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Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films

Issue 5 • Date Sep 2013

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Displaying Results 1 - 25 of 30
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  • Review of radiation damage in GaN-based materials and devices

    Page(s): 050801 - 050801-16
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    A review of the effects of proton, neutron, γ-ray, and electron irradiation on GaN materials and devices is presented. Neutron irradiation tends to create disordered regions in the GaN, while the damage from the other forms of radiation is more typically point defects. In all cases, the damaged region contains carrier traps that reduce the mobility and conductivity of the GaN and at high enough doses, a significant degradation of device performance. GaN is several orders of magnitude more resistant to radiation damage than GaAs of similar doping concentrations. In terms of heterostructures, preliminary data suggests that the radiation hardness decreases in the order AlN/GaN > AlGaN/GaN > InAlN/GaN, consistent with the average bond strengths in the Al-based materials. View full abstract»

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  • Atomically resolved force microscopy

    Page(s): 050802 - 050802-18
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    Atomic force microscopy (AFM) with atomic resolution has opened up a new “atom world” based on the chemical nanoscale force. In the noncontact regime where a weak attractive chemical force appears, AFM has successfully achieved atomically resolved imaging of various surfaces. In the near-contact regime, where a strong attractive chemical force or Pauli repulsive force appears, AFM can map the force and potential even on insulator surfaces, it can identify the chemical species of individual atoms using the chemical force, manipulate embedded heterogeneous atoms vertically and laterally, image individual chemical bonds using the Pauli repulsive force, and detect the energy gap opening induced by covalent bond formation in combination with scanning tunneling microscopy. View full abstract»

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  • Nanostructured materials for supercapacitors

    Page(s): 050803 - 050803-14
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    Supercapacitor is an energy storage device that attempts to combine the high power density of a capacitor with the high energy density of a battery. Conventional supercapacitors use carbon based electrodes, mostly graphite. In recent years, alternatives such as carbon nanotubes, graphene, and other nanostructured materials have been considered to construct supercapacitor electrodes. This article reviews the progress in this area in addition to presenting a brief background on supercapacitors as energy storage medium and nanomaterials. View full abstract»

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  • Nanoscale characterization and metrology

    Page(s): 050804 - 050804-10
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    This paper will take a “From the Lab to the FAB” approach for discussing the measurements and applications of nanoscale characterization and metrology. The nanoscale dimensions of features found in semiconductor materials and devices provide many challenges for characterization of physical properties as well as measurements for process control. The use of multiple measurement methods results in a more complete determination of the properties so that structure–function relationships can be elucidated. Here, the authors use pseudomorphic Si1−xGex on Si(001), nanoscale films of Ni, and nanoscale Hf oxide films to illustrate this principle. View full abstract»

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  • Advances in silicon carbide science and technology at the micro- and nanoscales

    Page(s): 050805 - 050805-18
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    Advances in silicon carbide microfabrication and growth process optimization for silicon carbide nanostructures are ushering in new opportunities for microdevices capable of operation in a variety of demanding applications, involving high temperature, radiation, or corrosive environment. This review focuses on the materials science and processing technologies for silicon carbide thin films and low dimensional structures, and details recent progress in manufacturing technology, including deposition, metallization, and fabrication of semiconductor microdevices, with emphasis on sensor technology. The challenges remaining in developing silicon carbide as a mainstay materials platform are discussed throughout. View full abstract»

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  • Gaseous mixtures in vacuum systems and microfluidics

    Page(s): 050806 - 050806-17
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    In vacuum technology, one deals with gaseous mixtures more frequently than with a single gas, but the information about transport phenomena in mixtures published in the open literature is very poor. Moreover, methods to model mixture flows are more complicated than those for single gas. The aim of this work is to review general approaches to modeling mass, heat, and momentum transfer through gaseous mixtures over the whole range of gas rarefaction. This review is written in an easy, accessible manner avoiding hard mathematical derivations, though an extensive list of references is provided for readers wishing to find more details about the field. Results for some classical problems such as velocity slip and temperature jump coefficients, Poiseuille flow, Couette flow, and heat transfer for gaseous mixtures are presented in graphical form. A comparison of these results with those corresponding to a single gas is presented, which shows the peculiarities of the transport phenomena in mixtures and gives us an idea how to model mixture flows in vacuum systems and microfluidics. View full abstract»

