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Top 20 Most Read Articles
June 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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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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Silicon nanoparticles: Absorption, emission, and the nature of the electronic bandgap J. Appl. Phys. 101, 103112 (2007); http://dx.doi.org/10.1063/1.2720095 (8 pages) Online Publication Date: 30 May 2007
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Silicon nanoparticles synthesized in the gas phase are studied. From time-resolved photoluminescence measurements we determine, quantitatively, the size-dependence of the oscillator strength of the nanoparticles. We investigate experimentally the absorption and photoluminescence emission of nanoparticle ensembles with a broad size distribution. Using a model which accounts for size-effects in both oscillator strength and quantum-confinement, we are able to calculate absorption and emission spectra of ensemble samples. From these results we have determined, whether silicon nanoparticles should be regarded as indirect or direct semiconductors. Moreover, we systematically study the influence of the particle size-distribution on the optical spectra.
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J. Appl. Phys. 101, 113101 (2007); http://dx.doi.org/10.1063/1.2740361 (6 pages) Online Publication Date: 4 June 2007
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Low temperature spatially resolved cathodoluminescence was carried out on GaN films grown by the epitaxial-lateral-overgrowth (ELO) technique with the nonpolar (11-20) and the semipolar (11-22) orientations on R- and M-sapphires, respectively. Defect related optical transitions were identified and their localization was correlated to different regions of ELO. The sample microstructure was further investigated by plan-view and cross-section transmission electron microscopies. It is shown that the defect related emissions are mainly localized in the seed of the samples, but different defects occur as well in the wings, especially in the case of nonpolar GaN. The structural defect densities are lowest in the overgrown wings of semipolar GaN. In particular, the [0001] wing region of semipolar ELO-GaN is almost defect-free with a cathodoluminescence spectrum dominated by the GaN band-edge emission at 3.476 eV.
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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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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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Electron screening in nanostructures J. Appl. Phys. 101, 104308 (2007); http://dx.doi.org/10.1063/1.2734954 (10 pages) Online Publication Date: 18 May 2007
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Distributions of electrical potential and carrier concentration, the contact capacity, and its voltage dependence are calculated for Schottky contacts to various types of nanostructures, including nanolayers and nanowires of different thickness, as well as their arrays. The results demonstrate a dramatic dependence on the nanostructure geometry. Single nanostructures and planar arrays of nanowires cannot provide effective screening of the contact potential, so that the total stored charge and the structure capacity depend on the separation between external contacts. On the contrary, for nanolayer and two-dimensional nanowire arrays, the mutual electrostatic interaction between different elements provides effective screening with the screening length equal to the interelement distance, which determines the contact capacity and its voltage dependence.
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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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Wave propagation characteristics of a figure-eight shaped nanoaperture J. Appl. Phys. 101, 103101 (2007); http://dx.doi.org/10.1063/1.2732412 (4 pages) Online Publication Date: 16 May 2007
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Wave propagation characteristics of apertures were analyzed to explain the light transmission of metallic nanoapertures. Based on Maxwell’s equations, the wave dispersion relations of wave propagation modes in nanoapertures were derived. The resonance frequency shift of the aperture and the variation of the spot size are explained with the dispersion relations. The relationship between near-field and far-field light transmission power throughput and spot size is also shown with the wave mode change predicted by the dispersion relations.
