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Top 20 Most Read Articles
October 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Intrinsic stress in sputtered thin films J. Vac. Sci. Technol. A 9, 2431 (1991); http://dx.doi.org/10.1116/1.577295 (6 pages)
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A review of the literature reveals that the intrinsic stress in sputtered thin films can be tensile or compressive depending on the flux and energy of particles striking the film. Stress data indicates that the normalized momentum P∗n=γ√ME (where γ is the energetic particle/atom flux ratio, M the mass, and E the energy) may be the appropriate stress scaling factor. The forward sputtering model predicts a √E dependence. An idealized stress–momentum curve is constructed consisting of three regimes:(1) a region of increasing tensile stress at low P∗n, due to a porous microstructure, followed by (2) a sharp transition from tensile to compressive stress at intermediate momentum, accompanied by a conversion to zone T‐type microstructure and (3) a saturation region at high momentum, due to plastic flow. For any deposition process the sign and magnitude of the stress depends on P∗n, which is a function of several deposition parameters. Calculations indicate that stress reversal in sputtered and ion‐assisted evaporated films occurs at normalized momentum P∗n of about 5–15×10−23 kg m/s/atom (Pn=0.3–1 √eV/atom). |
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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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Atomic-scale investigation of graphene formation on 6H-SiC(0001) J. Vac. Sci. Technol. A 26, 932 (2008); http://dx.doi.org/10.1116/1.2900661 (6 pages) Online Publication Date: 1 July 2008
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The growth of graphene on the silicon-terminated face of 6H-SiC(0001) was investigated by scanning tunneling microscopy (STM) measurements. The initial stages of ultrahigh vacuum graphitization resulted in the growth of individual graphene sheets on random SiC terraces. These initial graphene sheets contained few defects, and the regions of clean SiC were free of contamination, exhibiting a 6
 ×6R30° surface reconstruction. However, graphitization to multilayer thickness resulted in multiple defects, as observed with the STM. A high density of defects was observed, which may be attributed to the initial treatment of the SiC wafer. We characterize these defects, showing that they are located predominantly below the first layer of graphene. |
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J. Vac. Sci. Technol. A 26, 1101 (2008); http://dx.doi.org/10.1116/1.2949232 (8 pages) Online Publication Date: 30 July 2008
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Novel quaternary Si–B–C–N materials are becoming increasingly attractive because of their possible high-temperature and harsh-environment applications. In the present work, amorphous Si–B–C–N films were deposited on Si and SiC substrates by reactive dc magnetron cosputtering using a single C–Si–B or B4C–Si target in nitrogen-argon gas mixtures. A fixed 75% Si fraction in the target erosion areas, a rf induced negative substrate bias voltage of −100 V, a substrate temperature of 350 °C, and a total pressure of 0.5 Pa were used in the depositions. The corresponding discharge and deposition characteristics (such as the ion-to-film-forming particle flux ratio, ion energy per deposited atom, and deposition rate) are presented to understand complex relationships between process parameters and film characteristics. Films deposited under optimized conditions (B4C–Si target, 50% N2+50% Ar gas mixture), possessing a composition (in at. %) Si32–34B9–10C2–4N49–51 with a low (less than 5 at. %) total content of hydrogen and oxygen, exhibited extremely high oxidation resistance in air at elevated temperatures (even above 1500 °C). Formation of protective surface layers (mainly composed of Si and O) was proved by high-resolution transmission electron microscopy, Rutherford backscattering spectrometry, and x-ray diffraction measurements after oxidization. Amorphous structure of the Si–B–C–N films was maintained under the oxidized surface layers after annealing in air up to 1700 °C (a limit imposed by thermogravimetric analysis in oxidative atmospheres).
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J. Vac. Sci. Technol. A 26, 710 (2008); http://dx.doi.org/10.1116/1.2889434 (6 pages) Online Publication Date: 30 June 2008
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An organic thin film transistor (OTFT) with a back gate structure on a patterned pentacene active region was fabricated. The variations of electrical properties as a function of polyvinylcinnamate (PVCN) concentration used as a gate dielectric were evaluated. In addition, the morphology of the pentacene thin film was characterized by scanning probe microscope by simultaneously obtaining the topology and current image. Within the pentacene thin film, the current was observed to flow through grain rather than through the grain boundaries. Within marginal variations, an OTFT fabricated using 8% PVCN revealed the best electrical properties in terms of mobility, threshold voltage (VT), subthreshold swing, and on/off ratio.
