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Journal of Applied Physics

Issue 8 • Date Oct 2005

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Displaying Results 1 - 25 of 119
  • Issue Cover

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

    Page(s): toc1
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  • Characterization of a high-thermal-stability spiroanthracenefluorene-based blue-light-emitting polymer optical gain medium

    Page(s): 083101 - 083101-7
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    We report the solid-state optical gain characteristics of a spiroanthracenefluorene polymer, namely, poly(9-spiro(10,10-bis(2-ethylhexyl)-10H-anthracene)fluorene) (PEHSAF), specifically designed for thermal stability. An efficient stimulated emission occurs at λ=445 nm under amplified spontaneous emission conditions for asymmetric slab waveguide structures. The modal gain and propagation loss coefficients were found to be g≤38 cm-1 and α=0.8 cm-1, respectively. The PEHSAF stimulated emission characteristics are shown to be thermally stable in vacuo for temperatures up to 250 °C. Surface-emitting distributed feedback lasers have been fabricated by spin-coating PEHSAF onto one-dimensional grating structures. The lasers operate in the blue spectral region and exhibit low oscillation thresholds (≥18 nJ) and relatively high slope efficiencies (≤5%). Varying the PEHSAF film thickness allows the tuning of the emission wavelength within a 16 nm window. View full abstract»

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  • Dependence of plasma characteristics on dc magnetron sputter parameters

    Page(s): 083301 - 083301-5
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    Plasma discharge characteristics of a dc magnetron system were measured by a single Langmuir probe at the center axis of the dual-side process chamber. Plasma potential, floating potential, electron and ion densities, and electron temperature were extracted with varying dc power and gas pressure during sputter deposition of a metal target; strong correlations were shown between these plasma parameters and the sputter parameters. The electron density was controlled mostly by secondary electron generation in constant power mode, while plasma potential reflects the confinement space variation due to change of discharge voltage. When discharge pressure was varied, plasma density increases with the increased amount of free stock molecules, while electron temperature inversely decreased, due to energy-loss collision events. In low-pressure discharges, the electron energy distribution function measurements show more distinctive bi-Maxwellian distribution, with the fast electron temperature gradually decreases with increased gas pressure. View full abstract»

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  • Efficient electron heating in short-pulsed magnetron discharges

    Page(s): 083302 - 083302-11
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    The effect of the duty cycle of the cathode pulse on the electron energy distribution function and the plasma parameters was investigated by means of the time-resolved probe measurements in the pulsed and unbalanced magnetron discharges. The discharges were driven by the cathode pulse with the repetition frequency of 20 kHz in three regulation modes of constant voltage, constant current, and constant power. As the duty cycle was reduced, the electron temperature averaged during the pulse-on period rapidly increased irrespective of the regulation mode. The comparison of the measured electron energy distribution functions shows that the increase of the electron temperature is caused by the decrease of the population of trapped low-energy electrons and the increase of the population of drifting high-energy electrons. It will be shown that these results can be explained from efficient electron heating by the high-voltage cathode sheath more deeply penetrating into the bulk with the reduction of the duty cycle during the pulse-on period. View full abstract»

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  • Simulation of an asymmetric single dielectric barrier plasma actuator

    Page(s): 083303 - 083303-7
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    Continuity equations governing electron and ion density are solved with Poisson’s equation to obtain spatial and temporal profiles of electron density, ion density, and voltage. The motion of electrons and ions results in charge separation and generation of an electrostatic electric field. Electron deposition downstream of the overlap region of the electrode results in formation of a virtual negative electrode that always attracts the charge separation. The value of charge separation e(ni-ne) and the force per volume F=e(ni-ne)E have been obtained near the dielectric surface for the 50th cycle. Domain integration of the force F=e(ni-ne)E has been obtained for different plasma densities, frequencies, and rf voltage wave forms. The time average of the x force is positive and the y force is negative over the domain; therefore there is an average net force on the plasma in the positive x and negative y directions. This will result in a moving wave of plasma over the dielectric surface in the positive x direction, which can find application in flow control. View full abstract»

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  • Towards an optimal antenna for helicon waves excitation

