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Showing papers in "Applied Physics Letters in 1998"


Journal ArticleDOI
TL;DR: In this article, the authors fabricated field effect transistors based on individual single and multi-wall carbon nanotubes and analyzed their performance, showing that structural deformations can make them operate as field-effect transistors.
Abstract: We fabricated field-effect transistors based on individual single- and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations—in our case a collapsed tube—can make them operate as field-effect transistors.

2,771 citations


Journal ArticleDOI
TL;DR: In this article, room-temperature ultraviolet (UV) laser emission of ZnO microcrystallite thin films is reported, which represents an important step towards the development of nanometer photoelectronics.
Abstract: Room-temperature ultraviolet (UV) laser emission of ZnO microcrystallite thin films is reported. The hexagonal ZnO microcrystallites are grown by laser molecular beam epitaxy. They are self-assembled and parallelly arrayed on sapphire substrates. The facets of the hexagons form natural Fabry-Perot lasing cavities. The optical gain for the room-temperature UV stimulated emission is of an excitonic nature and has a peak value an order of magnitude larger than that of bulk ZnO crystal. The observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals represents an important step towards the development of nanometer photoelectronics. © 1998 American Institute of Physics.

1,788 citations


Journal ArticleDOI
TL;DR: In this paper, the mechanical behavior of multi-walled carbon nanotube/epoxy composites was studied in both tension and compression, and it was found that the compression modulus is higher than the tensile modulus, indicating that load transfer to the nanotubes in the composite is much higher in compression.
Abstract: The mechanical behavior of multiwalled carbon nanotube/epoxy composites was studied in both tension and compression. It was found that the compression modulus is higher than the tensile modulus, indicating that load transfer to the nanotubes in the composite is much higher in compression. In addition, it was found that the Raman peak position, indicating the strain in the carbon bonds under loading, shifts significantly under compression but not in tension. It is proposed that during load transfer to multiwalled nanotubes, only the outer layers are stressed in tension whereas all the layers respond in compression.

1,617 citations


Journal ArticleDOI
TL;DR: In this article, a wide gap II-VI semiconductor alloy, MgxZn1−xO, was proposed for the fabrication of heteroepitaxial ultraviolet light emitting devices based on ZnO.
Abstract: We propose a widegap II–VI semiconductor alloy, MgxZn1−xO, for the fabrication of heteroepitaxial ultraviolet light emitting devices based on ZnO. The c-axis oriented MgxZn1−xO films were epitaxially grown by pulsed laser deposition on ZnO epitaxial films and sapphire (0001) substrates using ceramic targets. Solid solution films were prepared with Mg content up to x=0.33, achieving a band gap of 3.99 eV at room temperature. MgO impurity phase segregated at x⩾0.36. Lattice constants of MgxZn1−xO films changed slightly (∼1%), increasing in a axis and decreasing in c-axis direction with increasing x. These films showed ultraviolet photoluminescence at energies from 3.36 (x=0) to 3.87 eV (x=0.33) at 4.2 K.

1,441 citations


Journal ArticleDOI
TL;DR: In this article, the conditions for the self-organized formation of ordered hexagonal structures in anodic alumina were investigated for both oxalic and sulfuric acid as an electrolyte.
Abstract: The conditions for the self-organized formation of ordered hexagonal structures in anodic alumina were investigated for both oxalic and sulfuric acid as an electrolyte. Highly ordered pore arrays were obtained for oxidation in both acids. The size of the ordered domains depends strongly on the anodizing voltage. This effect is correlated with a voltage dependence of the volume expansion of the aluminum during oxidation and the current efficiency for oxide formation. The resulting mechanical stress at the metal/oxide interface is proposed to cause repulsive forces between the neighboring pores which promote the formation of ordered hexagonal pore arrays.

1,334 citations


Journal ArticleDOI
TL;DR: In this paper, a hexagonally symmetric honeycomb surface texture was used to reduce reflection loss in multicrystalline silicon solar cells and increase the cell's effective optical thickness.
Abstract: Multicrystalline silicon wafers, widely used in commercial photovoltaic cell production, traditionally give much poorer cell performance than monocrystalline wafers (the previously highest performance laboratory devices have solar energy conversion efficiencies of 186% and 240%, respectively) A substantially improved efficiency for a multicrystalline silicon solar cell of 198% is reported together with an incremental improvement in monocrystalline cell efficiency to 244% The improved multicrystalline cell performance results from enshrouding cell surfaces in thermally grown oxide to reduce their detrimental electronic activity and from isotropic etching to form an hexagonally symmetric “honeycomb” surface texture This texture reduces reflection loss as well as substantially increasing the cell’s effective optical thickness by causing light to be trapped within the cell by total internal reflection

