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Journal of Applied Physics

Issue 8 • Date Oct 2007

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Displaying Results 1 - 25 of 158
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Internal photoemission at interfaces of high-κ insulators with semiconductors and metals

    Page(s): 081301 - 081301-28
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    Internal photoemission spectroscopy provides the most straightforward way to characterize the relative energies of electron states at interfaces of insulators with metals and semiconductors by measuring the spectral onset of electron/hole photoemission from one solid into another. The article reviews the application of this technique for characterization of advanced nanometer-thin insulators prospected to be used in microelectronic devices. Fundamental aspects and technical features of the internal photoemission experiments are discussed together with basic electronic properties of a number of investigated high-permittivity insulating films and their interfaces in semiconductor heterostructures. Significant differences are found in the electronic properties of nanometer-thin amorphous insulating layers as compared to the known bulk phase characteristics. The band alignment at the interfaces of these insulators with metals is found to be highly sensitive to the surface preparation procedures. By contrast, at semiconductor/oxide interfaces the parameters of occurring interlayers affect the energy barriers only marginally at least in the case of studied oxides with close bandgap width (5.6–5.9 eV). The latter finding is in favor of the models describing the band offsets at semiconductor/insulator interfaces on the basis of the bulk density of electron states. Deviation of metal/oxide interfaces from this simple behavior is explained by (unintentional) formation of a polarization layer at the interface which may contain uncompensated charges and dipoles affecting the barrier height. View full abstract»

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  • Z-scan theory of two-photon absorption saturation and experimental evidence

    Page(s): 083101 - 083101-5
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    In our article [B. Gu, Y. X. Fan, J. Wang, J. Chen, J. P. Ding, H. T. Wang, and B. Guo, Phys. Rev. A 73, 065803 (2006)] we have presented the theory of open-aperture Gaussian-beam Z-scan, based on the Adomian decomposition method, which is available only for the saturable absorption caused by single-photon absorption transition. In the present article, using the same technique (Adomian decomposition method, as a common technique), we develop an open-aperture Z-scan theory for evaluating the property of the saturable absorption originating from two-photon absorption (2PA) transition, when a spatial Gaussian beam is used as the excitation source. We find analytic polynomial expressions of the Z-scan traces for a continuous wave laser or a temporal Gaussian pulsed laser. As the experimental evidence, we investigate the saturable 2PA behaviors caused by the interband two-photon transition in the direct-gap II–VI semiconductors CdS, CdSe, ZnSe, and ZnTe, under the excitation condition of a femtosecond laser with a 140 fs pulse duration, a 1 kHz low repetition rate, and a 1.6 eV photon energy. View full abstract»

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  • Deformation potentials of Si-doped GaAs from microscopic residual stress fields

    Page(s): 083102 - 083102-9
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    Underlying physics has been put forward and an experimental verification given for in situ determination of deformation potentials in Si-doped GaAs through a quantitative assessment of micro-/nanoscopic surface stress fields. Highly localized spectroscopic stress assessments could be achieved using a field emission scanning electron microscope as an energy source for stimulating cathodoluminescence emission from a Si-doped GaAs wafer. The deformation potentials were obtained from the local (elastic) residual strain fields stored in the neighborhood of an indentation print. The three independent GaAs deformation potentials could be obtained from a single measurement set and from mixed strain fields including tensile and compressive strains, while all the previously published characterizations were made in compression and on different samples. For these reasons, the deformation potentials determined in this study may prove more reliable and valid in a wider strain range as compared to those from previously published study. The proposed experimental method is suitable for in situ assessments of epitaxially grown thin-film materials and other zinc-blende-like III-V semiconductor heterostructures and alloys. View full abstract»

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  • Enhanced gas sensing of Au nanocluster-doped or -coated zinc oxide thin films