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  • Electronic surface and dielectric interface states on GaN and AlGaN

    Page(s): 050807 - 050807-29
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    GaN and AlGaN have shown great potential in next-generation high-power electronic devices; however, they are plagued by a high density of interface states that affect device reliability and performance, resulting in large leakage current and current collapse. In this review, the authors summarize the current understanding of the gate leakage current and current collapse mechanisms, where awareness of the surface defects is the key to controlling and improving device performance. With this in mind, they present the current research on surface states on GaN and AlGaN and interface states on GaN and AlGaN-based heterostructures. Since GaN and AlGaN are polar materials, both are characterized by a large bound polarization charge on the order of 1013 charges/cm2 that requires compensation. The key is therefore to control the compensation charge such that the electronic states do not serve as electron traps or affect device performance and reliability. Band alignment modeling and measurement can help to determine the electronic state configuration. In particular, band bending can determine how the polarization bound charge is compensated; however, the band bending is extremely sensitive to the specific processing steps such as cleaning, dielectric or metal deposition, postdeposition or postmetallization treatments, which affect oxygen coverage, carbon contamination, structural defects, bonding configurations, defect states, absorbates, and Fermi pinning states. In many cases, the specific effects of these treatments on the surface and interface states are not entirely clear as the nature of the electronic states has been obscured in complexity and subtlety. Consequently, a more systematic and methodical approach may be required. View full abstract»

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  • Electron-stimulated surface chemical reactions on phosphors

    Page(s): 050808 - 050808-12
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    The range of phenomena occurring on the surface of phosphors during cathodoluminescence (CL) has been reviewed. In particular, the consequences of beam-stimulated dissociation, absorption, desorption, and reactions on the lifetime and maintenance of CL phosphors were illustrated by several case studies. The understanding of the atomistic mechanisms that lead to changes in the CL intensity, efficiency, and lifetime provide a basis for understanding the phenomenological law (Pfahnl's Law) used previously to predict lifetime of the phosphors. In particular, the electron-stimulated surface chemical reaction model describes many of the surface processes and allows quantitation of their rates based on parameters such as gas pressure, primary beam energy, temperature, and absorption energy. View full abstract»

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  • InGaN light-emitting diodes: Efficiency-limiting processes at high injection

    Page(s): 050809 - 050809-21
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    The authors discuss a relatively comprehensive theoretical and experimental study aimed on unveiling the dominant efficiency loss mechanism at high injection levels in InGaN light-emitting diodes (LEDs), which still limits their application for general lighting despite the breathtaking performance demonstration. A large body of theoretical and experimental data ascribes the observed efficiency loss to overflow of hot electrons aggravated by nonuniform distribution of carriers in the active region as the primary origin of the efficiency droop-phenomenon, but Auger recombination has also been invoked as the genesis of the efficiency loss. The electron overflow and the associated efficiency loss can be reduced substantially by inserting, in the n-side of the InGaN active region, an InGaN stair-case electron injector (SEI) with a step-like increased indium composition to operate as an “electron cooler.” In contrast to electron-blocking layer usually employed to prevent the electron leakage from the active region, the SEI does not impede hole injection due to the absence of valence band offset with p-GaN. Moreover, SEI does not generate piezoelectric polarization field in addition to differential spontaneous polarization field that pulls down the conduction band at the AlGaN/GaN interface aggravating the electron rollover. In terms of the active region design, owing to their high three-dimensional density of states, it is argued that double heterostructures (DHs) are more attractive for general-lighting LEDs than necessarily quantum wells. The authors demonstrate that DH-based LED active regions, particularly wide ones and those composed of multiple DHs separated by thin (3 nm) In0.06Ga0.94N barriers of reduced barrier height, meant to allow efficient hole transport across the active regions, naturally act as an electron cooler, thus considerably reducing the electron overflow at high injection. However, a wide separation of- electron and hole distribution functions in DHs wider than 6 nm substantially reduces the radiative recombination efficiency at injection current densities below ∼200 A/cm2. Consequently, the LEDs with dual 6 nm and quad (4×) 3 nm DHs separated by 3-nm In0.06Ga0.94N barriers exhibit the highest external quantum efficiency with substantially reduced efficiency degradation at injection current densities of special interest for low-voltage general-lighting applications. The authors conclude that, for achieving the highest possible LED efficiency, it is imperative that optimum the SEI and the active region should be designed to operate in unison. View full abstract»

