Design rules for dislocation filters

The efficacy of strained layer threading dislocation filter structures in single crystal epitaxial layers is evaluated using numerical modeling for (001) face-centred cubic materials, such as GaAs or Si(1-x)Ge(x), and (0001) hexagonal materials such as GaN. We find that threading dislocation densities decay exponentially as a function of the strain relieved, irrespective of the fraction of threading dislocations that are mobile. Reactions between threading dislocations tend to produce a population that is a balanced mixture of mobile and sessile in (001) cubic materials. In contrast, mobile threading dislocations tend to be lost very rapidly in (0001) GaN, often with little or no reduction in the immobile dislocation density. The capture radius for threading dislocation interactions is estimated to be approx. 40nm using cross section transmission electron microscopy of dislocation filtering structures in GaAs monolithically grown on Si. We find that the minimum threading dislocation density that can be obtained in any given structure is likely to be limited by kinetic effects to approx. 1.0e+04 to 1.0e+05 per square cm.Comment: 18 pages, 9 figure

Recent Progress of Quantum Dot Lasers Monolithically Integrated on Si Platform

With continuously growing global data traffic, silicon (Si)-based photonic integrated circuits have emerged as a promising solution for high-performance Intra-/Inter-chip optical communication. However, a lack of a Si-based light source remains to be solved due to the inefficient light-emitting property of Si. To tackle the absence of a native light source, integrating III-V lasers, which provide superior optical and electrical properties, has been extensively investigated. Remarkably, the use of quantum dots as an active medium in III-V lasers has attracted considerable interest because of various advantages, such as tolerance to crystalline defects, temperature insensitivity, low threshold current density and reduced reflection sensitivity. This paper reviews the recent progress of III-V quantum dot lasers monolithically integrated on the Si platform in terms of the different cavity types and sizes and discusses the future scope and application

The growth of low-threading-dislocation-density GaAs buffer layers on Si substrates

Monolithic integration of III-V optoelectronic devices on Si platform is gaining momentum, since it enables advantages of low cost, less complexity and high yield for mass production. With the aim of achieving advances in monolthic integration, the challenges associated with lattice mismatch between III-V layers and Si substrates must be overcome, as a low density of threading dislocations is a prerequisite for the robustness of the integrated devices. In this paper, we have investigated and compare different tyeps of dislocation filter layers (DFLs) from InGaAs asymmetric step-graded buffer layer (ASG), InGaAs/GaAs strained-layer superlattices, and quaternary alloy InAlGaAs ASG, on the functionlity of reducing threading dislocation density (TDD) for GaAs buffer layers on Si. Compared to other DFLs, the sample with InAlGaAs ASG buffer layer shows the lowest average TDD value and roughness, while the deccrease of TDD in the sample with InAlGaAs ASG buffer layer can be understood in terms of the hardening agent role of aluminium in the InAlGaAs ASG. By further optimising the InAlGaAs ASG through thermal cyclic annealing, we successfully demonstrate a low surface TDD of 6.3±0.1×106 /cm2 for a 2 µm GaAs/InAlGaAs ASG buffer layer grown on Si. These results could provide a thin buffer design for monolthic integration of various III-V devices on Si substrates

Monolithic quantum-dot distributed feedback laser array on silicon

Electrically-pumped lasers directly grown on silicon are key devices interfacing silicon microelectronics and photonics. We report here, for the first time, an electrically-pumped, room-temperature, continuous-wave (CW) and single-mode distributed feedback (DFB) laser array fabricated in InAs/GaAs quantum-dot (QD) gain material epitaxially grown on silicon. CW threshold currents as low as 12 mA and single-mode side mode suppression ratios (SMSRs) as high as 50 dB have been achieved from individual devices in the array. The laser array, compatible with state-of-the-art coarse wavelength division multiplexing (CWDM) systems, has a well-aligned channel spacing of 20 0.2 nm and exhibits a record wavelength coverage range of 100 nm, the full span of the O-band. These results indicate that, for the first time, the performance of lasers epitaxially grown on silicon is elevated to a point approaching real-world CWDM applications, demonstrating the great potential of this technology

A Cyber Physical System Crowdsourcing Inference Method Based on Tempering: An Advancement in Artificial Intelligence Algorithms

