Reactive sputtering: A method for controlling the stoichiometry and energy level structure of amorphous molybdenum oxide
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Reactive sputtering: A method for controlling the stoichiometry and energy level structure of amorphous molybdenum oxide films.
Jonathan Griffin1 and Alastair R. Buckley1. 1
The Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK
Abstract Thin films of molybdenum oxide have been deposited by reactive magnetron sputtering and characterised by photoelectron spectroscopy. Such films of MoOx are increasingly being used as electrode interfaces in both organic light emitting diodes and bulk heterojunction polymer-fullerene solar cells. Control of the Mo to oxygen stoichiometry has been gained by two methods of R.F magnetron sputtering. The first method is by controlling the proportion of oxygen in the sputtering chamber and the second is by controlling the level of surface oxidation of the sputter target prior to deposition, both methods have been used to control average film stoichiometry. In general the transition from metallic like film to oxide like film occurs at an oxygen chamber partial pressure of 15 %. UPS measurements show that such a transition, and resulting increase in the average oxidation state, leads to a shift in the Fermi level from 4.5eV to 5.2eV with respect to vacuum. The presence of specific oxidation states below +6 give rise to electronic structures that lie between the valence band edge and the Fermi level. For reduced samples +2 and +5 oxidation states peaks appear at binding energies of 0.4eV and 2.1eV respectively while a third peak at 1eV could be attributed to either the presence of +3 or +4 oxidation states.
Introduction In recent years molybdenum oxide has been of great interest within the field of organic electronics for use as an electrode buffer layer. [1-3] It has been shown that, when used as a hole extraction layer in organic photovoltaic devices, molybdenum oxide can be used to achieve highly efficient hole extraction with highest reported efficiencies reaching 7.2%. [3] MoO3 is also often used as an effective hole injection layer in organic light emitting diodes.[4] However some recent work that has seen much interest has also shown that a reduced form of molybdenum oxide, MoO2.7, can also be used as an electron injection layer within organic light emitting diodes.[5] In order to understand why molybdenum oxide can act as both a hole and an electron injection layer it is important to understand its electronic structure as a function of the presence of different oxidation states. To do this a method of deposition is required that is capable of varying the oxidation state of deposited molybdenum oxide films. This work details a reactive R.F magnetron sputtering technique that is capable of varying the overall oxidation state of deposited molybdenum oxide films along with characterisation of these thin films via XPS and UPS. Previously similar work has been done on the control of the oxidation state of molybdenum oxide films by reactive
sputtering.[6-7] However both of these papers show a limited ability to control the oxidation state of deposited
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