185 research outputs found
Controlled Crack Propagation for Atomic Precision Handling of Wafer-Scale Two-Dimensional Materials
No full textDue to copyright restrictions and/or publisher's policy full text access from Treasures at UT Dallas is limited to current UTD affiliates (use the provided Link to Article).Although flakes of two-dimensional (2D) heterostructures at the micrometer scale can be formed with adhesive-tape exfoliation methods, isolation of 2D flakes into monolayers is extremely time consuming because it is a trial-and-error process. Controlling the number of 2D layers through direct growth also presents difficulty because of the high nucleation barrier on 2D materials. We demonstrate a layer-resolved 2D material splitting technique that permits high-throughput production of multiple monolayers of wafer-scale (5-centimeter diameter) 2D materials by splitting single stacks of thick 2D materials grown on a single wafer. Wafer-scale uniformity of hexagonal boron nitride, tungsten disulfide, tungsten diselenide, molybdenum disulfide, and molybdenum diselenide monolayers was verified by photoluminescence response and by substantial retention of electronic conductivity. We fabricated wafer-scale van der Waals heterostructures, including field-effect transistors, with single-atom thickness resolution. © 2018 American Association for the Advancement of Science. All rights reserved.NSF grant no. CMMI-1825731, CMMI-1825256, DMR-1231319, DMR-1700137; ONR grant no. N00014-16-1-2657; Air Force contract no. FA8721-05-C-0002 and/or FA8702-15-D-0001; NIST through award no. 70NANB17H041Erik Jonsson School of Engineering and Computer Scienc
Chemical bonding and defect states of LPCVD grown silicon-rich Si3N4 for quantum dot applications
No full textSi-rich Si₃N₄ (SRN) thin films were investigated to understand the various defect states present within the SRN that can lead to reduced performance in quantum dot based devices made of these materials. The SRN films, deposited by low pressure chemical vapor deposition followed by furnace anneals over a range of temperatures, were determined to be comprised of two distinct phase separated SRN regions with different compositions (precipitates within a host matrix). Photoluminescence (PL) spectra showed multiple peaks convoluted together within the visible and near-visible range. Depending on deposition and annealing conditions, the films displayed changes in PL peak intensities which were correlated with chemical bonding utilizing x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, spectroscopic ellipsometry, and capacitance-voltage measurements. It is found that the PL originates from defect-state to defect-state and band edge to defect-state electronic transitions
Thomas Merton\u27s Gethsemani: Landscapes of Paradise
No full textFor twenty-seven years, renowned and beloved monk Thomas Merton (1915-1968) belonged to Our Lady of Gethsemani, a Trappist monastery established in 1848 amid the hills and valleys near Bardstown, Kentucky. In Thomas Merton\u27s Gethsemani, dramatic black-and-white photographs by Harry L. Hinkle and artful text by Merton scholar Monica Weis converge in a unique experience for lovers of Merton.
Hinkle was allowed unprecedented access to many areas inside the monastery and on its grounds that are generally restricted. His photographs invite the reader to experience the various knobs, lakes, woods, and hermitages Merton sought out for times of solitude and contemplation and for reading and writing. These unique images, each accompanied by a passage from Merton\u27s writings, evoke personal reflection and a deeper understanding of how and why Merton came to recognize himself as a part of his Kentucky landscape.
Woven throughout the book, Weis\u27s text explores Merton\u27s fascination with nature not only at Gethsemani, but during his early childhood, throughout his spiritual conversion to Roman Catholicism, and while a member of the Trappist community. She examines how Merton\u27s lifelong interaction with nature subtly revealed and informed his profound spiritual experiences and his writing about contemplation. Thomas Merton\u27s Gethsemani replicates Merton\u27s path on his solitary hikes in the woods and conveys the wonder of the landscapes that inspired him.
Harry L. Hinkle is a fine art photographer in Lexington, Kentucky.
Monica Weis, SSJ, a Sister of St. Joseph, is professor of English at Nazareth College in Rochester, New York.
