Browsing by Subject "CVD"
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Item Chemical vapor deposited two-dimensional material based high frequency flexible field-effect transistors(2018-06-20) Park, Saungeun; Akinwande, Deji; Banerjee, Sanjay K.; Shi, Li; Register, Leonard F.; Dodabalapur, AnanthFlexible nanoelectronics have attracted great attention due to interesting concepts such as wearable electronics and internet of things, which requires high speed and low power consumption flexible smart system with functions ranging from sensing, computing to wireless communicating. In this dissertation, transparent and solution processable nanoscale polyimide film for highly flexible gate dielectrics is demonstrated by in-situ opto-electro-mechanical measurement and utilized for two-dimensional nanomaterials based field-effect transistors (FETs). Graphene thin film transistor with the nanoscale polyimide dielectric on flexible glass is operated in extremely high frequency regime and shows the highest experimental saturation velocity (~8.4 × 10⁶ cm/s) in any materials in any flexible transistors. Molybdenum disulfide (MoS₂) based transistors with embedded gate structure on rigid substrate are demonstrated with enhancement mode operation, ON/OFF ratio over 10⁸, the highest transconductance (~ 70 µS/µm) and saturation velocity (~1.8 × 10⁶ cm/s). CVD MoS₂ FETs on flexible plastic substrates are also demonstrated, showing enhancement mode operation, ON/OFF radio over 10¹⁰ and transconductance (~6 µS/µm). The flexible CVD MoS₂ transistors with embedded gate structure were employed to study effects of substoichiometric doping by HfO [subscript 2-x]. After the doping layer, the flexible MoS₂ transistors show ×8 higher source-drain current density as well as more than ×2 mobility improvements. For the another first demonstration, GHz operation and flexibility of graphene and MoS₂ based FETs are realized on commercial available paper substrates, which indicates flexible two-dimensional material based nanoelectronics can be implemented on paper substrates for systems, sensors, and Internet of Things.Item Chemical vapor deposition graphene on polycrystalline copper foil(2014-05) Magnuson, Carl William; Akinwande, Deji; Ruoff, Rodney S.Graphene, a single atomic layer of sp²-bonded carbon, has been of significant interest to basic sciences and engineering. Among its unique properties are exceptional mechanical strength, from the strong carbon-carbon bond; high in-plane thermal conductivity; high carrier mobilities, since electrons and holes travel through graphene as mass-less Dirac fermions; and quantum effects (such as the quantum Hall effect), which can be observed at room temperature. In 2009, Li et al., of Professor Ruoff's research group at the University of Texas at Austin, published a seminal paper detailing the production of fairly high quality graphene grown on copper foils using chemical vapor deposition (CVD). The potential for scalability of graphene CVD processing is extremely attractive, and this is currently the most promising method for its commercial viability, particularly for transparent conductive electrodes (TCEs). Here, graphene-based TCEs are compared with TCEs made with multi-walled carbon nanotubes (MWCNTs). A novel technique to reduce the sheet resistance of MWCNT-based TCEs in half is described in detail. Even with these improvements, graphene-based TCEs outperform MWCNT-based TCEs. The decomposition of copper oxides at high temperatures in an oxygen deficient environment is characterized. The ability for the oxygen evolved from the copper foil during this decomposition to react with carbon on the surface of the copper substrate is verified. This phenomenon was used to develop a technique for getting clean pre-graphene growth copper substrates and allowing repeatable graphene nucleation results. A technique for growing large graphene domains inside a copper vapor trapping 'copper enclosure' is described. The quality of the graphene grown inside the copper enclosure is characterized and shown to be of very high quality. This technique can grow graphene domains over 0.5 mm across. Finally, a possible cause of graphene ad-layer growth on the copper surface is suggested. It is proposed that gas diffusing through the copper substrate at high temperature delaminates the graphene from the copper surface in some regions. This then allows carbon containing molecules to diffuse under the graphene and grow new graphene layers. The increased ad-layer growth in the presence of helium supports this.Item CVD MoS₂ for high speed devices and circuits(2018-05-03) Sanne, Atresh Murlidhar; Banerjee, Sanjay; Akinwande, Deji; Register, Leonard F; Sreenivasan, S.V.; Rao, RajeshTwo-dimensional layered materials (2DLMs) have been widely studied as a potential alternative to the complementary metal-oxide semiconducting field-effect transistor (CMOS FET) "switch." The atomically thin body of 2DLMs lends itself to improved electrostatic gate control, leading to a suppression of the short channel effects which limit the scalability of CMOS devices. While many experiments have examined 2DLMs as a low power solution