Browsing by Author "Wallace, Robert M."
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Item Accumulation Capacitance Frequency Dispersion of Ⅲ-Ⅴ Metal-Insulator-Semiconductor Devices due to Disorder Induced Gap States(American Institute of Physics Inc., 2014-07-07) Galatage, R. V.; Zhernokletov, Dmitry M.; Dong, Hong; Brennan, Barry; Hinkle, Christopher L.; Wallace, Robert M.; Vogel, E. M.The origin of the anomalous frequency dispersion in accumulation capacitance of metal-insulator-semiconductor devices on InGaAs and InP substrates is investigated using modeling, electrical characterization, and chemical characterization. A comparison of the border trap model and the disorder induced gap state model for frequency dispersion is performed. The fitting of both models to experimental data indicate that the defects responsible for the measured dispersion are within approximately 0.8nm of the surface of the crystalline semiconductor. The correlation between the spectroscopically detected bonding states at the dielectric/III-V interface, the interfacial defect density determined using capacitance-voltage, and modeled capacitance-voltage response strongly suggests that these defects are associated with the disruption of the III-V atomic bonding and not border traps associated with bonding defects within the high-k dielectric.Item Al₂O₃ on WSe₂ by Ozone Based Atomic Layer Deposition: Nucleation and Interface StudyAzcatl, Angelica; Wang, Qingxiao; Kim, Moon J.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); Azcatl, Angelica; Wang, Qingxiao; Kim, Moon J.; Wallace, Robert M.In this work, the atomic layer deposition process using ozone and trimethylaluminum (TMA) for the deposition of Al₂O₃ films on WSe₂ was investigated. It was found that the ozone-based atomic layer deposition enhanced the nucleation of Al₂O₃ in comparison to the water/TMA process. In addition, the chemistry at the Al₂O₃ / WSe₂ interface and the surface morphology of the Al₂O₃ films exhibited a dependence on the deposition temperature. A non-covalent functionalizing effect of ozone on WSe₂ at low deposition temperatures 30 ⁰C was identified which prevented the formation of pinholes in the Al₂O₃ films. These findings aim to provide an approach to obtain high-quality gate dielectrics on WSe₂ for two-dimensional transistor applications.Item Atomic Layer Deposition Application in Interconnect Technology: From Material Understanding to Area Selective Deposition(December 2021) Hwang, Su Min; Kim, Jiyoung; Kehtarnavaz, Nasser; Quevedo-Lopez, Manuel; Alavrez, Daniel; Wallace, Robert M.; Walker, Amy V.Atomic layer deposition (ALD) technique has been widely employed in the semiconductor industry. As the devices continuously scaling down to sub 3 nm, SiOx and SiNx thin films, for applications such as a spacer or an etch stopper, are expected to satisfy stringent requirements (e.g., precise thickness control, high bulk film density, high wet-etch resistance, conformality, and lowthermal budget) in the current back-end-of-line process. Besides the film qualities, challenges in the current “top-down” approach also need to be addressed to reduce the size of the devices. Moreover, the lack of a fundamental understanding of surface chemistry using in-situ characterization can further impede future interconnect technology. This dissertation focuses on the application of the ALD process for current and future interconnect technology applications. The first study is ALD of high-quality SiOx and SiNx films with lowtemperature feasibility. With the structural modification of conventional Si precursors (e.g., the addition of ligand or substitution of terminating groups), the molecular polarity of precursors is changed, resulting in the improvement of surface reactivities. By leveraging the unique structure of the Si sources, the film deposition at low temperature with enhanced film properties can be achieved. Secondly, this dissertation further identifies the correlation between the metal surface condition and physical/chemical stability of passivation materials in application to the areaselective deposition process. Using in-situ reflectance absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), and high-resolution TEM analysis, the issues arising with poor ALD selectivity are identified. After analyzing the issues, the potential