Volume 78, Issue 17, 23 April 2001
- lasers, optics, and optoelectronics
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
78(2001); http://dx.doi.org/10.1063/1.1367905View Description Hide Description
A proposal for a photonic transistor is made and some basic proving experiments are described. These experiments show that by focusing two laser beams (405 and 635 nm) in one small spot on a high-speed rotating optical disk, a large signal enhancement is observed. It was found that a plasmon interaction generated between a silver light-scattering center and recorded small marks in the optical disk with a super-resolution near-field structure produced the large signal amplification in the spot (<1 μm). A modulated signal of the blue laser was enhanced by 60 times by controlling the red laser power from 1.5 to 3.5 mW. It has been shown that the system has the potential to realize all-thin-films photonic transistors by using local plasmon amplification.
78(2001); http://dx.doi.org/10.1063/1.1368181View Description Hide Description
A 3.5 W fiber-based laser source at 772 nm is demonstrated by using quasiphase-matched second-harmonic generation of a 40 dBm seeded ytterbium–erbium fiber amplifier in PPKTP. A 40% conversion efficiency is achieved across the entire output power range of the EDFA. No optical damage has been observed in the PPKTP over long-term high-power exposure.
78(2001); http://dx.doi.org/10.1063/1.1365945View Description Hide Description
Antiferroelectricliquid-crystal cells usually show symmetric electro-optic switching response from the antiferroelectric state to two opposite ferroelectric states. Intermediate transmission levels (analog gray scale) can be stabilized, applying a constant dc bias voltage after switching. Modifying the manufacturing process and using narrow dynamic-range antiferroelectric materials, a fully asymmetric response has been achieved. This asymmetric switching allows the antiferroelectric cell to be driven as a ferroelectric cell, reducing or eliminating the bias voltage, and ultimately leading to analog optical multistability, i.e., devices whose optical transmission may be arbitrarily set and maintained without power supply.
78(2001); http://dx.doi.org/10.1063/1.1367901View Description Hide Description
We report the observation of the well defined oscillation of a picosecond tetrahertz (ps THz) pulse within a dielectric concentric cylindrical cavity, which is coupled to an incoming focused beam of ps THz pulses by the optical tunneling effect of frustrated total internal reflection. The tunneling barrier is an 18-μm-thick air slab, situated between the plane surfaces of a hyperhemicylindrical focusing lens and the cavity. The output of the quasioptic cavity consists of a train of ps THz pulses with a frequency spectrum from 0.1 to 1.0 THz. Good agreement between experiment and theory is obtained in both the time and frequency domains.
78(2001); http://dx.doi.org/10.1063/1.1362201View Description Hide Description
A ten-stacked self-assembled InAs/GaAs quantum-dot infrared photodetector operated in the 2.5–7 μm range by photovoltaic and photoconductive mixed-mode near-room-temperature operation (⩾250 K) was demonstrated. The specific peak detectivity is at 250 K. The use of high-band-gap barriers at both sides of the InAs quantum-dot structure and the long carrier recombination time are the key factors responsible for its near-room-temperature operation.
78(2001); http://dx.doi.org/10.1063/1.1367898View Description Hide Description
Rigorous electromagnetic theory has been used to optimize elastomeric phase masks for generating sub-100-nm parallel lines by means of near-field photolithography J. A. Rogers et al., Appl. Phys. Lett. 70, 2658 (1997). In the near-field region, the scattering effect is so strong that the scalar theory is no longer adequate: A bright line was found adjacent to the dark line previously predicted by the scalar theory, and the widths of both lines were found to be insensitive to the refractive index of the photoresist. The simulation results are in good agreement with experimental studies, which showed that the bright and dark lines could be used to generate trenches and lines in a positive-tone photoresist by controlling the exposure time. Our simulations also indicate that parallel lines as small as 50 nm can be generated by adjusting the parameters of the phase mask.
78(2001); http://dx.doi.org/10.1063/1.1366364View Description Hide Description
We present details of the fabrication, calculations, and transmission measurements for finite two-dimensional (2D) polymerphotonic crystal(PC)slab waveguides, which were fabricated from a benzocyclobutene polymer on a low refractive index substrate from Teflon. A square air hole lattice (500 nm lattice constant, 300 nm hole diameter) was realized by electron beam lithography and reactive ion etching.Polarization and wavelength dependent transmission results show TE-like and TM-like stop gaps at 1.3 μm excitation wavelengths and are in good agreement with the calculated data obtained by 2D and three-dimensional finite difference time domain methods. Transmission was suppressed by 15 dB in the center of the TE-like stop gap for a PC length of ten lattice constants.
