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1 Apr 2008

Volume 103, Issue 7, Articles (07xxxx)

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Phonon confinement and electron transport in GaAs-based quantum cascade structures

X. Gao, D. Botez, and I. Knezevic

J. Appl. Phys. 103, 073101 (2008); http://dx.doi.org/10.1063/1.2899963 (9 pages) | Cited 2 times

Online Publication Date: 2 April 2008

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We present a detailed investigation of the effects that optical-phonon confinement has on the electronic transport properties of GaAs-based midinfrared multiple-quantum-well (MQW) quantum cascade lasers (QCLs). The macroscopic dielectric continuum model is used to describe the interface (IF) and confined (CF) optical phonon modes. Dispersions of the IF modes are obtained by using the transfer matrix method with periodic boundary conditions. Normalization coefficients of the IF and CF potentials are derived in detail for MQW structures consisting of arbitrary combinations of binary and ternary alloys. Interstage and intrastage scattering rates due to all the IF and CF modes are calculated for both Γ- and X-valley electrons. The IF and CF scattering processes, in addition to the electron-electron and intervalley phonon scattering, are fully incorporated into the multivalley Monte Carlo simulation of a deep-active-well 6.7 μm GaAs-based MQW QCL. At both 77 K and room temperature, we find that phonon confinement enhances the electron-polar optical phonon scattering rates to a relatively small extent and induces minor corrections to the current, population inversion, and the electronic temperature with respect to the results obtained in the bulk-phonon approximation. Therefore, the bulk-phonon approximation in transport simulations of GaAs-based QCLs remains valuable due to its simplicity and high accuracy.
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63.22.Np Layered systems
73.63.Hs Quantum wells
81.07.St Quantum wells
72.20.Dp General theory, scattering mechanisms

Lateral and longitudinal mode pattern of broad ridge 405 nm (Al, In)GaN laser diodes

H. Braun, H.-M. Solowan, D. Scholz, T. Meyer, U. T. Schwarz, S. Brüninghoff, A. Lell, and U. Strauß

J. Appl. Phys. 103, 073102 (2008); http://dx.doi.org/10.1063/1.2902505 (3 pages) | Cited 6 times

Online Publication Date: 4 April 2008

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The lateral mode profile of pulsed broad ridge 405 nm (Al, In)GaN laser diodes grown on GaN and SiC substrates, respectively, is investigated by temporal and spectral resolved scanning near-field optical microscopy. During the first microsecond of the pulse, we observe changes both in the spatial mode profile and in the spectral regime caused by thermal and carrier induced modification of the waveguide refractive index, before stable filaments build up. In quasi-cw operation, a correlation between the lateral mode profile and the corresponding spatial resolved longitudinal mode pattern can be found. The results show that different filaments have different effective refractive indices and thus build up separate longitudinal mode combs.
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42.55.Px Semiconductor lasers; laser diodes
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
84.40.Az Waveguides, transmission lines, striplines
42.79.Gn Optical waveguides and couplers

Band anticrossing in diluted AlxGa1−xAs1−yNy (x ⩽ 0.37,y ⩽ 0.04)

S. Procz, M. Fiederle, M. Kunzer, K. Köhler, and J. Wagner

J. Appl. Phys. 103, 073103 (2008); http://dx.doi.org/10.1063/1.2895002 (6 pages) | Cited 1 time

Online Publication Date: 7 April 2008

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We show that the conduction band structure of dilute AlxGa1−xAs1−yNy with x ⩽ 0.37 and y ⩽ 0.04 can be described consistently by the experimentally motivated band anticrossing model. The interband transition energies E−, E−+Δ0, and E+ have been derived from a full line shape fit to photomodulated reflectance (PR) spectra recorded at room temperature. The PR data were taken (a) from a series of Al0.06Ga0.94As1−yNy samples with y ⩽ 0.04 and (b) from a set of AlxGa1−xAs0.99N0.01 layers with x ⩽ 0.37. The latter series covers the range of Al concentrations where the AlxGa1−xAs band gap energy EM is expected to cross the nitrogen-induced energy level EN. The resulting nitrogen- and Al-concentration dependent interband transition energies are described by the band anticrossing model using a matrix element for the coupling between the nitrogen-induced states and the extend lowest conduction band states of CMN = 2.32 eV and a nitrogen level energy EN = (1.625+0.069x) eV, the latter measured with respect to the GaAs valence band edge.
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71.20.Nr Semiconductor compounds
78.20.-e Optical properties of bulk materials and thin films

