Browsing by Subject "Oxides"

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An accurate and computationally-efficient model for boron implants through an overlying oxide layer into single-crystal (100) silicon
(1993) Lim, Hsuan-Yu, 1967-; Tasch, Al F., Jr.
In many of the current implant applications in integrated circuits, a thin overlying amorphous oxide layer is often used to reduce the depth of dopant profiles by reducing the amount of channeling. The oxide layer is believed to randomly scatter a well-collimated (≤ 0.5° divergence) ion beam into a cone-shape angular distribution prior to its entry into the underlying single-crystal silicon. In this manner, most of the ions are scattered away from major axial and planar channels. A computationally-efficient 1-D model for boron implantation into single-crystal silicon through a screen oxide layer was developed. This model is of great interest and importance to the semiconductor industry for understanding process control issues in manufacturing and for guiding technology development. In developing this model, approximately 400 SIMS experimental profiles were obtained. In addition, the UT-MARLOWE Monte Carlo ion implant code was improved to generate boron profiles for part of the implant parameter space. It has been observed that boron implanted profiles are significantly dependent on the implant dose, energy, oxide layer thickness, tilt angle and rotation angle. A curve-fitting software program that uses the Dual Pearson Distribution function was used to extract the nine parameter values which define each profile. The parameters extracted for each profile are arranged into a lookup table where each set of nine parameter values corresponds to the profile for a particular combination of implant dose, energy, tilt angle, rotation angle and oxide thickness. Linear interpolation functions are employed to generate profiles for which there are no explicit set of parameters. This computationally-efficient model is able to generate as-implanted profiles of boron implantation through thin overlying oxides ranging from bare silicon (with a native oxide of approximately I.6nm) to 40nm, implant energies ranging from 15keV to 80keV, doses up to 1x10¹⁶cm⁻², tilt angles ranging from 0°-10°, and rotation angles ranging from 0°-360°. This model is being implemented into SUPREM III, a widely used process simulation program used in the semiconductor industry in order to demonstrate the model. In this manner, the model will allow users to generate, in a highly computationally-efficient manner, accurate 1-D boron profiles as a function of implant dose, energy, oxide thickness, tilt angle and rotation angle
Atomic layer deposition of amorphous hafnium-based thin films with enhance thermal stabilities
(2010-12) Wang, Tuo, 1983-; Ekerdt, John G.; Demkov, Alexander A.; Hwang, Gyeong S.; Korgel, Brian A.; Mullins, C. B.
The continuous scaling of microelectronic devices requires high permittivity (high-k) dielectrics to replace SiO₂ as the gate material. HfO₂ is one of the most promising candidates but the crystallization temperature of amorphous HfO₂ is too low to withstand the fabrication process. To enhance the film thermal stability, HfO₂ is deposited using atomic layer deposition (ALD), and incorporated with various amorphizers, such as La₂O₃, Al₂O₃, and Ta₂O₅. The incorporation is achieved by growing multiple ALD layers of HfO₂ and one ALD layer of MO[subscript x] (M = La, Al, and Ta) alternately (denoted as [xHf + 1M]), and the incorporation concentration can be effectively controlled by the HfO₂-to-MO[subscript x] ALD cycle ratio (the x value). The crystallization temperature of 10 nm HfO₂ increases from 500 °C to 900 °C for 10 nm [xHf + 1M] film, where x = 3, 3, and 1 for M = La, Al, and Ta, respectively. The incorporation of La₂O₃, and Ta₂O₅ will not compromise the dielectric constant of the film because of the high-k nature of La₂O₃, and Ta₂O₅. Angle resolved X-ray photoelectron spectroscopy (AR-XPS) reveals that when the HfO₂-to-MO[scubscript x] ALD cycle ratio is large enough (x > 3 and 4 for La and Al, respectively), periodic structures exist in films grown by this method, which are comprised of repeated M-free HfO₂ ultrathin layers sandwiched between HfM[subscript x]O[scubscript y] layers. Generally, the film thermal stability increases with thinner overall thickness, higher incorporation concentration, and stronger amorphizing capability of the incorporated elements. When the x value is low, the films are more like homogeneous films, with thermal stabilities determined by the film thickness and the amorphizer. When the x value is large enough, the periodically-repeated structure may add an extra factor to stabilize the amorphous phase. For the same incorporation concentration, films with an appropriately high periodicity may have an increased thermal stability. The manner by which the periodic structure and incorporated element affect thermal stability is explored and resolved using nanolaminates comprised of alternating layers of [scubscript y]HfO₂ and [xHf + 1M] × n, where y varied from 2 to 20, x varied from 1 to 2, and n varied from 4 to 22.
