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Applied Physics Letters

Issue 9 • Date Mar 2002

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Displaying Results 1 - 25 of 60
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Time-resolved coherent anti-Stokes Raman scattering microscopy: Imaging based on Raman free induction decay

    Page(s): 1505 - 1507
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    A time-resolved coherent anti-Stokes Raman scattering (CARS) microscope allows three-dimensional imaging based on Raman free induction decay of molecular vibration with no requirement for labeling of the sample with natural or artificial fluorophores. A major benefit of the technique is the capability to completely remove nonresonant coherent background signal from the sample and the solvent, and thus increasing the detection sensitivity of CARS microscopy significantly. © 2002 American Institute of Physics. View full abstract»

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  • Holographic fabrication of multiple layers of grating inside soda–lime glass with femtosecond laser pulses

    Page(s): 1508 - 1510
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    Gratings have been holographically fabricated inside soda–lime glass by two-beam interference of a single 130 fs laser pulse at a wavelength of 800 nm. Because the grating was localized in the focal volume and the depth at which the grating was formed can be controlled by translation of the sample, multiple layers of grating can be fabricated. As an example, three layers of grating have been recorded at depths of ∼200, ∼400, and ∼600 μm, respectively. These gratings can be read by either of the two recording beams with reduced energy, which demonstrates their potential applications in integrated optics and optical storage. © 2002 American Institute of Physics. View full abstract»

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  • Strain relaxation effect in microdisk lasers with compressively strained quantum wells

    Page(s): 1511 - 1513
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    From lasing wavelengths observed for many samples of microdisk lasers with GaInAsP compressively strained quantum wells (CS-QWs), we estimate the gain peak redshifted by ∼10 meV. We explain this phenomenon as the strain relaxation in the CS-QWs at the disk wing exposed to the air. Band and rate equation analyses show that the built-in potential by strain relaxation accelerates the carrier diffusion toward the disk edge and reduces the threshold to 30%–60% of that without strain relaxation. This result indicates the advantage of CS-QWs not only for the microdisk laser but also for various microlasers with a membrane structure, e.g., photonic crystal slab lasers. © 2002 American Institute of Physics. View full abstract»

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  • Integrated platform for silicon photonic crystal devices at near-infrared wavelengths

    Page(s): 1514 - 1516
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    We demonstrate an integrated fiber-photonic crystal system with a uniform band gap of around 1550 nm with insertion loss as low as 3.5 dB. One-dimensional photonic crystals fabricated on this platform exhibit a higher-order photonic band gap with over 30 dB suppression of transmission from 1400 to 1700 nm that is consistent with simulation. These results may lead to true integration of photonic crystals into optical fiber communication systems as optical switches and routers. © 2002 American Institute of Physics. View full abstract»

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  • Experimental studies of two-dimensional coaxial Bragg structures for a high-power free-electron maser

    Page(s): 1517 - 1519
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    The experimental studies of two-dimensional (2D) coaxial Bragg structures are presented. These structures, which realize 2D distributed feedback, have been recently proposed as a method of producing gigawatt power level spatially coherent radiation from a free-electron maser driven by a large-size relativistic electron beam of annular geometry. The experimentally obtained frequency dependence of transmission coefficients for the 2D Bragg structures are in good agreement with theoretical predictions that demonstrates the operation of the two-dimensional Bragg scattering mechanism. © 2002 American Institute of Physics. View full abstract»

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  • Photonic band gap formation by microstrip ring: A way to reduce the size of microwave photonic band gap structures

    Page(s): 1520 - 1522
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    We show that a microstrip ring with a narrow gap exhibits a photonic band gap (PBG) that is generally believed to be created by a periodic array. The discontinuity of impedance at the narrow gap induces multiple reflections with fixed phase correlation necessary to make the PBG. We also discuss the PBG frequency tuning achieved by applying an external voltage on a varactor mounted on the gap and applications of this PBG ring as a tunable band stop filter and a microwave switch. © 2002 American Institute of Physics. View full abstract»

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  • Kinetics of surface relief gratings tailored by laser-induced photochemical deposition

    Page(s): 1523 - 1525
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    We theoretically describe and experimentally explore the real-time formation of gratings induced by photochemical deposition in liquids. Considering the diffusion and the surface adsorption on the deposit of photochemically produced molecules, we show that the pattern induced (1) is not necessarily an exact replica of the light intensity distribution and (2) can become thick. By implementing experiments with chromate solutions photoactivated by two interfering continuous wave Ar+ laser waves, we validate the model by analyzing the temporal behavior of the first three harmonics of the induced grating. © 2002 American Institute of Physics. View full abstract»

