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15 Apr 2011

Volume 109, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 109, 081101 (2011); http://dx.doi.org/10.1063/1.3566072 (9 pages)

Simeon Katz, Augustinas Vizbaras, Ralf Meyer, and Markus-Christian Amann

Injectorless quantum cascade lasers

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back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

The effect of Si-nanocrystal size distribution on Raman spectrum

Weiwei Ke, Xue Feng, and Yidong Huang

J. Appl. Phys. 109, 083526 (2011); http://dx.doi.org/10.1063/1.3569888 (5 pages) | Cited 3 times

Online Publication Date: 21 April 2011

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The effect of Si-nanocrystal (Si-nc) size distribution on Raman spectrum is studied in detail within the framework of a phonon confinement model. It is found that size distribution has little effect on Raman frequency shift, but greatly affects the width and shape of Raman spectrum. Si-nc Raman spectrum can be well explained by considering the size distribution. Furthermore, a set of simple relationships between Raman frequency shift/full width at half maximum (FWHM) and size distribution is set up based on the framework of a modified phonon confinement model, which can be applied to calculate Si-nc size distribution from the Raman spectrum.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.30.Am Elemental semiconductors and insulators
63.22.Kn Clusters and nanocrystals
61.46.Hk Nanocrystals

Implementation of faceted dendrite growth on floating cast method to realize high-quality multicrsytalline Si ingot for solar cells

Noritaka Usami, Isao Takahashi, Kentaro Kutsukake, Kozo Fujiwara, and Kazuo Nakajima

J. Appl. Phys. 109, 083527 (2011); http://dx.doi.org/10.1063/1.3576108 (4 pages) | Cited 2 times

Online Publication Date: 22 April 2011

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We carried out small-scale crystal growth experiment to apply faceted dendrite growth at the top of the melt to floating cast method in order to grow high-quality multicrystalline Si (mc-Si) ingot for solar cells. By appropriate cooling at the initial stage, the most part of the top surface was covered by a single faceted dendrite crystal followed by directional solidification from the top to the bottom. As a consequence, the cross-section of the ingot was dominated by a single crystal grain. The crystal grain was found to be almost free from subgrain boundaries as evidenced by spatially resolved single-peak x-ray rocking curve profiles. The minority carrier diffusion length was found to be much longer than that in a control sample grown from the bottom to the top presumably due to the minimized contact with the crucible wall. This shows that the floating cast method combined with formation of faceted dendrite crystals at the initial stage is a promising route to realize high-quality mc-Si ingot for high-efficiency solar cells.
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81.05.Cy Elemental semiconductors
68.70.+w Whiskers and dendrites (growth, structure, and nonelectronic properties)
81.10.Fq Growth from melts; zone melting and refining
81.30.Fb Solidification
61.72.Mm Grain and twin boundaries
88.40.jj Silicon solar cells

Photoluminescence of nanocrystalline titanium dioxide films loaded with silver nanoparticles

Jana Preclíková, Pavel Galář, František Trojánek, Bohuslav Rezek, Yvonne Němcová, and Petr Malý

J. Appl. Phys. 109, 083528 (2011); http://dx.doi.org/10.1063/1.3573486 (7 pages) | Cited 4 times

Online Publication Date: 22 April 2011

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We report on a systematic study of the photoluminescence properties of nanocrystalline titanium dioxide films loaded with silver nanoparticles under various ambient conditions: in the temperature interval of 10 to 300 K, under different values of ambient air pressure (5 to 105 Pa), and under visible light irradiation that causes the photochromic transformation of the film. Our results enable us to follow the electron transfer from titanium dioxide into silver nanoparticles and to observe the oxidation of silver nanoparticles during the photochromic transformation. We propose a microscopic model explaining the behavior of extinction and photoluminescence of the Ag–TiO2 film under different values of ambient air pressure.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.-m Photoluminescence, properties and materials
78.66.-w Optical properties of specific thin films

Influence of crystal polarity on crystal defects in GaP grown on exact Si (001)

Andreas Beyer, Igor Németh, Sven Liebich, Jens Ohlmann, Wolfgang Stolz, and Kerstin Volz