Characterization of low density oxide surface sites using fluorescent probes
(2013-12) McCrate, Joseph Michael; Ekerdt, John G.
Low density surface sites are believed to play an important role in processes occurring on oxide surfaces, including catalysis and particle and film nucleation. However, our understanding of the role and chemical nature of such sites play in these processes is limited by the inability to experimentally detect minority surface sites in many oxide systems. The research performed for this dissertation is focused on developing a surface science technique utilizing fluorescent molecules to titrate specific surface sites on planar fused silica surfaces in an ultra-high vacuum (UHV) environment. High sensitivity (low detection limit) is achieved by using derivatives of perylene, a high quantum yield fluorophore. High specificity is attained by employing perylene derivatives with functional groups designed to react chemically with and titrate various sites. In addition to titrating the well-studied hydroxyl sites with perylene-3-methanol (density ~ 10¹⁴ cm⁻²), which is used to establish the technique, the detection of strained siloxane sites (~ 10¹² cm⁻²), ) with perylene-3-methanamine and oxygen vacancy sites (~ 10¹¹ cm⁻²), ) with 3-vinyl perylene is demonstrated. Particle nucleation on oxides is suspected to involve defects that trap adatoms and form critical nuclei. Using this technique, the possible role strained siloxane and oxygen vacancy sites play in trapping adatoms during the nucleation of Ge nanoparticles on silica surfaces is examined.
The effect of epitaxial strain and R³+ magnetism on the interfaces between polar perovskites and SrTiO₃
(2011-05) Monti, Mark Charles; Markert, John T.; Markert, John T.; de Lozanne, Alex; Tsoi, Maxim; Yao, Zhen; Campion, Alan
We have embarked on a systematic study of novel charge states at oxide interfaces. We have performed pulsed laser deposition (PLD) growth of epitaxial oxide thin films on single crystal oxide substrates. We studied the effects of epitaxial strain and rare-earth composition of the metal oxide thin films. We have successfully created TiO₂ terminated SrTiO₃ (STO) substrates and have grown epitaxial thin films of LaAlO₃ (LAO), LaGaO₃ (LGO), and RAlO₃ on STO using a KrF pulsed excimer laser. Current work emphasizes the importance of understanding the effect of both epitaxial strain and R³+ magnetism on the interface between RAlO₃ and STO. We have demonstrated that the interfaces between LAO/STO and LGO/STO are metallic with carrier concentrations of 1.1 x 10¹⁴ cm[superscript -2] and 4.5 x 10¹⁴ cm[superscript −2], respectively. Rare-earth aluminate films, RAlO₃, with R = Ce, Pr, Nd, Sm, Eu, Gd, and Tb, were also grown on STO. Conducting interfaces were found for R = Pr, Nd and Gd, and the results indicate that for R [does not equal] La the magnetic nature of the R³+ ion causes increased scattering with decreasing temperature that is modeled by the Kondo effect. Epitaxial strain between the polar RAlO₃ films and STO appears to play a crucial role in the transport properties of the metallic interface, where a decrease in the R³+ ion size causes an increase in sheet resistance and an increase in the onset temperatures for increased scattering.
Emerging phenomena in oxide heterostructures
(2010-08) Lee, Jaekwang; Demkov, Alexander A.; Kleinman, Leonard; Chelikowsky, James R.; Macdonald, Allan H.; Hwang, Gyeong S.
