Optoelectronic Properties of Two-Dimensional Molybdenum Ditelluride

In this thesis the layered, two-dimensional material MoTe2 is examined experimentally for its optoelectronic properties, using a field effect transistor device configuration. MoTe2 experiences a strong light matter interaction, which is highly dependent on the conditions of the measurement, and the wavelength of light used. Light is able to: produce a photocurrent in MoTe2, desorb adsorbates from the surface, and even controllably thin by a single layer at a time. A theoretical study on MoTe2 also provides insights on the source of some of these interesting light matter interactions. MoTe2 is found to be a fast and responsive photodetector when illuminated with red laser light in ambient conditions, with increases in current stemming from the photovoltaic effect. Due to the generated charge carriers from the photovoltaic effect, conductivity can increase by increasing the Fermi energy of the material, or by a photogating effect where excited charges are trapped and behave as an artificial gate for the field effect transistor. The mechanisms of charge trapping are experimentally investigated due to their prevalence in the photodetection mechanisms. A theoretical study points towards the existence of two types of trap states, in not just MoTe2 but all transition metal dichalcogenides, with shallow traps closer to the valence band edge (τ ~ 500 s) and deeper traps (τ ~ 1000 s), further away from the valence band edge. MoTe2, under the effects of higher energy photons from blue and green lasers, showed different photocurrent mechanisms to red light. From the increased energy of the photons, photo-desorption of adsorbates on the surface of MoTe2 occurred causing a decrease in the overall current, in a rarely seen photocurrent mechanism. Again, both shallow and deep traps are evident from the experimental measurements, with the shallow traps being removed when illuminated by higher energy photons. Finally, a humidity assisted photochemical layer-by-layer etching process was developed with an in-situ Raman spectroscopy system, able to thin MoTe2 by a single layer at a time with 200 nm spatial resolution. MoTe2 FETs were created with thinned channels to examine the effect of the thinning technique on optoelectronic properties. Some improvement in optoelectronic performance (higher responsivity, higher mobility) was seen for the thinned channel devices, with great improvement observed for monolayer MoTe2.Engineering and Physical Sciences Research Council (EPSRC

Thermal Atomic Layer Etching of MoS\u3csub\u3e2\u3c/sub\u3e Using MoF\u3csub\u3e6\u3c/sub\u3e and H\u3csub\u3e2\u3c/sub\u3eO

Two-dimensional (2D) layered materials offer unique properties that make them attractive for continued scaling in electronic and optoelectronic device applications. Successful integration of 2D materials into semiconductor manufacturing requires high-volume and high-precision processes for deposition and etching. Several promising large-scale deposition approaches have been reported for a range of 2D materials, but fewer studies have reported removal processes. Thermal atomic layer etching (ALE) is a scalable processing technique that offers precise control over isotropic material removal. In this work, we report a thermal ALE process for molybdenum disulfide (MoS2). We show that MoF6 can be used as a fluorination source, which, when combined with alternating exposures of H2O, etches both amorphous and crystalline MoS2 films deposited by atomic layer deposition. To characterize the ALE process and understand the etching reaction mechanism, in situ quartz crystal microbalance (QCM), Fourier transform infrared (FTIR), and quadrupole mass spectrometry (QMS) experiments were performed. From temperature-dependent in situ QCM experiments, the mass change per cycle was −5.7 ng/cm2 at 150 °C and reached −270.6 ng/cm2 at 300 °C, nearly 50× greater. The temperature dependence followed Arrhenius behavior with an activation energy of 13 ± 1 kcal/mol. At 200 °C, QCM revealed a mass gain following exposure to MoF6 and a net mass loss after exposure to H2O. FTIR revealed the consumption of Mo−O species and formation of Mo−F and MoFx=O species following exposures of MoF6 and the reverse behavior following H2O exposures. QMS measurements, combined with thermodynamic calculations, supported the removal of Mo and S through the formation of volatile MoF2O2 and H2S byproducts. The proposed etching mechanism involves a two-stage oxidation of Mo through the ALE halfreactions. Etch rates of 0.5 Å/cycle for amorphous films and 0.2 Å/cycle for annealed films were measured by ex situ ellipsometry, Xray reflectivity, and transmission electron microscopy. Precisely etching amorphous films and subsequently annealing them yielded crystalline, few-layer MoS2 thin films. This thermal MoS2 ALE process provides a new mechanism for fluorination-based ALE and offers a low-temperature approach for integrating amorphous and crystalline 2D MoS2 films into high-volume device manufacturing with tight thermal budgets

