Ultraviolet detectors are of a great interest to a wide range of industrial, military, environmental and even biological
applications. This paper intends first to review some of the most relevant recent developments in the field of wide
bandgap semiconductor UV detectors, and to give an overview of their applications. A special focus is given on III-nitride
based devices, which more and more clearly represent to date one of the most promising and flexible technical
solutions for UV detection.
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.
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.
AlxGa1-xN material system, whose bandgap lies in the 3.42-6.2 eV range, is extremely interesting for visible and solar blind UV photodetector applications. This paper describes the device performances of AlxGa1-xN UV Schottky barrier photodetectors for visible-blind applications grown on c-oriented sapphire, with a detailed balance with the basic materials properties. Conventional low temperature grown AlN or GaN were used in all applications. High quality Schottky barrier photodiodes made of Epitaxial Lateral Overgrown (ELOG) GaN are also presented. All Schottky barrier devices show a fast time response, a high UV-visible rejection factor, and high absolute values of above bandgap responsivities. A new application of AlGaN UV Schottky barrier photodetectors to monitor the biological action of the solar UV radiation, as well as the device performance of high quality GaN and AlGaN Metal Semiconductor Metal with cutoff wavelengths as short as 310 nm, are described in detail.
In the recent years, the depletion of the stratospheric ozone layer has alerted the scientific community about the risks of a solar ultraviolet (UV) radiation overexposure. Biological research has confirmed the very important role of the UV-B (320 - 280 nm) and UV-A (400 - 320 nm) bands on the Earth biosystem. AlxGa1-xN semiconductor alloys, with a bandgap tunable between 3.4 eV and 6.2 eV, are the most suitable materials for the fabrication of solar UV detectors. In this paper we describe the fabrication and characteristics of AlGaN photoconductive and Schottky barrier photodetectors, with Al mole fractions up to 35%. Photoconductive detectors show very high gains, that decrease with increasing incident optical power. They present persistent photoconductivity effects, and a significant below-the-gap response. The physics of this behavior is discussed. On the other hand, AlGaN Schottky barrier photodetectors show a very fast response that is independent of the optical power, and their UV/visible rejection ratio is rather high. As the Al content increases, the evolution of the responsivity and cut-off wavelength is presented. Al0.22Ga0.78N Schottky barriers are very good candidates to monitor the UV-B band. The prospective applications of AlGaN photodiodes to determine the biological action of the solar UV radiation are also discussed.
High-quality GaAs-GalnP heterojunctions, quantum wells and
superlattices have been grown using low-pressure metalorganic chemical vapo
deposition. We showed using photoluminescence that the growth rate of the
thin layers can not be extrapolated from the thick layer growth rate. In situ
reflectance anisotropy (RA) measurements were used to monitor the growth.
Correlations were made between the RA signals and film quality. RA signals
were also measured for GaAs quantum wells of various thickness, as well as f