J. Appl. Phys. 109, 083529 (2011); http://dx.doi.org/10.1063/1.3567910 (6 pages) | Cited 5 times

Online Publication Date: 22 April 2011

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For the implementation of optoelectronic devices on silicon, which could be realized by a combination of Si and direct-bandgap III/V semiconductors, a defect free nucleation layer of GaP on Si is essential. This paper summarizes the results of structural investigations carried out by transmission electron microscopy on defects, which can be observed in GaP films grown by metal organic vapor phase epitaxy on exactly oriented (001) Si substrates. Under optimized growth conditions the anti phase domains (APDs), which arise in the III/V semiconductor at the monoatomic steps on the silicon surface, show a specific typical shape. They self-annihilate on {112} planes in the GaP and can be observed in [110] cross-section, looking perpendicular to the steps on the Si surface. In contrast to that, the anti phase boundaries (APBs) lie on {110} GaP planes in the [−110] direction, parallel to the steps on the Si surface. From convergent beam electron diffraction one can show, that the GaP has Ga-polarity in the [110] direction, viewing perpendicular to the steps on the Si-surface. With the knowledge of the polarity and the shape of the APDs, we suggest a model for chemical composition of their boundaries. According to this model the APBs, which lie on {110} and {112} planes, consist of an equal amount of Ga-Ga and P-P bonds. Furthermore, when stacking faults and twins are observed, they only occur in the Ga-polar [110] GaP direction, and consequently lie on {111}A planes. With the knowledge of the structure of the defects that arise at the GaP/Si interface we suggest growth conditions and an optimum Si surface structure, which guarantee a defect-free GaP overgrowth layer after several 10 nms of III/V material, even on exact Si substrates.
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81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
61.72.Nn Stacking faults and other planar or extended defects
61.72.Mm Grain and twin boundaries
61.50.Lt Crystal binding; cohesive energy

Engineering of nitrogen-vacancy color centers in high purity diamond by ion implantation and annealing

J. O. Orwa, C. Santori, K. M. C. Fu, B. Gibson, D. Simpson, I. Aharonovich, A. Stacey, A. Cimmino, P. Balog, M. Markham, D. Twitchen, A. D. Greentree, R. G. Beausoleil, and S. Prawer

J. Appl. Phys. 109, 083530 (2011); http://dx.doi.org/10.1063/1.3573768 (7 pages) | Cited 8 times

Online Publication Date: 22 April 2011

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The negatively-charged nitrogen-vacancy (NV) center is the most studied optical center in diamond and is very important for applications in quantum information science. Many proposals for integrating NV centers in quantum and sensing applications rely on their tailored fabrication in ultra pure host material. In this study, we use ion implantation to controllably introduce nitrogen into high purity, low nitrogen chemical vapor deposition diamond samples. The properties of the resulting NV centers are studied as a function of implantation temperature, annealing temperature, and implantation fluence. We compare the implanted NV centers with native NV centers present deep in the bulk of the as-grown samples. The results for implanted NV centers are promising but indicate, at this stage, that the deep native NV centers possess overall superior optical properties. In particular, the implanted NV centers obtained after annealing at 2000 °C under a stabilizing pressure of 8 GPa showed an ensemble linewidth of 0.17 nm compared to 0.61 nm after annealing at 1000 °C. Over the same temperature range, the ensemble NV−/NV0 ratio increased by a factor of ∼5, although this was accompanied by an overall decrease in the NV count.
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61.72.jd Vacancies
61.72.jn Color centers
61.72.up Other materials
81.40.Gh Other heat and thermomechanical treatments

Phase stability of intermetallic PtIn2 under pressure: An in-situ transport, structural and first principles investigations

Alka B. Garg, P. Modak, and V. Vijayakumar

J. Appl. Phys. 109, 083531 (2011); http://dx.doi.org/10.1063/1.3574404 (6 pages) | Cited 1 time

Online Publication Date: 22 April 2011

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The high pressure phase stability of intermetallic PtIn2 at ambient temperature has been investigated using the synchrotron based angle dispersive x-ray diffraction technique, electrical resistance measurements, and first principles electronic structure calculations. The electrical resistance falls continuously up to 5 GPa and then remains almost constant up to 10 GPa. Beyond 10 GPa, the resistance increases and continues to increase up to 21 GPa which is the highest pressure of the present measurements. Consistent with this, the x-ray diffraction measurements show a structural phase transition from the ambient cubic phase to a low symmetry orthorhombic phase beyond 10 GPa. Upon pressure release, the ambient cubic phase is obtained. Ab initio structural relaxations and total energy calculations predict the cubic to orthorhombic phase transitions at 10.5 GPa.
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64.70.kd Metals and alloys
62.50.-p High-pressure effects in solids and liquids
71.20.Be Transition metals and alloys
72.15.Eb Electrical and thermal conduction in crystalline metals and alloys
71.15.Nc Total energy and cohesive energy calculations
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder

