(Nov 15) Silicon Photonics and Germanium

Silicon is not viewed as a material for photonics or optoelectronics, compared to compound semiconductors because of lack of light emission or optical modulation which is attributed to its indirect band gap and very weak electro-optic effects. Therefore, “Silicon Photonics” would have sounded like an oxymoron before the early 2000s, in which a group of researchers at Intel demonstrated the world-first GHz optical modulator and Raman laser based on silicon. Since then, there has been a tremendous amount of investigation and demonstration of silicon photonic components and integration targeting optical communication or sensing as well as offering lower cost and scalability with mature silicon CMOS technologies. To that end, photodetection at near-infrared (NIR, 1.3-1.6um) is a must. Now is Germanium to shine thanks to high absorption at NIR and integrability with CMOS fabrication lines. Although, achieving a good germanium epi quality is challenging due to 4.2% lattice mismatch between the two elements which inevitably produces dislocations and in turn increases dark current of photodetectors. That is where MATERIAL SCIENTISTS and ENGINEERS should come to the rescue. Nonetheless, many groups have demonstrated Ge photodiodes with acceptable dark current, responsivity close to the quantum limit and bandwidth of >50GHz.