    Page(s): 083103 - 083103-6
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    We demonstrated that doping or covering with Au nanoclusters boosts gas sensing effectiveness of optical metal oxide sensors. The sensing response of pulsed laser deposited ZnO films as sensing element was tested by m-line technique for low concentration (1000 ppm) of butane in environmental N2. The optical interrogation was performed for three types of coatings: undoped ZnO, undoped ZnO structures partially covered with Au nanoclusters, or obtained from Au (0.5 wt %) doped ZnO targets. Nanocluster coating tripled the sensitivity, while doping resulted in an increase of up to 45% as compared with simple structures. View full abstract»

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  • Optical properties of conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites

    Page(s): 083104 - 083104-5
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    In this work, we present the evolution of optical constants as a function of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) concentration for conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites. The PCBM concentration of the utilized samples varies from 1 to 50 wt %. The dielectric functions for all these composites reveal electronic structural changes as a result of the addition of PCBM. We have deconvoluted the contribution of the substrate using a two-layer Fabry-Pérot structural model. The extracted optical properties contain crucial absorption peaks of singlet exciton states and vibronic sidebands for poly(3-hexylthiophene) (P3HT) conjugated polymer as well as two PCBM-related states at higher energies. With the addition of PCBM, we have observed a limit of 20 wt % PCBM beyond which two discrete energy levels (3.64 and 4.67 eV) appear in the spectrum. For the highest concentration composite, the results suggest that the interchain interactions provide a small excitonic contribution in the absorption spectrum at energies where the conjugated polymer absorbs (1.85–2.7 eV) and a strong rise of PCBM states (3.64 and 4.67 eV) which are responsible for the subsequent exciton dissociation. In addition, the energy gap between the higher occupied molecular orbitals and the lower unoccupied molecular orbitals of the highest concentration composite (50 wt %) is 1.85 eV. The tuning of the optical properties of P3HT with the addition of PCBM shows that el- lipsometry can be used to monitor layer concentration toward optimization of plastic solar cells. View full abstract»

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  • Optical characterization and ligand-field splitting of Er3+(4f11) energy levels in a fluorine containing tellurite glass

    Page(s): 083105 - 083105-6
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    An in-depth spectroscopic study is performed on Er3+(4f11) ions doped into a fluorine containing (lead, lanthanum)-tellurite glass host, containing nominal 1.93 at. % of Er3+. The standard Judd-Ofelt (JO) model is applied to the room temperature absorption intensities of Er3+(4f11) transitions in the tellurite glass host to obtain three phenomenological intensity parameters, Ω2, Ω4, and Ω6. These parameters are subsequently used to determine the radiative decay rates, radiative lifetimes, and branching ratios of the Er3+ transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds 2S+1LJ of Er3+(4f11) in the tellurite glass host. The emission cross sections of the intermanifold Er3+ 4I13/24I15/2 (1.5 μm) and 4S3/24I15/2 (540 nm) transitions have been determined. The room temperature fluorescence lifetimes of the 4I13/24I15/2 and 4S3/24I15/2 transitions in this tellurite glass host were also measured. From the radiative lifetime determined from the JO model and measured fluorescence lifetime, the quantum efficiency of this material was also determined. The spect- roscopic properties were compared with those of Er3+(4f11) in other halotellurite glass hosts. We also analyzed the band structure observed in the absorption spectra of Er3+ in the tellurite glass at 8 K. The structure can be interpreted in terms of the ligand-field splitting of the energy levels of the Er3+ ion in the local environment established by the glass matrix into which Er2O3 has been introduced. The spectroscopic analysis of Er3+(4f11) suggests that the tellurite glass is an excellent candidate for various photonic applications. View full abstract»

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  • Electroluminescent and carrier transport mechanisms of MgxZn1-xO/Si heterojunctions

