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Top 20 Most Read Articles
December 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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A comprehensive review of ZnO materials and devices J. Appl. Phys. 98, 041301 (2005); http://dx.doi.org/10.1063/1.1992666 (103 pages) Online Publication Date: 30 August 2005
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The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by
Bunn [Proc. Phys. Soc. London 47, 836 (1935)
], studies of its vibrational properties with Raman scattering in 1966 by
Damen et al. [Phys. Rev. 142, 570 (1966)
], detailed optical studies in 1954 by
Mollwo [Z. Angew. Phys. 6, 257 (1954)
], and its growth by chemical-vapor transport in 1970 by
Galli and Coker [Appl. Phys. Lett. 16, 439 (1970)
]. In terms of devices, Au Schottky barriers in 1965 by
Mead [Phys. Lett. 18, 218 (1965)
], demonstration of light-emitting diodes (1967) by
Drapak [Semiconductors 2, 624 (1968)
], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by
Minami et al. [Jpn. J. Appl. Phys. 13, 1475 (1974)
], ZnO/ZnSe n-p junctions (1975) by
Tsurkan et al. [Semiconductors 6, 1183 (1975)
], and Al/Au Ohmic contacts by
Brillson [J. Vac. Sci. Technol. 15, 1378 (1978)
] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by
Look and Claflin [Phys. Status Solidi B 241, 624 (2004)
]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures.
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Nanostructured ceramics by electrospinning J. Appl. Phys. 102, 111101 (2007); http://dx.doi.org/10.1063/1.2815499 (17 pages) Online Publication Date: 3 December 2007
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Nanostructured ceramics are attractive materials that find potential uses ranging from simple everyday applications like paints and pigments to sophisticated ones such as bioimaging, sensors, etc. The inability to economically synthesize nanoscale ceramic structures in a large scale and simultaneously achieve precise control of their size has restricted their real time application. Electrospinning is an efficient process that can fabricate nanofibers on an industrial scale. During the last 5 years, there has been remarkable progress in applying this process to the fabrication of ceramic nanorods and nanofibers. Ceramic nanofibers are becoming useful and niche materials in several applications owing to their surface dependant and size dependant properties. These advances are reviewed here. The various ceramic nanofiber systems that have been fabricated so far are presented. The physical and chemical property enhancements due to the nanosize have been discussed in detail and the various applications they fit into are outlined in this article.
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Recent progress in solution processable organic light emitting devices J. Appl. Phys. 102, 091101 (2007); http://dx.doi.org/10.1063/1.2804122 (21 pages) Online Publication Date: 13 November 2007
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Organic light emitting devices (OLEDs) have been the subject of intense research because of their potential for flat panel display and solid state lighting applications. While small molecule OLEDs with very high efficiencies have been demonstrated, solution processable devices are more desirable for large size flat panel display and solid state applications because they are compatible with low cost, large area roll-to-roll manufacturing process. In this review paper, we will present the recent progress made in solution processable OLEDs. The paper will be divided into three parts. In the first part of the paper, we will focus on the recent development of fluorescent polymer OLEDs based on conjugated polyfluorene copolymers. Specifically, we will present results of carrier transport and injection measurements, and discuss how the charge transport and injection properties affect the device performance. In the second part of the paper, we will focus on the recent progress on phosphorescent dye-dispersed nonconjugated polymer OLEDs. Specifically, we will present our recent results on high efficiency green and blue emitting devices based on the dye-dispersed polymer approach. Similar to fluorescent conjugated polymer OLEDs, charge transport and injection properties in dye-dispersed polymer OLEDs also play an important role in the device performance. In the third part of this paper, we will present our results on white emitting phosphorescent OLEDs. Two approaches have been used to demonstrate white emitting OLEDs. First, white emitting OLEDs were made using blue emitting OLEDs with downconversion phosphors. Second, white emitting OLEDs were made by dispersing red, green, and blue phosphorescent dyes into the light emitting layer. High efficiency devices have been demonstrated with both approaches.