for aggressively scaled digital devices, their feasibility study for use in high speed radio frequency (RF) devices and circuits is still in its infancy. Current technological trends such as the Internet of Things (IoT) and 5G communication have increased the demand for novel high speed devices to serve next-generation circuits and systems. Graphene, as a 2DLM, has garnered significant interest for its use in high speed radio frequency (RF) devices and circuits. A carrier mobility greater than 10,000 cm²/Vs, ambipolar transport, and excellent thermomechanical stability has afforded graphene cutoff frequencies greater than 400 GHz. Multi-transistor integrated circuits, including a fully integrated RF receiver have been demonstrated using graphene. However, graphene poses a limitation in high speed operation in that the Dirac cone band structure results in a zero bandgap, leading to semi-metallic transport behavior. As a result, graphene field-effect transistors (GFETs) exhibit a low ION/IOFF ratio and non-saturating output behavior. This translates to FETs showing reduced power and voltage gains, hindering the realization of high performance amplifiers, mixers, and other RF circuit elements. Another class of 2DLMs has generated renewed interest for its potential to replace silicon as the next-generation CMOS "switch." Transition metal dichalcogenides (TMDs) is a family of 2DLMs with the general chemical formula MX₂ (M = metal, X = chalcogen). Of the class of TMDs, molybdenum disulfide (MoS₂) is of special interest. With its thickness-dependent electronic properties, MoS₂ has been considered for applications in the fields of opto-electronics, flexible electronics, spintronics, and coupled electro-mechanics. Its single layer direct bandgap of ~1.8 eV allows for high ION/IOFF metal-oxide semiconducting FETs. More relevant for RF applications, theoretical studies predict MoS₂ can afford saturation velocities greater than 3×10⁶ cm/s. While the mobility of MoS₂ is lower than that of graphene, the intrinsic bandgap in MoS₂ has shown voltage gains, A [subscript v] = g [subscript m] /g [subscript ds], greater than 30. Thus far, most of the studies of graphene and MoS₂ have utilized crystalline exfoliated layers, which provide a convenient high quality source of material for laboratory experiments. However, for industrial scale applications, the mechanical cleavage process is not scalable and, thus far, there have been few studies on large area chemical vapor deposited (CVD) MoS₂ RF FETs. In this dissertation, the initial efforts to utilize CVD MoS₂ for RF FETs are presented. The RF figures-of-merit transit frequency, fT, and maximum frequency of oscillation are measured for CVD MoS₂. The effects of different substrates and superstrates on MoS₂ are investigated. In order to improve the cutoff frequencies, a combination of channel length scaling and device geometry modifications are applied. Simple RF circuits are demonstrated experimentally using CVD MoS₂ FETs. Additionally larger circuit building blocks are simulated using experimental data. The goal of this work is to provide a baseline of the RF performance achievable using CVD MoS₂. Hopefully, this work will motivate future studies directing MoS₂ towards industrial electronic applications.Item Design and analysis for roll-to-roll graphene transfer(2017-08) Xin, Hao; Li, Wei (Of University of Texas at Austin); Crawford, Richard; Cullinan, Michael; Lu, Nanshu; Sreenivasan, S.V.Graphene as the first 2D material ever discovered has attracted tremendous attention worldwide since its discovery in 2004. One of the goals of graphene research is to implement this wonder material into industrial applications, e.g., flexible transparent electrode. Chemical vapor deposition (CVD) was proven to be a viable method for growing large-scale, electronics-grade graphene on transition metal substrates. The next critical step is to transfer as-grown graphene from its growth substrate to a target for device fabrication. In this study, a roll-to-roll (R2R) system was developed to transfer as-grown graphene from copper foil to flexible polymer film by mechanical dry peeling. With the proposed R2R design, high-purity copper foil can be recycled for future graphene growth. This approach could considerably reduce graphene production cost. Furthermore, the high transfer speed and efficiency of R2R graphene dry peeling is superior to the traditional wet etching and the recently reported R2R electrochemical delamination method. The process parameters for R2R dry peeling were investigated in detail, and their individual effects on transferred graphene quality were quantified for process optimization. The success of R2R graphene dry peeling is dependent on the understanding of interfacial property of as-grown graphene on copper. A blister test was used to measure the interfacial adhesion energy of CVD graphene on copper foil. Prior to CVD graphene growth, acetic acid was used to remove surface oxidation on copper. Different treatment time was used to generate three levels of surface roughness. It was found that higher copper surface roughness led to increased interfacial adhesion