UC Riverside

Successful exfoliation of graphene and discoveries of its unique electrical and thermal properties have motivated searches for other quasi two-dimensional (2D) materials with interesting properties. The layered van der Waals materials can be cleaved mechanically or exfoliated chemically by breaking the relatively weak bonding between the layers. In this dissertation research I addressed a special group of inorganic van der Waals materials - layered transition metal dichalcogenides (MX₂, where M=Mo, W, Nb, Ta or Ti and X=S, Se or Te). The focus of the investigation was electronic properties of thin films of TaSe₂ and MoS₂ and their device applications. In the first part of the dissertation, I describe the fabrication and performance of all-metallic three-terminal devices with the TaSe₂ thin-film conducting channel. The layers of 2H-TaSe₂ were exfoliated mechanically from single crystals grown by the chemical vapor transport method. It was established that devices with nanometer-scale thickness channels exhibited strongly non-linear current-voltage characteristics, unusual optical response, and electrical gating at room temperature. It was found that the drain-source current in thin-film 2H-TaSe₂-Ti/Au devices reproducibly shows an abrupt transition from a highly resistive to a conductive state, with the threshold tunable via the gate voltage. Such current-voltage characteristics can be used, in principle, for implementing radiation-hard all-metallic logic circuits. In the second part of the dissertation, I describe the fabrication, electrical testing and measurements of the low-frequency 1/f noise in three-terminal devices with the MoS₂ thin-film channel (f is the frequency). Analysis of the experimental data allowed us to distinguish channel and contact noise contributions for both as fabricated and aged devices. The noise characteristics of MoS₂-Ti/Au devices are in agreement with the McWhorter model description. The latter is contrary to what is observed in graphene devices, where the noise spectral density does not follow the carrier number fluctuation model. The trap density extracted from the noise measurements is on the order of 1.5x10¹⁹ eV^-1cm^-3 and 2x10²⁰ eV^-1cm^-3 for as fabricated and aged samples, respectively. These values are of the same order of magnitude as those in high-k MOSFETS. These results of this dissertation research may lead to new applications of van der Waals materials.