Showing papers in "Applied Physics Letters in 1997"
TL;DR: In this paper, the authors reported the observation of optically pumped lasing in ZnO at room temperature using a plasma-enhanced molecular beam epitaxy on sapphire substrates.
Abstract: We report the observation of optically pumped lasing in ZnO at room temperature. Thin films of ZnO were grown by plasma-enhanced molecular beam epitaxy on (0001) sapphire substrates. Laser cavities formed by cleaving were found to lase at a threshold excitation intensity of 240 kW cm−2. We believe these results demonstrate the high quality of ZnO epilayers grown by molecular beam epitaxy while clearly demonstrating the viability of ZnO based light emitting devices.
2,126 citations
TL;DR: In this article, an ultrathin LiF layer adjacent to an electron-transporting layer and an aluminum outerlayer was used as an electrode for organic electroluminescent devices.
Abstract: A bilayer is used as an electrode for organic electroluminescent devices The bilayer consists of an ultrathin LiF layer adjacent to an electron-transporting layer and an aluminum outerlayer Devices with the bilayer electrode showed enhanced electron injection and high electroluminescence efficiency as compared with a Mg09Ag01 cathode Similar effects were observed when replacing MgO for LiF The improvements are attributed to band bending of the organic layer in contact with the dielectrics
1,471 citations
TL;DR: In this paper, a carbon nanotube sheet electrode with high power and long cycle life was used for a single cell device with 38 wt% H2SO4 as the electrolyte.
Abstract: Carbon nanotube sheet electrodes have been prepared from catalytically grown carbon nanotubes of high purity and narrow diameter distribution, centered around 80 A. Our study shows that the electrodes are free-standing mats of entangled nanotubes with an open porous structure almost impossible to obtain with activated carbon or carbon fiber. These properties are highly desirable for high power and long cycle life electrochemical capacitors. Specific capacitances of 102 and 49 F/g were measured at 1 and 100 Hz, respectively, on a single cell device with 38 wt % H2SO4 as the electrolyte. The same cell had a power density of >8000 W/kg.
1,410 citations
TL;DR: In this paper, a long-range ordered channel array with dimensions on the order of millimeters with a channel density of 1010 cm−2 was obtained, and the aspect ratio was over 150.
Abstract: The development of the ordered channel array in the anodic porous alumina was initiated by the textured pattern of the surface made by the molding process, and growth of an almost defect-free channel array can be achieved throughout the textured area. The long-range-ordered channel array with dimensions on the order of millimeters with a channel density of 1010 cm−2 was obtained, and the aspect ratio was over 150. The master for molding could be used many times, which makes it possible to overcome problems in the conventional nanolithographic technique, such as low through-put and high cost.
1,194 citations
TL;DR: In this article, structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/In0.05Ga 0.95N (6 nm) multiple quantum wells.
Abstract: Structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/ In0.05Ga0.95N (6 nm) multiple quantum wells, by employing transmission electron microscopy and energy-dispersive x-ray microanalysis, both of which are assessed from the cross-sectional direction. It was found that the contrast of light and shade in the well layers corresponds to the difference in In composition. The main radiative recombination was attributed to excitons localized at deep traps which probably originate from the In-rich region in the wells acting as quantum dots. Photopumped lasing was observed at the high energy side of the main spontaneous emission bands.
850 citations
TL;DR: In this article, permanent optical waveguides can be formed in various bulk glasses by photoinduced refractive index change with an ultrashort pulse laser, which can be fabricated by focusing the laser beam through an microscope objective and translating the sample parallel to the axis of the beam.
Abstract: We show that permanent optical waveguides can be formed in various bulk glasses by photoinduced refractive index change with an ultrashort pulse laser. The waveguides were fabricated by focusing the laser beam through an microscope objective and translating the sample parallel to the axis of the laser beam. From the observations of intensity distributions in the output of guided light by a CCD camera, we demonstrated that permanent optical waveguides can be successfully formed in various glasses. In addition, from the analysis of a near-field pattern, it was confirmed that single mode waveguides of the graded index type can be formed by a writing technique using the ultrashort pulse laser.