p-type K-doped ZnO nanorods for optoelectronic applications

Manoj K. Gupta, Nidhi Sinha, and Binay Kumar

J. Appl. Phys. 109, 083532 (2011); http://dx.doi.org/10.1063/1.3574656 (5 pages) | Cited 4 times

Online Publication Date: 22 April 2011

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Single crystalline p-type K-doped ZnO nanorods (NR) have been synthesized by convenient and low-cost solution technique. X-ray diffraction analysis confirmed the hexagonal system of K-doped ZnO nanorods and a preferred a axis orientation. The diameter and length were found to be 30–50 nm and 150–200 nm, respectively, in electron microscopic studies. The p-type nature and high hole density of 2.36×1019 per cm3 was demonstrated by Hall studies. A sharp lower cutoff and reduction in the bandgap was observed in UV-Vis study which is linked to its smaller size and p-type conduction in NR. In Photoluminescence studies, a near band emission in the UV range and a yellow emission with a blueshift were detected in the visible region in K-doped ZnO NR. The variation in intensity of peaks is correlated with the dimensions of NR. The redshift observed in Raman peaks is explained in terms of the stress developed in ZnO nanostructure due to lattice mismatch by K doping. These results provide better insight of K-doped ZnO nanostructures for both fundamental research and technological applications in optoelectronic devices.
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81.16.-c Methods of micro- and nanofabrication and processing
68.55.ag Semiconductors
78.55.Et II-VI semiconductors
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
78.66.Hf II-VI semiconductors
81.05.Dz II-VI semiconductors

Energy transfer rates and population inversion investigation of 1G4 and 1D2 excited states of Tm3+ in Yb:Tm:Nd:KY3F10 crystals

Horácio Marconi da Silva M. D. Linhares, André Felipe Henriques Librantz, Laércio Gomes, Lilia Coronato Courrol, Sonia Lícia Baldochi, and Izilda Marcia Ranieri

J. Appl. Phys. 109, 083533 (2011); http://dx.doi.org/10.1063/1.3552924 (11 pages)

Online Publication Date: 22 April 2011

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In this work we present the spectroscopic properties of KY3F10 (KY3F) single crystals activated with thulium and co-doped with ytterbium and neodymium ions. The most important processes that lead to the thulium up-conversion emissions in the blue and ultraviolet regions were identified. A time-resolved luminescence spectroscopy technique was employed to measure the luminescence decays and to determine the most important mechanisms involved in the up-conversion process that populates 1G4 and 1D2 (Tm3+) excited states. Analysis of the energy-transfer processes dynamics using selective pulsed-laser excitations in Yb:Tm:Nd, Tm:Nd, and Tm:Yb KY3F crystals show that the energy transfer from Nd3+ to Yb3+ ions is the mechanism responsible for the enhancement of the blue up-conversion efficiency in the Yb:Tm:Nd:KY3F when compared with the Yb:Tm system. A study of the energy transfer processes in Yb:Tm:Nd:KY3F crystal showed that the 1G4 excited level is mainly populated by a sequence of two nonradiative energy transfers that starts well after the Nd3+ and Tm3+ excitation at 797 nm according to: Nd3+ (4F3/2) → Yb3+ (2F7/2) followed by Yb3+ (2F5/2) → Tm (3H4) → Tm3+ (1G4). Results of numerical simulation of the rate equations system showed that a population inversion for 483.1 nm laser emission line is attained for a pumping rate threshold of 98 s−1, which is equivalent to an intensity of 3.3 KW cm−2 for a continuous laser pumping at 797 nm for Yb(30 mol%):Tm(0.5 mol%):Nd(1 mol%):KY3F. Nevertheless, best Yb3+ concentration for the laser emission near 483.1 nm was estimated to be within 40 and 50 mol%. On the other hand, a population inversion was not observed for the case of 960 nm (Yb3+) pumping.
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78.45.+h Stimulated emission
78.47.jd Time resolved luminescence
78.55.Hx Other solid inorganic materials
78.47.da Excited states

Probing the phonon confinement in ultrasmall silicon nanocrystals reveals a size-dependent surface energy

Iain F. Crowe, Matthew P. Halsall, Oksana Hulko, Andrew P. Knights, Russell M. Gwilliam, Maciej Wojdak, and Anthony J. Kenyon

