This paper investigates the role of the insertion of trapping layers, above and below the active region, in improving the reliability of 1.31 μm InAs quantum-dot laser diodes epitaxially grown on silicon substrate. The study is based on an extensive set of characterization and accelerated aging experiments carried out on two groups of quantum-dot lasers, featuring the same geometry and epitaxial structure, but differing in the presence or absence of defect trapping layers. The results of our work demonstrate that devices with trapping layers exhibit i) higher optical performance in terms of L-I characteristics, ii) longer lifetime, when aged at similar temperatures and identical current densities, and iii) similar degradation modes with respect to the devices without trapping layers. This latter point highlights the role of epitaxial structure optimization in the improvement of the lifetime of the IR optical sources for next-generation silicon photonics. The selective reduction in concentration of specific defects, misfit dislocations rather than threading dislocations in this case, can effectively improve the reliability of the devices.
We review the latest results on the physical cause for the degradation of heterogeneous and quantum-dot based infrared laser diodes. Such devices are a fundamental building block of photonic integrated circuits for silicon photonics applications.
One of the most critical fabrication steps is the integration of the III-V semiconductors active device and the silicon-on insulator wafer providing the waveguiding and additional electronic components. One option is to achieve heterogeneous integration by bonding the two wafers together. Devices fabricated in this way do not suffer from bonding-related reliability issues; degradation is mainly related to long-term performance drift caused by generation and/or diffusion of non-radiative recombination centers.
The second possible option is the epitaxial growth of active optical devices directly on top of the SOI wafer, used as a substrate. In this case, quantum-dot devices are preferable, since they limit the degradation caused by the highly defective heteroepitaxial material. In these devices the main degradation processes are related to the high dislocation density through a recombination-enhanced defect reaction (REDR) climb and/or growth. Operation in excited state versus ground state can also accelerate degradation, since it lowers the effective barrier for carrier escape from the quantum dots to the quantum wells, a region where the defect reaction process is taking place.
In this paper we review our recent progress on high performance mode locked InAs quantum dot lasers that are directly grown on CMOS compatible silicon substrates by solid-source molecular beam epitaxy. Different mode locking configurations are designed and fabricated. The lasers operate within the O-band wavelength range, showing pulsewidth down to 490 fs, RF linewidth down to 400 Hz, and pulse-to-pulse timing jitter down to 6 fs. When the laser is used as a comb source for wavelength division multiplexing transmission systems, 4.1 terabit per second transmission capacity was achieved. Self-mode locking is also investigated both experimentally and theoretically. The demonstrated performance makes those lasers promising light source candidates for future large-scale silicon electronic and photonic integrated circuits (EPICs) with multiple functionalities.
Direct epitaxial growth of III-V lasers on silicon provides the most economically favorable means of photonic integration but has traditionally been hindered by poor material quality. Relative to commercialized heterogeneous integration schemes, epitaxial growth reduces complexity and increases scalability by moving to 300 mm wafer diameters. The challenges associated with the crystalline mismatch between III-Vs and Si can be overcome through optimized buffer layers including thermal cyclic annealing and metamorphic layers, which we have utilized to achieve dislocation densities < 7×106 cm-2. By combining low defect densities with defect-tolerant quantum dot active regions, native substrate performance levels can be achieved. Narrow ridge devices with threshold current densities as low as ~130 A/cm2 have been demonstrated with virtually degradation free operation at 35°C over 11,000 h of continuous aging at twice the initial threshold current density (extrapolated time-to-failure >10,000,000 h). At 60°C, lasers with extrapolated time-to-failure >50,000 h have been demonstrated for >4,000 h of continuous aging. Lasers have also been investigated for their performance under optical feedback and showed no evidence of coherence collapse at back-reflection levels of 100% (minus 10% tap for measurement) due to the ultralow linewidth enhancement factor (αH < 0.2) and high damping of the optimized quantum dot active region.
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