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Journal of Applied Physics

Issue 7 • Date Oct 2002

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Displaying Results 1 - 25 of 117
  • Issue Cover

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

    Page(s): toc1
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  • Dynamic up-conversion population processes of erbium-doped pentaphosphate crystal

    Page(s): 3425 - 3435
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    Rate equations describing the dynamic processes in erbium (Er3+)-doped pentaphosphate crystals were developed to obtain the population processes of all levels of Er3+ ions in pentaphosphate ErP5O14 crystals under 650 nm laser excitation. The effects of pump power on the population (ρ) dynamic process were analyzed. The spontaneous radiative processes, multiphonon nonradiative processes, absorption of the pump photons, corresponding stimulated radiation processes, and all other possible energy transfer processes between Er3+ ions were included in the rate equations. The pump power was assumed to be constant with time (t). Considering that the equation ρ(t0+Δt)=ρ(t0)+(dρ/dt)Δt would be held if Δt was small enough and using the initial condition ρ1=1.0, ρi=0(i≠1), the time-dependent populations of all states could be obtained from the rate equations, from which the steady-state solution of the rate equation was deduced. The comprehensive study of the population dynamic process of all energy-level Er3+ ions in ErP5O14 crystal when excited by 100 mW–107W has resulted in the conclusion that the strong upconversion luminescence would behave excellently when 650 nm pump laser power reaches the level of 1000–105W. The representative population dynamic process when excited by 105W laser power indicates 4I15/2 and 4F9/2 levels have a kind of great takeover co-contribution in up-converting population to higher states. The steady-state population probability of 4I15/2 is as small as 0.0617. The population of the 4F9/2 state increases rapidly to about a 0.362 high level and sequentially decreases smoothly to a stable value of about a 0.0374 low level, which implies a population already having been upconverted to higher level. Excellent results clearly exhibit that the main up-conversion luminescent level 4S3/2 has a great population probability of about 0.212, which results in a strong up-conversion luminescence of 543 nm, and it is so exciting that a very strong 379 nm blue up-conversion luminescence could be achieved when 650 nm pump laser power is enhanced to the level of 1000–105W. The results of this article are significant both to science and applications. © 2002 American Institute of Physics. View full abstract»

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  • Intracavity second harmonic generation in the green from a diode-end-pumped Nd3+:Ca3Ga2Ge3O12 laser garnet crystal

    Page(s): 3436 - 3441
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    A diode-end-pumped intracavity frequency doubled Nd3+:Ca3Ga2Ge3O12/KTiOPO4 green laser is demonstrated. The infrared laser output and pump-to-green conversion efficiency have been investigated as a function of crystal length and Nd3+ concentration. Results show that optimum ion concentration is around 2–3 at. %, in excellent agreement with theoretical predictions. Due to the broad absorption bands of this crystal, output power does not depend on diode temperature within a 12 °C range. The influence of Nd3+ concentration on the 4F3/2 quantum efficiency has been also analyzed. We have found that for optimum ion concentration quantum efficiency is as high as 0.7. Finally, the spectral distribution of green light is explained in terms of a combination of both second harmonic and frequency mixing processes between the two infrared oscillating lines. © 2002 American Institute of Physics. View full abstract»

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  • Photochemical reaction of divalent-germanium center in germanosilicate glasses under intense near-ultraviolet laser excitation: Origin of 5.7 eV band and site selective excitation of divalent-germanium center

    Page(s): 3442 - 3446
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    The photochemical reaction of Ge2+ in 10GeO2-90SiO2 optical fiber preform and the origin of the 5.7 eV optical absorption band induced by intense ultraviolet laser excitation have been investigated using a near-ultraviolet XeF excimer laser (3.7 eV) as the main excitation source. Based on the previous and present experimental results, it is concluded that the 5.7 eV optical absorption band does not originate from the Ge(2) electron trapped center. In place of this model, we have proposed an alternative structural model for the origin of the 5.7 eV band taking into account the experimental fact that the corresponding center is diamagnetic. In addition, it is found that the site selective excitation of the Ge2+ center can be achieved by using laser sources with different photon energies. © 2002 American Institute of Physics. View full abstract»

