Alain Deneuville

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.

New UV detectors for solar observations

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 properties of wide bandgap materials (in particular diamond and Al-Ga-nitrides). The investigation is proposed in view of the Solar Orbiter (S.O.) UV instruments, for which the expected benefits of the new sensors -primarily visible blindness and radiation hardness- will be highly valuable. Despite various advances in the technology of imaging detectors over the last decades, the present UV imagers based on silicon CCDs or microchannel plates exhibit limitations inherent to their actual material and technology. Yet, the utmost spatial resolution, fast temporal cadence, sensitivity, and photometric accuracy will be decisive for the forthcoming solar space missions. The advent of imagers based on wide-bandgap materials will permit new observations and, by simplifying their design, cheaper instruments. As for the Solar Orbiter, the aspiration for wide-bandgap material (WBGM) based UV detectors is still more sensible because the spacecraft will approach the Sun where the heat and the radiation fluxes are high. We describe the motivations, and present the program to achieve revolutionary flight cameras within the Solar Orbiter schedule as well as relevant UV measurements.

Development of imaging arrays for solar UV observations based on wide band gap materials

Solar ultraviolet imaging instruments in space pose most demanding requirements on their detectors in terms of dynamic range, low noise, high speed, and high resolution. Yet UV detectors used on missions presently in space have major drawbacks limiting their performance and stability. In view of future solar space missions we have started the development of new imaging array devices based on wide band gap materials (WBGM), for which the expected benefits of the new sensors - primarily visible blindness and radiation hardness - will be highly valuable. Within this initiative, called “Blind to Optical Light Detectors (BOLD)”, we have investigated devices made of AlGa-nitrides and diamond. We present results of the responsivity measurements extending from the visible down to extreme UV wavelengths. We discuss the possible benefits of these new devices and point out ways to build new imaging arrays for future space missions.

Electronic properties, devices and applications of diamond thin films

The effect of the preparation conditions on p and n doping, the characteristics of the Schottky diodes, and the MESFET and MISFET are shown. The main applications of undoped and doped polycrystalline and homoepitaxial films (high temperature electronic, particule and UV solar blind detectors and electrodes for various electrochemical reactions) are described in connection with the characteristics of the devices and of the films.

Hydrogen Diffusion Mechanisms and Hydrogen-Dopant Interactions in Diamond

Electronic grade diamond is usually grown by Microwave Plasma assisted CVD from a hydrogen rich CH4/H2 mixture, hence hydrogen is likely to be incorporated during growth. It may thus affect the properties of the material. In this work, we present the state of the art on the understanding of the diffusion properties of hydrogen and of the hydrogen-dopant interactions in diamond. First, we show the existence of strong interactions between H and boron dopants in diamond. The formation of H-acceptor pairs results in the passivation of the acceptors. Further, we show that an excess of hydrogen in selected boron-doped diamond epitaxial layers can result in the creation of H and boron-containing donors with a ionization energy of 0.36 eV (about half the ionization energy of phosphorus). At 300 K, the n-type conductivity of hydrogenated borondoped diamond is several orders of magnitude higher than the conductivity of phosphorus-doped diamond. The formation process of these new donors is discussed.

Some Recent Advances on the n-Type Doping of Diamond

The n-type doping of diamond with phosphorus suffers from defects reducing the electron mobilities and inducing some degree of compensation. In addition, the relatively high ionization energy (0.6 eV) of phosphorus severely limits the electrical activity of the dopants. Here, we present two recent advances of the n-type doping of diamond. One is based on the significant reduction of the compensation ratio of highly compensated phosphorus-doped diamond by thermal annealings. The second one presents the possibility of converting p-type boron-doped diamond into n-type by deuterium diffusion and formation of deuterium-related shallow donors with ionization energy of 0.33 eV.