Person: Wang, Xinwei
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Wang
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Xinwei
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Wang, Xinwei
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Publication Atomic Layer Deposition of \(Sc_2O_3\) for Passivating AlGaN/GaN High Electron Mobility Transistor Devices(American Institute of Physics, 2012) Wang, Xinwei; Saadat, Omair I.; Xi, Bin; Lou, Xiabing; Molnar, Richard J.; Palacios, Tomás; Gordon, RoyPolycrystalline, partially epitaxial \(Sc_2O_3\) films were grown on AlGaN/GaN substrates by atomic layer deposition (ALD). With this ALD \(Sc_2O_3\) film as the insulator layer, the \(Sc_2O_3\)/AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors showed excellent electrical performance with a high I\(_{on}\)/I\(_{off}\) ratio of over 10\(^8\) and a low subthreshold slope of 75 mV/dec. The UV/NH\(_4\)OH surface treatment on AlGaN/GaN prior to ALD was found to be critical for achieving these excellent figures. In addition, the \(Sc_2O_3\)dielectric is found to be negatively charged, which facilitates the enhancement-mode operation. While bare \(Sc_2O_3\) suffers from moisture degradation, depositing a moisture blocking layer of ALD \(Al_2O_3\) can effectively eliminate this effect.Publication Synthesis of Vanadium Dioxide Thin Films on Conducting Oxides and Metal–Insulator Transition Characteristics(Elsevier, 2012) Cui, Yanjie; Wang, Xinwei; Zhou, You; Gordon, Roy; Ramanathan, ShriramWe report on growth and physical properties of vanadium dioxide \((VO_2)\) films on model conducting oxide underlayers (Nb-doped \(SrTiO_3\) and \(RuO_2\) buffered \(TiO_2\) single crystals). The \(VO_2\) films, synthesized by rf sputtering, are highly textured as seen from X-ray diffraction. The \(VO_2\) film grown on Nb doped \(SrTiO_3\) shows over two orders of magnitude metal–insulator transition, while \(VO_2\) film on \(RuO_2\) buffered \(TiO_2\) shows a smaller resistance change but with an interesting two step transition. X-ray photoelectron spectroscopy has been performed as a function of depth on both sets of structures to provide mechanistic understanding of the transition characteristics. We then investigate voltage-driven transition in the \(VO_2\) films grown on Nb-doped \(SrTiO_3\) substrate as a function of temperature. The present study contributes to efforts towards correlated oxide electronics utilizing phase transitions.Publication Epitaxial Growth of MgxCa1–xO on GaN by Atomic Layer Deposition(American Chemical Society (ACS), 2016) Lou, Xiabing; Zhou, Hong; Kim, Sang Bok; Alghamdi, Sami; Gong, Xian; Feng, Jun; Wang, Xinwei; Ye, Peide D.; Gordon, RoyWe demonstrate for the first time that a singlecrystalline epitaxial MgxCa1−xO film can be deposited on gallium nitride (GaN) by atomic layer deposition (ALD). By adjusting the ratio between the amounts of Mg and Ca in the film, a lattice matched MgxCa1−xO/GaN(0001) interface can be achieved with low interfacial defect density. High-resolution X-ray diffraction (XRD) shows that the lattice parameter of this ternary oxide nearly obeys Vegard’s law. An atomically sharp interface from cross-sectional transmission electron microscopy (TEM) confirmed the high quality of the epitaxy.High-temperature capacitance−voltage characterization showed that the film with composition Mg0.25Ca0.75O has the lowest interfacial defect density. With this optimal oxide composition, a Mg0.25Ca0.75O/AlGaN/GaN metal−oxide −semiconductor high-electron-mobility (MOS-HEMT) device was fabricated. An ultrahigh on/off ratio of 1012 and a near ideal SS of 62 mV/dec were achieved with this device.Publication Smooth, Low-Resistance, Pinhole-free, Conformal Ruthenium Films by Pulsed Chemical Vapor Deposition(Electrochemical Society, 2013) Wang, Xinwei; Gordon, RoyRuthenium (Ru) thin films were deposited by pulsed chemical vapor deposition with precursors bis(N,N′-di-tert-butylacetamidinato)ruthenium(II)dicarbonyl, ammonia and hydrogen. Low-resistance polycrystalline Ru films with bulk density were obtained. Good adhesion to \(SiO_2\) substrates was achieved by introducing a thin layer of WN in between the Ru and the \(SiO_2\). Ru films only \(\sim 2\) nm thick fully covered the WN layer without any pinholes. Deposition of Ru inside narrow holes showed that good conformality was obtained by lowering the deposition temperature. The film surface was smooth, and the rms roughness value did not increase too much after rapid thermal annealing at 700°C.Publication III-V 4D Transistors(Institute of Electrical and Electronics Engineers, 2012) Gu, J.J.; Wang, Xinwei; Shao, J.; Neal, A.T.; Manfra, M.J.; Gordon, Roy; Ye, P.D.We fabricated for the first time vertically and laterally integrated III-V 4D transistors. III-V gate-all-around (GAA) nanowire MOSFETs with \(3×4\) arrays show high drive current of \(1.35mA/ \mu m\) and high transconductance of \(0.85mS/ \mu m\). The vertical stacking of the III-V nanowires have provided an elegant solution to the drivability bottleneck of nanowire devices and is promising for future low-power logic and RF application.Publication Applications of Vapor Deposition in Microelectronics and Dye-Sensitized Solar Cells(2012-10-31) Wang, Xinwei; Gordon, Roy Gerald; Spaepen, Frans; Aziz, MichaelOver the past decades, vapor deposition of thin films has gained wide interest in both industry and academia, and a variety of its applications have been demonstrated. As one of the most promising vapor deposition techniques, atomic layer deposition (ALD) and its applications in microelectronics and dye-sensitized solar cells are extensively investigated in this dissertation. ALD has many distinct features including low temperature processing, self-limiting growth, and precise control of film composition and thickness. Thus, ALD is considered to be suitable for conformal coating of 3D nanostructures, such as nanoporous structures, high aspect-ratio trench or hole structures, and so forth. Additionally, pulsed chemical vapor deposition (CVD) and its applications in microelectronics are explored in this dissertation. Ruthenium (Ru) is a promising electrode material for next generation microelectronic devices. The ALD and pulsed CVD processes discussed in Chapter 2 provide several approaches to produce smooth, conformal, pin-hole free Ru metal thin films. High-quality Ru films can be made under either oxidizing ambient or reducing ambient, which provides more flexibility for applications in microelectronics. Conductive ruthenium dioxide \((RuO_2)\) is also considered as a promising microelectrode material. Chapter 3 demonstrates a pulsed CVD process of depositing pure, smooth \((RuO_2)\) films with reasonably low resistivity. Chapter 3 also demonstrates that \((RuO_2)\) can be epitaxially grown on rutile \(TiO_2(011)\) with a high-quality coherent heteroepitaxy structure. III-V MOSFET is now a very active area of growing interest to researchers and engineers in electronic industry and academia. Applications of ALD WN and high-k oxide materials for GaAs and GaN based devices are investigated in Chapters 4 and 5. Taking advantage of the conformal-coating feature of ALD, a stack of gate dielectric and metal gate can be coated uniformly around suspended nanowire structures, which is crucial for well-behaved gate-all-around MOSFETs. III-V MOSFETs also generally lack a suitable dielectric layer that has low interface trap density \((D_{it})\). Epitaxial ALD high-k dielectric lanthanum yttrium oxide, grown on GaAs(111)A, is found to have a fairly low \((D_{it})\), and therefore, the electrical properties are dramatically improved with its inclusion. This finding is very insightful for the applications of next generation III-V MOSFETs. In addition, a few ALD processes of candidate dielectric materials for GaN based devices are discussed. Dye-sensitized solar cells have great potential to compete with conventional p-n junction solar cells due to their relatively low cost. However, their efficiency is limited by the ease with which electrons collected by the nanoparticle framework can recombine with ions in solution. As discussed in Chapter 6, by depositing insulating and transparent \(SiO_2\) selectively onto the open areas of nanoparticulate \(TiO_2\) surface, while avoiding any deposition of \(SiO_2\) over or under the organic dye molecules, the solar cell efficiency can be significantly improved.