Effect of band gap on the red luminescence capability of Pr-doped InGaN layers grown by molecular beam epitaxy

Yaroslav E. Romanyuk, Lukas D. Kranz, and Stephen R. Leone

J. Appl. Phys. 103, 073104 (2008); http://dx.doi.org/10.1063/1.2903879 (6 pages) | Cited 5 times

Online Publication Date: 8 April 2008

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The effect of In concentration on the red photoluminescence (PL) from trivalent Pr rare-earth (RE) ions in InGaN layers is investigated. A series of Pr-doped InxGa1−xN thin layers with 0 ⩽ x ⩽ 0.15 is grown by radio-frequency plasma-assisted molecular beam epitaxy on GaN∕sapphire templates. Photoexcitation with a He–Cd laser results in characteristic red emission at 652 nm, corresponding to the 3P0−3F2 transition of Pr3+. The Pr PL is the highest for samples grown under slightly N-rich conditions, but the “yellow luminescence” caused by Ga vacancies is also present. The intensity of the characteristic Pr PL in InxGa1−xN:Pr decreases by one order of magnitude when increasing the In content from x = 0 to 0.15, even though the matching of the band gap to the Pr3+ energy states is better. Low-temperature PL measurements show that the de-excitation energy backtransfer process from excited Pr3+ ions is more severe for In-richer samples, although the amount of backtransfer cannot be the sole reason for the significant drop of the Pr3+ emission with the band gap reduction. The dissociation of bound excitons from Pr-related traps as well as the presence of In-related exciton localization centers are also likely to decrease the excitation of the Pr3+ ions. In view of ongoing efforts to search for efficient RE-doped III-V emitters, the results indicate that the formation∕dissociation rate of RE-bound excitons is one of the key factors that determine the RE emission yield.
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78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
71.20.Nr Semiconductor compounds
71.35.Gg Exciton-mediated interactions
61.72.jd Vacancies

Three-photon absorption saturation in ZnO and ZnS crystals

Bing Gu, Jun He, Wei Ji, and Hui-Tian Wang

J. Appl. Phys. 103, 073105 (2008); http://dx.doi.org/10.1063/1.2903576 (5 pages) | Cited 8 times

Online Publication Date: 8 April 2008

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We reported the observation of the saturation of interband three-photon absorption (3PA) in wide-gap semiconductors under intense femtosecond laser excitation. Theoretically, we developed a 3PA saturation model that is in agreement with the Z-scan experimental results. The characteristic saturation intensities were determined to be 44 and 210 GW/cm2 for ZnO and ZnS crystals, respectively. The 3PA saturation model is also consistent with the ultrafast dynamics of 3PA-generated charge carriers in ZnO and ZnS crystals, obtained from the femtosecond transient absorption measurements.
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78.47.J- Ultrafast spectroscopy (<1 psec)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Ey III-V and II-VI semiconductors
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Operating regimes of corona and silent discharges in coaxial cylindrical reactors

Ashraf Yehia

J. Appl. Phys. 103, 073301 (2008); http://dx.doi.org/10.1063/1.2901058 (7 pages) | Cited 1 time

Online Publication Date: 1 April 2008

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The physical mechanisms of the electrical discharges formed in coaxial cylindrical reactors have been investigated in this paper under different operating conditions for the reactors. The reactors were fed by natural air flowing with a constant rate at atmospheric conditions and stressed by an ac voltage. The wave form of the ac voltage applied to the reactors has been studied corresponding to the discharge current, over the whole range of the operating conditions. The results have shown that the operating conditions for the reactor modify the physical mechanism inside it in the form of either corona or silent discharges or spark over. The onset voltage of the different electrical discharges formed inside the reactors has agreed with Peek’s equation, whatever the operating conditions. Subsequently, empirical equations concerning the operating conditions for the reactors have been derived by using Peek’s and Townsend’s equations. The derived equations represent suitable criterions to predict the different types of the electrical discharges that will form inside the reactors under any operating conditions. The results have been illustrated by recorded oscillograms showing the wave form of the ac voltage applied to the reactors and the corresponding current pulses.
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52.80.Hc Glow; corona
52.50.-b Plasma production and heating

