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Applied Physics Letters

Issue 14 • Date Apr 1996

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Displaying Results 1 - 25 of 51
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Mechanisms of band‐edge emission in Mg‐doped p‐type GaN

    Page(s): 1883 - 1885
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    Time‐resolved photoluminescence has been employed to study the mechanisms of band‐edge emissions in Mg‐doped p‐type GaN. Two emission lines at about 290 and 550 meV below the band gap (Eg) have been observed. Their recombination lifetimes, dependencies on excitation intensity, and decay kinetics have demonstrated that the line at 290 meV below Eg is due to the conduction band‐to‐impurity transition involving shallow Mg impurities, while the line at 550 meV below Eg is due to the conduction band‐to‐impurity transition involving doping related deep‐level centers (or complexes). © 1996 American Institute of Physics. View full abstract»

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  • Net optical gain at 1.53 μm in Er‐doped Al2O3 waveguides on silicon

    Page(s): 1886 - 1888
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    A 4 cm long Er‐doped Al2O3 spiral waveguide amplifier was fabricated on a Si substrate, and integrated with wavelength division multiplexers within a total area of 15 mm2. When pumped with 9 mW 1.48 μm light from a laser diode, the amplifier shows 2.3 dB net optical gain at 1.53 μm. The gain threshold was 3 mW. The amplifier was doped with Er by ion implantation to a concentration of 2.7×1020 cm-3. The data agree well with calculations based on a model which includes the effects of cooperative upconversion and excited state absorption. For an optimized amplifier, net optical gain of 20 dB is predicted. © 1996 American Institute of Physics. View full abstract»

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  • Time‐resolved D‐band luminescence in strain‐relieved SiGe/Si

    Page(s): 1889 - 1891
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    Temporal decay characteristics of dislocation‐related luminescence bands (D1–D4) were explored in strain‐relieved epitaxial SiGe/Si(100). Close similarity of the decay profiles was observed not only between D1 and D2 bands but also between D3 and D4 bands. The decay transients of the D1 and D2 bands at low temperatures are characterized by long decay times, τ≳200 ns, whereas the D3 and D4 bands exhibit even sharper transients with τ≪60 ns. Temperature dependence of ‘‘radiative’’ lifetimes implies a free‐to‐bound nature of the D1 and D2 bands, while a bound‐to‐bound character of the luminescence origins for the D3, D4 bands. © 1996 American Institute of Physics. View full abstract»

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  • Temperature dependence of the threshold current in gain‐coupled distributed feedback lasers with periodically etched quantum wells: Mechanism for an appearance of infinite T0

    Page(s): 1892 - 1894
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    A phenomenological model describing the temperature dependence of the threshold current of 1.3 μm gain‐coupled distributed feedback (DFB) lasers with periodically etched strained‐layer quantum wells is presented and compared to experimental results. The model can predict the minimum threshold current as a function of temperature (infinite T0), which has been observed experimentally. The mechanism for the appearance of this minimum is explained by the decrease of the detuning of the DFB‐mode wavelength from the material gain‐peak wavelength as the temperature increases. The condition for single‐mode operation is also discussed with use of the effective gain of DFB laser structures. © 1996 American Institute of Physics. View full abstract»

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  • Continuous‐wave operation of a blue vertical‐cavity surface‐emitting laser based on second‐harmonic generation

    Page(s): 1895 - 1897
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    An InGaAs/GaAs vertical‐cavity surface‐emitting laser has been fabricated on a (311)B GaAs substrate. Pulsed lasing operation is obtained at room temperature, and continuous‐wave lasing operation is obtained at less than 270 K. From the device, blue laser emission based on second‐harmonic generation is observed. The wavelength of the blue laser emission is 482 nm. At 135 K, its output power is 1 nW under continuous‐wave operation and more than 10 nW under pulsed operation. © 1996 American Institute of Physics. View full abstract»

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  • Kinetics of thermal oxidation of AlAs in water vapor

    Page(s): 1898 - 1900
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    We have investigated the lateral thermal oxidation of AlAs in water vapor in vertical cavity surface emitting laser structures. At low temperatures and short oxidation times, oxide growth was found to be reaction rate limited. Conversely, diffusion across the oxide was the rate controlling mechanism at higher temperatures and longer oxidation times. Lasers are typically processed at intermediate values of temperatures and time. The observed growth can be modeled by rate equations by which the two component growth mechanisms can be separated. Activation energies of 1.6 and 0.8 eV were determined for the reaction rate and diffusion limited mechanisms, respectively. © 1996 American Institute of Physics. View full abstract»

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  • Atomic resolution x‐ray hologram

