Low pressure MOVPE grown AlGaN for UV photodetector applications

The low pressure Metalorganic Vapor Phase Epitaxy (LP-MOVPE) growth conditions of AlGaN epilayers on c-oriented sapphire have been optimized with a view to application to UV photodetectors both on GaN and AlN nucleation layers, for aluminium mole fractions lying typically in the range 0–25%. Good structural, electrical and optical properties were obtained for AlGaN alloys on (0001) oriented sapphire substrates, for both undoped and n-type doped epilayers. A typical full width at half maximum (FWHM) of 670–700 arc-s is measured for the (0002) X-ray double diffraction peak in the ω-configuration of 1 μm-thick AlGaN epilayers grown on a GaN nucleation layer, and a typical electron mobility of 50–90 cm2 V−1 s−1 is measured at T=300 K on 1018 cm−3 n-type doped AlGaN epilayers. The low temperature photoluminescence (T=9 K) performed on non intentionally doped AlGaN epilayers with low Al contents (5 and 10%) exhibits reproducibly a sharp exciton-related peak, associated with two phonon replica and a total absence of low photon energy transitions. Optical transmission as well as absorption coefficient measurements using the photothermal deflection spectroscopy (PDS) clearly show that the variation of the energy gap of AlGaN with the aluminium concentration is linear. Solar-blind AlGaN-based photoconductors and Schottky barrier photodiodes with good operating characteristics have been fabricated with these materials.