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Journal of Applied Physics

Issue 3 • Date Feb 2007

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Displaying Results 1 - 25 of 183
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Wavelength switching dynamics in injected fiber ring lasers

    Page(s): 033101 - 033101-5
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    We experimentally and theoretically investigate the wavelength switching dynamics of an injection-seeded Er doped fiber ring laser. The fiber ring laser is seeded by an external cavity laser diode that is amplitude modulated with a long period 50% duty cycle wave form so that the return to steady state can be investigated. It is found that the laser suffers from severe large-amplitude relaxation oscillations and slow decay times that limit the available wavelength switching speed. View full abstract»

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  • Efficient blue upconversion emission in Tm3+ via energy transfer from Yb3+ doped in lithium modified tellurite glass

    Page(s): 033102 - 033102-4
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    Intense blue and red upconversion emissions were obtained in Tm3+ doped tellurite glass when codoped with Yb3+ ions on pumping with 798 nm radiation. The highest intensity of the blue emission was found for glasses with 0.5 mol % Tm3+ and 6.0 mol % Yb3+. From a study of the dependence of the upconversion intensity on excitation intensity, it is concluded that emissions are due to Tm3+ ions and involve cooperative energy transfer as well as energy transfer from Yb3+ to Tm3+ ions. Lifetime of the Tm3+ level, which is responsible for the intense blue emission, has been measured for various concentrations of Yb3+. View full abstract»

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  • Efficient blue lasers based on gain structure optimizing of vertical-external-cavity surface-emitting laser with second harmonic generation

    Page(s): 033103 - 033103-4
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    We report on the demonstration of highly efficient blue lasers based on intracavity frequency doubling vertical-external-cavity surface-emitting lasers (VECSELs). By optimizing the number of InGaAs quantum wells and employing Al0.3Ga0.7As carrier blocking layers in resonant periodic gain structures, we observed the pump-power-limited output power of 4.5 W at 920 nm for an InGaAs/GaAs quantum well VECSEL. With a frequency doubling LiB3O5 crystal inside the cavity, 1.9 W continuous-wave 460 nm blue output was demonstrated. Power conversion efficiencies (=output power/pump input power) of 22.5% and 9.5% are realized for λ∼920 nm and λ∼460 nm, respectively. View full abstract»

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  • Improvements of external quantum efficiency of InGaN-based blue light-emitting diodes at high current density using GaN substrates

    Page(s): 033104 - 033104-5
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    InGaN-based blue light-emitting diodes (LEDs) with different quantum well (QW) thicknesses were grown on freestanding GaN substrates with low threading dislocation densities (TDDs) and on c-plane sapphire substrates. In the case of thin QWs of 3 nm in thickness, the external quantum efficiencies (EQEs) of LEDs on GaN substrates, as well as those on sapphire substrates, decreased with increasing forward current, indicating that carrier localization is in play on both types of substrates. For thicker 5-nm-thick QWs, the EQEs of LEDs grown on GaN substrates improved at high current densities, while those on sapphire substrates decreased even at low current densities. The LED with 5-nm-thick QWs on the GaN substrate mounted p-side down and molded with epoxy showed EQE as high as 26% at 125 A/cm2. Cathodoluminescence observations of the active layers on GaN substrates revealed that the expansion of nonradiative areas related to TDDs, which are responsible for the deterioration of the EQE of the LED on the sapphire substrate, had been suppressed. View full abstract»

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  • Picosecond blue-light-induced infrared absorption in single-domain and periodically poled ferroelectrics