    Page(s): 1901 - 1903
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    Unlike crystals producing sharp peaks under the x‐ray diffraction, disordered materials show slowly varying patterns presenting little structural information. This is due to the fact that only the scattering intensity can be recorded, and the loss of the phase information makes the Fourier inversion of the amplitude impossible. However, when an amorphous sample is placed on a crystal, the x‐ray intensity diffracted by the assembly can produce a hologram which contains the spatial modulation of both density functions. Therefore, with known crystal structure the electron density of the amorphous specimen can be resolved by holographic technique. Moreover, it is found that only the amorphous scattering data are needed, and the Bragg peak(s) can be dropped. This gives the scheme the same noise tolerance as by the ordinary amorphous diffraction. © 1996 American Institute of Physics. View full abstract»

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  • Room‐temperature quantum well infrared modulator using a Schottky diode

    Page(s): 1904 - 1906
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    A reverse biased Schottky diode is used to deplete the population in quantum wells and to modulate the intersubband absorption coefficient. This middle infrared electro‐optic modulator works at room temperature. A waveguide geometry is demonstrated, with only six quantum wells. The 14% modulation could easily be enhanced by optimizing the quantum well overlap with the optical guided mode of the waveguide. © 1996 American Institute of Physics. View full abstract»

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  • Measurements of the spectral line shape function and frequency width of longitudinal mode in a multimode gas laser

    Page(s): 1907 - 1909
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    In terms of the quasiperiodicity of temporal coherence g(1)(τ) in a multimode gas laser, we present measurements of both spectral line shape function and frequency width of longitudinal mode in a multimode laser. The experimental result shows that the spectral line shape of longitudinal mode is Lorentzian line‐shape function for a freely operated multimode gas laser with inhomogeneous broadening, it is not affected by the drift effect of longitudinal mode frequency. However, the frequency width of longitudinal mode is broadened greatly because of the influence of the drift effect of longitudinal mode frequency. © 1996 American Institute of Physics. View full abstract»

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  • Temperature sensitivity and thermal expansion coefficient of benzocyclobutene thin films studied with ellipsometry

    Page(s): 1910 - 1912
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    Temperature induced changes in thickness and refractive index of thin films of benzocyclobutene (BCB) have been studied with ellipsometry in the temperature range 22–190 °C. The ellipsometric parameters Ψ and Δ, which depend on both the thickness and the optical constants of the BCB films, change significantly with temperature with short response times. The temperature sensitivity, resolution, and dynamic range depend on film thickness and wavelength. For a film with thickness 4.73 μm, a sensitivity of 0.2°/°C with a resolution of 0.25 °C is achieved within a range of 170 °C. Thermal expansion coefficients (TEC) of BCB films with different thicknesses evaluated from spectroscopic ellipsometric measurements are in the range 6.1–6.5×10-5/°C. Based on the findings here, a real‐time detection system for remote optical monitoring of surface temperature variation can be developed. A technique for the determination of TEC of some organic films is also feasible. © 1996 American Institute of Physics. View full abstract»

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  • Stable optical storage and high‐order diffraction in a liquid‐crystal polymer film by two‐wave mixing

    Page(s): 1913 - 1914
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    Optical storage with long‐term stability and up to third‐order diffraction have been observed in a liquid‐crystal polymer film by two‐wave mixing. After turning off the pump beam, the diffraction of the probe beam remains. While the probe beam is turned off, the intensities of the diffraction beams of the pump beam does not decrease significantly. A brilliant hologram spot is observed on the film during the experiments and the trace of the spot can be maintained over 120 days without obvious change. © 1996 American Institute of Physics. View full abstract»

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  • Absolute density measurements of ammonia synthetized in N2–H2 mixture discharges

    Page(s): 1915 - 1917
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    Measurements of absolute density of ammonia in nitrogen–hydrogen mixture dc glow discharges were performed by employing photofragment translational spectroscopy associated with laser induced fluorescence (LIF) of the H ground state atoms. The dissociation energy of ammonia using the photolysis at 205 nm was determined to be (4.34±0.07) eV. The H atom LIF measurements were calibrated by vacuum ultraviolet absorption spectroscopy on the Lyman‐α (121 nm) line. The ammonia synthesized in the discharge has a maximum density of about 8×1012 cm-3 for a 75% H2–25% N2 mixture, a discharge current of 50 mA and a pressure of 2 Torr. © 1996 American Institute of Physics. View full abstract»

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  • Ion focusing by an expanding, two‐dimensional plasma sheath

    Page(s): 1918 - 1920
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    The temporal evolution of the collisionless, pulsed plasma sheath around a square bar is studied using a two‐dimensional particle‐in‐cell code. It is found that the incident ion dose is peaked near to, but not on, the corner. This effect is explained physically by considering ion trajectories across the sheath, yielding an estimate for the dose in good agreement with the simulation results. The present results are compared to those from a fluid simulation [T. E. Sheridan and M. J. Alport, Appl. Phys. Lett. 64, 1783 (1994)], and it is found that the fluid simulation erroneously underestimates the ion dose near the corner of the bar. © 1996 American Institute of Physics. View full abstract»

