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Showing papers in "Journal of Applied Physics in 2009"


Journal ArticleDOI
TL;DR: A review of defects in ZnO is presented in this paper, with an emphasis on the physical properties of point defects in bulk crystals, and the problem of acceptor dopants remains a key challenge.
Abstract: Zinc oxide (ZnO) is a wide band gap semiconductor with potential applications in optoelectronics, transparent electronics, and spintronics. The high efficiency of UV emission in this material could be harnessed in solid-state white lighting devices. The problem of defects, in particular, acceptor dopants, remains a key challenge. In this review, defects in ZnO are discussed, with an emphasis on the physical properties of point defects in bulk crystals. As grown, ZnO is usually n-type, a property that was historically ascribed to native defects. However, experiments and theory have shown that O vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature. Group-III (B, Al, Ga, and In) and H impurities account for most of the n-type conductivity in ZnO samples. Interstitial H donors have been observed with IR spectroscopy, while substitutional H donors have been predicted from first-principles calculations but not observed directly. Despite numerous reports, reliable p-t...

995 citations


Journal ArticleDOI
TL;DR: The International Nanofluid Property Benchmark Exercise (INPBE) as mentioned in this paper was held in 1998, where the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or "nanofluids" was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady state methods, and optical methods.
Abstract: This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

942 citations


Journal ArticleDOI
TL;DR: In this paper, the bandgap of InN was revised from 1.9 eV to a much narrower value of 0.64 eV, which is the smallest bandgap known to date.
Abstract: Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...

871 citations


Journal ArticleDOI
TL;DR: In this paper, the thermal conductivity and viscosity of various shapes of alumina nanoparticles in a fluid consisting of equal volumes of ethylene glycol and water were investigated and accompanied by theoretical modeling.
Abstract: The thermal conductivity and viscosity of various shapes of alumina nanoparticles in a fluid consisting of equal volumes of ethylene glycol and water were investigated. Experimental data were analyzed and accompanied by theoretical modeling. Enhancements in the effective thermal conductivities due to particle shape effects expected from Hamilton–Crosser equation are strongly diminished by interfacial effects proportional to the total surface area of nanoparticles. On the other hand, the presence of nanoparticles and small volume fractions of agglomerates with high aspect ratios strongly increases viscosity of suspensions due to structural constrains. Nanoparticle surface charge also plays an important role in viscosity. It is demonstrated that by adjusting pH of nanofluid, it is possible to reduce viscosity of alumina nanofluid without significantly affecting thermal conductivity. Efficiency of nanofluids (ratio of thermal conductivity and viscosity increase) for real-life cooling applications is evaluate...

735 citations


Journal ArticleDOI
TL;DR: In this paper, fundamental and technological issues associated with the development and exploitation of third-generation IR photon detectors are discussed, and the most recently developed focal plane arrays based on type-II strained-layer superlattices and quantum dot IR photodetectors are considered.
Abstract: Hitherto, two distinct families of multielement detector arrays have been used for infrared (IR) imaging system applications: linear arrays for scanning systems (first generation) and two-dimensional arrays for staring systems (second generation). Nowadays, third-generation IR systems are being developed which, in the common understanding, provide enhanced capabilities such as larger numbers of pixels, higher frame rates, better thermal resolution, multicolor functionality, and/or other on-chip signal-processing functions. In this paper, fundamental and technological issues associated with the development and exploitation of third-generation IR photon detectors are discussed. In this class of detectors the two main competitors, HgCdTe photodiodes and quantum-well photoconductors, are considered. This is followed by discussions focused on the most recently developed focal plane arrays based on type-II strained-layer superlattices and quantum dot IR photodetectors. The main challenges facing multicolor devi...

