Showing papers in "Applied Physics Letters in 1996"
TL;DR: In this paper, the superelastic motion of twin boundaries in the martensitic phase of Ni2MnGa with magnetic fields of 8 kOe applied at 265 K was found to be associated with the staining.
Abstract: Strains of nearly 0.2% have been induced along [001] in unstressed crystals of Ni2MnGa with magnetic fields of 8 kOe applied at 265 K. These stains are associated with the superelastic motion of twin boundaries in the martensitic phase that is stable below about 274 K.
2,442 citations
TL;DR: Forouhi and Bloomer as mentioned in this paper presented a parameterization of the optical functions of amorphous semiconductors and insulators in which the imaginary part of the dielectric function e2 is determined by multiplying the Tauc joint density of states by the e2 obtained from the Lorentz oscillator model.
Abstract: A parameterization of the optical functions of amorphous semiconductors and insulators is presented in which the imaginary part of the dielectric function e2 is determined by multiplying the Tauc joint density of states by the e2 obtained from the Lorentz oscillator model. The real part of the dielectric function e1 is calculated from e2 using Kramers–Kronig integration. The parameters of this model are fit to n and k data for amorphous Si (2 data sets), SiO, As2S3, and Si3N4. Comparative fits are made with a similar parameterization presented earlier by Forouhi and Bloomer [Phys. Rev. B 34, 7018 (1986)]. In all cases, the new parameterization fits the data better.
2,090 citations
TL;DR: In this article, a new GaAs-based diluted magnetic semiconductor, (Ga,Mn)As, was prepared by molecular beam epitaxy and the lattice constant was determined by x-ray diffraction and shown to increase with the increase of Mn composition, x.
Abstract: A new GaAs‐based diluted magnetic semiconductor, (Ga,Mn)As, was prepared by molecular beam epitaxy. The lattice constant of (Ga,Mn)As films was determined by x‐ray diffraction and shown to increase with the increase of Mn composition, x. Well‐aligned in‐plane ferromagnetic order was observed by magnetization measurements. Magnetotransport measurements revealed the occurrence of anomalous Hall effect in the (Ga,Mn)As layer.
2,072 citations
TL;DR: By combining electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy, a strong correlation is observed between the green 510 nm emission, the free-carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO phosphor powders as mentioned in this paper.
Abstract: By combining electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy, a strong correlation is observed between the green 510 nm emission, the free‐carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO phosphor powders. From these results, we demonstrate that free‐carrier depletion at the particle surface, and its effect on the ionization state of the oxygen vacancy, can strongly impact the green emission intensity. The relevance of these observations with respect to low‐voltage field emission displays is discussed.
1,888 citations
TL;DR: In this paper, the electrical characteristics of field effect transistors using solution cast regioregular poly(3-hexylthiophene) are discussed and the authors demonstrate that both high field effect mobilities (ca. 0.045 cm2/V) and relatively high on/off current ratios (greater than 103) can be achieved.
Abstract: The electrical characteristics of field‐effect transistors using solution cast regioregular poly(3‐hexylthiophene) are discussed. We demonstrate that both high field‐effect mobilities (ca. 0.045 cm2/V s in the accumulation mode and 0.01 cm2/V s in the depletion mode), and relatively high on/off current ratios (greater than 103) can be achieved. We find that the film quality and field‐effect mobility are strongly dependent on the choice of solvents. In addition, treating a film with ammonia or heating to 100 °C under N2 can increase the on/off ratio without decreasing the mobility.
1,669 citations
TL;DR: In this paper, a new memory structure using threshold shifting from charge stored in nanocrystals of silicon (≊5nm in size) was described, which utilizes direct tunneling and storage of electrons in the nanocrystal.
Abstract: A new memory structure using threshold shifting from charge stored in nanocrystals of silicon (≊5nm in size) is described. The devices utilize direct tunneling and storage of electrons in the nanocrystals. The limited size and capacitance of the nanocrystals limit the numbers of stored electrons. Coulomb blockade effects may be important in these structures but are not necessary for their operation. The threshold shifts of 0.2–0.4 V with read and write times less than 100’s of a nanosecond at operating voltages below 2.5 V have been obtained experimentally. The retention times are measured in days and weeks, and the structures have been operated in an excess of 109 cycles without degradation in performance. This nanomemory exhibits characteristics necessary for high density and low power.