771 citations
TL;DR: The extent and microstructural characteristics of the lateral overgrowth were a strong function of stripe orientation and threading dislocations, originating from the interface of the underlying GaN with the AlN buffer layer were contained in the GaN grown in the window regions.
Abstract: Organometallic vapor phase lateral epitaxy and coalescence of GaN layers originating from GaN stripes deposited within 3-μm-wide windows spaced 3 μm apart and contained in SiO2 masks on GaN/AlN/6H–SiC(0001) substrates are reported. The extent and microstructural characteristics of the lateral overgrowth were a strong function of stripe orientation. A high density of threading dislocations, originating from the interface of the underlying GaN with the AlN buffer layer, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The coalesced layers had a rms surface roughness of 0.25 nm.
744 citations
TL;DR: In this paper, an indium tin oxide anode contact to an organic light emitting device via oxygen plasma treatment was shown to improve the performance of single-layer doped-polymer devices.
Abstract: We demonstrate the improvement of an indium tin oxide anode contact to an organic light emitting device via oxygen plasma treatment. Enhanced hole-injection efficiency improves dramatically the performance of single-layer doped-polymer devices: the drive voltage drops from >20 to <10 V, the external electroluminescence quantum efficiency (backside emission only) increases by a factor of 4 (from 0.28% to 1%), a much higher drive current can be applied to achieve a much higher brightness (maximum brightness ∼10,000 cd/m2 at 1000 mA/cm2), and the forward-to-reverse bias rectification ratio increases by orders of magnitude (from 102 to 106–107). The lifetime of the device is also enhanced by two orders of magnitude.
734 citations
TL;DR: In this article, micro-explosions inside fused silica, quartz, sapphire, and other transparent materials using tightly focused 100 fs laser pulses are investigated, where material is ejected from the center, forming a cavity surrounded by a region of compacted material.
Abstract: We initiate micro-explosions inside fused silica, quartz, sapphire, and other transparent materials using tightly focused 100 fs laser pulses. In the micro-explosions, material is ejected from the center, forming a cavity surrounded by a region of compacted material. We examine the resulting structures with optical microscopy, diffraction, and atomic force microscopy of internal cross sections. We find the structures have a diameter of only 200–250 nm, which we attribute to strong self-focusing of the laser pulse. These experiments probe a unique regime of light propagation inside materials at intensities approaching 1021 W/m2, the electron ionization that accompanies it, and the material response to extreme pressure and temperature conditions. The micro-explosions also provide a novel technique for internal microstructuring of transparent materials.
678 citations
TL;DR: In this paper, the absorption coefficient for a 0.4-μm-thick GaN layer grown on a polished sapphire substrate was determined from transmission measurements at room temperature.
Abstract: The absorption coefficient for a 0.4-μm-thick GaN layer grown on a polished sapphire substrate was determined from transmission measurements at room temperature. A strong, well defined exciton peak for the A and B excitons was obtained. The A, B, and C excitonic features are clearly defined at 77 K. At room temperature, an energy gap Eg=3.452±0.001 eV and an exciton binding energy ExA,B=20.4±0.5 meV for the A and B excitons and ExC=23.5±0.5 meV for the C exciton were determined by analysis of the absorption coefficient. From this measured absorption coefficient, together with the detailed balance approach of van Roosbroek and Shockley, the radiative constant B=1.1×10−8 cm3/s was obtained.
665 citations
TL;DR: The thermal conductivity of Si-Ge superlattices with relatively longer periods, L>130 A, has smaller thermal conductivities than the short-period samples.
Abstract: The thermal conductivity of Si–Ge superlattices with superlattice periods 30 2 × 109 W m−2 K−1 at 200 K. Superlattices with relatively longer periods, L>130 A, have smaller thermal conductivities than the short-period samples. This unexpected result is attributed to a strong disruption of the lattice vibrations by extended defects produced during lattice-mismatched growth.