78(2001); http://dx.doi.org/10.1063/1.1367900View Description Hide Description
In this letter a thin film of diamond-like carbon (DLC) deposited by pulse laser deposition(PLD) is used as an electron injection layer in organic light emitting diodes. The heterojunction structures of these devices consist of: indium tin oxide (ITO)/conducting polymer/DLC/nickel, or ITO/DLC/P3OT/Pt. Poly(3-octylthiophene) conjugated polymer (P3OT) is used as the emission layer. In all the realized diodes, the current is two orders of magnitude larger than in the conventional ITO/P3OT/aluminum structure, and the driving voltage is drastically reduced. However, the light emission is observed only in the ITO/DLC/P3OT/Pt structure. These results can be interpreted in terms of a highly efficient electron injection from the DLC into the conducting polymer and a DLC/P3OT interface.
- STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER
78(2001); http://dx.doi.org/10.1063/1.1366360View Description Hide Description
Phase shifts in the intensity oscillation of reflection high-energy electron diffraction spots provide evidence for monolayer island formation on AlSb that is caused by sudden changes in surface stoichiometry. High-resolution scanning tunneling microscopy confirms the interpretation of the phase shift. These results are consistent with a previous structural assignment of the and surface reconstructions and provide guidelines for producing smooth interfaces in antimonide-based heterostructures.
78(2001); http://dx.doi.org/10.1063/1.1366647View Description Hide Description
The strain relaxation of GaN nucleation layers grown on sapphire (0001) during rapid thermal annealing was studied in a synchrotron x-ray scattering experiment. The as-grown GaN nucleation layer is compressively strained. Upon annealing to 750 °C, the lattice strain first changes to tensile. This tensile strain is released progressively as the annealing temperature increases. The nucleation layer sublimates significantly at 1050 °C where it becomes mostly strain-free hexagonal GaN.
78(2001); http://dx.doi.org/10.1063/1.1367288View Description Hide Description
The effect of postdeposition annealing in vacuum and in dry on the atomic transport and chemical stability of chemical vapor depositedfilms on Si is investigated. Rutherford backscattering spectrometry, narrow nuclear resonance profiling, and low energy ion scattering spectroscopy were used to obtain depth distributions of Si, O, and Zr in the films. The chemical environment of these elements in near-surface and near-interface regions was identified by angle-resolved x-ray photoelectron spectroscopy. It is shown that although the interface region is rather stable, the surface region presents an accumulation of Si after thermal annealing.
78(2001); http://dx.doi.org/10.1063/1.1366649View Description Hide Description
The friction behavior of a diamond-like carbon coating was studied in reciprocating sliding contact at speeds from 0.01 to 5 mm/s, in dry nitrogen. “Superlow” friction coefficients of 0.003–0.008 were obtained in continuous sliding at the higher speeds (>1 mm/s). However, friction coefficients rose to values typical of diamond-like carbon in dry and ambient air (0.01–0.1) at lower speeds (<0.5 mm/s) as well as in time-delayed, higher speed tests. The rise of the friction coefficients in both speed and time-delay tests was in good quantitative agreement with gas adsorption kinetics predicted by the Elovich equation for adsorption onto carbon. More generally, superlow friction could be sustained, suppressed, and recovered as a function of exposure time, demonstrating that duty cycle cannot be ignored when predicting performance of superlow friction coatings in devices.
78(2001); http://dx.doi.org/10.1063/1.1368186View Description Hide Description
High-purity silica that contains a high concentration of dissolved molecular hydrogen is found to undergo expansion, rather than densification, as a consequence of exposure to deep ultraviolet irradiation. Concurrent with the decreased density, we suggest that a positive photorefractive effect occurs, which increases the index. The contributions of the density change and the photorefractive effect can be observed by comparison of the birefringence and the wave front change obtained through interferometry. Expansion and the photorefractive effect are interpreted in terms of the photolytic formation of SiOH.
78(2001); http://dx.doi.org/10.1063/1.1368182View Description Hide Description
This work concerns the experimental analysis of the influence of the surface charge on the anchoring energy of a nematic liquid crystal(NLC). The study has been carried out on sandwich like cells in which the semitransparent indiumtin oxide electrodes were covered by a sputtered tungsten trioxide film to supply an extra amount of surface charge. We found that the huge amount of tungsten-oxide protons injected at the oxide-NLC interface play a key role in weakening the surface anchoring energy. To estimate the surface anchoring energy during a bias-controlled injection process, a photopolarimetric technique has been used. The result highlights the importance of the polarized surface on the anchoring breaking mechanism.
