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Top 20 Most Read Articles
June 2012
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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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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J. Appl. Phys. 111, 113709 (2012); http://dx.doi.org/10.1063/1.4725475 (10 pages) Online Publication Date: 7 June 2012
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Injection-dependent minority carrier lifetime measurements are a valuable characterisation method for semiconductor materials, particularly those for photovoltaic applications. For a sample containing defects which obey Shockley-Read-Hall statistics, it is possible to use such measurements to determine (i) the location of energy levels within the band-gap and (ii) the ratios of the capture coefficients for electrons and holes. In this paper, we discuss a convenient methodology for determining these parameters from lifetime data. Minority carrier lifetime is expressed as a linear function of the ratio of the total electron concentration to the total hole concentration for p-type (or vice versa for n-type) material. When this is plotted on linear scales, a single-level Shockley-Read-Hall centre manifests itself as a straight line. The gradient and intercepts of such a plot can be used to determine recombination parameters. The formulation is particularly instructive when multiple states are recombination-active in a sample. To illustrate this, we consider oxide precipitates in silicon as a case study and analyse lifetime data for a wide variety of p-type and n-type samples as a function of temperature. We fit the data using both a single two-level defect and two independent single-level defects and find the latter can fit the lifetime curves in all cases studied. The first defect is at EV + 0.22 eV and has a capture coefficient for electrons ∼157 times greater than that for holes at room temperature. The second defect is at EC − 0.08 eV and has a capture coefficient for holes ∼1200 times greater than that for electrons at room temperature. We find that the presence of dislocations and stacking faults around the precipitates acts to increase the density of both states without introducing new levels. Using the analysis method described, we present a parameterisation of the minority carrier lifetime in silicon containing oxide precipitates.
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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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Resistive switching in silicon suboxide films J. Appl. Phys. 111, 074507 (2012); http://dx.doi.org/10.1063/1.3701581 (9 pages) Online Publication Date: 6 April 2012
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We report a study of resistive switching in a silicon-based memristor/resistive RAM (RRAM) device in which the active layer is silicon-rich silica. The resistive switching phenomenon is an intrinsic property of the silicon-rich oxide layer and does not depend on the diffusion of metallic ions to form conductive paths. In contrast to other work in the literature, switching occurs in ambient conditions, and is not limited to the surface of the active material. We propose a switching mechanism driven by competing field-driven formation and current-driven destruction of filamentary conductive pathways. We demonstrate that conduction is dominated by trap assisted tunneling through noncontinuous conduction paths consisting of silicon nanoinclusions in a highly nonstoichiometric suboxide phase. We hypothesize that such nanoinclusions nucleate preferentially at internal grain boundaries in nanostructured films. Switching exhibits the pinched hysteresis I/V loop characteristic of memristive systems, and on/off resistance ratios of 104:1 or higher can be easily achieved. Scanning tunneling microscopy suggests that switchable conductive pathways are 10 nm in diameter or smaller. Programming currents can be as low as 2 μA, and transition times are on the nanosecond scale.
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Aperiodic arrays of active nanopillars for radiation engineering J. Appl. Phys. 111, 113101 (2012); http://dx.doi.org/10.1063/1.4723564 (9 pages) Online Publication Date: 1 June 2012
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We engineer aperiodic nanostructures for enhanced omnidirectional light extraction and coupling of 1.55 μm radiation to distinctive optical resonances carrying of orbital angular momentum (OAM) using light emitting Si-based materials. By systematically studying nanopillar arrays with varying pillar separations and increasing degree of rotational symmetry in Fourier space, we show that omnidirectional extraction is achieved with circularly symmetric Fourier space, leading to best light emission enhancement from planar devices such as LEDs or lasers. To demonstrate the potential of active aperiodic structures with azimuthally isotropic k-space, we fabricate nanopillar arrays of erbium doped silicon-rich nitride using electron beam lithography and reactive ion etching. Experimental results obtained using leaky-mode photoluminescence spectroscopy prove over 10 times extraction enhancement at 1.55 μm from aperiodic golden angle spirals (GA spirals), in good agreement with design based on analytical Bragg scattering and finite difference time domain calculations. In addition, by imaging Er radiation in direct and reciprocal space, we demonstrate that GA spiral arrays support angularly isotropic emission patterns and distinctive optical resonances with a well-defined azimuthal structure carrying OAM. These findings offer unique opportunities for the engineering of novel active structures that leverage isotropic emission patterns and structured light for secure optical communication, sensing, imaging, and light sources on a Si platform.