Oxide interfaces have attracted considerable attention in recent years due to emerging novel properties that do not exist in the corresponding parent compounds. Furthermore, modern atomic-scale growth and probe techniques enable the formation and study of new artificial interface states distinct from the bulk state. A central issue in controlling the novel behavior in oxide heterostructures is to understand how various physical variables (spin, charge, lattice and/or orbital hybridization) interact with each other. In particular, density function theory (DFT) has provided significant insight into underlying physics of materials at the atomic level, giving quantitative results consistent with experiment. In this dissertation using density functional theory methods, we explore the electronic, magnetic and structural properties developed near the interface in SrTiO3/LaAlO3, EuO/LaAlO3, Fe/PbTiO3/Pt, Fe//BaTiO3/Pt and Cs/SrTiO3 heterostructures. We study the interplay between physical interactions, and quantify parameters that determine physical properties of hetetrostructures. These theoretical studies help understanding how physical variables couple with each other and how they determine new properties at oxide interfaces.
First principle study of transition metal oxide (catalytic) electrodes for electrochemical energy technologies
(2017-08-08) Tsai, Yu-Hao; Hwang, Gyeong S.; Manthiram, Arumugam; Yu, Guihua; Ferreira, Paulo
TTo fulfill the needs for developing the alternative energy technologies, searching for the adequate electrode materials which catalyze the electrochemical reactions utilized in devices such as fuel cell, Li-ion batteries, and related applications such as hydrogen generation and storage, has been a longstanding challenge. Among various catalysts, transition metal oxides (TMO) draw great attentions due to their low-cost, high stability, and, most importantly, a great variety of structures and electrical properties. Nonetheless, studying electrochemical reactions catalyzed by TMO is a challenging task due to the possible multivalent systems, flexible coordinations of lattice atoms, adjustable surface structures and diverse surface species. In the past decades, many innovative approaches have been explored with encouraging results; however, the mechanisms of incorporating the bulk/surface TMO structures in various chemical reactions still remain unclear. In this dissertation, using quantum mechanical calculations, we attempt to improve the fundamental understandings of how structures and electronic properties of TMO materials facilitate the electrochemical reactions of interest. To identify the possible causes for CuO and Cu structures having different selectivity in catalysis, in Chapter 3, we study the CO₂ reduction reaction (CO2RR) catalyzed by CuO (111) surface structure, and compare the results with the more widely studied Cu (100) surface. The roles played by the electronic properties of two materials in their selectivity are elucidated. In Chapter 4 and 5, we study the oxygen evolution reaction (OER) for LiCoO₂ surface structure. The structures and stabilities of Li-, O-, and H-terminated surface are investigated comprehensively. Based on the results, the formation of H-terminated surface results from Li/H exchange at the solid/liquid interface is proposed (Chapter 4). Along with the findings, we explore the possible mechanisms for the OER for non-metal terminated LiCoO₂ surface (Chapter 5). In Chapter 6, we study the oxygen reduction reaction (ORR) for Co₃O₄ (111) H-terminated surface structure. The possible reaction steps for both four-electron and two-electron pathway are investigated. In Chapter 7, the PO₄-decicient LiFePO₄/FePO₄ structures are investigated to understand how the presence of polyanion defects in the matrices could potentially improve the performance of the materials as electrodes in Li-ion batteries.
First principles-based atomistic modeling of the structural properties of silicon-oxide nanomaterials
(2010-08) Lee, Sangheon, 1978-; Hwang, Gyeong S.; Chelikowsky, James R.; Banerjee, Sanjay K.; Ekerdt, John G.; Mullins, C. B.
