Reliable, efficient electrically pumped silicon-based lasers would enable full integration of photonic and electronic circuits, but have previously only been realized by wafer bonding. Here, we demonstrate the first continuous-wave InAs/GaAs quantum-dot lasers directly grown on silicon substrates with a low threshold current density of 62.5 A/cm 2 , a room-temperature output power exceeding 105 mW, lasing operation up to 120 o C, and over 3,100 hours of continuous-wave operating data collected, giving an extrapolated mean time to failure of over 100,158 hours. The realization of highperformance quantum-dot lasers on silicon is due to the achievement of a low density of threading dislocations on the order of 10 5 cm -2 in the III-V epilayers by combining a nucleation layer and dislocation filter layers with in-situ thermal annealing. These results are a major advance towards silicon-based photonics and photonic-electronic integration, and could provide a route towards reliable and cost-effective monolithic integration of III-V devices on silicon.Increased data throughput between silicon processors in modern information processing demands unprecedented bandwidth and low power consumption beyond the capability of conventional copper interconnects. To meet these requirements, silicon photonics has been under intensive study in recent years 1,2 . Despite rapid progress being made in silicon-based light modulation and detection technology and low-cost silicon optoelectronic integrated devices enabled by the mature CMOS technology 3,4 , an efficient reliable electrically pumped laser on a silicon substrate has remained an unrealized scientific challenge 5 . Group IV semiconductors widely used in integrated circuits, e.g. silicon and germanium, are inefficient light-emitting materials due to their indirect bandgap, introducing a major barrier to the development of silicon photonics. Integration of IIIÐV materials on a silicon platform has been one of the most promising techniques for generating coherent light on silicon. IIIÐV semiconductors with superior optical properties, acting as optical gain media, can be either bonded or epitaxially grown on silicon substrates [6][7][8][9][10][11] , with the latter approach being more attractive for large scale, low-cost, and streamlined fabrication. However, until now, material lattice mismatch and incompatible thermal expansion coefficients between IIIÐV materials and silicon substrates have fundamentally limited the monolithic growth of IIIÐV lasers on silicon substrates by introducing high-density threading dislocations (TDs) 12 .Lasers with active regions formed from III-V quantum dots (QDs), nano-size crystals, can not only offer low threshold current density (J th ) but also reduced temperature sensitivity [13][14][15][16][17] . As shown in Figure 1a, within less than 10 years, the performance of QD lasers has surpassed state-of-the-art quantum-well (QW) lasers developed over the last few decades in terms of J th . QD lasers have now been demonstrated with nearly constan...
The addition of elevated temperature steps (annealing) during the growth of InAs/GaAs quantum dot (QD) structures on Si substrates results in significant improvements in their structural and optical properties and laser device performance. This is shown to result from an increased efficacy of the dislocation filter layers (DFLs); reducing the density of dislocations that arise at the Si/III-V interface which reach the active region. The addition of two annealing steps gives a greater than three reduction in the room temperature threshold current of a 1.3 μm emitting QD laser on Si. The active region of structures grown on Si have a room temperature residual tensile strain of 0.17%, consistent with cool down from the growth temperature and the different Si and GaAs thermal expansion coefficients. This strain limits the amount of III-V material that can be grown before relaxation occurs.
Initial age-related degradation mechanisms for InAs quantum dot lasers grown on silicon substrates emitting at 1.3 µm are investigated. The rate of degradation is observed to increase for devices operated at higher carrier densities and is therefore dependent on gain requirement or cavity length. While carrier localization in quantum dots minimizes degradation, an increase in the number of defects in the early stages of aging can increase the internal optical-loss that can initiate rapid degradation of laser performance due to the rise in threshold carrier density. Population of the two-dimensional states is considered the major factor for determining the rate of degradation, which can be significant for lasers requiring high threshold carrier densities. This is demonstrated by operating lasers of different cavity lengths with a constant current and measuring the change in threshold current at regular intervals. A segmented-contact device, which can be used to measure the modal absorption and also operate as a laser, is used to determine how the internal optical-loss changes in the early stages of degradation. Structures grown on silicon show an increase in internal optical loss, whereas the same structure grown on GaAs shows no signs of increase in internal optical loss when operated under the same conditions.
We describe an approach to detect the presence of a nonuniform distribution of carriers between the different wells of multi-quantum-well laser diodes by measuring the gain and spontaneous emission spectra and demonstrate its application to a five-well sample that has a nonuniform carrier distribution at low temperatures.
aThe use of SU-8 photoresist as a structuring material for portable capillary-flow cytometry devices has been restricted by the nearhydrophobic nature of the SU-8 surface. In this work, we evaluate the use of chemical and plasma treatments to render the SU-8 surface hydrophilic and characterise the resulting surface utilising a combination of techniques including contact angle goniometry, atomic force microscopy and X-ray photoelectron spectroscopy. In particular, for low-power plasma treatments, we find that the chemistry of the plasma used to modify the SU-8 surface and the incorporation of O 2 on that modified surface are paramount for improved surface wettability, whilst plasma-induced surface roughness is not a necessary requirement. We demonstrate a technique to obtain a hydrophilic SU-8 surface with contact angle as low as 7°whilst controlling and significantly reducing the level of surface roughness generated via the applied plasma. An additional chemical treatment step is found to be essential to stabilise the activated SU-8 surface, and incubation of the samples with ethanolamine is demonstrated as an effective second-stage treatment. Application of the optimised two-stage surface treatment to cross-linked SU-8 is shown to result in a smooth hydrophilic surface that remains stable for over 3 months.
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