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Journal of Applied Physics

Issue 9 • Date Nov 2006

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Displaying Results 1 - 25 of 137
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

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  • Investigation of liquid/solid interface waves with laser excitation and photoelastic effect detection

    Page(s): 093101 - 093101-6
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    Theoretical and experimental investigations on transparent liquid (water)/solid (aluminum or steel) interface waves generated by laser pulse and detected with photoelastic effect are reported. When the detection beam of a laser interferometer is skimmed over the water/solid interface and conjoined with the interface wave propagated in water, an extra optical phase shift is produced. The output signal from the interferometer is proportional to the acoustic pressure in water. The characteristic equation of the liquid/solid interface wave is derived from elastic wave theory, and the transient response of acoustic pressure in water is simulated by means of inverse Laplace and Hankel transforms. The experimental results of acoustic velocity and wave form for all interface waves are in good agreement with theoretical predictions. They show that the optical detecting method based on photoelastic effect is very powerful for the research of liquid/solid interface waves. View full abstract»

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  • Spectral function of InAs/InGaAs quantum dots in a well detector using Green’s function

    Page(s): 093102 - 093102-6
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    Theoretical modeling of an InAs/InGaAs quantum dot-in-a well (DWELL) detector is reported. The DWELL structure consists of pyramidal-shaped InAs quantum dots with dimensions of 11 nm (base) and 6.5 nm (height) placed on the top half of an InGaAs quantum well of 11 nm width, which is buried in a GaAs matrix. The Green’s function method is used to calculate the spectral function and the density of states of the DWELL. 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 responsivity of the DWELL can be estimated. The calculated energy eigenvalues are compared with the experimentally measured responsivity of a DWELL detector. View full abstract»

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  • Optical damage in x-cut proton exchanged LiNbO3 planar waveguides

    Page(s): 093103 - 093103-7
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    The optical damage of different proton exchanged LiNbO3 planar waveguides has been experimentally studied by measuring the intensity output of a single beam as a function of the intensity input. Parallel measurements of photovoltaic currents (sometimes referred to as photocurrents) have been carried out with the same setup as a function of the input intensity and they have been correlated to the optical damage data. The following proton exchanged phases have been studied and compared with the substrate: α, β1, β2, and reverse proton exchanged (RPE). The greatest intensity thresholds for optical damage, about 2×103 times greater than that of the substrate, have been obtained in RPE guides supporting ordinary polarization and in β1,2 guides which support extraordinary polarization. On the other hand, the lowest photovoltaic currents have been measured in β1,2 phases. As a function of the light intensity, the strong superlinear behavior exhibited by the photovoltaic current in the α phase is almost absent in the others. In addition, it has been experimentally ascertained that a scattering increase produces a decrease of the threshold intensity for optical damage. The role of those different effects is discussed extensively together with the electro-optic properties of the different guides. View full abstract»

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  • Carrier distribution and its dependence on barrier thickness in InGaAsP/InP asymmetric multiple quantum well lasers

    Page(s): 093104 - 093104-7
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    The carrier distribution and its dependence on barrier thickness were investigated experimentally using the transition cavity length method and the temperature varying photoluminescence (PL) spectra for the mirror image asymmetric multiple quantum well structures. The carriers were found to be richer in the long wavelength wells rather than in the p-side wells, and the nonuniformity of the carrier distribution depends on the barrier thickness. A rate equation model was developed to simulate the room-temperature PL spectra and it was found that the carrier distribution is very sensitive to the ratio of the capture times among different quantum wells and to the diffusion time across the barrier while the escape time plays a minor role on the carrier distribution. The large sensitivity of the carrier distribution on the diffusion time explains the dependence of the carrier distribution on the barrier thickness. View full abstract»

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  • Dependence of the vibronic emission on self-absorption and reemission processes in conjugated polymers

    Page(s): 093105 - 093105-5
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    Photoluminescence spectra from the edge of thin films of a conjugated polymer were obtained by varying the self-absorption level at different temperatures. This technique together with the use of a Franck-Condon analysis enabled a clear identification of the vibrational relaxation processes, providing a way to recognize the participating vibrational modes. The higher order vibronic bands were observed to shift to lower energies than those predicted theoretically. This was interpreted as an effect of self-absorption processes, which lead to the loss of energy by intramolecular interactions due to the increase of thermal disorder. View full abstract»