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Fluorocarbon high‐density plasmas. II. Silicon dioxide and silicon etching using CF4 and CHF3 J. Vac. Sci. Technol. A 12, 333 (1994); http://dx.doi.org/10.1116/1.578877 (12 pages)
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We report a study of the application of CF4 and CHF3 electron cyclotron resonance (ECR) discharges to selective etching of SiO2 over Si. Due to significant fluorocarbon film deposition for plasma operation without rf sample bias in the pressure range below 10 mTorr, rf biasing is required for etching of SiO2 and Si. The rf threshold voltage for etching is 55 V for CHF3 and 35 V for CF4 at a pressure of 1 mTorr. At 100 V rf bias, silicon dioxide etch rates were greater than ≂600 nm/min in CF4 and 450 nm/min for 1000 W plasmas at 1 mTorr pressure. A plot of the oxide etch rate vs rf bias exhibits a fluorocarbon film suppression regime at low rf voltages and an oxide sputtering regime at higher rf voltages. In the fluorocarbon suppression regime, the etch rate is primarily determined by fluorocarbon deposition which results in a thin fluorocarbon film being present on the SiO2 surface during steady‐state etching. In the oxide sputtering regime, the oxide etch rate increases linearly with the ion current to the wafer and the square root of the ion energy. The etch yields decrease with increasing microwave power and decreasing pressure and are in the range 0.5–2 atoms per incoming ion. The silicon etch rate is much lower in CHF3 than in CF4, which translates into better SiO2‐to‐Si etch selectivity in CHF3 (≂15) than in CF4 (≂5). The lower Si etch rate in CHF3 is due to a greater thickness of the fluorocarbon film present on the silicon surface during steady‐state etching. The fluorocarbon film thickness is ≂5.5 nm in CHF3 as compared to ≂2.5 nm in a CF4 discharge (at a rf bias of 100 V). The oxide surface is free of fluorocarbon film for the same conditions. The etch depth of ≂2.5 μm deep contact holes etched using 1 mTorr CHF3 plasmas into photoresist patterned SiO2 was measured by scanning electron microscopy as a function of the feature width. The etch depth decreased by ≂10% as the feature size was reduced from 1.3 to 0.6 μm. |
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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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Rarefied gas flow through short tubes into vacuum J. Vac. Sci. Technol. A 26, 228 (2008); http://dx.doi.org/10.1116/1.2830639 (11 pages) Online Publication Date: 24 January 2008
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A rarefied gas flow into vacuum through a tube of finite length is investigated over the whole range of gas rarefaction by the direct simulation Monte Carlo method. The nonequilibrium effects at the inlet and outlet of the tube have been considered by including in the computational domain large volumes of the upstream and downstream reservoirs. Results for the dimensionless flow rate and for the flow field are presented for a wide range of the gas rarefaction and for various values of the length to radius ratio in the range from 0 to 10. The influence of the gas-surface interaction model, as well as the effect of the intermolecular potential model on the gas flow, is examined. A good agreement has been obtained between the present numerical results and the corresponding experimental ones available in the literature.
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J. Vac. Sci. Technol. A 26, 893 (2008); http://dx.doi.org/10.1116/1.2836425 (5 pages) Online Publication Date: 1 July 2008
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Photocatalytic TiO2 films were deposited by a hollow cathode gas-flow sputtering method using two Ti metal targets mounted parallel to each other. The Ar and O2 flow rates were 3000 and 0–50 SCCM (SCCM denotes cubic centimeter per minute at STP), respectively, and total gas pressure during the deposition was maintained at 45 Pa. The highest deposition rate for the photocatalytic TiO2 films was 162 nm/min at 30 SCCM of O2 flow. The as-deposited films and postannealed films, annealed in air at 300 °C for 1 h, were used to carry out photocatalytic decomposition of acetaldehyde (CH3CHO). In particular, the postannealed films showed extremely high photocatalytic activity compared to the photocatalytic activity of films deposited by conventional reactive sputtering.