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  • Semiconductor surface functionalization for advances in electronics, energy conversion, and dynamic systems

    Page(s): 050810 - 050810-12
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    Semiconductors have played a tremendous role in the development of electronics since the inception of the electronics revolution more than 60 years ago. Over this period, the performance of semiconductors relied on the development of robust and reliable surface passivation and functionalization schemes. As the size of the individual components in microelectronics has decreased, the role of surface chemistry has become even more important. Moreover, in the development of fields such as sensing and energy conversion, the surface chemistry of the component semiconductor materials has often driven the functionality of devices and applications. Available functionalization chemistries take advantage of the localized and covalent nature of the semiconductor surfaces to form organic layers that can passivate the surface, assemble nanopatterns, influence subsequent deposition, or change the nature of interfacial electron transfer. Despite an established toolkit already available for semiconductor surface functionalization, new applications will require better, cheaper, faster, and more controllable ways to produce surfaces that are well-defined, responsive to external factors, ready for further treatment, and adaptable for use on unusually shaped features or on nanostructures. Two major directions are identified in the field of semiconductor surface functionalization based on the nature of the surface response required: “static” in which surface properties are modified by classical thermodynamic and kinetic control approaches and then maintain these properties following the modification, and “dynamic” in which surfaces adapt to the conditions needed and respond to the application of external stimuli. Possible opportunities within these two development pathways are presented in this article. View full abstract»

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  • Charge transfer processes at the interface between plasmas and liquids

    Page(s): 050811 - 050811-13
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    Charge transfer processes at the interface of plasmas and liquid electrolytes have been studied for over 100 years. Both plasmas and liquid electrolytes contain charged species, and interactions between the two phases represent a unique combination of physics, chemistry, and materials science that is fundamentally different than that between solid electrodes and liquid electrolytes. Unfortunately, scientific progress over the last century has been slow because of several key challenges including the inability to stabilize nonthermal plasmas at atmospheric pressure, and the lack of fundamental understanding of the mechanisms for charge transfer. Within the last decade, significant strides have been made to overcome these challenges. Here, the authors review this fascinating area of study, highlighting the history, development of experimental systems, mechanistic aspects, and emerging applications. View full abstract»

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  • Material design and discovery with computational materials science

    Page(s): 050812 - 050812-5
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    The discovery and design of new materials to improve existing technologies or enable new applications is a driving force for much of the research that takes place in multiple disciplines, including materials science and engineering, condensed matter physics, and materials chemistry. This article reviews the way in which computational methods are being applied to achieve the promise of “materials by design.” In particular, the article reviews the technologies that have enabled the evolution of computational materials science as a field and its integration with cutting-edge experimental methods. Illustrative applications are discussed where traditional computational methods and materials informatics approaches were applied to design new materials. The article concludes with a discussion of the future outlook of computational materials science within the context of material design and discovery. View full abstract»

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  • Progress and issues in e-beam and other top down nanolithography

    Page(s): 050813 - 050813-9
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    In this 60th anniversary year of the American Vacuum Society (AVS), this paper is one in a series of topical reviews of science and technology represented by the various AVS Divisions. The focus of the paper is on trends, frontier advancement, and issues remaining in nanolithography. The manuscript highlights, in particular, the progress made in electron beam lithography system development, advancement in materials and methods used to pattern down to a few nanometers, and the prospects for multiple beam systems as high throughput alternatives. Also discussed are the underlying rationales for observed trends in lithography tool development. Invariably the discussion about emerging lithography solutions bifurcates depending on whether one is examining the roadmap for the silicon integrated circuit industry or everything else. The emphasis here is mostly on everything else but also explores the boundary. View full abstract»

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  • Development of molecular beam epitaxy technology for III–V compound semiconductor heterostructure devices