    Page(s): 083106 - 083106-6
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    MgxZn1-xO (x≈0.19) films were, respectively, deposited by reactive sputtering on the heavily arsenic-doped (n+), lightly phosphorus-doped (n-), heavily boron-doped (p+), and lightly boron-doped (p-) silicon substrates. The sputtered MgxZn1-xO films were n-type with an electron concentration of 4.1×1017 cm-3. Moreover, the energy-band gap of the MgxZn1-xO films was determined to be ∼3.67 eV. The MgxZn1-xO/n--Si and MgxZn1-xO/p--Si heterojunctions showed rectifying behavior to different degrees while the MgxZn1-xO/n+-Si and MgxZn1-xO/p+-Si heterojunctions did not possess rectifying function. The MgxZn1-xO/n+-Si heterojunction exhibited UV near-band-edge and visible defect-related electroluminescence (EL) under the forward bias but only visible EL under reverse bias; while the MgxZn1-xO/n--Si heterojunction only emitted relatively weak UV and visible EL under the forward bias. On the other hand, the MgxZn1-xO/p+-Si heterojunction was electroluminescent in the visible region under the forward bias while in both the UV and visible regions under the reverse bias; and the MgxZn1-xO/p--Si heterojunction did not exhibit detectable EL under either forward or reverse bias. The mechanisms of the EL and carrier transport for the above-mentioned heterojunctions have been tentatively explained in terms of their energy-band structures. View full abstract»

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  • Linewidth characteristics of a filterless tunable erbium doped fiber ring laser

    Page(s): 083107 - 083107-6
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    The linewidth characteristics of a continuous wave erbium doped fiber ring laser whose tunability is controlled by intracavity loss are studied in this work. The spectral linewidth of such a filterless laser is measured experimentally, and its characteristics are analyzed using the existing analytical model of fiber laser. The long length of the cavity results in lasing into multiple longitudinal modes, thus leading to a broad linewidth. The extent of this broadening is found to depend on the intracavity attenuation and pump powers. Under the given experimental conditions, for a fiber of length 12 m, the linewidth increases up to 4.5 nm corresponding to an additional cavity loss of 5 dB, while it increases up to 8 nm for a loss of 1.2 dB for a fiber of length 4.6 m. The linewidth decreases with further increase in intracavity loss for both cases. In this work, the linewidth dependence on the intracavity loss is directly linked to the spectral dependence of the threshold power for each length. The linewidth increases with pump power irrespective of the intracavity attenuation, and this is explained through the nature of the amplified spontaneous emission from the fiber at different pump powers. The experimental results and the subsequent analysis are useful for the design of a filterless tunable laser with a narrow linewidth using erbium doped fiber in the ring configuration. View full abstract»

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  • Spectral function and responsivity of resonant tunneling and superlattice quantum dot infrared photodetectors using Green’s function

    Page(s): 083108 - 083108-12
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    Theoretical modeling of resonant tunneling (RT) and superlattice (SL) quantum dot infrared photodetectors (QDIPs) using Green’s function is reported. The RT QDIP gives very low dark current which improves the detectivity of the device and allows for high temperature operation. The SL QDIP gives high responsivity and is suitable for low-level signal detection. The theoretical model is based on Green’s function method which is used to calculate the spectral function and the density of states of the two detectors. The kinetic equation that governs Green’s functions is solved numerically using the method of finite differences. From the information obtained from the density of states, the possible energy transitions are obtained. The bound states are calculated by solving the eigenvalue problem using the method of finite differences, while the continuum states localized in the quantum dot region are calculated using Green’s functions. Using the first order dipole approximation and Fermi golden rule, the eigenstates are used to calculate the responsivity of the detectors which is compared with available experimental results. The theoretical model is then used for studying the effect of changing the quantum dot height-to-diameter ratio on the normal incidence responsivity of the SL structure. View full abstract»

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  • Analysis of optically pumped compact laterally coupled distributed feedback lasers with three symmetric defect regions

    Page(s): 083109 - 083109-8
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    This article analyzes compact laterally coupled distributed feedback (DFB) lasers with three defect regions. These devices are more flexible and smaller than conventional DFB lasers, having typical lengths between 20 and 50 μm and a width less than 1 μm (lateral gratings are inserted in a single-mode waveguide). We optimize the defect regions to achieve an improved performance. In other words, an adequate choice of phase shifts may lead to single-mode operation, lower threshold optical power, higher quantum differential efficiency, and more uniform field distribution. This device is designed to operate under optical pumping. View full abstract»