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  • Light-emitting diodes based on phosphorescent guest/polymeric host systems

    Page(s): 3447 - 3453
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    Fabrication of polymer light-emitting diodes based on emission from the phosphorescent molecule fac tris(2-phenylpyridine) iridium doped into a poly(vinyl carbazole) host are reported. Several spin-coating solvents were evaluated for deposition of the polymer layer; toluene and chlorobenzene were found to consistently produce device-quality films with sufficient incorporation of the dopant. For single-layered devices with Mg0.9Ag0.1 cathodes, the luminance efficiency at 20 mA/cm2 was measured to be 8.7 Cd/A for devices processed from chlorobenzene. This efficiency could be increased by over a factor of two with a trilayered device geometry consisting of the doped polymer layer, a hole-blocking layer, and electron transport layer. Further increases in efficiency, up to 30 Cd/A and 8.5% external quantum efficiency, were observed when a second dopant of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole was added to the polymer emitter layer. © 2002 American Institute of Physics. View full abstract»

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  • Chemi-ionization in neon plasma

    Page(s): 3454 - 3457
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    The density of electrons formed in binary collisions of 2p53s neon atoms was measured in the afterglow of a low pressure glow discharge by observing the high energy tail of the electron energy distribution function (EEDF) using a Langmuir probe. In the afterglow, the bulk plasma electrons thermalize but the density of 2p53s neon atoms remains significant. In an ionizing collision, a pair of these atoms releases high energy (∼11.6 eV) electrons which form a characteristic peak in the EEDF. Simultaneously with the chemi-ionization electrons, the densities of 1s2, 1s3, 1s4, and 1s5 neon atoms were independently measured using diode laser absorption spectroscopy. It was found that the data obtained are described well by a single chemi-ionization reaction when the 2p53s configuration is considered a single state. The corresponding rate coefficient, found to be (3.2±0.4)×10-10 cm3s-1 at a temperature of 310 K, is recommended for use in discharge modeling. © 2002 American Institute of Physics. View full abstract»

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  • High resolution measurements of ion temperatures in z-pinch plasmas

    Page(s): 3458 - 3462
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    The Doppler broadening of ion line profiles emitted by z-pinch plasma provides information about the thermalization of the implosion kinetic energy and the radiation efficiency of the pinch. Measurements of these line profiles are often complicated by source broadening in the instrument and opacity broadening of the emitted radiation. A high resolution concave crystal spectrometer in the Johann geometry was used to record the time averaged spectra of optically thin trace elements in the load. An imaging slit provided radially resolved but axially averaged spectra. The measurements indicate that lower ion temperatures (3–5 keV) are observed for Al wire loads on both the Saturn and Double EAGLE accelerators in the short current pulse mode (60–100 ns) than in the long pulse mode (125–225 ns) where values of 6.3–9.5 keV are observed. These values are smaller than those observed on Saturn by others. Furthermore, the wavelength at the line center of axially resolved ion line profiles on the DM-2 accelerator at Titan was observed to vary about some average value which implies an axially varying fluid motion of the plasma column transverse to the pinch axis. © 2002 American Institute of Physics. View full abstract»

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  • Langmuir probe diagnostic studies of pulsed hydrogen plasmas in planar microwave reactors

    Page(s): 3463 - 3471
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    Langmuir probe techniques have been used to study time and spatially resolved electron densities and electron temperatures in pulse-modulated hydrogen discharges in two different planar microwave reactors (fmicrowave=2.45 GHz, tpulse= 1 ms). The reactors are (i) a standing-wave radiative slotted waveguide reactor and (ii) a modified traveling-wave radiative slotted waveguide reactor, which generate relatively large plasmas over areas from about 350 to 500 cm2. The plasma properties of these reactor types are of particular interest as they have been used for basic research and for plasma processing; for example, for surface treatment and layer deposition. In the present study the pressures and microwave powers in the reactors were varied between 33 and 55 Pa and 600 and 3600 W, respectively. In regions with high electromagnetic fields, shielded Langmuir probes were used to avoid disturbances of the probe characteristic. Close to the microwave windows of the reactors both the electron density and the electron temperature showed strong inhomogeneities. In the standing-wave reactor the inhomogeneity was found to be spatially modulated by the position of the slots. The maximum value of the electron temperature was about 10 eV, and the electron density varied between 0.2 and 14×1011 cm-3. The steady-state electron temperature in a discharge pulse was reached within a few tens of microseconds, whereas the electron density needed some hundreds of microseconds to reach a steady state. Depending on the reactor the electron density reached a maximum between 80 and 200 μs after the beginning of the pulse. © 2002 American Institute of Physics. View full abstract»