TL;DR: In this article, a giant magnetocaloric effect (ΔSmag) has been discovered in the Gd5(SixGe1−x)4 pseudobinary alloys, where x⩽0.5 is the largest order phase transformation.
Abstract: A giant magnetocaloric effect (ΔSmag) has been discovered in the Gd5(SixGe1−x)4 pseudobinary alloys, where x⩽0.5. For the temperature range between ∼50 and ∼280 K it exceeds the reversible (with respect to alternating magnetic field) ΔSmag for any known magnetic refrigerant material at the corresponding Curie temperature by a factor of 2–10. The two most striking features of this alloy system are: (1) the first order phase transformation, which brings about the large ΔSmag in Gd5(SixGe1−x)4, is reversible with respect to alternating magnetic field, i.e., the giant magnetocaloric effect can be utilized in an active magnetic regenerator magnetic refrigerator; and (2) the ordering temperature is tunable from ∼30 to ∼276 K by adjusting the Si:Ge ratio without losing the giant magnetic entropy change.
TL;DR: In this article, the combinatorial optimization simulated annealing algorithm is applied to the analysis of Rutherford backscattering data, which is fully automatic and does not require time-consuming human intervention.
Abstract: The combinatorial optimization simulated annealing algorithm is applied to the analysis of Rutherford backscattering data. The analysis is fully automatic, i.e., it does not require time-consuming human intervention. The algorithm is tested on a complex iron-cobalt silicide spectrum, and all the relevant features are successfully determined. The total analysis time using a PC 486 processor running at 100 MHz is comparable to the data collection time, which opens the way for on-line automatic analysis.
TL;DR: In this article, the authors used the third harmonic generation near the focal point of a tightly focused beam to probe microscopical structures of transparent samples, which can resolve interfaces and inhomogeneities with axial resolution comparable to the confocal length of the beam.
Abstract: Third harmonic generation near the focal point of a tightly focused beam is used to probe microscopical structures of transparent samples. It is shown that this method can resolve interfaces and inhomogeneities with axial resolution comparable to the confocal length of the beam. Using 120 fs pulses at 1.5 μm, we were able to resolve interfaces with a resolution of 1.2 μm. Two-dimensional cross-sectional images have also been produced.
TL;DR: In this paper, SiO2 films containing Si nanocrystals (nc-Si) and Er were studied and their photoluminescence properties were assigned to electron-hole recombination in nc-Si and the intra-4f transition in Er3+, respectively.
Abstract: SiO2 films containing Si nanocrystals (nc-Si) and Er were prepared and their photoluminescence (PL) properties were studied. The samples exhibited luminescence peaks at 0.81 and 1.54 μm, which could be assigned to the electron-hole recombination in nc-Si and the intra-4f transition in Er3+, respectively. Correlation between the intensities of the two luminescence peaks was studied as functions of Er concentration and excitation power. The present results clearly demonstrate that excitation of Er3+ occurs through the recombination of photogenerated carriers spatially confined in nc-Si and the subsequent energy transfer to Er3+.
TL;DR: In this article, a self-assembled monolayer of 4-thioacetylbiphenyl with nanoscale area was investigated and the experimental results showed that thermal emission of electrons over a barrier of 0.22 eV dominates electron injection from Ti into the organic layer while the transport for electron injection was satisfied by the formula for hopping conduction.
Abstract: We present the investigation of novel metal/organic monolayer/metal heterostructure diodes. Our technique provides well-defined, stable, and reproducible metallic contacts to a self-assembled monolayer of 4-thioacetylbiphenyl with nanoscale area. Electronic transport measurements show a prominent rectifying behavior arising from the asymmetry of the molecular heterostructure. Variable-temperature measurements reveal that thermal emission of electrons over a barrier of 0.22 eV dominates for electron injection from Ti into the organic layer while the transport for electron injection from Au into the organic layer satisfies the formula for hopping conduction.
