Strain compensated InGaAs/AlAs triple barrier resonant tunnelling structures for THz applications

We report a theoretical study of InGaAs/AlAs triple barrier resonant tunnelling heterostructures which are optimised for operation in the terahertz frequency range, and compare these to current state of the art double barrier structures realised in the literature. We consider the effect of strain introduced due to the large lattice mismatch of the substrate, quantum well and potential barrier materials and describe designs with strain compensated active regions. Constraints have been imposed on the designs to minimise charge accumulation in the emitter quantum well which is often associated with more complex triple barrier structures. The use of a triple barrier structure suppresses the off resonance leakage current, thus increasing the maximum output power density, with � 3 mW�

InP quantum dot mode-locked lasers and materials studies

InP/GaInP quantum dot laser structures exhibiting broad optical gain spectra suitable for mode-locking have been demonstrated. Two-section narrow ridge passive mode-locked lasers were fabricated from this material. Mode-locking conditions have been investigated for devices with different cavity lengths, with maximum frequency of 15.21 GHz

Critical state alignment and charge accumulation in triple barrier resonant tunnelling structures

We report observations of resonant tunnelling features in the current-voltage (I(V)) characteristics of a series of triple barrier resonant tunnelling structures (TBRTS) due to the critical alignment of the n=1 confined states of the two quantum wells within the active region. Charge accumulation in the first QW of these structures has a significant effect on the I(V) characteristics of the resonances. A nominally symmetric TBRTS and asymmetric TBRTS, with decreasing second well widths, have been studied, with observations of charge accumulation affecting the critical alignment in both symmetric and asymmetric designs. We demonstrate that in highly asymmetric structures the critical alignment can occur coincident to the Fermi level in the emitter, and remains on resonance at higher bias than is expected due to charge accumulation in the structure. With great renewed interest in tunnelling structures for high frequency (THz) operation, the understanding of device transport and charge accumulation is critical

Monolithic growth of InAs quantum dots lasers on (001) silicon emitting at 1.55 μm

Broad-area 1.55 μm InAs quantum dots (QDs) lasers were fabricated based on monolithic growth of InAs/InAlGaAs/InP active structures on nano-patterned (001) silicon substrates. Device optoelectronic properties and materials' optical gain and absorption features were studied to provide experimental support for further optimizations in laser design

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

12.5-GHz InP quantum dot monolithically mode-locked lasers emitting at 740 nm

Monolithic InP/GaInP quantum dot passively mode-locked lasers, designed using gain and absorption measurements, are realised for the first time, emitting at 740 nm with 12.5 GHz repetition frequency