Recent advances in electronic and optoelectronic Devices Based on Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides (2D TMDCs) offer several attractive features for use in next-generation electronic and optoelectronic devices. Device applications of TMDCs have gained much research interest, and significant advancement has been recorded. In this review, the overall research advancement in electronic and optoelectronic devices based on TMDCs are summarized and discussed. In particular, we focus on evaluating field effect transistors (FETs), photovoltaic cells, light-emitting diodes (LEDs), photodetectors, lasers, and integrated circuits (ICs) using TMDCs

Impact of adjacent dielectrics on the high-frequency performance of graphene field-effect transistors

Transistors operating at high frequencies are the basic building blocks of millimeter wave communication and sensor systems. The high velocity and mobility of carriers in graphene can open ways for development of ultra-fast group IV transistors with similar or even better performance than that achieved with III-V based semiconductors. However, the progress of high-speed graphene transistors has been hampered by limitations associated with fabrication, influence of adjacent materials and self-heating effects.This thesis work presents results of the comprehensive analysis of the influence of material imperfections, self-heating and limitations of the charge carrier velocity, imposed by adjacent dielectrics, on the transit frequency, fT, and the maximum frequency of oscillation, fmax, of graphene field-effect transistors (GFETs). The analysis allowed for better understanding and developing a strategy for addressing the limitations.In particular, it was shown that the GFET high-frequency performance can be enhanced by utilizing the gate and substrate dielectric materials with higher optical phonon (OP) energy, allowing for higher saturation velocity and, hence, higher fT and fmax. This approach was experimentally verified by demonstration of enhancement in the fT and fmax in GFETs with graphene channel encapsulated by the Al2O3 layers. As a further step, GFETs on diamond, material with highest OP energy and thermal conductivity, were introduced, developed and fabricated, showing the extrinsic fmax up to 50 GHz, at the gate length of 0.5 \ub5m, which is highest reported so far among the best published graphene and semiconductor counterparts.The main achievements of this thesis work are as follows: (i) comprehensive study of correlations between graphene-dielectric material quality, small-signal equivalent circuit parameters and high-frequency performance of the GFETs; (ii) experimental verification of the concept of improving the GFET high- frequency performance via selection of adjacent dielectric materials with high OP energy; (iii) introducing the diamond as a most promising dielectric material for high-frequency GFETs; (iv) development of technology and demonstration of fully integrated X and Ku band GFET IC amplifiers with state-of-the art performance.In conclusion, the routes of future development depicted in this thesis work may allow for enhancing the high-frequency performance of GFETs up to the level or even higher than that of the modern III-V semiconductor counterparts

Frequency Response of Graphene Electrolyte-Gated Field-Effect Transistors

This work develops the first frequency-dependent small-signal model for graphene electrolyte-gated field-effect transistors (EGFETs). Graphene EGFETs are microfabricated to measure intrinsic voltage gain, frequency response, and to develop a frequency-dependent small-signal model. The transfer function of the graphene EGFET small-signal model is found to contain a unique pole due to a resistive element, which stems from electrolyte gating. Intrinsic voltage gain, cutoff frequency, and transition frequency for the microfabricated graphene EGFETs are approximately 3.1 V/V, 1.9 kHz, and 6.9 kHz, respectively. This work marks a critical step in the development of high-speed chemical and biological sensors using graphene EGFETs.United States. Office of Naval Research (Grant N00014-12-1-0959)United States. Office of Naval Research (Grant N0014-16-1-2230)United States. National Aeronautics and Space Administration (Award NNX14AH11A)United States. Army Research Office (Contract W911NF-13-D-0001

Design, fabrication and characterization of field-effect transistors based on two-dimensional materials and their circuit applications

The field of two-dimensional layered materials has witnessed extensive research activities during the past decade, which commenced with the seminal work of isolating graphene from bulk graphite. In addition to providing a rich playground for scientific experiments, graphene has soon become a material of technological interest for many of its fascinating electrical, thermal, mechanical and optical properties. The controllability of carrier density with electric field in graphene, along with very high carrier mobility and saturation velocity, has motivated the use of graphene channel in field-effect devices. Also, the two-dimensional layered materials family has grown very rapidly with the application of the graphene exfoliation technique and many of these elemental and compound materials are considered useful for transistor applications. In this work, various aspects of the use of two-dimensional layered materials for transistor applications were analyzed. Starting with material synthesis, field-effect transistors (FETs) were designed, fabricated and tested for their DC and high frequency performances. Through the detailed electrical and spectroscopic investigations of several processing techniques for enhanced FET performance, numerous insights were obtained into the FET operation and performance bottlenecks. The reduction of charged impurity scattering in graphene FET by Hexamethyldisilazane interaction improved field-effect mobility and reduced residual carrier concentration. This technique was also shown to be promising for other two-dimensional materials based FET. A useful technique for reducing the thickness of black phosphorus flake with oxygen plasma etching was developed. Both back-gated and top-gated FETs were implemented with good performances. Secondary ion mass spectroscopy and x-ray photoelectron spectroscopy revealed vital structural information about layered black phosphorus. Lastly, these exotic materials based FETs were characterized for their high frequency performance, resulting in gigahertz range transit frequency and operated in a variety of important circuit configurations such as frequency multiplier, amplifier, mixer and AM demodulator.Electrical and Computer Engineerin

