Open Access Dissertation
Date of Award
Physics and Astronomy
Transition metal dichalcogenides (TMDCs) as the semiconductor counterparts of gra-phene have emerged as promising channel materials for flexible electronic and optoelectronic devices. The 2D layer structure of TMDCs enables the ultimate scaling of TMDC-based devices down to atomic thickness. Furthermore, the absence of dangling bonds in these materials helps to form high quality heterostructures with ultra-clean interfaces. The main objective of this work is to develop novel approaches to fabricating TMDC-based 2D electronic devices such as diodes and transistors. In the first part, we have fabricated 2D p-n junction diodes through van der Waals assembly of heavily p-doped MoS2 (WSe2) and lightly n-doped MoS2 to form vertical homo-(hetero-) junctions, which allows to continuously tune the electron concentration on the n-side for a wide range. In sharp contrast to conventional p-n junction diodes, we have observed nearly exponential dependence of the reverse-current on gate-voltage in our 2D p-n junction devices, which can be attributed to band-to-band tunneling through a gate-tunable tunneling barrier. In the second part, we developed a new strategy to engineer high-κ dielectrics by con-verting atomically thin metallic 2D TMDCs into high-κ dielectrics because it remains a signifi-cant challenge to deposit uniform high-κ dielectric thin films on TMDCs with ALD due to the lack of dangling bonds on the surfaces of TMDCs. In our study, we converted mechanically ex-foliated atomically thin layers of a 2D metal, TaS2 (HfSe2) into a high-κ dielectric, Ta2O5 (HfO2) by thermal oxidation. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and atomic force microscopy (AFM) were used to understand the phase conversion process. Capacitance-voltage (C-V) measure-ments were carried out to determine the dielectric constant of thermally oxidized dielec-trics. We fabricated MoS2 field-effect transistors (FETs) with thermally oxidized ultra-thin and ultra-smooth Ta2O5 as top-gate and bottom-gate high-κ dielectric layers. We observed promis-ing device performance, including a nearly ideal subthreshold swing of ~ 61 mV/dec at room temperature, negligible hysteresis, drain-current saturation in the output characteristics, a high on/off ratio ~ 106, and a room temperature field-effect mobility exceeding 60 cm2/Vs. To fur-ther reduce the leak current and improve the device performance, we have also investigated the chemical transformation of HfSe2 to HfO2 high-κ dielectric, which has significantly larger band gap than Ta2O5.
Paudel, Sagar Prasad, "Doped And Chemically Transformed Transition Metal Dichalcogenides (tmdcs) For Two-Dimensional (2d) Electronics" (2018). Wayne State University Dissertations. 2056.