J.-F. Hochedez

Assessment of GaN metal–semiconductor–metal photodiodes for high-energy ultraviolet photodetection

We report on the fabrication and characterization of low dark-current GaN metal–semiconductor–metal (MSM) photodiodes. Their quantum efficiency in the vacuum-ultraviolet range has been analyzed, demonstrating that these devices are an excellent choice for high-energy photodetection. Models to explain and control the performance as a function of residual doping and geometry are applied to GaN-based MSM photodiodes.

193nm deep-ultraviolet solar-blind cubic boron nitride based photodetectors

Deep-ultraviolet (DUV) solar-blind photodetectors based on high-quality cubic boron nitride (cBN) films with a metal/semiconductor/metal configuration were fabricated. The design of interdigitated circular electrodes enables high homogeneity of electric field between pads. The DUV photodetectors present a peak responsivity at 180nm with a very sharp cutoff wavelength at 193nm and a visible rejection ratio (180 versus 250nm) of more than four orders of magnitude. The characteristics of the photodetectors present extremely low dark current, high breakdown voltage, and high responsivity, suggesting that cBN films are very promising for DUV sensing.

Stationary parts of an EIT and Moreton wave : a topological model

Context. EIT and Moreton waves came into focus in 1997, when a propagating disturbance on a large area of the solar disc was discovered. The process generating the EIT and Moreton waves has been frequently discussed. Aims. On May 2, 1998, a halo CME was observed related to an EIT wave, a Moreton wave, a X1 flare, radio emission sources, and dimmings. We studied this event to find the relation between all these structures. Methods. We use and co-align multi-wavelength observations and the online potential field source surface (pfss) package. Results. The observed EIT and Moreton waves present some brightenings that remain at the same location. We relate the connectivity of the coronal potential magnetic field to the stationary brightenings. We find that the areas where the magnetic field lines have drastic jumps of connectivity are cospatial to the stationary brightenings of the waves. Conclusions. We conclude that the EIT and Moreton waves may be due to Joule heating resulting from the generation of electric currents in the neighboring area of the drastic jumps of magnetic connectivity, while the magnetic field lines are opening during a CME.

Fast and robust segmentation of solar EUV images: algorithm and results for solar cycle 23

Context. The study of the variability of the solar corona and the monitoring of coronal holes, quiet sun and active regions are of great importance in astrophysics as well as for space weather and space climate applications. Aims. In a previous work, we presented the spatial possibilistic clustering algorithm (SPoCA). This is a multi-channel unsupervised spatially-constrained fuzzy clustering method that automatically segments solar extreme ultraviolet (EUV) images into regions of interest. The results we reported on SoHO-EIT images taken from February 1997 to May 2005 were consistent with previous knowledge in terms of both areas and intensity estimations. However, they presented some artifacts due to the method itself. Methods. Herein, we propose a new algorithm, based on SPoCA, that removes these artifacts. We focus on two points: the definition of an optimal clustering with respect to the regions of interest, and the accurate definition of the cluster edges. We moreover propose methodological extensions to this method, and we illustrate these extensions with the automatic tracking of active regions. Results. The much improved algorithm can decompose the whole set of EIT solar images over the 23rd solar cycle into regions that can clearly be identified as quiet sun, coronal hole and active region. The variations of the parameters resulting from the segmentation, i.e. the area, mean intensity, and relative contribution to the solar irradiance, are consistent with previous results and thus validate the decomposition. Furthermore, we find indications for a small variation of the mean intensity of each region in correlation with the solar cycle. Conclusions. The method is generic enough to allow the introduction of other channels or data. New applications are now expected, e.g. related to SDO-AIA data.

Diamond UV detectors for future solar physics missions

Despite their steady improvement over the last decades, the present UV detectors exhibit some limitations inherent to their silicon technology. Yet, the utmost spatial resolution, temporal cadence, sensitivity and photometric accuracy will be decisive for the forthcoming space solar missions. The advent of novel diamond or nitride imagers would surmount many current weaknesses, thus opening up new prospects and making the instruments cheaper. As for projects like the Solar Probe of NASA, or the Solar Orbiter of ESA, the aspiration for diamond UV detectors is still more sensible since these spacecrafts will approach very near to the Sun where the heat and the radiation fluxes are tremendously high. This triggered the initiative of an original R and T programme entitled BOLD described in this paper. We depict motivations and intentions and report on dedicated experiments with several devices under EUV synchrotron light, NUV laser and micro-Raman spectroscopy.