This handsome book combines words and photographs that every Merton admirer will appreciate. . . . Hinkle\u27s roaming camera preserves the essence of Gethsemani —Booklist
Will live beyond books that have treated Mertons poetry, religious commentary and social criticism in a vacuum. It brings together in one volume the images that inspired the words and the words drawn from the images. —Bowling Green (KY) Daily News
Both the casual visitor to Gethsemani and the Merton scholar will find this book a valuable addition to their library. —Catholic Telegraph
Hinkle\u27s sublime artwork is impressive and inspiring. . . . Weis has produced the text which artfully conveys how Merton\u27s lifelong interaction with nature revealed and inspired his spiritual experiences, his contemplations, and writings. —Chevy Chaser/Southsider
Combines some of Merton\u27s photographs and writings with Hinkle\u27s dramatic black and white photographs and the narrative of three gifted writers to convey the contemplative setting that was Merton\u27s home for twenty-seven years. —Contemplation & Action
This beautify volume documents Merton\u27s environment, both monastic and natural. . . .Those interested in American nature writing, art photography, spirituality, and the influence of place upon personal development will welcome this beautifully produced, insightfully written, and contemplative volume. —ISLE
Hinkle\u27s stunning photographs, and the text which accompanies them, allow us to see Merton\u27s monastic world with that fresh eye which Thomas Merton himself insisted was the gift of contemplation. —Lawrence S. Cunningham, University of Notre Dame
In this life, on this earth, Gethsemani was Merton\u27s paradise. Hinkle and Weis artfully show how Merton learned and grew spiritually in this paradise, and, by implication, how others might do so, too. —Lexington Herald-Leader
Lovingly recreates the Trappist monastery where Merton lived for 27 years. —Louisville Courier-Journal
An evocative book of photographs memorably wed with a remarkable essay. —Merton Seasonal
Harry Hinkle\u27s superb photographs do for \u27Merton Country\u27 what Herbert Gleason did many years ago for Thoreau-provide a vivid record of the landscapes and natural phenomena that inspired the author-and Monica Weis\u27 lucid tracing of both the chronological breadth and spiritual depth of Merton\u27s reflections on his environment reveal why and how the natural world served Merton as a revelation of the Creator. —Patrick F. O\u27Connell, coeditor of The Thomas Merton Encyclopedia
An artful combination of lovely photographs by Harry L. Hinkle, illuminated by Merton\u27s own words, and an insightful essay by Monica Weis help the reader experience Merton\u27s spiritual interaction with his physical environment. —Register of the Kentucky Historical Society
A fine contribution not only to art photography, but to Merton scholarship and will be welcomed by those unlikely to visit Merton\u27s Gethsemani and those who know it well. —Spiritualityhttps://uknowledge.uky.edu/upk_history_of_religion/1005/thumbnail.jp
Covalent nitrogen doping in molecular beam epitaxy-grown and bulk WSe2
No full textCovalent p-type doping of WSe2 thin films grown by molecular beam epitaxy and WSe2 exfoliated from bulk crystals is achieved via remote nitrogen plasma exposure. X-ray photoelectron and Raman spectroscopies indicate covalently bonded nitrogen in the WSe2 lattice as well as tunable nitrogen concentration with N2 plasma exposure time. Furthermore, nitrogen incorporation induces compressive strain on the WSe2 lattice after N2 plasma exposure. Finally, atomic force microscopy and scanning tunneling microscopy reveal that N2 plasma treatment needs to be carefully tuned to avoid any unwanted strain or surface damage
Molecular Beam Epitaxy Growth of Transition Metal Dichalcogenides
No full textThe exponential growth of Si-based technology has finally reached its limit, and a new generation of devices must be developed to continue scaling. A unique class of materials, transition metal dichalcogenides (TMD), have attracted great attention due to their remarkable optical and electronic properties at the atomic thickness scale. Over the past decade, enormous efforts have been put into TMD research for application in low-power devices. Among these studies, a high-quality TMD synthesis method is essential. Molecular beam epitaxy (MBE) can enable high-quality TMD growth by combining high purity elemental sources and an ultra-high vacuum growth environment, together with the back-end-of-line compatible growth temperatures. Although many TMD candidates have been grown by MBE with promising microstructure, the limited grain size (< 200 nm) for the MBE-grown TMDs reported in the literature thus far is unsuitable for high-performance device applications.
