Phonon and Electron Properties of Transition Metal Dichalcogenides – Applications in High-Temperature Electronics

Transition metal dichalcogenides are layered van der Waals materials with a number of unique electrical and thermal properties. These materials are of interest from both fundamental science and practical application points of view. In this dissertation, I describe results of my research of the phonon and electron properties of layered transition metal dichalcogenides as well as devices based on these materials. In the first part of this dissertation, I report on the phonon and thermal properties of thin films of tantalum diselenide grown by chemical vapor transport method. The Raman optothermal measurements revealed that the room temperature thermal conductivity in these films decreases with decreasing thickness suggesting strong phonon – boundary scattering. The measurements of electrical resistivity of the field-effect devices with TaSe2 channels have indicated that heat conduction is dominated by acoustic phonons in these van der Waals films. In the framework of this dissertation research, I fabricated of MoS2 thin-film transistors and investigated their high-temperature current-voltage characteristics. The measurements revealed that MoS2 transistors remain functional to temperatures of at least as high as 500 K and after two month of aging. The comparison of the direct current and pulse measurements has demonstrated that the direct current sub-linear and super-linear output characteristics of MoS2 thin-films devices result from the Joule heating and the interplay of the threshold voltage and mobility temperature dependences. At temperatures above 450 K, a kink in the drain current occurs at zero gate voltage irrespective of the threshold voltage value. This intriguing phenomenon was attributed to the slow relaxation processes in thin films. The results of this dissertation contribute to better understanding of properties of two-dimensional materials, and reveal their potential for electronic applications