Activity selection is critical for the smart environment and Cyber-Physical Systems (CPSs) that can provide timely and intelligent services, especially as the number of connected devices is increasing at an unprecedented speed. As it is important to collect labels by various agents in the CPSs, crowdsourcing inference algorithms are designed to help acquire accurate labels that involve high-level knowledge. However, there are some limitations in the algorithm in the existing literature such as incurring extra budget for the existing algorithms, inability to scale appropriately, requiring the knowledge of prior distribution, difficulties to implement these algorithms, or generating local optima. In this paper, we provide a crowdsourcing inference method with variational tempering that obtains ground truth as well as considers both the reliability of workers and the difficulty level of the tasks and ensure a local optimum. The numerical experiments of the real-world data indicate that our novel variational tempering inference algorithm performs better than the existing advancing algorithms. Therefore, this paper provides a new efficient algorithm in CPSs and machine learning, and thus, it makes a new contribution to the literature

Modelling the effects of p-modulation doping in InAs/InGaAs quantum dot devices

A modelling routine has been developed to quantify effects present in p-modulation doped 1.3 μm InAs/InGaAs quantum dot laser and modulator devices. Utilising experimentally verified parameters, calculated modal absorption is compared to measurements, prior to simulation of structures under reverse and forward bias. Observed broadening and a reduction of absorption in p-doped structures is attributed primarily to increased carrier scattering rates and can bring benefit when structures are configured as optical modulators with enhancements in the figure of merit. However, increased carrier scattering limits the maximum modal gain that can be achieved for lasers. The state filling caused by p-doping only marginally reduces absorption but assists laser operation with increased differential gain and gain magnitude at lower current densities

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

High performance waveguide uni-travelling carrier photodiode grown by solid source molecular beam epitaxy

The first waveguide coupled phosphide-based UTC photodiodes grown by Solid Source Molecular Beam Epitaxy (SSMBE) are reported in this paper. Metal Organic Vapour Phase Epitaxy (MOVPE) and Gas Source MBE (GSMBE) have long been the predominant growth techniques for the production of high quality InGaAsP materials. The use of SSMBE overcomes the major issue associated with the unintentional diffusion of zinc in MOVPE and gives the benefit of the superior control provided by MBE growth techniques without the costs and the risks of handling toxic gases of GSMBE. The UTC epitaxial structure contains a 300 nm n-InP collection layer and a 300 nm n++-InGaAsP waveguide layer. UTC-PDs integrated with Coplanar Waveguides (CPW) exhibit 3 dB bandwidth greater than 65 GHz and output RF power of 1.1 dBm at 100 GHz. We also demonstrate accurate prediction of the absolute level of power radiated by our antenna integrated UTCs, between 200 GHz and 260 GHz, using 3d full-wave modelling and taking the UTC-to-antenna impedance match into account. Further, we present the first optical 3d full-wave modelling of waveguide UTCs, which provides a detailed insight into the coupling between a lensed optical fibre and the UTC chip.Comment: 19 pages, 24 figure

Electrically pumped continuous-wave O-band quantum-dot superluminescent diode on silicon

High-power, broadband quantum-dot (QD) superluminescent diodes (SLDs) are ideal light sources for optical coherence tomography (OCT) imaging systems but have previously mainly been fabricated on native GaAs- or InP-based substrates. Recently, significant progress has been made to emigrate QD SLDs from native substrates to silicon substrates. Here, we demonstrate electrically pumped continuous-wave InAs QD SLDs monolithically grown on silicon substrates with significantly improved performance thanks to the achievement of a low density of defects in the III-V epilayers. The fabricated narrow-ridge-waveguide device exhibits a maximum 3 dB bandwidth of 103 nm emission spectrum centered at the O-band together with a maximum single facet output power of 3.8 mW at room temperature. The silicon-based SLD has been assessed for application in an OCT system. Under optimized conditions, a predicted axial resolution of ∼5.3µm is achieved with a corresponding output power of 0.66 mW/facet

From Challenges to Solutions, Heteroepitaxy of GaAs-Based Materials on Si for Si Photonics

Monolithic growth of III-V materials onto Si substrates is appealing for realizing practical on-chip light sources for Si-based photonic integrated circuits (PICs). Nevertheless, the material dissimilarities between III-V materials and Si substrates inevitably lead to the formation of crystalline defects, including antiphase domains (APBs), threading dislocations (TDs), and micro-cracks. These nontrivial defects lead to impaired device performance and must be suppressed to a sufficiently low value before propagating into the active region. In this chapter, we review current approaches to control the formation of defects and achieve high-quality GaAs monolithically grown on Si substrates. An APB-free GaAs on complementary-metal-oxide semiconductor (CMOS)-compatible Si (001) substrates grown by molecular beam epitaxy (MBE) only and a low TD density GaAs buffer layer with strained-layer superlattice (SLS) and asymmetric step-graded (ASG) InGaAs layers are demonstrated. Furthermore, recent advances in InAs/GaAs quantum dot (QD) lasers as efficient on-chip light sources grown on the patterned Si substrates for PICs are outlined