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Top 20 Most Read Articles
April 2012
The 20 articles with the most full-text downloads during the month, in descending order.
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GaN based nanorods for solid state lighting J. Appl. Phys. 111, 071101 (2012); http://dx.doi.org/10.1063/1.3694674 (23 pages) Online Publication Date: 2 April 2012
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In recent years, GaN nanorods are emerging as a very promising novel route toward devices for nano-optoelectronics and nano-photonics. In particular, core-shell light emitting devices are thought to be a breakthrough development in solid state lighting, nanorod based LEDs have many potential advantages as compared to their 2 D thin film counterparts. In this paper, we review the recent developments of GaN nanorod growth, characterization, and related device applications based on GaN nanorods. The initial work on GaN nanorod growth focused on catalyst-assisted and catalyst-free statistical growth. The growth condition and growth mechanisms were extensively investigated and discussed. Doping of GaN nanorods, especially p-doping, was found to significantly influence the morphology of GaN nanorods. The large surface of 3 D GaN nanorods induces new optical and electrical properties, which normally can be neglected in layered structures. Recently, more controlled selective area growth of GaN nanorods was realized using patterned substrates both by metalorganic chemical vapor deposition (MOCVD) and by molecular beam epitaxy (MBE). Advanced structures, for example, photonic crystals and DBRs are meanwhile integrated in GaN nanorod structures. Based on the work of growth and characterization of GaN nanorods, GaN nanoLEDs were reported by several groups with different growth and processing methods. Core/shell nanoLED structures were also demonstrated, which could be potentially useful for future high efficient LED structures. In this paper, we will discuss recent developments in GaN nanorod technology, focusing on the potential advantages, but also discussing problems and open questions, which may impose obstacles during the future development of a GaN nanorod based LED technology.
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J. Appl. Phys. 111, 063918 (2012); http://dx.doi.org/10.1063/1.3698346 (5 pages) Online Publication Date: 29 March 2012
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We have systematically investigated the compositional dependence of the magnetic and magnetocaloric properties of La0.7-xPrxSr0.3MnO3 (LPSMO). Polycrystalline samples of LPSMO with 0.02 ≤ x ≤ 0.30 were prepared by a standard solid-state reaction method with phase purity and structure confirmed using x-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transitions in each sample with Curie temperature decreasing progressively with increasing Pr content from ∼350 K for x = 0.02 to ∼295 K for x = 0.30. Magnetic entropy change (ΔSM) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. In the sample with x = 0.30, the maximum value of −ΔSM reaches ∼2.08 J/kg K at 295 K for a field change of 1.5 T. Reduced Pr content corresponds to larger values of entropy change, reaching −ΔSM ∼2.79 J/kg K for the x = 0.02 doping. The refrigeration capacity for each composition reached sizable values of 33–48 J/kg for a small applied field of 1.5 T.
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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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Cantilever deflection associated with hybridization of monomolecular DNA film J. Appl. Phys. 111, 074310 (2012); http://dx.doi.org/10.1063/1.3698204 (9 pages) Online Publication Date: 6 April 2012
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Recent experiments show that specific binding between a ligand and surface immobilized receptor, such as hybridization of single stranded DNA immobilized on a microcantilever surface, leads to cantilever deflection. The binding-induced deflection may be used as a method for detection of biomolecules, such as pathogens and biohazards. Mechanical deformation induced due to hybridization of surface-immobilized DNA strands is a commonly used system to demonstrate the efficacy of microcantilever sensors. To understand the mechanism underlying the cantilever deflections, a theoretical model that incorporates the influence of ligand/receptor complex surface distribution and empirical interchain potential is developed to predict the binding-induced deflections. The cantilever bending induced due to hybridization of DNA strands is predicted for different receptor immobilization densities, hybridization efficiencies, and spatial arrangements. Predicted deflections are compared with experimental reports to validate the modeling assumptions and identify the influence of various components on mechanical deformation. Comparison of numerical predictions and experimental results suggest that, at high immobilization densities, hybridization-induced mechanical deformation is determined, primarily by immobilization density and hybridization efficiency, whereas, at lower immobilization densities, spatial arrangement of hybridized chains need to be considered in determining the cantilever deflection.
