Solar-blind AlGaN-based UV photodetectors grown on Si (111) substrates

In this work, efficient solar-blind metal-semiconductor-metal (MSM) photodiodes grown on Si (I 11) by molecular beam epitaxy are reported. Growth details are described, including the use of different kinds of buffer layers. AlGaN samples using an AlGaN/GaN superlattice (SL) as a buffer showed the presence of cracks, while AlGaN samples on an AlN buffer were crack-free. The additional strain introduced by the SL and the increase of the lattice mismatch between Si and AlGaN when the Al content increases, are responsible for the cracking. MSM photodiodes were fabricated and characterized using such layers. UV detectors obtained on the sample with cracks presented a dark current above 100 pA at 5 V, while in the crack-free photodiodes the dark current was below 10 pA at 30 V. The ultraviolet/visible contrast was also reduced in order of magnitude due to the presence of cracks. Peak responsivity values of 14 mA/W at 5 V and of 16 mA/W at 10 V were obtained for the photodetectors with cracks and for the crack-free photodetectors, respectively. The spectral noise density was 1 x 10(-24) A(2)/Hz at 5 V for the detectors with cracks, showing at low frequencies a 1/f-type behaviour. For the crack-free photodetectors, the spectral noise density value was below the system detection limits (1 x 10(-26) A(2)/HZ) at 10 V. A detectivity value of 5.2 x 10(10) cm.Hz.W-1 at 10 V was estimated for the crack-free photodiodes.