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  • Nonlinear transformation of electromagnetic waves in suddenly created cold magnetized plasma: Longitudinal propagation

    Page(s): 3472 - 3479
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    The transformation of a circularly polarized electromagnetic source wave in a suddenly created cold magnetized and weakly nonlinear plasma is considered. For the case of longitudinal propagation (a source wave propagating along an external static magnetic field) when the ion motion is neglected, a circularly polarized source wave splits into four longitudinal oscillating modes. In the case of a linearly polarized source wave due to the interaction between its right and left circularly polarized components additional stationary and traveling electron wave modes are created. Amplitude distributions of all created modes are determined in closed form and presented in corresponding diagrams for different values of source wave and electron cyclotron frequencies. © 2002 American Institute of Physics. View full abstract»

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  • Influence of opacity on gain coefficients in static, and fast moving neon-like krypton plasmas

    Page(s): 3480 - 3486
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    X-ray laser gains and the level populations in collisionally pumped neon (Ne)-like krypton (Kr XXVII) ions have been investigated considering 27 levels of the 2p6, 2p5 3s, 2p5 3p, and 2p5 3d configurations in fast moving cylindrical plasma. Effects of opacity of the 3d 1P1→2p61S0, 3d 3D1→2p61S0, 3d 3P1→2p61S0, 3s 3P1→2p61S0, and 3s 1P1→2p61S0 transitions on the gain coefficients are considered using escape probability factors in both the static, and dynamic plasmas under the Sobolev approximation, that includes effect of the large velocity gradient. Results indicate that the gain values are significantly affected by plasma opacity, and the velocity gradient allows us to obtain a broader gain region in dense plasmas. © 2002 American Institute of Physics. View full abstract»

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  • Continuum model of thin film deposition incorporating finite atomic length scales

    Page(s): 3487 - 3494
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    We show that surface evolution resulting from the deposition of discrete particles is intrinsically different from that produced by continuum processes. The atomistic effects have major consequences, even when observed at macroscopic length scales. We have elucidated some of the atomistic effects by comparing: (i) numerical simulations of thin film deposition using the continuum model, (ii) atomistic (Monte Carlo) models, and (iii) experiments on the sputter deposition of Ta onto a substrate containing etched vias. We have therefore developed a continuum model which incorporates finite atomic length scales. The model incorporates effects of atomic interactions, which lead to the capture of impinging atoms that pass near a point on the film. This capture effect results in “breadloafing” at sharp convex corners where the curvature is high. We have validated our model in idealized two-dimensional simulations and obtained improved qualitative agreement with both experiment and Monte Carlo atomistic simulations. In the case of deposition into a trench, the model predicts that the protruding material from breadloafing eventually merges above the trench, leaving an enclosed void. This effect is observed in experiments, but is not reproduced when using the standard continuum model. Finally, we have also developed and implemented a more general three-dimensional model which successfully results in the breadloafing effect. © 2002 American Institute of Physics. View full abstract»

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  • Fluorene–fluorenone copolymer: Stable and efficient yellow-emitting material for electroluminescent devices