    Page(s): 033105 - 033105-5
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    Picosecond blue-light-induced infrared absorption is investigated in bulk and periodically poled ferroelectrics known to have high photorefractive resistance (KTiOPO4, MgO-doped congruent LiNbO3, MgO-doped stoichiometric LiNbO3, MgO-doped stoichiometric LiTaO3, and KNbO3). KNbO3 and MgO-doped congruent and stoichiometric LiNbO3 show the lowest susceptibility to induced absorption. Periodic poling slightly increases the susceptibility to the induced absorption in all materials but most noticeably in KTiOPO4, MgO-doped stoichiometric LiTaO3, and KNbO3. Different dynamics of induced absorption are investigated. Relatively high thresholds for induced absorption were observed in MgO-doped stoichiometric LiTaO3 and KNbO3. By increasing the peak power intensity of blue light, the induced absorption for LiNbO3, KTiOPO4, and K- NbO3 are saturated, while in MgO-doped stoichiometric LiTaO3, the induced absorption increases almost linearly with the blue peak intensity. The low susceptibility to induced absorption observed in KNbO3 corroborates well with the earlier proposed existence of very shallow traps close to the valence band. View full abstract»

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  • Analysis of plume following ultraviolet laser ablation of doped polymers: Dependence on polymer molecular weight

    Page(s): 033106 - 033106-6
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    This work investigates the effect of polymer molecular weight MW on the plume characteristics of poly(methyl methacrylate) (PMMA) and polystyrene (PS) films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI) following irradiation in vacuum at 248 nm. Laser-induced fluorescence probing of the plume reveals the presence of ArH products (NapH and PhenH from, respectively, NapI- and PhenI-doped films). While a bimodal translational distribution of these products is observed in all cases, on average, a slower translational distribution is observed in the low MW system. The extent of the observed dependence is reduced as the optical absorption coefficient of the film increases, i.e., in the sequence NapI/PMMA, PhenI/PMMA, and PS-doped films. Further confirmation of the bimodal translational distributions is provided by monitoring in situ the temporally resolved attenuation by the plume as it expands in vacuum of a continuous wave helium–neon laser propagating parallel to the substrate. Results are discussed in the framework of the bulk photothermal model, according to which ejection requires that a critical number of bonds are broken. View full abstract»

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  • Monte Carlo ray trace simulation for micro-ball-lens-integrated high-speed InGaAs p-i-n photodiodes

    Page(s): 033107 - 033107-9
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    To widen the alignment tolerance of a 10 Gbytes/s InGaAs p-i-n photodiode, which typically only has an optical coupling aperture of 30 μm in diameter, we have developed a self-positioning ball-lens-on-chip scheme for enlarging the effective coupling aperture. According to the measured results, the 4.5- and 5.3-fold improvements along the transverse and optical axes, respectively, in alignment tolerance have been demonstrated without sacrificing the diode efficiency as a commercially available ruby micro-ball-lens (MBL) with a 300 μm lens diameter integrated on the high-speed photodiode. To further explore the aperture enlargement capability and optimize the structural design, we have constructed a ray trace model based on the Monte Carlo method for the optical coupling system as well. Using this well-constructed simulation model, we further predict that, by integrating a 250 μm diametric ruby MBL on the photodiode with a 20 μm high lens socket, the alignment tolerance can have 7.1- and 10-fold improvements along the transverse and optical axes, respectively. Such a large alignment tolerance permits the usage of conventional passive scheme for photodiode package. View full abstract»

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  • 1.84 μm emission of Tm3+ sensitized by Yb3+ ions in monoclinic KGd(WO4)2 single crystals

    Page(s): 033108 - 033108-6
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    By exciting at 940 nm, we have characterized the 1.84 μm near infrared emission of trivalent thulium ions in Yb3+, Tm3+:KGd(WO4)2 single crystals as a function of the dopant concentration and temperature, from 10 K to room temperature. An overall 3H6 Stark splitting of ∼470 cm-1 for the Tm3+ ions in the Yb3+, Tm3+:KGd(WO4)2 was obtained. We also studied the blue emission at 476 nm (Tm3+) and the near infrared emissions at 1.48 μm (Tm3+) and 1 μm (Yb3+) as a function of the dopant concentration. Experimental decay times of the 1G4, 3H4, and 3F4 (Tm3+) and 2F5/2 (Yb3+) excited states have been measured as a function of Yb3+ and Tm3+ ion concentrations. For the 3F43H6 transition - of Tm3+ ions, we used the reciprocity method to calculate the maximum emission cross section of 3.07×10-20 cm2 at 1.84 μm for the polarization parallel to the Nm principal optical direction. View full abstract»