In this dissertation, the synthesis of TMDs by MBE and their implementation in device structures were investigated. van der Waals epitaxial growth of these TMDs (HfSe2, WTe2, WSe2, WTexSe2-x), due to the relaxed interactions at the interface, have been demonstrated on large lattice-mismatched substrates without strain and misfit dislocations. The fundamental nucleation and growth behavior of WSe2 was investigated through a detailed experimental design, combined with on-lattice, diffusion-based first principles kinetic modeling. Over one order of magnitude improvement in grain size was achieved through this study. Results from both experiment and simulation showed that reducing the growth rate, enabled by high growth temperature and low metal flux, is vital to nucleation density control. Meanwhile, providing a chalcogen-rich growth environment will promote larger grain lateral growth by suppressing vertical growth. Applying the knowledge learned from the nucleation study, we sucessfully integrated the MBE-grown WSe2 into Si complementary metal-oxide-semiconductor (CMOS) compatible field-effect transistors (FETs). Excellent transport properties, such as field effect hole mobilities (40 cm2/V∙s) with orders of magnitude improvement over the reported values of MBE-grown TMDs, are shown. These studies provide a comprehensive understanding of the MBE synthesis of TMDs and devices, indicating the great potential of integrating TMDs into CMOS process flows for the future electronics
The impact of the strong euro on the real effective exchange rates of the two Francophone African CFA Zones
No full textThe author estimates the degree of misalignment of the CFA franc since the introduction of the euro in 1999. Using a relative purchasing power parity-based methodology, he develops a monthly panel time series dataset for both the Economic and Monetary Community of Central Africa (CEMAC) zone and the West African Economic and Monetary Union (UEMOA) zone to compute a trade-weighted real effective exchange rate indexed series from January 1999 to December 2004. The author's main finding is that the real effective exchange rate appreciated by close to 8 percent in UEMOA and 7 percent in CEMAC, influenced by volatility in the euro-dollar bilateral exchange rate and conservative monetary policies in the two zones, resulting in a partial loss of competitiveness in export markets. The lower appreciation in Central Africa can be explained by lower inflation in CEMAC than in UEMOA and by the greater trade with higher inflation East Asian countries, partially offset by the peg to the dollar. However, the inclusion of"unrecorded trade"results in an appreciation of only 6 percent in the UEMOA zone and 6 percent in the CEMAC zone due to higher inflation in the two countries with unmonitored cross-border flows, Ghana and Nigeria. Using time series econometrics, an Engle-Granger two stage procedure for cointegration, and an error correction framework, a single equation modeling of the real exchange rate from 1970 to 2005 as a function of terms of trade, economic openness, aid inflows, and a dummy representing the 1994 devaluation, the author finds little statistical evidence of a long-run equilibrium exchange rate that is a vector of economic fundamentals. The dummy explains most of the real exchange rate behavior in the two zones, while openness in UEMOA has contributed to an appreciation of the real effective exchange rate.Economic Stabilization,Economic Theory&Research,Macroeconomic Management,Fiscal&Monetary Policy,Free Trade
Molecular Beam Epitaxy of van der Waals Materials for Applications in Novel Logic and Memory Devices
No full textThe continued scaling toward high-density, low-power devices has pushed the current Si-based
technology to the fundamental limit. To overcome these limitations, new types of materials are
being researched and developed for implementation in the next-generation of devices. The ability
to produce uniform crystals in nanoscale dimensions presents major challenges in device design
and fabrication. However, van der Waals materials offer the advantage of crystallizing in a
planar monolayer where strong in-plane covalent bonds along with saturated surface bonds allow
for scaling beyond the current limit. To investigate the potential of certain van der Waals
materials for novel device applications, this research utilizes molecular beam epitaxy (MBE) to
study the formation of thin films with close control of the deposition conditions. This dissertation
examines a number of van der Waals materials (HfSe₂, WSe₂, WTe₂, WSe(2-x)Tex, hBN, Bi₂Se₃)
to determine the potential advantages and challenges of device integration. The results show that
chalcogen-rich deposition conditions are essential in the crystallization of stoichiometric
transition metal dichalcogenides (HfSe₂, WSe₂, WTe₂). However, in the pursuit of more complex
alloys, a study of the MBE growth of WSe(2-x)Tex at 250°C reveals an energetic barrier of
formation that results in phase-separated films from 14% to 79% Te concentrations.
Furthermore, research investigating an insulating member of the van der Waals materials family,
hexagonal boron nitride (hBN), revealed the necessity of high growth temperatures to form
large-area crystals with monolayer thickness control. Finally, a number of studies surrounding
the topological insulator Bi2Se3 showed the formation of stoichiometric, large-area crystals at
low substrate temperatures (320°C) are very robust to physical damage. These studies shows that
sputtering with He⁺ ions preferentially removed Se atoms from the crystal during the destructive
technique, but failed to disrupt the topologically-protected surface states. Furthermore,
inductively coupled plasma etching with chlorine and fluorine-based recipes revealed the
necessity for strategic selection of etching constituents. Whereas fluorine-based recipes result in
chemical reactions that produced insulating bismuth fluoride, chlorine-based recipes had little
reactivity and resulted in films that had minimal changes to the surface states. Finally, a study
showing the regeneration effects of Bi₂Se₃ demonstrated the ability to “heal” the damage induced
during conventional processing. The totalities of these results reveal the potential of van der
Waals materials for applications in novel logic and memory devices while highlighting the
challenges associated with their synthesis. This dissertation serves as a guide for the design of
next-generation devices where these van der Waals materials might be crucial components
Comprehensive Capacitance-Voltage Analysis Including Quantum Effects for High-K Interfaces on Germanium and Other Alternative Channel Materials
No full textHigh mobility alternative channel materials to silicon are critical to the continued scaling of metal oxide semiconductor (MOS) devices. However, before they can be incorporated into advanced devices, some major issues need to be solved. The high mobility materials suffer from lower allowable thermal budgets compared to Si (before desorption and defect formation becomes an issue) and the absence of a good quality native oxide has further increased the interest in the use of high-k dielectrics. However, the high interface state density and high electric fields at these semiconductor/high-k interfaces can significantly impact the capacitance-voltage (C-V) profile, and current C-V modeling software cannot account for these effects. This in turn affects the parameters extracted from the C-V data of the high mobility semiconductor/high-k interface, which are crucial to fully understand the interface properties and expedite process development.
