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�

Mixed Grazing Increases Abundance of Arbuscular Mycorrhizal Fungi in Upland Welsh Grasslands

Grasslands play a crucial role in exchanges between global ecosystems and the atmosphere and form an integral part of the agricultural industry. Arbuscular mycorrhizal fungi (AMF) are mutualistic symbionts of most grassland plant species and thereby influence the functional capacity of grassland systems. Agricultural grasslands are primarily used for livestock farming and are subjected to various management practices designed to increase production, but which also alter both plant and soil communities in the process. This research investigated the effects of a selection of management practices and environmental factors on the presence and abundance of AMF in upland Welsh grasslands. The aim was to identify how these management practices affected the abundance of AMF, assessed through microscopic observations of four AMF structures: spores, hyphae, vesicles and arbuscules. The results suggest grazing sheep and cattle together had the highest overall influence on AMF abundance compared to grazing sheep or cattle separately. High plant diversity correlated with high arbuscule and vesicle abundance, but conversely, the application of lime reduced vesicle abundance. These findings offer new insights into the effects of management practices on AMF. Mixing livestock, increasing plant diversity and reducing lime applications are shown here to improve the abundance of AMF and could, therefore, help to inform sustainable farm management decisions in the future

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

High field magneto-transport in high mobility gated InSb/InAlSb quantum well heterostructures

We present high field magneto-transport data from a range of 30nm wide InSb/InAlSb quantum wells. The low temperature carrier mobility of the samples studied ranged from 18.4 to 39.5 m2V-1s-1 with carrier densities between 1.5x1015 and 3.28x1015 m-2. Room temperature mobilities are reported in excess of 6 m2V-1s-1. It is found that the Landau level broadening decreases with carrier density and beating patterns are observed in the magnetoresistance with non-zero node amplitudes in samples with the narrowest broadening despite the presence of a large g-factor. The beating is attributed to Rashba splitting phenomenon and Rashba coupling parameters are extracted from the difference in spin populations for a range of samples and gate biases. The influence of Landau level broadening and spin-dependent scattering rates on the observation of beating in the Shubnikov-de Haas oscillations is investigated by simulations of the magnetoconductance. Data with non-zero beat node amplitudes are accompanied by asymmetric peaks in the Fourier transform, which are successfully reproduced by introducing a spin-dependent broadening in the simulations. It is found that the low-energy (majority) spin up state suffers more scattering than the high-energy (minority) spin down state and that the absence of beating patterns in the majority of (lower density) samples can be attributed to the same effect when the magnitude of the level broadening is large

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