724 citations


Journal ArticleDOI
TL;DR: In this article, a mathematical definition of a memristive device provides the framework for understanding the physical processes involved in bipolar switching and also yields formulas that can be used to compute and predict important electrical and dynamical properties of the device.
Abstract: Memristive devices are promising components for nanoelectronics with applications in nonvolatile memory and storage, defect-tolerant circuitry, and neuromorphic computing. Bipolar resistive switches based on metal oxides such as TiO2 have been identified as memristive devices primarily based on the “pinched hysteresis loop” that is observed in their current-voltage (i-v) characteristics. Here we show that the mathematical definition of a memristive device provides the framework for understanding the physical processes involved in bipolar switching and also yields formulas that can be used to compute and predict important electrical and dynamical properties of the device. We applied an electrical characterization and state-evolution procedure in order to capture the switching dynamics of a device and correlate the response with models for the drift diffusion of ionized dopants (vacancies) in the oxide film. The analysis revealed a notable property of nonlinear memristors: the energy required to switch a me...

688 citations


Journal ArticleDOI
TL;DR: In this paper, the origin of visible luminescence from ZnO layers deposited on p-Si substrates by various growth methods using temperature dependent photoluminescence measurements was examined.
Abstract: This study examined the origin of visible luminescence from ZnO layers deposited on p-Si substrates by various growth methods using temperature dependent photoluminescence measurements. The deep level emissions of ZnO layers are found to be strongly dependent on the growth conditions and growth methods used. For the samples grown by sputtering, the visible emission consisted of violet, green, and orange-red regions, which corresponded to zinc interstitial (Zni), oxygen vacancy (VO), and oxygen interstitial (Oi) defect levels, respectively. In contrast, the deep level emissions of metal organic chemical vapor deposition grown samples consisted of blue and green emissions and blue and orange-red emissions at low and high oxygen flow rates, respectively. The ZnO nanorods synthesized by thermal evaporation showed a dominant deep level emission at the green region, which is associated with oxygen vacancies (VO).

678 citations


Journal ArticleDOI
TL;DR: In this paper, the formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSSss) on single-crystalline silicon upon irradiation with single or multiple femtosecond-laser pulses (pulse duration τ=130
Abstract: The formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSSs) on single-crystalline silicon upon irradiation with single or multiple femtosecond-laser pulses (pulse duration τ=130 fs and central wavelength λ=800 nm) in air is studied experimentally and theoretically. In our theoretical approach, we model the LIPSS formation by combining the generally accepted first-principles theory of Sipe and co-workers with a Drude model in order to account for transient intrapulse changes in the optical properties of the material due to the excitation of a dense electron-hole plasma. Our results are capable to explain quantitatively the spatial periods of the LIPSSs being somewhat smaller than the laser wavelength, their orientation perpendicular to the laser beam polarization, and their characteristic fluence dependence. Moreover, evidence is presented that surface plasmon polaritons play a dominant role during the initial stage of near-wavelength-sized periodic surface structures in fem...

555 citations


Journal ArticleDOI
TL;DR: In this article, a simple and effective method of enhancing light trapping in solar cells with thin absorber layers by tuning localized surface plasmons in arrays of Ag nanoparticles is presented.
Abstract: Effective light management is imperative in maintaining high efficiencies as photovoltaic devices become thinner. We demonstrate a simple and effective method of enhancing light trapping in solar cells with thin absorber layers by tuning localized surface plasmons in arrays of Ag nanoparticles. By redshifting the surface plasmon resonances by up to 200 nm, through the modification of the local dielectric environment of the particles, we can increase the optical absorption in an underlying Si wafer fivefold at a wavelength of 1100 nm and enhance the external quantum efficiency of thin Si solar cells by a factor of 2.3 at this wavelength where transmission losses are prevalent. Additionally, by locating the nanoparticles on the rear of the solar cells, we can avoid absorption losses below the resonance wavelength due to interference effects, while still allowing long wavelength light to be coupled into the cell. Results from numerical simulations support the experimental findings and show that the fraction ...