1,624 citations
TL;DR: In this article, a stable organic electroluminescent devices based on vapor-deposited Alq thin films have been achieved, which are derived from several factors including: (1) a multilayer thin-film structure with a CuPc stabilized hole-injection contact, (2) a hole-transport diamine layer using a naphthyl substituted benzidine derivative, and (3) an ac drive wave form.
Abstract: Highly stable organic electroluminescent devices based on vapor‐deposited Alq thin films have been achieved. The improvement in stability is derived from several factors including: (1) a multilayer thin‐film structure with a CuPc stabilized hole‐injection contact, (2) a hole‐transport diamine layer using a naphthyl‐substituted benzidine derivative, and (3) an ac drive wave form. These emissive devices have shown an operational half‐lifetime of about 4000 h from an initial luminance of 510 cd/m2.
1,559 citations
TL;DR: In this paper, a simple method for implementing the steady-state photoconductance technique for determining the minority-carrier lifetime of semiconductor materials is presented, using a contactless instrument.
Abstract: A simple method for implementing the steady‐state photoconductance technique for determining the minority‐carrier lifetime of semiconductor materials is presented. Using a contactless instrument, the photoconductance is measured in a quasi‐steady‐state mode during a long, slow varying light pulse. This permits the use of simple electronics and light sources. Despite its simplicity, the technique is capable of determining very low minority carrier lifetimes and is applicable to a wide range of semiconductor materials. In addition, by analyzing this quasi‐steady‐state photoconductance as a function of incident light intensity, implicit current–voltage characteristic curves can be obtained for noncontacted silicon wafers and solar cell precursors in an expedient manner.
1,522 citations
TL;DR: In this paper, a field-effect transistor made of transparant oxidic thin films, showing an intrinsic memory function due to the usage of a ferroelectric insulator.
Abstract: Operation is demonstrated of a field‐effect transistor made of transparant oxidic thin films, showing an intrinsic memory function due to the usage of a ferroelectric insulator. The device consists of a high mobility Sb‐doped n‐type SnO2 semiconductor layer, PbZr0.2Ti0.8O3 as a ferroelectric insulator, and SrRuO3 as a gate electrode, each layer prepared by pulsed laser deposition. The hysteresis behavior of the channel conductance is studied. Using gate voltage pulses of 100 μs duration and a pulse height of ±3 V, a change of a factor of two in the remnant conductance is achieved. The dependence of the conductance on the polarity of the gate pulse proves that the memory effect is driven by the ferroelectric polarization. The influence of charge trapping is also observed and discussed.
1,175 citations
TL;DR: In this paper, the authors investigated the emission mechanisms of InGaN single quantum well blue and green light emitting diodes and multiquantum well structures by means of modulation spectroscopy and assigned the static electroluminescence peak to recombination of excitons localized at certain potential minima in the quantum well.
Abstract: Emission mechanisms of InGaN single quantum well blue and green light emitting diodes and multiquantum well structures were investigated by means of modulation spectroscopy. Their static electroluminescence (EL) peak was assigned to the recombination of excitons localized at certain potential minima in the quantum well. The blueshift of the EL peak caused by the increase of the driving current was explained by combined effects of the quantum‐confinement Stark effect and band filling of the localized states by excitons.
1,089 citations
TL;DR: In this article, the authors investigated native defects and native defect impurity complexes as candidate sources for the yellow luminescence in GaN and found strong evidence that the Ga vacancy (VGa) is responsible.
Abstract: We have investigated native defects and native defect‐impurity complexes as candidate sources for the yellow luminescence in GaN. Using state‐of‐the‐art first‐principles calculations, we find strong evidence that the Ga vacancy (VGa) is responsible. The dependence of the VGa formation energy on Fermi level explains why the yellow luminescence is observed only in n‐type GaN. The VGa defect level is a deep acceptor state, consistent with recent pressure experiments. Finally we show that the formation of VGa is enhanced by the creation of complexes between VGa and donor impurities.
TL;DR: In this article, the x-ray line profiles of an ultrafine grained copper crystal, produced by equal-channel angular pressing, were measured by a special high resolution diffractometer with negligible instrumental line broadening.
Abstract: The x‐ray line profiles of an ultrafine grained copper crystal, produced by equal‐channel angular pressing, were measured by a special high resolution diffractometer with negligible instrumental line broadening. The analysis of the line breadths and the Fourier coefficients have shown that taking into account the contrast caused by dislocations on line profiles gives new scaling factors in the Williamson–Hall plot and in the Warren–Averbach analysis, respectively. When strain is caused by dislocations the new procedure proposed here enables a straightforward determination of particle size and strain, the latter in terms of the dislocation density.
