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Top 20 Most Read Articles
May 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Photovoltaic manufacturing: Present status, future prospects, and research needs J. Vac. Sci. Technol. A 29, 030801 (2011); http://dx.doi.org/10.1116/1.3569757 (16 pages) Online Publication Date: 29 March 2011
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In May 2010 the United States National Science Foundation sponsored a two-day workshop to review the state-of-the-art and research challenges in photovoltaic (PV) manufacturing. This article summarizes the major conclusions and outcomes from this workshop, which was focused on identifying the science that needs to be done to help accelerate PV manufacturing. A significant portion of the article focuses on assessing the current status of and future opportunities in the major PV manufacturing technologies. These are solar cells based on crystalline silicon (c-Si), thin films of cadmium telluride (CdTe), thin films of copper indium gallium diselenide, and thin films of hydrogenated amorphous and nanocrystalline silicon. Current trends indicate that the cost per watt of c-Si and CdTe solar cells are being reduced to levels beyond the constraints commonly associated with these technologies. With a focus on TW/yr production capacity, the issue of material availability is discussed along with the emerging technologies of dye-sensitized solar cells and organic photovoltaics that are potentially less constrained by elemental abundance. Lastly, recommendations are made for research investment, with an emphasis on those areas that are expected to have cross-cutting impact.
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Status and potential of atmospheric plasma processing of materials J. Vac. Sci. Technol. A 29, 020801 (2011); http://dx.doi.org/10.1116/1.3559547 (17 pages) Online Publication Date: 4 March 2011
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This paper is a review of the current status and potential of atmospheric plasma technology for materials processing. The main focus is the recent developments in the area of dielectric barrier discharges with emphasis in the functionalization of polymers, deposition of organic and inorganic coatings, and plasma processing of biomaterials. A brief overview of both the equipment being used and the physicochemical reactions occurring in the gas phase is also presented. Atmospheric plasma technology offers major industrial, economic, and environmental advantages over other conventional processing methods. At the same time there is also tremendous potential for future research and applications involving both the industrial and academic world.
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Comparison of the sputter rates of oxide films relative to the sputter rate of SiO2 J. Vac. Sci. Technol. A 28, 1060 (2010); http://dx.doi.org/10.1116/1.3456123 (13 pages) Online Publication Date: 2 September 2010
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There is a growing interest in knowing the sputter rates for a wide variety of oxides because of their increasing technological importance in many different applications. To support the needs of users of the Environmental Molecular Sciences Laboratory, a national scientific user facility, as well as our research programs, the authors made a series of measurements of the sputter rates from oxide films that have been grown by oxygen plasma-assisted molecular beam epitaxy, pulsed laser deposition, atomic layer deposition, electrochemical oxidation, or sputter deposition. The sputter rates for these oxide films were determined in comparison with those from thermally grown SiO2, a common reference material for sputter rate determination. The film thicknesses and densities for most of these oxide films were measured using x-ray reflectivity. These oxide films were mounted in an x-ray photoelectron or Auger electron spectrometer for sputter rate measurements using argon ion sputtering. Although the primary objective of this work was to determine relative sputter rates at a fixed angle, the measurements also examined (i) the angle dependence of the relative sputter rates, (ii) the energy dependence of the relative sputter rates, and (iii) the extent of ion beam induced reduction for some oxides. Oxide films examined include SiO2, Al2O3, CeO2, Cr2O3, Fe2O3, HfO2, In–Sn oxide, Ta2O5, TiO2 (anatase, rutile, and amorphous), and ZnO. The authors found that the sputter rates for the oxides can vary up to a factor of 2 (usually lower) from that observed for SiO2. The ratios of sputter rates relative to those of SiO2 appear to be relatively independent of ion beam energy in the range of 1–4 kV and for incident angles <50°. As expected, the extent of ion beam induced reduction of the oxides varies with the sputter angle.
