Advancing silicon photonics through germanium based devices and 3D integration

Silicon Photonics is considered to be essential for the sustained growth of semiconductor industry moving forward. The ubiquitous mobile devices and Internet of Things (IoT) are driving the data needs of the end user exponentially which has led to numerous data centers and perilously large power consumption in each of them. More than 3.4 billion people in the world have access to internet today and this number is increasing steadily day by day. Together, we generate more than 50 Terabytes (50,000 Gigabytes) of internet traffic per second at any given point of time. This number was just 100 Gigabytes per second in 2002 and it is expected to grow much faster going into the future. As for the power consumption, US data centers alone consumed about 70 billion kilowatt-hours of electricity in 2014, representing 2 percent of the country's total energy consumption, according to a study. That's equivalent to the amount consumed by about 6.4 million average American homes that year. This is a 4 percent increase in total data center energy consumption from 2010 to 2014, and a huge change from the preceding five years, during which total US data center energy consumption grew by 24 percent, and an even bigger change from the first half of last decade, when their energy consumption grew nearly 90 percent. It is well established that the copper cables which transfer data from one end of the data-center to the other are the bottlenecks reducing the overall bandwidth of the system and skyrocketing the power consumption on the whole. This bottleneck is getting worse day by day owing to the ever-increasing data needs. Silicon Photonics based 'optical interconnects' are the best solution to remove this bottleneck. Optical interconnects use photons instead of electrons for communication and therefore have the potential to offer very large bandwidths at minimal power consumption. In the very near future all the copper wires in the data-center ecosystem will have to be replaced by these optical interconnects if we must meet the data needs within the prescribed power budget. In order to build such a platform where conventional machines in the data center work in tandem with novel interconnects based on photon-devices, all the optical components need to be integrated seamlessly on a silicon chip. Modulators are the most important optical component of such a platform since they act as optical switches which control the flow of photons. In the first part of the dissertation, a silicon compatible germanium (Ge) electro-absorption modulator with the best reported energy-delay product is demonstrated. The figure-of-merits along with the design principles are discussed in detail while the fabrication methodology is briefly touched upon. Experimentally measured characteristics are then shown to be the best-in-class and ones that match the data requirements of the data-centers with minimal energy consumption. In the second part of the dissertation, we focus on developing an efficient silicon-compatible light emitter based on strained Ge technology. Detailed theoretical calculations lay down a roadmap for room-temperature lasing from Ge. These calculations also prove that the loss mechanisms involved in the light emission process from Ge have been inadequately modeled until now and shows that a particular loss mechanism known as the inter-valence-band absorption is a major barrier in the realization of a strained Ge laser. CMOS compatible fabrication techniques to introduce large uniaxial strain in Ge are then discussed. Finally, a low-threshold Ge laser at a temperature of 83 K is demonstrated. In the final part of the dissertation, the first demonstration of a 'truly' silicon compatible three-dimensional (3D) photonic crystals is discussed. Using the methodology developed, a broadband omnidirectional reflector is also demonstrated on silicon. This methodology is also shown to be particulary well suited for 3D waveguides and optical cavities.

Keywords: Electrical Engineering.
Advisor: Saraswat Krishna 
Harris J. S. (James Stewart)
Vuckovic Jelena