    Page(s): 083304 - 083304-6
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    Helicon sources are known to produce high-density plasmas and have found many applications. Different types of antenna have been used for helicon excitation but none of them generate a radio-frequency (rf) field that matches the helicon wave field determined by the dispersion equation. We show that this match can be obtained to a very good approximation by using a birdcage type antenna. Our plasma experiments show that a helicon regime with electron densities up to 5×1012 cm-3 is obtained for very low rf power injection (typically 200 W), and at an unusual operating pressure up to 25 Pa. View full abstract»

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  • Energy balance in laser ablation of metal targets

    Page(s): 083305 - 083305-5
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    Laser-generated plasma was induced on metallic targets glued to a piezoelectric microphone and placed between the plates of a planar charged capacitor. The plasma generates a temporal redistribution of electric charge on the plates that can easily be measured by a resistor connected to the ground plate; this signal is proportional to the total number of ions removed by breakdown. Both the absorbed and scattered energies were simultaneously monitored by the photoacoustic signal and an energy meter. From these signals it was possible to determine the energy involved in each of the processes. Just above the ablation threshold most of the delivered energy is absorbed and the acoustic signal prevails compared to other contributions. Above this region, the electric signal, which is proportional to the energy involved in the ablation process, becomes dominant. View full abstract»

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  • Comparison between Langmuir probe and microwave autointerferometry measurements at intermediate pressure in an argon surface wave discharge

    Page(s): 083306 - 083306-9
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    This paper is devoted to the validation of Langmuir probe technique by microwave autointerferometry in a surface wave discharge at medium pressure (from 0.1 up to 10 Torr). Temperatures of neutrals, obtained from Rayleigh scattering, and electrons, given by double probe measurements, have been used to estimate the mean free path and the sheath length. Hence, the number of collisions in the sheath has been obtained. We have verified that two different phenomena occur in the sheath, while the pressure increases. These phenomena are depicted, in literature, as two different coefficients used to modify the value of the noncollisional current of Laframboise. Coefficients given by various authors have been investigated with accuracy knowing the number of collisions in the sheath. Very good agreements are obtained between Zakrzewski and Kopiczynski theory and autointerferometry measurements which are unaffected by collisional phenomena. View full abstract»

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  • Fabrication and nonlinear optical properties of monodomain polymers derived from bent-core mesogens

    Page(s): 083501 - 083501-4
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    Monodomain polymers derived from bent-core mesogens were obtained using the in situ photopolymerization technique under an in-plane electric field. The orientation of the samples was characterized using x-ray diffraction. It was found that the samples presented homeotropic alignment and ferroelectric ordering. Stable second-harmonic generation response was found at room temperature and under no field. The second-order susceptibility tensor was fully determined. View full abstract»

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  • Influence of excitation density on photoluminescence of zinc oxide with different morphologies and dimensions

    Page(s): 083502 - 083502-5
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    Photoluminescence from zinc oxide (ZnO) nanoparticles, nanorods, microparticles, and single crystals was measured under various conditions of ultraviolet excitation. Near-band-gap and deep-level emissions were observed from all samples. The intensities of both emissions from each ZnO sample exhibited a different nonlinear dependence on the excitation density. These differences could be interpreted by the variation of the light scattering and the specific surface areas of the ZnO morphologies. The intensity ratio of the near-band-gap emission to the deep-level emission from the ZnO samples strongly depended on the density of the excitation light. Our results indicated that the intensity ratio of two emissions depends on the sample type (such as the dimension, specific surface area, etc.), as well as the experimental conditions (such as excitation density, radiation area, etc.). Therefore, this ratio could not be simply employed to unequivocally evaluate the quality of the ZnO. View full abstract»

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  • An electronic criterion for the intrinsic embrittlement of structural intermetallic compounds

    Page(s): 083503 - 083503-4
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    Development of intermetallics for high-temperature structural applications has long been impeded by their brittleness. To overcome the brittleness, efforts have been devoted to understand the atomic bonding nature of intermetallics. However, there are no simple theoretical or experimental means for investigating the electronic origin of the brittleness. We find a simple experimental method, which employs the Kelvin-probe technique to measure the electron work function. The typical intermetallics in three alloy systems (Ni–Al, Fe–Al, and Ti–Al) were chosen as examples. An electronic criterion was therefore proposed for judging intrinsic brittleness. This criterion could provide some principles for improving the ductility of intermetallics and is also expected to be extendable to other materials (e.g., quasicrystalline alloys and metallic glasses). View full abstract»