    Page(s): 050814 - 050814-10
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    Molecular beam epitaxy (MBE) is a versatile ultrahigh vacuum technique for growing multiple epitaxial layers of semiconductor crystals with high precision. The extreme control of the MBE technique over composition variation, interface sharpness, impurity doping profiles, and epitaxial layer thickness to the atomic level makes it possible to demonstrate a wide variety of novel semiconductor structures. Since its invention nearly 40 years ago, the MBE technique has evolved from a laboratory apparatus for exploring new materials and novel devices to a favored tool for the mass production of III–V high-speed devices. This paper will review some of the past developments in this technology and propose an outlook of future developments. View full abstract»

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  • Tobacco mosaic virus: A biological building block for micro/nano/bio systems

    Page(s): 050815 - 050815-24
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    Tobacco mosaic virus (TMV) has the potential to be an ideal candidate for a building block of the next-generation micro/nano/bio systems. The TMV virion is a high-aspect ratio rigid nanotube that is robust and compatible with some conventional microfabrication processes. TMV can be chemically and genetically modified to enhance its physical properties and tailor them to specific applications. This review covers the use of TMV nanostructures in a wide range of micro/nano/bio systems. TMV has been utilized in the production of nanowires, nanostructured thin films, biomimetic surfaces, novel sensors, high performance microbatteries, solid-state electronics, and engineered biosystems. The work highlighted here is meant to give a perspective of the entire breadth of the properties of these virions, from their synthesis and functionalization to assembly and patterning, as well as feature works that represent key milestones in the field of biofabrication and biomaterial integration. The advantages already demonstrated by the integration of TMV nanostructures, even at this early stage of development, suggest that the applications for this micro/nano/bio systems building block will continue to grow. View full abstract»

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  • Toward a new world of molecular devices: Making metallic contacts to molecules

    Page(s): 050816 - 050816-19
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    The incorporation of molecular layers into electronic devices has many applications from sensing to energy harvesting. While many devices have been demonstrated and some are close to market, there are many challenges that must be overcome before molecular assemblies are incorporated in every device. Perhaps the most important is the development of reliable, reproducible metallic contacts to connect molecular layers with other device components. In order for this to happen, the following must be developed: an understanding of the interaction between the electrode and the different parts of the molecular layer, methods for the large-scale integration of molecules into devices, and eliminate variability in the contact. In this paper, the progress and remaining challenges in making metallic contacts to molecules will be discussed. Methods employed range from those that measure single molecules to ensembles of molecules, as well as those that could be employed in manufacturing processes. View full abstract»

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  • Case studies in surface photochemistry on metal nanoparticles

    Page(s): 050817 - 050817-13
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    The authors give a survey of their work on photochemical processes at silver nanoparticles carried out in Berlin in the past decade. Using well established procedures for the preparation of silver nanoparticles (Ag NPs) supported on ultrathin alumina layers on NiAl single crystals, they have investigated the photoreactions of adsorbed (NO)2 and of Xe induced by laser pulses. The authors examined the influences of photon energy (2.3, 3.5, and 4.7 eV) and polarization, mean particle size (2–10 nm), and pulse length (5 ns and 100 fs) on yields and cross sections, and on photoreaction mechanisms. Comparison with Ag(111) was made throughout. For the NO dimer layer, the authors find general agreement with known results on bulk Ag(111) in terms of possible reactions (NO desorption and NO monomer formation as well as conversion into adsorbed N2O and O) and predominant mechanism (via transient negative ion formation, TNI); NO desorption is the strongest channel. However, on the NPs, the cross sections show selective enhancement in particular under conditions of excitation of the Mie plasmon due to the field enhancement caused by it, but—more weakly—also under off-resonant conditions which the authors interpret by excitation confinement in the NPs. For ns laser pulses, the desorption yield responds linearly to photon flux so that the cross sections are independent of laser fluence. Using fs laser pulses, nonlinear yield response is found under plasmon excitation which is interpreted as due to re-excitation of hot electrons in the NPs during a single laser pulse. The dynamics of the individual process, however, stay the same under almost all conditions, as indicated by constant energy distributions over translational, rotational, and vibrational energies of the desorbing NO molecules, even in the nonlinear range. Only for the highest photon energy (i.e., off-resonance) and the smallest particles, a new chan- el is observed with higher translational energy which is believed to proceed via transient positive ions. The branching into the various reaction channels is found to be different on Ag NPs from that on Ag(111) which is ascribed to differing enhancements for the various channels. While these results show that for a typical molecular reaction only the yields are modified on NPs, very different behavior is observed for desorption of adsorbed Xe. Here, low intensity excitation of the Mie plasmon leads to chaotic response which must be due to hot spot formation. As in this case no simple desorption mechanism (via transient negative or positive ions, or direct HOMO–LUMO excitation of the adsorbate) is expected, a direct action of the plasmon excitation is postulated. Some general conclusions are drawn from these case studies. View full abstract»