TL;DR: In this article, β-Ga2O3 single crystals were grown by the floating zone method and their conductivity along the b axis was controlled from <10−9 to 38 Ω−1 1 cm−1 by changing the growth atmosphere.
Abstract: β-Ga2O3 single crystals were grown by the floating zone method and their conductivity along the b axis was controlled from <10−9 to 38 Ω−1 cm−1 by changing the growth atmosphere. By using feed rods doped with Sn, the grown crystal became highly conductive even under oxidative atmosphere. The optical transmission spectra showed that the β-Ga2O3 single crystal with 0.32 mm was transparent in the visible and ultraviolet region, with 20% transmittance at the fourth-harmonic wave of the Nd:YAG laser (266 nm). The band-gap widening was observed with the increasing of the carrier concentration. It is expected that the light of the KrF laser can be transmitted in the heavily doped β-Ga2O3.
TL;DR: In this article, the initial stages of GaAs overgrowth over self-assembled coherently strained InAs quantum dots (QDs) are studied, and it is shown that surface QDs with 5 nm or more GaAs are remarkably insensitive to surface recombination effects.
Abstract: The initial stages of GaAs overgrowth over self-assembled coherently strained InAs quantum dots (QDs) are studied. For small GaAs coverages (below 5 nm), atomic force microscopy (AFM) images show partially covered island structures with a regular size distribution which are elongated in the [011] direction. Analysis of the AFM profiles show that a large anisotropic redistribution of the island material is taking place during the initial GaAs overgrowth. Short time annealing experiments together with photoluminescence spectroscopy on annealed QDs are consistent with a Ga and In intermixing during the overgrowth. Surface QDs capped with 5 nm or more GaAs show a strong luminescence intensity indicating that surface QDs are remarkably insensitive to surface recombination effects.
TL;DR: In this paper, an external-cavity tunable semiconductor laser was used to detect the modulation signal at 1.3 µm using a laser heterodyne system.
Abstract: Electro-optic modulation up to 113 GHz has been demonstrated using traveling wave polymer modulators. The modulation signal was directly detected at 1.3 μm using a laser heterodyne system with an external-cavity tunable semiconductor laser. The device optical response variation, as a function of frequency over the whole W band, was within 3 dB. A well-matched coplanar probe was used to launch W band millimeter wave driving power into the microstrip line electrode on the device. Based upon these measurements, high speed electrodes with integrated millimeter wave transitions had been fabricated and tested.
TL;DR: In this paper, the microstructure and the lateral epitaxy mechanism of formation of homoepitaxially and selectively grown GaN structures within windows in SiO2 masks have been investigated by transmission electron microscopy (TEM) and scanning electron microscope.
Abstract: The microstructure and the lateral epitaxy mechanism of formation of homoepitaxially and selectively grown GaN structures within windows in SiO2 masks have been investigated by transmission electron microscopy (TEM) and scanning electron microscopy. The structures were produced by organometallic vapor phase epitaxy for field emission studies. A GaN layer underlying the SiO2 mask provided the crystallographic template for the initial vertical growth of the GaN hexagonal pyramids or striped pattern. The SiO2 film provided an amorphous stage on which lateral growth of the GaN occurred and possibly very limited compliancy in terms of atomic arrangement during the lateral growth and in the accommodation of the mismatch in the coefficients of thermal expansion during cooling. Observations with TEM show a substantial reduction in the dislocation density in the areas of lateral growth of the GaN deposited on the SiO2 mask. In many of these areas no dislocations were observed.
TL;DR: In this article, the relationship between microstructure and luminescence efficiency for heteroepitaxial films of GaN grown on c-axis sapphire substrates by metalorganic chemical-vapor deposition was discussed.