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  • Direct measurement of curvature-dependent ion etching of GaN

    Page(s): 083504 - 083504-7
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    The evolution of nanoscale pores or dimples during ion etching of GaN was used to measure the magnitude of the curvature-dependent roughening. GaN(0001) surfaces were ion etched with glancing-incident, 300 eV Ar and nitrogen ions using a beam flux of 3.6×1014 ions cm-2 s-1. The samples were rotated during the etching, and the sample temperatures maintained between room temperature and 600 °C. This etch process smoothened the surface but left nanoscale dimples or pores with diameters between 30 and 800 nm. The density of these dimples remained constant during the etch process but the dimples were observed to grow larger in size until coalescence occurred. The formation of these ion-induced, nanoscale features was analyzed in terms of a continuum model that included a curvature-dependent roughening term and a smoothening term. The integral of the removed material was measured in order to directly determine the curvature dependence of the sputter yield. From the evolution of the dimple dimensions, we measured the roughening coefficient as 43±5 and 28±4 nm2/s at 460 and 315 °C, respectively, which are four orders of magnitude larger than that calculated using curvature-dependent sputtering alone. Preliminary measurements of the roughening coefficient versus temperature show a minimum at a temperature of about 365 °C. View full abstract»

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  • Optical properties of magnetron-sputtered and rolled aluminum

    Page(s): 083505 - 083505-9
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    The optical properties of magnetron-sputtered aluminum and AA1050 aluminum alloy sheet have been examined qualitatively using total reflectance and quantitatively by means of visible spectroscopic ellipsometry (VISSE). Significant changes in reflectance and optical constants are observed, which are related to the incorporation of oxide in the aluminum bulk. The role of such oxide was determined by VISSE using the Bruggeman effective-medium approximation, with the findings validated by x-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy. View full abstract»

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  • The kinetics of point defects in low-power pulsed laser annealing of ion-implanted CdTe/CdMnTe double quantum well structures

    Page(s): 083506 - 083506-7
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    Double quantum wells of CdTe in CdMnTe were implanted with argon ions to create vacancies and interstitials. This destroyed the photoluminescence (PL) emission from the top well and reduced the intensity from the bottom well. Pulsed radiation from an excimer laser emitting at 308 nm, with a full width at half maximum pulse lengths of 26 ns, was used to anneal the implantation damage and restore the luminescence. An optimum fluence close to 50 mJ cm-2 exists for laser annealing, with the best results being obtained if single pulses are employed. Prior irradiation at lower fluences prevents full recovery of the luminescence when the higher fluence pulse is applied, and irradiation at lower fluences on unimplanted material causes a reduction in the luminescence from the top well. These results are interpreted in terms of vacancy creation and annihilation during the laser pulse. Calculations of the total number of vacancies created suggest that annihilation of the Te vacancies is the limiting step in the recovery of the PL in implanted material. It is proposed that loss of material from the surface, amounting to less than a monolayer, leads to the effective diffusion of vacancies into the solid. View full abstract»

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  • Energy transfer between Co2+ and Fe2+ ions in diffusion-doped ZnSe

    Page(s): 083507 - 083507-6
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    Absorption, photoluminescence (PL), and time-resolved PL measurements were applied to samples of polycrystalline ZnSe which were diffusion doped with iron or with cobalt or codoped with both iron and cobalt. Characteristic emission and absorption features due to Co2+ and Fe2+ were observed in the visible and infrared regions. In the spectral region near 3 μm, a Co2+ emission overlaps completely with an Fe2+ absorption and energy transfer occurs. Using PL lifetime data, a decrease in the Co2+ lifetime of ∼300 μs to less than 100 μs was observed. This decrease in lifetime indicates that the nature of the energy-transfer process is nonradiative. The energy-transfer rate decreases with temperature, as expected for the nonradiative electric dipole-dipole interaction. The temperature dependences of both Co2+ and Fe2+ emissions are described. View full abstract»

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  • Femtosecond nonlinear coherence spectroscopy of carrier dynamics in porous silicon