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Critical review: Plasma-surface reactions and the spinning wall method J. Vac. Sci. Technol. A 29, 010801 (2011); http://dx.doi.org/10.1116/1.3517478 (25 pages) Online Publication Date: 3 January 2011
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This article reviews methods for studying reactions of atoms and small molecules on substrates and chamber walls that are immersed in a plasma, a relatively unexplored, yet very important area of plasma science and technology. Emphasis is placed on the “spinning wall” technique. With this method, a cylindrical section of the wall of the plasma reactor is rotated, and the surface is periodically exposed to the plasma and then to a differentially pumped mass spectrometer, to an Auger electron spectrometer, and, optionally, to a beam of additional reactants or surface coatings. Reactants impinging on the surface can stick and react over time scales that are comparable to the substrate rotation period, which can be varied from ∼ 0.5 to 40 ms. Langmuir–Hinshelwood reaction probabilities can be derived from a measurement of the absolute desorption product yields as a function of the substrate rotation frequency. Auger electron spectroscopy allows the plasma-immersed surface to be monitored during plasma operation. This measurement is critical, since wall “conditioning” in the plasma changes the reaction probabilities. Mass spectrometer cracking patterns are used to identify simple desorption products such as Cl2, O2, ClO, and ClO2. Desorption products also produce a measurable pressure rise in the second differentially pumped chamber that can be used to obtain absolute desorption yields. The surface can also be coated with films that can be deposited by sputtering a target in the plasma or by evaporating material from a Knudsen cell in the differentially pumped wall chamber. Here, the authors review this new spinning wall technique in detail, describing both experimental issues and data analysis methods and interpretations. The authors have used the spinning wall method to study the recombination of Cl and O on plasma-conditioned anodized aluminum and stainless steel surfaces. In oxygen or chlorine plasmas, these surfaces become coated with a layer containing Si, Al, and O, due to slow erosion of the reactor materials, in addition to Cl in chlorine plasmas. Similar, low recombination probabilities were found for Cl and O on anodized Al versus stainless steel surfaces, consistent with the similar chemical composition of the layer that forms on these surfaces after long exposure to the plasma. In chlorine plasmas, weakly adsorbed Cl2 was found to inhibit Cl recombination, hence the Cl recombination probability decreases with increasing Cl2-to-Cl number density ratios in the plasma. In mixed Cl2/O2 plasmas, Cl and O recombine to form Cl2 and O2 with probabilities that are similar to those in pure chlorine or oxygen plasmas, but in addition, ClO and ClO2 form on the surface and desorb from the wall. These and other results, including the catalytic enhancement of O recombination by monolayer amounts of Cu, are reviewed.
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Dry etching of polydimethylsiloxane for microfluidic systems J. Vac. Sci. Technol. A 20, 975 (2002); http://dx.doi.org/10.1116/1.1460896 (8 pages) Online Publication Date: 7 May 2002
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A fluorine-based reactive ion etch (RIE) process has been developed to anisotropically dry etch the silicone elastomer polydimethylsiloxane (PDMS). This technique complements the standard molding procedure that makes use of forms made of thick SU-8 photoresist to produce features in the PDMS. Total gas pressure and the ratio of O2 to CF4 were varied to optimize etch rate. The RIE recipe developed in this study uses a 1:3 mixture of O2 to CF4 gas resulting in a highly directional and stable etch rate of approximately 20 μm per hour. Selective dry etching can be performed through a photolithographically patterned metal etch mask providing greater precision and alignment with preexisting molded features. The dry etch process is presented in this article along with a brief comparison to recently reported wet etch approaches. © 2002 American Vacuum Society. |
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Work function of fluorine doped tin oxide J. Vac. Sci. Technol. A 29, 011019 (2011); http://dx.doi.org/10.1116/1.3525641 (4 pages) Online Publication Date: 5 January 2011
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Fluorine doped tin oxide (FTO) is a commonly used transparent conducting oxide in optoelectronic device applications. The work function of FTO is commonly cited as 4.4 eV, which is incommensurate with recent device performance results. Using x-ray photoelectron spectroscopy, the authors measured the work function of commercial FTO to be 5.0±0.1 eV. UV ozone treatment was found to increase the work function by ∼ 0.1 eV due to surface band bending. The origins of the much lower work function previously reported are also discussed and are found to be a result of carbon contamination and UV induced work function lowering.