We have developed continuous random network (CRN) model based Metropolis Monte Carlo simulation tools which are capable of predicting the structural properties of amorphous semiconductor and oxide materials as well as their interface. To bolster the reliability of the CRN model, we have developed force fields based on gradient corrected density functional theory (DFT) calculations. Our in-house CRN-MMC tools have been massively parallelized, which allows us to create fairly large model structures within a reasonable computational time. Using the integrated CRN-MMC tools, we have elucidated the complex growth and structure of self-interstitial and vacancy clusters in silicon and the effect of strain on the structure and stability of the defect clusters. Our work for vacancy clusters suggests that small vacancy defects exclusively favor fourfold-coordination thermodynamically with no significant kinetic limitation rather than void-like structure formation, which has widely been adapted to explain the behavior and properties of vacancy defects. Our results also highlight the identification of stable high-symmetry fourfold-coordinated V₁₂ and V₃₂ clusters that could be expected to exist to a large extent in a vacancy rich region although its direct characterization appears impractical at present. Our work for self-interstitial clusters provides the first theoretical support for earlier experiments which suggest a shape transition from compact to elongated structures around n = 10. When the cluster size is smaller than 10, the stable I₄ and I₈ compact clusters are found to inhibit the formation of elongated defects, whereas the newly discovered fourfold-coordinated I₁₂ state is found to serve as an effective nucleation center for large extended defects. Our CRN-MMC approach also enabled us to elucidate the underlying mechanisms of synthesis and manipulation of Si rich insulators as well as the fundamental understanding of the relationship between the atomic structure and properties. We developed a valence force field based on a modified Keating model for the structure and energetics of amorphous Si rich oxide materials. In particular, our work emphasizes the importance of correctly describing the wide Si-O-Si angle distribution. Our work also suggests that the relative rigidity between Si and SiO₂ matrices is critical in determination of the Si/SiO₂ interface structure. The present potential model coupled with the CRN-MMC method can be used to create structural models (free of coordination defects) for complex a-SiO[subscript x]-based materials, which will further allow thorough studies of the properties of these materials.
Mixing of t_{2g}-e_{g} orbitals in 4d and 5d transition metal oxides
(2018-02-26) Stamokostas, Georgios; Fiete, Gregory A.
Using exact diagonalization, we study the spin-orbit coupling and interaction-induced mixing between t2g and eg d-orbital states in a cubic crystalline environment, as commonly occurs in transition metal oxides. We make a direct comparison with the widely used t2g only or eg only model, depending on electronic ﬁlling. We consider all electron ﬁllings of the d-shell and compute the total magnetic moment, the spin, the occupancy of each orbital, and the eﬀective spin-orbit coupling strength (renormalized through interaction eﬀects) in terms of the bare interaction parameters, spin-orbit coupling, and crystal ﬁeld splitting, focusing on the parameter ranges relevant to 4d and 5d transition metal oxides. In various limits we provide perturbative results consistent with our numerical calculations. We ﬁnd that the t2g-eg mixing can be large, with up to 20% occupation of orbitals that are nominally “empty”, which has experimental implications for the interpretation of the branching ratio in experiments, and can impact the eﬀective local moment Hamiltonian used to study magnetic phases and magnetic excitations in transition metal oxides. Our results can aid the theoretical interpretation of experiments on these materials, which often fall in a regime of intermediate coupling with respect to electron-electron interactions.
Multiscale modeling of formation and structure of oxide embedded silicon and germanium nanocrystals
(2005) Yu, Decai; Hwang, Gyeong S.
This thesis research involves the development of theoretical foundations for studying the synthesis and structure of oxide-embedded silicon and germanium nanocrystals, by integrating various state-of-the-art theoretical techniques at different time and length scales. The primary focus was placed on (1) investigation of mechanisms underlying the formation of silicon and germanium nanocrystals in an oxide matrix and (2) development of kinetic models capable of predicting the structural properties of the silicon-germanium-oxide nanosystem under various processing conditions. The discovery of efficient room temperature luminescence has generated significant interest in silicon and germanium nanocrystals embedded in an oxide matrix because of their potential applications in electronic, optoelectronic, and optical devices in Si-compatible technology. Earlier experimental investigations have suggested the absorption and luminescence properties of the embedded nanocrystal systems would be governed by a complex combination of: nanocrystal sizes, shapes, and size distributions; crystal-matrix interface structures, bonding, and defects; and matrix structure and composition. This may imply that atomic-level control of such structural properties would offer great opportunities in the development of silicon/germanium nanocrystal based novel devices. However, even the fundamental mechanics of the growth and structure of embedded silicon and germanium nanocrystals are still unclear. Using multiscale modeling and simulation, we have identified mechanisms underlying the formation of silicon and germanium nanocrystals in a silicon-rich oxide matrix. Our multiscale approach combines: first principles quantum mechanical calculations of fundamental processes; Metropolis Monte Carlo simulations of amorphous structures; and kinetic Monte Carlo simulations of long-time scale growth. We find that the formation of oxide embedded silicon clusters is primarily attributed to a chemical phase separation to silicon and silicon dioxide, which is mainly driven by suboxide penalty, with a minor contribution of strain. The phase separation turns out to be primarily controlled by oxygen out-diffusion from silicon-rich regions, rather than excess silicon diffusion and agglomeration. From kinetic Monte Carlo simulations based on these fundamental findings we identify two growth characteristics: “coalescence-like” and “pseudo Ostwald ripening”. The simulation results agree well with experimental observations of strong dependence of the cluster size on the initial Si supersaturation and rapid formation of Si clusters at the early stages of annealing with very slow ripening. On the other hand, we find that the formation of gemanium nanocrystals in a silicon dioxide matrix is attributed to the diffusion and agglomeration of germanium precipitates. We have also determined the atomic structure and stability of embedded silicon nanocrystals and the mechanisms of silicon oxidation. While current experimental techniques are still limited to providing complementary atomic-level, real-space information, our comprehensive multiscale modeling based on first-principles quantum mechanics contributes greatly to understanding the fundamental behavior and properties of the silicon-germanium-oxide nanosystems.