solution to provide a high-quality SAM monolayer is demonstrated. Lastly, a cleaning process using a noble metal cleaning agent, in which a clean metal surface at low temperature (< 200 o C) can be achieved, is developed. The unique cleaning process could pave the way for the implementation of the consecutive organic-free area-selective-deposition process.Item Atomically Thin Resonant Tunnel Diodes Built from Synthetic van der Waals Heterostructures(Nature Pub. Group) Lin, Yu-Chuan; Ghosh, Ram Krishna; Addou, Rafik; Lu, Ning; Eichfeld, Sarah M.; Zhu, Hui; Li, Ming-Yang; Peng, Xin; Kim, Moon J.; Li, Lain-Jong; Wallace, Robert M.; Datta, Suman; Robinson, Joshua A.; A-5283-2008 (Wallace, RM); A-2297-2010 (Kim, MJ)Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂). The realization of MoS₂-WSe₂-graphene and WSe₂-MoS₂-graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.;Item Characterizing and Engineering the Metal Contact Interface in 1D and 2D Chalcogenide Systems(2018-12) Smyth, Christopher M.; 0000-0003-4668-9555 (Smyth, CM); Wallace, Robert M.; Hinkle, Christopher L.The 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.Item Chemical and Electrical Characterization of the HfO2/InAlAs InterfaceBrennan, Barry; Galatage, Rohit V.; Thomas, K.; Pelucchi, E.; Hurley, P. K.; Kim, Jiyoung; Hinkle, Christopher L.; Vogel, E. M.; Wallace, Robert M.InAlAs has the potential to be used as a barrier layer in buried channel quantum well field effect transistor devices due to favorable lattice-matching and carrier confinement properties with InGaAs. Field effect device structures of this nature may also require a high-k oxide deposited on the InAlAs surface to reduce leakage current. This study investigates the impact of surface preparations and atomic layer deposition of HfO2 on these surfaces using x-ray photoelectron spectroscopy to analyse the chemical interactions taking place, as well as the electrical performance of associated capacitor devices. A large concentration of As related surface features is observed at the InAlAs surface, and is attributed to a large Dit response in electrical measurements.Item Covalent Nitrogen Doping in Molecular Beam Epitaxy-Grown and Bulk WSe₂(American Institute of Physics Inc, 2018-10-22) Khosravi, Ava; Addou, Rafik; Smyth, Christopher M.; Yue, Ruoyu; Cormier, Christopher R.; Kim, Jiyoung; Hinkle, Christopher L.; Wallace, Robert M.; 0000-0002-5454-0315 (Addou, R); 0000-0003-2781-5149 (Kim, J); 0000-0001-5566-4806 (Wallace, RM); 70133685 (Kim, J); Addou, Rafik; Smyth, Christopher M.; Yue, Ruoyu; Cormier, Christopher R.; Kim, Jiyoung; Hinkle, Christopher L.; Wallace, Robert M.Covalent 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.Item A Crystalline Oxide Passivation for Al₂O₃/AlGaN/GaN(American Institute of Physics Inc.) Qin, Xiaoye; Dong, Hong; Kim, Jiyoung; Wallace, Robert M.; A-5283-2008 (Wallace, RM)In situ X-ray photoelectron spectroscopy and low energy electron diffraction are performed to study the formation of a crystalline oxide on the AlGaN surface. The oxidation of the AlGaN surface is prepared by annealing and remote N₂+O₂ plasma pretreatments resulting in a stable crystalline oxide. The impact of the oxide on the interface state density is studied by capacitance voltage (C-V) measurements. It is found that a remote plasma exposure at 550⁰ C shows the smallest frequency dispersion. Crystalline oxide formation may provide a novel passivation method for high quality AlGaN/GaN devices.Item A Crystalline Oxide Passivation on In₀․₅₃Ga₀․₄₇As (100)(American Institute of Physics Inc, 2018-09-24) Qin, Xiaoye; Wang, W. -E; Droopad, R.; Rodder, M. S.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); Qin, Xiaoye; Wallace, Robert M.The passivation of In₀․₅₃Ga₀․₄₇As surfaces is highly desired for transistor performance. In this study, the feasibility of a crystalline oxide passivation on In₀․₅₃Ga₀․₄₇As (100) is demonstrated experimentally. The (3 × 1) and (3 × 2) crystalline oxide reconstructions are formed on the de-capped In₀․₅₃Ga₀․₄₇As (100) surfaces through the control of the surface oxidation states. By monitoring the evolution of chemical states and associated structures of the In₀․₅₃Ga₀․₄₇As (100) surfaces upon O₂ and subsequent atomic hydrogen exposure, we find that the control of the Ga oxide states is critical to the formation of the