    Page(s): 083508 - 083508-5
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    Multidimensional nonlinear coherence spectroscopy based on spectrally resolved femtosecond two-color three-pulse photon echo measurements are used to investigate carrier dynamics and energy structures in porous silicon samples, an indirect band-gap material. Short time scales for electron localization (∼500 fs) and electron hopping (∼3 ps) are observed which are dependent on the porosity of the samples. A spin-orbit splitting for the conduction band of 4–5 meV is deduced. The observed energy splittings of 18 and 22 meV for the 48% porosity sample and 21 and 28 meV for the 70% porosity sample are assigned to spin-orbit splitting for the valence band. View full abstract»

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  • Formation rates of iron-acceptor pairs in crystalline silicon

    Page(s): 083509 - 083509-5
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    The characteristic association time constant describing the formation of iron-acceptor pairs in crystalline silicon has been measured for samples of various p-type dopant concentrations and species (B, Ga, and In) near room temperature. The results show that the dopant species has no impact on the pairing kinetics, suggesting that the pairing process is entirely limited by iron diffusion. This conclusion was corroborated by measurement of the activation energy of pair formation, which coincides with the migration enthalpy of interstitial iron in silicon. The results also indicate that the pair-formation process occurs approximately twice as fast as predicted by a commonly used expression. View full abstract»

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  • Evaluation of photoinduced change in refractive index of a polymer film doped with an azobenzene liquid crystal by means of a prism-coupling method

    Page(s): 083510 - 083510-5
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    The photoinduced change in refractive index of poly(methyl methacrylate) films doped with an azobenzene liquid crystal was measured by the prism-coupling method. Upon irradiation of the film with a high-pressure mercury lamp at 366 nm, the coupling angles shifted and then recovered to the initial position by turning off the light. The change in refractive index was found to be 2×10-3, which is attributed to the reversible photoisomerization of the azobenzene moieties. View full abstract»

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  • Luminescence spectra of an Al/SiO2/p-Si tunnel metal-oxide-semiconductor structure

    Page(s): 083511 - 083511-12
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    We present both theoretical and experimental results of our investigation into hot-electron luminescence in p-Si tunnel metal-oxide-semiconductor structures. The luminescence spectra of such structures contrast considerably with those of other silicon-based devices due to the fundamentally different method of charge injection, and we discuss several of their interesting features. The potential for direct modulation of low-intensity, low-efficiency infrared emission between 1.24 and 1.6 μm is also demonstrated. Additionally, we investigate the role of self-heating, the influence of crystal axis orientation, and the possibility of anisotropy, and find the effect of each to be relatively small. View full abstract»

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  • Electroluminescence of type II broken-gap p-Ga0.84In0.16As0.22Sb0.78/p-InAs heterostructures with a high-mobility electron channel at the interface

    Page(s): 083512 - 083512-5
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    Ga0.84In0.16As0.22Sb0.78 quaternary solid solutions, lattice matched to InAs, were grown by liquid phase epitaxy on (100)-oriented p-InAs substrates from In-rich melt. The p-type Ga0.84In0.16As0.22Sb0.78 layers were intentionally undoped or slightly doped with Sn to the hole concentration about p∼5×1016 cm-3. This allowed us to obtain a high-mobility∼(3.5-5.0)×104 cm2 V-1 s-1 electron channel at the type II broken-gap p-Ga0.84In0.16As0.22Sb0.78/p-InAs heterointerface. Low-temperature (T=5 K) electroluminescence spectra exhibited two pronounced emission bands 1=0.372 eV and 2=0.400 eV under forward bias. The emission band 2 was split into two lines and was attributed to interband transitions through acceptor and valence-band states in the bulk InAs, whereas emission band 1 was ascribed to interface-related radiative transitions of electrons from the two-dimensional electron channel to the interface states at the p-Ga0.84In0.16As0.22Sb0.78/p-InAs heteroboundary. View full abstract»

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  • Behavior of Ga atoms on Si(001) surface at high temperature