Oxide-metal nanoparticles using laser ablation of microparticle aerosols
(2009-08) Nahar, Manuj; Kovar, Desiderio; Becker, Miachel F.
We have studied a continuous aerosol process for producing oxide nanoparticles with sizes of 10-60 nm that are decorated with smaller 1-3 nm metallic nanoparticles. Such particles may be useful in a number of areas including catalysis and as contrast enhancement agents in biomarkers. To produce the oxide nanoparticle carriers, an aerosol of 1-10 [micrometer] oxide particles are ablated using an excimer laser. The resulting oxide nanoparticle aerosol is then mixed with 1-2 [micrometer] metallic particles and this mixed aerosol is ablated a second time. The oxide nanoparticles are too small to ablate but act as seeds for the nucleation of metallic nanoparticles on the surface of the oxide. The nanoparticle sizes can be varied by changing the gas type or gas pressure in the aerosol. We demonstrate the feasibility of such an approach using two oxides, SiO₂ and TiO₂, and two metals, Au and Ag.
Wide band gap oxide-semiconductor heterostructures grown by molecular beam epitaxy
(2020-12-04) Hadamek, Tobias; Demkov, Alexander A.; De Lozanne, Alejandro L; Ekerdt, John G; Lai, Keji; Tsoi, Maxim
Wide band gap oxides and semiconductors will have tremendous impact on future energy efficient and environmentally sustainable electronics. Wide gap semiconductors like GaN and AlGaN are important in light emitting diode applications and for high-frequency telecom and radar applications like base stations for upcoming 5G networks. Further, these materials along with some wide band gap semiconducting oxides like Ga₂O₃ may prove to be invaluable for medium to high power electronics applications, starting from switching power supplies used to charge batteries in consumer devices like smartphones and laptops, to high-power supplies that can charge electric car batteries and are suitable for electric grid and power transmission line applications. Basic materials studies of these material systems are therefore in high demand. In this dissertation I will present materials studies on wide band gap oxide thin films grown by molecular beam epitaxy on crystalline semiconductor substrates. The oxide thin films are characterized with regards to epitaxial structure and electronic structure by electron and x-ray diffraction techniques, by photoelectron spectroscopy and in collaboration with researchers from UT Dallas, Arizona State University, Rutgers University and University of Turku by transmission electron microscopy, inverse photoemission spectroscopy and scanning tunneling spectroscopy. Two materials systems are discussed in detail: 1. The rare-earth sesquioxide Eu₂O₃ in regards to potential gate-dielectric applications on the wide band gap semiconductor GaN. The focus of the studies were interface quality, structural quality, and band offsets; and the electronic structure of Eu₂O₃ to determine the band gap and understand the influence of Eu 4f states on the band gap of Eu₂O₃. 2. The structural integration of ultra-wide band gap oxide semiconductor Ga₂O₃ on a standard Si semiconductor substrate. Epitaxial integration of Ga₂O₃ with the workhorse of semiconductors Si can enable cost-reduction & monolithically-integrated devices. The focus of the studies was the structural characterization of the Ga₂O₃ layers grown by plasma-assisted molecular beam epitaxy.