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High-κ gate dielectrics: Current status and materials properties considerations J. Appl. Phys. 89, 5243 (2001); http://dx.doi.org/10.1063/1.1361065 (33 pages)
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Many materials systems are currently under consideration as potential replacements for SiO2 as the gate dielectric material for sub-0.1 μm complementary metal–oxide–semiconductor (CMOS) technology. A systematic consideration of the required properties of gate dielectrics indicates that the key guidelines for selecting an alternative gate dielectric are (a) permittivity, band gap, and band alignment to silicon, (b) thermodynamic stability, (c) film morphology, (d) interface quality, (e) compatibility with the current or expected materials to be used in processing for CMOS devices, (f) process compatibility, and (g) reliability. Many dielectrics appear favorable in some of these areas, but very few materials are promising with respect to all of these guidelines. A review of current work and literature in the area of alternate gate dielectrics is given. Based on reported results and fundamental considerations, the pseudobinary materials systems offer large flexibility and show the most promise toward successful integration into the expected processing conditions for future CMOS technologies, especially due to their tendency to form at interfaces with Si (e.g. silicates). These pseudobinary systems also thereby enable the use of other high-κ materials by serving as an interfacial high-κ layer. While work is ongoing, much research is still required, as it is clear that any material which is to replace SiO2 as the gate dielectric faces a formidable challenge. The requirements for process integration compatibility are remarkably demanding, and any serious candidates will emerge only through continued, intensive investigation. © 2001 American Institute of Physics. |
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Small molecular weight organic thin-film photodetectors and solar cells J. Appl. Phys. 93, 3693 (2003); http://dx.doi.org/10.1063/1.1534621 (31 pages) Online Publication Date: 21 March 2003
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In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illumination, devices with external power conversion efficiencies of 2.4%±0.3% are reported. In addition, we show that by using materials with extended exciton diffusion lengths LD, highly efficient double heterojunction photovoltaic cells are obtained, even in the absence of a light trapping geometry. Using C60 as an acceptor material, double heterostructure external power conversion efficiencies of 3.6%±0.4% under 1 sun AM1.5 illumination are obtained. Stacking of single heterojunction devices leads to thin film multiple heterojunction photovoltaic and photodetector structures. Thin bilayer photovoltaic cells can be stacked with ultrathin (∼5 Å), discontinuous Ag layers between adjacent cells serving as efficient recombination sites for electrons and holes generated in the neighboring cells. Such stacked cells have open circuit voltages that are n times the open circuit voltage of a single cell, where n is the number of cells in the stack. In optimized structures, the short circuit photocurrent remains approximately constant upon stacking thin cells, leading to higher achievable power conversion efficiencies, as confirmed by modelling optical interference effects and exciton migration. A 2.5%±0.3% power efficiency under 100 mW/cm2 AM1.5 illumination conditions is obtained by stacking two ∼1% efficient devices. Alternatively, when the contact layers between the stacked cells are eliminated, a multilayer structure consisting of alternating films of donor and acceptor-type materials is obtained. Since the thicknesses of the individual layers (∼5 Å) can be substantially smaller than the exciton diffusion length, nearly 100% of the photogenerated excitons are dissociated, and the resulting free charges are detected. In addition, the ultrathin organic layers facilitate electron and hole transport through the multilayer stack by tunneling. When these devices are operated as photodetectors under applied fields >106 V/cm, the carrier collection efficiency reaches 80%, leading to external quantum efficiencies of 75%±1% across the visible spectrum in cells containing the thinnest layers. We find that due to the fast carrier tunneling process, the temporal response of these multilayer detectors is a direct measure of exciton dynamics. Response times of 720±50 ps are achieved, leading to a 3 dB bandwidth of 430±30 MHz. A summary of representative results obtained for both polymer and small molecule photovoltaic cells and photodetectors is included in this review. Prospects for further improvements in organic solar cells and photodetectors are considered. © 2003 American Institute of Physics. |
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Band parameters for III–V compound semiconductors and their alloys J. Appl. Phys. 89, 5815 (2001); http://dx.doi.org/10.1063/1.1368156 (61 pages)
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We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other. © 2001 American Institute of Physics. |
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Microstructured surface design for omnidirectional antireflection coatings on solar cells J. Appl. Phys. 102, 103105 (2007); http://dx.doi.org/10.1063/1.2817470 (9 pages) Online Publication Date: 27 November 2007
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We recently demonstrated a new process for the formation of partially spherical structures as an omnidirectional antireflection coating (omni-AR). In this paper, we report the simulation results of the angular and spectral dependences of the total reflectivity on various microstructured surfaces based on the rigorous coupled-wave analysis. Close to zero reflection can be achieved in these microstructured surfaces over an extended spectral region for large ranges of light incident angles. The impact of feature size, density, shape, and refractive index has all been investigated. The experimental results agree reasonably well with the theoretical work. Such an omni-AR structure offers an attractive solution to current crystalline silicon solar cells, as well as future thin film, quantum dot, and organic solar cells.