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J. Appl. Phys. 111, 114102 (2012); http://dx.doi.org/10.1063/1.4724332 (9 pages) Online Publication Date: 1 June 2012
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(001)-oriented BiFeO3 (BFO) thin films were grown on SrxCa1−xRuO3- (SCRO; x = 1, 0.67, 0.33, 0) buffered SrTiO3 (001) substrates using pulsed laser deposition. The microstructural, electrical, ferroelectric, and piezoelectric properties of the thin films were considerably affected by the buffer layers. The interface between the BFO films and the SCRO-buffer layer was found to play a dominant role in determining the electrical and piezoelectric behaviors of the films. We found that films grown on SrRuO3-buffer layers exhibited minimal electrical leakage while films grown on Sr0.33Ca0.67RuO3-buffer layers had the largest piezoelectric response. The origin of this difference is discussed.
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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, 114505 (2012); http://dx.doi.org/10.1063/1.4723831 (8 pages) Online Publication Date: 5 June 2012
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The use of InGaN photovoltaic devices as a top cell in a tandem solar cell has the potential to improve the power conversion efficiency of multi-junction devices. The effects of the InGaN top cell’s external quantum efficiency, voltage offset, and fill factor on the integrated III-nitride/non-III-nitride solar cell’s power conversion efficiency are presented. The results are summarized into the III-nitride device parameter requirements for top cell applications. The minimum acceptable area ratio between the III-nitride and non-III-nitride subcells in a 3- or 4-terminal device is also determined.
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Stress Relaxation Studies of the Viscoelastic Properties of Polymers J. Appl. Phys. 27, 673 (1956); http://dx.doi.org/10.1063/1.1722465 (13 pages) Online Publication Date: 14 May 2004
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Extensive studies of the viscoelastic properties of polymers undertaken in the author's laboratory by means of the method of stress relaxation are here reviewed. The discussion is divided into four parts: chemical stress relaxation, stress relaxation in amorphous polymers, stress relaxation in crystalline polymers, and stress relaxation in certain natural polymers and polyelectrolytes. Mathematical description of the phenomena are presented in simple form. The relation between structure and viscoelastic properties of polymers are discussed and a rather complete over‐all picture of these phenomena seems to be emerging. |
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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, 114306 (2012); http://dx.doi.org/10.1063/1.4725489 (7 pages) Online Publication Date: 5 June 2012
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Electrochemical functionalization and possible hydrogenation of treated epitaxial graphene samples on 6H-SiC are presented. To attract H+ ions to react with the exposed working cathode, a 10% sulfuric acid electrolyte was used with a Pt counter anode. Functionalization was determined using Raman spectroscopy and measured by a marked increase in I(D)/I(G) ratio and introduction of C-H bond peak at ∼2930 cm−1. There was also a marked increase in fluorescence background, which clearly differentiates functionalization from lattice damage in the graphene. Quantifying the fluorescence, we estimate that H-incorporation as high as 50% was achieved based on results on hydrocarbons, although other functional groups cannot be excluded. We further distinguished these functionalization signatures from lattice damage through measurements on nanocrystalline graphene on a and m plane SiC, which displayed very different surface morphologies and no measureable fluorescence. Finally, we show that the extent of functionalization is strongly substrate dependent by using samples cut from three semi-insulating 6H-SiC substrates with similar resistivity but orientations varying from on-axis (∼0.02°), 0.5° to 1.0° off-axis. This functionalization was found to be thermally reversible at ∼1000 °C. Scanning tunneling spectroscopy indicates the presence of sp3-like localized states not present in the starting graphene, further supporting the assertion that functionalization has occurred.
