Heterogeneous Silicon Photonics: Photodiodes Based on Heteroepitaxy and Adhesive Bonding, and Micro-Transfer Printed Modulators

Silicon photonics has attracted considerable research interest in recent years. Leveraging complementary metal-oxide semiconductor (CMOS) infrastructure for the fabrication of photonic integrated circuits (PICs), silicon photonics has been increasingly adopted for optical data communications, microwave photonics, sensing, imaging, biomedical, and AI applications. A primary challenge for silicon photonics has been the absence of high-performance active components, which predominantly rely on group III-V materials. Given that high-speed photodetectors and modulators are fundamental to photonic circuits, my thesis focuses on the integration of high-performance III-V photodetectors and modulators onto silicon photonic platforms using different integration techniques. My dissertation is structured around several key devices: III-V waveguide photodetectors on silicon by heteroepitaxy, high-speed photodetectors integrated on silicon nitride (SiN) platform via heterogeneous integration, micro-transfer printable Mach-Zehnder modulators, and foundry-enabled silicon PICs.

In the project concerning photodetectors via III-V-on-Si heteroepitaxy, I led both fabrication and characterization efforts. We demonstrated the first high-speed III–V waveguide (WG) photodiodes (PDs) epitaxially grown on Si having high responsivity of 0.78 A/W and a 3-dB bandwidth of 28 GHz. Also, an open non-return-zero (NRZ) 40 Gbit/s eye diagram was detected by our device.

In the work on high-speed photodetectors integrated with microresonators on the SiN platform, I led the device design, fabrication, and characterization. The resulting PD exhibited a responsivity of 0.45 A/W, a 53-GHz 3 dB bandwidth, and a maximum radio frequency (RF) output power of -8.9 dBm at 50 GHz. Balanced PDs of this type achieved a record-high 3-dB bandwidth of 30 GHz and a common-mode rejection ratio (CMRR) of 26 dB. Notably, the photodetector's fabrication process that I developed had negligible impact on the microresonator's performance, demonstrating its suitability for integration into high-speed applications on the SiN platform.

A significant part of my thesis addresses high-speed transfer-printable O-band Mach-Zehnder modulators (MZMs). Here, I devised a completely new fabrication process for modulator chiplets, and developed the measurement methodology in our lab. The multi quantum well (MQW) MZM demonstrated remarkable performance metrics: a halfwave voltage-length product Vπ·L of 3 V·mm, a bandwidth exceeding 67 GHz, and an extinction ratio of up to 17 dB. Moreover, our modulator chiplets were successfully transfer-printed onto a SiN waveguide chip with a promising yield.

Finally, another topic of my PhD work centered on silicon PIC design: the presented optoelectronic frequency mixer showed a 3-dB bandwidth of 30 GHz, with potential scalability for multi-channel applications.