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Review of zincblende ZnO: Stability of metastable ZnO phases J. Appl. Phys. 102, 071101 (2007); http://dx.doi.org/10.1063/1.2787957 (12 pages) Online Publication Date: 1 October 2007
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Common II-VI compound semiconducting materials are stable thermodynamically with zincblende phase, while the II-O materials such as zinc oxide (ZnO) and beryllium oxide (BeO) are stable with wurtzite phase, and cadmium oxide (CdO) and magnesium oxide (MgO) are stable in rocksalt phase. This phase disharmony in the same material family laid a challenge for the basic physics and in practical applications in optoelectronic devices, where ternary and quaternary compounds are employed. Thermodynamically the zincblende ZnO is a metastable phase which is free from the giant internal electric fields in the [001] directions and has an easy cleavage facet in the ⟨110⟩ directions for laser cavity fabrication that combined with evidence for the higher optical gain. The zincblende materials also have lower ionicity that leads to the lower carrier scattering and higher doping efficiencies. Even with these outstanding features in the zincblende materials, the growth of zincblende ZnO and its fundamental properties are still limited. In this paper, recent progress in growth and fundamental properties of zincblende ZnO material has been reviewed.
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J. Appl. Phys. 102, 113101 (2007); http://dx.doi.org/10.1063/1.2817954 (5 pages) Online Publication Date: 3 December 2007
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In this paper, we present four GaN based polar quantum structures grown on c-plane embedded in p-i-n diode architecture as a part of high-speed electroabsorption modulators for use in optical communication (free-space non-line-of-sight optical links) in the ultraviolet (UV): the first modulator incorporates ∼ 4–6 nm thick GaN/AlGaN quantum structures for operation in the deep-UV spectral region and the other three incorporate ∼ 2–3 nm thick InGaN/GaN quantum structures tuned for operation in violet to near-UV spectral region. Here, we report on the design, epitaxial growth, fabrication, and characterization of these quantum electroabsorption modulators. In reverse bias, these devices exhibit a strong electroabsorption (optical absorption coefficient change in the range of 5500–13 000 cm−1 with electric field swings of 40–75 V/μm) at their specific operating wavelengths. In this work, we show that these quantum electroabsorption structures hold great promise for future applications in ultraviolet optoelectronics technology such as external modulation and data coding in secure non-line-of-sight communication systems.
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Ferroelectric thin films: Review of materials, properties, and applications J. Appl. Phys. 100, 051606 (2006); http://dx.doi.org/10.1063/1.2336999 (46 pages) Online Publication Date: 12 September 2006
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An overview of the state of art in ferroelectric thin films is presented. First, we review applications: microsystems’ applications, applications in high frequency electronics, and memories based on ferroelectric materials. The second section deals with materials, structure (domains, in particular), and size effects. Properties of thin films that are important for applications are then addressed: polarization reversal and properties related to the reliability of ferroelectric memories, piezoelectric nonlinearity of ferroelectric films which is relevant to microsystems’ applications, and permittivity and loss in ferroelectric films—important in all applications and essential in high frequency devices. In the context of properties we also discuss nanoscale probing of ferroelectrics. Finally, we comment on two important emerging topics: multiferroic materials and ferroelectric one-dimensional nanostructures.
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Surface plasmon enhanced silicon solar cells J. Appl. Phys. 101, 093105 (2007); http://dx.doi.org/10.1063/1.2734885 (8 pages) Online Publication Date: 7 May 2007
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Thin-film solar cells have the potential to significantly decrease the cost of photovoltaics. Light trapping is particularly critical in such thin-film crystalline silicon solar cells in order to increase light absorption and hence cell efficiency. In this article we investigate the suitability of localized surface plasmons on silver nanoparticles for enhancing the absorbance of silicon solar cells. We find that surface plasmons can increase the spectral response of thin-film cells over almost the entire solar spectrum. At wavelengths close to the band gap of Si we observe a significant enhancement of the absorption for both thin-film and wafer-based structures. We report a sevenfold enhancement for wafer-based cells at λ = 1200 nm and up to 16-fold enhancement at λ = 1050 nm for 1.25 μm thin silicon-on-insulator (SOI) cells, and compare the results with a theoretical dipole-waveguide model. We also report a close to 12-fold enhancement in the electroluminescence from ultrathin SOI light-emitting diodes and investigate the effect of varying the particle size on that enhancement.