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  • Heat flux distribution on a substrate in capacitively coupled radio-frequency discharges

    Page(s): 1526 - 1528
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    This letter investigates the heat flux distribution on a substrate in capacitively coupled radio-frequency argon glow discharges. The heat fluxes were determined from substrate temperature distributions measured by a high-sensitivity laser interferometer method with a LiNbO3 birefringent substrate. This method is based on monitoring the variation of refractive index with temperature. The distribution was obtained by sliding the 1-cm long LiNbO3 substrate between two semicircular glass substrates placed on a disk electrode, 12 cm in diameter. The gap length was fixed at 2 cm and gas pressure was 100 mTorr. For the substrate placed on the powered electrode, the heat flux increases radically from the glow center to the radial electrode edge from 40 to 58 mW/cm2. The heat flux toward the grounded electrode increases radically from 15 to 21 mW/cm2. The total measured heat flux to the electrode is in good agreement with electrical power consumption determined from voltage and current measurements. The radial electron density profile in the discharge chamber was measured using a Langmuir probe. The results show that the maximum electron density observed at the electrode edge coincides with the maximum heat flux. © 2002 American Institute of Physics. View full abstract»

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  • Realization of band gap above 5.0 eV in metastable cubic-phase MgxZn1-xO alloy films

    Page(s): 1529 - 1531
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    We report on the realization of wide band gap (5–6 eV), single-phase, metastable, and epitaxial MgxZn1-xO thin-film alloys grown on sapphire by pulsed laser deposition. We found that the composition, structure, and band gaps of the MgxZn1-xO thin-film alloys depend critically on the growth temperature. The structural transition from hexagonal to cubic phase has been observed for (Mg content greater than 50 at. %) (1≥x≥0.5) which can be achieved by growing the film alloys in the temperature range of 750 °C to room temperature. Interestingly, the increase of Mg content in the film has been found to be beneficial for the epitaxial growth at relatively low growth temperature in spite of a large lattice mismatch between sapphire and cubic MgZnO alloys. © 2002 American Institute of Physics. View full abstract»

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  • Mechanism of fatigue in micron-scale films of polycrystalline silicon for microelectromechanical systems

    Page(s): 1532 - 1534
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    Reported nearly a decade ago, cyclic fatigue failure in silicon thin films has remained a mystery. Silicon does not display the room-temperature plasticity or extrinsic toughening mechanisms necessary to cause fatigue in either ductile (e.g., metals) or brittle (e.g., ceramics and ordered intermetallics) materials. This letter presents experimental evidence for the cyclic fatigue of silicon via a conceptually different mechanism termed reaction-layer fatigue. Based on mechanical testing, electron microscopy, and self-assembled monolayers, we present direct observation of fatigue-crack initiation in polycrystalline silicon, the mechanism of crack initiation, and a method for altering fatigue damage accumulation. © 2002 American Institute of Physics. View full abstract»

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  • Green phosphorescence of CaAl2O4:Tb3+,Ce3+ through persistence energy transfer

    Page(s): 1535 - 1537
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    Single-crystal CaAl2O4 fibers doped with Ce3+ or Tb3+ or double doped with Tb3+ and Ce3+ were prepared by the laser-heated pedestal-growth method. Ce3+-doped CaAl2O4 was found to have persistent phosphorescence in the blue violet lasting more than 10 h, while Tb3+-doped CaAl2O4 showed only a weak afterglow. However, long persistent green phosphorescence from Tb3+ was observed in the doubly doped fiber, CaAl2O4:Tb3+,Ce3+. It was found that the enhanced green phosphorescence from Tb3+ is due to persistence energy transfer from the thermally released electrons to Ce3+ followed by energy transfer to Tb3+. The finding suggests a promising approach for controlling the color of persistent phosphors. © 2002 American Institute of Physics. View full abstract»

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  • Tuning polar anchoring energy through chemical modification of photodimerized surfaces

    Page(s): 1538 - 1540
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    A detailed study of the dependence of the polar anchoring energy of liquid crystals on the chemical structure of photoaligning layers is presented. The monolayer alignment layers consist of different chemical groups at the terminus so that the interaction of the alignment layer with the liquid-crystal molecules is systematically varied. The results demonstrate the ability to tune the polar anchoring energy by chemical modification of the alignment layers. View full abstract»

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  • Strain relaxation behavior of InxGa1-xAs quantum wells on vicinal GaAs (111)B substrates