538 citations


Journal ArticleDOI
TL;DR: In this article, Zhang et al. investigated the mechanism of the giant unipolar strain recently observed in a lead-free piezoceramic, 0.92(Bi0.5Na 0.93(Bi 0.5 Na 0.9)TiO3-0.06BaTiO(3) -0.02(K0.1(K 0.2)NbO3)
Abstract: The mechanism of the giant unipolar strain recently observed in a lead-free piezoceramic, 0.92(Bi0.5Na0.5)TiO3-0.06BaTiO(3)-0.02(K0.5Na0.5)NbO3 [S.-T. Zhang, A. B. Kounga, E. Aulbach, H. Ehrenberg, and J. Rodel, Appl. Phys. Lett. 91, 112906 (2007) was investigated. The validity of the previously proposed mechanism that the high strain comes both from a significant volume change during the field-induced phase transition, from an antiferroelectric to a ferroelectric phase and the domain contribution from the induced ferroelectric phase was examined. Monitoring the volume changes from the simultaneously measured longitudinal and transverse strains on disk-shaped samples showed that the phase transition in this specific material does not involve any notable volume change, which indicates that there is little contribution from a volume change due to the phase transition to the total strain response. Temperature dependent hysteresis measurements on unpoled samples of a nearby ferroelectric composition, 0.93(Bi0.5Na0.5)TiO3-0.06BaTiO(3) -0.01(K0.5Na0.5)NbO3 demonstrated that the origin of the large strain is due to the presence of a nonpolar phase that brings the system back to its unpoled state once the applied electric field is removed, which leads to a large unipolar strain.

531 citations


Journal ArticleDOI
TL;DR: In this article, spectrally resolved photoluminescence quenching was used to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices.
Abstract: We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices. We show that aggregation and crystal orientation influence the anisotropy of the diffusion length for vacuum-deposited polycrystalline films. The measurement of the singlet diffusion lengths is found to be in agreement with diffusion by Forster transfer, whereas triplet diffusion occurs primarily via Dexter transfer.

Journal ArticleDOI
TL;DR: In this article, it was shown that the triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics.
Abstract: We have demonstrated that the exemplary red fluorescent organic light-emitting diodes (OLEDs) gain as much as half of their electroluminescence from annihilation of triplet states generated by recombining charge carriers. The magnitude of triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics, which was independently confirmed by studying magnetic field effects on delayed luminescence. We determined the value of 1.3 for the ratio of the rate constants of singlet and triplet channels of annihilation, which is indeed substantially higher than the value of 0.33 expected for a purely statistical annihilation process. It is, however, in an excellent quantitative agreement with the extent of the experimental contribution of delayed luminescence to steady-state electroluminescence. The ...

Journal ArticleDOI
TL;DR: In this article, the UHV multichamber photoelectron gun was used to study the removal of surface pollutants and the degraded (Cs,O)-activation layer during the cleaning procedure.
Abstract: Atomic hydrogen, produced by thermal dissociation of H2 molecules inside a hot tungsten capillary, is shown to be an efficient tool for multiple recleaning of degraded surfaces of high quantum efficiency transmission-mode GaAs photocathodes within an ultrahigh vacuum (UHV) multichamber photoelectron gun. Ultraviolet quantum yield photoemission spectroscopy has been used to study the removal of surface pollutants and the degraded (Cs,O)-activation layer during the cleaning procedure. For photocathodes grown by the liquid-phase epitaxy technique, the quantum efficiency is found to be stable at about 20% over a large number of atomic hydrogen cleaning cycles. A slow degradation of the quantum efficiency is observed for photocathodes grown by metal-organic chemical vapor deposition, although they reached a higher initial quantum efficiency of about 30%–35%. Study of the spatial distributions of photoluminescence intensity on these photocathodes proved that this overall degradation is likely due to insertion o...

