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Top 20 Most Read Articles
April 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Application of porcelain enamel as an ultra-high-vacuum-compatible electrical insulator J. Vac. Sci. Technol. A 18, 1751 (2000); http://dx.doi.org/10.1116/1.582418 (4 pages)
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Many accelerator vacuum system components require electrical insulation internal to the vacuum system. Some accelerator components at Brookhaven National Laboratory are installed in ultra-high-vacuum systems which require the insulation to have excellent vacuum characteristics, be radiation resistant, and be able to withstand high temperatures when used on baked systems. Porcelain enamel satisfies all these requirements. This article describes the process and application of coating metal parts with porcelain enamel to provide electrical insulation. The mechanical and vacuum testing of Marman flanges coated with porcelain and using metal Helicoflex seals to form a zero-length electrical break are detailed. The use of porcelain enameled parts is attractive since it can be done quickly, is inexpensive and environmentally safe, and most of all satisfies stringent vacuum system requirements. © 2000 American Vacuum Society. |
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Metal–organic interface and charge injection in organic electronic devices J. Vac. Sci. Technol. A 21, 521 (2003); http://dx.doi.org/10.1116/1.1559919 (11 pages) Online Publication Date: 18 March 2003
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Charge injection at the interface between metallic electrodes and organic semiconductors plays a crucial role in the performance of organic (opto-)electronic devices. This article discusses the current understanding of the formation of the metal–organic contact and the parameters which control the injection current. Organic semiconductors differ significantly from their inorganic counterparts, primarily because they are amorphous van der Waals solids. As a result the electronic states are highly localized, and charge transport is by site-to-site hopping. Organics can also form clean interfaces with many metals, free of interface states in the gap. Nevertheless, there is generally found to be a significant vacuum level offset, the origins of which are not yet fully understood. Organic semiconductors are frequently free of donor and acceptor dopants, and as a result the depletion depth is larger than the organic layer thickness. Thus the Fermi level in the organic and the charge injection barriers depend most directly on the interface offset. The charge injection process is described as thermally assisted tunneling from the delocalized states of the metal into the localized states of the semiconductor, whose energy includes contributions from the mean barrier height, the image potential, the energetic disorder, and the applied electric field. There is no completely satisfactory analytic theory for the field and temperature dependence of the injection current, which, for well characterized interfaces, exhibits behavior relating to both thermionic emission and field-induced tunneling. © 2003 American Vacuum Society. |
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J. Vac. Sci. Technol. A 25, 557 (2007); http://dx.doi.org/10.1116/1.2730513 (9 pages) Online Publication Date: 20 April 2007
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AlN films were deposited on microscopy glass slide and silicon (111 orientation) substrates by reactive ac magnetron sputtering using two nitrogen concentrations and three discharge powers of 1.5, 2.5, and 5.0 kW. X-ray diffraction studies showed that films prepared on glass and Si substrates were of hexagonal wurtizite phase. Films on Si substrates also contained small amounts of the cubic phase of AlN besides the predominantly hexagonal wurtizite phase. AlN coatings on glass substrates were textured towards the (00∙2) plane; this preferred orientation of crystals was found to decrease with increase in sputtering power. Scanning electron microscopy studies showed that AlN films prepared at higher nitrogen concentration have a microstructure consisting of pebblelike crystals, some of which were hexagonal in shape. The crystal size in the coatings increased with sputtering power and was in the range of 70–230 nm.
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Nucleation theory and the early stages of thin film growth J. Vac. Sci. Technol. A 21, S96 (2003); http://dx.doi.org/10.1116/1.1600454 (14 pages) Online Publication Date: 2 September 2003
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A review is given of nucleation and growth models as applied to the earliest stages of thin film growth. Rate equations, kinetic Monte Carlo, and level set simulations are described in some detail, with discussion of remaining uncertainties, in particular the functional form of the so-called capture numbers in rate equations. Recent examples are given of sub-monolayer nucleation at surface defects, attachment-limited capture, and Ostwald ripening. The experimental literature is cited, and experiment–theory comparisons are made where possible. Emphasis is given to fast computational models that can span a large range of length and time scales, which might be further developed in the direction of on-line process control. © 2003 American Vacuum Society. |
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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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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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J. Vac. Sci. Technol. A 24, 1197 (2006); http://dx.doi.org/10.1116/1.2167077 (6 pages) Online Publication Date: 21 June 2006
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Ultrathin coatings of fluorosilane films for silicon and polydimethylsiloxane (PDMS) nanochannels are desirable to control the hydrophobicity of the surface and reduce or prevent undesired protein adsorption or cell interactions critical for the performance of most biomedical micro/nanodevices. Surface modifications using vapor-phase deposition become increasingly important for some biomedical nanodevices and have advantages over liquid-phase deposition since the vapor phase can permeate more efficiently into silicon nanochannels. In this study, vapor-phase deposition was used to deposit ultrathin films of four fluorosilanes on silicon and PDMS and identify deposition conditions for an optimal process. The films were characterized by means of a contact angle analyzer for hydrophobicity, an ellipsometer for film thickness, and an atomic force microscope for surface roughness of these films. Results of this study and relevant mechanisms are the subject of this article.