energy. To find its root cause, the surface of as-grown graphene samples were characterized with atomic force microscope (AFM), scanning electron microscope (SEM), and optical profilometer to identify the surface morphology on various length scales. Additionally, an analytical model was developed to describe the interfacial adhesion using both global mode mixity and local kinking angle at the interfacial delamination front. The analytical model was validated by experimental dataItem Design and analysis of precursors for CVD of Ru thin films and Li-ion batteries with MoP₄ anode materials(2013-08) De Pue, Lauren Joy; Jones, Richard A., 1954-The chemical vapor deposition growth of amorphous metallic alloys is currently of interest for potential uses in electronic devices. We have explored the use of ligands having Ru-H, Ru-N, and Ru-P bonds to study the effects of ligand selection. The synthesis and design of novel Ru dinuclear complexes using volatile ligands such as 3,5-bis-trifluoromethylpyrazolate and trimethylphosphine will be presented as well as materials characterization studies on grown films. Another class of functional materials of interest is the transition metal phosphides (TMPs) which have found applications in Li-ion batteries. Current research on TMPs is focused on obtaining materials with improved or new compositions and morphologies and on improving Li insertion/de-insertion reactions and charge carrying capacities. Traditional routes to these materials involve the use of high temperatures and pressures. The work presented here will focus on a synthetic route which employs relatively mild conditions. Surface analysis studies and the electrochemical performance of mesoporous MoP₄ for use as anode materials in Li-ion batteries will be described.Item Electrical and physical comparison of MoS₂ monolayers synthesized via CVD in different carrier gas environments(2020-05-06) Depoy, Jessica Marie; Banerjee, SanjayTransition metal dichalcogenides (TMDs) are an important family of two- dimensional (2D) materials currently being studied for implementation in next-generation electronics. Like graphene, this group of materials is a promising alternative to silicon as scaling effects begin to degrade silicon-based device performances. However, contrary to graphene, this family of 2D materials has an indirect bandgap in bulk form, and many members experience a shift to a direct bandgap in monolayers. MoS₂ is one of the most highly researched members of TMDs. With a direct bandgap of 1.8eV, the material is suited for applications from optoelectronics to spin electronics. One major difficulty in using MoS₂ in future devices is the struggle to produce large area, single crystalline layers. Chemical vapor deposition is one method of synthesizing single crystal MoS2. However, the large number of growth parameters makes optimization difficult. This study varies one parameter of growth, the carrier gas used in the synthesis of MoS₂, and compares the resulting electrical and surface characteristics of the synthesized MoS₂ layers. Hydrogen, nitrogen, and argon were the three carrier gases compared in this study. The samples grown using each carrier gas were also characterized with Raman spectroscopy and imaged with a scanning electron microscope to compare surface morphology and crystallinity. Back-gated field effect transistors were fabricated on each sample grown with each different carrier gas in order to compare electrical properties of the respective monolayers. Hydrogen is less commonly used as a carrier gas, but the resulting crystalline layers grown with H₂ displayed higher mobility and lower contact resistance than the layers grown with N₂ and Ar carrier gases. Other electrical properties compared included threshold voltage, channel resistance, and on/off ratio.Item Epitaxial germanium via Ge:C and its use in non-classical semiconductor devices(2015-12) Mantey, Jason Christopher; Banerjee, Sanjay; Lee, Jack C; Register, Leonard F; Akinwande, Deji; Ferreira, Paulo JThe microelectronics industry has been using Silicon (Si) as the primary material for complementary metal-oxide-semiconductor (CMOS) chip fabrication for more than six decades. Throughout this time, these CMOS devices have gotten exponentially smaller, faster, and cheaper. While new materials and fabrication processes have been slowly added over the years, the CMOS device of today is largely the same as it was decades ago. However, field-effect transistors (FETs) have now scaled so far that Si is approaching physical limits. Thus, new channel materials and new fundamental device structures are being investigated to replace traditional CMOS. Germanium is one of the prime candidates to replace Si in the FET channel, with its increased electron and hole mobilities compared to Si. Perhaps more importantly, it is compatible with the existing Si manufacturing techniques by epitaxially growing thin layers of Ge crystal on the starting Si wafer. Because these two crystals do not share a lattice constant, there will inevitably be crystal defects in the thin Ge layer that can be catastrophic for device functionality. Several approaches have been