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  • Influence of proximity effects in electron-beam lithography on the optical properties of planar photonic-crystal waveguides

    Page(s): 083110 - 083110-7
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    To measure the influence of proximity effects in electron-beam lithography on the optical properties of planar photonic crystal (PPC) waveguides we propose a PPC structure called the “PECmeter.” The PECmeter consists of nearly identical PPC waveguides which only differ in the number of rows of holes along the waveguide. The difference in the number of rows does not influence the modal properties directly but changes the diameter of the holes neighboring the waveguide through the proximity effect. The operation principle of the PECmeter is demonstrated using energy-intensity simulations of a W3 waveguide (three missing rows of holes) mini stop band. The principle is confirmed experimentally with structures fabricated in the InP-based material system and measured by the end-fire transmission technique. The results clearly show that the application of proximity-effect correction (PEC) is crucial for the fabrication of PPC waveguides. We demonstrate that when using the midpoint-equalization PEC method a near-to-perfect correction with sub-nm hole-radius uniformity can be achieved. We show the PECmeter to be sensitive enough to detect hole-radius changes as small as ΔR=0.4 nm. View full abstract»

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  • Surface plasmon excitation at second harmonic over a rippled surface

    Page(s): 083301 - 083301-4
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    A high power laser of frequency ω incident on a rippled metal surface gives rise to oscillatory electron velocity v at the second harmonic in the skin layer. Its coupling with the surface ripple (modeled as a density ripple) of suitable wave number produces a nonlinear current, driving a surface plasma wave at frequency . View full abstract»

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  • Beryllium deposition on International Thermonuclear Experimental Reactor first mirrors: Layer morphology and influence on mirror reflectivity

    Page(s): 083302 - 083302-7
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    Metallic mirrors will be essential components of the optical diagnostic systems in the International Thermonuclear Experimental Reactor (ITER). Reliability of these systems may be affected by mirror reflectivity changes induced by erosion and/or deposition of impurities (carbon, beryllium). The present study aims to assess the effect of beryllium (Be) deposition on the reflectivity of metallic mirrors and to collect data on the optical quality of these layers in terms of morphology, roughness, etc. Mirrors from molybdenum and copper were exposed in the PISCES-B linear plasma device to collect eroded material from graphite and beryllium targets exposed to beryllium-seeded deuterium plasma. After exposure, relative reflectivity of the mirrors was measured and different surface analysis techniques were used to investigate the properties of the deposited layers. Be layers formed in PISCES-B exhibit high levels of porosity which makes the reflectivity of the Be layers much lower than the reflectivity of pure Be. It is found that if Be deposition occurs on ITER first mirrors, the reflectivity of the coated mirrors will strongly depend on the layer morphology, which in turn depends on the deposition conditions. View full abstract»

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  • Emission and shock visualization in nonequilibrium nitrogen afterglow plasma

    Page(s): 083303 - 083303-8
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    Kinetic modeling of propagating and stationary normal shocks in nonequilibrium nitrogen afterglow plasma is used to simulate the results of shock emission measurements in nitrogen afterglow. Emission intensity overshoot behind the shock predicted by the model is in satisfactory agreement with the experimental results and is consistent with previous analytic estimates. The model demonstrates that the first and the second positive band emission overshoot behind the shock are produced by energy transfer processes among the triplet electronic states of nitrogen generated in the electric discharge. On the other hand, charge separation and ambipolar electric field produced across the shock layer do not result in electron heating and additional electron impact excitation of electronic states. The calculations show that emission overshoot makes possible accurate detection of a stationary shock layer in supersonic flowing afterglow experiments. View full abstract»

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  • Cleaning properties of atomic oxygen excited to metastable state 2s22p4(1S0)