Journal ArticleDOI
TL;DR: In this paper, an x-ray study revealed that a tetragonal phase exists in the temperature range between 330 and 480°C in BNT ceramic as well as BNT single crystals.
Abstract: Stoichiometric and nonstoichiometric (Bi05Na05)TiO3 (BNT) ceramics were prepared by a conventional ceramic fabrication process This study revealed that the high conductivity of BNT ceramics is associated with Bi vaporization during sintering An x-ray study revealed that a tetragonal phase exists in the temperature range between 330 and 480 °C in BNT ceramic as well as BNT single crystals In addition, the depolarization temperature Td, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature Tm of the maximum dielectric constant were determined to be 187, approximately 300, and 325 °C, respectively, from the temperature dependences of dielectric properties using unpoled and poled specimens The piezoelectric properties of all vibration modes and the temperature dependences of the piezoelectric properties were measured using fully poled BNT ceramics It was also revealed that BNT ceramics exhibit three thermal depoling processes at Td, between Td and TR-T, and between TR-T and Tm

Journal ArticleDOI
TL;DR: In this paper, LiPSS with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150-200 fs, 800 nm) in air.
Abstract: Laser-induced periodic surface structures (LIPSS) (ripples) with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150–200 fs, 800 nm) in air. For normal incident laser radiation, low spatial frequency LIPSS (LSFL) with a period (630–730 nm) close to the wavelength and an orientation perpendicular to the laser polarization have been found in the fluence range between ∼0.7 and ∼0.8 J/cm2 and predominantly for pulse numbers up to N=100. For lower fluences (0.5–0.7 J/cm2), a sharp transition from the LSFL features toward the formation of high spatial frequency LIPSS (HSFL) appears at any given pulse number below N=100. The HSFL are always parallel to the LSFL, exhibit spatial periods between 200 and 280 nm, and completely substitute the LSFL for pulse numbers N>100. Additionally, the influence of the angle of incidence has been studied experimentally for both LIPSS types revealing a different b...

Journal ArticleDOI
TL;DR: In this paper, the effect of shunting in the framework of the Dyakonov-Shur plasma-wave theory was theoretically studied, with the following key results: in the quasistatic limit, the capacitive shunt induces the longitudinal high-frequency field neede...
Abstract: In search of novel detectors of electromagnetic radiation at terahertz frequencies, field-effect transistors (FETs) have recently gained much attention. The current literature studies them with respect to the excitation of plasma waves in the two-dimensional channel. Circuit aspects have been taken into account only to a limited degree. In this paper, we focus on embedding silicon FETs in a proper circuitry to optimize their responsivity to terahertz radiation. This includes impedance-matched antenna coupling and amplification of the rectified signal. Special attention is given to the investigation of high-frequency short-circuiting of the gate and drain contacts by a capacitive shunt, a common approach of high-frequency electronics to induce resistive mixing in transistors. We theoretically study the effect of shunting in the framework of the Dyakonov–Shur plasma-wave theory, with the following key results. In the quasistatic limit, the capacitive shunt induces the longitudinal high-frequency field neede...

Journal ArticleDOI
TL;DR: In this article, particle swarm optimization (PSO) was applied to extract the solar cell parameters from illuminated currentvoltage characteristics, and the performance of the PSO was compared with the genetic algorithms (GAs) for the single and double diode models.
Abstract: In this article, particle swarm optimization (PSO) was applied to extract the solar cell parameters from illuminated current-voltage characteristics. The performance of the PSO was compared with the genetic algorithms (GAs) for the single and double diode models. Based on synthetic and experimental current-voltage data, it has been confirmed that the proposed method can obtain higher parameter precision with better computational efficiency than the GA method. Compared with conventional gradient-based methods, even without a good initial guess, the PSO method can obtain the parameters of solar cells as close as possible to the practical parameters only based on a broad range specified for each of the parameters.

Journal ArticleDOI
TL;DR: Based on the measured XRD pattern and magnetization curve, Wang et al. as discussed by the authors deduced that the CoFeMnSi compound is crystallized in the structure where half metallicity occurs.
Abstract: A good field to develop new materials with half metallicity is the quaternary Heusler alloys. The preferred route is to combine the compounds that have been already grown in Heusler structure. As a typical example, the quaternary LiMgPdSb-type CoFeMnSi have been investigated in detail. For the quaternary LiMgPdSb-type compounds, three nonequivalent structures exist. From the calculated density of state (DOS) results, it can be seen that one superstructure shows half metallicity. The second superstructure has a pseudogap at the Fermi level. The third superstructure shows metallic behavior. Based on the analysis of the measured XRD pattern and magnetization curve, we can deduce that the CoFeMnSi compound is crystallized in the structure where half metallicity occurs.

