Solar blind (Al,Ga)N metal-semiconductor-metal devices for high performance flame detection

In this work we report on solar blind (Al,Ga)N photovoltaic metal-semiconductor-metal (MSM) detectors with a cutoff wavelength as short as similar to270 nm. (Al,Ga)N heterostructures, that allow backside illumination, were grown on sapphire substrates by both metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). We observed that both the interdigitated electrodes and the contact areas contribute to the overall photocurrent. In order to avoid the parasitic current due to the contact pads, we developed a new process where the electrodes are deposited on the (Al,Ga)N surface whereas the contact pads are deposited on an insulator (dielectric) layer besides the electrodes. Some layers develop microcracks related to excess stress. In that case, we showed that both the dark current and the responsivity strongly depend on the crack density. By using our two-level process, we could reduce the parasitic effects of cracks on the dark current. Several dielectrics were tested and our results are reported; values of dark current < 10 fA have been measured at 10 V bias voltage. An extremely high performance can then be reached in these ultraviolet solar blind detectors.