    Page(s): 083513 - 083513-7
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    The growth processes and structures of Ga layers formed on a Si(001) surface have been studied by scanning tunneling microscopy and low-energy electron-diffraction analysis. Si(001) wafers cut at 0.5° toward the [110] direction and showing a double-domain surface structure were used as substrates. Ga atoms from a Knudsen cell were deposited on substrates maintained at 600 °C. In a Ga coverage range between 0.15 and 0.30 monolayer (ML), a Si(001)2×3–Ga structure composed of Ga ad-dimers rows was formed. In this coverage range, the ratio of the surface area of the TA terrace to that of the TB terrace (terraces where Si dimer rows run parallel and perpendicular to the step, respectively) was changed by step rearrangement, and biatomic steps were formed partially. Such step rearrangement can be explained by the substitution of Si atoms by Ga atoms at kinks. On the TB terrace, Ga ad-dimer rows along the SA step filled the terrace, while those along the SB step (an SA step where Si dimer rows on the upper terrace run parallel and perpendicular to the SB step) terminated partly. At a Ga coverage of about 0.48 ML, a Si(001)2×2–Ga single-domain structure occupied almost the entire surface (accounts for 97% of the whole surface area). On the terraces on the surface, Ga ad-dimer rows run parallel to step edges. Bright irregular protrusions were observed on biatomic step edges, while small hazy protrusions attributed to adatoms were observed on the ledge and edge of the step. View full abstract»

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  • Strong enhancement of band-edge photoluminescence in CdS quantum dots prepared by a reverse-micelle method

    Page(s): 083514 - 083514-4
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    We have investigated the effects of surface modification on photoluminescence (PL) properties of CdS quantum dots (QDs) prepared by a reverse-micelle method. It is demonstrated that the modification of QD surface with a Cd(OH)2 layer leads to a strong enhancement of the band-edge PL intensity of CdS QDs in reverse micelles. The PL-decay profile before the surface modification exhibits a fast decay component less than 50 ps. After the modification, the fast-decay component disappears. The drastic change of the PL-decay profiles before and after the surface modification corresponds to the marked increase of the PL intensity. This suggests that the strong enhancement of the band-edge PL intensity originates from the remarkable reduction of nonradiative recombination processes, which usually result in a very short decay time, due to surface defects of QDs. View full abstract»

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  • A theoretical and experimental study of atomic-layer-deposited films onto porous dielectric substrates

    Page(s): 083515 - 083515-9
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    Gaseous precursors for the atomic layer deposition (ALD) process can penetrate and deposit inside porous dielectrics. Understanding the relationship between the substrate porosity and the size and shape of precursor molecules used for the ALD of dielectric or metal films on these substrates is crucial for the formation and fabrication of reliable nanostructures. Sealing the surface pores is highly desirable to prevent the deposition of gaseous ALD precursors inside the porous network. In this study, in-diffusion of precursors is investigated using x-ray reflectivity, transmission electron microscopy, and Rutherford backscattering spectrometry for different porous substrates and binary (TaN) and ternary (WNC) ALD systems. Experimental observations are complemented with computational molecular modeling performed in the framework of the density-functional-theory formalism to determine the size and shape of ALD precursors. Pore sealing treatments are used to prevent a precursor penetration and to provide a suitable starting surface for the nucleation and growth of ALD films. View full abstract»

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  • Mechanism for improvements of optical properties of 1.3-μm InAs/GaAs quantum dots by a combined InAlAsInGaAs cap layer

    Page(s): 083516 - 083516-7
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    The optical and structural properties of InAs quantum dots (QDs) with a thin InAlAsInGaAs composite cap layer have been systematically investigated by photoluminescence and transmission electron microscopy (TEM). A number of improvements in the optical properties are observed with the use of an InAlAsInGaAs cap layer, instead of InGaAs. These include a redshift of the emission, a reduction of the photoluminescence linewidth, an increased separation between the ground- and first-excited-state transitions, and an enhancement of the photoluminescence intensity at room temperature. To understand these optical improvements, the structural characteristics of the dots are studied by cross-sectional TEM. The height of the QDs is found to increase with increasing InAlAs thickness in the InAlAsInGaAs cap layer. In addition, scanning TEM is used to qualitatively map the Al distribution in the vicinity of the QDs. These studies indicate that Al atoms are not deposited directly above the QDs in the present structures and hence that the InAlAs cap layer mainly affects the lateral potential barrier of the QDs. The improvements of the QD optical properties can thus be explained in terms of the increased QD height and lateral potential barrier. A consideration of mass transport during the capping process provides a possible growth mechanism responsible for the formation of taller InAs dots when capped with a thin Al-containing layer. 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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Editor
P. James Viccaro
Argonne National Laboratory