Ultraviolet photodiode based on p-Mg0.2Zn0.8O/n-ZnO heterojunction with wide response range

P-Mg0.2Zn0.8O/n-ZnO heterojunction ultraviolet photodiode was fabricated on a sapphire substrate by plasma-assisted molecular beam epitaxy. The current–voltage measurement indicates that the heterojunction has a weak rectifying behaviour with a turn-on voltage of ~5 V. The spectral response measurement shows that the photodiode has a peak responsivity at around 340 nm, and it has a wide detection range in the ultraviolet region from 400 to 320 nm. The response in the long and short wavelength region is due to the contribution of the n-ZnO and p-MgZnO layers, respectively. The ultraviolet–visible rejection ratio (R340 nm/R500 nm) of two orders of magnitude was obtained at a reverse bias of 8 V.