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J. Vac. Sci. Technol. A 29, 041601 (2011); http://dx.doi.org/10.1116/1.3585665 (7 pages) Online Publication Date: 13 May 2011
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For the torus of the nuclear fusion project ITER (originally the International Thermonuclear Experimental Reactor, but also Latin: the way), eight high-performance large-scale customized cryopumps must be designed and manufactured to accommodate the very high pumping speeds and throughputs of the fusion exhaust gas needed to maintain the plasma under stable vacuum conditions and comply with other criteria which cannot be met by standard commercial vacuum pumps. Under an earlier research and development program, a model pump of reduced scale based on active cryosorption on charcoal-coated panels at 4.5 K was manufactured and tested systematically. The present article focuses on the simulation of the true three-dimensional complex geometry of the model pump by the newly developed ProVac3D Monte Carlo code. It is shown for gas throughputs of up to 1000 sccm (∼1.69 Pa m3/s at T = 0° C) in the free molecular regime that the numerical simulation results are in good agreement with the pumping speeds measured. Meanwhile, the capture coefficient associated with the virtual region around the cryogenic panels and shields which holds for higher throughputs is calculated using this generic approach. This means that the test particle Monte Carlo simulations in free molecular flow can be used not only for the optimization of the pumping system but also for the supply of the input parameters necessary for the future direct simulation Monte Carlo in the full flow regime.
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High rate deep Si etching for through-silicon via applications J. Vac. Sci. Technol. A 29, 021009 (2011); http://dx.doi.org/10.1116/1.3543635 (6 pages) Online Publication Date: 28 January 2011
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High rate deep Si etching for through-silicon via (TSV) applications is reported. The requirements for the Si etch process is discussed from the viewpoint of TSV size and productivity, and the effective processes are described. For “small” TSV a few microns in diameter and up to 10 μm deep, profile control is the most important requirement, For “large” TSV with diameters of more than 50 μm and depths up to 100 μm and more, an ultrahigh Si etch rate is indispensable. The “medium” TSV with diameters and depths several tens of microns requires both high etch rate and profile control. Capacitively coupled plasma MERIE at high pressure is shown to be effective, by using HBr gas chemistry for small TSV, and by using SF6 gas chemistry and high rf frequency for large and medium TSV where an extremely high etch rate can be obtained.
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Nanosphere lithography: A materials general fabrication process for periodic particle array surfaces J. Vac. Sci. Technol. A 13, 1553 (1995); http://dx.doi.org/10.1116/1.579726 (6 pages)
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In this article nanosphere lithography (NSL) is demonstrated to be a materials general fabrication process for the production of periodic particle array (PPA) surfaces having nanometer scale features. A variety of PPA surfaces have been prepared using identical single‐layer (SL) and double‐layer (DL) NSL masks made by self‐assembly of polymer nanospheres with diameter, D=264 nm, and varying both the substrate material S and the particle material M. In the examples shown here, S was an insulator, semiconductor, or metal and M was a metal, inorganic ionic insulator, or an organic π‐electron semiconductor. PPA structural characterization and determination of nanoparticle metrics was accomplished with atomic force microscopy. This is the first demonstration of nanometer scale PPA surfaces formed from molecular materials. © 1995 American Vacuum Society |
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Transparent conductive Al-doped ZnO thin films grown at room temperature J. Vac. Sci. Technol. A 29, 031505 (2011); http://dx.doi.org/10.1116/1.3565462 (6 pages) Online Publication Date: 16 March 2011
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Aluminum-doped ZnO (ZnO:Al, AZO) thin films were prepared on glass substrates by dc reactive magnetron sputtering from a Zn–Al alloy target at room temperature. The effects of the Ar-to-O2 partial pressure ratios on the structural, electrical, and optical properties of AZO films were studied in detail. AZO films grown using 100:4 to 100:8 Ar-to-O2 ratio result in acceptable quality films with c-axis orientated crystals, uniform grains, 10−3 Ω cm resistivity, greater than 1020 cm−3 electron concentration, and high transmittance, 90%, in the visible region. The lowest resistivity of 4.11×10−3 Ω cm was obtained under the Ar-to-O2 partial pressure ratio of 100:4. A relatively strong UV emission at ∼ 3.26 eV was observed in the room-temperature photoluminescence spectrum. X-ray photoelectron spectroscopy analysis confirmed that Al was introduced into ZnO and substitutes for Zn and doped the film n-type.