    Page(s): 3495 - 3502
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    We have synthesized and characterized a new fluorene copolymer exhibiting bright yellow luminescence. In order to ensure a complete π-stacking of the active layer, a 9-fluorenone monomeric unit (FOne) has been used as comonomer in conjunction with the more classical 9,9-di-n-nonylfluorene unit. As expected with fluorene-based materials, when excited at 370 nm, the corresponding dilute copolymer solution photoluminescence spectra exhibit a main peak centered at 450 nm in the blue part of the visible spectrum. However, in the solid state, immediate structural reorganization of the layer occurs, leading to a red-shifted emission (bright yellow emission) centered at 550 nm. The origin of the emitted light has been attributed to excimers and/or aggregates based on short FOne segments and involves mainly exciton transfer between nonaggregated fluorene segments and aggregated ones. It is noteworthy that organic light-emitting devices based on these new materials exhibit no spectral evolution upon device operation. However, although stacking leads generally to a detrimental quenching of the luminescence in the solid state, as for regular poly(alkyl-fluorene), the luminescence efficiency of the fluorene–fluorenone copolymer remains suitable for device preparation. High material stability is attributed to an efficient and fast structural reorganization of the active layer, triggered by the small proportion of fluorenone. High electroluminescence efficiency, when compared to aggregated regular poly(alkyl-fluorene), results from an improved electron injection, a better carrier transport, and the conjunction of an efficient energy transfer from fluorene segments to excimers and/or aggregates with the implication of spin triplet, which is often lacking when using regular semiconducting polymers. © 2002 American Institute of Physics. View full abstract»

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  • Micro-Raman investigation of strain in GaN and AlxGa1-xN/GaN heterostructures grown on Si(111)

    Page(s): 3503 - 3510
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    Using micro-Raman spectroscopy, we have studied the vibrational properties of GaN and Al0.5Ga0.5N/GaN long period superlattices (SLs) grown on Si(111). Crack-free areas of GaN layers grown on Si(111) exhibit residual tensile stress, which is evidenced by the red shift of the frequency of E2(TO) phonon. We have derived the strain cartography in GaN and Al0.5Ga0.5N/GaN long period SLs, which shows that cracking leads to strain relaxation. In addition, the AlGaN layers on GaN introduce an additional component of compressive strain into the GaN layers in these SLs. The amount of strain is quantified using micro-Raman analyses and by taking into account the elastic properties of GaN and AlGaN. By introducing a thin, low temperature InGaN interlayer, we could significantly reduce the crack density of the GaN layer. © 2002 American Institute of Physics. View full abstract»

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  • Stimulated emission due to the inelastic scattering from the heavy-hole exciton to the light-hole exciton in CuI thin films

    Page(s): 3511 - 3516
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    We have investigated high-excitation photoluminescence properties of CuI thin films with a thickness of 100 nm grown on a NaCl substrate by vacuum deposition. The energies of the heavy-hole and light-hole excitons in the CuI thin film are split at low temperatures by the thermal strain resulting from the difference in the thermal expansion coefficients of CuI and NaCl: the heavy-hole exciton is lower in energy than the light-hole exciton. Under resonant excitation of the heavy-hole exciton, a new photoluminescence band appears on the low-energy side of the heavy-hole exciton. The energy spacing between the new photoluminescence band and the heavy-hole exciton is almost equal to the splitting energy of the heavy-hole and light-hole excitons. The photoluminescence intensity exhibits an almost quadratic dependence on the excitation power. These results indicate that the new photoluminescence band originates from an inelastic scattering process from the heavy-hole-exciton state to the light-hole-exciton state. We have confirmed the existence of optical gain in the energy region of the new photoluminescence band by using a variable-stripe-length method. This fact demonstrates that the inelastic scattering process from the heavy-hole-exciton state to the light-hole-exciton state may lead to stimulated emission. © 2002 American Institute of Physics. View full abstract»

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  • Surface enhanced Raman spectroscopy of carbon nanotubules deposited on a silver self-affine fractal surface

    Page(s): 3517 - 3523
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    The observed Raman intensity measured from carbon nanotubules with well-defined diameters and wall thicknesses of ∼50 and ∼4 nm, respectively, deposited on a self-affine fractal silver surface was found to be enhanced by a factor of ∼1.4–1.7, relative to that of freestanding nanotubules in the absence of silver. Using an analysis in which the calculated distribution of tubules-to-silver distances determined by atomic force microscopy, was convolved with the expected nanotubule-silver distance dependence of the surface enhanced Raman (SERS) enhancement, this observation was reconciled with typical SERS enhancements observed in most silver-adsorbate systems. The analysis also took into account the attenuation of the incident laser intensity, which was determined from the reduction in the resonant Raman background signal that accompanies SERS. © 2002 American Institute of Physics. View full abstract»