977 citations


Journal ArticleDOI
TL;DR: In this article, the authors report the observation of single nanotube fragmentation, under tensile stresses, using nanotubes-containing thin polymeric films, and they estimate that the multi-wall multi-nanotube-matrix stress transfer efficiency is at least one order of magnitude larger than in conventional fiber-based composites.
Abstract: We report the observation of single nanotube fragmentation, under tensile stresses, using nanotube-containing thin polymeric films. Similar fragmentation tests with single fibers instead of nanotubes are routinely performed to study the fiber-matrix stress transfer ability in fiber composite materials, and thus the efficiency and quality of composite interfaces. The multiwall nanotube-matrix stress transfer efficiency is estimated to be at least one order of magnitude larger than in conventional fiber-based composites.

897 citations


Journal ArticleDOI
TL;DR: In this article, the transport properties of polycrystalline Ge clathrates with general composition Sr8Ga16Ge30 are reported in the temperature range 5'K⩽T'⦽300'K.
Abstract: Transport properties of polycrystalline Ge clathrates with general composition Sr8Ga16Ge30 are reported in the temperature range 5 K⩽T⩽300 K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 1017–1018 cm−3. The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, ZT, at room temperature for the present samples is 14 that of Bi2Te3 alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that ZT>1 at T>700 K, thus exceeding that of most known materials.

861 citations


Journal ArticleDOI
TL;DR: In this paper, a reproducible fabrication of field effect transistors in which the semiconducting, conducting, and insulating parts are all made of polymers is described, using spin-coating of electrically active precursors and pattern-wise exposure of the deposited films.
Abstract: A technology has been developed to make all polymer integrated circuits. It involves reproducible fabrication of field-effect transistors in which the semiconducting, conducting and insulating parts are all made of polymers. The fabrication on flexible substrates uses spin-coating of electrically active precursors and patternwise exposure of the deposited films. In the whole process stack integrity is maintained. Vertical interconnects are made mechanically. As a demonstrator functional 15-bit programmable code generators are fabricated. These circuits still operate when the foils are sharply bent. Due to the limited number of process steps the technology is potentially inexpensive.

840 citations


Journal ArticleDOI
TL;DR: In this paper, a method was presented to measure the energy dissipated by the tip-sample interaction in tapping-mode atomic force microscopy (AFM) using a 4 N/m cantilever with an initial amplitude of 25 nm tapping on a hard substrate at 74 kHz.
Abstract: A method is presented to measure the energy dissipated by the tip–sample interaction in tapping-mode atomic force microscopy (AFM). The results show that if the amplitude of the cantilever is held constant, the sine of the phase angle of the driven vibration is then proportional to changes in the tip–sample energy dissipation. This means that images of the cantilever phase in tapping-mode AFM are closely related to maps of dissipation. The maximum dissipation observed for a 4 N/m cantilever with an initial amplitude of 25 nm tapping on a hard substrate at 74 kHz is about 0.3 pW.

836 citations


Journal ArticleDOI
Thomas R. Hebner1, Chung-Chih Wu1, Duane Marcy1, M. H. Lu1, James C. Sturm1 
TL;DR: Ink-jet printing was used to directly deposit patterned luminescent doped polyvinyl carbazol (PVK) films as mentioned in this paper, and light emitting diodes with low turn-on voltages were also fabricated in PVK doped with coumarin 6 (C6).
Abstract: Ink-jet printing was used to directly deposit patterned luminescent doped-polymer films. The luminescence of polyvinylcarbazol (PVK) films, with dyes of coumarin 6 (C6), coumarin 47 (C47), and nile red was similar to that of films of the same composition deposited by spin coating. Light emitting diodes with low turn-on voltages were also fabricated in PVK doped with C6 deposited by ink-jet printing.

Journal ArticleDOI
TL;DR: In this paper, a GaN multi-quantum well-structure laser diodes with Al0.14Ga0.86N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN (ELOG) substrate was demonstrated to have a lifetime of more than 1150 h under room-temperature continuous-wave operation.
Abstract: InGaN multi-quantum-well-structure laser diodes with Al0.14Ga0.86N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN (ELOG) substrate was demonstrated to have a lifetime of more than 1150 h under room-temperature continuous-wave operation. After 4 μm etching of the ELOG substrate, the etch pit density was about 2×108 cm2 in the region of the 4-μm-wide stripe window, but almost zero in the region of the 7-μm-wide SiO2 stripe.