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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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Thin film deposition with physical vapor deposition and related technologies J. Vac. Sci. Technol. A 21, S74 (2003); http://dx.doi.org/10.1116/1.1600450 (14 pages) Online Publication Date: 2 September 2003
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The properties of thin films depend critically on how they are made. For the most part, thin films are assembled in ways very different from the production of bulk materials. Thin films are usually deposited on existing, bulk surfaces using techniques based on atomic or molecular scale physics and chemistry. Physical vapor deposition (PVD) of thin films relies on the removal of atoms from a solid or a liquid by energetic means, and the subsequent deposition of those atoms on a nearby surface. Variations of PVD processes include thermal evaporation, physical sputtering, laser ablation, and arc-based emission. Additional modifications to physical sputter deposition have been made to enhance the chemical and/or structural nature of the deposited films. These modifications include reactive sputter deposition, the unbalanced magnetron, collimated and ionized sputter deposition. Each of these systems and techniques will be described as well as some of the current day applications of the films produced. © 2003 American Vacuum Society. |
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J. Vac. Sci. Technol. A 25, 411 (2007); http://dx.doi.org/10.1116/1.2712196 (4 pages) Online Publication Date: 7 March 2007
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A thin passivation layer of aluminum oxide was deposited on polyimide by using the combined plasma immersion ion implantation and deposition (PIII&D) and cathodic vacuum arc technique. X-ray photoelectron spectroscopy C 1s spectra showed that the carbonyl bond (CO) and ether group (C–O–C and C–N–C) presented in pristine polyimide were damaged by implantation of aluminum ions and deposition of an aluminum oxide passivation layer. O 1s and Al 2p spectra confirmed the formation of a thin aluminum oxide passivation layer. This passivation layer can be implemented in aerospace engineering where polyimide may suffer degradation from fast atomic oxygen in the low-earth-orbit environment. To test the protection of this passivation layer to energetic oxygen ions, a plasma-enhanced chemical vapor deposition system was used to simulate the oxygen-ion irradiation, and the results showed that a higher weight occurred for passivated samples compared to pristine ones. X-ray diffraction showed that Al peaks were presented on the surface region, but no aluminum oxide peak was detected. The authors then concluded that Al clusters were formed in polyimide besides aluminum oxide, which was in an x-ray amorphous state. Furthermore, contact-angle measurements showed a reduced contact angle for passivated polyimide from a pristine value of 78° to 20° by using deionized water. Several discussions have been made on the surface chemical and structural property changes by using the combined PIII&D and cathodic vacuum arc technique.
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J. Vac. Sci. Technol. A 21, S207 (2003); http://dx.doi.org/10.1116/1.1600446 (9 pages) Online Publication Date: 2 September 2003
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In this the 50th anniversary year of the AVS and the AVS Symposium, this article is offered as one in a series of topical review articles to celebrate the role of this community to the progress in nanofabrication technology. The emphasis of the article is on the principles and limits of the various pattern formation techniques which have emerged as important tools in the research of nanoscale devices and structures. Topics such as e-beam lithography, proximal probes, imprint lithography, self assembly, and directed assembly are all discussed. © 2003 American Vacuum Society. |
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On the electrochemical etching of tips for scanning tunneling microscopy J. Vac. Sci. Technol. A 8, 3570 (1990); http://dx.doi.org/10.1116/1.576509 (6 pages)
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The sharpness of tips used in scanning tunneling microscopy (STM) is one factor which affects the resolution of the STM image. In this paper, we report on a direct‐current (dc) drop‐off electrochemical etching procedure used to sharpen tips for STM. The shape of the tip is dependent on the meniscus which surrounds the wire at the air–electrolyte interface. The sharpness of the tip is related to the tensile strength of the wire and how quickly the electrochemical reaction can be stopped once the wire breaks. We have found that the cutoff time of the etch circuit has a significant effect on the radius of curvature and cone angle of the etched tip; i.e., the faster the cutoff time, the sharper the tip. We have constructed an etching circuit with a minimum cut‐off time of 500 ns which uses two fast metal–oxide semiconductor field effect transistors (MOSFET) and a high‐speed comparator. The radius of curvature of the tips can be varied from approximately 20 to greater than 300 nm by increasing the cutoff time of the circuit. |
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Crystalline alumina coatings by reactive ac magnetron sputtering J. Vac. Sci. Technol. A 25, L5 (2007); http://dx.doi.org/10.1116/1.2431353 (4 pages) Online Publication Date: 29 January 2007
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Alumina coatings were deposited on silicon (111 orientation) substrates by reactive ac magnetron sputtering. Film deposition was done using Al targets and three O2/Ar gas flow rate ratios at 5 kW power. X-ray diffraction studies showed that films were crystalline and contained several phases of alumina. Secondary ion mass spectroscopy analyses were used to measure O/Al atomic ratio and Ar and H concentrations in the films. Hydrogen content in the coatings depended on the O2 partial pressure used during sputtering and also on the arrival rate of Al and O species on the substrates and seemed to influence the crystallinity of the coatings.