TL;DR: In this article, the temperature dependence of the binodal and spinodal lines in the Ga1−xInxN system was calculated using a modified valence force field model where the lattice is allowed to relax beyond the first nearest neighbor.
Abstract: The large difference in interatomic spacing between GaN and InN is found to give rise to a solid phase miscibility gap. The temperature dependence of the binodal and spinodal lines in the Ga1−xInxN system was calculated using a modified valence‐force‐field model where the lattice is allowed to relax beyond the first nearest neighbor. The strain energy is found to decrease until approximately the sixth nearest neighbor, but this approximation is suitable only in the dilute limit. Assuming a symmetric, regular‐solutionlike composition dependence of the enthalpy of mixing yields an interaction parameter of 5.98 kcal/mole. At a typical growth temperature of 800 °C, the solubility of In in GaN is calculated to be less than 6%. The miscibility gap is expected to represent a significant problem for the epitaxial growth of these alloys.
TL;DR: Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high‐Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.
Abstract: A method to obtain long lengths of flexible, biaxially oriented substrates with smooth, chemically compatible surfaces for epitaxial growth of high‐temperature superconductors is reported. The technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield chemically compatible surfaces. Epitaxial YBa2Cu3Ox films grown on such substrates have critical current densities exceeding 105 A/cm2 at 77 K in zero field and have field dependencies similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high‐Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.
TL;DR: In this article, the authors demonstrate that the anomalously low (002) x-ray rocking curve widths for epitaxial hexagonal GaN films on (001) sapphire are a result of a specific threading dislocation geometry.
Abstract: In this letter we demonstrate that the anomalously low (002) x‐ray rocking curve widths for epitaxial hexagonal GaN films on (001) sapphire are a result of a specific threading dislocation (TD) geometry. Epitaxial GaN films were grown on c‐plane sapphire by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) in a horizontal flow reactor. Films were grown with (002) rocking curves (ω‐scans) widths as low as 40 arcsec and threading dislocation densities of ∼2×1010 cm−2. The threading dislocations in this film lie parallel to the [001] direction and within the limit of imaging statistics, all are pure edge with Burgers vectors parallel to the film/substrate interface. These TDs will not distort the (002) planes. However, distortion of asymmetric planes, such as (102), is predicted and confirmed in (102) rocking curve widths of 740 arcsec. These results are compared with films with (002) rocking curves of ∼270 arcsec and threading dislocation densities of ∼7×108 cm−2.
TL;DR: In this article, the photovoltaic response of two-layer photocells formed with layers of conjugated polymer poly(phenylenevinylene), PPV and fullerene, C60, formed between indium-tin oxide and aluminum electrodes was measured under short-circuit conditions.
Abstract: We report measurements of the photovoltaic response of two‐layer photocells formed with layers of the conjugated polymer poly(phenylenevinylene), PPV and fullerene, C60, formed between indium‐tin oxide and aluminum electrodes. Peak quantum efficiencies of up to ∼9% (electrons collected per incident photon) were measured under short‐circuit conditions. We model the photovoltaic response as arising from excitons photogenerated in the PPV layer which are able to diffuse to the interface with the C60 layer where they are ionized. We obtain a value for the exciton diffusion range of 7±1 nm, both from the spectral response and from the absolute efficiency. We demonstrate that the branching ratio for the creation of singlet excitons from absorbed photons is close to unity.
TL;DR: In this paper, a scheme for exploiting the nonresonant second-order nonlinearities in electrooptic media to extend the bandwidth of coherent spectroscopy in the far-infrared using ultrafast laser pulses was presented.
Abstract: We present a scheme for exploiting the nonresonant second‐order nonlinearities in electro‐optic media to extend the bandwidth of coherent spectroscopy in the far‐infrared using ultrafast laser pulses. Using optical rectification and electro‐optic sampling in 〈110〉 ZnTe for the generation and coherent detection of freely propagating THz radiation, respectively, we have demonstrated spectral sensitivity beyond 3 THz. This was accomplished by achieving phase matching for both optical rectification and electro‐optic sampling over a broad range of THz frequencies.
TL;DR: In this paper, an organic field effect transistors that employ copper phthalocyanine (Cu-Pc) as the semiconducting layer can function as p-channel accumulation mode devices.
