Electronic characterization of single-layer MoS 2 sheets exfoliated on SrTiO 3
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Electronic characterization of single-layer MoS2 sheets exfoliated on SrTiO3 Benedict Kleine Bußmann1, Kolyo Marinov1, Oliver Ochedowski1, Nils Scheuschner2, Janina Maultzsch2 and Marika Schleberger1 1 Fakultät für Physik and CeNIDE, Universität Duisburg-Essen, 47057 Duisburg, Germany 2
Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany
ABSTRACT Single layer regions of MoS2 on SiO2 and SrTiO3 were identified by Raman spectroscopy and μphotoluminescence before Kelvin probe force microscopy was performed. For the already known system MoS2/SiO2 we find 1.839 eV for the direct bandgap, in good agreement with earlier results. On MoS2/SrTiO3 the direct bandgap was determined to be 1.829 eV. From our Kelvin probe data we infer that the SrTiO3 substrate leads to a dipole layer at the interface of the MoS2 single layer. The corresponding µ-PL measurements however show no significant decrease of the bandgap. This shows, that in the case of MoS2 the carrier type as well as concentration is not significantly influenced by the choice of SrTiO3 as the substrate compared to SiO2. INTRODUCTION Single layers of graphite are discussed as a substitute of silicon in future transistors. The disadvantage though is that single layer graphene lacks an intrinsic bandgap which is required to obtain large current on/off ratios. In contrast, single layers of MoS2 are semiconducting and have therefore attracted much attention recently. In any insulator gated field effect transistor (IGFET) the charge exchange between the insulator and the semiconductor plays a crucial role. To investigate the effect of charge exchange in MoS2/insulator systems, we have performed Kelvin probe measurements of thin MoS2 sheets, prepared by mechanical exfoliation. MoS2 is a layered transition metal dichalcogenide. Its structure consists of stacked planes where covalently bound S-M-S atoms are closely packed in a hexagonal arrangement. These planes are weakly bond to each other by van der Waals forces. This makes it possible, like in the case of graphene, to prepare thin MoS2 layers on insulating substrates by mechanical exfoliation [1]. Electronically bulk MoS2 is a semiconductor with an indirect bandgap of 1.23 eV [2]. In the case of single layer thickness though a direct bandgap of 1.8 eV is observed [3] in contrast to single layer graphene which is a semimetal with no bandgap at all. In IGFETS the choice of the insulating material plays an important role with respect to the doping of the gate material as well as its function to be able to provide the electric fields without the risk of unwanted leakage currents. This problem especially occurs when the gate lengths are becoming smaller than 100 nm as it is the case today. One solution for this is to find insulating materials that exhibit high dielectric constants. The insulator with one of the highest dielectric constants at room temperature is SrTiO3 with εr = 300 [4] and is therefore a promising candidate to replace SiO2 as the insulator between the gate and channel region in