J. Appl. Phys. 109, 083534 (2011); http://dx.doi.org/10.1063/1.3575181 (8 pages) | Cited 7 times

Online Publication Date: 22 April 2011

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We validate for the first time the phenomenological phonon confinement model (PCM) of H. Richter, Z. P. Wang, and L. Ley [Solid State Commun. 39, 625 (1981)] for silicon nanostructures on the sub-3 nm length scale. By invoking a PCM that incorporates the measured size distribution, as determined from cross-sectional transmission electron microscopy (X-TEM) images, we are able to accurately replicate the measured Raman line shape, which gives physical meaning to its evolution with high temperature annealing and removes the uncertainty in determining the confining length scale. The ability of our model to explain the presence of a background scattering spectrum implies the existence of a secondary population of extremely small (sub-nm), amorphous silicon nanoclusters which are not visible in the X-TEM images. Furthermore, the inclusion of an additional fitting parameter, which takes into account the observed peak shift, can be explained by a size-dependent interfacial stress that is minimized by the nanocluster/crystal growth. From this we obtain incidental, yet accurate estimates for the silicon surface energy and a Tolman length, δ ≈ 0.15 ± 0.1 nm using the Laplace-Young relation.
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63.22.Kn Clusters and nanocrystals
78.66.Jg Amorphous semiconductors; glasses
78.30.Hv Other nonmetallic inorganics
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
61.43.Dq Amorphous semiconductors, metals, and alloys

Enhanced effect of Ce3+ ions on 2 μm emission and energy transfer properties in Yb3+/Ho3+ doped fluorophosphate glasses

Ying Tian (田颖), Rongrong Xu (徐茸茸), Liyan Zhang (张丽艳), Lili Hu (胡丽丽), and Junjie Zhang (张军杰)

J. Appl. Phys. 109, 083535 (2011); http://dx.doi.org/10.1063/1.3569855 (6 pages) | Cited 3 times

Online Publication Date: 22 April 2011

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Effect of Ce3+ ions introduction on 2 μm emission properties and energy transfer mechanism in Yb3+/Ho3+-doped fluorophosphate glass has been investigated. From the measured fluorescence spectra, strong emission near 2 μm is demonstrated due to the sensitization of Yb3+ and Ce3+. Based on the absorption spectra, the Judd-Ofelt parameters and radiative properties were calculated and compared with those of other glass hosts. In addition, the energy transfer coefficient from Yb3+ to Ho3+ is 9.52 × 10−40 cm6/s in Yb3+/Ho3+/Ce3+ triply doped sample and is three times larger than that in undoped Ce3+ sample. These results suggest that this Yb3+/Ho3+/Ce3+ fluorophosphate glass with excellent thermal stability and efficient energy transfer from Yb3+ to Ho3+ is a good candidate for 2 μm laser.
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78.55.Qr Amorphous materials; glasses and other disordered solids
61.72.up Other materials

Pressure-dependent Hugoniot elastic limit of Gd3Ga5O12 single crystals

Xianming Zhou, Jun Li, Williams J. Nellis, Xiang Wang, Jiabo Li, Hongliang He, and Qiang Wu

J. Appl. Phys. 109, 083536 (2011); http://dx.doi.org/10.1063/1.3575330 (5 pages)

Online Publication Date: 22 April 2011

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Single-crystal Gd3Ga5O12 has been studied at high dynamic pressures generated with plate impacts. Shock-wave profiles and Hugoniot points were measured with a picosecond time-resolved Doppler Pin System. For final shock pressures in the range 8.52-113 GPa, a two-wave structure is observed below 59.3 GPa, a three-wave structure at ∼88.5 GPa, and a single shock wave is observed at ∼113 GPa. Our data show that the Hugoniot elastic limit (HEL) of single-crystal Gd3Ga5O12 is strongly dependent on final shock pressure. The HEL increases from 7.65 to 24.2 GPa as final pressure increases from 8.52 to 88.5 GPa. A shock-induced phase transformation is observed at a pressure of ∼75.9 GPa, which is a little higher than the value reported previously (Mashimo et al., Phys. Rev. Lett. 96, 105504, 2006), but is consistent with previous DAC work (Mao et al., Phys. Rev. B 83, 054114, 2011).
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
62.50.Ef Shock wave effects in solids and liquids
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
64.70.kt Molecular crystals

