Effect of oxidation temperature on the properties of NiOx layers for application in optical sensors

J.R. Castillo-Saenz, N. Nedev, B. Valdez-Salas, M. Bernechea, E. Martínez-Guerra, I. Mendivil-Palma, M. Curiel-Alvarez, D. Mateos, O. Perez-Landeros. Thin Solid Films, 2021, 734, 138849

Thin NiOx layers were prepared by oxidation at 400, 500 and 600 ◦C of metallic Ni deposited by electron beam evaporation. The bandgap of NiOx determined from optical measurements increases from 3.6 to 3.7 eV with the increase of oxidation temperature from 400 to 500◦C. Higher temperature leads to larger grain sizes, 12.6 nm at 400 ◦C, 15.3 nm at 500 ◦C and 16.1 nm 600 ◦C. Photodiodes based on NiOx/Si heterostructure were fabricated by evaporation of semitransparent Au top contacts. The structures with NiOx obtained at 500 ◦C showed superior diode characteristics compared to the other two types of devices with dark current < 3.5 × 10-8 A (current density J < 3.9 × 10− 4 A/cm2) for reverse biases between 0 and -2 V and forward current of 3.3 × 10− 5 A (J = 0.36 A/cm2) at bias of 1.5 V. The responsivity of these devices at 0 V is between 0.85 and 0.55 A/W for wavelengths in the 365 – 635 nm range, values higher than those previously reported for self-powered NiOx/Si photodiodes. The responsivity at reverse bias of -1.5 V increases more than 60 times to values of 33, 32, 50 and 59 A/W for red, green, blue, and ultraviolet light, values among the highest reported for broadband photodetectors. The excellent properties of the photodiodes with NiOx obtained at 500 ◦C are the result of a higher quality of the Si/NiOx interfacial region, which leads to lower recombination rate of the photogenerated carriers