Journal ArticleDOI
TL;DR: In this article, a large scale and low-cost method for the synthesis of single-walled carbon nanotubes (SWNTs) using an improved floating catalyst method was proposed.
Abstract: Rope-like bundles of single-walled carbon nanotubes (SWNTs) similar to those obtained by laser vaporization and electric-are techniques were synthesized on a relatively large scale and at low cost by the catalytic decomposition of hydrocarbons at a temperature of about 1200 degrees C using an improved floating catalyst method. The SWNTs thus obtained have larger diameters and are self-organized into ropes. The addition of thiophene was found to be effective in promoting the growth of SWNTs and in increasing the yield of either SWNTs or multiwalled carbon nanotubes under different growth conditions. (C) 1998 American Institute of Physics. [S0003-6951(98)01125-5].

Journal ArticleDOI
TL;DR: In this paper, the authors report that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10kJ/m2 fluence in 500-Torr SF6 or Cl2.
Abstract: We report that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in 500-Torr SF6 or Cl2. The spikes are up to 40-μm tall, and taper to about 1-μm diam at the tip. Irradiation of silicon surfaces in N2, Ne, or vacuum creates structured surfaces, but does not create sharp conical spikes.

Journal ArticleDOI
TL;DR: In this article, the authors explored the charge-carrier transport mechanism in the organic semiconductor pentacene using thin-film transistor structures and found that the variation of the field-effect mobility with temperature differs from sample to sample, ranging from thermally activated to temperature-independent behavior.
Abstract: The charge-carrier transport mechanism in the organic semiconductor pentacene is explored using thin-film transistor structures. The variation of the field-effect mobility with temperature differs from sample to sample, ranging from thermally activated to temperature-independent behavior. This result excludes thermally activated hopping as the fundamental transport mechanism in pentacene thin films, and suggests that traps and/or contact effects may strongly influence the observed characteristics. These results also indicate that field-effect transistors may not be appropriate vehicles for illuminating basic transport mechanisms in organic materials.

Journal ArticleDOI
TL;DR: In this article, the ground state of an InGaAs/GaAs quantum-dot ensemble was obtained at 1.31 μm with a threshold current density of 270 A/cm2 using high-reflectivity facet coatings.
Abstract: Room-temperature lasing at the wavelength of 1.31 μm is achieved from the ground state of an InGaAs/GaAs quantum-dot ensemble. At 79 K, a very low threshold current density of 11.5 A/cm2 is obtained at a wavelength of 1.23 μm. The room-temperature lasing at 1.31 μm is obtained with a threshold current density of 270 A/cm2 using high-reflectivity facet coatings. The temperature-dependent threshold with and without high-reflectivity end mirrors is studied, and ground-state lasing is obtained up to the highest temperature investigated of 324 K.

Journal ArticleDOI
TL;DR: In this paper, the emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power, a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature.
Abstract: The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kW cm−2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kW cm−2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.

Journal ArticleDOI
TL;DR: In this article, the topmost oxide layer was observed to hydroxylate after immersion in water, which was used to enhance the etch-selectivity of SrO relative to TiO2 in a buffered HF solution.
Abstract: In recent years, well-defined and nearly perfect single crystal surfaces of oxide perovskites have become increasingly important. A single terminated surface is a prerequisite for reproducible thin film growth and fundamental growth studies. In this work, atomic and lateral force microscopy have been used to display different terminations of SrTiO3. We observe hydroxylation of the topmost SrO layer after immersion of SrTiO3 in water, which is used to enhance the etch-selectivity of SrO relative to TiO2 in a buffered HF solution. We reproducibly obtain perfect and single terminated surfaces, irrespective of the initial state of polished surfaces and the pH value of the HF solution. This approach to the problem might be used for a variety of multi-component oxide single crystals. True two-dimensional reflection high-energy electron diffraction intensity oscillations are observed during homo epitaxial growth using pulsed laser deposition on these surfaces.

Journal ArticleDOI
TL;DR: In this article, the authors report on the field emission properties of single-wall carbon nanotube films, with emphasis on current-versus-voltage (I-V) characteristics and current stability.
Abstract: We report on the field emission properties of single-wall carbon nanotube films, with emphasis on current–versus–voltage (I–V) characteristics and current stability. The films are excellent field emitters, yielding current densities higher than 10 mA cm−2 with operating voltages that are far lower than for other film emitters, but show a significant degradation of their performances with time. The observed deviations from the Fowler-Nordheim behavior in the I–V characteristics point to the presence of a nonmetallic density of states at the tip of the nanotubes.