Electrolyte gate dependent high-frequency measurement of graphene field-effect transistor for sensing applications

We performed radiofrequency (RF) reflectometry measurements at 2.4 GHz on electrolyte-gated graphene field-effect transistors (GFETs) utilizing a tunable stub-matching circuit for impedance matching. We demonstrate that the gate voltage dependent RF resistivity of graphene can be deduced even in the presence of the electrolyte which is in direct contact with the graphene layer. The RF resistivity is found to be consistent with its DC counterpart in the full gate voltage range. Furthermore, in order to access the potential of high-frequency sensing for applications, we demonstrate time-dependent gating in solution with nanosecond time resolution.Comment: 14 pages, 4 figure

Lithium-ion electrolytic substrates for sub-1V high-performance transition metal dichalcogenide transistors and amplifiers

Electrostatic gating of two-dimensional (2D) materials with ionic liquids (ILs), leading to the accumulation of high surface charge carrier densities, has been often exploited in 2D devices. However, the intrinsic liquid nature of ILs, their sensitivity to humidity, and the stress induced in frozen liquids inhibit ILs from constituting an ideal platform for electrostatic gating. Here we report a lithium-ion solid electrolyte substrate, demonstrating its application in high-performance back-gated n-type MoS2 and p-type WSe2 transistors with sub-threshold values approaching the ideal limit of 60 mV/dec and complementary inverter amplifier gain of 34, the highest among comparable amplifiers. Remarkably, these outstanding values were obtained under 1 V power supply. Microscopic studies of the transistor channel using microwave impedance microscopy reveal a homogeneous channel formation, indicative of a smooth interface between the TMD and underlying electrolytic substrate. These results establish lithium-ion substrates as a promising alternative to ILs for advanced thin-film devices

Graphene Transistor Based Nanoelectronic and Nanophotonic Applications.

Over the past few decades, electronics and photonics have made significant impacts on every aspect of our daily life. Importantly, as the technology advancing and moving forward, the development of these devices not only relies on deeper fundamental understanding but also requires novel materials with unique properties as well as new device architecture to achieve higher performance with more diverse functionalities. In this regards, low dimensional materials inherently possess properties that are conceptually different from those of bulk materials in most aspects. The capability to tailor these nanomaterials as well as their unique properties is essential to achieve unconventional devices with revolutionary impacts. In this dissertation work, our aim is to develop novel nanoelectronics and nanophotonics by exploiting the extraordinary characteristics of purely two-dimensional (2D) monolayer graphene and its heterostructures. Firstly, we design and propose the dual-gate graphene ambipolar transistor that can operate as either common mode or differential mode amplifier by properly tuning the gate biases. Our device can also achieve high noise rejection amplification with common mode rejection ration (CMRR) as high as 80 dB, which is comparable to a commercial operational amplifier (op-amp). Secondly, we demonstrate the hyperbolic metamaterials (HMMs) by using precisely controlled periodic graphene-dielectric multilayer nanostructures to investigate the optical topological transition from elliptical to hyperbolic dispersion in mid-infrared regime. Thirdly, we propose the graphene-SOI heterojunction broadband photodetector design to improve the device on-off operation speed, strengthen the photo-gating effect, as well as minimize the dark current. We further fabricate the single pixels into 32 x 32 matrix arrangement to demonstrate the proof-of-concept image array readout, opening up the development of graphene-based ultra-broadband image sensor array applications. Lastly, we propose the all-graphene transparent photodetector design for light-field imaging and demonstrate the proof-of-concept one-dimensional (1D) ranging by using two stacked single-pixel transparent photodetectors. The results should lay the stepping stones and foundation for the new generation of graphene-based light-field photodetectors and image sensors.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135842/1/chehung_1.pd

Performance of Monolayer Graphene Nanomechanical Resonators with Electrical Readout

The enormous stiffness and low density of graphene make it an ideal material for nanoelectromechanical (NEMS) applications. We demonstrate fabrication and electrical readout of monolayer graphene resonators, and test their response to changes in mass and temperature. The devices show resonances in the MHz range. The strong dependence of the resonant frequency on applied gate voltage can be fit to a membrane model, which yields the mass density and built-in strain. Upon removal and addition of mass, we observe changes in both the density and the strain, indicating that adsorbates impart tension to the graphene. Upon cooling, the frequency increases; the shift rate can be used to measure the unusual negative thermal expansion coefficient of graphene. The quality factor increases with decreasing temperature, reaching ~10,000 at 5 K. By establishing many of the basic attributes of monolayer graphene resonators, these studies lay the groundwork for applications, including high-sensitivity mass detectors