Micro-cleaved ridge lasers for optoelectronic integration on silicon

Abstract

This thesis addresses one of the last hurdles to optoelectronic integration on silicon, namely the incorporation of room-temperature, electrically-pumped edge-emitting laser diodes. To this end, thin (-6 pm) InP-based multiple quantum well (MQW) ridge laser platelets emitting at a wavelength of 1550 nm have been manufactured and integrated by metal-to-metal bonding onto silicon substrates. These laser platelets can be thought of as freestanding optoelectronic building blocks that can be integrated as desired on diverse substrates. These blocks are fully processed lasers, with both top side and bottom side electrical contacts. The thinness of these optoelectronic building blocks and the precision with which their dimensions are defined are conducive to assembling them in dielectric recesses on a substrate, such as silicon, as part of an end-fire coupled, coaxial alignment optoelectronic integration strategy. They are assembled by a micro-scale pick and place technique that allows the blocks to be picked up individually and placed as desired on any substrate. Integration is accomplished by metal-to-metal solder bonding. To enable the manufacture of these laser blocks, a novel micro-cleaving process technology has been developed. This novel micro-cleaving process is used to simultaneously obtain both smooth end laser facets and precisely defined laser cavity lengths. As a proof of concept, this process has been shown to achieve nominal cavity lengths of 300 pm +/- 1.25 pm. It is believed that this micro-cleaving process could be used in the future to make thin platelet lasers having much shorter cavity lengths and with better than 1.25 pm length precision. For the 300 pm long, 6 ,am thin, micro-cleaved ridge platelet lasers integrated onto silicon substrates, continuous-wave lasing at temperatures as high as 55 "C and pulsed lasing at temperatures to at least 80 "C have been achieved.

(cont.) These lasers have output powers as high as 26.8 mW (at T = 10.3 oC), differential efficiencies as high as 81% (at T = 10.3 oC), and threshold currents as low as 18 mA (at T = 10.3 oC). The characteristic temperatures, To and T1, of the lasers on silicon were measured to be 43 K and 85 K, respectively. To put the performance of these integrated micro-cleaved ridge lasers on silicon in perspective, conventionally cleaved multiple quantum well (MQW) ridge lasers on their native InP substrate were also fabricated and tested. The thin micro-cleaved ridge platelet lasers integrated onto silicon outperformed the conventional lasers on InP in terms of thermal characteristics (maximum operating temperature, To, and T1), output power, and differential efficiency. The structure of this thesis is as follows. First, the motivation for this work and the historical evolution of the optoelectronics field are briefly described. Next, the various optoelectronic integration techniques that have been pursued over the years and their limits are presented. The novel fabrication processes developed to manufacture these platelet lasers is then described in detail. Specifics on the characterization methods and measurement results of both the micro-cleaved ridge lasers on silicon and the conventionally cleaved ridge lasers on native InP substrates are presented. A technique, Magnetically Assisted Statistical Assembly, that could be potentially used to scale the integration technology to ultra-high densities of optoelectronic components is then theoretically described. Finally, the thesis concludes with a comparison with other state of the art results in the literature and proposes further directions for this research effort.