crystalline oxide reconstructions. The stability of the crystalline oxide layers upon the atomic layer deposition of HfO₂ is investigated as well. Furthermore, the capacitance voltage behavior of metal oxide semiconductor capacitors with an HfO₂ dielectric layer reveals that the crystalline oxide reconstructions result in a decrease in the density of interface traps (D_{it}) from ∼1 × 10¹³ cm⁻² eV⁻¹ to ∼1 × 10¹² cm⁻² eV⁻¹ compared with the de-capped surface. The crystalline oxide passivation offers a platform to develop In₀․₅₃Ga₀․₄₇As devices with a low density of interface states.Item Development of Probe-based Devices and Methods for Applications in Micro/Nano Characterization and Fabrication(2022-12-01T06:00:00.000Z) Alemansour, Hamed; Fahimi, Babak; Moheimani, S.O. Reza; Wallace, Robert M.; Ruths, Justin; Summers, Tyler; Randall, JohnThe ability to investigate mechanical and material properties of micro- and nano-sized devices and surfaces is of significance in many fields of science and technology. Scanning tunneling microscope (STM), atomic force microscope (AFM), and MEMS force sensors are in high demand for this purpose, whereas their functionality is limited in many cases. This dissertation focuses on novel scanning methods, MEMS devices, and control design methodologies that facilitate micro/nano characterization and fabrication of devices and surfaces. High-speed and high resolution scanning tunneling spectroscopy (STS), as well as atomically precise fabrication of silicon quantum electronic devices are still among challenging topics in the STM. It is due to the limitation of conventional methods that are used for these purposes. Besides the STM hardware specifications, control methods contribute significantly to achieving the ultimate goal of ultrafast STS and atomically precise manufacturing (APM). The STM control system needs to be improved to harness the STS full potential, as well as to provide adequate precision and robustness during lithography. In part I of this dissertation, we detail our research on the STM-based probe microscopy for applications in surface characterization and atomically precise manufacturing. Modulation technique is conventionally used to obtain dnI/dV n STS images of a surface. However, images that are obtained from this method suffer from noise, which is mainly due to the small amplitude of the modulation voltage. The modulation technique is a powerful tool which enables us to utilize the unused frequency band of the STM for other purposes without disturbing its normal operation. We exploit this technique in different STS and APM methods throughout the part I of this dissertation. First, we propose a modified STM feedback loop to improve the SNR of STS images. Then, a novel STM imaging mode is introduced, which is based on keeping dI/dV constant. We also propose an ultrafast STS method that can provide an I–V curve for every pixel of the image simultaneously with the topography image. This method significantly reduces the spectroscopy time. Finally, we introduce a method for hydrogen depassivation lithography (HDL) with STM. Unlike the conventional approach, where a positive bias voltage is applied to the sample, we have developed an automated scheme to perform HDL at negative bias voltages. We show that our proposed lithography method significantly decreases chance of a tip-sample crash and can potentially increase the lithography precision. Probe-based force measurement systems are also widely utilized to investigate characteristics of micro- and nano-sized devices and surfaces. These systems work based on measuring the interaction forces between a known probe and an unknown surface. In part II of this dissertation, we proceed by proposing novel design and control methods for two well-known probe-based force measurement systems: AFM and MEMS force sensor. AFM plays a crucial role in a myriad of applications in science and technology. It is the most widely used tool for imaging and manipulating matter at the nanoscale. The AFM utilizes a microcantilever with a very sharp tip that interacts with a sample surface. The use of this technology alone does not guarantee its efficient functionality. It is of significant importance to harness the full functionality of an AFM by employing efficient control methods. We implement a positive position