Journal ArticleDOI
TL;DR: Using first-principles all-electron band structure method, the authors systematically calculated the natural band offsets ΔEv between all II-VI and separately between III-V semiconductor compounds and found that coupling between anion p and cation d states plays a decisive role in determining the absolute position of the valence band maximum.
Abstract: Using first-principles all-electron band structure method, we have systematically calculated the natural band offsets ΔEv between all II–VI and separately between III–V semiconductor compounds Fundamental regularities are uncovered: for common-cation systems ΔEv decreases when the cation atomic number increases, while for common-anion systems ΔEv decreases when the anion atomic number increases We find that coupling between anion p and cation d states plays a decisive role in determining the absolute position of the valence band maximum and thus the observed chemical trends

Journal ArticleDOI
TL;DR: In this paper, a method to fabricate polymer-based composites with aligned carbon nanotubes is described, and a procedure to determine the nanotube orientation and the degree of alignment is presented.
Abstract: We report a method to fabricate polymer-based composites with aligned carbon nanotubes, and a procedure to determine the nanotube orientation and the degree of alignment. The composites were fabricated by casting a suspension of carbon nanotubes in a solution of a thermoplastic polymer and chloroform. They were uniaxially stretched at 100 °C and were found to remain elongated after removal of the load at room temperature. The orientation and the degree of alignment were determined by x-ray diffraction. The dispersion and the alignment of the nanotubes were also studied by transmission electron microscopy.

Journal ArticleDOI
TL;DR: In this paper, a matrix addressable diode flat panel display has been fabricated using a carbon nanotube-epoxy composite as the electron emission source and field-emission uniformity has been confirmed by measuring the I-V curves of pixels across the panel.
Abstract: A matrix addressable diode flat panel display has been fabricated using a carbon nanotube–epoxy composite as the electron emission source. Field-emission uniformity has been confirmed by measuring the I–V curves of pixels across the panel. This prototype display demonstrates well-lit pixels under ±150 V biasing signals. The “on” and “off” of the pixels are well controlled by the half voltage “off-pixel” method. Further improvement of this technology may lead to easy-to-make and inexpensive flat panel displays.

Journal ArticleDOI
TL;DR: In this article, a nematic liquid crystal cell associated with a homogeneously aligned to twisted transition of a liquid crystal director was fabricated, which exhibits a high transmittance ratio as well as a wide viewing angle.
Abstract: We have fabricated a nematic liquid crystal cell associated with a homogeneously aligned to twisted transition of a liquid crystal director. In the absence of an electric field, the liquid crystal molecule is homogeneously aligned under the crossed polarizers, and thus the cell appears to be black. When a fringe field induced by interdigital electrodes is applied, liquid crystal molecules rotate in plane even above electrodes and thus the cell transmits light. The device exhibits a high transmittance ratio as well as a wide viewing angle, which solves a long standing problem of low transmittance existing in the conventional in-plane switching mode. We show that the distance between electrodes smaller than the width of an electrode and cell gap is required for generating fringe field with applied voltage and rotating molecules above electrodes. We also investigate the mechanism of fringe-field switching and dependence of electro-optic effect on different cell conditions and dielectric anisotropy of liquid ...

Journal ArticleDOI
TL;DR: In this article, cyclic voltammetry measurements for the blue electroluminescent conjugated polymer poly(9,9-dioctylfluorene) were obtained and both oxidation and reduction potentials were determined and estimates of both the ionization potential Ip and electron affinity Ea of the polymer were obtained for the same sample under the same experimental conditions.
Abstract: We report cyclic voltammetry measurements for the blue electroluminescent conjugated polymer poly(9,9-dioctylfluorene). Both oxidation and reduction potentials are determined and thus estimates of both the ionization potential Ip and electron affinity Ea of the polymer are obtained for the same sample under the same experimental conditions. We estimate Ip=5.80 eV and Ea=2.12 eV. These results disagree with the common assumption that Ea is, to good approximation, given by the difference between Ip and the optical gap. Measurements on indium tin oxide/polyfluorene/calcium light emitting diode structures are consistent with the deductions from the electrochemical data.

Journal ArticleDOI
TL;DR: In this article, temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition were performed.
Abstract: We report temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. We observed anomalous emission behavior, specifically an S-shaped (decrease–increase–decrease) temperature dependence of the peak energy (Ep) for InGaN-related PL with increasing temperature: Ep redshifts in the temperature range of 10–70 K, blueshifts for 70–150 K, and redshifts again for 150–300 K with increasing temperature. In addition, when Ep redshifts, the spectral width is observed to narrow, while when Ep blueshifts, it broadens. From a study of the integrated PL intensity as a function of temperature, it is found that thermionic emission of photocarriers out of local potential minima into higher energy states within the wells is the dominant mechanism leading to the thermal quenching of the InGaN-related PL. We demonstrate that the temperature-induced S-shaped PL shift is caused by a change in the carrier dyna...