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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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A deep acceptor defect responsible for the yellow luminescence in GaN and AlGaN J. Appl. Phys. 111, 113105 (2012); http://dx.doi.org/10.1063/1.4725484 (7 pages) Online Publication Date: 5 June 2012
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In the present study, the electrical and optical properties of deep defects in p-i-n GaN junction and AlGaN/GaN heterojunction are investigated by means of the deep level transient spectroscopy (DLTS), Laplace DLTS, and electroluminescence (EL) techniques. We demonstrate that in both structures the yellow luminescence (YL) is a dominant band in the EL spectra recorded at room temperature. We correlate the YL band with the minority DLTS peaks observed at about 370 K. A gallium vacancy-related defect seems to be a probable candidate as to the origin of the defect. Another dominant majority peak observed in the DLTS studies was concluded to be linked with a donor-like defect in the upper half of the bandgap. The origin of the defect is discussed.
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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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Characterization of InGaN-based nanorod light emitting diodes with different indium compositions J. Appl. Phys. 111, 113103 (2012); http://dx.doi.org/10.1063/1.4725417 (7 pages) Online Publication Date: 5 June 2012
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Blue and green InGaN/GaN-based nanorod array light emitting diodes (LEDs) with superior performance have been realized using self-assembled Ni nano-mask and dry etching techniques. Temperature-dependent photoluminescence measurement shows that the internal quantum efficiencies (IQEs) of the LED nanorods are significantly improved in comparison to the as-grown epiwafer, with enhancement factors of 2.8 and 1.5 for the green LED nanorods and blue LED nanorods, respectively. It is in good agreement with a theoretical calculation based on the reduction in the internal electrical field due to strain relaxation in strained InGaN/GaN QWs. As compared to the planar LEDs fabricated using the same wafer, the emission of both nanorod LEDs is greatly improved. More significant enhancement in the light output power is observed for the green nanorod LED, manifesting that the emission enhancement is mainly attributed to a significant enhancement in the IQE. Furthermore, the current-voltage characteristics of nanorod LEDs exhibit two distinct regions at moderate forward bias, in which diffusion-recombination process is involved to a large extent in later period in spite that tunnelling transport dominates over a wide range of bias. The reverse leakage current of nanorod LEDs is about one order of magnitude higher as compared to the planar ones.
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Design of acoustic beam aperture modifier using gradient-index phononic crystals J. Appl. Phys. 111, 123510 (2012); http://dx.doi.org/10.1063/1.4729803 (4 pages) Online Publication Date: 19 June 2012
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This article reports the design concept of a novel acoustic beam aperture modifier using butt-jointed gradient-index phononic crystals (GRIN PCs) consisting of steel cylinders embedded in a homogeneous epoxy background. By gradually tuning the period of a GRIN PC, the propagating direction of acoustic waves can be continuously bent to follow a sinusoidal trajectory in the structure. The aperture of an acoustic beam can therefore be shrunk or expanded through change of the gradient refractive index profiles of the butt-jointed GRIN PCs. Our computational results elucidate the effectiveness of the proposed acoustic beam aperture modifier. Such an acoustic device can be fabricated through a simple process and will be valuable in applications, such as biomedical imaging and surgery, nondestructive evaluation, communication, and acoustic absorbers.
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J. Appl. Phys. 34, 1793 (1963); http://dx.doi.org/10.1063/1.1702682 (11 pages) Online Publication Date: 9 June 2004
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A formula is derived for the electric tunnel effect through a potential barrier of arbitrary shape existing in a thin insulating film. The formula is applied to a rectangular barrier with and without image forces. In the image force problem, the true image potential is considered and compared to the approximate parabolic solution derived by Holm and Kirschstein. The anomalies associated with Holm's expression for the intermediate voltage characteristic are resolved. The effect of the dielectric constant of the insulating film is discussed in detail, and it is shown that this constant affects the temperature dependence of the J‐V characteristic of a tunnel junction. |
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J. Appl. Phys. 108, 071301 (2010); http://dx.doi.org/10.1063/1.3460809 (38 pages) Online Publication Date: 13 October 2010
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Since its discovery in less than five years ago, graphene has become one of the hottest frontiers in materials science and condensed matter physics, as evidenced by the exponential increase in number of publications in this field. Several reviews have already been published on this topic, focusing on single and multilayer graphene sheets. Here, we review the recent progresses in this field by extending the scope to various types of two-dimensional carbon nanostructures including graphene and free-standing carbon nanowalls/nanosheets. After a brief overview of the electronic properties of graphene, we focus on the synthesis, characterization and potential applications of these carbon nanostructures.
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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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