Loss-Coupled DFB Nano-Ridge Laser Monolithically Grown on a Standard 300-mm Si Wafer

Abstract

The integration of III-V semiconductors on Silicon is one of the key processes needed to build a versatile Silicon Photonics platform incorporating active and passive components. The direct epitaxy of III-V materials on Si can be considered the ultimate strategy to achieve this but, depending on the approach, still faces challenges in reaching sufficient crystalline quality, and/or interfacing with other silicon photonics components, and/or demonstrating electrical injection. Here, we present a loss-coupled DFB nano-ridge laser that is easier to process than our earlier demonstrated devices and opens the road towards electrical injection of these nanoscale lasers. The nano-ridges are epitaxially grown on a standard 300-mm Si wafer using the aspect ratio trapping (ART) technique to suppress defects, for details see [1, 2]. The high crystalline quality of the nano-ridges was proven before through characterization of their material gain [3] and the demonstration of optically-pumped lasing from a DFB cavity with etched gratings [4]. Electrical injection could not yet be demonstrated as depositing a continuous metal contact directly on top of the nano-ridge, only 300nm away from the active QWs, would result in a prohibitively large loss (>300dB/cm). Here, patterned metal gratings are deposited on the top of the nano-ridge instead. This allows to form a pure loss-coupled DFB cavity without the need to etch the III-V material, simplifying the processing and avoiding damaged interfaces. At the same time, this patterned metal grating in the future might serve as a pathway for electrical injection. The optical field of the low loss cavity mode concentrates in the dielectric part of the metal grating. This reduces the simulated loss of this mode to 23 dB/cm. The duty cycle of the gratings is designed to be 0.4 (0.37 as fabricated) for a good trade-off between the coupling strength provided by the periodically modulated metal layer and the cavity loss of the lasing mode. Following epitaxy of the GaAs/InGaAs nano-ridges, only a single ebeam and lift-off step is required to deposit the 5nm/40nm Ti/Au metallic grating layer with 164nm period. Fig. 1 (a) shows the cross-section of as-grown nano-ridges and (b) presents a top view of the loss-coupled DFB laser with metallic gratings on top.