Date on Master's Thesis/Doctoral Dissertation

5-2022

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Sumanasekera, Gamini

Committee Co-Chair (if applicable)

Sergio, Mendes

Committee Member

Sergio, Mendes

Committee Member

Yu, Ming

Committee Member

Fu, Xiao-An

Author's Keywords

Tunneling; resonant tunneling; PECVD; single barrier; double barrier; Josephson junction

Abstract

Two-dimensional electron gas (2DEG) systems have played a vital role in the development of superior electronic devices including tunnel junctions consisting of two such 2DEG systems. With the advent of the new 2D electronic material systems, it has opened a new route for 2D–2D tunneling in such extended systems. In this study, we have utilized a plasma enhanced chemical vapor deposition (PECVD) technique to directly deposit graphene (nanowalls) and h-BN on Si/SiO2 substrates to construct two-dimensional material based, vertically stacked electron tunneling devices free of expensive and cumbersome microfabrication steps. In the first study, we fabricated direct quantum tunneling devices by depositing atomically thin tunnel barriers of h-BN as the tunneling barrier with equally doped (p-doped under ambient conditions) graphene nanowalls as the active electrode layers (top and bottom) on Si/SiO2 substrates. Current-voltage (I-V) measurements for varying h-BN thicknesses of these single barrier tunneling devices showed linear I-V characteristics at low bias but an exponential dependence at higher bias. Our measurements of the electron tunnel current through the barrier demonstrated that the h-BN films act as a good tunnel barrier. The barrier thickness dependent tunneling current was in good agreement with the tunnelling currents computed using the Bardeen transfer Hamiltonian approach with equally doped top and bottom graphene electrodes. Presence of negative differential resistance (NDR) is characteristic of the current–voltage relationship of a resonant tunneling device, enabling many unique applications. NDR arises at a voltage bias corresponding to aligned band structures of the 2D systems, causing a sharp peak in the tunnelling current. The existence of devices with NDR has been reported since the late 1950's in devices that contained degenerately doped p-n junctions with thin oxide barriers (tunnel diodes) and double barrier heterojunction devices where quantum tunneling effects are utilized. The NDR in the I-V characteristics of these devices has been used in many applications involving microwave/millimeter wave oscillators, high speed logic devices and switches. We investigated NDR phenomenon in our graphene/h-BN systems in two different routes. In the first case, graphene/h-BN/graphene single barrier device, the bottom and top graphene layers were unequally doped. One of the graphene layers was n-type doped using ammonia or hydrazine. Nitrogen doping using ammonia was accomplished during the growth by incorporating ammonia in the PECVD system. Hydrazine doping was accomplished by exposing the graphene to hydrazine vapor in vacuum. The unequal doping of graphene causes alignment of the band structures of graphene systems giving rise to NDR. The tunnelling devices consisting of unequally doped graphene with a single barrier shows resonant quantum tunneling with the presence of a pronounced peak in the current corresponding to NDC whose peak current value and the voltage value depend on the doping levels. The results are explained according to the modified Bardeen tunneling model. Next, resonant tunneling behavior was demonstrated in Graphene/h-BN/Graphene/h-BN/Graphene double barrier (DB) devices by directly depositing graphene and h-BN successive layers on Si/SiO2 substrates using PECVD. DB Tunneling junctions with various barrier widths were investigated (by varying the thickness of the second graphene layer). The I-V parameters of tunneling current at room temperature demonstrated resonant tunneling with negative differential conductance. A quantum mechanical double barrier tunneling model was used to explain the phenomenon, by solving the Schrödinger's equation in either side of the system. A systematic behavior of the current peak values and the corresponding voltage values in I-V curves were seen to be in good agreement with the transmission coefficient calculated using a quantum mechanical model. Josephson tunneling is a different kind of tunneling phenomenon in superconductors, in which superconducting cooper pairs tunnel across a thin insulating barrier. A supercurrent can flow between two superconductors that are separated by a narrow insulating barrier. The current is influenced by the phase difference between the two superconductors. We fabricated Josephson junctions with atomically thin tunnel barriers by combining h-BN with magnesium diboride (MgB2) active electrode layers on a Si/SiO2 substrate using a PECVD (for h-BN) and a Hybrid Physical-Chemical Vapor Deposition (HPCVD) (for Mg ). The I-V characteristics were measured above and below the transition temperature Tc (37 K). A measurable supercurrent was detected below Tc.

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