Journal ArticleDOI
TL;DR: In this paper, the electrical and magnetic properties of polycrystalline samples of Bi0.90La0.10(Fe1−xMnx)O3 (x=0, 0, 0.05,0.15, and 0.20) were prepared using a novel mechanical activation followed by a conventional solid-state reaction technique.
Abstract: Polycrystalline samples of Bi0.90La0.10(Fe1−xMnx)O3 (x=0, 0.05, 0.10, 0.15, and 0.20) were prepared using a novel mechanical activation followed by a conventional solid-state reaction technique. The formation of the desired materials was confirmed using x-ray diffraction. The electrical and magnetic properties of the materials were investigated at different Mn concentrations as a function of temperature. Both dielectric constant and loss tangent increased with the increase in Mn content in the system. The grain and grain boundary contributions have been estimated using impedance spectroscopy analysis. Both grain and grain boundary conductivity increased with a rise in temperature for all Mn concentrations. The value of activation energy for both grain and grain boundary is nearly the same, and decreased with an increase in Mn concentration. There is a systematic increase in the value of magnetization on increasing Mn concentration.

Journal ArticleDOI
TL;DR: In this paper, the experimental characteristics of polarization fatigue in thin-film, bulk ceramic, and single-crystalline ferroelectrics have been reviewed in detail, together with the methods of improving the fatigue endurance in ferroelectric materials.
Abstract: The experimental characteristics of polarization fatigue in thin-film, bulk ceramic, and single-crystalline ferroelectrics have been reviewed in detail. Various scenarios and models proposed for fatigue in ferroelectric materials during the past few decades have been discussed, together with our own model developed very recently [Phys. Rev. Lett. 97, 177601 (2006); Phys. Rev. B 75, 244104 (2007)]. Interpretations for the experimental data reviewed in this paper but untreated in our previous work [Phys. Rev. B 75, 244104 (2007)] as well as the methods of improving the fatigue endurance in ferroelectrics are given based on this model. Finally, the results on polarization fatigue in some special classes of ferroelectrics such as multiferroics (e.g., BiFeO3-based materials), ferroelectric polymers, and antiferroelectric thin films and ceramics are discussed and accounted for in light of our model.

Journal ArticleDOI
TL;DR: In this article, the authors used poly(3-hexylthiophene):[6, 6]-phenyl-C61-butyric acid methyl ester as their model polymer system and investigate different device structures using ultraviolet photoelectron spectroscopy as their primary tool to investigate the reason for this S-shaped kink.
Abstract: A kink is sometimes seen in the I-V curves for organic solar cells. In literature charge blocking has been speculated to be responsible for such kind of anomalous features. In this manuscript, we use poly(3-hexylthiophene):[6, 6]-phenyl-C61-butyric acid methyl ester as our model polymer system and investigate different device structures using ultraviolet photoelectron spectroscopy as our primary tool to investigate the reason for this S-shaped kink. We attribute this anomalous feature to the presence of strong interface dipoles. We further propose a model based on the standard set of Poisson equation, continuity equation, and current density equations including both drift and diffusion components.

Journal ArticleDOI
TL;DR: In this article, the effect of the electrical double layer created and its influence on viscosity increase has been investigated for alumina-water nanofluids, which are electrostatically stabilized.
Abstract: Nanofluids have shown remarkable attraction in heat transfer community due to its reported enhanced thermal properties. One factor which can restrict nanofluids in heat transfer application is the increased viscosity value (compared to classical predictions). Particle aggregation occurring was the major reason for this observation. Even though majority of the aqueous nanofluids prepared in literature were stabilized electrostatically by adjusting the pH, studies on the effect of the electrical double layer thus created and its influence on viscosity increase has not been investigated for these nanofluids so far. Thus, in the present paper, rheological properties of alumina-water nanofluids, which are electrostatically stabilized, are measured and the increase in suspension viscosity due to presence of this electrical double layer causing additional electroviscous effects is brought out. Based on dynamic light scattering studies, particle agglomeration and its subsequent effect in increasing the viscosity ...