Time-correlated force production measurements of the dielectric barrier discharge plasma aerodynamic actuator

C. L. Enloe, M. G. McHarg, and T. E. McLaughlin

J. Appl. Phys. 103, 073302 (2008); http://dx.doi.org/10.1063/1.2896590 (7 pages) | Cited 22 times

Online Publication Date: 1 April 2008

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An atmospheric-pressure dielectric barrier discharge (DBD) plasma, operated as a surface-mode discharge with a single encapsulated electrode and an asymmetric electrode alignment, is known to couple momentum into the surrounding neutral fluid and through this coupling has shown considerable promise as an aerodynamic flow control device. Several different models, often with conflicting explanations, have been offered to explain the process of this momentum coupling. The DBD is known to proceed in two stages during the discharge cycle, one on the positive-going portion of the applied ac high-voltage waveform and the other on the negative-going portion. By using the actuator to drive a second-order mechanical system, we show here that the great majority (97%) of the momentum coupling occurs during the negative-going portion of the discharge cycle and we relate this behavior to dramatic differences in the structure of the discharge revealed with high-speed photography. This information is critical in evaluating descriptions of the momentum-coupling processes in the plasma.
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52.75.-d Plasma devices
52.80.-s Electric discharges

Plasma damage mechanisms for low-k porous SiOCH films due to radiation, radicals, and ions in the plasma etching process

Saburo Uchida, Seigo Takashima, Masaru Hori, Masanaga Fukasawa, Keiji Ohshima, Kazunori Nagahata, and Tetsuya Tatsumi

J. Appl. Phys. 103, 073303 (2008); http://dx.doi.org/10.1063/1.2891787 (5 pages) | Cited 28 times

Online Publication Date: 7 April 2008

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Low dielectric constant (low-k) films have been widely used as insulating materials in ultra-large-scale integrated circuits. Low-k films receive heavy damage during the plasma processes of etching or ashing, resulting in an increase in their dielectric constant. In order to realize damage-free plasma processes for low-k films, it is essential to determine the influence of radiation, radicals, and ions emitted in the plasma process on the characteristics of low-k films. We have developed a technique to evaluate the influence of radiation, radicals, ions, and their synergies on films in real plasma processes and have named it pallet for plasma evaluation (PAPE). Using the PAPE, plasma-induced damage on porous SiOCH films were investigated in dual-frequency capacitively coupled H2/N2 plasmas. The damage was characterized by ellipsometry, Fourier-transform infrared spectroscopy, and thermal desorption spectroscopy. On the basis of the results, the damage mechanisms associated with vacuum ultraviolet (VUV) and UV radiation, radicals, and ions were clarified. The damage was caused not only by ions and radicals but also by VUV and UV radiation emitted by the plasmas. Moreover, it was found that the synergy between the radiation and the radicals enhanced the damage.
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81.65.Cf Surface cleaning, etching, patterning
77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
78.66.Nk Insulators
78.30.Hv Other nonmetallic inorganics
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
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Transmittance and optical constants of Ce films in the 6–1200 eV spectral range

Mónica Fernández-Perea, José A. Aznárez, Juan I. Larruquert, José A. Méndez, Luca Poletto, Denis Garoli, A. Marco Malvezzi, Angelo Giglia, and Stefano Nannarone

J. Appl. Phys. 103, 073501 (2008); http://dx.doi.org/10.1063/1.2901137 (7 pages) | Cited 9 times

Online Publication Date: 1 April 2008

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The optical constants of Ce films were obtained in the 6–1200 eV range from transmittance measurements obtained at room temperature. Thin films of Ce were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Ce films were deposited onto grid-supported, thin C films. Transmittance measurements of various film thicknesses were used to obtain the extinction coefficient k of Ce films at each individual photon energy investigated. The refractive index n of Ce was calculated with the Kramers–Krönig analysis using the current k data, which were extended toward smaller and larger energies with available data from literature and extrapolations. Ce has a low-absorption band right below O2,3 edge, with lowest absorption at 16.1 eV. This makes Ce a promising material for the development of new filters and multilayer coatings below Ce O2,3 edge, in which few developments have been performed due to the lack of low-absorption materials. A good consistency of the data was evaluated through f and inertial sum rules.
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78.66.Bz Metals and metallic alloys
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.40.Kc Metals, semimetals, and alloys

