Preface for the special issue on Microscopy of Semiconducting Materials 2019

This issue contains selected invited and contributed presentations from the 21st international conference on 'Microscopy of Semiconducting Materials' held at Fitzwilliam College, University of Cambridge, on 9–12 April 2019. The meeting was organised by the Institute of Physics, supported by the Royal Microscopical Society, the European Microscopy Society, attolight (Platinum sponsor), JEOL (Gold sponsor) and ThermoFisher Scientific (Silver sponsor)

X-ray reflectivity method for the characterization of InGaN/GaN quantum well interface

A method to characterize the interface of InGaN/GaN quantum wells by X-ray reflectivity is presented. The interface roughness can be obtained from the ratio of diffuse to specular scatterings obtained on a transverse ω-scan. Rotation around the azimuthal ϕ angle allows for information about the directionality of the roughening mechanisms to be obtained. The method allows for quick identification of the presence or absence of gross well width fluctuations in the quantum well, providing that the interface is chemically sharp. When the interface exhibits chemical grading, compositional fluctuations across the terraced structure of the quantum well surface lead to aggravated roughness as the barrier is grown, which may be misinterpreted as gross well width fluctuations. This method carries promises for complementing analysis by transmission electron microscopy as it is non-destructive, fast, and allows multi-directional characterization of the roughness. It would therefore be particularly useful to detect process deviation in a production line, where prior knowledge of the sample is already available.This work has been funded in part by the EPSRC (under EP/H0495331) and the ERC (grant agreement no. 279361 (MACONS))

Acidification des eaux de surface sous l'influence des précipitations acides : rôle de la végétation et du substratum, conséquences pour les populations de truites. Le cas des ruisseaux des Vosges

L'étendue de l'acidification des ruisseaux des Vosges a été étudié. L'influence des précipitations atmosphériques acides, accentuée par la présence de résineux et de roches mères pauvres en minéraux altérables, a provoqué l'acidification de certains ruisseaux. L'acidité (pH 200 ppb) constituent des facteurs limitants à la présence de truites dans ces ruisseaux

Water quality assessment by means of HFNI valvometry and high-frequency data modeling

International audienceThe high-frequency measurements of valve activity in bivalves (e.g., valvometry) over a long period of time and in various environmental conditions allow a very accurate study of their behaviors as well as a global analysis of possible perturbations due to the environment. Valvom- etry uses the bivalve's ability to close its shell when exposed to a contaminant or other abnormal environmental conditions as an alarm to indicate possible perturbations in the environment. The modeling of such high-frequency serial valvom- etry data is statistically challenging, and here, a nonparametric approach based on kernel estima- tion is proposed. This method has the advantage of summarizing complex data into a simple den- sity profile obtained from each animal at every 24-h period to ultimately make inference about time effect and external conditions on this profile. The statistical properties of the estimator are pre- sented. Through an application to a sample of 16 oysters living in the Bay of Arcachon (France), we demonstrate that this method can be used to first estimate the normal biological rhythms of permanently immersed oysters and second to de- tect perturbations of these rhythms due to changes in their environment. We anticipate that this ap- proach could have an important contribution to the survey of aquatic systems

Velocity estimation of valve movement in oysters for water quality surveillance

International audienceThe measurements of valve opening activity in a population of oysters under natural environmental conditions are used to estimate the velocity of their valve movement activity. Three different differentiation schemes were used to estimate the velocity, namely an algebraic-based differentiator method, a non-homogeneous higher order sliding mode differentiator and a homogeneous finite-time differentiator. The estimated velocities were then used to compare the performances of these three different differentiators. We demonstrate that this estimated velocity can be used for water quality monitoring as the differentiators can detect very rapid change in valve movements of the oyster population resulting from some external stimulus or common input

Cathodoluminescence hyperspectral imaging of trench-like defects in InGaN/GaN quantum well structures

Optoelectronic devices based on the III-nitride system exhibit remarkably good optical efficiencies despite suffering from a large density of defects. In this work we use cathodoluminescence (CL) hyperspectral imaging to study InGaN/GaN multiple quantum well (MQW) structures. Different types of trench defects with varying trench width, namely wide or narrow trenches forming closed loops and open loops, are investigated in the same hyperspectral CL measurement. A strong redshift (90 meV) and intensity increase of the MQW emission is demonstrated for regions enclosed by wide trenches, whereas those within narrower trenches only exhibit a small redshift (10 meV) and a slight reduction of intensity compared with the defect-free surrounding area. Transmission electron microscopy (TEM) showed that some trench defects consist of a raised central area, which is caused by an increase of about 40% in the thickness of the InGaN wells. The causes of the changes in luminescences are also discussed in relation to TEM results identifying the underlying structure of the defect. Understanding these defects and their emission characteristics is important for further enhancement and development of light-emitting diodes

The effects of Si-doped prelayers on the optical properties of InGaN/GaN single quantum well structures

In this paper, we report on the effects of including Si-doped (In)GaN prelayers on the low temperature optical properties of a blue-light emitting InGaN/GaN single quantum well. We observed a large blue shift of the photoluminescence peak emission energy and significant increases in the radiative recombination rate for the quantum well structures that incorporated Si-doped prelayers. Simulations of the variation of the conduction and valence band energies show that a strong modification of the band profile occurs for the quantum wells on Si-doped prelayers due to an increase in strength of the surface polarization field. The enhanced surface polarization field opposes the built-in field across the quantum well and thus reduces this built-in electric field. This reduction of the electric field across the quantum well reduces the Quantum Confined Stark Effect and is responsible for the observed blue shift and the change in the recombination dynamics.This work was carried out with the financial support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Nos. EP/I012591/1 and EP/ H011676/1.This is the accepted manuscript version of the article. The final version is available from AIP at http://scitation.aip.org/content/aip/journal/apl/105/9/10.1063/1.4894834

Évènements Rares sur des Séries Temporelles Environnementales

International audienceLa mesure de l'activité de mollusques bivalves est un moyen d'enregistrer le comportement de bivalves in situ et donc d'évaluer des changements de la qualité de l'eau. Nous proposons une méthode statistique basée sur la théorie des valeurs extrêmes permettant d'estimer des changements globaux (pollution, changement de température) et ainsi d'aider à la surveillance de systèmes aquatiques

Effect of QW growth temperature on the optical properties of blue and green InGaN/GaN QW structures

In this paper we report on the impact that the quantum well growth temperature has on the internal quantum efficiency and carrier recombination dynamics of two sets of InGaN/GaN multiple quantum well samples, designed to emit at 460 and 530 nm, in which the indium content of the quantum wells within each sample set was maintained. Measurements of the internal quantum efficiency of each sample set showed a systematic variation, with quantum wells grown at a higher temperature exhibiting higher internal quantum efficiency and this variation was preserved at all excitation power densities. By investigating the carrier dynamics at both 10 K and 300 K we were able to attribute this change in internal quantum efficiency to a decrease in the non-radiative recombination rate as the QW growth temperature was increased which we attribute to a decrease in incorporation of the point defects.This work was carried out with the financial support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Nos. EP/I012591/1 and EP/H011676/1.This is the final version of the article. It first appeared from Wiley via https://doi.org/10.1002/pssc.20151018