Fabrication and Characterizations of Two-Dimensional Charge-Density-Wave Devices Operating at Room Temperature
Abstract
The charge-density-wave (CDW) phase is a macroscopic quantum state consisting of a periodic modulation of the electronic charge density which is accompanied by a periodic distortion of the atomic lattice in quasi-one-dimensional (1D) or layered quasi-two-dimensional (2D) metallic crystals. Several layered transition metal dichalcogenides (TMDs) reveal the CDW phase transitions at rather high temperatures. These transitions can be affected by environmental conditions, film thickness and applied electric bias. This thesis reports results of investigation focused on developing and optimizing fabrication procedures for devices implemented with a specific TMD - 1T polytype of TaS2. This material undergoes the transition from a nearly commensurate CDW phase to incommensurate CDW phase at 350 K. This phase transition is accompanied by a lattice reconstruction, which results in strong modifications of the material's electrical properties. The change in the electrical characteristics in the transition point can be used for fabricating oscillator devices operating at room temperature. In this thesis, I briefly outline CDW theoretical background and then, describe in details the fabrication steps required for such devices. Specifically, I describe exfoliation of the 2D material, protection from oxidation, transfer process, spin coating, electron beam lithography, metal deposition and lift-off process. The current-voltage characteristics of the resulting devices show reproducibly an abrupt change in electrical current and a hysteresis loop. The hysteresis loop can be used for building a voltage controlled oscillator. The low-frequency noise measurements have been conducted in a wide temperature range in order to elucidate the physical mechanism of the phase transition and electronic transport. The noise spectral density shows 1/f type spectrum with signature of the generation – recombination (G-R) bulges at some bias points (f is the frequency). The obtained results are important for developing the 2D electronic device technology based on CDW effects.