Overlayer effects in the critical-point analysis of ellipsometric spectra: Application to InxGa1−xAs alloys

T. H. Ghong, T. J. Kim, Y. W. Jung, Y. D. Kim, and D. E. Aspnes

J. Appl. Phys. 103, 073502 (2008); http://dx.doi.org/10.1063/1.2902502 (5 pages) | Cited 2 times

Online Publication Date: 1 April 2008

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We investigate the effect of incomplete removal of semiconductor overlayers on critical-point (CP) parameters determined from the analysis of ellipsometric spectra. An approximate analytic expression shows that CP energies and broadening parameters should be relatively unaffected for isolated CPs if the dielectric response of the overlayer varies slowly with energy. The results are confirmed by model calculations for InAs, which show that the energies of the E1 and E1+Δ1 CP structures that are commonly used for compositional analysis of semiconductor alloys are relatively unaffected. We also analyze overlayer-removal data for a series of InxGa1−xAs alloy samples. Consistent with the above, the amplitudes and phases are affected significantly for all CPs, while the energies of the well-separated E1 and E1+Δ1 transitions are relatively invariant. The results show that accurate values of composition can be obtained from the analysis of the E1 and E1+Δ1 CP structures, even if complete removal of overlayers is not achieved.
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78.66.Fd III-V semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

High temperature thermoelectric transport properties of p-type Ba8Ga16AlxGe30−x type-I clathrates with high performance

Shukang Deng, Xinfeng Tang, Peng Li, and Qingjie Zhang

J. Appl. Phys. 103, 073503 (2008); http://dx.doi.org/10.1063/1.2902504 (6 pages) | Cited 8 times

Online Publication Date: 1 April 2008

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Using group-III atom Al as doping element, Ba8Ga16AlxGe30−x (x = 1.0, 2.0, 3.0, 4.0, and 5.0) type-I clathrates with different Al content were synthesized by combining melting reaction with spark plasma sintering method. The effects of Al content on thermoelectric properties are investigated. X-ray diffraction patterns and Rietveld analysis reveal that the compounds prepared by this method are type-I clathrates and Al atom preference for the 6c site. The Al substitutions do not affect the atomic displacement parameters (ADPs) of framework atoms (Ge/Ga) and filled atoms (Ba) compared to that of Ba8Ga16Ge30, and the ADPs of Al are nearly equivalent to that of other framework Ge/Ga atoms. All specimens exhibit the behavior of the p-type conduction. The carrier concentration and electrical conductivity increase while Seebeck coefficient decreases with the increasing Al content for the specimens with ⩽ 4.0. Ba8Ga16Al3.0Ge27.0 compound possesses the relatively lower lattice thermal conductivity κL due to the mass fluctuation between Al atoms and other atoms in the framework; it is as low as 0.96 W/mK at 300 K. The maximum ZT value of 0.61 is obtained at 760 K for Ba8Ga16Al3.0Ge27.0.
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72.15.Jf Thermoelectric and thermomagnetic effects
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling
61.66.Dk Alloys

Carrier concentration tuning of bandgap-reduced p-type ZnO films by codoping of Cu and Ga for improving photoelectrochemical response

Sudhakar Shet, Kwang-Soon Ahn, Yanfa Yan, Todd Deutsch, Kevin M. Chrustowski, John Turner, Mowafak Al–Jassim, and Nuggehalli Ravindra

J. Appl. Phys. 103, 073504 (2008); http://dx.doi.org/10.1063/1.2888578 (5 pages) | Cited 15 times

Online Publication Date: 1 April 2008

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In this study, the synthesis of p-type ZnO films with similar bandgaps but varying carrier concentrations through codoping of Cu and Ga is reported. The ZnO:(Cu,Ga) films are synthesized by rf magnetron sputtering in O2 gas ambient at room temperature, followed by postdeposition annealing at 500 °C in air for 2 h. The bandgap reduction and p-type conductivity are caused by the incorporation of Cu. The tuning of carrier concentration is realized by varying the Ga concentration. The carrier concentration tuning does not significantly change the bandgap and crystallinity. However, it can optimize the carrier concentration to significantly enhance the photoelectrochemical response for bandgap-reduced p-type ZnO thin films.
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73.61.Ga II-VI semiconductors
73.50.-h Electronic transport phenomena in thin films
72.20.-i Conductivity phenomena in semiconductors and insulators
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.uj III-V and II-VI semiconductors
82.45.Vp Semiconductor materials in electrochemistry