Abstract: We discuss the relationship between microstructure and luminescence efficiency for heteroepitaxial films of GaN grown on c-axis sapphire substrates by metalorganic chemical-vapor deposition. We directly characterize the correlation between threading dislocations as observed by transmission electron microscopy, surface morphology as observed by atomic force microscopy, and wavelength-resolved cathodoluminescence imaging. We show that the inhomogeneity in the luminescence intensity of these films near band edge can be accounted for by a simple model where nonradiative recombination at threading dislocations causes a deficiency of minority carriers and results in dark regions of the epilayer. An upper bound for average diffusion length is estimated to be 250 nm.
TL;DR: In this article, a broad range of temperatures and pumping levels of high-brightness light-emitting AlGaN and InGaN/GaN single-quantum-well structures were studied over a broad spectrum of spectra, and a quantitative description of the blue temperature-induced shift was proposed assuming a Gaussian shape of the band tail.
Abstract: Electro- and photoluminescence spectra of high-brightness light-emitting AlGaN/InGaN/GaN single-quantum-well structures are studied over a broad range of temperatures and pumping levels. Blue shift of the spectral peak position was observed along with an increase of temperature and current. An involvement of band-tail states in the radiative recombination was considered, and a quantitative description of the blue temperature-induced shift was proposed assuming a Gaussian shape of the band tail.
TL;DR: In this article, an n-type semiconducting diamond thin film was obtained by microwave enhanced plasma chemical vapor deposition using phosphine (PH3) as a dopant source, and the activation energy of carriers was 0.43 eV.
Abstract: An n-type semiconducting diamond thin film was obtained by microwave enhanced plasma chemical vapor deposition using phosphine (PH3) as a dopant source. A homoepitaxial diamond thin film with a thickness of about 300 nm was grown on the {111} surface of a type Ib diamond with a variety of dopant concentrations. Over a wide range of dopant concentrations (PH3/CH4: 1000–20 000 ppm), the n-type conduction was confirmed by Hall-effect measurements. The activation energy of carriers was 0.43 eV. The Hall mobility of about 23 cm2/V s has been obtained at around 500 K for the 1000 ppm sample. No significant increase of hydrogen has been observed by secondary-ion-mass-spectroscopy analysis for the phosphorous doped layers.
TL;DR: In this article, stable Pt electrodes with a spacing down to 4 nm have been fabricated and a new deposition technique, electrostatic trapping, which can be used to bridge the electrodes in a controlled way with a single conducting nanoparticle such as a conjugated or metal cluster molecule.
Abstract: For molecular electronics, one needs the ability to electrically address a single conducting molecule. We report on the fabrication of stable Pt electrodes with a spacing down to 4 nm and demonstrate a new deposition technique, i.e., electrostatic trapping, which can be used to bridge the electrodes in a controlled way with a single conducting nanoparticle such as a conjugated or metal–cluster molecule. In electrostatic trapping, nanoparticles are polarized by an applied electric field and are attracted to the gap between the electrodes where the field is maximum. The feasibility of electrostatic trapping is demonstrated for Pd colloids. Transport measurements on a single Pd nanoparticle show single electron tunneling coexisting with tunnel-barrier suppression.
TL;DR: In this article, the growth of InGaN thick films and InGal−xN/GaN double heterostructures by molecular beam epitaxy at the substrate temperatures 700-800°C was reported.
Abstract: We report the growth of InGaN thick films and InGaN/GaN double heterostructures by molecular beam epitaxy at the substrate temperatures 700–800 °C, which is optimal for the growth of GaN. X-ray diffraction and optical absorption studies show phase separation of InN for InxGal−xN thick films with x>0.3. On the other hand, InxGal−xN/GaN double heterostructures show no evidence of phase separation within the detection capabilities of our methods. These observations were accounted for using Stringfellow’s model on phase separation, which gives a critical temperature for miscibility of the GaN–InN system equal to 2457 K.
TL;DR: In this article, defect structures in GaN films grown selectively in hydride vapor-phase epitaxy (HVPE) were characterized by transmission electron microscopy (TEM) defect structures.