To address this issue, we developed a model which takes into account quantum corrections
which can be applied to a number of these alternative channel materials including SixGe1−x, Ge, InGaAs, and GaAs. The C-V simulation using this QM correction model is orders of magnitude faster compared to a full band Schrodinger-Poisson solver. The simulated C-V is directly benchmarked to a self consistent Schrodinger-Poisson solution for each bulk semiconductor material, and from the benchmarking process the QM correction parameters are extracted. The full program, C-V Alternative Channel Extraction (CV ACE), incorporates a quantum mechanical correction model, along with the interface state density model, and can extract device parameters such as equivalent oxide thickness (EOT), doping density and flat band voltage (Vfb) as well as the interface state density profile using multiple measurements performed at different frequencies and temperatures, simultaneously. The program was used to analyze experimentally measured C-V profiles and the extracted device parameters show excellent agreement with the known device structure and previously published results.
CV ACE has been applied in the development of a process flow for germanium interface passivation in Ge based MOS devices using a GeOx interlayer. A post atomic layer deposition (ALD) plasma oxidation (PPO) process was developed using radio frequency (RF) plasma in a plasma enhanced chemical vapor deposition (PECVD) chamber and demonstrated significant surface passivation. Various gases were investigated and 1% O2/Ar was found to reduce the growth rate and provide excellent control over the degradation of EOT. A 100 W plasma with 1% O2/Ar was found to provide the best combination of EOT and low Dit and is concluded to be the optimum process for PPO of germanium surfaces.
CV ACE and PPO were also utilized to investigate other process development challenges. A study of the impact of low temperature anneals on Ge-based MOS devices was found to result in a degradation of the electrical thickness and a change in fixed charge, indicating that the process window is very narrow and at much lower temperatures than for Si
Characterizing and Engineering the Metal Contact Interface in 1D and 2D Chalcogenide Systems
No full textThe layered ransition metal dichalcogenides (TMDs) exhibit unique phase- and thicknessdependent electronic, photonic, and magnetic properties intriguing for future device technologies.
Historically, contact engineering in silicon devices has relied on a detailed understanding of the
relationships between contact chemistry, phase, and resistance. However, similar relationships in
metal–TMD systems are not yet understood and high contact resistance critically limits TMD
device performance. This dissertation employs a variety of materials characterization techniques,
such as in–situ photoelectron spectroscopies, Raman spectroscopy, and scanning probe
microscopy, to study the metal–TMD and metal–Te interface chemistries, structures, and band
alignments as a function of pre-metallization, in-situ metallization, and post-metallization
processing conditions. The band alignments of similarly processed Schottky diodes and fieldeffect transistors are extracted analytically and corroborated with chemical and structural changes
during processing. Process recommendations for consistent, high-performance contacts to MoS2
and WSe2 are provided
Understanding the Impact of Annealing on Interface and Border Traps in the Cr/HfO2/Al2O3/MoS2 System
No full textTop-gated, few-layer MoS2 transistors with HfO2 (6 nm)/Al2O3 (3 nm) gate dielectric stacks are fabricated and electrically characterized by capacitance-voltage (C-V) measurements to study electrically active traps (D-it) in the vicinity of the Al2O3/MoS2 interface. Devices with low D-it and high D-it are both observed in C-V characterization, and the impact of H-2/N-2 forming gas annealing at 300 and 400 degrees C on the D-it density and distribution is studied. A 300 degrees C anneal is able to reduce the D-it significantly, while the 400 degrees C anneal increases defects in the gate stack. Simulation with modeled defects suggests a sizable decrease in D-it, half the amount of positive fixed charge in the dielectric, and slightly increased unintentional doping in MoS2 after a 300 degrees C anneal. In the as-fabricated devices displaying high D-it levels, the energy distribution of the D-it located at the Al2O3/MoS2 interface is continuous from the conduction band edge of MoS2 down to 0.13-0.35 eV below the conduction band edge. A plausible D-it origin in our experiments could come from the unexpected oxygen atoms that fill the sulfur vacancies during the UV-O-3 functionalization treatment. The border trap concentration in Al2O3 is the same, both before and after the anneal, suggesting a different origin of the border traps, possibly due to the low-temperature atomic-layer-deposited process
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