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Work function determination of zinc oxide films J. Vac. Sci. Technol. A 15, 428 (1997); http://dx.doi.org/10.1116/1.580502 (3 pages)
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Zinc oxide-silicon heterojunctions were fabricated using both n- and p-type silicon. The zinc oxide films were deposited by the magnetron sputtering process at various substrate temperatures to form these devices. The electrical properties of these devices were measured and the work function of the zinc oxide was evaluated from these properties. © 1997 American Vacuum Society. |
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Effect of oxygen deficiency on the photoresponse and reactivity of mixed phase titania thin films J. Vac. Sci. Technol. A 29, 031508 (2011); http://dx.doi.org/10.1116/1.3574350 (7 pages) Online Publication Date: 8 April 2011
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Nonstoichiometric mixed phased titania nanocomposites (TiO2−x) were deposited by reactive direct current magnetron sputtering. The authors explored the role of nonstoichiometry (as defined by oxygen deficiency in synthesis) in mixed phase titania thin films and its effects on the photoresponse and photocatalytic performance for CO2 reduction to methane under UV and visible light. Oxygen partial pressure was varied during film deposition, yielding different levels of oxygen deficiency in the films. Optimized nonstoichiometric films showed a strong redshift. The authors have identified an optimum set of synthesis conditions for TiO2−x films that produce a relative maximum in photocatalytically produced methane under both UV and visible light.
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J. Vac. Sci. Technol. A 29, 030601 (2011); http://dx.doi.org/10.1116/1.3562167 (5 pages) Online Publication Date: 11 March 2011
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Microstructural evolution during film growth J. Vac. Sci. Technol. A 21, S117 (2003); http://dx.doi.org/10.1116/1.1601610 (12 pages) Online Publication Date: 2 September 2003
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Atomic-scale control and manipulation of the microstructure of polycrystalline thin films during kinetically limited low-temperature deposition, crucial for a broad range of industrial applications, has been a leading goal of materials science during the past decades. Here, we review the present understanding of film growth processes—nucleation, coalescence, competitive grain growth, and recrystallization—and their role in microstructural evolution as a function of deposition variables including temperature, the presence of reactive species, and the use of low-energy ion irradiation during growth. © 2003 American Vacuum Society. |
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J. Vac. Sci. Technol. A 29, 030602 (2011); http://dx.doi.org/10.1116/1.3570818 (4 pages) Online Publication Date: 28 March 2011
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The authors compare the effects of vacuum ultraviolet (VUV) irradiation on pristine and UV-cured low-k porous organosilicate glass (SiCOH). The authors find that during VUV irradiation, more trapped charges are generated in UV-cured SiCOH as compared to pristine SiCOH. VUV is also used as a tool to investigate effects of UV curing. From comparison of VUV spectroscopy and photoinjection current of the two samples, the authors find that UV curing reduces the number of defect states in SiCOH. The authors also find that UV-cured SiCOH has higher photoconductivity and intrinsic conductivity from VUV spectroscopy and trapped-charge decay rate, respectively.
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MgZnO/ZnO quantum well nanowire heterostructures with large confinement energies J. Vac. Sci. Technol. A 29, 03A104 (2011); http://dx.doi.org/10.1116/1.3531709 (5 pages) Online Publication Date: 11 January 2011
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Mg0.25Zn0.75O/ZnO-quantum well nanowire heterostructures were grown with a three-step pulsed laser deposition process. To avoid shadowing effects during the coating, the ZnO nanowires were grown with a low area density on a ZnO buffer layer deposited on an a-plane sapphire substrate. By using spatially resolved cathodoluminescence measurements, the luminescence of axial and radial quantum wells were clearly distinguished. The large bandgap energy of the Mg0.25Zn0.75O barrier material ( ≈ 3.85 eV) made it possible to tune the energy of quantum wells from 3.4 to 3.7 eV. The homogeneity of the radial quantum well along the wire axis was probed, revealing that only small fluctuations of about 4 meV are found in the main part of the nanowire. Near the tip of the nanowire, the energy of the radial quantum well increases due to locally modified growth conditions reducing the growth rate by up to 15%. Furthermore, the growth rates of the axial and radial quantum wells were determined, showing that the one in axial direction is a factor of about 2 larger than the one in radial direction.