Future Diamond UV Imagers For Solar Physics

Despite their steady improvement over the last decades, the UV imaging detectors exhibit some limitations ± inherent to their silicon technology ± that become critical in the context of space missions where the highest spatial resolution, temporal cadence, and photometric accuracy are required. The use of diamond imagers would allow to overcome many of the drawbacks, opening new perspectives in particular for solar observations. It will also make the instruments cheaper. As for projects like the Solar Probe of NASA or the Solar Orbiter of ESA, the case for diamond UV detectors is even stronger, since these missions approach very near to the Sun where the temperature and the radiation fluxes are extremely high. These are the main motivations behind the new R&T programme approved by the French space agency (CNES) in August 1999 which is dedicated to demonstrate the feasability of UV imagers made of semiconducting diamond. In this paper, we report on recent experiments led at the Super-ACO synchrotron facility of LURE in the 150 to 600 eV photon energy range in order to address the influence of surface effects and electrode geometries on the detector behaviour. Quantum Efficiency (QE) values as high as 20% have been measured on prototype detectors over the investigated energy range.

Characterization of AlN metal-semiconductor-metal diodes in the spectral range of 44–360nm: Photoemission assessments

The absolute responsivity of a metal-semiconductor-metal (MSM) photodiode based on high quality AlN material has been tested from the vacuum ultraviolet (vuv) to the near UV wavelength range (44–360nm). The metal finger Schottky contacts have been processed to 2μm in width with spacing between the contacts of 4μm. In the vuv wavelength region, the measurement methodology is described in order to distinguish the contribution of the photoemission current from the internal diode signal. In the wavelength range of interest, AlN MSM is sensitive and stable under brief vuv irradiation. The MSM shows a 200∕360nm rejection ratio of more than four orders of magnitude and demonstrates the advantages of wide band gap material based detectors in terms of high rejection ratio and high output signal for vuv solar observation missions.

Radiometric characteristics of new diamond PIN photodiodes

New PIN photodiode devices based on CVD diamond have been produced showing high responsivity in a narrow bandpass around 200 nm. A set of measurement campaigns was carried out to obtain their XUV-to-VIS characterization (responsivity, stability, linearity, homogeneity). The responsivity has been measured from the XUV to the NIR, in the wavelength range of 1 nm to 1127 nm (i.e. 1240 to 1.1 eV). The diamond detectors exhibit a high responsivity of 10 to 30 mA W−1 around 200 nm and demonstrate a visible rejection ratio (200 nm versus 500 nm) of six orders of magnitude. We show that these PIN diamond photodiodes are sensitive sensors in the 200 to 220 nm range, stable under brief irradiation with a good linearity and homogeneity. They will be used for the first time in a solar physics space instrument LYRA, the Large Yield RAdiometer.

New developments on diamond photodetector for VUV solar observations

A new large-size metal–semiconductor–metal photoconductor device of 4.6 mm in diameter based on diamond material has been reprocessed and characterized in the vacuum-ultraviolet (VUV) wavelength range. The metal finger contacts have been processed to 2 µm in width with spacing between the contacts of 5 µm for a bias voltage of 5 V. The responsivity, stability, linearity and homogeneity have been tested. Solutions and progresses on diamond processing are identified and are reported. In the VUV wavelength range of interest, the diamond photodetector is sensitive with a maximum response of 48 mA W−1 at 210 nm with a corresponding external quantum efficiency of 42%, homogenous and stable under short irradiation. It indicates a 200–400 nm rejection ratio of more than four orders of magnitude and demonstrates the advantages of diamond-based detectors in terms of high rejection ratio and high output signal for VUV solar observation missions.

Recent progresses of the BOLD investigation towards UV detectors for the ESA Solar Orbiter

Abstract BOLD (Blind to the Optical Light Detectors) is an international initiative dedicated to the development of novel imaging detectors for UV solar observations. It relies on the diamond and nitride materials that have lately undergone key advances. The investigation is proposed in view of Solar Orbiter UV instruments, for which the expected properties of the new sensors—visible blindness and radiation hardness—will be highly beneficial. Solar Orbiter is a selected Flexi mission of the European Space Agency (ESA). Despite various improvements over the last few decades, the present UV detectors exhibit limitations inherent to their actual technology. Yet the utmost spatial resolution, temporal cadence, sensitivity, and photometric accuracy will be decisive for the forthcoming space solar missions. The advent of imagers made of a large bandgap semiconductor would surmount many weaknesses, thus opening up new prospects and making the instruments cheaper. As for the ESA Solar Orbiter, the aspiration for wide bandgap semiconductor-based UV detectors is still more sensible, for the spacecraft will approach the Sun where the heat and the radiation fluxes are high. We depict motivations and present activities and programme to achieve revolutionary flight cameras within the Solar Orbiter schedule.