Effects of processing conditions on the work function and energy-level alignment of NiO thin films

We have investigated NiO thin films prepared by in situ and ex situ ozone oxidation, as well as air-exposed and vacuum-annealed NiO films. The core-level and valence-level photoemission spectra, as well as the work function and energy-level alignment with a common hole-injection material, have been measured using X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. We found that in situ oxidation results in the formation of a purely NiO film, while ex situ oxidation and air exposure result in a hydroxide-terminated NiO film. Work functions as high as 6.7 eV can be achieved for in situ-oxidized NiO; however, the work function decreases rapidly with time due to adsorption of residual gases in vacuum. The work functions of ex situ and air-exposed NiO were significantly lower, between 5.2 and 5.6 eV, due to hydroxylation of the oxide surface. We have examined the rate at which the work function decreases with air exposure and found there to be a very rapid initial decrease in work function, followed by a much slower continual decline. Despite the decrease in work function, energy-level alignment of α-NPD is not affected until the work function drops below a threshold value. We have also examined the effect of vacuum annealing of NiO and found that it becomes highly defective with oxygen vacancies, causing the Fermi level position of the oxide to move away from the valence band maximum and decreasing the work function.