    Page(s): 083304 - 083304-14
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    A possibility to treat surfaces of various materials by bombardment with neutral atomic oxygen excited to the metastable state 2s22p4(1S0), (4.1891 V from the ground) is described. The oxygen dissociation and excitation was developed in plasmas generated in reactors of capacitively coupled dielectric barrier discharge configurations comprised of a quartz or ceramic tube passing throughout two annular electrodes and containing a gas mixture of ∼98% Ar with ∼2% O2 ionized at the atmospheric pressure by the radio frequency discharge at 13.56 MHz and flowing at about 6 m/s. As shown in the experiments, the transition 2s22p4(1S0)-2s22p4(1D2) with a high degree of probability is responsible for the yellow color of afterglow products, and the lifetime of the metastable state can be as long as 5.4 ms in certain cases, allowing one to separate the excited atomic medium from plasma that it produced. It has been shown that the cleaning process included significant Van der Waals bonds liberation to the depth of several hundred angstroms of surfaces treated, which drastically reduced the percentage of carbon-containing contaminants and changed, to some degree, the chemical structure of surfaces containing chemically bonded oxygen. View full abstract»

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  • Communication through plasma sheaths

    Page(s): 083305 - 083305-14
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    We wish to transmit messages to and from a hypersonic vehicle around which a plasma sheath has formed. For long distance transmission, the signal carrying these messages must be necessarily low frequency, typically 2 GHz, to which the plasma sheath is opaque. The idea is to use the plasma properties to make the plasma sheath appear transparent. View full abstract»

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  • Loss current minimization during ion extraction from an inductively coupled plasma

    Page(s): 083306 - 083306-5
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    In ion sources with a grid-type acceleration system, the output current often decreases with increasing power at a constant extraction voltage despite the fact that the carrier density inside the source increases linearly. At this point, the loss and output currents have the same magnitude. This paper proposes a mechanism for loss current minimization when restrictions are placed on the beam focusing efficiency with increasing rf power. The influence of the plasma potential on the magnitude of the loss current was examined. The beam intensity was found to increase considerably when the plasma potential was sufficiently high to suppress sheath curvature growth. View full abstract»

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  • Effect of thickness on the self-positioning of nanostructures

    Page(s): 083501 - 083501-5
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    Atomic-scale modeling of self-positioning GaAs–InAs nanostructures is performed. Curvature radius values obtained by the atomic-scale finite element method are compared with those obtained by a continuum mechanics solution under plane strain conditions. Atomic-scale modeling and continuum mechanics solution predict the same curvature radius for structures with large thickness. However, atomic-scale modeling shows significant decrease of the curvature radius for structures with thickness less than 40 nm. View full abstract»

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  • Ultrasonic characteristics of porous Pd42.5Cu30Ni7.5P20 glassy alloys

    Page(s): 083502 - 083502-7
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    Using ultrasonics, the acoustic characteristics of porous Pd42.5Cu30Ni7.5P20 glassy alloys were examined in terms of complex elasticity. Bulk modulus (K) and Lamè parameter (λ) decrease in a parabolic manner as porosity increases, while Young (E) and shear (G) moduli decrease linearly. Consequently, the order of K≫λ≫E≫G, which is monopolistically characterized by nonporous glassy alloys, changes to the order of E≫K≫λ≥G at 58% porosity. The decrease in Poisson’s ratio and the increase in G/K ratio up to 58% porosity indicate crystallinelike characteristics that resist three-dimensional volume-nonpreserving deformation. An increase in porosity induces frequency convergence for longitudinal waves and an increment of the imaginary parts in the complex waves. The viscoelasticity of the porous glassy alloys is predominated by shear motion. In contrast to the continuous increment in dilational damping, the porosity-dependent increment in shear damping could be elucidated based on the accumulation of strains induced by the formation of pores. View full abstract»

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  • A shock-induced phase transformation in a LiTaO3 crystal