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Detailed Balance Limit of Efficiency of p‐n Junction Solar Cells J. Appl. Phys. 32, 510 (1961); http://dx.doi.org/10.1063/1.1736034 (10 pages) Online Publication Date: 11 June 2004
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In order to find an upper theoretical limit for the efficiency of p‐n junction solar energy converters, a limiting efficiency, called the detailed balance limit of efficiency, has been calculated for an ideal case in which the only recombination mechanism of hole‐electron pairs is radiative as required by the principle of detailed balance. The efficiency is also calculated for the case in which radiative recombination is only a fixed fraction fc of the total recombination, the rest being nonradiative. Efficiencies at the matched loads have been calculated with band gap and fc as parameters, the sun and cell being assumed to be blackbodies with temperatures of 6000°K and 300°K, respectively. The maximum efficiency is found to be 30% for an energy gap of 1.1 ev and fc = 1. Actual junctions do not obey the predicted current‐voltage relationship, and reasons for the difference and its relevance to efficiency are discussed. |
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Measurement of semiconductor surface potential using the scanning electron microscope J. Appl. Phys. 111, 046103 (2012); http://dx.doi.org/10.1063/1.3684556 (3 pages) Online Publication Date: 21 February 2012
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We calibrate the secondary electron signal from a standard scanning electron microscope to voltage, yielding an image of the surface or near-surface potential. Data on both atomically abrupt heterojunction GaInP/GaAs and diffused homojunction Si solar cell devices clearly show the expected variation in potential with position and applied bias, giving depletion widths and locating metallurgical junctions to an accuracy better than 10 nm. In some images, distortion near the p-n junction is observed, seemingly consistent with the effects of lateral electric fields (patch fields). Reducing the tube bias removes this distortion. This approach results in rapid and straightforward collection of near-surface potential data using a standard scanning electron microscope.
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Electromigration study of copper lines on steps prepared by a plasma-based etch process J. Appl. Phys. 111, 064909 (2012); http://dx.doi.org/10.1063/1.3698296 (6 pages) Online Publication Date: 29 March 2012
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The electromigration phenomenon of the copper line etched with a plasma-based process over the SiNx step has been investigated. Two important factors, i.e., the dielectric topography and the stress temperature, were examined using the accelerated isothermal electromigration method. The activation energy of 0.73 eV to 0.89 eV indicates two possible mass transport pathways: interfacial and copper surface diffusions. The copper line on the SiNx step has a shorter lifetime and a smaller activation energy than the copper line on the flat surface has. For the former, voids were formed at the cusp region and perpendicular to the current flow direction. For the latter, voids were formed in series and parallel to the current flow direction. The “neck” structure at the cusp region, which is a result of the inappropriate etching condition, further decreased the lifetime and the activation energy. The lifetime of the “neck-free” copper line over the SiNx step was estimated to be 7.1 × 109 s under the high-speed IC operation condition. The thermal stress mismatch between the copper layer and TiW barrier layer as well as the underneath dielectric layer facilitated the void formation. The step effect on the lifetime was reduced when the test temperature was high because of the change of the local stress. In summary, the topography and the test temperature are critical factors for the copper line’s lifetime.
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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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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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Multiferroic magnetoelectric composites: Historical perspective, status, and future directions J. Appl. Phys. 103, 031101 (2008); http://dx.doi.org/10.1063/1.2836410 (35 pages) Online Publication Date: 5 February 2008
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Multiferroic magnetoelectric materials, which simultaneously exhibit ferroelectricity and ferromagnetism, have recently stimulated a sharply increasing number of research activities for their scientific interest and significant technological promise in the novel multifunctional devices. Natural multiferroic single-phase compounds are rare, and their magnetoelectric responses are either relatively weak or occurs at temperatures too low for practical applications. In contrast, multiferroic composites, which incorporate both ferroelectric and ferri-/ferromagnetic phases, typically yield giant magnetoelectric coupling response above room temperature, which makes them ready for technological applications. This review of mostly recent activities begins with a brief summary of the historical perspective of the multiferroic magnetoelectric composites since its appearance in 1972. In such composites the magnetoelectric effect is generated as a product property of a magnetostrictive and a piezoelectric substance. An electric polarization is induced by a weak ac magnetic field oscillating in the presence of a dc bias field, and/or a magnetization polarization appears upon applying an electric field. So far, three kinds of bulk magnetoelectric composites have been investigated in experimental and theoretical, i.e., composites of (a) ferrite and piezoelectric ceramics (e.g., lead zirconate titanate), (b) magnetic metals/alloys (e.g., Terfenol-D and Metglas) and piezoelectric ceramics, and (c) Terfenol-D and piezoelectric ceramics and polymer. The elastic coupling interaction between the magnetostrictive phase and piezoelectric phase leads to giant magnetoelectric response of these magnetoelectric composites. For example, a Metglas/lead zirconate titanate fiber laminate has been found to exhibit the highest magnetoelectric coefficient, and in the vicinity of resonance, its magnetoelectric voltage coefficient as high as 102 V/cm Oe orders has been achieved, which exceeds the magnetoelectric response of single-phase compounds by many orders of magnitude. Of interest, motivated by on-chip integration in microelectronic devices, nanostructured composites of ferroelectric and magnetic oxides have recently been deposited in a film-on substrate geometry. The coupling interaction between nanosized ferroelectric and magnetic oxides is also responsible for the magnetoelectric effect in the nanostructures as was the case in those bulk composites. The availability of high-quality nanostructured composites makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry, and interfacial coupling. In this review, we discuss these bulk and nanostructured magnetoelectric composites both in experimental and theoretical. From application viewpoint, microwave devices, sensors, transducers, and heterogeneous read/write devices are among the suggested technical implementations of the magnetoelectric composites. The review concludes with an outlook on the exciting future possibilities and scientific challenges in the field of multiferroic magnetoelectric composites.