    Page(s): 1541 - 1543
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    A number of reports have suggested that InGaAs/GaAs (111)B strained layer epitaxy has the prospect of reaching a higher critical layer thickness than that which can be achieved for (001) substrates. This has motivated a study of the relaxation mechanism of InGaAs/GaAs (111)B quantum wells with high In content (0.12≪x≪0.35). Transmission electron microscopy has revealed the existence of a different misfit dislocation (MD) configuration for high In contents (x≫0.25), which, we believe, has not been reported until now. For such compositions, plastic relaxation takes place through a polygonal network of MDs, which have Burgers vectors in the interface plane. The origin of this network is an unusual dislocation source that occurs through the formation of a three-pointed star-shaped configuration. The characteristics of this misfit dislocation network, which has a higher misfit relieving component and a glide plane coincident with the interface plane, imply a reduction of the previous critical layer thickness estimates for high In content InGaAs/GaAs (111)B heterostructures. However, we observe that none of the (111)B samples shows evidence of a transition to a three-dimensional growth mode, which represents a significant advantage compared to the behavior of high In content quantum wells on (001) substrates. © 2002 American Institute of Physics. View full abstract»

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  • Closely stacked InAs/GaAs quantum dots grown at low growth rate

    Page(s): 1544 - 1546
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    We present a systematic study of closely stacked InAs/GaAs quantum dots (QDs) grown at low growth rates. Transmission electron microscopy reveals that for thin spacer layers vertically aligned QDs merge into one large QD. After capping the initial QD layer the GaAs surface is decorated with well-developed nanostructures, which act as nucleation centers for the QDs deposited in the second layer. Despite the size increase, photoluminescence (PL) experiments show a systematic blueshift up to 103 meV of the QD related signal with decreasing spacer thickness. We explicitly show that this significant blueshift cannot fully be ascribed to specific growth phenomena, but instead is caused by the actual presence of the second dot layer. We report a PL linewidth as narrow as 16 meV at low temperature for a sample with 5 nm spacer thickness. © 2002 American Institute of Physics. View full abstract»

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  • Stress-based prediction of dislocation generation in GaN during lateral epitaxial overgrowth

    Page(s): 1547 - 1549
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    Finite element models were developed to simulate the stress history during an entire gallium nitride lateral epitaxial overgrowth process, from seed layer deposition on a substrate to GaN overgrowth above an SiO2 mask at 1100 °C and then cooldown to 20 °C. Lattice and thermal expansion mismatches between GaN and sapphire were considered simultaneously. Shearing stresses on a series of crystal planes were transiently analyzed. Based on the computational results, it is possible to predict the locations of high-density dislocations and when they develop, as well as Burgers vectors, dislocation types and their directions. © 2002 American Institute of Physics. View full abstract»

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  • Misfit defect formation in thick GaN layers grown on sapphire by hydride vapor phase epitaxy

    Page(s): 1550 - 1552
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    The nucleation and microstructure of large-scale columnar domains present in hydride vapor phase epitaxial (HPVE)-GaN layers grown directly on sapphire have been studied using cathodoluminescence and transmission electron microscopy. The domains are distributed in a quasicontinuous layer close to the GaN/sapphire interface. The domain boundaries are found to be associated with stacking mismatch defects. They are initiated at steps on the sapphire surface and are formed between nucleation islands growing on adjacent terraces. The formation of these domains in the initial stages of HVPE-GaN heteroepitaxial growth is proposed to play an important role in the strain relaxation mechanism. © 2002 American Institute of Physics. View full abstract»

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  • Phonon excitations and thermodynamic properties of cubic III nitrides

    Page(s): 1553 - 1555
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    A comprehensive study of phonon excitation and thermodynamic properties of cubic GaN and AlN is reported using a rigid-ion model (RIM) in the quasiharmonic approximation. The data on elastic constants and phonon modes at critical points at ambient and high pressures have allowed us to optimize RIM parameters to obtain accurate values of phonon dispersions, one-phonon density of states, mode Grüneisen parameters γ(q), specific heat Cv(T), and thermal expansion α(T) coefficients. Despite the small softening of TA modes in GaN and AlN, the variations of thermal expansion coefficients are seen much like that of Cv(T), and unlike other III–V compounds it exhibits no negative values at lower temperatures. © 2002 American Institute of Physics. View full abstract»

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  • Factors influencing glass formation in rapidly solidified Si,Ge–Ni and Si,Ge–Ni–Nd alloys