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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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Effects of molecular interface modification in hybrid organic-inorganic photovoltaic cells J. Appl. Phys. 101, 114503 (2007); http://dx.doi.org/10.1063/1.2737977 (12 pages) Online Publication Date: 6 June 2007
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We have systematically investigated the effects of surface modification of titania (TiO2) in hybrid TiO2/regioregular poly(3-hexylthiophene) (P3HT) photovoltaic cells. By employing a series of para-substituted benzoic acids with varying dipoles and a series of multiply substituted benzene carboxylic acids, the energy offset at the TiO2/polymer interface and thus the open-circuit voltage of devices can be tuned systematically by 0.25 V. Transient photovoltage measurements showed that the recombination kinetics was dominated by charge carrier concentration in these devices and were closely associated with the dark current. The saturated photocurrent of TiO2/P3HT devices exhibited more than a twofold enhancement when molecular modifiers with large electron affinity were employed. The ability of modifiers to accept charge from polymers, as revealed in photoluminescence quenching measurement with blends of polymers, was shown to be correlated with the enhancement in device photocurrent. A planar geometry photoluminescence quenching measurement showed that TiO2 substrates modified by these same molecules that accept charge quenched more excitons in regioregular P3HT than bare TiO2 surfaces. An exciton diffusion length in P3HT as large as 6.5−8.5 nm was extracted. By measuring the external quantum efficiency (EQE) of working devices, it was found that all of the excitons that were quenched were accountable as extracted photocurrent. EQE was effectively increased from 5% to 10%−14% with certain surface modifiers; consequently exciton harvesting was more than doubled. The use of ruthenium (II) sensitizing dyes with good exciton harvesting property coupled with suppression of the recombination kinetics improved the efficiency of optimized bilayer TiO2/P3HT devices from 0.34% to 0.6% under AM 1.5 solar illuminations. The implication of this work is directly relevant to the design of nanostructured bulk heterojunction inorganic-organic cells, in which efficient exciton harvesting and control of the recombination kinetics are key to achieving high efficiency.
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J. Appl. Phys. 101, 113102 (2007); http://dx.doi.org/10.1063/1.2736312 (5 pages) Online Publication Date: 4 June 2007
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Ultraviolet (UV) light-emitting diodes with AlxGa1−xN/AlyGa1−yN multiple quantum well active regions, doped in the barriers with different Si doping levels, show a sharp near-band edge emission line (UV luminescence). Some samples have a broad subband gap emission band centered at about 500 nm (green luminescence) in addition to the near-band edge emission. The electroluminescence intensities of the UV and green emission line are studied as a function of the injection current. For the sample grown on the AlN substrate under optimized growth conditions, the UV luminescence intensity increases linearly with the injection current, following a power law with an exponent of 1.0, while the green luminescence intensity increases sublinearly with the injection current. On the contrary, the samples grown on the sapphire substrate show a superlinear (to the power of 2.0) and linear (to the power of 1.0) dependence on the injection current for the UV and green luminescence, respectively. A theoretical model is proposed to explain the relationship between the luminescence intensities and the injection current. The results obtained from the model are in excellent agreement with the experimental results. The model provides a method to evaluate the dominant recombination process by measuring the exponent of the power-law dependence.
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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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The work function of the elements and its periodicity J. Appl. Phys. 48, 4729 (1977); http://dx.doi.org/10.1063/1.323539 (5 pages) Online Publication Date: 26 August 2008
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A new compilation, based on a literature search for the period 1969–1976, is made of experimental data on the work function. For these 44 elements, preferred values are selected on the basis of valid experimental conditions. Older values, which are widely accepted, are given for 19 other elements on which there is no recent literature, and are so identified. In the data for the 63 elements, trends that occur simultaneously in both the columns and the rows of the periodic table are shown to be useful in predicting correct values and also for identifying questionable data. Several illustrative examples are given, including verifications of predictions published in 1950. |
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Magnetic phase diagram of ultrathin films J. Appl. Phys. 101, 113904 (2007); http://dx.doi.org/10.1063/1.2738461 (6 pages) Online Publication Date: 4 June 2007
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By micromagnetic simulations and analytically we study the nonhomogeneous magnetization states of ultrathin films with perpendicular and in-plane anisotropy. Ground and metastable states are mapped onto a (K1,K2) phase diagram (where K1 and K2 are the first and second anisotropy constants, accordingly). It is shown that in the part of the phase diagram where K2<0, on increasing K1 or K2 the initial homogeneous in-plane magnetization distribution evolves in two sequential steps: (i) the appearance of two-phase metastable states with gradually decreasing in-plane domain fraction and (ii) a jump to a perpendicular domain state reaching 50% of the in-plane domain fraction. In the metastability area of the phase diagram, the possibility of topological frustrations in two-phase domain patterns is shown.