Residual stresses in the elastoplastic multilayer thin film structures: The cases of Si/Al bilayer and Si/Al/SiO2 trilayer structures

X. C. Zhang, B. S. Xu, and F. Z. Xuan

J. Appl. Phys. 103, 073505 (2008); http://dx.doi.org/10.1063/1.2832751 (7 pages) | Cited 2 times

Online Publication Date: 1 April 2008

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A theoretical model was developed to predict the thermal residual stresses within the elastoplastic multilayer thin film structures. The plastic deformation of one of the films was considered. Special analyses were made on the bilayer structures, i.e., a film overlaid on a substrate, as they are more of practical interest. Closed-form solutions were derived to estimate the residual stresses in the films and curvature of the multilayer film structure, and the relationship between the temperature difference and the thickness of the plastic zone. The cases of Si/Al bilayer and Si/Al/SiO2 multilayer structures were studied to illustrate the implementation of this model. In these structures, Al film was assumed to be plastically deformed. Results showed that, for both structures, there was a linear relationship existing between the thickness of the plastic zone in Al film and the temperature difference. SiO2 layer deposited on the aluminum film had an obvious influence on the critical temperature at which Al film started to be plastically deformed.
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68.60.Bs Mechanical and acoustical properties
68.35.Gy Mechanical properties; surface strains
68.65.Ac Multilayers
81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity

Investigation of stress and morphology in electrodeposited copper nanofilms by cantilever beam method and in situ electrochemical atomic force microscopy

S. Ahmed, T. T. Ahmed, M. O’Grady, S. Nakahara, and D. N. Buckley

J. Appl. Phys. 103, 073506 (2008); http://dx.doi.org/10.1063/1.2890995 (11 pages) | Cited 1 time

Online Publication Date: 1 April 2008

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Both stress and atomic force microscopy (AFM) measurements were carried out in situ during potentiostatic electrodeposition of copper on gold in 0.05 mol dm−3 CuSO4 in 0.1 mol dm−3 H2SO4 with and without additives. With no additives, compressive stress generally developed initially and films subsequently underwent a compressive-to-tensile (C-T) transition. With increasing negative potential, the time for the C-T transition decreased rapidly as the rate of coalescence of nuclei (measured by AFM) increased rapidly. This is consistent with models that attribute the C-T transition to increasing tensile stress due to coalescence of nuclei. Furthermore, at a potential of −75 mV (Cu/Cu2+), where AFM showed very little coalescence of nuclei, no C-T transition was observed, again consistent with these models. The nucleation density measured by AFM increased from 2.7×107 cm−2 at −75 mV to 2.5×109 cm−2 at −300 mV. Stress measurements with a combination of three additives [1×10−3 mol dm−3 Cl−, 8.82×10−5 mol dm−3 polyethylene glycol, and 1×10−5 mol dm−3 3-mercapto-1-propanesulfonic acid sodium salt (MPSA)] also showed that compressive stress generally developed initially and its magnitude was greater than in additive-free electrolyte. At less negative potentials, even though the rate of coalescence of nuclei was rapid, as observed by AFM, the stress continued to evolve in the compressive direction. At intermediate potentials (−90 to−150 mV), classical compressive-tensile-compressive (C-T-C) behavior was observed, while at more negative potentials the stress continued to evolve in the tensile direction. Similar results were obtained with a combination of two additives (1×10−3 mol dm−3 Cl− and 1×10−5 mol dm−3 MPSA), but in that case the compressive stress appeared to be greater, and consequently the T-C transition was observed even at −500 mV. The results are consistent with enhancement of a compressive component of stress in the presence of additives.
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81.15.Pq Electrodeposition, electroplating
82.80.Fk Electrochemical methods
64.70.K- Solid-solid transitions
64.60.Q- Nucleation

Characterization of Al–As codoped p-type ZnO films by magnetron cosputtering deposition