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J. Appl. Phys. 102, 113509 (2007); http://dx.doi.org/10.1063/1.2819367 (4 pages) Online Publication Date: 6 December 2007
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Nitrogen-doped ZnO thin films with different nitrogen concentrations were fabricated by plasma-assisted molecular beam epitaxy. Hall effect measurements show p-type conduction for samples with low doping concentration. In highly doped ZnO, the p-type conduction converted to high resistance or unstable p-type behavior. This result indicates that highly doped samples are heavily compensated. In the low temperature photoluminescence spectrum, a donor-acceptor pair (DAP) emission band shows a strong redshift and broadening with increasing nitrogen doping concentration. The large shift of the DAP emission is explained by the Coulomb-potential fluctuation model related to compensated semiconductors.
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Structural and magnetic properties of Zn1−xMnxO (0 ≤ x ≤ 0.40) nanoparticles J. Appl. Phys. 102, 103907 (2007); http://dx.doi.org/10.1063/1.2815647 (6 pages) Online Publication Date: 21 November 2007
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Zn1−xMnxO nanoparticles synthesized by a combustion method with Mn concentration x up to 0.40 are carefully characterized by means of x-ray powder diffraction and high resolution transmission electron microscopy. Single-phase solid solution Zn1−xMnxO is achieved with a solubility limit of ∼ 30 at. % Mn. An almost linear dependence of lattice constant on the Mn content is observed, indicating the homogeneous substitution of Mn2+ for the Zn2+ in wurtzite ZnO. Magnetization at T = 5 K and H = 5 T exhibits a maximum around the Mn doping concentration of xm = 0.125. Room temperature ferromagnetism with low coercive field (Hc ∼ 70±5 Oe) is obtained for the sample with x = 0.01. The observed magnetic properties of Zn1−xMnxO can be well understood within the framework of the donor impurity band exchange model.
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The properties of ZnO photoluminescence at and above room temperature J. Appl. Phys. 102, 116105 (2007); http://dx.doi.org/10.1063/1.2822156 (3 pages) Online Publication Date: 11 December 2007
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A study of the photoluminescence characteristics of a ZnO single crystal at the temperature range 173–823 K is presented. The analysis employed the electron-phonon interaction model as well as the radiative recombination rate model. Both studies indicate that at ∼ 700 K the photoluminescence character undergoes a transition from being a free exciton emission to a band gap recombination, implying a breakup of excitons into free carriers is occurring. The transition temperature corresponds to ∼ 60 meV, which is consistent with the binding energy of the free exciton in ZnO.
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Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures J. Appl. Phys. 98, 011101 (2005); http://dx.doi.org/10.1063/1.1951057 (10 pages) Online Publication Date: 11 July 2005
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We review the basic physics of surface-plasmon excitations occurring at metal/dielectric interfaces with special emphasis on the possibility of using such excitations for the localization of electromagnetic energy in one, two, and three dimensions, in a context of applications in sensing and waveguiding for functional photonic devices. Localized plasmon resonances occurring in metallic nanoparticles are discussed both for single particles and particle ensembles, focusing on the generation of confined light fields enabling enhancement of Raman-scattering and nonlinear processes. We then survey the basic properties of interface plasmons propagating along flat boundaries of thin metallic films, with applications for waveguiding along patterned films, stripes, and nanowires. Interactions between plasmonic structures and optically active media are also discussed.