feedback controller on a previously designed active microcantilever (Coskun et al., 2017) to achieve a faster cantilever response. Then, we exploit MEMS technology to realize an on-chip MEMS AFM. The tracking performance of the device is enhanced by implementing different control methods. Finally, we propose a MEMS-based force sensor. On the contrary to the AFM that mainly relies on its microcantilever for force measurements, a variety of mechanisms can be incorporated in a MEMS force sensor. This makes it possible to readily adjust the force sensor parameters. Our force sensor features built-in electrostatic actuation, piezoresistive displacement sensor, and stiffness adjustment mechanism. It works in the closed-loop, which mitigates the adverse effect of flexural nonlinearities on the precision of the force measurement.Item Dual-Gate MoS₂ Transistors with Sub-10 NM Top-Gate High-K Dielectrics(American Institute of Physics Inc.) Bolshakov, Pavel; Khosravi, Ava; Zhao, Peng; Hurley, P. K.; Hinkle, Christopher L.; Wallace, Robert M.; Young, Chadwin D.; 0000-0002-3530-6400 (Zhao, P); 0000-0001-5566-4806 (Wallace, RM); 0000-0003-0690-7423 (Young, CD); Bolshakov, Pavel; Khosravi, Ava; Zhao, Peng; Hinkle, Christopher L.; Wallace, Robert M.; Young, Chadwin D.High quality sub-10 nm high-k dielectrics are deposited on top of MoS₂ and evaluated using a dual-gate field effect transistor configuration. Comparison between top-gate HfO₂ and an Al₂O₃/HfO₂ bilayer shows significant improvement in device performance due to the insertion of the thin Al₂O₃ layer. The results show that the Al₂O₃ buffer layer improves the interface quality by effectively reducing the net fixed positive oxide charge at the top-gate MoS₂/high-k dielectric interface. Dual-gate sweeping, where both the top-gate and the back-gate are swept simultaneously, provides significant insight into the role of these oxide charges and improves overall device performance. Dual-gate transistors encapsulated in an Al₂O₃ dielectric demonstrate a near-ideal subthreshold swing of ~60 mV/dec and a high field effect mobility of 100 cm²/V·s.Item Engineering The Palladium-WSe₂ Interface Chemistry for Field Effect Transistors with High-Performance Hole Contacts(Amer Chemical Soc, 2018-12-07) Smyth, Christopher M.; Walsh, Lee A.; Bolshakov, Pavel; Catalano, Massimo; Addou, Rafik; Wang, Luhua; Kim, Jiyoung; Kim, Moon J.; Young, Chadwin D.; Hinkle, Christopher L.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); 0000-0003-0690-7423 (Young, CD); 0000-0003-2781-5149 (Kim, J); 0000-0002-6688-8626 (Walsh, LA); 0000-0002-5485-6600 (Hinkle, CD); 0000-0002-5454-0315 (Addou, R); 70133685 (Kim, J); Smyth, Christopher M.; Walsh, Lee A.; Bolshakov, Pavel; Catalano, Massimo; Addou, Rafik; Wang, Luhua; Kim, Jiyoung; Kim, Moon J.; Young, Chadwin D.; Hinkle, Christopher L.; Wallace, Robert M.Palladium has been widely employed as a hole contact to WSe₂ and has enabled, at times, the highest WSe₂ transistor performance. However, there are orders of magnitude variation across the literature in Pd-WSe₂ contact resistance and I-ON/I-OFF ratios with no true understanding of how to consistently achieve high-performance contacts. In this work, WSe₂ transistors with impressive I-ON/I-OFF ratios of 10(6) and Pd-WSe₂ Schottky diodes with near-zero variability are demonstrated utilizing Ohmic-like Pd contacts through deliberate control of the interface chemistry. The increased concentration of a PdSeₓ intermetallic is correlated with an Ohmic band alignment and concomitant defect passivation, which further reduces the contact resistance, variability, and barrier height inhomogeneity. The lowest contact resistance occurs when a 60 min post-metallization anneal at 400 degrees C in forming gas (FG) is performed. X-ray photoelectron spectroscopy indicates this FG anneal produces 3x the concentration of PdSeₓ and an Ohmic band alignment, in contrast to that detected after annealing in ultrahigh vacuum, during which a 0.2 eV hole Schottky barrier forms. Raman spectroscopy and scanning transmission electron microscopy highlight the necessity of the fabrication step to achieve high-performance contacts as no PdSeₓ forms, and WSe₂ is unperturbed by room temperature Pd deposition. However, at least one WSe₂ layer is consumed by the necessary interface reactions that form PdSeₓ requiring