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  • Analytic determination of n, κ, and d of an absorbing film from polarimetric data in the thin-film limit

    Page(s): 033109 - 033109-11
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    We obtain a solution of the three-phase model in the limit d/λ≪1, where the complex refractive index ñ=n+iκ and thickness d of an isotropic film on an isotropic substrate are given analytically at any single wavelength λ from polarimetric data Δρ/ρ and ΔR/R, where ρ is the complex reflectance ratio and R is either the p- or s-polarized reflectance. We describe several procedures for extending the range of validity of the solution. Analysis of correlations shows that the uncertainty δ(ΔR/R) of ΔR/R is significantly more important than the δ(Δρ/ρ) of Δρ/ρ, which allows us to obtain an expression for the uncertainties δn, δκ, and δd of n, κ, and d, and to identify conditions that optimize the determination of the layer parameters. We find that the relative uncertainties δn/n and δd/d are not equal, as would be expected if they were determined by the optical thickness nd measured by ellipsometry, but that ΔR/R breaks the connection. We verify our results by measurements of H2O reversibly physisorbed on oxidized GaAs, finding, for example, that for our conditions δκ is determined more accurately than δn, and δn more accurately than δd. These data and model calculations show that flu- ctuations in parameters, particularly d, are asymmetric, leading in principle to inaccurate average values. However, we show that the importance of the ΔR/R data together with the remaining high correlation between n and d allows us to define a characteristic curve that can be used to correct the results for this nonlinearity. Finally, we extend our analysis to determine the orthogonal linear combinations of n, κ, and d that the data actually determine, which explains why the data fit the characteristic curve so well. Our results will be useful in various contexts for the analysis of films less than 1 nm thick, for example, in applications involving preparation of next-generation electronic and optoelectronic devices with complicated multilayer structures, real-time control of deposition, and the identification of physisorbed and chemisorbed layers on the monolayer scale. View full abstract»

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  • Self-assembled spontaneous structures induced by a pulsed laser on a surface of azobenzene polymer film

    Page(s): 033110 - 033110-5
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    The paper presents the self-assembled regularly patterned unique structures induced by the irradiation of pulsed laser beam on a surface of azobenzene polymer film. Self-assembled patterns depend on the polarization of laser beam. Linearly polarized beam gave rise to the ripple structures whose grating vector is perpendicular to the polarization of laser beam. Pitch of the ripple structures (Λ) was related to the wavelength (λ) of irradiated laser beam; Λ=λ/n. Λ=λ/1.05 for λ=266 and 355 nm and Λ=λ/1.3 for λ=420–540 nm were obtained. These results implied that the ripple structures were produced by the interference of the incident beam with the optical wave scattered in a surface (waveguide mode light). The difference of n value is explained by the wavelength dispersion of refractive indices in the azobenzene polymeric matrix. Circularly polarized beam gave rise to the concentric wavelike structure. Pitch of wavelike structure was analyzed using two dimensional Fourier transform imaging techniques. Pitch of wavelike structure increases with increasing the number of laser shots. Furthermore, with increasing irradiation time, well-defined honeycomblike structure was also developed. View full abstract»

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  • Resonant control of the characteristic temperatures T0 and T1 of AlInGaAs 0.8 μm semiconductor lasers with delta-doped tunneling quantum wells

    Page(s): 033111 - 033111-6
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    It is demonstrated experimentally that AlInGaAs/GaAs lasers with Te n-type δ-doped resonant tunneling quantum wells (QWs), tuned to operate at resonance at elevated temperature, exhibit higher external efficiency and improved temperature stability. These lasers were designed to ensure that the decrease in the band bending (which results from the increased current density at threshold due to the elevated temperature) would cause the ground-state levels in the two QWs to coincide, thus obtaining enhanced coupling between the QWs. The enhanced coupling increases the efficiency and reduces the transparency current which curbs the increase of the threshold current due to the elevated temperature. View full abstract»

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  • Broadband emission in Er–Tm codoped Al2O3 films: The role of energy transfer from Er to Tm