introduced to reduce defects, but most of them are wastefully thick (>1 um) or require complex manufacturing methods. In this work, we utilize an extremely thin (~10 nm) buffer layer of carbon-doped Ge (Ge:C) to grow Ge and SiGe layers for FET and virtual substrate applications with improved crystalline quality and reduced surface roughnesses. These thin Ge layers not only offer new pathways for MOSFETs, but can also be used in non-classical structures. Semiconductor nanowires (NWs) and tunnel-FETs (TFETs) are two of the most promising device architectures, and both can be used with Ge. This dissertation presents a simulated Si/Ge heterostructure interface TFET that can be fabricated on a virtual substrate made with the Ge:C buffer layer. Detailed analysis on device operation is given. Also in this work is the fabrication process for individually addressable Ge NW-FETs. The NWs offer excellent electrostatic gate control through reduced dimensions and offer another potential pathway for Ge in a post-CMOS world.Item Growth optimization of WSe₂ and its sulfurization to WS₂(2019-07-09) Liu, Chison Qishan; Banerjee, SanjayTungsten diselenide has gained much interest within recent years after it was reported to have both p-type and ambipolar transport properties. And because most other transition metal dichalcogenides exhibit n-type transport properties, tungsten diselenide would help to further realize CMOS technology if one could find a more reliable way to synthesize it in large areas with high quality crystallinity. In this report I will be detailing my work on successfully synthesizing WSe₂, its sulfurization into WS₂, and discussing what I’ve observed in both the crystal quality and growth mechanisms. My goal is to provide a better understanding of the growth process in hopes of moving forward with improving future growth recipesItem Novel organometallic precursors for the Chemical Vapor Deposition of metal thin films(2010-08) Rivers, Joseph Henry; Jones, Richard A., 1954-; Cowley, Alan H.; Holliday, Bradley J.; Magnus, Philip D.; Ekerdt, John G.With the growing demand for miniaturization of devices and for new materials with useful properties, the use of Chemical Vapor Deposition (CVD) for the manufacture of thin films is receiving growing attention. The synthesis of potentially volatile metal complexes and investigation of their use as CVD precursors is an important part of this process. The research presented addresses several major areas of this process, (i) the identification and synthesis of ligands which can impart volatility to a metal complex, (ii) the synthesis, characterization, and assessment of volatility of metal complexes containing these ligands, and (iii) the full materials characterization of thin films grown with these complexes. The use of trimethylphosphine, bis(trifluoromethyl)pyrazolate, and bis(trifluoromethyl)pyrrolyl ligands have been successfully used to synthesize volatile new complexes of cobalt, rhodium, and nickel, some of which show promise for use as potential CVD precursors.Item Precursor design for materials applications(2011-12) McCarty, William Jeffrey; Jones, Richard A., 1954-The importance of platinum group metals for catalytic and microelectronic applications has prompted research into the development of novel molecular precursors for chemical vapor deposition of thin films of these metals. A variety of molecular architectures, ligand systems, as well as deposition conditions are investigated and related to the morphology and composition of the resultant films. For example, amorphous thin films of ruthenium and phosphorus alloys are deposited using single source metal hydride precursors, while use of the 3,5-di-substituted pyrazolate ligand in conjunction with various rhodium starting materials leads to a variety of different volatile monomeric and dimeric complexes. Synthesis of pyrazole and pyrazolate complexes of tungsten and palladium are also explored. In a related research area, progress towards the development of novel synthetic routes to mesostructured transition metal phosphides and borates for Li-ion battery electrode applications is summarized. Traditional routes to these materials involve high-temperature syntheses, allowing limited control over morphology. Identification of low-temperature reaction conditions necessary to afford a desired composition, morphology and electrochemical performance of the bulk material are the main goals of this project, and results are discussed with various early transition metals.Item Solid electrolyte substrates for two-dimensional transition metal dichalcogenide growth, transistors and circuits(2021-08-13) Alam, Md Hasibul; Akinwande, Deji; Banerjee, Sanjay K; Dodabalapur, Ananth; Incorvia, Jean Anne; Lai, KejiThe high surface charge carrier densities, accumulated by the electrostatic gating of two-dimensional (2D) materials with ionic liquids (ILs), have often been exploited in 2D transistors and devices. However, the intrinsic liquid nature, sensitivity to humidity, and the stress induced in frozen liquids prevent them from forming an ideal platform for electrostatic gating and surface probe techniques. This