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  • Modeling paraxial wave propagation in free-electron laser oscillators

    Page(s): 093106 - 093106-8
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    Modeling free-electron laser (FEL) oscillators requires calculation of both the light-beam interaction within the undulator and the light propagation outside the undulator. We have developed a paraxial optical propagation code that can be combined with various existing models of gain media, for example, Genesis 1.3 for FELs, to model oscillators with full paraxial wave propagation within the resonator. A flexible scripting interface is used both to describe the optical resonator and to control the codes for propagation and amplification. To illustrate its capabilities, we numerically investigate two significantly different FEL oscillators: the free-electron laser for infrared experiments (FELIX) system and the vacuum-ultraviolet (VUV)-FEL oscillator of the proposed high-gain fourth generation light source. For the FELIX system, we find that diffraction losses are a considerable part of the single-pass cavity loss (at a wavelength of 40 μm). We also demonstrate that a resonator with hole coupling may be a viable alternative to a standard resonator with transmissive optics for the high gain VUV-FEL oscillator. View full abstract»

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  • A model for the temperature dependence of photoluminescence from self-assembled quantum dots

    Page(s): 093107 - 093107-7
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    Photoexcited carriers, distributed among the localized states of self-assembled quantum dots, often show very anomalous temperature dependent photoluminescence characteristics. The temperature dependence of the peak emission energy may be nonmonotonic and the emission linewidth can get narrower with increasing temperature. This paper describes a quasithermodynamic model that naturally explains these observations. Specifically, we introduce a temperature dependent function to parametrize the degree of thermalization of carriers. This function allows us to continuously interpolate between the well-defined low and high temperature limits of the carrier distribution function and to describe the observed anomalies in the photoluminescence spectra with just two fitting parameters. We show that the description is equivalent to assuming that the partially thermalized carriers continue to be described by equilibrium statistics, but with a higher effective temperature. Our treatment of the problem is computationally simpler than the usually employed rate equation based analyses [e.g., S. Sanguinetti etal, Phys. Rev. B 60, 8276 (1999)], which typically also have many more underdetermined fitting parameters. The model is extended to quantum dots with a bimodal size distribution. View full abstract»

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  • Time-resolved postdischarge absolute silicon monoxide density measurement by resonant absorption spectroscopy in a nonthermal atmospheric plasma

    Page(s): 093301 - 093301-9
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    In this study we present the technique of resonant absorption spectroscopy diagnostic developed to estimate the density of silicon monoxide (SiO) molecules during the postdischarge of an atmospheric dielectric barrier discharge plasma. The ultraviolet (0,0) rovibrational band of the SiO(A 1Π-X 1Σ+) electronic transition was investigated. Effective values of absorption coefficient and absorption cross section for the rotational transitions under consideration were calculated. The SiO concentration was estimated by comparison between experimental and computed spectra. The self-absorption in the probe reactor was taken into account in the computed spectra. View full abstract»

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  • Heat flux measurements and plasma composition

    Page(s): 093302 - 093302-4
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    Measurements of heat flux into a probe in plasma, combined with measurement of electron temperature, are used to extract information about the plasma composition. In particular, such measurements in oxygen and nitrogen plasmas at a pressure of several millitorrs indicate that these plasmas are composed mostly of molecular ions. The measurement is based on comparing the rates of heating and cooling of a probe during its exposure to and isolation from the plasma flow. The measured heat flux into the negatively biased probe is in good agreement with the calculated heat flux carried by the impinging plasma ions. View full abstract»

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  • N, NH, and NH2 radical densities in a remote ArNH3SiH4 plasma and their role in silicon nitride deposition

