30 research outputs found
Control of InGaAs facets using metal modulation epitaxy (MME)
Control of faceting during epitaxy is critical for nanoscale devices. This
work identifies the origins of gaps and different facets during regrowth of
InGaAs adjacent to patterned features. Molecular beam epitaxy (MBE) near SiO2
or SiNx led to gaps, roughness, or polycrystalline growth, but metal modulated
epitaxy (MME) produced smooth and gap-free "rising tide" (001) growth filling
up to the mask. The resulting self-aligned FETs were dominated by FET channel
resistance rather than source-drain access resistance. Higher As fluxes led
first to conformal growth, then pronounced {111} facets sloping up away from
the mask.Comment: 18 pages, 7 figure
Selective-area chemical beam epitaxy of in-plane InAs one-dimensional channels grown on InP(001), InP(111)B, and InP(110) surfaces
We report on the selective-area chemical beam epitaxial growth of InAs
in-plane, one-dimensional (1-D) channels using patterned SiO-coated
InP(001), InP(111)B, and InP(110) substrates to establish a scalable platform
for topological superconductor networks. Top-view scanning electron micrographs
show excellent surface selectivity and dependence of major facet planes on the
substrate orientations and ridge directions, and the ratios of the surface
energies of the major facet planes were estimated. Detailed structural
properties and defects in the InAs nanowires (NWs) were characterized by
transmission electron microscopic analysis of cross-sections perpendicular to
the NW ridge direction and along the NW ridge direction. Electrical transport
properties of the InAs NWs were investigated using Hall bars, a field effect
mobility device, a quantum dot, and an Aharonov-Bohm loop device, which reflect
the strong spin-orbit interaction and phase-coherent transport characteristic
in the selectively grown InAs systems. This study demonstrates that
selective-area chemical beam epitaxy is a scalable approach to realize
semiconductor 1-D channel networks with the excellent surface selectivity and
this material system is suitable for quantum transport studies
The 2021 WHO catalogue of Mycobacterium tuberculosis complex mutations associated with drug resistance: a genotypic analysis.
Background: Molecular diagnostics are considered the most promising route to achievement of rapid, universal drug susceptibility testing for Mycobacterium tuberculosis complex (MTBC). We aimed to generate a WHO-endorsed catalogue of mutations to serve as a global standard for interpreting molecular information for drug resistance prediction. Methods: In this systematic analysis, we used a candidate gene approach to identify mutations associated with resistance or consistent with susceptibility for 13 WHO-endorsed antituberculosis drugs. We collected existing worldwide MTBC whole-genome sequencing data and phenotypic data from academic groups and consortia, reference laboratories, public health organisations, and published literature. We categorised phenotypes as follows: methods and critical concentrations currently endorsed by WHO (category 1); critical concentrations previously endorsed by WHO for those methods (category 2); methods or critical concentrations not currently endorsed by WHO (category 3). For each mutation, we used a contingency table of binary phenotypes and presence or absence of the mutation to compute positive predictive value, and we used Fisher's exact tests to generate odds ratios and Benjamini-Hochberg corrected p values. Mutations were graded as associated with resistance if present in at least five isolates, if the odds ratio was more than 1 with a statistically significant corrected p value, and if the lower bound of the 95% CI on the positive predictive value for phenotypic resistance was greater than 25%. A series of expert rules were applied for final confidence grading of each mutation. Findings: We analysed 41 137 MTBC isolates with phenotypic and whole-genome sequencing data from 45 countries. 38 215 MTBC isolates passed quality control steps and were included in the final analysis. 15 667 associations were computed for 13 211 unique mutations linked to one or more drugs. 1149 (7·3%) of 15 667 mutations were classified as associated with phenotypic resistance and 107 (0·7%) were deemed consistent with susceptibility. For rifampicin, isoniazid, ethambutol, fluoroquinolones, and streptomycin, the mutations' pooled sensitivity was more than 80%. Specificity was over 95% for all drugs except ethionamide (91·4%), moxifloxacin (91·6%) and ethambutol (93·3%). Only two resistance mutations were identified for bedaquiline, delamanid, clofazimine, and linezolid as prevalence of phenotypic resistance was low for these drugs. Interpretation: We present the first WHO-endorsed catalogue of molecular targets for MTBC drug susceptibility testing, which is intended to provide a global standard for resistance interpretation. The existence of this catalogue should encourage the implementation of molecular diagnostics by national tuberculosis programmes. Funding: Unitaid, Wellcome Trust, UK Medical Research Council, and Bill and Melinda Gates Foundation
Improved breakdown voltages for type I InP/InGaAs DHBTs
We have investigated the base-collector breakdown voltage of type I InGaAs/InP DHBTs, which is shown to be dominated by band-to-band tunneling in the base-collector grade. By optimizing the grade we obtain a 20% increase in the breakdown voltage compared with traditional grades