P. Muret

Photoelectric study of β‐FeSi2 on silicon: Optical threshold as a function of temperature

Investigation of the photoelectric properties of several metal/β‐FeSi2/Si heterostructures is presented. For thin silicide samples (200 A), the photocurrent follows a Fowler’s law with a threshold Φ1 lower than the silicide band gap Eg. For thicker silicide samples (2500 A), the behavior of the photocurrent is different because the optical absorption within the silicide can no longer be neglected: a maximum of the photocurrent is observed instead at Eg. The variations of Eg and Φ1 with temperature are compellingly similar and show the strong effect of the electron‐phonon coupling. We suggest that the threshold Φ1 corresponds to transitions between a trap localized near the heterojunction and the silicide conduction band.

Zr/oxidized diamond interface for high power Schottky diodes

High forward current density of 103 A/cm2 (at 6 V) and a breakdown field larger than 7.7 MV/cm for diamond diodes with a pseudo-vertical architecture, are demonstrated. The power figure of merit is above 244 MW/cm2 and the relative standard deviation of the reverse current density over 83 diodes is 10% with a mean value of 10−9 A/cm2. These results are obtained with zirconium as Schottky contacts on the oxygenated (100) oriented surface of a stack comprising an optimized lightly boron doped diamond layer on a heavily boron doped one, epitaxially grown on a Ib substrate. The origin of such performances are discussed.

Band bending, electronic affinity and density of states at several (100) surfaces of boron-doped homoepitaxial diamond thin films

Using ultraviolet and x-ray photoelectron spectroscopies, we investigate four types of the (100) surface of homoepitaxial diamond films, either doped with boron or undoped. We measure the position of the valence band maximum at the surface, the electronic affinity and the density of states. In p-type doped films, conductivity measurements indicate that the bulk Fermi level is pinned close to 0.4 eV above the valence band edge and subtracting this value from the valence band maximum at the surface permits us to derive the band bending. The different (100) surfaces of the p-type diamond samples are (i) hydrogenated with the 2 × 1 reconstruction (H-surface), (ii) free with the 2 × 1 reconstruction, and either (iii) post-oxygenated or (iv) oxygenated during the growth, both with the 1 × 1 reconstruction (O-surface). All these surfaces show a downward band bending, which implies hole depletion, ranging from 0.3 eV for the H-surface to 1.2 eV for the O-surface. Only the H-surface exhibits a negative electronic affinity (−0.9 eV) whereas the others display positive electronic affinities in the range 0.9–2.2 eV. Oxygen passivates the diamond surface, giving both very low conductivity and density of states in the bandgap, and increasing the band bending. These results, obtained in homoepitaxial diamond thin films, are shown to be relevant for implementing diamond-based electronic devices.

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.

Band discontinuities at beta -FeSi2/Si heterojunctions as deduced from their photoelectric and electrical properties

Experimental photoresponses and electrical characteristics of metal/ beta -FeSi2/Si structures are presented. Three kinds of samples are compared: two with a thin epitaxial silicide layer (180 AA), prepared by two different methods, and one with a thick polycrystalline silicide layer (2500 AA). The rectifying behaviour and the photoelectric response of the three kinds of samples are different. In the thin samples these properties are governed by those of the beta -FeSi2/Si interface, whereas for thick samples bulk mechanisms dominate. Analysis of the photocurrent in one kind of thin sample shows that two contributions exist. Their intensities follow similar temperature behaviours but the two transition thresholds do not. These considerations allow assignment of the initial and final states of the transitions, and the upper threshold is shown to correspond to an internal photoemission effect at the beta -FeSi2/Si interface. The conduction band offset is deduced from the difference between the two thresholds. The valence band discontinuity is less than 50 meV between 360 K and 260 K, whereas it changes sign when the temperature decreases below 260 K, the two bandgaps becoming nested within each other. These properties are also discussed for the other kinds of sample and related to the mechanisms which are responsible for the electrical characteristics.

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.

Potential barrier heights at metal on oxygen-terminated diamond interfaces

Electrical properties of metal-semiconductor (M/SC) and metal/oxide/SC structures built with Zr or ZrO2 deposited on oxygen-terminated surfaces of (001)-oriented diamond films, comprised of a stack of lightly p-doped diamond on a heavily doped layer itself homoepitaxially grown on an Ib substrate, are investigated experimentally and compared to different models. In Schottky barrier diodes, the interfacial oxide layer evidenced by high resolution transmission electron microscopy and electron energy losses spectroscopy before and after annealing, and barrier height inhomogeneities accounts for the measured electrical characteristics until flat bands are reached, in accordance with a model which generalizes that by Tung [Phys. Rev. B 45, 13509 (1992)] and permits to extract physically meaningful parameters of the three kinds of interface: (a) unannealed ones, (b) annealed at 350 °C, (c) annealed at 450 °C with the characteristic barrier heights of 2.2–2.5 V in case (a) while as low as 0.96 V in case (c). Pos...

Semiconducting iron silicide thin films on silicon (111) with large Hall mobility and low residual electron concentration

Unprecedented Hall mobility, electron concentration and photoconductivity are demonstrated in semiconducting - thin films prepared on Si(111) surfaces by co-sputtering of iron and silicon followed by post-anneal. Characterization of the silicide as a function of the initial temperature and post-treatment shows that annealing temperatures above C are needed to obtain single phase -. Reactive deposition on substrates heated at C leads to textured films. Majority carriers are electrons in all these unintentionally doped films. Hall concentrations between and electrons and respective Hall mobilities from 290 to are measured at room temperature, involving two different conduction band minima in these two extreme cases. Only deep centres exist in the samples having the lower carrier concentration. In such a situation, raw data must be corrected for the substrate contribution to extract values which are relevant for the - film alone. Photoconductivity also takes place in these samples: at 80 K, it shows a maximum value at the direct band gap of - while at 296 K a step still appears at the same energy. Such results are a consequence of the important decrease of the residual impurity concentration in comparison to values previously published.