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J. Appl. Phys. 111, 061301 (2012); http://dx.doi.org/10.1063/1.3688435 (24 pages) Online Publication Date: 16 March 2012
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Carbon (C) is a crucial material for many branches of modern technology. A growing number of demanding applications in electronics and telecommunications rely on the unique properties of C allotropes. The need for microwave absorbers and radar-absorbing materials is ever growing in military applications (reduction of radar signature of aircraft, ships, tanks, and targets) as well as in civilian applications (reduction of electromagnetic interference among components and circuits, reduction of the back-radiation of microstrip radiators). Whatever the application for which the absorber is intended, weight reduction and optimization of the operating bandwidth are two important issues. A composite absorber that uses carbonaceous particles in combination with a polymer matrix offers a large flexibility for design and properties control, as the composite can be tuned and optimized via changes in both the carbonaceous inclusions (C black, C nanotube, C fiber, graphene) and the embedding matrix (rubber, thermoplastic). This paper offers a perspective on the experimental efforts toward the development of microwave absorbers composed of carbonaceous inclusions in a polymer matrix. The absorption properties of such composites can be tailored through changes in geometry, composition, morphology, and volume fraction of the filler particles. Polymer composites filled with carbonaceous particles provide a versatile system to probe physical properties at the nanoscale of fundamental interest and of relevance to a wide range of potential applications that span radar absorption, electromagnetic protection from natural phenomena (lightning), shielding for particle accelerators in nuclear physics, nuclear electromagnetic pulse protection, electromagnetic compatibility for electronic devices, high-intensity radiated field protection, anechoic chambers, and human exposure mitigation. Carbonaceous particles are also relevant to future applications that require environmentally benign and mechanically flexible materials.
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J. Appl. Phys. 93, 793 (2003); http://dx.doi.org/10.1063/1.1524305 (26 pages) Online Publication Date: 27 December 2002
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Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology. © 2003 American Institute of Physics. |
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ZnO Schottky barriers and Ohmic contacts J. Appl. Phys. 109, 121301 (2011); http://dx.doi.org/10.1063/1.3581173 (33 pages) Online Publication Date: 23 June 2011
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ZnO has emerged as a promising candidate for optoelectronic and microelectronic applications, whose development requires greater understanding and control of their electronic contacts. The rapid pace of ZnO research over the past decade has yielded considerable new information on the nature of ZnO interfaces with metals. Work on ZnO contacts over the past decade has now been carried out on high quality material, nearly free from complicating factors such as impurities, morphological and native point defects. Based on the high quality bulk and thin film crystals now available, ZnO exhibits a range of systematic interface electronic structure that can be understood at the atomic scale. Here we provide a comprehensive review of Schottky barrier and ohmic contacts including work extending over the past half century. For Schottky barriers, these results span the nature of ZnO surface charge transfer, the roles of surface cleaning, crystal quality, chemical interactions, and defect formation. For ohmic contacts, these studies encompass the nature of metal-specific interactions, the role of annealing, multilayered contacts, alloyed contacts, metallization schemes for state-of-the-art contacts, and their application to n-type versus p-type ZnO. Both ZnO Schottky barriers and ohmic contacts show a wide range of phenomena and electronic behavior, which can all be directly tied to chemical and structural changes on an atomic scale.
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Quantum confinement in Si and Ge nanostructures J. Appl. Phys. 111, 034307 (2012); http://dx.doi.org/10.1063/1.3680884 (9 pages) Online Publication Date: 8 February 2012
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We apply perturbative effective mass theory as a broadly applicable theoretical model for quantum confinement (QC) in all Si and Ge nanostructures including quantum wells (QWs), wires (Q-wires), and dots (QDs). Within the limits of strong, medium, and weak QC, valence and conduction band edge energy levels (VBM and CBM) were calculated as a function of QD diameters, QW thicknesses, and Q-wire diameters. Crystalline and amorphous quantum systems were considered separately. Calculated band edge levels with strong, medium, and weak QC models were compared with experimental VBM and CBM reported from X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS), or photoluminescence (PL). Experimentally, the dimensions of the nanostructures were determined directly, by transmission electron microscopy (TEM), or indirectly, by x-ray diffraction (XRD) or by XPS. We found that crystalline materials are best described by a medium confinement model, while amorphous materials exhibit strong confinement regardless of the dimensionality of the system. Our results indicate that spatial delocalization of the hole in amorphous versus crystalline nanostructures is the important parameter determining the magnitude of the band gap expansion, or the strength of the quantum confinement. In addition, the effective masses of the electron and hole are discussed as a function of crystallinity and spatial confinement.