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  • Photoelectron spectra enhanced by x‐ray total reflection and diffraction from periodic multilayer

    Page(s): 1921 - 1923
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    Photoelectron spectra from a periodic multilayer of (Mo/B4C)30/Si were measured by changing the x‐ray glancing angle. The angular dependencies of the photoelectron intensities of O 1s and Mo 3d revealed the atomic depth profiles of the Mo oxide overlayer, and the compositions of the MoO3 upper layer and MoO2 lower layer. Furthermore, we observed enhancement of photoelectron emission by means of x‐ray standing waves generated near the multilayer surface during x‐ray diffraction from the superlattice. © 1996 American Institute of Physics. View full abstract»

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  • Nondestructive measurement of interfacial SiO2 films formed during deposition and annealing of Ta2O5

    Page(s): 1924 - 1926
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    It is demonstrated that infrared absorption spectroscopy of the transverse optic and longitudinal optic vibrational modes of the asymmetric bridging oxygen stretch in amorphous SiO2 may be used to conveniently estimate film thickness in sandwiched structures. Measurements have been made on the SiO2 film formed at the substrate/dielectric interface during deposition of Ta2O5 on Si substrates and the evolution of its thickness during subsequent annealing. The accuracy of the film thickness determined is compared with electron microscopy observations. © 1996 American Institute of Physics. View full abstract»

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  • Diamond film orientation by ion bombardment during deposition

    Page(s): 1927 - 1929
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    The influence of ion bombardment during microwave plasma chemical vapor deposition (CVD) on diamond film orientation has been investigated. Two interesting findings were obtained: (1) The [001] axes of the grown diamond grains are always along the ion flow direction, perpendicular to the substrate and independent of the crystal orientation of the substrates and (2) for the crystallites which are homoepitaxially grown on the (001) diamond faces parallel to the substrate slight misorientations were found. These new findings confirm the role of ion impact in diamond CVD and can help us to understand the basic mechanism responsible for the crystal orientation in heteroepitaxial diamond films prepared using bias‐enhanced nucleation. © 1996 American Institute of Physics. View full abstract»

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  • Structures and dielectric properties of thin polyimide films with nano‐foam morphology

    Page(s): 1930 - 1932
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    Thin polyimide films with dispersed nano‐foam morphology have been prepared for the purpose of obtaining low dielectric polymer insulators for microelectronic applications. They were obtained by utilizing micro phase‐separated triblock copolymers where the thermally stable polyimide matrix component was derived from pyromellitic dianhydride (PMDA) with 1,1‐bis(4‐aminophenyl)‐1‐phenyl‐2,2,2‐trifluoroethane (3F) and a thermally labile poly(propylene oxide)(PO) component comprised the outside block of the ABA triblock architecture. TEM studies show that the initial irregular nanoscale phase‐separated morphology of polyimide triblock copolymers are mostly maintained in the final nano‐foam films upon thermal decomposition of the dispersed PO component. The nano‐foam polyimide films exhibit significantly lower dielectric constants ϵ′ (e.g., 2.3 at 19% porosity) as compared with ϵ′≊2.9 for the homopolymer, as predicted by Maxwell–Garnett theory, with the nano‐pore structures remaining stable at 350 °C. © 1996 American Institute of Physics. View full abstract»

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  • Surface nucleation of Ti silicides at elevated temperatures

    Page(s): 1933 - 1935
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    The nucleation of Ti silicide at the surfaces of Si was studied. Deposition of Ti and codeposition of TiSix at elevated temperatures on single crystal and amorphous Si led to the direct growth of silicides. The temperature and composition of the deposition and the crystallinity of the substrate were found to have a strong effect on the phases(s) of the silicide layer. A remarkably low nucleation temperature, ∼500 °C, for the low‐resistivity C54‐TiSi2 phase was observed on amorphous Si. Stoichiometric and uniform TiSi2 layers were grown with the depositions of pure Ti. On crystalline Si, uniform TiSi2 layers were also grown at ∼500 °C with the deposition of essentially Ti. The significant difference between silicide formation in the present scheme and that under conventional silicide processing was discussed in terms of a possible circumvention of precursor amorphous salicide phases during surface nucleation. © 1996 American Institute of Physics. View full abstract»

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  • Laser induced impact ionization in semiconductors: A Monte Carlo study for silicon

    Page(s): 1936 - 1938
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    Monte Carlo computer simulations are carried out to study impact ionization due to a sinusoidal field present in high‐power laser pulses. As an application we study the impact ionization coefficient, α, for electrons in silicon as a function of the field frequency, pulse width, and the rms value of the field. In all cases we stay below the frequency values where band‐to‐band absorption would create electron‐hole pairs. As is the case for constant (dc) fields, log α is found to be linear with field strength. For fields oscillating at frequencies much below the inverse of the carrier scattering rate, the impact ionization coefficient is found to have the same value as in the constant field case with the rms field replacing the dc value. At higher frequencies the impact ionization rate decreases. The dependence of α on field frequency and pulse width is studied. © 1996 American Institute of Physics. View full abstract»

