Photoreflectance study of the role of a thin AlOx layer in a GaAs surface passivation and the Schottky barrier forming

We studied UN⁺-type GaAs(100) samples covered with a thin AlO_x passivation layer and with Ni metallization of variable thicknesses. Photoreflectance spectroscopy was used to measure the electric field in the undoped part of the samples. The position of the surface Fermi level, the amount of the electric charge trapped at the surface, and the effect of the pinning were investigated. The AlO_x layer did not change the pinning compared to the UN⁺ sample without AlO_x. After metallisation, the barrier height was markedly higher in the MIS structures with AlO_x, indicating a much weaker pinning than in the structures with native oxides. Several samples covered with metallisation layers of different thickness were prepared. The highest barrier was reached for the 3 nm Ni / 3 nm Au stack.