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  • Effective electron mass and plasma filter characterization of n-type InGaAs and InAsP

    Page(s): 3524 - 3529
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    We measured the infrared reflectance of thin films of degenerate n-type InxGa1-xAs and n-InAsyP1-y as a function of doping for compositions that correspond to x=0.53, 0.66, and 0.78 (band gaps of 0.74, 0.60, and 0.50 eV, respectively) and y=0.00, 0.31, 0.52, and 0.71 (band gaps of 1.34, 1.00, 0.75, and 0.58 eV, respectively). We then used the Drude theory and Hall measurements to determine the effective electron mass for these samples, and checked the results using Raman spectroscopy. The effective electron mass for these compositions increases abruptly as a function of free-electron density and converges at 5×1019 electrons/cm3. Consequently, it is difficult to attain plasma edges at wavelengths shorter than 5 μm using these materials, and the plasma edge is nearly independent of composition at large electron density levels. Results from similar studies on InP, InAs, and GaAs have been compiled and compared with our data. It is clear that the Kane band model offers an accurate description of the conduction-band nonparabolicity. © 2002 American Institute of Physics. View full abstract»

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  • Acoustic and optical properties of (Li2O)0.2-x(Na2O)x(TeO2)0.8 glasses

    Page(s): 3530 - 3539
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    We study the ultrasonic properties of five different samples of Na2O-Li2O-TeO2 glass system within the temperature range 140–420 K (for longitudinal polarization) and 140–300 K (for transverse polarization). The variation of ultrasonic velocity of all samples shows negative gradient with temperature and has been explained by the existing theories. No well defined broad peak has been observed in the temperature variation of attenuation and a mixed alkali type peak starts growing at temperature ≈350 K. Composition dependence of elastic properties and density shows maximum or minimum at a critical concentration of Na2O. Optical transmission and reflection spectra were recorded in the UV-visible region at room temperature. The physical quantities such as optical energy gap, refractive index and optical dielectric constant were evaluated from the spectra. The theoretical fitting of the optical absorption indicates that the present glass system behaves as indirect gap semiconductor. © 2002 American Institute of Physics. View full abstract»

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  • Interdiffusion studies for HfSixOy and ZrSixOy on Si

    Page(s): 3540 - 3550
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    Metal incorporation into silicon substrates, and thermal stability of alternate gate dielectric candidates HfSixOy and ZrSixOy films after aggressive thermal annealing are reported. Considerable Zr incorporation is observed after furnace and rapid thermal annealing. No detectible Hf incorporation is observed for HfSixOy films annealed with the same conditions as the ZrSixOy films. Sputter deposited Hf silicate films showed superior thermal stability compared with chemical vapor deposited Zr silicate films. An alternate approach to obtain sub-nm resolution depth profiling of impurities in Si is also reported. Device performance associated with Zr incorporation into the channel is also discussed. © 2002 American Institute of Physics. View full abstract»

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  • Refractive index profiles of LiB3O5 waveguides formed by MeV He ion irradiation

    Page(s): 3551 - 3553
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    LiB3O5 crystals were irradiated with MeV He+ ions of different fluences and energies. The refractive index profiles, nx, ny, and nz in LiB3O5 waveguides are given. The results show that: (1) the thickness of the ion irradiated waveguides can be estimated by the mean projected range of irradiated ions, and the refractive index changes in ion irradiated waveguides are mainly caused by nuclear collisions; (2) after annealing at 200 °C for 40 min., the height of the optical barrier in the LiB3O5 waveguide is decreased; (3) the attenuation measured is 0.87 dB/cm for m=0 in the LiB3O5 waveguide irradiated with 2.8 MeV He+ to 5×1016ions/cm2; and (4) it is possible to make double waveguides in LiB3O5 by MeV He+ ion irradiation. © 2002 American Institute of Physics. View full abstract»

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  • Ion-beam-produced damage and its stability in AlN films