Journal ArticleDOI
TL;DR: Gilbert damping for epitaxial Co2FeAl Heusler alloy films was investigated by analyzing the data of ferromagnetic resonance measured at the frequency of 2.20 GHz as discussed by the authors.
Abstract: Gilbert damping for the epitaxial Co2FeAl Heusler alloy films was investigated. Gilbert damping constant for the films was evaluated by analyzing the data of ferromagnetic resonance measured at the frequency of 2–20 GHz. Gilbert damping constant for the film without annealing was rather large, while it decreased remarkably with postannealing. Gilbert damping constant for the film annealed at 600 °C was ≃0.001. These behavior of Gilbert damping constant can be well explained by the fact that the density of states calculated from first principles decreases with increasing the degree of B2 order.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the Raman spectra of single-layer and multilayer graphene under ultraviolet laser excitation at the wavelength λ =325 nm and found that the 2D band intensity undergoes severe quenching.
Abstract: We investigated Raman spectra of single-layer and multilayer graphene under ultraviolet laser excitation at the wavelength λ=325 nm. It was found that while graphene’s G peak remains pronounced in UV Raman spectra, the 2D-band intensity undergoes severe quenching. The evolution of the ratio of the intensities of the G and 2D peaks, I(G)/I(2D), as the number of graphene layers n changes from n=1 to n=5, is different in UV Raman spectra from that in conventional visible Raman spectra excited at the 488 and 633 nm wavelengths. The 2D band under UV excitation shifts to larger wave numbers and is found near 2825 cm−1. The observed UV Raman features of graphene were explained by invoking the resonant scattering model. The obtained results contribute to the Raman nanometrology of graphene by providing an additional metric for determining the number of graphene layers and assessing its quality.

Journal ArticleDOI
TL;DR: In this article, the influence of silver nanoparticles on light absorption in organic solar cells based on poly(3-exylthiophene):(6,6)-phenyl-C61-butyric-acid-methyl ester is studied by means of finite element method simulations.
Abstract: The influence of silver nanoparticles on light absorption in organic solar cells based on poly(3-exylthiophene):(6,6)-phenyl-C61-butyric-acid-methyl ester is studied by means of finite element method simulations. The metallic nanoparticles are embedded directly inside the active layer. We investigate the enhancement mechanism and the influence of factors such as the spacing between neighboring nanoparticles, the particle diameter, and the coating thickness. The plasmonic resonance of the particles has a wideband influence on the absorption, and we observe a rich interaction between plasmonic enhancement and the absorption characteristics of the active layer material. An enhancement with a factor of around 1.56 is observed for nanoparticles with a diameter of 24 nm and a spacing of 40 nm, bringing the structure to the absorption level of much thicker active layers without nanoparticles. In addition, a significant effect of the particle coating thickness is observed.

Journal ArticleDOI
TL;DR: In this article, a two-dimensional simulation of electrical properties of the radio frequency (RF) sputter amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) is presented.
Abstract: We reported on a two-dimensional simulation of electrical properties of the radio frequency (rf) sputter amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). The a-IGZO TFT used in this work has the following performance: field-effect mobility (μeff) of ∼12 cm2/V s, threshold voltage (Vth) of ∼1.15 V, subthreshold swing (S) of ∼0.13 V/dec, and on/off ratio over 1010. To accurately simulate the measured transistor electrical properties, the density-of-states model is developed. The donorlike states are also proposed to be associated with the oxygen vacancy in a-IGZO. The experimental and calculated results show that the rf sputter a-IGZO TFT has a very sharp conduction band-tail slope distribution (Ea=13 meV) and Ti ohmic-like source/drain contacts with a specific contact resistance lower than 2.7×10−3 Ω cm2.