    Page(s): 083503 - 083503-5
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    The high-pressure phase transformation behavior of LiTaO3 crystal has been studied by both Hugoniot measurements and first-principle calculations. We observe a discontinuity in shock velocity (D) versus particle velocity (UP) relation, a two-wave structure below 37.9 GPa, and a three-wave structure above 37.9 GPa. These data confirm that a shock-induced phase transformation of LiTaO3 occurs. The onset pressure of the phase transformation (37.9 GPa) defined by our new shock compression data is higher than the early shock wave value (19 GPa) reported by Stanton and Graham [P. L. Stanton and R. A. Graham, J. Appl. Phys. 50, 6892 (1979)]. A first-principle calculation of the zero degree isotherm for rhombohedral phase (R3c space group) is in good agreement with our low-pressure experimental data. The calculated zero degree isotherm for orthorhombic phase (Pbnm space group) is in concord with our high-pressure shock compression data. View full abstract»

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  • Annealing effects on the nanoscale indium and nitrogen distribution in Ga(NAs) and (GaIn)(NAs) quantum wells

    Page(s): 083504 - 083504-6
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    III/V semiconductors containing dilute amounts of nitrogen are metastable and need to be thermally treated after growth to optimize optoelectronic properties. The influence of thermal annealing on the nitrogen depth profile in metal organic vapor phase epitaxygrown Ga(NAs)/GaAs as well as (GaIn)(NAs)/GaAs heterostructures is examined on a nanometer scale by combining several high resolution transmission electron microscopy techniques, also with Rutherford backscattering spectrometry. Annealing conditions, which are optimized for quaternary alloys with respect to photoluminescence intensity, do not result in element redistribution for the In containing material. Contrary to the quaternary material, the result of annealing the ternary Ga(NAs) is a pronounced pileup of the nitrogen profile without any out diffusion of nitrogen. These findings have important influence on device structures, which often contain Ga(NAs) barriers for strain-compensation purposes together with (GaIn)(NAs) active regions. In the light of metastability considerations for the ternary and quaternary alloy, one can conclude that the In contained in the quaternary material stabilizes the material and suppresses phase separation. Consequently (GaIn)(NAs) is more stable than its ternary counterpart Ga(NAs). View full abstract»

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  • Optical patterning of silver nanoparticle Langmuir-Blodgett films

    Page(s): 083505 - 083505-4
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    We report on the optical patterning of Ag nanoparticle Langmuir-Blodgett (LB) films deposited on glass substrates. Ag nanoparticles could be quickly desorbed from the substrate without surface damage, when exposed to a nanosecond Nd:YAG laser pulse over threshold energy density. Well-aligned one-dimensional and two-dimensional patterns at the micrometer scale were fabricated in LB films by a simple spatial modulation of the laser beam. This parallel process enables the period and feature size of the pattern to be independently controlled, and may provide an effective method for nanoparticle pattering. View full abstract»

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  • Effects of the quantum dot ripening in high-coverage InAs/GaAs nanostructures

    Page(s): 083506 - 083506-8
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    We report a detailed study of InAs/GaAs quantum dot (QD) structures grown by molecular beam epitaxy with InAs coverages θ continuously graded from 1.5 to 2.9 ML. The effect of coverage on the properties of QD structures was investigated by combining atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence, capacitance-voltage, and deep level transient spectroscopy. In the 1.5–2.9 ML range small-sized coherent QDs are formed with diameters and densities that increase up to 15 nm and 2×1011 cm-2, respectively. For θ≫2.4 ML large-sized QDs with diameters of 25 nm and densities ranging from 2×108 to 1.5×109 cm-2 coexist with small-sized QDs. We explain the occurrence of large-sized QDs as the inevitable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with θ, leading to the formation of V-shaped defects at the interface between QDs and upper confining layers that propagate toward the surface. Island relaxation substantially affects the properties of QD structures: (i) free carrier concentration is reduced near the QD plane, (ii) the QD photoluminescence intensity is significantly quenched, and (iii) deep levels show up with typical features related to extended structural defects. View full abstract»

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Aims & Scope

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