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�

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

Thermally activated resonant tunnelling in GaAs/AlGaAs triple barrier tunnelling structures

A thermally activated resonant tunnelling feature has been observed in the current-voltage characteristics (I(V)) of triple barrier resonant tunnelling structures (TBRTS) due to alignment of the n=1 confined states in the two quantum wells (QWs) within the active region. With rising sample temperature, the tunnelling current of the resonant feature increases in magnitude, showing a small negative differential resistance region which is discernable even at 293K. This behaviour is unique to multiple barrier devices and cannot be observed in conventional double barrier resonant tunnelling structures. Symmetric TBRTS, of nominal well widths 67Å and asymmetric QW, with decreasing second well widths, nominally 64Å to 46Å, have been studied with temperature dependent resonant tunnelling behaviour observed in both symmetric and asymmetric designs. Activation energies have been extracted from Arrhenius plots of the magnitude of the thermally activated peak current for each device design. This activation energy decreases as the second well width is decreased due to alignment occurring at increasingly greater bias and as such at energies closer to the Fermi level in the emitter region of the devices. Experimentally determined activation energies are in good agreement with theoretical values obtained by modelling the device I(V) characteristic

Design and Characterisation of Multi-Mode Interference Reflector Lasers for Integrated Photonics

InAs quantum dot ridge waveguide lasers comprising single-port multi-mode-interference-reflectors (MMIR) and single-cleaved reflectors are designed, fabricated, and characterised, to demonstrate capability for optoelectronic-integrated-circuits. Simulations of an MMIR show high values of fundamental mode reflectivity ($> 80\% )$ and good selectivity against higher order modes. Deep-etched MMIR lasers fabricated with 0.5 mm long cavities have a threshold current of 24 mA, compared to 75 mA for standard Fabry–Perot cleaved–cleaved FP-RWG lasers of the same length, both at 25 °C, and 56 mA compared to 102 mA at 55 °C. MMIR lasers exhibit stable ground state operation up to 50 °C and show promise as small footprint sources for integrated photonics

Determination of the transport lifetime limiting scattering rate in InSb/Al<inf>x</inf>In<inf>1−x</inf> Sb quantum wells using optical surface microscopy

We report magnetotransport measurements of InSb/Al1−xInxSb quantum well structures at low temperature (3 K), with evidence for 3 characteristic regimes of electron carrier density and mobility. We observe characteristic surface structure using differential interference contrast DIC (Nomarski) optical imaging, and through use of image analysis techniques, we are able to extract a representative average grain feature size for this surface structure. From this we deduce a limiting low temperature scattering mechanism not previously incorporated in transport lifetime modelling of this system, with this improved model giving strong agreement with standard low temperature Hall measurements. We have demonstrated that the mobility in such a material is critically limited by quality from the buffer layer growth, as opposed to fundamental material scattering mechanisms. This suggests that the material has immense potential for mobility improvement over that reported to date

Thermally activated resonant tunnelling in GaAs/AlGaAs triple barrier heterostructures

Abstract We report the observation of a thermally activated resonant tunnelling feature in the currentvoltage characteristics (I(V)) of triple barrier resonant tunnelling structures (TBRTS) due to the alignment of the n = 1 confined states of the two quantum wells within the active region. With great renewed interest in tunnelling structures for high frequency (THz) operation, the understanding of device transport and charge accumulation as a function of temperature is critical. With rising sample temperature, the tunnelling current of the observed low voltage resonant feature increases in magnitude showing a small negative differential resistance region which is discernible even at 293 K and is unique to multiple barrier devices. This behaviour is not observed in conventional double barrier resonant tunnelling structures where the transmission coefficient at the Fermi energy is predominantly controlled by an electric field, whereas in TBRTS it is strongly controlled by the 2D to 2D state alignment