dissertation reports a lithium-ion (Li-ion) solid electrolyte substrate (or simply Li-ion glass) alternative to ILs, by demonstrating its application in high-performance transistors and circuits using 2D transition metal dichalcogenide (TMD). The back-gated n-type MoS² and p-type WSe² transistors resulted in sub-threshold values approaching the ideal limit of 60 mV/dec while maintaining a high ON/OFF ratio (> 10⁶) and a complementary inverter amplifier gain of 34 under a 1 V supply, the highest among comparable solid-state amplifiers. Microscopic studies using microwave impedance microscopy clearly show a uniform and homogeneous channel formation, indicating a smooth interface between the TMD and the underlying electrolytic substrate. This dissertation also reports the direct growth of few-layer (3-4L) single-crystal MoS² on lithium-ion solid electrolyte substrate by chemical vapor deposition (CVD) and demonstrates efficient gate control in the as-grown crystal via electrolytic gating. The gating efficiency of the transistors fabricated on the as-grown crystals, and back-gated by the solid electrolyte, are comparable to the devices with exfoliated and transferred material with an additional gain in mobility value. Field-effect mobility in the range of 42-49 cm²V⁻¹s⁻¹ with current densities as high as 120 μA/μm with 0.5 μm channel length has been achieved, as expected from devices free from material transfer-related damage and impurity. This CVD growth method can potentially be extended for other 2D TMDs to realize high-mobility transistors and study intrinsic device properties. To sum up, the dissertation demonstrates solid electrolytes as an ideal platform for 2D TMD synthesis, advanced thin-film transistors, and circuits, otherwise difficult to achieve with liquid electrolytes. The results, therefore, further establish solid electrolytes as a promising alternative to ILs for surface science experiments and advanced thin-film devices. Finally, based on the work presented in this dissertation, some future research directions have been proposedItem Spectroscopic studies of boron carbo-nitride(2010-12) Ahearn, Wesley James; Ekerdt, John G.; Hwang, Gyeong S.BCxNy films were characterized as a potential pore sealing layer for low κ dielectrics. The changes in chemical bonding were studied as a function of growth temperature to aid in understanding the variation in electrical performance of these films. Thermal chemical vapor deposition of BCxNy using dimethylamine borane and ethylene were deposited on porous methylsilsesquioxane substrates at 335 °C and 1 Torr. BCxNy was able to seal the porous interface with a thickness of 3.9 nm for both blanket and patterned substrates. BCxNy films deposited over a temperature range of 300-400 °C with dimethylamine borane and either ethylene or ammonia coreactant gas were characterized. Films deposited with ethylene became more concentrated in B at the expense of C with increasing temperature. These films favored C-B intermixing over C-C and B-B bonding at higher temperature. H was detected in the form of B-H and C-H bonds. Films deposited with ammonia became concentrated in N at the expense of B, and favored B-N viii bonding at higher temperatures. H was found in the films as B-H, C-H, and N-H bonds. The amount of H in the films decreased with increasing growth temperature for both ethylene and ammonia coreacted films. The valence band offset of C-rich films increased from 0.17 ± 0.22 eV to 0.32 ± 0.22 eV when deposited at 300°C and 400 °C. For the Nrich films, the valence band offset shifted from 0.26 ± 0.28 at 300 °C to -0.15 ± 0.24 eV at the same deposition conditions. High temperature annealing from 400-800 °C in forming gas caused all BCxNy films to decrease in thickness up to 30%. At the same time, the index of refraction increased, and likely, the dielectric constant. X-ray photoelectron spectroscopy revealed little change in the constituent bonding, suggesting that volatile –H containing species were removed during the annealing process.Item Synthesis and Raman spectroscopy characterization of long Carbon Nanotubes(2023-08) Cantu, Rodolfo, Jr.; Shi, Li, Ph. D.Carbon Nanotubes (CNTs) are promising nanomaterials with outstanding thermal, electrical, and mechanical properties. Composed of carbon atoms arranged in thin cylindrical walls, these nanostructures have promising applications in nanoelectronics, ultra-strong fibers, and thermal management solutions. Chemical Vapor Deposition (CVD) has been used to grow CNTs from catalytical nanoparticles at low pressure and high temperatures. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization of the structures of CNTs can potentially induce defects due to the high electron beam energy. This thesis reports an effort to establish Raman spectroscopy mapping as a relatively non-invasive technique for locating individual suspended and long single-walled carbon nanotubes (SWCNTs) grown by catalytic CVD before they are transferred for subsequent measurements of their thermal transport and other properties. The g-band and radial breathing mode in the Raman spectrum is used to identify SWCNTs and determine their diameters.