Optimization and integration of ultrathin e-beam grown HfO2 gate dielectrics in MoS2 transistors

Integration of high-k dielectrics with two-dimensional (2D) layered semiconductors is one of the bottleneck in realization of the devices in a viable technology. In this work we report on the optimization and integration of electron beam (e-beam) evaporated, ultrathin (5 nm) HfO2 thin films on a few layer (5-7 nm) MoS2 substrate, without the need for any functionalization and buffer layers. The effect of HfO2 film thickness on optical, compositional and electrical properties of the material has been evaluated using ellipsometry, Rutherford backscattering, x-ray photoelectron spectroscopy and electrical measurements. The estimated dielectric constant and leakage current density values have been correlated with the refractive index and density of the films. It has been observed that the dielectric constant decreased drastically in ultrathin (5 nm) HfO2 film in case of Al as a gate electrode due to the formation of interfacial layer. The effect of gate electrode material and its processing on dielectric properties of ultra-thin films has been studied in detail and it is found that e-beam evaporated Ni seems to be a promising gate electrode. Equivalent oxide thickness of 1.2 nm and leakage current density (J) of 1.1 x 10(-4) A cm(-2) have been achieved for 5 nm films with Ni as gate electrode. Top gated MoS2 transistors with, 5 nm HfO2 gate dielectric show very good performance with I (ON) to I (OFF) ratio of 10(6). We thus demonstrate e-beam evaporation as a promising technique to directly integrate high-k dielectrics (HfO2) with 2D materials.