High Performance Solar Blind Detectors based on AlGaN grown by MBE and MOCVD
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High Performance Solar Blind Detectors based on AlGaN grown by MBE and MOCVD Jean-Yves DUBOZ1, Jean-Luc Reverchon, Mauro Mosca, Nicolas Grandjean1, Franck Omnes1 Device Department, Thales TRT, Orsay, France 1 CRHEA-CNRS, Valbonne, France ABSTRACT Solar blind detectors based on AlGaN grown by Molecular Beam Epitaxy and Metal Organic Vapor Phase Epitaxy have been fabricated and characterized. Metal Semiconductor Metal (MSM) detectors and vertical Schottky detectors have been realized, with a design that allows back side illumination. The growth was optimized in order to improve the layer quality, avoid crack formation, and provide the best detector performance. The technological process was also optimized in order to reduce the dark currents and improve the spectral rejection ratio, which is a key factor for solar blind detection. As a result, a rejection ratio of 5 decades between the UV (below 300 nm) and 400 nm, and a steep cut off limited by alloy fluctuations have been obtained. A noise equivalent power below 10 fW is obtained in MSM detectors. INTRODUCTION AlGaN alloys have a direct band gap that can be tuned from 3.45 to 6.2 eV by varying the Al content. They are thus good candidates for ultraviolet detection, and especially in the 250-300 nm range, where the background solar radiations are filtered out by the ozone (so called solar blind detection as detectors with a photo-response below 300 nm only become insensitive to the sun light). The development of nitrides for other applications such as light emitters or transistors [1-3] has been very beneficial for the development of AlGaN UV detectors and solar blind detectors have already been reported [1-4]. We report here on our latest results on Metal Semiconductor Metal (MSM) and Schottky devices and present detectors with a state of the art performance. DETECTOR DESIGN The AlGaN layers were grown by Metal Organic Vapor Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). For imaging applications, arrays have to be fabricated and hybridized to a silicon read out circuit on the front side. Thus, the illumination has to be incident from the back side, and we focus here on devices that are compatible with such a back side illumination. This implies that AlGaN is grown on a transparent substrate. Hence, the most obvious choice is to use sapphire as a substrate. The lattice mismatch between nitrides and sapphire is large and leads to the formation of a large density of dislocations. It has been demonstrated in the early times [5] that a better material quality could be obtained if the growth was initiated with a low temperature buffer layer, and this technique is now widely used. The buffer layer is typically 25 to 50 nm thick, and can be based on GaN, AlN or even AlGaN. We have tried to use either GaN or AlN buffer layers. In MOVPE, the AlN buffer layer tends to lead to better results in terms of material quality, leakage currents or spectral response, so that we adopted the AlN buffer. In MBE, the situation is more complicated. On the one hand, we foun
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