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Electrical and microstructural properties of N+ ion-implanted ZnO and ZnO:Ag thin films J. Vac. Sci. Technol. A 29, 03A108 (2011); http://dx.doi.org/10.1116/1.3554836 (7 pages) Online Publication Date: 14 February 2011
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ZnO and Ag-doped ZnO films were grown on sapphire (0001) substrates by pulsed-laser deposition in vacuum both with and without oxygen at 700 °C. N+ ions were implanted in these films at room temperature and at 300 °C to a dose of 1×1014 cm−2 at 50 keV. Hall measurements indicate that ZnO films deposited in vacuum without oxygen and implanted with N+ at elevated temperatures are p-type with a hole-carrier concentration of 6×1016 cm−3, a mobility of 2.1 cm2 V−1 s−1, and a resistivity of 50 Ω cm. Both scanning-electron microscopy and transmission-electron microscopy studies on the implanted films reveal microstructural differences in grain size, surface roughness, and the nature of defects, which may impact the activation of N atoms as p-type carriers. Low-energy ion implantation at elevated temperatures is shown to be an effective method to introduce p-type N dopants into ZnO, which minimizes defect clustering and promotes defect annihilation during implantation.
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J. Vac. Sci. Technol. A 25, 1317 (2007); http://dx.doi.org/10.1116/1.2764082 (19 pages) Online Publication Date: 30 July 2007
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Physical vapor deposition under conditions of obliquely incident flux and limited adatom diffusion results in a film with a columnar microstructure. These columns will be oriented toward the vapor source and substrate rotation can be used to sculpt the columns into various morphologies. This is the basis for glancing angle deposition (GLAD), a technique for fabricating porous thin films with engineered structures. The origin of the columnar structure characteristic of GLAD films is discussed in terms of nucleation processes and structure zone models. As deposition continues, the columnar structures are influenced by atomic-scale ballistic shadowing and surface diffusion. Competitive growth is observed where the tallest columns grow at the expense of smaller features. The column shape evolves during growth, and power-law scaling behavior is observed as shown in both experimental results and theoretical simulations. Due to the porous nature of the films and the increased surface area, a variety of chemical applications and sensor device architectures are possible. Because the GLAD process provides precise nanoscale control over the film structure, characteristics such as the mechanical, magnetic, and optical properties of the deposited film may be engineered for various applications. Depositing onto prepatterned substrates forces the columns to adopt a planar ordering, an important requirement for photonic crystal applications.
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Plasma enhanced atomic layer deposition of SiNx:H and SiO2 J. Vac. Sci. Technol. A 29, 041501 (2011); http://dx.doi.org/10.1116/1.3584790 (9 pages) Online Publication Date: 24 May 2011
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As the nanoelectronics industry looks to transition to both three dimensional transistor and interconnect technologies at the <22 nm node, highly conformal dielectric coatings with precise thickness control are increasingly being demanded. Plasma enhanced chemical vapor deposition (PECVD) currently fills this role for most applications requiring low temperature processing but does not always meet step coverage and thickness precision requirements. The authors present results for a hybrid technique, plasma enhanced atomic layer deposition (PEALD), which utilizes typical PECVD process gases and tooling while delivering improved topography coverage and thickness control. Specifically, the authors show that alternating SiH4 gas/N2 plasma exposures applied in an atomic layer deposition sequence can be used to deposit SiNx:H films in a self-limiting fashion with improved conformality and superior performance as a moisture barrier. PEALD of SiO2 using alternating SiH4 and CO2 plasma exposures is further demonstrated.
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Preparation of clean Bi2Te3 and Sb2Te3 thin films to determine alignment at valence band maxima J. Vac. Sci. Technol. A 29, 031403 (2011); http://dx.doi.org/10.1116/1.3581053 (5 pages) Online Publication Date: 15 April 2011
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The thermoelectric application of Bi2Te3 and Sb2Te3 thin film structures relies on the relative alignment of the valence band maxima for good electrical conduction. In order to determine the valence band maxima of the bulk films, the authors propose a simple repeatable treatment of a chemical etching in dilute hydrochloric acid solution and a subsequent annealing at ∼ 150 °C under ultrahigh vacuum to prepare clean surfaces of Bi2Te3 and Sb2Te3 thin films. High-resolution photoemission spectroscopy using synchrotron radiation is used to investigate the chemical states of epitaxial Bi2Te3 and Sb2Te3 thin films grown on GaAs by low-temperature metal-organic chemical vapor deposition. The valence band and core-level photoemission spectra indicate that the surface contaminations and oxides were removed. After chemical etching in acid solution, elemental Te was observed on the surface; a follow-up anneal in ultrahigh vacuum creates a stoichiometric oxide-free surface.
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