    Page(s): 093303 - 093303-10
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    The densities of N, NH, and NH2 radicals in a remote ArNH3SiH4 plasma used for high-rate silicon nitride deposition were investigated for different gas mixtures and plasma settings using cavity ringdown absorption spectroscopy and threshold ionization mass spectrometry. For typical deposition conditions, the N, NH, and NH2 radical densities are on the order of 1012 cm-3 and the trends with NH3 flow, SiH4 flow, and plasma source current are reported. We present a feasible reaction pathway for the production and loss of the NHx radicals that is consistent with the experimental results. Furthermore, mass spectrometry revealed that the consumption of NH3 was typically 40%, while it was over 80% for SiH4. On the basis of the measured N densities we deduced the recombination and sticking coefficient for N radicals on a silicon nitride film. Using this sticking coefficient and reported surface reaction probabilities of NH and NH2 radicals, we conclude that N and NH2 radicals are mainly responsible for the N incorporation in the silicon nitride film, while Si atoms are most likely brought to the surface in the form of SiHx radicals. View full abstract»

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  • Ion attachment mass spectrometry of nonequilibrium atmospheric-pressure pulsed remote plasma for SiO2 etching

    Page(s): 093304 - 093304-5
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    An etching process for SiO2 that employs atmospheric-pressure pulsed remote plasma with a mixture gas (CF4/Ar) has been demonstrated. The etch rate increased by increasing the pulse frequency and also increased rapidly with the addition of a small amount of O2 gas. A SiO2 etch rate of 400 nm/min was obtained without any bias supplied to the substrate, and more than 8 μm/min was obtained with H2O addition. The exhaust gas from the plasma source was investigated using ion attachment mass spectrometry (IAMS). With IAMS, the polymerized species were measured without the generation of fragment ions. The results suggest that HF molecules are generated by H2O addition to CF4 and that the etch rate of SiO2 depends on the density of HF molecules. View full abstract»

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  • Spatial distribution of fluorescent light emitted from neon and nitrogen excited by low energy electron beams

    Page(s): 093305 - 093305-6
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    Side-view intensity profiles of fluorescent light were measured for neon and nitrogen excited with 12 keV electron beams at gas pressures from 250 to 1400 hPa. The intensity profiles were compared with theoretical profiles calculated using the CASINO program which performs Monte Carlo simulations of electron scattering. It was assumed that the spatial distribution of fluorescent intensity is directly proportional to the spatial distribution of energy loss by primary electrons. The comparison shows good correlation of experimental data and the results of numeric simulations. View full abstract»

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  • Particle emission from tantalum plasma produced by 532 nm laser pulse ablation

    Page(s): 093306 - 093306-7
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    A study of visible laser ablation of tantalum in vacuum by using 3 ns Nd:YAG laser radiation at high pulse energy is reported. Nanosecond pulsed ablation, at an intensity on the order of 109 W/cm2, produces high nonisotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, coupled to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species within the plume as a function of the incident laser energy. Neutrals show typical Boltzmann distributions while ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Surface profiles of the craters and microscopy investigations permitted to study the ablation threshold, ablation yields, and deposition rates of thin films on silicon substrates. The multicomponent structure of the plume emission is rationalized in terms of charge state, ion and neutral equivalent temperatures, and plasma density. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated at the nonequilibrium plasma conditions. The angular distributions of the neutral and ion species are also presented and discussed. View full abstract»

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  • Temperature dependence of coherent A1g and Eg phonons of bismuth

    Page(s): 093501 - 093501-6
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    Bismuth has been a model material in the study of femtosecond dynamics of coherent lattice oscillations. The generation mechanism was first proposed to be displacive for the symmetric A1g mode, which was the only mode observed as a coherent phonon. The absence of the other Raman active mode Eg has not been fully explained, but was phenomenologically attributed to the exclusive coupling of the hot electrons at k∼0 and high symmetry phonons. In the present study, we demonstrate that both A1g and Eg modes are excited as coherent phonons at low temperature and confirm that the coherent phonons are generated via a Raman process in bismuth. We found a puzzling π/2 difference in the initial phases of the two coherent phonons, which suggests that the initial phase cannot be a clear-cut index for the generation mechanism in absorbing media. View full abstract»

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  • Effect of polyimide layer surfaces on pretilt angles and polar anchoring energy of liquid crystals

