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�

Resonant tunnelling in GaAs/AlGaAs triple barrier heterostructures

This thesis describes experimental and theoretical research into triple barrier resonant tunnelling structures which are attractive as potential high frequency oscillators in the terahertz frequency range. A lack of practical and coherent radiation emitters in this frequency range has resulted in it being named the \terahertz gap". However resonant tunnelling structures are seen as potential sources for practical solid state emitters which operate in this frequency range at room temperature. A series of symmetric and asymmetric GaAs/Al0:33Ga0:67As triple barrier resonant tunnelling structures have been studied at low temperatures to investigate the tunnelling electrical behaviour and origin of the current resonances observed in the current-voltage characteristics of these structures. The effect of charge accumulation in the emitter quantum well has been investigated, and has been found to significantly alter the behaviour of the electrical characteristics of the structures. These investigations have provided a thorough understanding of the behaviour of these structures and has allowed for optimisation of the triple barrier design with a view to being utilised as a high frequency emitter. The current-voltage characteristics have also been studied as a function of temperature and a novel temperature dependent resonant tunnelling mechanism has been observed. The magnitude of the observed current resonance, which is associated with the energetic alignment to the n = 1 quasi-bound subband states increases with increasing sample temperature which is rare behaviour in systems dominated by quantum mechanics. Finally, the maximum oscillation frequency and output power of these resonant tunnelling structures has been calculated and an optimised triple barrier structure in which charge accumulation in the emitter quantum well does not occur has been designed. Simulated current-voltage characteristics for this design shows it improves the maximum oscillation frequency and maximum output power reported in current state of the art double barrier resonant oscillator structures

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