Abstract: Organic field‐effect transistors that employ copper phthalocyanine (Cu–Pc) as the semiconducting layer can function as p‐channel accumulation‐mode devices. The charge carrier mobility of such devices is strongly dependent on the morphology of the semiconducting thin film. When the substrate temperature for deposition of Cu–Pc is 125 °C, a mobility of 0.02 cm2/V s and on/off ratio of 4×105 can be obtained. These features along with the highly stable chemical nature of Cu–Pc make it an attractive candidate for device applications.
TL;DR: In this paper, the transport properties of electrons and holes in polydialkoxy-p‐phenylene vinylene (PPV) were investigated by currentvoltage measurements using Ca as an electron and indium-tinoxide as a hole injecting contact.
Abstract: The transport properties of electrons and holes in poly(dialkoxy‐p‐phenylene vinylene) (PPV) are investigated by current–voltage measurements using Ca as an electron and indium‐tin‐oxide as a hole injecting contact Both the electron and hole currents are dominated by the bulk conduction properties of the PPV, in contrast to previous reports The hole current is governed by bulk space‐charge limited conductivity and a hole mobility of 05×10−6 cm2/V s is determined The electron current is strongly reduced by the presence of traps with a total density of 1018 cm−3
TL;DR: In this paper, the valence band discontinuities at various wurtzite GaN, AlN, and InN heterojunctions were measured by means of x-ray photoemission spectroscopy.
Abstract: The valence‐band discontinuities at various wurtzite GaN, AlN, and InN heterojunctions were measured by means of x‐ray photoemission spectroscopy. A significant forward–backward asymmetry was observed in the InN/GaN–GaN/InN and InN/AlN–AlN/InN heterojunctions. The asymmetry was understood as a piezoelectric strain effect. We report the valence band discontinuities for InN/GaN=1.05±0.25 eV, GaN/AlN=0.70±0.24 eV, and InN/AlN=1.81±0.20 eV, all in the standard type I lineup. These values obey transitivity to within the experimental accuracy. Tables of photoemission core level binding energies are reported for wurtzite GaN, AlN, and InN.
TL;DR: In this paper, the authors used ultraviolet and x-ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films.
Abstract: We used ultraviolet and x‐ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films. We obtained a work function of 4.4–4.5 eV which is lower than the commonly cited value. These values do not change substantially by heating and Ar ion sputtering. The atomic concentrations of each element in ITO, measured with XPS, are also quite stable under heat treatment and ion sputtering.
TL;DR: In this article, transparent zinc oxide (ZnO) films were cathodically deposited on conductive glasses from a simple aqueous zinc nitrate electrolyte kept at 335 K. ZnO films prepared had a wurtzite structure and exhibited an optical band gap energy of 3.3 eV.
Abstract: Transparent zinc oxide (ZnO) films has been cathodically deposited on conductive glasses from a simple aqueous zinc nitrate electrolyte kept at 335 K. ZnO films prepared had a wurtzite structure and exhibited an optical band gap energy of 3.3 eV which is characteristic of ZnO. A 2‐μm‐thick ZnO film with an optical transmittance of 72% has been deposited by electrolysis only for approximately 20 minutes at the cathodic potential of −1.0 V compared with the Ag/AgCl reference electrode.
TL;DR: In this article, a method to fabricate nanometer scale mechanical structures from bulk, single-crystal Si substrates is presented, which does not require low pressure chemical vapor deposition of intermediate masking layers, and the final step in the processing uses a dry etch technique, avoiding the difficulties encountered from surface tension effects when wet processing mechanically delicate or large aspect ratio structures.
Abstract: We report on a method to fabricate nanometer scale mechanical structures from bulk, single-crystal Si substrates. A technique developed previously required more complex fabrication methods and an undercut step using wet chemical processing. Our method does not require low pressure chemical vapor deposition of intermediate masking layers, and the final step in the processing uses a dry etch technique, avoiding the difficulties encountered from surface tension effects when wet processing mechanically delicate or large aspect ratio structures. Using this technique, we demonstrate fabrication of a mechanical resonator with a fundamental resonance frequency of 70.72 MHz and a quality factor of 2 x 10^(4).
TL;DR: In this paper, a continuous-wave (cw) operation of InGaN multi-quantum-well structure laser diodes (LDs) was demonstrated at room temperature (RT).
