Degradation of III-V quantum dot lasers grown directly on silicon substrates

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 localisation in quantum dots minimises degradation, an increase in the number of defects in the early stages of ageing can increase the internal optical-loss which, can initiate rapid degradation of laser performance due to the rise in threshold carrier density. Population of the 2-D 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 show no signs of increase in internal optical-loss when operated under the same conditions

A two-stage surface treatment for the long-term stability of hydrophilic SU-8

The use of SU-8 photoresist as a structuring material for portable capillary-flow cytometry devices has been restricted by the near-hydrophobic 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 O2 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

Faraday-cage-assisted etching of suspended gallium nitride nanostructures

We have developed an inductively coupled plasma etching technique using a Faraday cage to create suspended gallium-nitride devices in a single step. The angle of the Faraday cage, gas mix, and chamber condition define the angle of the etch and the cross-sectional profile, which can feature undercut angles of up to 45°. We fabricate singly- and doubly-clamped cantilevers of a triangular cross section and show that they can support single optical modes in the telecom C-band

Non-uniform carrier distribution in multi-quantum-well lasers

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 temperature

Improving the optical bandwidth of passively mode-locked InAs quantum dot lasers

We examine in detail the relation between the optical gain spectra, mode-locked optical emission spectra, and temporal optical pulse widths as a function of temperature between 80 and 300 K in passively mode-locked InAs quantum dot lasers. By increasing the length of the active region, we can decrease the threshold gain requirement for mode locking. At 300 K, where the dot states and wetting layer are close to thermal equilibrium, the bandwidth of the optical emission spectra and temporal optical pulse width remain largely unaffected when the threshold gain requirement is reduced. At 80 K, where the dots are randomly populated, there is a near doubling of the optical bandwidth for the same reduction of the threshold gain requirement and a corresponding decrease in the temporal optical pulse width. Rate equations, which take explicit account of the photon density in the cavity, are used to qualitatively highlight the key parameters, which are responsible for increasing the optical bandwidth in the random population regime

Analysing radiative and non-radiative recombination in InAs QDs on Si for integrated laser applications

Three InAs quantum dot (QD) samples with dislocation filter layers (DFLs) are grown on Si substrates with and without in-situ annealing. Comparison is made to a similar structure grown on a GaAs substrate. The three Si grown samples have different dislocation densities in their active region as revealed by structural studies. By determining the integrated emission as a function of laser power it is possible to determine the power dependence of the radiative efficiency and compare this across the four samples. The radiative efficiency increases with decreasing dislocation density; this also results in a decrease in the temperature quenching of the PL. A laser structures grown on Si and implementing the same optimum DFL and annealing procedure exhibits a greater than 3 fold reduction in threshold current as well as a two fold increase in slope efficiency in comparison to a device in which no annealing is applied. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Direct-write projection lithography of quantum dot micropillar single photon sources

We have developed a process to mass-produce quantum dot micropillar cavities using direct-write lithography. This technique allows us to achieve high volume patterning of high aspect ratio pillars with vertical, smooth sidewalls maintaining a high quality factor for diameters below 2.0 μm. Encapsulating the cavities in a thin layer of oxide (Ta2O5) prevents oxidation in the atmosphere, preserving the optical properties of the cavity over months of ambient exposure. We confirm that single dots in the cavities can be deterministically excited to create high purity indistinguishable single photons with interference visibility (96.2 ± 0.7)%