Heterogeneously Integrated Photodiodes on Silicon

High speed, high efficiency and high power photodiodes are key components in digital and analog photonic systems. To fully exploit the benefits of optical systems, high performance photodiodes are needed. Silicon photonics, as a promising enabler for low-cost and high performance passive photonic devices with potential deployment in next generation data center and many other applications, has attracted vast research and funding during the past decade. However, due to the fact that silicon is transparent at telecommunication wavelengths and its indirect bandgap, it is challenging to achieve active devices, such as lasers and photodiodes, out of silicon. One solution is to heterogeneously integrate group III-V materials onto a SOI/silicon platform.
The objective of this dissertation is to study and demonstrate high performance photodiodes heterogeneously integrated onto a silicon photonics platform. Chapter 1, 2 and 3 review photodiode fundamentals, heterogeneous integration approaches, optical coupling schemes, and fabrication and characterization techniques. Chapters 4, 5 and 6 focus on device design, fabrication and characterization of molecular bonded waveguide photodiodes. Chapters 7 and 8 describe an alternative bonding process and a waveguide photodiode design using SU8 as adhesive.
In my dissertation I demonstrate modified uni-traveling carrier (MUTC) photodiodes with top p-contact heterogeneously integrated on silicon-on-insulator (SOI) nano-waveguides. Single photodiodes have very low dark current of 1 nA and a high bandwidth of up to 65 GHz. At 70 GHz, a record-high RF output power of -2 dBm at 20 mA was measured. Balanced photodiodes of this type reached 20 GHz bandwidth and a CMRR of 20 dB. In Chapter 7, InGaAsP/InP high-power photodiode structure was bonded onto a silicon die using SU8 and surface normal photodiodes were fabricated. These photodiodes have very low dark currents and are similar to photodiodes on native InP substrate. The responsivity was found to be 0.4 A/W, which is close to the calculated value. Based on my findings in Chapter 7, I developed a design of an adhesively integrated high efficiency waveguide photodiode on SOI that promises a bandwidth of up to 160 GHz.