Energy level alignment at Co/AlOx/pentacene interfaces

Abstract

X-ray and ultraviolet photoemission spectroscopy ͑XPS and UPS͒ 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 ͑AlOx͒ 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 ͑FL͒ 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.

Simple tight-binding theory for magnetoresistance in perfect sandwiched structures

“…27 It is known from x-ray and ultraviolet photoemission spectroscopies that some charge-transfer effects also appear at the metal-insulator interface, leading to the formation of an interfacial charge-dipole whose magnitude is defined by the localized states at interfaces. 28 Since this dipole di-rectly affects the efficiency of tunneling, it is also important to evaluate its effect in the magnetoresistance.…”

A synchrotron-based photoemission study of the MoO3/Co interface

“…Due to the exceptionally long spin coherent length inherent to organic materials, π -conjugated organic molecules are attractive candidates for spin-transport layers in spintronic devices. 8,9 Therefore, it is important to search for alternative tunneling barrier oxides with improved performance and functionalities. 6 This problem can be solved by inserting a tunneling barrier such as transition metal oxide in between.…”

Electronic Properties of Organic Semiconductor/Electrode Interfaces: The Influence of Contact Contaminations on the Interface Energetic

“…In the case of ex-situ cleaned Al 2 O 3 /Co surfaces a work function ranging from 3.1 eV to 3.5 eV was measured. In contrast to the interfaces of the OSC pentacene to clean Al 2 O 3 /Co and clean Co contacts [89,105] an increase of the hole injection barrier for the 3 nm thick Al 2 O 3 tunnel barrier was observed. For all investigated interfaces a very small, short range interface dipole was observed suggesting that the interface dipole is hardly influenced by the introduction of the thin tunnel barrier of Al 2 O 3 .…”