78(2001); http://dx.doi.org/10.1063/1.1367307View Description Hide Description
X-rayabsorption fine-structure spectroscopy is used to determine the site of residual As in wet-oxidized In a -oxide film removed from its GaAs substrate, the remaining As atoms are found to be coordinated with oxygen in the form of amorphous-As oxides, with a mixture of and sites that are locally similar to and No evidence of interstitial or substitutional As, As precipitates, or GaAs is seen, implying that less than 10% of the As atoms are in these forms.
78(2001); http://dx.doi.org/10.1063/1.1367302View Description Hide Description
We investigate the atomic interface structure and the residual strain state of ferromagnetic α (hexagonal) MnAs layers on cubic GaAs(001) by means of high-resolution transmission electron microscopy and electron diffraction. Despite the different symmetries of the adjacent planes at the heterointerface and the large and orientation-dependent lattice mismatch, the hexagonal MnAs grows epitaxially on GaAs(001) with the prism plane parallel to the cubic substrate. The atomic arrangement at the interface, which is defined by the accommodation of the large lattice mismatch, explains this extreme case of heteroepitaxial alignment. The anisotropic residual strain distribution is discussed with respect to the particular process of lattice misfit relaxation in the presence of the ferromagnetic phase transition.
Temperature dependence of excitonic absorption spectra in multiquantum wells grown on lattice-matched substrates78(2001); http://dx.doi.org/10.1063/1.1367300View Description Hide Description
The excitonic properties of high-quality multiquantum wells grown by laser-molecular-beam epitaxy were investigated using temperature-dependent optical absorptionspectra from 5 K to room temperature. The strength of exciton-longitudinal-optical (LO) -phonon coupling was deduced from the temperature dependence of the linewidth of the fundamental excitonic peak. Effective reduction of the exciton-LO-phonon coupling with decreasing the well width was observed, which is consistent with the confinement-induced enhancement of the exciton binding energy. The thermal shift of the lowest excitonic energy is independent of well width, indicating that the strain effect is negligible for this material.
78(2001); http://dx.doi.org/10.1063/1.1365418View Description Hide Description
Devising ultrathin barrier layers to prevent Cudiffusion into dielectrics is a major challenge that must be met to increase the speed, number density, and performance of microelectronics devices. Here, we demonstrate the use of near-zero-thickness (<2-nm-thick) self-assembled molecular monolayers (SAMs) as candidates for this application. metal–oxide–semiconductor capacitors, with and without SAMs at the interface, were annealed at 200 °C in a 2 MV cm−1 electrical field. Capacitance–voltage and current–voltage measurements of SAM-coated capacitors indicate that SAMs with aromatic terminal groups inhibit Cudiffusion into They consistently show more than four-orders-of-magnitude lower leakage currents and a factor-of-4 higher time to failure when compared with the corresponding values from samples without SAMs at the interface. SAMs with short tail lengths or aliphatic terminal groups are ineffective in hindering Cudiffusion, indicating that the molecular length and chemical configuration are key factors determining the efficacy of SAMs as barriers. We propose that the steric hindrance offered by the terminal groups in the SAMs are responsible for the barrier properties.
Fluorescence x-ray absorption fine structure study on local structures around Fe atoms heavily doped in GaN by low-temperature molecular-beam epitaxy78(2001); http://dx.doi.org/10.1063/1.1368184View Description Hide Description
A local structural transition in heavily Fe-doped GaNfilms related to the magnetic properties has been revealed by fluorescencex-rayabsorption fine structure (XAFS) analysis. The structural transition is explained (or considered to be induced) by the change in the degree of hybridization between Fe and N states, which can be evaluated by x-ray absorption near edge structurespectra. The XAFS analysis indicates that the present diluted magnetic semiconductor based on GaN can be fabricated by electron cyclotron resonance microwave plasma-assisted low-temperature molecular-beam epitaxy.
78(2001); http://dx.doi.org/10.1063/1.1368192View Description Hide Description
We report the molecular-beam-epitaxy growth and characterization of epilayers and quantum-well (QW)structures on InP substrates. Good optical properties and high crystalline quality were established using photoluminescence and x-ray diffraction measurements. Narrow x-ray rocking curves with line widths down to 49 arc sec were obtained for closely lattice-matched to InP. A strong luminescence emission with energy of 2.072 eV and a full width at half maximum of 27 meV at 77 K was obtained from a QWstructure with a 48-Å-thick QW. Strong room-temperature luminescence was also observed from the QW. A linear dependence of the QWphotoluminescence intensity on the excitation laser density and an absence of shift in the emission energy indicates that the QW emission has an excitonic behavior. Based on these results and on the expected lattice-hardening properties of BeSe, we propose that BeCdSe is an attractive quantum-well material for visible light-emitting diodes.