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Making waves: Kinetic processes controlling surface evolution during low energy ion sputtering J. Appl. Phys. 101, 121301 (2007); http://dx.doi.org/10.1063/1.2749198 (46 pages) Online Publication Date: 20 June 2007
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When collimated beams of low energy ions are used to bombard materials, the surface often develops a periodic pattern or “ripple” structure. Different types of patterns are observed to develop under different conditions, with characteristic features that depend on the substrate material, the ion beam parameters, and the processing conditions. Because the patterns develop spontaneously, without applying any external mask or template, their formation is the expression of a dynamic balance among fundamental surface kinetic processes, e.g., erosion of material from the surface, ion-induced defect creation, and defect-mediated evolution of the surface morphology. In recent years, a comprehensive picture of the different kinetic mechanisms that control the different types of patterns that form has begun to emerge. In this article, we provide a review of different mechanisms that have been proposed and how they fit together in terms of the kinetic regimes in which they dominate. These are grouped into regions of behavior dominated by the directionality of the ion beam, the crystallinity of the surface, the barriers to surface roughening, and nonlinear effects. In sections devoted to each type of behavior, we relate experimental observations of patterning in these regimes to predictions of continuum models and to computer simulations. A comparison between theory and experiment is used to highlight strengths and weaknesses in our understanding. We also discuss the patterning behavior that falls outside the scope of the current understanding and opportunities for advancement.
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Imaging properties of a metallic photonic crystal J. Appl. Phys. 101, 113105 (2007); http://dx.doi.org/10.1063/1.2737771 (5 pages) Online Publication Date: 7 June 2007
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Imaging effects in metallic photonic crystals (PCs) are examined theoretically based on the finite difference time-domain method. The analysis shows that, in metallic PC-based systems, far-field images do form at the opposite side of the PC “lens” and more importantly, follow the rule of geometric optics with respect to the changes in the source position as a direct proof of negative refraction. However, the comparison of ideal left-handed media with a metallic PC suggests that the focusing effect in the PC based system is different from that of the ideal left-handed media in many aspects, due to the inhomogeneous nature of the PC. Particularly, strong dependence on the individual geometry as well as the frequency in the PC-based system renders the effective index sensitive to the variations and potentially limits its application as a superlens.
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Physics of strain effects in semiconductors and metal-oxide-semiconductor field-effect transistors J. Appl. Phys. 101, 104503 (2007); http://dx.doi.org/10.1063/1.2730561 (22 pages) Online Publication Date: 18 May 2007
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A detailed theoretical picture is given for the physics of strain effects in bulk semiconductors and surface Si, Ge, and III–V channel metal-oxide-semiconductor field-effect transistors. For the technologically important in-plane biaxial and longitudinal uniaxial stress, changes in energy band splitting and warping, effective mass, and scattering are investigated by symmetry, tight-binding, and k⋅p methods. The results show both types of stress split the Si conduction band while only longitudinal uniaxial stress along 〈110〉 splits the Ge conduction band. The longitudinal uniaxial stress warps the conduction band in all semiconductors. The physics of the strain altered valence bands for Si, Ge, and III–V semiconductors are shown to be similar although the strain enhancement of hole mobility is largest for longitudinal uniaxial compression in 〈110〉 channel devices and channel materials with substantial differences between heavy and light hole masses such as Ge and GaAs. Furthermore, for all these materials, uniaxial is shown to offer advantages over biaxial stress: additive strain and confinement splitting, larger two dimensional in-plane density of states, smaller conductivity mass, and less band gap narrowing.
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X-ray diffraction study on an InGaN/GaN quantum-well structure of prestrained growth J. Appl. Phys. 101, 113503 (2007); http://dx.doi.org/10.1063/1.2736860 (6 pages) Online Publication Date: 1 June 2007
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We compare the x-ray diffraction (XRD) results of two InGaN/GaN quantum-well (QW) structures to observe the effects of prestrained growth by depositing a low-indium QW before the growth of five high-indium QWs. From the results of reciprocal space mapping, we observe the fully strained condition in the QWs of the control sample. However, in the sample of prestrained growth, the average strain is partially relaxed. By using an XRD fitting algorithm for calibrating QW parameters, we obtain reasonable values for the compositions and thicknesses of the QWs in both samples. In particular, by assuming a nonuniform strain relaxation distribution among the five high-indium QWs in the prestrained sample, we obtain reasonable composition variations among the QWs. The high-indium QW closest to the low-indium one is most strain-relaxed and has the highest indium incorporation, leading to the longest-wavelength emission. The observed red shift with increasing electron penetration depth in the cathodo-luminescence spectra of the prestrained sample is consistent with the distributions of calibrated strain relaxation and indium composition. The results of high-resolution transmission electron microscopy and effective band gap calculation also agree with the above conclusions.
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