unit cell comprised of multiple planar epitaxial layers of Si and Ge grown on (001) Si0.4Ge0.6 (4). Others are based on silicon-germanium layers grown epitaxially on silicon in such a way as to form multiple layer three-dimensional NSs (quantum dots) (5, 6). We have also developed a simple and efficient electrochemical process that combines galvanic reaction and focused-ion-beam lithography to selectively synthesize gold nanoparticles that are consequently used for the growth of ordered SiGe nanowire arrays with predefined diameter (200 nm) and position (7, 8). Here, the light emitting properties of these and other recent similar Si/Ge NSs that have been found to luminesce efficiently at wavelengths in the important spectral range of 1.1–1.6 μm are compared for possible applications in optoelectronics and photonics.

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