Enhancing the Performance of NiO-Based Metal-Semiconductor-Metal Ultraviolet Photodetectors Using a MgO Capping Layer

Nickel oxide (NiO) based metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors (PDs) are fabricated using a radio-frequency (RF) magnetron sputtering system. In this study, the surface of NiO is capped with magnesium oxide (MgO) using the same RF sputtering system. The effects of the MgO capping layer on the characteristics of the prepared MSM-PDs are studied. Using X-ray diffraction, a smaller grain size and tensile strain on the MgO-capped NiO is observed, which is due to the introduction of O atoms from MgO. The MgO capping layer effectively reduced the dark current by three orders of magnitude compared to the MSM-PDs without the MgO capping layer. In the MSM-PDs without the MgO capping layer, the photo-to-dark current ratio is only 100, and the ratio is enhanced by three orders of magnitude upon capping the MSM-PDs with MgO. Both the MSM-PDs, with and without the MgO capping layer, exhibit a UV/visible rejection ratio as high as 800, demonstrating the fabricated MSM-PDs are good visible-blind photodetectors. X-ray photoelectron spectroscopy illustrates that the introduced Mg atoms from MgO react with NiO to form strong Mg
_x
Ni
_1−x
O and/or MgO bonding, which passivate the surface defects on NiO surface, thus significantly reducing the dark current and enhancing the performance of the prepared MSM-PDs.