Abstract: Continuous‐wave (cw) operation of InGaN multi‐quantum‐well structure laser diodes (LDs) was demonstrated at room temperature (RT). The threshold current and voltage of the LD were 130 mA and 8 V, respectively. The threshold carrier density was 9 kA/cm2. The lifetime of the LDs under RT cw operation was 1 s due to large heat generation. Mode hopping of the emission wavelength of the LDs was observed. The average wavelength drift due to temperature increase was 0.066 nm/K between 20 and 70 °C, because of the temperature dependence of the gain profile due to band‐gap narrowing of the InGaN active layer.
TL;DR: In this article, the authors report the demonstration of transparent organic light emitting devices (OLEDs) which are ∼70% transparent throughout the visible spectrum when switched off, and emit light from both sides with a total external quantum efficiency of ∼0.1% when turned on.
Abstract: We report the demonstration of transparent organic light emitting devices (OLEDs) which are ∼70% transparent throughout the visible spectrum when switched off, and emit light from both sides with a total external quantum efficiency of ∼0.1% when turned on. The devices are Alq3‐based single heterostructure OLEDs grown on an ITO‐coated glass substrate with a top electrode composed of a very thin layer of Mg–Ag and an overlaying ITO film. The top electrode is both electron injecting and transparent. The transparent OLEDs are expected to be useful in high‐resolution full‐color displays, as well as for helmet‐mounted, windshield‐mounted, or other ‘‘head‐up’’ display applications.
TL;DR: In this paper, a new metallization scheme was developed for obtaining very low Ohmic contact to n-GaN, which was shown to have a specific resistivity value of 8.9×10−8 Ω'cm2 or lower for a doping level of 4×1017 cm−3.
Abstract: A new metallization scheme has been developed for obtaining very low Ohmic contact to n‐GaN. The metallization technique involves the deposition of a composite metal layer Ti/Al/Ni/Au (150 A/2200 A/400 A/500 A) on n‐GaN preceded by a reactive ion etching (RIE) process which most likely renders the surface highly n type. Of the several attempts and with annealing at 900 °C for 30 s, contacts with specific resistivity values of ρs=8.9×10−8 Ω cm2 or lower for a doping level of 4×1017 cm−3 were obtained. The physical mechanism underlying the realization of such a low resistivity is elucidated.
TL;DR: In this article, the authors present measurements of the broadband detection capability of copropagating electro-optic ZnTe field detectors for the characterization of free-space pulsed electromagnetic radiation.
Abstract: We present our measurements of the broadband detection capability of copropagating electro‐optic ZnTe field detectors for the characterization of free‐space pulsed electromagnetic radiation. To demonstrate ultrawide detection bandwidth, radiation from both gigahertz and terahertz bandwidth pulsed microwave sources has been characterized. The high frequency limitation of the sensor is the first TO phonon resonance (5.31 THz). The ultrashort temporal resolution is demonstrated by the detection of a 177 fs (full width at half‐maximum) pulse generated via THz optical rectification in 〈111〉 GaAs.
TL;DR: In this paper, a model based on the a−C:H:N acting as a space charge interlayer on the n++−Si cathodes was proposed to explain the emission at low electric fields.
Abstract: Field emission measurements using 0.3 μm thick nitrogen containing hydrogenated amorphous carbon films (a‐C:H:N) on n++‐Si cathodes are reported. Onset emission fields as low as 4 V μm−1 have been obtained using a flat plate anode configuration. Uniform emission is observed over the entire cathode area at current densities below 7×10−2 mA cm−2. At higher current density preferential emission from spots is observed. The spot emission is imaged using the ITO coated plate anode. A model based on the a‐C:H:N acting as a space charge interlayer on the n++‐Si is proposed to explain the emission at low electric fields.
TL;DR: InAs nanocrystal quantum dots have been prepared via colloidal chemical synthesis using the reaction of InCl3 and As[Si(CH3)3]3.
Abstract: InAs nanocrystal quantum dots have been prepared via colloidal chemical synthesis using the reaction of InCl3 and As[Si(CH3)3]3. Sizes ranging from 25 to 60 A in diameter are produced and isolated with size distributions of ±10%–15% in diameter. The nanocrystals are crystalline and generally spherical with surfaces passivated by trioctylphosphine giving them solubility in common organic solvents. The dots have been structurally characterized by transmission electron microscopy (TEM) and powder x‐ray diffraction (XRD) and the optical absorption and emission have been examined. Quantum confinement effects are evident with absorption onsets well to the blue of the bulk band gap and size dependent absorption and emission features. The emission is dominated by band edge luminescence. These quantum dots are particularly interesting as they provide an opportunity to make important comparisons with comparably sized InAs quantum dots synthesized by molecular beam epitaxy techniques.