Eui-Jung Yun, Hyeong-Sik Park, Kyu H. Lee, Hyoung G. Nam, and Myunghee Jung

J. Appl. Phys. 103, 073507 (2008); http://dx.doi.org/10.1063/1.2901050 (4 pages) | Cited 9 times

Online Publication Date: 2 April 2008

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We report the preparation of Al–As codoped p-type ZnO films by rf magnetron cosputtering deposition. The p-type conductivity of the films was revealed by Hall measurements, x-ray photoelectron spectroscopy (XPS), and photoluminescence measurements after being annealed in O2. It was observed by XPS that Al content increased with increasing AlAs target power from 80 to 160 W and reached a maximum value at an AlAs target power of 160 W. Hole concentration decreased with increasing Al content. With increasing AlAs target power greater than 160 W, the samples exhibit increases in As and O contents and decreases in Al and Zn contents, which contribute to the increase in hole concentration. A high hole concentration of 2.354×1020 cm−3, a low resistivity of 2.122×10−2 Ω cm, and a Hall mobility of 0.13 cm2/V s for the films with high As content of 16.59% were obtained. XPS has also been used to address the unresolved issues related to the p-type formation mechanism of As-doped ZnO, supporting that the acceptor is AsZn–2VZn.
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81.15.Cd Deposition by sputtering
68.55.ag Semiconductors
73.61.Ga II-VI semiconductors
78.66.Hf II-VI semiconductors
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
78.55.Et II-VI semiconductors

Differential sputter yields in Si1−xGex

M. Zubaer Hossain, Jonathan B. Freund, and H. T. Johnson

J. Appl. Phys. 103, 073508 (2008); http://dx.doi.org/10.1063/1.2896451 (4 pages) | Cited 4 times

Online Publication Date: 2 April 2008

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Sputter yields for ion bombardment of Si1−xGex alloys are computed using molecular dynamics simulations with the target material modeled using the Stillinger–Weber empirical potential. The results show that Si atoms are preferentially sputtered relative to Ge atoms for all x. The parameters of the Stillinger–Weber potential are altered to investigate the relative importance of atomic mass, bonding energy, atomic number, and atomic density leading to the observed differential sputter yields. The results explain an experimentally observed nonlinear dependence of total sputter yields on composition in Si1−xGex [ Tuboltsev et al., Phys. Rev. B 72, 205434 (2005) ].
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79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Nonequilibrium solidification in undercooled Ti45Al55 melts

H. Hartmann, P. K. Galenko, D. Holland–Moritz, M. Kolbe, D. M. Herlach, and O. Shuleshova

J. Appl. Phys. 103, 073509 (2008); http://dx.doi.org/10.1063/1.2903920 (9 pages) | Cited 4 times

Online Publication Date: 3 April 2008

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Ti–Al alloys are of high technological interest as light-weight high-performance materials. When produced by solidification from the liquid state, the material properties of as-solidified materials are strongly dependent on the conditions governing the solidification process. Nonequilibrium solidification from the state of an undercooled liquid may result to the formation of metastable solid materials. On the one hand undercooling under special cases may influence the phase selection behavior during solidification, and on the other hand during rapid growth of solid phases in undercooled melts nonequilibrium effects such as solute trapping and disorder trapping may occur. In the present work containerless processing by electromagnetic levitation is used to undercool Ti45Al55 melts deeply below the liquidus temperature. The dendrite growth velocity during the solidification is measured as a function of undercooling by application of a high-speed video camera. In situ diffraction experiments at ESRF in Grenoble and microstructure investigations are performed in order to identify the primary solidified phases. The experimental findings are interpreted within current theoretical models for dendritic growth and solute trapping.
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81.05.Bx Metals, semimetals, and alloys
81.30.Fb Solidification
64.70.D- Solid-liquid transitions

Optical energies of AlInN epilayers

K. Wang, R. W. Martin, D. Amabile, P. R. Edwards, S. Hernandez, E. Nogales, K. P. O’Donnell, K. Lorenz, E. Alves, V. Matias, A. Vantomme, D. Wolverson, and I. M. Watson

J. Appl. Phys. 103, 073510 (2008); http://dx.doi.org/10.1063/1.2898533 (3 pages) | Cited 21 times

Online Publication Date: 4 April 2008

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Optical energy gaps are measured for high-quality Al1−xInxN-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of ≈ 6 eV and differences with earlier reports are discussed. Very large Stokes’ shifts of 0.4–0.8 eV are observed in the composition range 0.13<x<0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field.
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78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Enhanced ultraviolet emission from ZnO nanocrystals embedded in a hybrid polymer composite layer

Fushan Li, Dong-Ick Son, Jae-Hyeon Leem, Tae Whan Kim, Wenguo Dong, and Young-Ho Kim