    Page(s): 033112 - 033112-6
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    Er–Tm codoped a-Al2O3 thin films prepared by alternate pulsed laser deposition show a broad photoluminescence band with two characteristic peaks at 1540 and at 1640 nm, respectively, related to Er3+ and Tm3+ emissions. Two series of films have been prepared. For the first series, the Tm concentration [Tm] has been increased while keeping constant the Er concentration [Er]. The results show that the photoluminescence intensity at 1640 nm (I1640) to that at 1540 nm (I1540) decreases and that at 1640 nm (I1640) increases, i.e., the (I1640/I1540) ratio increases as [Tm] is increased. For [Tm]/[Er]=3, a fairly flat emission spectrum (I1640/I1540∼1) with a full width at half maximum of 230 nm is achieved. For the second series both [Tm] and [Er] are increased while keeping the [Tm]/[Er] ratio constant. The I1640/I1540 ratio tends to be constant with a full width at half maximum of 150 nm. The lifetime values decrease in all cases as [Tm] increases. The evolution of the I1640/I1540 and lifetimes as a function of [Tm] as well as the analysis of the latter considering Er as a donor and Tm as an acceptor evidence that there is an efficient energy transfer from Er3+ - to Tm3+. These results suggest that enhanced performance with a flat broadband emission useful for planar integrated devices is achievable by further dopant engineering in the nanoscale. View full abstract»

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  • Fabry-Perot effects in InGaN/GaN heterostructures on Si-substrate

    Page(s): 033113 - 033113-4
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    A strong intensity modulation is found in spatially and angular resolved photoluminescence spectra of InGaN/GaN heterostructures and quantum wells epitaxially grown on Si(111) substrates. This Fabry-Perot effect results from the high refractive index contrasts at the GaN/Si and the Air/InGaN interfaces. It can be used for a wavelength stabilization of the sample upon temperature change and, e.g., in the case of light emitting diodes, to additionally reduce the blueshift at increasing injection currents. A simple geometric approach has been chosen to calculate the influence of layer thickness, absorption and refractive indices, as well as detection angle. The cavity can be described quantitatively by a simple three layer Fabry-Perot model. An analytical expression is derived for the external luminescence line shape. Microphotoluminescence measurements at samples with the silicon substrate locally removed corroborate the model. View full abstract»

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  • Theoretical study of quantum well infrared photodetectors with asymmetric well and barrier structures for broadband photodetection

    Page(s): 033114 - 033114-7
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    The n-type InGaAs/AlxGa1-xAs quantum well infrared photodetector (QWIP) with asymmetric graded barriers for broadband detection has been investigated theoretically based on the eight-band kp model. It is found that the intersubband transitions from the ground state to all bound and continuum excited states contribute to the overall absorption and the bound-to-continuum (B-to-C) transitions dominate. The superposition of the bound-to-bound and B-to-C transitions results in a broad detection bandwidth, and both the detected wavelength and bandwidth can be tuned by the applied voltage. The analysis method is also applicable to the GaAs/InxGa1-xAs/InyGa1-yAs QWIP with step quantum wells. The calculated results are consistent with the reported experimental observations. View full abstract»

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  • Third-order nonlinear optical properties of bismuth-borate glasses measured by conventional and thermally managed eclipse Z scan

    Page(s): 033115 - 033115-7
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    Third-order nonlinearity one order of magnitude larger than silica is measured in bismuth-borate glasses presenting a fast response (≪200 fs). The results for the sign and magnitude of the nonlinearity were obtained using a combination of the eclipse Z scan with thermal nonlinearity managed Z scan, whereas the Kerr shutter technique was employed to obtain the electronic time response of the nonlinearity, all performed with 76 MHz repetition rate 150 fs pulses at 800 nm. Conventional Z scans in the picosecond regime at 532 and 1064 nm were also independently performed, yielding the values of the third-order nonlinear susceptibilities at those wavelengths. The results obtained for the femtosecond response, enhanced third-order nonlinearity of this glass (with respect to silica), place this glass system as an important tool in the development of photonics devices. Electro-optical modulators, optical switches, and frequency converters are some of the applications using second-order nonlinear properties of the Bi-glass based on the rectification model. View full abstract»