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The microstructure of sputter‐deposited coatings J. Vac. Sci. Technol. A 4, 3059 (1986); http://dx.doi.org/10.1116/1.573628 (7 pages)
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Microstructure is a critical consideration when polycrystalline or amorphous thin films are used for applications such as microcircuit metallization layers and diffusion barriers. The trend in device fabrication toward lower processing temperatures means that such coatings must often be deposited at substrate temperatures T that are low relative to the coating material melting point Tm. The structure of vapor deposited coatings grown under these conditions consists typically of a columnar growth structure, defined by voided open boundaries, which is superimposed on a microstructure which may be polycrystalline (defined by metallurgical grain boundaries) or amorphous. The voided growth structure is clearly undesirable for most applications. Its occurrence is a fundamental consequence of atomic shadowing acting in concert with the low adatom mobilities that characterize low T/Tm deposition, and its formation can be enhanced by the surface irregularities which are common to microcircuit fabrication. This paper reviews some of the recent developments in understanding the fundamental aspects of the relationship between the deposition conditions and the microstructure of sputter‐deposited thin films, with particular emphasis on the origin of the growth structure and its suppression through energetic particle bombardment. |
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J. Vac. Sci. Technol. A 8, 735 (1990); http://dx.doi.org/10.1116/1.576956 (29 pages)
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A quantitative surface analysis by Auger‐electron spectroscopy (AES) or x‐ray photoelectron spectroscopy (XPS) requires a series of operations that typically includes instrument setup, specimen positioning, data acquisition, data manipulation, and data analysis. These operations involve a sequence of measurements which are combined and/or compared with other data to yield an analysis. The final result has an estimated uncertainty that reflects the sum of the separate random and systematic errors in the various measurements and sources of data. We identify the major steps in typical analyses and comment on the major sources of error leading to estimates of uncertainty. Systematic errors generally exceed those of a random nature and are associated with the complex morphology of typical specimens, with parameters of instrument performance, and with limitations of current methodology and data. We review general measurement principles for surface analysis, the development of a suitable analytical strategy, and identify and discuss many of the sources of error. The discussion is specific to AES and XPS but many of the issues are relevant to other techniques of surface analysis. Finally, two examples are presented to illustrate the sources and magnitudes of some of the errors and the final uncertainties in some common examples of surface analyses. |
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Reduction of thin oxidized copper films using a hot-filament hydrogen radical source J. Vac. Sci. Technol. A 25, 415 (2007); http://dx.doi.org/10.1116/1.2712197 (6 pages) Online Publication Date: 7 March 2007
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This article aims to provide practical information on the performance of a hot-filament radical source, with which it becomes feasible for metallization in ultralarge-scale integrated circuits, which has not been studied in detail thus far. A very simple arrangement using this technique allows the highly efficient generation of hydrogen radicals and a quick recovery of oxidized Cu surfaces to their original metallic state. The amount of CuO reduction was evaluated by measuring sheet resistance and also by transmission microscopy. The reduction started when the specimen temperature exceeded 100 °C, and several tens of nanometers Cu of film was formed without resulting in a serious increase in the specimen temperature. The amount of reduction was found to increase almost proportionately to the hydrogen radical flux. The Cu layer had a very flat surface topography showing no trace of self-agglomeration of Cu. The reduction of a thermally oxidized Cu specimen was also demonstrated.
