Thermal profiles within the channel of planar gunn diodes using micro-particle sensors

The paper describes the use of a novel microparticle sensor (~3 μm diameter) and infra-red (IR) microscopy to measure the temperature profile within the active channel (typically 3 μm length and 120 μm width) of planar Gunn diodes. The method has enabled detailed temperature measurements showing an asymmetrical temperature profile along the active width of these devices. The asymmetrical temperature profile suggests a similar behaviour in the channel current density, which may contribute to the lower than expected RF output power

Impact ionisation electroluminescence in planar GaAs-based heterostructure Gunn diodes:Spatial distribution and impact of doping nonuniformities

When biased in the negative differential resistance regime, electroluminescence (EL) is emitted from planar GaAs heterostructure Gunn diodes. This EL is due to the recombination of electrons in the device channel with holes that are generated by impact ionisation when the Gunn domains reach the anode edge. The EL forms non-uniform patterns whose intensity shows short-range intensity variations in the direction parallel to the contacts and decreases along the device channel towards the cathode. This paper employs Monte Carlo models, in conjunction with the experimental data, to analyse these non-uniform EL patterns and to study the carrier dynamics responsible for them. It is found that the short-range lateral (i.e., parallel to the device contacts) EL patterns are probably due to non-uniformities in the doping of the anode contact, illustrating the usefulness of EL analysis on the detection of such inhomogeneities. The overall decreasing EL intensity towards the anode is also discussed in terms of the interaction of holes with the time-dependent electric field due to the transit of the Gunn domains. Due to their lower relative mobility and the low electric field outside of the Gunn domain, freshly generated holes remain close to the anode until the arrival of a new domain accelerates them towards the cathode. When the average over the transit of several Gunn domains is considered, this results in a higher hole density, and hence a higher EL intensity, next to the anode

Cathodoluminescence characterization of ZnO/ZnS nanostructures anodized under hydrodynamic conditions

ZnO/ZnS nanostructures were successfully synthesized by a simple electrochemical anodization of zinc in a glycerol based electrolyte containing sulfide-ammonium fluoride. The influence of different hydrodynamic conditions and anodization potentials during anodization on the morphological and electronic properties of the obtained ZnO/ZnS nanostructures was studied. The anodized samples were characterized using confocal Raman microscopy, X-Ray Diffraction (XRD), Field Emission Scanning Electronic Microscopy (FE-SEM), cathodoluminescence (CL), and photoelectrochemical water splitting tests under standard AM 1.5 conditions. The results showed that hydrodynamic conditions and higher potentials promoted the formation of ZnO/ZnS nanotubes with both higher sulphur content and crystalline defect density, which reduces the near band edge transition value of the materials and improves the photoelectrochemical activity for water splitting. Additionally, the higher photocurrent densities for water splitting were obtained for the samples anodized at the highest anodization potential and under hydrodynamic conditions, increasing in a 71% for the nanostructures anodized at 1000 rpm when the anodization potential changes from 20 to 40 V

Circumventing UV light induced nano-morphology disorder to achieve long lifetime PTB7-Th:PCBM based solar cells

Large area flexible electronics rely on organic or hybrid materials prone to degradation limiting the device lifetime. For many years, photo-oxidation has been thought to be one of the major degradation pathways. However, intense illumination may lead to a burn-in or a rapid decrease in performance for devices completely isolated from corrosive elements as oxygen or moisture. The experimental studies we present in here indicate that a plausible triggering for the burn-in is a spin flip after a UV photon absorption leading to the accumulation of electrostatic potential energy that initiates a rapid destruction of the nano-morpholgy in the fullerene phase of a polymer cell. To circumvent this and achieve highly stable and efficient devices, we induce a robust nano-crystalline ordering in the PCBM phase prior to UV illumination. In that event, PTB7-Th:PC71BM cells are shown to exhibit T80 lifetimes larger than 1.6 years under a continuous UV-filtered 1-sun illumination, equivalent to 7 years for sunlight harvesting at optimal orientation and 10 years for vertical applications.Peer ReviewedPostprint (author's final draft

Near infrared high efficiency InAs/GaAsSb QDLEDs: band alignment and carrier recombination mechanisms

The development of high efficiency laser diodes (LD) and light emitting diodes (LED) covering the 1.0 to 1.55 μm region of the spectra using GaAs heteroepitaxy has been long pursued. Due to the lack of materials that can be grown lattice-macthed to GaAs with bandgaps in the 1.0 to 1.55 μm region, quantum wells (QW) or quantum dots (QD) need be used. The most successful approach with QWs has been to use InGaAs, but one needs to add another element, such as N, to be able to reach 1.3/1.5μm. Even though LDs have been successfully demonstrated with the QW approach, using N leads to problems with compositional homogeneity across the wafer, and limited efficiency due to strong non-radiative recombination. The alternative approach of using InAs QDs is an attractive option, but once again, to reach the longest wavelengths one needs very large QDs and control over the size distribution and band alignment. In this work we demonstrate InAs/GaAsSb QDLEDs with high efficiencies, emitting from 1.1 to 1.52 μm, and we analyze the band alignment and carrier loss mechanisms that result from the presence of Sb in the capping layer

Intersubband plasmons induced negative refraction at mid-IR frequency in heterostructured semiconductor metamaterials

We theoretically and experimentally demonstrate negative refraction in a semiconductor system operating at mid-infrared wavelengths. Such effect is generic and realized by electrons quantum confinement in quantum wells, acting as an adjustable resonance