strategic exploitation of a sacrificial WSe₂ layer during device fabrication. The interface chemistry and structural properties are correlated with Pd-WSe₂ diode and transistor performance, and the recommended processing steps are provided to enable reliable high-performance contact formation.Item Enhancing Interconnect Reliability and Performance by Converting Tantalum to 2D Layered Tantalum Sulfide at Low Temperature(Wiley-VCH Verlag, 2019-06-11) Lo, C. -L; Catalano, Massimo; Khosravi, Ava; Ge, W.; Ji, Y.; Zemlyanov, D. Y.; Wang, Luhua; Addou, Rafik; Liu, Y.; Wallace, Robert M.; Kim, Moon J.; Chen, Z.; 0000-0001-5566-4806 (Wallace, RM); Catalano, Massimo; Khosravi, Ava; Wang, Luhua; Addou, Rafik; Wallace, Robert M.; Kim, Moon J.The interconnect half-pitch size will reach ≈20 nm in the coming sub-5 nm technology node. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be >4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultrascaled interconnect will be compromised by the thick bilayer. Therefore, 2D layered materials have been explored as diffusion barrier alternatives. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back-end-of-line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued. Here, a 2D layered tantalum sulfide (TaSₓ) with ≈1.5 nm thickness is developed to replace the conventional TaN/Ta bilayer. The TaSx ultrathin film is industry-friendly, BEOL-compatible, and can be directly prepared on dielectrics. The results show superior barrier/liner properties of TaSₓ compared to the TaN/Ta bilayer. This single-stack material, serving as both a liner and a barrier, will enable continued scaling of interconnects beyond 5 nm node. ©2019 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimItem GaSb Oxide Thermal Stability Studied by Dynamic-XPS(AVS: Science & Technology of Materials, Interfaces, and Processing) McDonnell, Steven; Brennan, Barry; Bursa, Emin; Wallace, Robert M.; Winkler, K.; Baumann, P.The thermal decomposition of the native GaSb oxides is studied using time resolved x-ray photoelectron spectroscopy with a temperature resolution of better than 1 K. The expected transfer of oxygen from Sb-O to Ga-O before the eventual desorption of all oxides is observed. However, an initial reaction resulting in the reduction of Sb₂O₃ along with the concurrent increase in both Ga₂O₃ and Sb₂O₄ is detected in the temperature range of 450-525 K. Using the relative changes in atomic concentrations of the chemical species observed; the initial reaction pathway is proposed.Item High Quality HfO₂/p-GaSb(001) Metal-Oxide-Semiconductor Capacitors with 0.8 Nm Equivalent Oxide ThicknessBarth, Michael; Rayner, G. Bruce,Jr.; McDonnell, Stephen; Wallace, Robert M.; Bennett, Brian R.; Engel-Herbert, Roman; Datta, SumanWe investigate in-situ cleaning of GaSb surfaces and its effect on the electrical performance of p-type GaSb metal-oxide-semiconductor capacitor (MOSCAP) using a remote hydrogen plasma. Ultrathin HfO₂ films grown by atomic layer deposition were used as a high permittivity gate dielectric. Compared to conventional ex-situ chemical cleaning methods, the in-situ GaSb surface treatment resulted in a drastic improvement in the impedance characteristics of the MOSCAPs, directly evidencing a much lower interface trap density and enhanced Fermi level movement efficiency. We demonstrate that by using a combination of ex-situ and in-situ surface cleaning steps, aggressively scaled HfO₂/p-GaSb MOSCAP structures with a low equivalent oxide thickness of 0.8 nm and efficient gate modulation of the surface potential are achieved, allowing to push the Fermi level far away from the valence band edge high up into the band gap of GaSb. (c) 2014 AIP Publishing LLC.Item High-κ Dielectric on ReS₂: In-Situ Thermal Versus Plasma-Enhanced Atomic Layer Deposition of Al₂O₃(MDPI AG, 2019-03-30) Khosravi, Ava; Addou, Rafik; Catalano, Massimo; Kim, Jiyoung; Wallace, Robert M.; 0000-0003-2781-5149 (Kim, J); 0000-0001-5566-4806 (Wallace, RM); 0000-0001-9901-9809 (Khosravi, A); 0000-0002-5454-0315 (Addou, R); 70133685 (Kim, J); Khosravi, Ava; Addou, Rafik; Catalano, Massimo; Kim, Jiyoung; Wallace, Robert M.We report an excellent growth behavior of a high-κ dielectric on ReS₂ , a two-dimensional (2D) transition metal dichalcogenide (TMD). The atomic layer deposition (ALD) of an Al₂O₃ thin film on the