J. Appl. Phys. 103, 073511 (2008); http://dx.doi.org/10.1063/1.2903937 (4 pages) | Cited 1 time

Online Publication Date: 4 April 2008

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Self-assembled ZnO nanocrystals embedded in a p-phenylene-biphenyltetracarboximide (BPDA-PDA)-type polyimide (PI) layer or in a hybrid (BPDA-PDA) PI and poly-N-vinylcarbazole (PVK) matrix were formed by curing Zn thin films in a nitrogen atmosphere. Transmission electron microscopy images showed that ZnO nanocrystals were formed inside in the hybrid BPDA-PDA PI and PVK matrix. Photoluminescence (PL) spectra for the ZnO nanocrystals embedded in the BPDA-PDA PI layer and for the ZnO nanocrystals embedded in the hybrid polymer layer showed that addition of PVK significantly increased the luminescence intensity corresponding to the ultraviolet (UV) emission from the ZnO nanocrystals. The luminescence mechanisms related to the green and the UV bands for the ZnO nanocrystals embedded in the BPDA-PDA PI and PVK hybrid layer are described on the basis of the PL results.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.-m Photoluminescence, properties and materials

Linear and nonlinear optical absorption coefficients and refractive index changes in spherical quantum dots: Effects of impurities, electric field, size, and optical intensity

İbrahim Karabulut and Sotirios Baskoutas

J. Appl. Phys. 103, 073512 (2008); http://dx.doi.org/10.1063/1.2904860 (5 pages) | Cited 61 times

Online Publication Date: 4 April 2008

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In the present work, the case of a spherical quantum dot with parabolic confinement subjected to an external electric field with the presence of an impurity, the linear and third-order nonlinear optical absorption coefficients as well as refractive index changes have been calculated. The numerical method we are using for the calculation of the energy levels and the corresponding wave functions is the potential morphing method in the effective mass approximation. As our results indicate an increase of the electric field and/or the position of the impurity and/or the quantum dot radius redshifts the peak positions of the total absorption coefficient and total refractive index changes. Additionally, an increase of the position of the impurity and/or the quantum dot radius decreases the total absorption coefficient and increases the total refractive index changes. An increase also of the electric field decreases the total absorption coefficient but does not significantly affect the peak values of the total refractive index changes. Finally, an increase of the optical intensity considerably changes the total absorption coefficient as well as the total refractive index changes.
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42.65.-k Nonlinear optics
73.21.La Quantum dots
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.67.Hc Quantum dots
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
71.15.-m Methods of electronic structure calculations

Diffusion of beryllium in Ge and Si–Ge alloys

O. Koskelo, P. Pusa, J. Räisänen, U. Köster, and I. Riihimäki

J. Appl. Phys. 103, 073513 (2008); http://dx.doi.org/10.1063/1.2903297 (4 pages) | Cited 2 times

Online Publication Date: 4 April 2008

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Diffusion of implanted 7Be in Si1−xGex (x = 0.20,0.65,1.00) systems has been studied under intrinsic conditions in the temperature range of 460–720 °C by the modified radiotracer technique. Arrhenius-type behavior with activation enthalpies of 2.0 eV for Ge and 2.5 eV for the Si–Ge alloys was noted. Unexpectedly, the diffusivity of beryllium is higher in the Si0.80Ge0.20 material than in Si0.35Ge0.65 which is discussed in terms of possible prevailing diffusion mechanisms. It is proposed that Be diffusion in Si1−xGex systems is dissociative mechanism dominated for germanium rich materials and the kick-out (or interstitialcy) mechanism dominates in silicon rich materials.
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66.30.J- Diffusion of impurities
65.40.G- Other thermodynamical quantities

Effect of ultrathin buffer on the microstructure of highly mismatched epitaxial ZnO films on Al2O3 (0001)