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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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Light scattering at textured back contacts for n-i-p thin-film silicon solar cells J. Appl. Phys. 111, 083101 (2012); http://dx.doi.org/10.1063/1.3703572 (5 pages) Online Publication Date: 16 April 2012
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The angular resolved light scattering at textured back contacts for n-i-p thin-film silicon solar cells is investigated experimentally in air. These results are compared to simulations performed by a scalar model for reflection with excellent agreement. Furthermore, light scattering is modeled for the transmission and reflection inside the silicon solar cell. It is found that the reflection at the back contact dominates the light scattering in the absorber layer. From these simulations, a quantity is derived that successfully predicts the external quantum efficiencies of solar cells on different textures.
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First-principles calculations for defects and impurities: Applications to III-nitrides J. Appl. Phys. 95, 3851 (2004); http://dx.doi.org/10.1063/1.1682673 (29 pages) Online Publication Date: 31 March 2004
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First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be attributed to nitrogen vacancies, but is due to unintentional incorporation of donor impurities. Native point defects may play a role in compensation and in phenomena such as the yellow luminescence, which can be attributed to gallium vacancies. In the section on impurities, specific attention will be focused on dopants. Oxygen, which is commonly present as a contaminant, is a shallow donor in GaN but becomes a deep level in AlGaN due to a DX transition. Magnesium is almost universally used as the p-type dopant, but hole concentrations are still limited. Reasons for this behavior are discussed, and alternative acceptors are examined. Hydrogen plays an important role in p-type GaN, and the mechanisms that underlie its behavior are explained. Incorporating hydrogen along with acceptors is an example of codoping; a critical discussion of codoping is presented. Most of the information available to date for defects and impurities in nitrides has been generated for GaN, but we will also discuss AlN and InN where appropriate. We conclude by summarizing the main points and looking towards the future. © 2004 American Institute of Physics. |
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Memory effects in superparamagnetic and nanocrystalline Fe50Ni50 alloy J. Appl. Phys. 111, 033919 (2012); http://dx.doi.org/10.1063/1.3684624 (7 pages) Online Publication Date: 14 February 2012
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Nanocrystalline Fe50Ni50 alloy is prepared using the sol-gel route in an amorphous silica host at different volume fractions (φ). The average particle size is 8.9 nm having log-normal distribution = 0.19, which is confirmed by transmission electron microscopy for φ = 1%. The blocking temperature (TB) is 30 K, as observed in dc magnetization. The frequency-dependent peak-shift in ac susceptibility satisfies Néel-Arrhenius formalism with more reasonable physical parameters than Vogel-Fulcher and dynamical scaling formalisms. Analysis of the relaxation dynamics below TB points toward weak interparticle interaction, signifying superparamagnetic behavior. Relaxation dynamics following stretched exponential function implies that it is involved with the activation against multiple anisotropy barriers, which is correlated to the distribution of particle size. Memory effects are observed in different experimental protocols below TB, which has been discussed pertaining to superparamagnetic behavior.
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J. Appl. Phys. 111, 073101 (2012); http://dx.doi.org/10.1063/1.3699197 (4 pages) Online Publication Date: 3 April 2012
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We experimentally demonstrated a controllable electromagnetically induced transparency (EIT)-like spectral response at microwave frequencies in a planar metamaterial consisting of two identical split-ring resonators (SRRs) with side-by-side symmetry. In our scheme, phase coupling between the two SRRs (serving as the bright mode), which were excited strongly by the incident wave, was employed, and it was found that the EIT-like spectral response could be controlled by simply adjusting the incident angle. Thus, our scheme may be used for electromagnetic-wave switching. A high group index for slow-light application and a high quality factor could be obtained by simply controlling the incident angle.
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Optical and electrical characteristics of asymmetric nanowire solar cells J. Appl. Phys. 111, 073102 (2012); http://dx.doi.org/10.1063/1.3699212 (5 pages) Online Publication Date: 4 April 2012
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We propose an asymmetric radial structure developed via simulation that improves the optical and electrical characteristics of silicon nanowire (SiNW) solar cells. This nanostructure is designed by shrinking the bottom core diameter and holding the top core diameter fixed in the SiNW, which results in a total reflection of the incident light in the outer wall of the shell due to the difference in the refractive index. The reflection enhances light trapping and concentration, which results in a 10 times higher optical generation rate and greater optical absorption in the high energy regime as compared with the fundamental symmetric radial structure. Further, we found that the efficiency is increased by over 10% when the bottom core diameter is decreased. The proposed structure has great potential to effectively improve the efficiency in concert with optimizing the design parameters.
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