Surface Oxidation Induced P-doping in Transition Metal Dichalcogenides*

Transition metal dichalcogenides (TMDCs) has received tremendous attention due to their fascinating electrical properties, such as a high carrier mobility, high on/off ratio and tunable bandgap. However, TDMC-based electronic devices do not show their pristine properties due to the contact resistance between the metal electrode and TMDCs. Surface oxidation has been known to induce a p-doping and it improves the contact resistance. With optimum oxidation, highly improved contact was evidently observed in field-effect transistors, while it is rather worsen with non-optimum conditions. To elucidate the origin of improvement, the changes in the electronic structure of TMDCs upon the surface oxidation should be understood. In this regard, the electronic structure of TMDCs was studied with ultraviolet and x-ray photoemission and inverse photoemission spectroscopy (UPS/XPS/IPES). TMDCs were treated with UV-ozone and the changes in oxidation states of TMDCs were investigated with XPS. Then, the changes of work function and band edges were directly observed with UPS and IPES. The surface oxidation can control the position of band edges efficiently.