UV-Ozone pretreated surface of ReS₂ yields a pinhole free and conformal growth. In-situ half-cycle X-ray photoelectron spectroscopy (XPS) was used to monitor the interfacial chemistry and ex-situ atomic force microscopy (AFM) was used to evaluate the surface morphology. A significant enhancement in the uniformity of the Al₂O₃ thin film was deposited via plasma-enhanced atomic layer deposition (PEALD), while pinhole free Al₂O₃ was achieved using a UV-Ozone pretreatment. The ReS₂ substrate stays intact during all different experiments and processes without any formation of the Re oxide. This work demonstrates that a combination of the ALD process and the formation of weak S-O bonds presents an effective route for a uniform and conformal high-κ dielectric for advanced devices based on 2D materials. © 2019 by the authors.Item Impurity and Silicate Formation Dependence on O₃ Pulse Time and the Growth Temperature in Atomic-Layer-Deposited La₂O₃ Thin Films(American Institute of Physics Inc, 2018-09-24) Park, Tae Joo; Byun, Young-Byun; Wallace, Robert M.; Kim, Jiyoung; Park, Tae Joo; Byun, Young-Byun; Wallace, Robert M.; Kim, JiyoungAtomic-layer-deposited La₂O₃ films were grown on Si with different O₃ pulse times and growth temperatures. The interfacial reactions and impurity behaviors were observed using in situ X-ray photoelectron spectroscopy. Longer pulse time of O₃ formed the solid SiO₂ interfacial barrier layer, which suppressed La-silicate formation. Meanwhile, the carboxyl compound acting as an impurity phase was replaced with LaCO₃ on increasing the O₃ pulse time due to further oxidation and reaction of La. Higher growth temperatures enhanced La-silicate formation by mixed diffusion of Si and La₂O₃, during which most of the La₂O₃ phase was consumed at 400 °C. C and N impurities decreased with increasing growth temperature and completely disappear at 400 °C.Item In situ atomic layer deposition half cycle study of Al2O 3 growth on AlGaN(American Institute of Physics, 2012-11-10) Brennan, Barry; Qin, Xiaoye; Dong, Hong; Kim, Jiyoung; Wallace, Robert M.; 70133685 (Kim, J)Use the DOI address to see the article abstract. A subscription or fee may be necessary to view the article.Item In Situ Characterization of Phase Transition and Defect Dynamics in Molybdenum Ditelluride(2020-04-17) Wang, Qingxiao; Kim, Moon J.; Wallace, Robert M.Transition metal dichalcogenides (TMDs) are regarded as promising materials for emerging applications, including electronic devices, photonic devices, biosensors, and energy storage, etc. Owing to their novel structures and extraordinary properties, they have provided the researchers with an excellent platform to explore low-dimensional physics. However, some challenges need to be resolved before their practical application. The phases and defects in TMDs can significantly affect their properties. Therefore, understanding the phase transition and defects in TMDs would be of great importance to advance their further application. This dissertation focuses on the identification and characterization of a novel phase transition from two dimensional MoTe2 phase to one dimensional Mo6Te6 nanowire phase during the vacuum annealing. Furthermore, the thermal stability of MoTe2 is extensively investigated. In particular, the inversion domain boundaries formed during the vacuum annealing are identified to be a defective interface in MoTe2. The role of Te vacancy to the evolution of inversion domain boundaries is extensively studied. Also, a possible strategy to improve its thermal stability is demonstrated.Item In Situ Plasma Enhanced Atomic Layer Deposition Half Cycle Study of Al₂O₃ on AlGaN/GaN High Electron Mobility Transistors(American Institute of Physics Inc.) Qin, Xiaoye; Wallace, Robert M.A half cycle study of plasma enhanced atomic layer deposited (PEALD) Al₂O₃ on AlGaN is investigated using in situ X-ray photoelectron spectroscopy, low energy ion scattering, and ex situ electrical characterizations. A faster nucleation or growth is detected from PEALD relative to purely thermal ALD using an H₂O precursor. The remote O₂ plasma oxidizes the AlGaN surface slightly at the initial stage, which passivates the surface and reduces the OFF-state leakage. This work demonstrates that PEALD is a useful strategy for Al₂O₃ growth on AlGaN/GaN devices. © 2015 AIP Publishing LLC.