Journal ArticleDOI
TL;DR: In this paper, a method of controlling threading dislocation densities in Ge on Si involving graded SiGe layers and chemical-mechanical polishing (CMP) is presented.
Abstract: A method of controlling threading dislocation densities in Ge on Si involving graded SiGe layers and chemical-mechanical polishing (CMP) is presented. This method has allowed us to grow a relaxed graded buffer to 100% Ge without the increase in threading dislocation density normally observed in thick graded structures. This sample has been characterized by transmission electron microscopy, etch-pit density, atomic force microscopy, Nomarski optical microscopy, and triple-axis x-ray diffraction. Compared to other relaxed graded buffers in which CMP was not implemented, this sample exhibits improvements in threading dislocation density and surface roughness. We have also made process modifications in order to eliminate particles due to gas-phase nucleation and cracks due to thermal mismatch strain. We have achieved relaxed Ge on Si with a threading dislocation density of 2.1×106 cm−2, and we expect that further process refinements will lead to lower threading dislocation densities on the order of bulk Ge su...

Journal ArticleDOI
TL;DR: In this article, a dioctyl-substituted polyfluorene was used as the emissive layer in combination with a polymeric triphenyldiamine hole transport layer.
Abstract: Efficient blue electroluminescence, peaked at 436 nm, is demonstrated from polymer light-emitting diodes operating at high brightness A dioctyl-substituted polyfluorene was used as the emissive layer in combination with a polymeric triphenyldiamine hole transport layer The luminance reaches 600 cd/m2 at a current density of 150 mA/cm2 for a bias voltage of 20 V, corresponding to an efficiency of 025 cd/A and a luminosity of 004 lm/W These values are optimized at a critical emissive layer thickness

Journal ArticleDOI
TL;DR: In this paper, a metal-doped organic layer was used as an electron-injecting layer at the interface between the cathode and the emitter layer of a bright organic electroluminescent device.
Abstract: Bright organic electroluminescent devices were developed using a metal-doped organic layer as an electron-injecting layer at the interface between the cathode and the emitter layer. The typical device structure is a glass substrate/indium-tin oxide/arylamine/tris(8-quinolinolato)Al (Alq)/metal-doped Alq/Al. Dopant metals are highly reactive metals such as Li, Sr, and Sm. A device with Li-doped Alq layer showed high luminance of over 30 000 cd/m2, while a device without the metal-doped Alq layer exhibited only 3400 cd/m2. These results suggest that the Li doping to the Alq layer generates the radical anions of Alq serving as intrinsic electron carriers, which result in low barrier height for electron injection and high electron conductivity of the Li-doped Alq layer.

Journal ArticleDOI
TL;DR: In this article, the piezoelectric field points from the growth surface to the substrate and its magnitude is 1.2 MV/cm for Ga0.84In0.16N/GaN quantum wells on sapphire substrate.
Abstract: We have identified piezoelectric fields in strained GaInN/GaN quantum well p-i-n structures using the quantum-confined Stark effect. The photoluminescence peak of the quantum wells showed a blueshift with increasing applied reverse voltages. This blueshift is due to the cancellation of the piezoelectric field by the reverse bias field. We determined that the piezoelectric field points from the growth surface to the substrate and its magnitude is 1.2 MV/cm for Ga0.84In0.16N/GaN quantum wells on sapphire substrate. In addition, from the direction of the field, the growth orientation of our nitride epilayers can be determined to be (0001), corresponding to the Ga face.

Journal ArticleDOI
TL;DR: In this paper, a novel technique is employed which simplifies the interpretation of the data and increases the imaging speed by at least one order of magnitude compared to previous implementations, and the variation of the imaging signal with distance fits well to a Hertzian contact model.
Abstract: Force sensors are key elements of atomic force microscopes and surface profilometers Sensors with an integrated deflection meter are particularly desirable Here, quartz tuning forks as used in watches are utilized as force sensors A novel technique is employed which simplifies the interpretation of the data and increases the imaging speed by at least one order of magnitude compared to previous implementations The variation of the imaging signal with distance fits well to a Hertzian contact model Images of compact discs and calibration gratings, which have been obtained with scanning speeds up to 230 μm/s, are presented