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J. Vac. Sci. Technol. A 15, 3154 (1997); http://dx.doi.org/10.1116/1.580860 (4 pages)
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Ellipsometry, contact angle goniometry, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS) are used to study native oxide growth on SiGe films (with Ge content of 0%, 20%, 40%, and 100%) after a chemical clean. Ellipsometry suggests that the presence of Ge affects the initial oxide thickness right after the clean but it does not affect the rate of native oxide growth. Roughness of SiGe samples as measured by AFM does not appear to be affected by the native oxide growth or the Ge content in the film. The decrease in advancing (and receding) contact angles of SiGe samples after the chemical clean is apparently the result of both increasing oxide thickness and oxide solid phase composition. XPS results suggest that increasing Ge content in the film increases the oxidation of SiGe surface atoms. The chemical shifts in the Si 2p and Ge 3d spectra suggest that both Si and Ge react with oxygen to form SiO2 and GeO2. Such data suggests that contact angle measurements could be a rapid method to determine the state and passivation characteristics of a silicon substrate surface as a function of time; however, such a technique would not be as effective for SiGe films, the chemical composition of their native oxides of which would also change as a function of time. © 1997 American Vacuum Society. |
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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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Structural and electronic properties of bilayer epitaxial graphene J. Vac. Sci. Technol. A 26, 938 (2008); http://dx.doi.org/10.1116/1.2944257 (6 pages) Online Publication Date: 1 July 2008
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Scanning tunneling microscopy and scanning tunneling spectroscopy (STS) are used to study the structural and electronic properties of bilayer epitaxial graphene on SiC(0001). Topographic images reveal that graphene conforms to the SiC interface morphology and is observed to be continuous across steps separating adjoining terraces. Bilayer epitaxial graphene is shown to be Bernal stacked as is evidenced by bias-dependent topographic imaging. STS maps of the differential conductance show that graphene lattice defects cause scattering of charge carriers near the Fermi level. An analysis of stationary scattering patterns observed in the conductance maps determines the energy-momentum dispersion relation within 100 meV of the Fermi level. In contrast to lattice defects, disorder at the SiC interface and at subsurface steps plays a much lesser role in the scattering of charge carriers.
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J. Vac. Sci. Technol. A 21, 274 (2003); http://dx.doi.org/10.1116/1.1538370 (10 pages) Online Publication Date: 20 December 2002
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The information depth (ID) is a measure of the sampling depth for the detected signal in Auger-electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) while the mean escape depth (MED) is a measure of surface sensitivity. We report ID and MED calculations for Si 2s, Si 2p3/2, Cu 2s, Cu 2p3/2, Au 4s, and Au 4f7/2 photoelectrons excited by Mg Kα x rays. These calculations were made for various electron emission angles and for a common XPS configuration. Similar calculations were made for Si L3VV, Si KL23L23, Cu M3VV, Cu L3VV, Au N7VV, and Au M5N67N67 Auger transitions. The IDs and MEDs were derived from an analytical expression for the signal-electron depth distribution function obtained from a solution of the kinetic Boltzmann equation within the transport approximation. The ratios of the IDs and the MEDs to the corresponding values found if elastic-electron scattering were assumed to be negligible, RID and RMED, were less than unity and varied slowly with electron emission angle α for emission angles less than 50°. For larger emission angles, these ratios increased rapidly with α. For α⩽50°, average values of RID and RMED varied linearly with the single-scattering albedo, ω, a simple function of the electron inelastic mean-free path and transport mean-free path. For α=70° and α=80°, RID also varied linearly with ω but RMED showed a quadratic variation. The albedo is thus a useful measure of the magnitude of elastic-scattering effects on the ratios RID and RMED. As a result of the elastic scattering of the signal electrons, AES and XPS measurements at α=80° are less surface sensitive than would be expected if elastic scattering had been neglected. Conversely, AES and XPS measurements made for α⩽50° are more surface sensitive as a result of elastic-scattering effects. © 2003 American Vacuum Society. |
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J. Vac. Sci. Technol. A 26, 1213 (2008); http://dx.doi.org/10.1116/1.2966425 (5 pages) Online Publication Date: 6 August 2008
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The deposition rate and quality of alumina thin films fabricated by plasma-enhanced chemical vapor deposition (PECVD) increased significantly when square wave power modulation was applied at low frequency ( ∼ 1 Hz). The pulsed PECVD rate was enhanced by a factor of ∼ 3 relative to continuous wave operation, and the quantity of impurities was dramatically attenuated. Deposition experiments on trenches with aspect ratios ranging from 4 to infinity demonstrated that the technique achieves a high degree of conformality. Important reactor design and operating considerations are described. Pulsed PECVD produced similar quality improvements for Ta2O5, TiO2, and ZnO, suggesting that the approach has widespread potential for metal oxide synthesis.