    Page(s): 3554 - 3558
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    Structural characteristics of single-crystal wurtzite AlN epilayers (grown on sapphire substrates) bombarded with 300 keV 197Au+ ions at room and liquid-nitrogen temperatures (RT and LN2) are studied by a combination of Rutherford backscattering/channeling spectrometry and cross-sectional transmission electron microscopy. Results reveal extremely strong dynamic annealing of ion-beam-generated defects in AlN. Lattice amorphization is not observed even for very large doses of keV heavy ions at LN2. An increase in irradiation temperature from LN2 to RT has a relatively small effect on the production of stable structural damage in AlN. In contrast to the case of AlxGa1-xN with x≤0.6, neither damage saturation in the crystal bulk (below the random level) nor preferential surface disordering is revealed for AlN. Results also show that structural lattice disorder produced in AlN by high-dose keV heavy-ion bombardment is stable to rapid thermal annealing at temperatures as high as 1000 °C. © 2002 American Institute of Physics. View full abstract»

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  • Atomic force microscopy study of surface roughening of sputter-deposited TiN thin films

    Page(s): 3559 - 3563
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    The kinetic surface roughening of TiN thin films sputter-deposited on silicon substrates at room temperature was studied. Scaling analyses were made by surface measurements of atomic force microscopy (AFM). The roughness exponent α and growth exponent β that characterize scaling behaviors of surface growth were calculated using the height–height correlation function H(r) and power spectra P(f). The exponent values of α=∼0.98 and β=∼0.28 indicated that the surface growth behavior of sputtered TiN thin films could be adequately explained by a simple linear growth model showing surface diffusion as a smoothing effect and shot noise as a roughening mechanism. An inverse Fourier transformation technique was also used to generate the evolution of theoretical surface profiles that showed good agreement with AFM measurements. © 2002 American Institute of Physics. View full abstract»

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  • Luminescence studies of a Si/SiO2 superlattice

    Page(s): 3564 - 3568
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    Photoluminescence and electroluminescence from a Si/SiO2 superlattice have been measured. They show similar characteristics and exhibit an inhomogeneously broadened photoluminescence band peaked at 2.06 eV. The excitation spectrum indicates that excitations occur in the Si layers. The insensitivity of the luminescence spectrum and decay to temperature and excitation wavelength suggests that luminescence originates from transitions between localized defect states. These localized states are most likely defect states residing at the Si/SiO2 interfaces, because there should be a significant concentration of defects at the interface and SiO2 due to the large lattice mismatch and the amorphous state. The close proximity of these states offers a more rapid transition path for the excited electrons. An energy band diagram of the superlattice is constructed based on our results. © 2002 American Institute of Physics. View full abstract»

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  • Chemical and structural aspects of annealed ZnSe/GaAs(001) heterostructures

    Page(s): 3569 - 3572
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    The thermal evolution of a ZnSe epilayer grown by molecular beam epitaxy on GaAs(001) has been studied by high resolution x-ray diffraction as well as photoelectron and Raman spectroscopies. Sequential annealing of a relaxed epilayer reveals a fast migration of Ga towards the ZnSe cap layer with a significant accumulation of As atoms near the ZnSe-reacted interface. A Ga2Se3 compound appears as a predominant byproduct whereas Zn atoms are probably diffusing from the reacted interface into the GaAs substrate. © 2002 American Institute of Physics. View full abstract»

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  • Longitudinal-optical-phonon-assisted energy relaxation in self-assembled CdS quantum dots embedded in ZnSe

    Page(s): 3573 - 3578
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    The energy relaxation processes of CdS self-assembled quantum dots (QDs) embedded in ZnSe were investigated. Longitudinal-optical (LO)-phonon resonant structures were discerned in the photoluminescence spectra under the CdS selective excitation, especially under nearly resonant excitation conditions. Strong energy selectivity among the Gaussian distributed energy levels of the QDs are observed in energy relaxation processes, in which excitons are dominantly injected into the QDs whose ground state energies with respect to the excitation energy are equal to the multi-LO-phonon energy of ZnSe. Detection-energy dependent photoluminescence excitation (PLE) measurements also suggest that the coupling strength between the excitons and LO phonons is strongly size-dependent, i.e., coupling is strongly enhanced in smaller QDs. In addition, type-II band alignment of the CdS/ZnSe heterostructure is experimentally confirmed directly by PLE measurements. © 2002 American Institute of Physics. View full abstract»

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Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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P. James Viccaro
Argonne National Laboratory