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J. Appl. Phys. 97, 011101 (2005); http://dx.doi.org/10.1063/1.1819976 (28 pages) Online Publication Date: 9 December 2004
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This article reviews the history and current progress in high-mobility strained Si, SiGe, and Ge channel metal-oxide-semiconductor field-effect transistors (MOSFETs). We start by providing a chronological overview of important milestones and discoveries that have allowed heterostructures grown on Si substrates to transition from purely academic research in the 1980’s and 1990’s to the commercial development that is taking place today. We next provide a topical review of the various types of strain-engineered MOSFETs that can be integrated onto relaxed Si1−xGex, including surface-channel strained Si n- and p-MOSFETs, as well as double-heterostructure MOSFETs which combine a strained Si surface channel with a Ge-rich buried channel. In all cases, we will focus on the connections between layer structure, band structure, and MOS mobility characteristics. Although the surface and starting substrate are composed of pure Si, the use of strained Si still creates new challenges, and we shall also review the literature on short-channel device performance and process integration of strained Si. The review concludes with a global summary of the mobility enhancements available in the SiGe materials system and a discussion of implications for future technology generations.
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J. Appl. Phys. 102, 114903 (2007); http://dx.doi.org/10.1063/1.2817255 (7 pages) Online Publication Date: 4 December 2007
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N-doped ZnO films were prepared by annealing zinc oxynitride films deposited by rf reactive sputtering. Two Raman peaks were observed at 274 and 580 cm−1. According to the variation of the integral intensity of these two peaks, the nitrogen activation at 500 °C [the activation temperature (AT)] has been obtained. Below the AT, the integral intensities of them show a different variation trend. X-ray photoelectron spectroscopy (XPS) indicates the N chemical state variation for them and finds the activated Zn-N bond. Further analyses by photoluminescence (PL) spectra and spectroscopic ellipsometry (SE) have been carried out. The activated sample exhibits a symmetric emission peak at 3.22 eV assigned to be the A0X emission at room temperature. SE investigation takes account of samples within the different temperature span divided by the AT. Different factors, such as nitrogen dopant (N)O and the nanocrystal growth, which affect the redshift of the absorption edges, have been discussed.
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J. Appl. Phys. 102, 103909 (2007); http://dx.doi.org/10.1063/1.2812541 (5 pages) Online Publication Date: 27 November 2007
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The spin dynamics of sub-100-nm Ni80Fe20 nanomagnets are directly measured using the magneto-optic Kerr effect and a broadband detection scheme. Elliptical dots approximately 68 nm in diameter and 10 nm thick were fabricated in 20×20 μm2 arrays. There is approximately a factor of 2 increase in the effective linewidth when compared to a 20 μm diameter continuous disk of the same material. Using micromagnetic simulations, we model the effect of dot-to-dot size variation on the effective linewidth and find that 2 nm size variations are more than sufficient to account for the effective increase in linewidth.
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Physics of the krypton fluoride laser J. Appl. Phys. 51, 2406 (1980); http://dx.doi.org/10.1063/1.328010 (15 pages) Online Publication Date: 9 July 2008
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The extremely complex kinetics of pumping, quenching, and absorption in krypton fluoride excimer lasers is now relatively well understood. A single comprehensive kinetic model, solved approximately by analytic means and more exactly by a computer code of moderate size, is capable of simulating with great accuracy the performance of KrF lasers pumped by e‐beam sustained discharge, and self‐sustained discharge. We present the results of applications of this model to interpret and predict the limiting behavior and scaling of KrF lasers. A measure of remaining uncertainties is discussed. |
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Barrier inhomogeneity and electrical properties of Pt/GaN Schottky contacts J. Appl. Phys. 102, 113701 (2007); http://dx.doi.org/10.1063/1.2817647 (8 pages) Online Publication Date: 3 December 2007
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The temperature dependence of the electrical properties of Pt/GaN Schottky barrier was studied. In particular, a Schottky barrier height of 0.96 eV and an ideality factor of 1.16 were found after a postdeposition annealing at 400 °C. Nanoscale electrical characterization was carried out by the conductive biased tip of an atomic force microscope both on the bare GaN surface and on the Pt/GaN contacts. The presence of a lateral inhomogeneity of the Schottky barrier, with a Gaussian distribution of the barrier height values, was demonstrated. Moreover, GaN surface defects were demonstrated to act as local preferential paths for the current conduction. The temperature dependent electrical characteristics of the diodes were discussed in terms of the existing models on inhomogeneous barriers and correlated to the nanoscale electrical characterization of the barrier. In this way, the anomalous electrical behavior of the ideality factor and of the Schottky barrier and the low experimental value of the Richardson’s constant were explained.
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