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Sculptured thin films and glancing angle deposition: Growth mechanics and applications J. Vac. Sci. Technol. A 15, 1460 (1997); http://dx.doi.org/10.1116/1.580562 (6 pages)
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Sculptured thin films with three dimensional microstructure controlled on the 10 nm scale were fabricated with an evaporation technique. Glancing angle deposition (GLAD) and substrate motion were employed to “sculpt” columnar thin film microstructure into desired forms ranging from zigzag shaped to helical to four-sided “square” helical. Computer control of substrate motion was used to accurately position the substrate and to achieve the desired film structures. The growth mechanics of this novel thin film deposition technique are investigated with density measurements, scanning electron microscopy analysis, and measurements of effective refractive index. Adatom diffusion and atomic shadowing are the dominant growth mechanisms with glancing angle deposition conditions creating extreme shadowing. With controlled rotation of the substrate about two axes during deposition, a dense capping layer can be produced on top of the porous sculptured films. The success of the capping process was found to be strongly dependent on the technique used, with an exponential decrease(θ∝[1−A⋅eB⋅t]) with time of incident flux angle found to be the best to reduce filling of the porous film and fracturing of the capping film. The GLAD technique was found to have potentially promising application in optical, biological, and chemical devices and materials. © 1997 American Vacuum Society. |
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Microfabrication of cantilever styli for the atomic force microscope J. Vac. Sci. Technol. A 8, 3386 (1990); http://dx.doi.org/10.1116/1.576520 (11 pages)
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Atomic force microscopy (AFM) is a newly developed high resolution microscopy technique which is capable of mapping forces near surfaces or, by means of these forces, the topography of the surface itself. In one mode of operation, AFM can resolve individual atoms on both conducting and insulating surfaces. A crucial component for the AFM is a flexible force‐sensing cantilever stylus, whose properties should include, among other things: a sharp tip, a low force constant, and a high mechanical resonance frequency. These requirements can be met by reducing the size of the cantilever stylus through microfabrication techniques and employing novel methods to construct a sharp tip. Presented here are a number of microfabrication processes for constructing cantilever styli with properties ideal for the AFM. These fabrication processes include (1) a method for producing thin film SiO2 or Si3N4 cantilevers without tips, (2) a method for producing Si3N4 cantilevers with integrated pyramidal tips formed by using an etch pit on the (100) surface of Si as a mold, (3) a method for producing SiO2 cantilevers with conical tips formed by a combination of isotropic and anisotropic plasma etching of a small Si post, and (4) a method for producing SiO2 cantilevers with integrated tetrahedral tips formed by anisotropically etching a corner of a small Si post to a sharp point. Each of these processes uses a (100) Si wafer as a substrate and relies on conventional batch fabrication techniques. The quality (i.e., sharpness) of the tips produced by the above methods matches or exceeds that of conventional tips used in the AFM or scanning tunneling microscope (STM). Alternative methods for producing tips by evaporation of material through an orifice or by selective chemical vapor deposition of W metal into a pyramidal etch pit in Si have been demonstrated, but these methods have not yet been successfully used in cantilever assemblies. |
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Plasma deposition of optical films and coatings: A review J. Vac. Sci. Technol. A 18, 2619 (2000); http://dx.doi.org/10.1116/1.1314395 (27 pages)
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Plasma enhanced chemical vapor deposition (PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin film systems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasma systems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others. © 2000 American Vacuum Society. |
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J. Vac. Sci. Technol. A 23, 1208 (2005); http://dx.doi.org/10.1116/1.1897697 (7 pages) Online Publication Date: 28 June 2005
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CdTe and CuIn1−xGaxSe2−ySy (CIGSS) are ideal candidates for thin-film solar cells. Present photovoltaic (PV) conversion efficiencies of champion thin-film solar cells are: CuIn1−xGaxSe2 (CIGS) 19.5%, CdTe 16.5%, and a‐Si:H 12.4%. Thin-film PV modules could spearhead production growth of photovoltaics in the United States because of their added production capacity. For this purpose, module efficiencies must be improved to the 13%–15% range. Obtaining Ohmic contacts is difficult, especially for CdTe, because of the inherently low p-type doping level. Therefore, increasing the p-type doping level is important. Growth of CIGSS film must be controlled carefully as it transitions from Cu-rich to In-rich composition. Other issues for CIGSS cells are minimizing indium consumption, and increasing process throughput of selenization∕sulfurization and transparent conducting oxide deposition. Development of all-dry processing for CdS deposition would be beneficial for both cells. This paper discusses basic devices and related issues.
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