During this two-year project, the specific objectives of ALDUV have been: a) to obtain device quality AlxGa1-xN epitaxial layers; b) to determine the Al mole fraction (x) required to fabricate AlGaN photodetectors to monitor the UV-B band; c) to fabricate various types of photodetectors based on AlGaN, in order to decide which structure is the most adequate for monitoring the solar UV-B radiation; d) by using the most appropriate photodetectors for UV-B monitoring, to conduct outdoors calibration and assessment studies, and to set up a complete monitoring system.

Measurements of the UV-B radiation should be made with the AlGaN photodetectors without the use of any optical filter.

AlxGa1-xN-based photodetectors are excellent candidates for the detection of the UV radiation. By a proper choice of the Al mole fraction, x, photodetectors with threshold energies from 3.4 eV (360nm, GaN) up to 6.2 eV (200nm, AlN) can be fabricated. A significant part of the difficulties in growing high quality AlGaN layers derive from the fact that neither GaN nor AlGaN bulk material are available to be used as substrates for epitaxial growth. From the experience of the partners with GaN layers, two types of substrates were selected: sapphire, already well proven in GaN light emitting diodes, and Si (111) wafers, available in large sizes and at lower cost, although much less experienced for this application.

Concerning the epitaxial growth technology, two available and well-established technologies have been used to grow AlGaN layers: metal organic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE). MOVPE is already a proven technology, used in all commercial GaN devices. MBE is potentially capable of a lower impurity incorporation and is free of hydrogen-related effects, because of its lower growth temperature and high vacuum environment. Thus, the technical approach has been to use sapphire and MOVPE epitaxial technology at CNRS-CRHEA, and silicon substrates and MBE technology at UPM.

When the ALDUV project started, very limited worldwide experience was available regarding AlGaN-based UV photodetectors. So, ALDUV had to determine not only the Al mole fraction, but also the more adequate photodetector structures for the project objective, and their processing technology (cleaning procedures, metals for ohmic contacts and Schottky barriers, etc.). During the two years that the project has been running, the consortium has fabricated AlGaN photodetectors with practically all the standard photodetector structures. These various structures were characterised regarding the dependence of their responsivity with wavelength (UV/visible contrast) and incident irradiance (dynamic range), time response (speed), stability with temperature, stability with time, low frequency noise properties, and dependence on bias voltage. Monitoring the UV-B band does not pose on the photodetector very high requirements regarding speed or responsivity (sensitivity). However, specifications are very strict when considering UV/visible contrast (several orders of magnitude), linearity with irradiance, and stability of the device response with time and temperature in typical outdoors range. Regarding these characteristics ALDUV has clearly proven that AlGaN-based photodetectors are superior to any other solution based on alternative semiconductors. or other detector technology.