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  • History of atomic layer deposition and its relationship with the American Vacuum Society

    Page(s): 050818 - 050818-11
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    This article explores the history of atomic layer deposition (ALD) and its relationship with the American Vacuum Society (AVS). The authors describe the origin and history of ALD science in the 1960s and 1970s. They also report on how the science and technology of ALD progressed through the 1990s and 2000s and continues today. This article focuses on how ALD developed within the AVS and continues to evolve through interactions made possible by the AVS, in particular, the annual International AVS ALD Conference. This conference benefits students, academics, researchers, and industry practitioners alike who seek to understand the fundamentals of self-limiting, alternating binary surface reactions, and how they can be applied to form functional (and sometimes profitable) thin film materials. The flexible structure of the AVS allowed the AVS to quickly organize the ALD community and create a primary conference home. Many new research areas have grown out of the original concepts of “Atomic Layer Epitaxy” and “Molecular Layering,” and some of them are described in this article. The people and research in the ALD field continue to evolve, and the AVS ALD Conference is a primary example of how the AVS can help a field expand and flourish. View full abstract»

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  • SIMS of organics—Advances in 2D and 3D imaging and future outlook

    Page(s): 050819 - 050819-14
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    Secondary ion mass spectrometry (SIMS) has become a powerful technique for the label-free analysis of organics from cells to electronic devices. The development of cluster ion sources has revolutionized the field, increasing the sensitivity for organics by two or three orders of magnitude and for large clusters, such as C60 and argon clusters, allowing depth profiling of organics. The latter has provided the capability to generate stunning three dimensional images with depth resolutions of around 5 nm, simply unavailable by other techniques. Current state-of-the-art allows molecular images with a spatial resolution of around 500 nm to be achieved and future developments are likely to progress into the sub-100 nm regime. This review is intended to bring those with some familiarity with SIMS up-to-date with the latest developments for organics, the fundamental principles that underpin this and define the future progress. State-of-the-art examples are showcased and signposts to more in-depth reviews about specific topics given for the specialist. View full abstract»

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  • Surface characterization of nanomaterials and nanoparticles: Important needs and challenging opportunities

    Page(s): 050820 - 050820-34
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    This review examines characterization challenges inherently associated with understanding nanomaterials and the roles surface and interface characterization methods can play in meeting some of the challenges. In parts of the research community, there is growing recognition that studies and published reports on the properties and behaviors of nanomaterials often have reported inadequate or incomplete characterization. As a consequence, the true value of the data in these reports is, at best, uncertain. With the increasing importance of nanomaterials in fundamental research and technological applications, it is desirable that researchers from the wide variety of disciplines involved recognize the nature of these often unexpected challenges associated with reproducible synthesis and characterization of nanomaterials, including the difficulties of maintaining desired materials properties during handling and processing due to their dynamic nature. It is equally valuable for researchers to understand how characterization approaches (surface and otherwise) can help to minimize synthesis surprises and to determine how (and how quickly) materials and properties change in different environments. Appropriate application of traditional surface sensitive analysis methods (including x-ray photoelectron and Auger electron spectroscopies, scanning probe microscopy, and secondary ion mass spectroscopy) can provide information that helps address several of the analysis needs. In many circumstances, extensions of traditional data analysis can provide considerably more information than normally obtained from the data collected. Less common or evolving methods with surface selectivity (e.g., some variations of nuclear magnetic resonance, sum frequency generation, and low and medium energy ion scattering) can provide information about surfaces or interfaces in working environments (operando or in situ) or information not provided by more traditional methods. Although these methods may r- quire instrumentation or expertise not generally available, they can be particularly useful in addressing specific questions, and examples of their use in nanomaterial research are presented. View full abstract»