Non-invasive digital etching of van der Waals semiconductors

The capability to finely tailor material thickness with simultaneous atomic precision and non-invasivity would be useful for constructing quantum platforms and post-Moore microelectronics. However, it remains challenging to attain synchronized controls over tailoring selectivity and precision. Here we report a protocol that allows for non-invasive and atomically digital etching of van der Waals transition-metal dichalcogenides through selective alloying via low-temperature thermal diffusion and subsequent wet etching. The mechanism of selective alloying between sacrifice metal atoms and defective or pristine dichalcogenides is analyzed with high-resolution scanning transmission electron microscopy. Also, the non-invasive nature and atomic level precision of our etching technique are corroborated by consistent spectral, crystallographic and electrical characterization measurements. The low-temperature charge mobility of as-etched MoS$_2$ reaches up to $1200\,$cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$, comparable to that of exfoliated pristine counterparts. The entire protocol represents a highly precise and non-invasive tailoring route for material manipulation.Comment: 46 pages, 4 figures, with S

Optimization Of Transition-Metal Dichalcogenides Based Field- Effect- Transistors Via Contact Engineering

ABSTRACT Optimization of Transition-Metal Dichalcogenides based Field- Effect-Transistors via contact engineering by Meeghage M Perera September , 2016 Advisor : Dr. Zhixian Zhou Major: Physics (Condensed mater physics/nano-electronics) Degree: Doctor of Philosophy Layered transition Metal Dichalcogenides (TMDs) have demonstrated a wide range of remarkable properties for applications in next generation nano-electronics. These systems have displayed many “graphene-like” properties including a relatively high carrier mobility, mechanical flexibility, chemical and thermal stability, and moreover offer the significant advantage of a substantial band gap. However, the fabrication of high performance field-effect transistors (FETs) of TMDs is challenging mainly due to the formation of a significant Schottky barrier at metal/TMD interface in most cases. The main goal of this study is to develop novel contact engineering strategies to achieve low-resistance Ohmic contacts. Our first approach is to use Ionic Liquid (IL) gating of metal contacted MoS2 FETs to achieve highly transparent tunneling contacts due to the strong band banding at metal/MoS2 interface. The substantially reduced contact resistance in ionic-liquid-gated bilayer and few-layer MoS2 FETs results in an ambipolar behavior with high ON/OFF ratios, a near-ideal subthreshold swing, and significantly improved field-effect mobility. Remarkably, the mobility of a 3-nm-thick MoS2 FET with an IL gate was found to increase from ~ 100 cm2V-1s-1 to ~ 220 cm2V-1s-1 as the temperature decreased from 180 K to 77 K. This finding is in quantitative agreement with the true channel mobility measured by four-terminal measurement, suggesting that the mobility is predominantly limited by phonon-scattering. To further improve the contacts of TMD devices, graphene was used as work function tunable electrodes. In order to achieve low Schottky barrier height, both IL gating and surface charge transfer doping were used to tune the work function of graphene electrodes close to the conduction band edge of MoS2. As a result, the performance of our graphene contacted MoS2 FETs is limited by the channel rather than contacts, which is further verified by four-terminal measurements. Finally, degenerately doped TMDs are used as drain/source electrodes to form 2D/2D van der Waals contacts, which are air and thermally stable. WSe2 devices with 2D/2D contacts and 0.01% Nb doped WSe2channel show a high ON/OFF ratio and high field-effect mobility of 175 cm2V-1S-1 at room temperature, which increases to 654 cm2V-1S-1 at cryogenic temperatures. As the doping concentration increases, both the ON/OFF ratio and mobility decrease. These contact engineering strategies overcome a major challenge in the development of electronics based on 2D materials beyond graphene

Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding

We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro- PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high temperature vacuum annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of non-radiative recombination of excitons at defect sites as verified by low temperature PL measurements. First principle calculations reveal a strong binding energy of ~2.395 eV for oxygen molecule adsorbed on an S vacancy of MoS2. The chemical adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physical adsorbed oxygen on ideal MoS2 surface. We also demonstrate that the defect engineering and oxygen bonding could be easily realized by oxygen plasma irradiation. X-ray photoelectron spectroscopy further confirms the formation of Mo-O bonding. Our results provide a new route for modulating the optical properties of two dimensional semiconductors. The strong and stable PL from defects sites of MoS2 may have promising applications in optoelectronic devices.Comment: 23 pages, 9 figures, to appear in ACS Nan

Kaasufaasidepositioidun grafeenin ja siirtymämetallidikalkogenidi nanohiutaleiden heterorakenteet