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  • Optical limiting behavior of zinc phthalocyanines in polymeric matrix

    Page(s): 033116 - 033116-5
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    Phthalocyanines are promising molecules which present very attractive properties for nonlinear optical applications of both second and third orders. Different solutions of zinc phthalocyanine in toluene at increasing concentration were investigated by spectroscopic analysis in order to get the best conditions for the realization of films in poly(methyl methacrylate). Some films of zinc phthalocyanine in polymeric matrix were realized and their nonlinear optical behavior was characterized. The ratio by weight of the compound to the host polymer was varied between 0.004% and 0.15%. In this range all samples show a low degree of aggregation. The nonlinear optical behavior of the films was evaluated by measuring nonlinear transmission. The measurements of nonlinear transmission were carried out at 532 nm using a nanosecond Nd:YAG laser followed by an optical parametric oscillator, and a nonlinear response was observed for the two highest concentrations (0.083% and 0.15%). Finally, the nonlinear absorption coefficient β was retrieved by fitting experimental data and found to be 29 and 43 cm/GW, respectively. View full abstract»

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  • Broadband SiGe/Si quantum dot infrared photodetectors

    Page(s): 033117 - 033117-4
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    The broadband absorption of metal-oxide-semiconductor SiGe/Si quantum dot infrared photodetectors is demonstrated using boron δ doping in the Si spacer. The peak at 3.7–6 μm results from the intersubband transition in the SiGe quantum dot layers. The other peak at 6–16 μm mainly comes from the intraband transition in the boron δ-doping wells in the Si spacers. Since the atmospheric transmission windows are located at 3–5.3 and 7.5–14 μm, broadband detection is feasible using this device. The δ doping in SiGe quantum dots and Si0.9Ge0.1 quantum wells is also investigated to identify the origin of the absorption. View full abstract»

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  • Microscopic electroabsorption line shape analysis for Ga(AsSb)/GaAs heterostructures

    Page(s): 033118 - 033118-8
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    A series of Ga(AsSb)/GaAs/(AlGa)As samples with varying GaAs spacer width are studied by electric-field modulated absorption (EA) and reflectance spectroscopy and modeled using a microscopic theory. The analysis of the Franz–Keldysh oscillations of GaAs capping layer and of the quantum-confined Stark shift of the lowest quantum well (QW) transitions shows the strong inhomogeneity of the built-in electric field indicating that the field modulation due to an external bias voltage differs significantly for the various regions of the structures. The calculations demonstrate that the line shape of the EA spectra of these samples is extremely sensitive to the value of the small conduction band offset between GaAs and Ga(AsSb) as well as to the magnitude of the internal electric field changes caused by the external voltage modulation in the QW region. The EA spectra of the entire series of samples are modeled by the microscopic theory. The good agreement between experiment and theory allows us to extract the strength of the modulation of the built-in electric field in the QW region and to show that the band alignment between GaAs and Ga(AsSb) is of type II with a conduction band offset of approximately 40 meV. View full abstract»

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  • Analysis of semiconductor laser dynamics under gigabit rate modulation

    Page(s): 033119 - 033119-9
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    A theoretical study of the dynamics of semiconductor lasers subjected to pseudorandom digital modulation at gigabit rates is presented. The eye diagram, turn-on jitter (TOJ), and power fluctuations in the modulated laser wave form are analyzed. The study is based on numerical large-signal analysis of the laser rate equations. Influences of the biasing and modulation currents on the eye diagram and TOJ are examined. The degree of eye opening is measured in terms of a Q factor of the laser signal analogous to the Q factor determining the bit-error rate in transmission systems. Influence of optimizing both the sampling and decision times on the signal Q factor is modeled. We show that the most eye opening corresponds to shortening the sampling time associated with lengthening the decision time. We also assess the relative contributions of the laser intrinsic noise and pseudorandom bit pattern to the TOJ. The results show that the bit pattern is the major contributor to the TOJ when the setting time of the relaxation oscillation is longer than the bit slot. View full abstract»