    Page(s): 1556 - 1558
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    In this letter we compare the glass-forming capabilities of binary Si,Ge–Ni and ternary Si,Ge–Ni–Nd alloys. The addition of Nd caused the formation of an amorphous single phase in the Ge–Ni–Nd alloy. The structure and crystallization behavior of the amorphous alloy were studied by differential scanning calorimetry and x-ray diffractometry. No amorphous phase formed in any of the Nd-free alloys or in the Si–Ni–Nd alloy. The higher glass-forming capability of the Ge–Ni–Nd alloy compared to Si–Ni–Nd cannot be explained on the basis of the widely used geometrical and chemical factors, viscosity, or diffusion data. The importance of the electronic structure characteristics is suggested to be a reason. © 2002 American Institute of Physics. View full abstract»

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  • Er-related luminescence in Er,O-codoped InGaAs/GaAs multiple-quantum-well structures grown by organometallic vapor phase epitaxy

    Page(s): 1559 - 1561
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    We have grown Er,O-codoped InGaAs/GaAs multiple-quantum-well (MQW:Er,O) structures by low-pressure organometallic vapor phase epitaxy (OMVPE), and investigated their luminescence properties. The MQW structures are designed to emit light at 0.98 μm for direct excitation of Er ions. Degradation of the structures due to introduction of Er and oxygen is not observed in x-ray diffraction patterns. Er ions doped with oxygen exhibit a sharp and well-ordered photoluminescence spectrum predominantly from one kind of Er center (Er-2O center). Photoluminescence excitation measurements on MQW:Er,O and Er,O-codoped GaAs samples reveal that under below-GaAs-band-gap excitation (830–940 nm), the Er-related luminescence is observed only in the MQW:Er,O sample. It indicates that the luminescence originates from Er ions in InGaAs QWs by means of trap-mediated excitation process. © 2002 American Institute of Physics. View full abstract»

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  • Formation of the GaN conversion layer in GaP (111)B wafers using ammonia gas

    Page(s): 1562 - 1564
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    The surface layer of GaP(111)B wafers is converted into α-GaN phase (wurtzite) by annealing at 900 °C for 6 h under NH3 gas flow. The converted layer extends to ∼10 μm from the surface. X-ray diffraction analysis shows that the conversion layer is a highly c-axis oriented GaN, supported by the appearance of an E2 phonon mode originated from the c face of α-GaN observed by a Raman scattering method. The photoluminescence spectra at 20 K consist of the zero-phonon-line transition at 3.34 eV followed by two phonon replicas observed 100 and 180 meV lower in energy. The possible origin of the emission is discussed. © 2002 American Institute of Physics. View full abstract»

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  • Effect of metal clusters on the swelling of gold–fluorocarbon–polymer composite films

    Page(s): 1565 - 1567
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    We have investigated the swelling of acetone in fluorocarbon polymer films doped with different gold concentrations below the percolation threshold. The presence of the gold clusters in the polymer improves the mixing between the polymer and the acetone, which is not a good solvent for this kind of polymer. To explain our results, the stoichiometry and the morphology of the polymer–metal system have been studied and a modified Flory–Huggins model has been developed. © 2002 American Institute of Physics. View full abstract»

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  • Neutron irradiation effects on visible-blind Au/GaN Schottky barrier detectors grown on Si(111)

    Page(s): 1568 - 1570
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    Neutron irradiation effects on the Au/sputtered–GaN Schottky barrier photodetectors grown on Si(111) were investigated. Evidence showed that the optimal fluence (1×1013 n/cm2) of neutron irradiation could not only promote the crystallinity of GaN films, but also effectively repress the occurrence of nitrogen–vacancy-related Et1 traps. We suggested that these two reasons are the main contributing factors to the superior rectifying current–voltage characteristics as well as the enhanced spectral response of Au/sputtered–GaN Schottky detectors. © 2002 American Institute of Physics. View full abstract»

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  • Band anticrossing in group II-OxVI1-x highly mismatched alloys: Cd1-xMnyOxTe1-x quaternaries synthesized by O ion implantation

    Page(s): 1571 - 1573
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    Highly mismatched group II-OxVI1-x alloys have been synthesized by oxygen implantation into Cd1-yMnyTe crystals. In crystals with y≫0.02, incorporation of O causes a large decrease in the band gap. The band gap reduction increases with y; the largest value observed is 190 meV in O-implanted Cd0.38Mn0.62Te. This striking behavior is consistent with the band anticrossing model which predicts that repulsive interaction between localized states of O and the extended states of the conduction band causes the band gap reduction. These large, O-induced effects provide a unique opportunity by which to control the optical and electronic properties in II–VI alloys. © 2002 American Institute of Physics. View full abstract»

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Aims & Scope

Applied Physics Letters, published by the American Institute of Physics, features concise, up-to-date reports on significant new findings in applied physics.

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Editor
Nghi Q. Lam
Argonne National Laboratory