Energy level alignment at Co/AlOx/pentacene interfaces

Abstract

X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) experiments were performed in order to study the energy level alignment and electronic structure at Co/AlOx/pentacene interfaces as a function of the aluminum oxide (AlOx) tunnel barrier thickness and the oxidation state of Co. XPS was used to determine the oxygen exposure for the optimum oxidation of 6, 8, and 10 Å thin layers of Al deposited on Co. The Fermi level (FL) position in the band gap of AlOx depends on the oxidation state of the underlying Co and on the thickness of the tunnel barrier. The energy level alignment at Co/AlOx interfaces is consistent with an interfacial dipole, its magnitude being sensitive to the oxidation of Co, and band bending phenomena in the thin AlOx tunnel barrier. UPS experiments revealed no chemical interaction at Co/AlOx/pentacene interface in contrast with hybridization effects found at Co/pentacene interface. The vacuum level of pentacene aligns with that of AlOx, following the position of AlOx energy levels with respect to FL. The hole injection barrier was found to increase with the thickness of the tunnel barrier and to decrease with the oxidation of Co at a fixed thickness of the AlOx layer.