Date of Award

2017

Document Type

Open Access Dissertation

Department

Electrical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

MVS Chandrashekhar

Abstract

Since the discovery of graphene, there has been an increase in two-dimensional (2D) materials research for their scalability down to atomic dimensions. Among the analogs of graphene, transition metal dichalcogenides (TMDs) are attractive due to their exceptional electronic and optoelectronic properties. MoS2, a TMD, has several advantages over graphene and the industry workhorse Si, and has been reported to demonstrate excellent transistor performances. The key obstacle in the commercialization of MoS2 technology is low carrier mobility over large areas for top-down devices. Although there were several early reports on synthesis of atomically thin MoS2 with moderate mobility, transferring large area grown films to a substrate of choice leads to interface charges that degrade mobility. In our work, a top-down growth technique for synthesizing large area, 3-5 monolayers (ML) thick MoS2 film have been presented by pre-oxidation of metallic Mo instead of direct sulfidation. The growth temperature was significantly reduced in this method, eliminating free sulfur-induced degradation of the SiO2 gate dielectric. As a result, the leakage current was suppressed by a factor of >108, when compared to a single step direct sulfidation method. Using these thin films, back-gated field effect transistors have been demonstrated with accumulation electron mobility >80 cm2/Vs, on/off >105, and subthreshold swing of 84 mV/dec; which are among the best results for MoS2 based transistors on SiO2 substrate. A hypothesis on current saturation has also been presented, attributing it to charge control rather than velocity saturation.

The second part of our work aims at utilizing the best properties both graphene and MoS2 simultaneously by forming a heterojunction of these two atomically thin materials. Interestingly, these two materials have certain contrasting properties, for example, graphene based FETs have poor switching performance while MoS2 based FETs can outperform many state-of-the-art ultra-low power transistors. Fabricating a Schottky diode made of graphene and MoS2 allows the unique properties of these two materials to be combined and has been shown to be useful. A key property of these 2D heterojunctions is that each constituent of the heterojunction is so thin that it may not be able to completely screen an electric field from the second constituent, i.e. the Debye screening length can be greater than the layer thicknesses, so that voltage-induced interfacial tuning is achievable. This capability is unique to thin layers, most practically achieved in 2D heterojunctions, and has been exploited in recent “barristors”, which are 3-terminal devices with Schottky diodes where the barrier height can be tuned by an insulated gate. Such a tunable Schottky diode, similar to a triode vacuum tube is attractive for applications in RF circuits, photodetection and chemical sensing, analog and digital electronics, etc, with all the advantages of solid state devices e.g. high speed, low-cost and compactness. In this work, a graphene/MoS2 heterojunction on SiO2 dielectric has been fabricated to demonstrate a functional barristor device. By varying the gate bias between -20 V and +10 V, the barrier height could be modulated by >0.65 eV, potentially enabling current control over 10 orders of magnitude at room temperature. Using the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of this device, we have also extracted the Richardson’s coefficient and electronic effective mass in MoS2 using a thermionic emission model, which are very important parameters required for proper engineering of these devices. After that, various applications of the barristor device have also been explored. The high optical response of the barristor has demonstrated the presence of photoconductive gain, and has been consistent with the changes in Schottky barrier height caused by the back-gate. The barristor has also been successful as gate-tunable toxic gas sensors, with lowest level detection lying around 100 ppb (parts per billion) for NO2 and 1 ppm (parts per million) for NH3. These observations highlight the potential applications of the graphene/MoS2 barristor for various electronic, optoelectronic and sensing applications.

Finally, a mixed dimensional barristor made of graphene/InN nanowire heterojunction with a backgate has been demonstrated. The surface passivation of InN and the tunnel barrier formation at the graphene/NW interface have been achieved through controlled O2 plasma exposure, which has allowed an otherwise ohmic contact to turn into a gate tunable Schottky junction with >1 eV barrier height. This device has been demostrated to perform sub-ppb level trace gas detection, photo-detection with very high sensitivity and a novel gate-controllable memristive action through longer O2 plasma exposure.

Available for download on Thursday, August 15, 2019

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