Examination Committee

Prof Shing Yu LEUNG, MATH/HKUST (Chairperson)
Prof Kei May LAU, ECE/HKUST (Thesis Supervisor)
Prof Ching Ping WONG, Department of Electronic Engineering, The Chinese University of Hong Kong (External Examiner)
Prof Joyce POON, Department of Electrical and Computer Engineering, University of Toronto (External Examiner)
Prof Andrew Wing On POON, ECE/HKUST
Prof Jianan QU, ECE/HKUST
Prof Kam Sing WONG, PHYS/HKUST

Abstract

Analogous to the historical scaling of CMOS technology governed by “Moore’s Law”, there is also an enduring and increasing need for miniaturization and large-scale cost-effective integration of photonic components on the Si platform for datacom and other emerging applications. Currently, direct hetero-epitaxial growth of III–V laser structures on Si using quantum dots (QDs) as the active region is a vibrant field of research, with the potential of low-cost, high-yield, long-lifetime and high-temperature operation. By confining light to small volumes with resonant recirculation, micro-lasers with low-loss, high-quality whispering gallery modes (WGMs) also hold great promise for ultralow-threshold lasing that is not limited by the challenges of gratings or Fabry-Pérot (FP) facets for optical feedback.
 
This thesis is thus devoted to reporting development of high-performance QD micro-cavity lasers directly grown on exact (001) Si substrates. By combining high-quality WGM and 3D confinement of injected carriers in the QD micro-disk structures, lasing operation from 10 K up to room temperature was achieved for 4-μm diameter micro-disk lasers (MDLs) under continuous optical pumping. Subwavelength MDLs as small as 1 μm in diameter have been demonstrated in the 1.2-μm wavelength range at 10 K. A systematic comparison of lasing dynamics shows that the MDLs on Si substrates compare favorably with devices fabricated on native GaAs substrates and state-of-the-art work reported elsewhere.
 
A full extension towards practical electrically injected laser configurations was further explored. The world’s first electrically pumped quantum-dot micro-ring lasers epitaxially grown on (001) Si were reported. Continuous-wave (CW) lasing up to 100°C was achieved at the 1.3 µm communication wavelength in micro-rings with a radius of 50 µm. Scaling the micro-sized WGM cavity to a radius of 5 µm gives rise to a sub-milliamp threshold of 0.6 mA. Both the thresholds and footprints are much smaller than those previously reported lasers epitaxially grown on Si. 
 
Furthermore, co-integration of various optoelectronic devices on Si was designed. Ultra-low dark current upon -1 V bias was measured to be 1.5e-10 A for the on-chip photodiode. For the in-plane waveguide coupling micro-ring laser with a 25-μm outer-ring radius, the measured photocurrent as a function of the injection current suggests a very low lasing threshold of around 3 mA. Through evanescent coupling, the feasibility of integrating active and passive devices was simulated, with preliminary experimental results. This is promising to increase integration density on this integrated Si platform by combining different components on the same chip to create increased functionality, speed and capacity.