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J. Vac. Sci. Technol. A 26, 1115 (2008); http://dx.doi.org/10.1116/1.2949234 (5 pages) Online Publication Date: 30 July 2008
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High-rate, low temperature deposition is an essential requirement for industrial fabrication technology to be suitable for the deposition of optical and protective coatings. High-density, low-pressure plasmas have received significant attention for such applications due to their ability to create large and controllable ion fluxes onto the substrate. In this study, the high-rate deposition of silica films from a silane and oxygen gas mixture in a high-density plasma system based on a matrix distributed electron cyclotron resonance (MDECR) plasma source is investigated using directional jet injection of undiluted silane. The influence of process parameters such as the microwave power, radio frequency biasing of the substrate holder, and gas flows on the OH content of the oxide films is studied using phase-modulated spectroscopic ellipsometry (SE), Fourier transform infrared (FTIR) spectroscopy, and transmission measurements. The results of the measurements, taken at various points across the wafer, show a decrease in the thickness-normalized OH concentration in the areas of higher deposition rates. The corresponding gas phase composition is investigated using optical emission spectroscopy and compared to the FTIR, transmission and SE measurement results in order to validate our findings and ultimately optimize the deposition process. It is found that the primary silane flux onto the surface, which depends on the positioning of the jet injection point and gas flow rate, plays an important role not only on the deposition rate but also on the OH content of the films. The authors conclude that high-density plasma deposition systems such as the MDECR plasma enhanced chemical vapor deposition system cannot be considered as well mixed for gases with dissociation products that have high sticking coefficients, contrary to the accepted paradigm.
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J. Vac. Sci. Technol. A 20, 492 (2002); http://dx.doi.org/10.1116/1.1450585 (7 pages)
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In this article we present a comprehensive study of microcrystalline silicon (μc-Si:H) p-i-n solar cells prepared by using plasma-enhanced chemical vapor deposition (PECVD) at 13.56 MHz excitation frequency. In the first step the cell development was performed in a small area PECVD reactor showing the relationship between the deposition process parameters and the resulting solar cell performance. Subsequent up-scaling to a substrate area of 30×30 cm2 confirmed the scalability of optimized deposition parameters to large area reactors. We investigated the deposition regime of high rf power Prf (0.25–0.7 W/cm2) and high deposition pressure pdep (1–11 Torr) for the μc-Si:H i layer. Furthermore, the influence of silane concentration and deposition temperature was studied. A transition between amorphous and microcrystalline growth could be achieved by a variation of either deposition pressure, plasma power, or silane concentration. The best microcrystalline silicon solar cells were prepared close to the transition to amorphous growth. A high deposition pressure was a prerequisite for obtaining high quality material at a high growth rate. The best solar cell efficiencies achieved so far are 8.1% and 6.6% at i-layer growth rates of 5 and 10 Å/s, respectively, for μc-Si:H single junction cells. Applied in a-Si:H/μc-Si:H tandem cells a stabilized efficiency of 10.0% was achieved. © 2002 American Vacuum Society. |
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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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Main determinants for III–V metal-oxide-semiconductor field-effect transistors (invited) J. Vac. Sci. Technol. A 26, 697 (2008); http://dx.doi.org/10.1116/1.2905246 (8 pages) Online Publication Date: 30 June 2008
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Lacking a suitable gate insulator, practical GaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) have remained all but a dream for more than four decades. The physics and chemistry of III–V compound semiconductor surfaces or interfaces are problems so complex that our understanding is still limited even after enormous research efforts. Most research is focused on surface pretreatments, oxide formation, and dielectric materials; less attention is paid to the III–V substrate itself. The purpose of this article is to show that device physics more related to III–V substrates is as important as surface chemistry for realizing high-performance III–V MOSFETs. The history and present status of III–V MOSFET research are briefly reviewed. A model based on the charge neutrality level is proposed to explain all experimental work he performed on III–V MOSFETs using ex situ atomic-layer-deposited high-k dielectrics. This model can also explain all reported experimental observations on III–V MOSFETs using in situ molecular-beam-expitaxy-grown Ga2O3(Gd2O3) as a gate dielectric. Related perspectives are also discussed to understand III–V MOS capacitance-voltage measurements.
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Effect of substrate temperatures on amorphous carbon nitride films prepared by reactive sputtering J. Vac. Sci. Technol. A 26, 966 (2008); http://dx.doi.org/10.1116/1.2919140 (4 pages) Online Publication Date: 1 July 2008
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Amorphous carbon nitride, a-CNx, films were deposited by reactive radio frequency magnetron sputtering of a graphite target in nitrogen gas. This kind of films could be used as novel electric and optical devices. The authors investigated effects of the substrate temperature up to 873 K on the films in this study. The films were characterized with x-ray photoelectron spectroscopy (XPS), ellipsometry, atomic force microscopy, and nanoindentation tests. XPS studies show that the decreasing tendency in the composition ratio of carbon to nitrogen in a-CNx films with the substrate temperature is observed; however, the bonding fraction of sp3C–N increases depending on the substrate temperature. The nanoindentation tests reveal that the film hardness increases from 2 to 12 GPa as the substrate temperature increases from room temperature to 823 K. These results suggest that the film hardness is closely related to the bonding states between carbon and nitrogen.
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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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