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  • Band offsets, Schottky barrier heights, and their effects on electronic devices

    Page(s): 050821 - 050821-18
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    The authors review the band line-ups and band offsets between semiconductors, dielectrics, and metals, including the theory, experimental data, and the chemical trends. Band offsets have been critical in the choice of high dielectric constant oxides to be used in advanced metal oxide semiconductor field effect transistors. It turns out that band offsets are also critical in the theory of doping limits, design of transparent conducting oxides, organic semiconductors, and electrodes to use in light emitting devices, photovoltaic cells, and photochemical cells. It is shown how band line-ups can be understood in terms of charge neutrality levels. These are also related to states due to interstitial hydrogen. View full abstract»

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  • Recent search for new superhard materials: Go nano!

    Page(s): 050822 - 050822-33
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    High elastic moduli do not guarantee high hardness because upon finite shear electronic instabilities often occur that result in transformation to softer phases. Therefore, the author concentrates on the extrinsically superhard nanostructured materials, which are the most promising. Decreasing crystallite size results in strengthening and hardening because the grain boundaries impede the plasticity (e.g., Hall–Petch strengthening in case of dislocation activity). However, this hardening is limited to a crystallite size down to 10–15 nm below which softening due to grain boundary shear dominates. This softening can be reduced by forming low energy grain boundaries or a strong interfacial layer. In such a way, much higher hardness enhancement can be achieved. The emphasis will be on the understanding of the mechanisms of the hardness enhancement. A special section deals with examples of the present industrial applications of such coatings on tools for machining in order to illustrate that these materials are already in large-scale use. In the last section, the author summarizes the open questions and limitations for the preparation of the super- and ultrahard nanocomposite coatings and possible ways on how to overcome them. View full abstract»

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  • Tailored ion energy distributions on plasma electrodes

    Page(s): 050823 - 050823-14
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    As microelectronic device features continue to shrink approaching atomic dimensions, control of the ion energy distribution on the substrate during plasma etching and deposition becomes increasingly critical. The ion energy should be high enough to drive ion-assisted etching, but not too high to cause substrate damage or loss of selectivity. In many cases, a nearly monoenergetic ion energy distribution (IED) is desired to achieve highly selective etching. In this work, the author briefly reviews: (1) the fundamentals of development of the ion energy distribution in the sheath and (2) methods to control the IED on plasma electrodes. Such methods include the application of “tailored” voltage waveforms on an electrode in continuous wave plasmas, or the application of synchronous bias on a “boundary electrode” during a specified time window in the afterglow of pulsed plasmas. View full abstract»

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  • Strain induced microstructural and ordering behaviors of epitaxial Fe38.5Pd61.5 films grown by pulsed laser deposition

    Page(s): 050824 - 050824-13
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    Epitaxial films of Fe38.5Pd61.5 at the L10-L12 eutectoid composition have been grown on MgO (001) oriented substrates by pulsed laser deposition. The effect of deposition temperature on the magnetic, microstructural, and crystallographic natures of these films are discussed. The films in this study exhibit atomic ordering with increasing deposition temperature, transitioning from the disordered face centered cubic (FCC) phase to an L12 ordered phase, which is tetragonally distorted due to epitaxial strain. This distortion leads to a perturbation in the Fe occupancy of the Pd superlattice sites at nonstoichiometric compositions. Additionally, Fe38.5Pd61.5 films grown at 550 °C have been found in an unique two-phase microstructure of prismatic, Fe60Pd40 disordered FCC secondary phases with 10–100 nm facets oriented along the 〈110〉 substrate directions, embedded within a nearly stoichiometric ordered L12-Fe27Pd73 matrix. These secondary phase precipitates exhibit single domain magnetic axis rotation, while the ordered L12 matrix has a magnetic easy axis aligned in-plane. View full abstract»

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Aims & Scope

The Journal of Vacuum Science and Technology A is devoted to reports of original research, review articles, and Critical Review articles.

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G. Lucovsky
North Carolina State University