    Page(s): 093502 - 093502-6
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    Considering the surface distribution of polyimide chains, the properties of the unidirectional out-of-plane nematic liquid crystal tilt angle are investigated in view of the van der Waals interaction between nematic liquid crystal molecules and two polyimide layers: the anisotropic upper layer and the isotropic lower layer. The dependence of the pretilt angle upon the thickness of the upper polyimide layer oriented by rubbing is estimated from experiment and theory. Another experimental result shows that the relation between the anchoring energy and the thickness of the polyimide layer could be dependent on lower material. This was also explained in view of the nonretarded van der Waals interaction. View full abstract»

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  • Optical characterization of digital alloy In0.49Ga0.51P/In0.49(Ga0.6Al0.4)0.51P multi-quantum-wells grown by molecular beam epitaxy

    Page(s): 093503 - 093503-5
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    An In0.49Ga0.51P/In0.49(Ga0.6Al0.4)0.51P multi-quantum-well (MQW) structure grown by molecular beam epitaxy using a digital alloy method was parametrically investigated by photoluminescence (PL) measurement performed in a temperature range of 10–290 K. The PL peak energies did not change with increasing temperature up to 60 K, while the PL peak energy monotonously decreased with increasing temperature beyond 60 K. From the curve fit of the linewidth full width at half maximum of the PL peak, it was observed that the homogeneous broadening of In0.49Ga0.51P/In0.49(Ga0.6Al0.4)0.51P MQW with digital alloy barriers due to scattering by longitudinal optical phonons was smaller than that of InGaAs/InGaAlAs MQW with digital alloy barriers. This is in accordance with the existence of a relatively weak phonon-related PL peak in the PL spectrum of InGaAlP digital alloy, as compared with InGaAlAs digital alloy. The fit of the integrated PL intensity shows the occurrence of a nonradiative recombination process with an activation energy E1=24.4 meV up to 60 K. On the other hand, the process of nonradiative recombination with an activation energy E2=109 meV occurred above 60 K, which is in good agreement with one-half of the calculated total confinement energy ΔE of the electron-hole pair in the quantum well (∼108 meV). The In0.49Ga0.51P/In0.49(Ga0.6Al0.4)0.51P MQW structure with digital alloy barriers has larger activation energy (E2=109 meV) than In0.49Ga0.51P/In0.49(Ga0.6Al0.4)0.51P MQW (E2=90 meV) with analog alloy barriers. Therefore, the thermal emission of carriers into the barrier can be reduced at temperatures above 60 K due to the high effective barrier height. View full abstract»

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  • Structure of a pentacene monolayer deposited on SiO2: Role of trapped interfacial water

    Page(s): 093504 - 093504-5
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    In situ synchrotron x-ray reflectivity is used to probe the early stages of pentacene growth in real time, under conditions relevant to the fabrication of organic thin film transistors. The results reveal that there is an interfacial water layer initially present on the SiO2 substrate and that this water layer is still present at the interface after the deposition of a pentacene thin film. The thickness of the trapped interfacial water layer does not significantly change subsequent to film deposition, even after exposure to atmospheric pressure or during vacuum annealing at 70 °C. However, a water layer is observed to form on the free surface of pentacene after sufficient exposure to water vapor, and the thickness of this layer can be reduced by subsequent vacuum annealing. These observations are correlated with organic thin film transistor mobilities measured at atmospheric pressure versus under vacuum. View full abstract»

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  • Modeling the accelerated cyclic annealing kinetics

    Page(s): 093505 - 093505-5
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    Cyclic thermal processing results in accelerated kinetics [Sahay etal Acta Mater. 51, 339 (2003)], with significant impact on the productivity and energy consumption. Considering the commercial significance of thermal processing operations, it is imperative that the mechanism of such behavior be understood and quantitatively modeled. In the present work, the cyclic annealing kinetics has been quantitatively modeled by incorporating an additional nonisothermal rate constant to the Arrhenius equation. The model closely describes the experimental recrystallization and grain growth kinetics data carried out under cyclic conditions. View full abstract»

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  • Valence and conduction intersubband transitions in SiGe, Ge-rich, quantum wells on [001] Si0.5Ge0.5 substrates: A tight-binding approach