Transition Metal Dichalcogenides (TMDs) offer new and complementary properties to those of graphene. It is of much interest to manufacture heterostructures of these materials to fully exploit their properties. Traditionally these heterostructures are manufactured by mechanically exfoliating small flakes from large bulk crystal and then manually aligning the flakes. This is a slow and cumbersome process. In this thesis, it is analyzed whether graphene can be directly grown on top of chemically exfoliated TMD flakes via Chemical Vapor Deposition (CVD) on copper substrates in order to significantly increase throughput. At first the thermal stability of the following TMDs were tested: Molybdenum Disulfide (MoS2), Tungsten Disulfide (WS2) and hexagonal Boron Nitride (hBN). It was concluded that only hBN has the thermal stability to be used in a standard methane based CVD graphene process. Initial experiments with chemically exfoliated hBN flakes gave inconclusive results due to the confocal Raman spectrography not offering resolutions high enough to map the surface of the flakes after CVD growth. However, the experiments lead to the conclusion that CVD graphene does not grow under the flakes via intercalation or precipitation. Lorentzian-peak center-position filter was developed to distinguish small hBN nanoflakes from the midst of defective graphene. Mechanically exfoliated hBN flakes were used to overcome the resolution limitation of confocal Raman spectroscopy. The results indicate that CVD graphene can grow on the flakes only if there are defects on the hBN flake surface. However, graphene growth is inconsistent and does not fully cover the flake.Siirtymämetallidikalkogenidit (TMDt) tarjoavat grafeenia täydentäviä ominaisuuksia ja näiden materiaalien välisillä heterorakenteilla voidaan yhdistää materiaalien parhaita puolia. Tavallisesti heterorakenteiden valmistaminen tapahtuu mekaanisesti eksfolioimalla (kuorimalla) ohuita hiutaleita suuremmasta kiderakenteesta ja manuaalisesti asettamalla hiutaleet paikoilleen. Tämä on hidas ja vaivalloinen prosessi. Tässä työssä tutkitaan grafeenin kasvattamista kaasufaasidepositiolla suoraan kemiallisesti eksfolioitujen TMD-hiutaleiden päälle, mikä onnistuessaan helpottaisi ja nopeuttaisi heterorakenteiden valmistamista. Ensimmäisessä vaiheessa tutkittiin TMD-materiaalien lämpötilakestävyyttä. Tutkitut materiaalit olivat molybdeenidisulfidi (MoS2), wolframdisulfidi (WS2) ja heksagonaalinen boorinitridi (hBN). Osoittautui, että vain hBN kestää korkean lämpötilan, jonka grafeenin kasvatus metaanipohjaisella kaasufaasidepositiolla vaatii. Grafeenin kasvamista kemiallisesti eksfolioitujen nanokokoisten hBN-hiutaleiden pinnalle ei saatu analysoitua, koska työssä käytetyn konfokaalisen Raman-spektroskopian resoluutio ei ollut riittävä hiutaleiden pinnan kuvantamiseen kasvatusprosessin jälkeen. Näitä hBN-hiutaleita pystyttiin kuitenkin hyödyntämään, kun tutkittiin grafeenin kasvua niiden alle. Tulosten perusteella voidaan sanoa, että grafeeni ei kasva nanokokoisten hBN-hiutaleiden alle. Työn ohessa kehitettiin Lorentz-sovitukseen perustuva suodatin, jolla saadaan erotettua toisistaan hBN nanohiutaleet huonolaatuisesta grafeenista. Grafeenin kasvatuksen jatkoanalyysissä käytettiin mekaanisesti eksfolioituja hBN-hiutaleita aikaisemmin mainitun konfokaalisen Raman-spektroskopian resoluutiorajoitteen takia. Jatkoanalyysissä havaittiin, että grafeeni voi kasvaa TMD-hiutaleiden päälle vain, jos niissä on rakenteellisia virheitä

Surface-Bound and Volatile Mo Oxides Produced During Oxidation of Single MoS2 Crystals in Air and High Relative Humidity

We report on the MoO3 oxides and their derivatives on microscopic 2H MoS2 flakes oxidized in air and high relative humidity at a moderate temperature range below 410 °C. We combine XPS and AFM measurements such as topography, friction, creation of nanoscale ripples and scratches on the MoS2 flakes deposited on Si substrates. We detect MoO3 oxides mostly by measuring selected nanomechanical properties of the MoO3 layer, such as its compressive mechanical stress at the plastic yield. We discuss basal surface coverage of the single MoS2 flakes by the MoO3 oxides. We discuss conditions for appearance of all possible MoO3 oxide derivatives, such as molybdenum(VI) hydroxyoxides and MoO3 hydrates. Our findings agree with an expected mechanistic switch in thermal oxidation in water vapors vs. air