In-Woo Kim and Kyu-Mann Lee

J. Appl. Phys. 103, 073514 (2008); http://dx.doi.org/10.1063/1.2905323 (4 pages)

Online Publication Date: 7 April 2008

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The effect of the ultra thin (4 nm) ZnO buffer layer grown at a low temperature of 300 °C on the microstructural evolution of highly mismatched ZnO/Al2O3(0001) films was investigated. Based on the real time synchrotron x-ray scattering, atomic force microscopy, and high resolution electron microscopy, it was shown that the ultrathin two-dimensional (2D) layers play a critical role for improving the ZnO layer quality by inducing 2D growth mode instead of three-dimensional mode at 500 °C in early stage. The ZnO films grown on the ultrathin buffer exhibited structural coherence between the surface and the interface in the substrate normal direction in early stage. The great enhancement of the microstructural quality was attributed to the strain accommodation by the 2D ultrathin buffer.
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68.55.ag Semiconductors
68.55.J- Morphology of films
68.35.bg Semiconductors
81.15.Cd Deposition by sputtering

Applied and residual stresses in brittle cylindrical multilayered composites

Milan Ambrožič and Tomaž Kosmač

J. Appl. Phys. 103, 073515 (2008); http://dx.doi.org/10.1063/1.2903062 (5 pages)

Online Publication Date: 7 April 2008

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We have made a theoretical study of the stress state in brittle multilayered composites with variable numbers, compositions, and thicknesses of concentric cylindrical layers. We used the linear theory of elasticity to calculate the stresses in the materials as they were subjected to a three-point bending test, including the thermal residual stresses that result from the fabrication process. As an example of the application of our model, we studied alumina/zirconia ceramic composites, wherein we indicated the way of optimizing individual layer compositions and thicknesses to get the highest composite strength. When the thicknesses of the layers are optimized for a given composition, more than one layer tends to be simultaneously damaged at the threshold force.
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68.35.Gy Mechanical properties; surface strains
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.D- Elasticity
62.20.mj Brittleness

Reversible radial deformation up to the complete flattening of carbon nanotubes in nanoindentation

Majid Minary-Jolandan and Min-Feng Yu

J. Appl. Phys. 103, 073516 (2008); http://dx.doi.org/10.1063/1.2903438 (5 pages) | Cited 12 times

Online Publication Date: 7 April 2008

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The reversible flattening up to 60% deformation of individual multiwalled carbon nanotubes (MWCNTs) was revealed with nanoindentation. The nanoindentation induced, in terms of indentation force versus indentation depth, two linear deformation responses and a nonlinear one in between in the measurement of an ∼ 9 nm diameter MWCNT having six walls. A continuum shell model was applied and found to be fully capable of describing the observed behavior and extracting accurate mechanical properties of the MWCNT. The measured linear deformation persisted up to an indentation depth twice the shell thickness of the MWCNT, a behavior much like a macroscopic thin shell in classical shell theory. Nonlinear deformation was subsequently introduced due to the extended flattening of the MWCNT in the axial direction and the formation of high curvature “bulbs” along the edges of a squashed MWCNT. Finally, the elastic deformation of such bulbs initiated the second linear deformation response. The deformation behavior was found to be fully reversible, manifesting the remarkable mechanical performance of CNT as a nanoscale elastic shell in sustaining severe mechanical deformation.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity

Membrane-enhanced surface acoustic wave analysis of grafted polymer brushes

David A. Brass and Kenneth R. Shull

J. Appl. Phys. 103, 073517 (2008); http://dx.doi.org/10.1063/1.2903880 (13 pages)

Online Publication Date: 8 April 2008

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An analysis is developed for the frequency response of a quartz crystal resonator (often referred to as a quartz crystal microbalance) that is modified with a grafted solvent-swollen polymer brush and placed in contact with a membrane capping layer. The shear wave generated at the resonator surface couples into the membrane layer with an efficiency that is strongly dependent on the thickness of the swollen brush layer. As a result, the resonant frequency changes by a maximum amount that is closely approximated by the Sauerbrey shift for the capping layer. The calculated shift substantially decreases for increases in the brush thickness of approximately 10 nm, which gives a net frequency response that is extremely sensitive to the degree of swelling of the polymer brush. An optimum capping layer thickness is determined by balancing the Sauerbrey shift against dissipative effects that weaken the crystal resonance. This optimum membrane thickness depends only weakly on the properties of the membrane material and is in the micron range. Detailed multilayer calculations are presented for the specific case of a poly(ethylene glycol) brush swollen with water and brought into contact with an elastomeric water-permeable membrane. These calculations confirm that the method is sensitive to the properties of the brush layer in the experimentally relevant thickness regime. Connections are also made to conceptually simpler two and three layer models of the acoustic impedance of the material systems that are brought into contact with the resonator.
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68.35.Iv Acoustical properties
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