(Gordon E. Moore Medal for Outstanding Achievement in Solid State Science and Technology Award Address) Towards Silicon-Based Photonic Integrated Circuits: The Quest for Compatible Light Sources
Authors: Lockwood David J 
Published: February 2020

Optoelectronics and photonics are playing an essential role in many aspects of daily life, including information and communication technologies, environmental and green technologies, mechanical and chemical sensing, consumer electronics, and biomedicine. So far, the use of optical components in communication systems has been mainly limited to direct replacement of electrical cables by optical cables. With the continual increase in link bit rates, optical cables are now replacing electrical cables for shorter and shorter interconnect lengths. Optoelectronic and photonic technologies are becoming less costly and more integrated, and there is currently an opportunity for optics to move “inside the box” and change the interconnect topology at all levels. 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. Such optical interconnects will help in extending the life of Moore’s Law [1].
Currently, most optoelectronic devices are fabricated as discrete components. This approach is based on serial (e.g., step-by-step) fabrication and packaging, and it makes optoelectronic technology drastically different compared to microelectronics, where the domination of parallel fabrication made possible ultra large scale integration. Also, discrete assembly reduces the optoelectronic system reliability and decreases the manufacturing yield. Additional complications arise due to materials issues: in microelectronics the major material is elemental silicon, while traditional semiconductor materials for optoelectronics are III-V compound alloys with much more complex technological requirements. Finally, optical waveguides and waveguide based devices are very bulky compared to electron devices; thus, the densities of electron devices in integrated circuits are many orders of magnitude greater compared to that in integrated optoelectronic systems.
Silicon photonics, where photonics devices are fabricated by using silicon or silicon compatible materials and where the manufacturing is based on the available microelectronics infrastructure, is emerging as the technology that can face all these challenges [2]. Silicon photonics is booming and growing at an incredible pace with many breakthroughs appearing day by day [3,4]. Speed, integration density, active components, logic, nonlinear optics, etc., are all surpassed frontiers, which silicon photonics has continuously moved apart. Many devices enabled by silicon photonics are already on the market and new ones are emerging continually [5]. Despite these advances, the major deficiency in such optoelectronic and photonic devices remains the lack of suitable silicon-based light emitters and especially lasers, which would enable a fully integrated silicon platform. In order to be commercially viable, these light emitters need to be efficient, fast, operational at room temperature, and, perhaps most importantly, be compatible with mainstream CMOS technology. Another important requirement is in the emission wavelength, which should match the optical waveguide low-loss spectral region of 1.3–1.6 μm.
The main problem in utilizing silicon as a light emitter is its indirect electronic band gap that results in inefficient carrier recombination and a long radiative lifetime. Many quite different approaches to alleviating the miserable light emission in bulk silicon (~10-4 quantum efficiency at 300 K) have been proposed and are actively being explored [6]. Some, such as Si1-xGex quantum well or Si/SiO2 superlattice structures, rely on band structure engineering, while others rely on quantum confinement effects in lower dimensional structures, as typified by silicon quantum dots or porous silicon. Still another approach is impurity-mediated luminescence from, for example, isoelectronic substitution or by the addition of rare earth or transition metal ions to silicon.
In this presentation, the use of the quantum confinement approach we have employed to producing efficient light emission in silicon and germanium will be reviewed. Nanostructured systems that will be covered include porous silicon, silicon quantum wells and wires, super unit cells, and arrays of silicon-germanium quantum dots.

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