Abstract: We have characterized by transmission electron microscopy (TEM) defect structures in GaN films grown selectively in hydride vapor-phase epitaxy (HVPE). In this experiment, growth was achieved on SiO2-stripe-patterned GaN layers that had been grown by metalorganic vapor-phase epitaxy (MOVPE) on sapphire substrates. Cross-sectional TEM revealed unambiguously that most of the dislocations, which originated from threading dislocations vertically aligned in the MOVPE-grown layer, propagated laterally around the SiO2 mask in the HVPE-grown film before the film thickness amounted to about 5 μm. This change of the propagation direction prevented the dislocations from crossing the film to the surface region and thus principally led to a drastic reduction in the threading dislocation density in thicker films.
TL;DR: In this article, a doped emitter consisting of 8-hydroxyquinoline aluminum (Alq) as host and N,N-dimethylquinacridone (DMQA) as emissive dopant was used for an organic electroluminescent (EL) device with a constant current of 20 mA/cm2 and starting at a high luminance of 1400 cd/m2.
Abstract: Remarkable improvement in stability has been demonstrated in an organic electroluminescent (EL) device using a doped emitter consisting of 8-hydroxyquinoline aluminum (Alq) as the host and N,N-dimethylquinacridone (DMQA) as the emissive dopant. A luminance half-life on the order of about 7500 h has been achieved in the DMQA/Alq EL device, operating under a constant current of 20 mA/cm2 and starting at a high luminance of 1400 cd/m2. The improved stability was attributed to the elimination of intermolecular hydrogen bonding between the dopant molecules.
TL;DR: In this article, the authors examined the kinetics and mechanism of local oxidation of silicon and how factors such as the strength of the electric field, ambient humidity, and thickness of the oxide affect its rate and resolution.
Abstract: Atomic force microscope induced local oxidation of silicon is a process with a strong potential for use in proximal probe nanofabrication. Here we examine its kinetics and mechanism and how such factors as the strength of the electric field, ambient humidity, and thickness of the oxide affect its rate and resolution. Detection of electrochemical currents proves the anodization character of the process. Initial very fast oxidation rates are shown to slow down dramatically as a result of a self-limiting behavior resulting from the build up of stress and a reduction of the electric field strength. The lateral resolution is determined by the defocusing of the electric field in a condensed water film whose extent is a function of ambient humidity.
TL;DR: In this article, a wide supercooled liquid region before crystallization was found in Fe(Co,Ni) and Fe(Zr,Nb,Ta,Mo,W)-B systems with diameters up to 6 mm.
Abstract: New bulk amorphous alloys exhibiting a wide supercooled liquid region before crystallization were found in Fe–(Co,Ni)–(Zr,Nb,Ta)–(Mo,W)–B systems. The Tg is as high as about 870 K and the supercooled liquid region reaches 88 K. The high thermal stability of the supercooled liquid enabled the production of bulk amorphous alloys with diameters up to 6 mm. These bulk amorphous alloys exhibit a high compressive strength of 3800 MPa, high Vickers hardness of 1360, and high corrosion resistance. Besides, the amorphous alloys exhibit a high magnetic-flux density of 0.74–0.96 T, low coercivity of 1.1–3.2 A/m, high permeability exceeding 1.2×104 at 1 kHz, and low magnetostriction of about 12×10−6.
TL;DR: In this paper, the fabrication of nanotube field emitters with an onset field as low as 0.8 V/μm is described and the low-field electron emission mechanism is discussed.
Abstract: The fabrication of nanotube field emitters with an onset field as low as 0.8 V/μm is described and the low-field electron emission mechanism is discussed. These emitters are made using nanotube cathode deposit with the addition of epoxy resin. The preferred orientation of nanotubes in nanotube bundles of the deposit is preserved. The nanotube tips are sharpened by exposing the nanotube bundle surface to a microwave oxygen plasma. The local-field enhancement factor is estimated to be 8000 by using the Fowler–Nordheim equation. The low onset field is attributed to the well-distributed, highly orientated sharp tips at the sample surface.