    Page(s): 093506 - 093506-6
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    Electronic and optical properties of germanium-rich Si/SiGe quantum wells grown on Si0.5Ge0.5 substrates are investigated by a nearest neighbor tight-binding Hamiltonian. The basis set includes spds* orbitals with both spin states. Appropriate scaling laws account for strain effects. We present full electronic band structure calculations both for valence and conduction bands. Confinement effects on the electronic states are considered in detail. Optical spectra related to hole and electron intersubband transitions are derived. Our results for optical absorption due to valence intersubband transitions show excellent agreement with experimental spectra and previous kp calculations. For the same quantum well samples, spectra due to conduction intersubband absorption are provided here. View full abstract»

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  • A view of the implanted SiC damage by Rutherford backscattering spectroscopy, spectroscopic ellipsometry, and transmission electron microscopy

    Page(s): 093507 - 093507-5
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    4H-SiC single crystalline substrates were implanted at room temperature with 150 keV Al+ ions using fluences of 4×1014, 1×1015, and 2×1015 cm-2 with current density of 2.5 μA cm-2. The samples were subsequently annealed at 1100 °C in N2 for 1 h in order to analyze their structural recovery. The disorder induced in both sublattices by the Al+ ions was studied by backscattering spectrometry in channeling geometry with a 3.5 MeV He2+ beam. The results were compared with the optical properties of the samples measured by spectroscopic ellipsometry. In a previous work, we concluded that during the postimplantation annealing of a highly damaged SiC crystalline material the short distance order can be recovered, while the long distance disorder remains. We also presented the possibility to have grains of different polytypes oriented faraway from the original direction. Now, this alternative is confirmed by the cross-sectional transmission and high resolution electron microscopy studies, carried out to obtain information about the crystal structure. View full abstract»

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  • Stress dependence of the cathodoluminescence spectrum of N-doped 3C-SiC

    Page(s): 093508 - 093508-8
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    The stress dependence of the room-temperature cathodoluminescence spectrum of N-doped cubic silicon carbide has been evaluated in a field-emission-gun scanning electron microscope, using the electron beam as an excitation source for luminescence emission. The electron-stimulated spectrum was dominated by only one broad band centered at about 544 nm, with a broad shoulder centered at a slightly lower energy level (≈572 nm). The cathodoluminescence spectrum, which was attributed to the four-particle N-bound excitonic transition, arose from substitutional N in the cubic silicon carbide lattice. Using experimentally measured probe response functions and energy shift magnitude collected near the tip of a Vickers indentation microcrack, it was possible to retrieve the actual magnitude of the piezospectroscopic coefficient [i.e., the slope of a linear plot of spectral band shift versus the trace of the stress tensor: Π=0.61±0.02 nm/GPa] of the N-bound exciton (cumulative) band of cubic silicon carbide. View full abstract»

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  • Stability of rocksalt phase of zinc oxide under strong compression: Synchrotron x-ray diffraction experiments and first-principles calculation studies

    Page(s): 093509 - 093509-5
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    The stability of rocksalt (B1) phase of ZnO under high pressure up to 2 Mbars was studied using angular dispersive x-ray diffraction at room temperature and compared to the structurally analogous CdO where both theoretical and experimental results are available. First-principles total energy calculations predict the B1-to-B2 (CsCl-type) phase transition in ZnO and CdO to occur at 261 and 83 GPa, respectively. The bulk moduli, K0, of the B1 structure of ZnO were measured and the possible high pressure CsCl-type (B2) structure was looked for. The potential application of ZnO as an internal pressure marker in x-ray diffraction study at the megabar range is proposed. View full abstract»

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  • X-ray diffraction study of crystal plane distortion in silicon carbide substrates

    Page(s): 093510 - 093510-5
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    Bulk growth of 4H-SiC is challenging due to the required high growth temperatures and gradients used in sublimation physical vapor transport that are difficult to control, particularly over large diameter boules. We used x-ray diffraction mapping to show concave crystal plane curvature in substrates from five commercial suppliers with two suppliers producing wafers with ≤2° curvature. The extent of curvature varied little for substrates from any particular supplier. Maps of peak position and full width at half maximum from symmetric and asymmetric reflections were used to identify defective regions in the crystal. Closer examination of the rocking curves in the defective regions found a low density of low angle grain boundaries only in substrates from one supplier. 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