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  • Effect of encapsulant material on the diffusion of beryllium in molecular beam epitaxy gallium arsenide

    Page(s): 1939 - 1941
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    Experiments were done that illustrate the role of the wafer surface in the transient diffusion of Be in GaAs. Samples were doped during molecular beam epitaxial growth and annealed at 900 °C for 15 min and 2 h under two different caps. In some of the annealed samples, the dopant was initially located near the surface. Other samples had the dopant initially located in a buried layer. Both types of samples were analyzed by secondary ion mass spectrometry measurement. The variations in the diffusion behavior for these different experimental conditions can all be qualitatively explained by a model which accounts for three important effects: the transient evolution of point defect populations; the injection of Ga vacancies by an oxide cap; and the efficiency of the surface in restoring point defect equilibria. © 1996 American Institute of Physics. View full abstract»

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  • Electronic structure of (GaAs)m/(Ge2)n (001) superlattices with 1⩽(m,n)⩽20

    Page(s): 1942 - 1944
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    The semiempirical tight‐binding method with second‐nearest‐neighbor interactions has been used to investigate the electronic structure of (GaAs)m/(Ge2)n (001) superlattices (SLs) with 1≤(m,n)≤20. We have found no correspondence between the calculated band structures of these (GaAs)/(Ge2) SLs and (GaAs)1-x(Ge2)x random alloys. The inclusion of second‐nearest‐neighbor interactions raised the predicted energies of the conduction band confined states while lowering that of the highest‐lying interface state. For small values of m, the (GaAs)m/(Ge2)n SLs are predicted to have indirect band gaps regardless of the valence band offset used in the calculation. For larger values of m, however, the predicted SL band gaps become direct for large values of valence band offset. © 1996 American Institute of Physics. View full abstract»

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  • Ca and O ion implantation doping of GaN

    Page(s): 1945 - 1947
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    p‐ and n‐type doping of GaN have been realized by ion implantation of Ca and O, respectively. Rapid thermal annealing at 1100 °C or higher is required to achieve p‐type conduction in Ca or Ca+P implanted samples with an estimated ionization level of 169 meV and a corresponding activation efficiency of ∼100%. This is the first experimental report of an acceptor species in GaN, other than Mg, with an ionization energy level less than 180 meV. O‐implanted GaN displays an ionization level of ∼29 meV but with an activation efficiency of only 3.6% after a 1050 °C anneal that may result from insufficient vacancy generation for the lighter O ion or from the existence of a second, deeper O energy level. Neither Ca or O displayed measurable redistribution, based on secondary ion mass spectrometry measurements, even after a 1125 °C anneal. © 1996 American Institute of Physics. View full abstract»

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  • Sealing AlAs against oxidative decomposition and its use in device fabrication

    Page(s): 1948 - 1950
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    Data are presented demonstrating that an exposed AlAs layer can be sealed against further decomposition as would otherwise occur due to, for example, wet oxidation. A critical processing step consists of a brief 500–600 °C anneal in forming gas of a previously room ambient exposed AlGaAs surface. In the brief high‐temperature anneal, a thin but dense surface barrier layer forms which further blocks diffusing oxygen species. The effectiveness of the surface layer as a barrier is demonstrated through its use as a mask against wet oxidation in the fabrication of an all‐epitaxial selectively oxidized multimode AlAs/AlGaAs vertical‐cavity laser. © 1996 American Institute of Physics. View full abstract»

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  • A spectroscopic study on the luminescence of Er in porous silicon

    Page(s): 1951 - 1953
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    We report a spectroscopic study on the photoluminescence (PL) of erbium implanted into porous silicon (Er:PSi). Two different porous Si samples were implanted with a dose of 1015 Er/cm2 at 380 keV and annealed at 650 °C for 30 min under identical conditions. Both samples exhibited Er3+ luminescence at 1.54 μm, which was quenched by less than a factor of two between 15 K and room temperature. Visible PL studies of Er implanted and annealed porous Si samples showed broad spectra which peaked at ∼700 nm for sample A and peaked at ∼660 nm for sample B. Sample A showed a four times stronger Er3+ luminescence than that observed from sample B. In contrast, temperature quenching of the Er3+ luminescence was found to be similar or slightly weaker from sample B than from sample A. The spectroscopic data will be discussed in terms of the excitation mechanisms of Er3+ in porous Si nanostructures. © 1996 American Institute of Physics. View full abstract»

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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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Nghi Q. Lam
Argonne National Laboratory