Journal ArticleDOI
TL;DR: In this paper, a boundary condition based on the periodic heat flux with constant virtual wall temperature is developed for the studied periodic structures, which enable the simulation of the lattice thermal conductivities with a minimum computational domain.
Abstract: In this work, phonon transport in two-dimensional (2D) porous silicon structures with aligned pores is investigated by Monte Carlo simulations considering the frequency-dependent phonon mean free paths (MFPs). A boundary condition based on the periodic heat flux with constant virtual wall temperature is developed for the studied periodic structures. Such periodic boundary conditions enable the simulation of the lattice thermal conductivities with a minimum computational domain. For the 2D case, it is found that phonon size effects caused by the periodically arranged pores can be remarkable even when the pore size and spacing are much larger than the averaged phonon MFPs. Our results show the importance of considering the frequency dependence of phonon MFPs in the analysis of micro- and nanostructured materials.

Journal ArticleDOI
TL;DR: In this article, the first and second-order Raman scattering of zinc blende and wurtzite ZnS was studied and the origins of these vibration modes in the second order Raman spectra from these two polymorphs were identified for the first time.
Abstract: We have conducted an experimental and theoretical study on first- and second-order Raman scattering of zinc blende and wurtzite ZnS. Based on the calculated phonon band structure, phonon density of states, and symmetry selection rules, we have clearly identified for the first time the origins of these vibration modes in the second-order Raman spectra from these two polymorphs. For zinc blende ZnS, it is found that the previously estimated frequency of the LA mode at X point in the Brillouin-zone boundary is much smaller than the value obtained from other experiments and our calculation. Considering the involvement of LA phonon at X point, we reassign the second-order Raman active modes and some other modes which have not yet been understood so far. This work clarifies some of the controversial Raman mode assignments in zinc blende and wurtzite ZnS.

Journal ArticleDOI
TL;DR: In this article, an Al2O3 film with a thickness of only 5 nm on a SiN PECVD film with thickness of 100 nm was shown to achieve a water vapor transmission rate of 5×10−5g/m2
Abstract: Thin films grown by Al2O3 atomic layer deposition (ALD) and SiN plasma-enhanced chemical vapor deposition (PECVD) have been tested as gas diffusion barriers either individually or as bilayers on polymer substrates. Single films of Al2O3 ALD with thicknesses of ≥10 nm had a water vapor transmission rate (WVTR) of ≤5×10−5 g/m2 day at 38 °C/85% relative humidity (RH), as measured by the Ca test. This WVTR value was limited by H2O permeability through the epoxy seal, as determined by the Ca test for the glass lid control. In comparison, SiN PECVD films with a thickness of 100 nm had a WVTR of ∼7×10−3 g/m2 day at 38 °C/85% RH. Significant improvements resulted when the SiN PECVD film was coated with an Al2O3 ALD film. An Al2O3 ALD film with a thickness of only 5 nm on a SiN PECVD film with a thickness of 100 nm reduced the WVTR from ∼7×10−3 to ≤5×10−5 g/m2 day at 38 °C/85% RH. The reduction in the permeability for Al2O3 ALD on the SiN PECVD films was attributed to either Al2O3 ALD sealing defects in the SiN PE...

Journal ArticleDOI
TL;DR: In this paper, the influence of the tip to electrode distance and specimen temperature on the parameters of APT tomography data sets is explored and the results indicate that the reconstruction parameters are specific to each specimen.
Abstract: Modern wide field-of-view atom probes permit observation of a wide range of crystallographic features that can be used to calibrate the tomographic reconstruction of the analyzed volume. In this study, methodologies to determine values of the geometric parameters involved in the tomographic reconstruction of atom probe data sets are presented and discussed. The influence of the tip to electrode distance and specimen temperature on these parameters is explored. Significantly, their influence is demonstrated to be very limited, indicating a relatively wide regime of experimental parameters space for sound atom probe tomography (APT) experiments. These methods have been used on several specimens and material types, and the results indicate that the reconstruction parameters are specific to each specimen. Finally, it is shown how an accurate calibration of the reconstruction enables improvements to the quality and reliability of the microscopy and microanalysis capabilities of the atom probe.