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  • Linearly polarized lasing in one-dimensional hybrid photonic crystal containing cholesteric liquid crystal

    Page(s): 033120 - 033120-4
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    We have achieved single-mode laser action in a one-dimensional hybrid photonic crystal (HPC) containing cholesteric liquid crystal (CLC) as a defect and experimentally investigated the polarization characteristics of the laser light. The laser light emitted from the HPC containing the CLC was linearly polarized, which was different from that of the simple CLC, despite the CLC being contained in this structure. We have theoretically calculated the optical propagation to justify the experimental result. The calculated result was in good agreement with the experimental result and showed the difference in optical characteristics between the simple CLC and the HPC containing the CLC. View full abstract»

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  • Modeling photoreflectance of quantum well heterostructures: A comprehensive approach

    Page(s): 033121 - 033121-7
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    We present a comprehensive approach to modeling the photoreflectance (PR) spectra of semiconductor quantum wells embedded in layered heterostructures. Near-gap PR spectra are obtained directly from the calculated variation of surface reflectance spectra induced by modulation of an internal electric field. The field-dependent reflectance spectra are themselves obtained from a transfer matrix model of a quantum well heterostructure (QWH) in which the quantum well layer is treated in detail using electric-field-dependent optical absorption calculations and all surrounding higher-gap layers are treated as lossless dielectric slabs. The model is described in detail and is applied to unstrained GaAs/AlGaAs and compressively strained InGaAs/GaAs single-well QWHs for which both experimental data and other calculations are available for comparison. This model can serve as a tool for interpretation of experimental PR spectra, and should be particularly useful for analysis of dense spectra with overlapping features that would be difficult to analyze using empirical fitting schemes. The approach can be used to model electroreflectance without modification. View full abstract»

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  • Improvement of GaInNAs p-i-n photodetector responsivity by antimony incorporation

    Page(s): 033122 - 033122-5
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    Deep-level transient spectra (DLTS) and photoresponsivity were measured for Ga0.90In0.10N0.033As0.967/GaAs and Ga0.96In0.04N0.028As0.967Sb0.005/GaAs p-i-n photodetector structures. The GaInNAs and GaInNAsSb layers were grown closely lattice matched to GaAs substrate at 460 °C using molecular beam epitaxy. Two hole-trap levels were observed in the DLTS spectra of the GaInNAs sample with activation energies of 0.152 and 0.400 eV (labeled as H-1 and H-2 peak, respectively). The lower activation energy is believed to be associated with nitrogen-related defects and the higher activation energy is associated with arsenic antisite defects (AsGa). Following the incorporation of Sb into GaInNAs, the H-1 peak vanished from the DLTS spectra of the GaInNAsSb sample, and the AsGa defect-related DLTS signal was significantly reduced. Analysis of the DLTS data also showed that the trap concentration related to AsGa was reduced from 2.15×1015 to 2.58×1014 cm-3. The DLTS results are in good agreement with the photoresponsivity results, in which the GaInNAsSb sample showed 10× higher photoresponse compared to the GaInNAs sample. This indicates the incorporation of Sb into GaInNAs has effectively improved the p- -i-n photodetector device performance. View full abstract»

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  • Calculated optical properties of wurtzite InN

    Page(s): 033123 - 033123-4
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    We report ab initio, self-consistent calculations of the dielectric function of wurtzite indium nitride (w-InN). Our calculations employed a local density approximation (LDA) potential, a linear combination of atomic orbital basis set, and the Bagayoko-Zhao-Williams (BZW) method. Our findings agree very well with recent measurements up to photon energies of 6 eV. This excellent agreement shows the correct description, by the LDA-BZW method, of the relative separations between upper valence bands and low-lying conduction bands, in general, and corroborates our previous result of 0.88 eV for the intrinsic, fundamental band gap of w-InN, in particular. We also report results of simulations of the effect of high electron doping on the optical properties of InN. 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