Anomalously large bend elastic constant and faster electro-optic response in anisotropic gels formed by a dipeptide

R. Bhargavi, Geetha G. Nair, S. Krishna Prasad, Rashmi Prabhu, and C. V. Yelamaggad

J. Appl. Phys. 109, 083537 (2011); http://dx.doi.org/10.1063/1.3553413 (7 pages) | Cited 2 times

Online Publication Date: 22 April 2011

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We report rheological, static, and dynamic Freedericksz transformation measurements on an anisotropic thermoreversible gel formed by gelation of a nematic liquid crystal (NLC) with a monodisperse dipeptide. The storage and loss modulii obtained from a low strain oscillatory shear experiment display that the material forms a weak anisotropic gel, and undergoes a sharp thermal transition to an anisotropic sol state. Freedericksz transformation studies employing an electric field for the reorientation of the molecules present a surprising result: the gel possesses a very large Frank bend elastic constant value, which is orders of magnitude higher than that for the high temperature sol state as well as that for the neat NLC used. On the other hand, the splay elastic constant shows relatively a small increase. Further, these elastic constants show systematic but nonlinear variation with the concentration of the gelator. Attractive features of the electro-optic switching when the sol transforms to the gel state are the vanishing of the undesirable backflow effect, and nearly an order of magnitude decrease in the switching speed. In both the gel and sol states the extracted rotational viscosities are comparable to the values of the neat NLC at corresponding temperatures. In contrast, the bulk dynamic viscosity is more than three orders of magnitude higher in the gel. The studies also demonstrate that the anisotropic gel to anisotropic sol transition seen in this weak gel can be tracked by simply monitoring the static or the dynamic Freedericksz transformation.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
66.20.-d Viscosity of liquids; diffusive momentum transport
82.70.Gg Gels and sols
61.30.-v Liquid crystals
62.20.dq Other elastic constants

Anomalous temperature-dependency of phonon line widths probed by Raman scattering from β-FeSi2 thin films

H. F. Liu, A. Huang, and D. Z. Chi

J. Appl. Phys. 109, 083538 (2011); http://dx.doi.org/10.1063/1.3573671 (5 pages) | Cited 2 times

Online Publication Date: 26 April 2011

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Micro-Raman scattering and its temperature dependencies have been used to investigate lattice dynamic properties of β-FeSi2 thin films epitaxially grown on Si (111) substrates by sputtering at elevated temperatures. A linear correlation between the blueshift in phonon energies and the lattice expansions in the growth direction of β-FeSi2 is observed. The shifting rate of the Raman mode at 248 cm−1 (13.6 cm−1/%) is much larger than that of the mode at 194 cm−1 (8.4 cm−1/%) due to the involvement of radial bonding vibrations between Fe and Si atoms. Temperature-dependent Raman scattering, which is hitherto unknown for β-FeSi2, reveals an extremely small phonon line width broadening in the range of 80-480 K. It is revealed that the broadening in the phonon line widths contributed by phonon-phonon scattering is weaker and is masked by phonon-hole couplings in β-FeSi2, while the effect of phonon-hole coupling on phonon line width broadening is saturated at the studied temperatures due to the high impurity densities.
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78.30.Hv Other nonmetallic inorganics
81.15.Cd Deposition by sputtering
63.20.kg Phonon-phonon interactions
68.55.ag Semiconductors
78.66.Li Other semiconductors

Optical properties of SnCl2 phosphor

Jun-ichi Nara and Sadao Adachi

J. Appl. Phys. 109, 083539 (2011); http://dx.doi.org/10.1063/1.3576118 (7 pages) | Cited 1 time

Online Publication Date: 27 April 2011

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A white SnCl2 phosphor is synthesized by the chemical etching of Sn shots in an aqueous HCl solution. The optical properties of this phosphor are investigated using diffuse reflectance, photoluminescence (PL) analysis, PL excitation spectroscopy, and PL lifetime measurements. The SnCl2 phosphor exhibits PL in the blue (∼460 nm) and red spectral regions (∼600 nm) under ultraviolet excitation (≥4.1 eV). The differences in the PL features observed using Nd:YAG (λ = 266 nm) and He-Cd (λ = 325 nm) lasers as excitation light sources are interpreted using the configurational-coordinate model, taking into account the band-gap energy (∼3.8 eV) of SnCl2 and the excited-state energies of the Sn2+ ions. A reliable energy level diagram for Sn2+ ions in the SnCl2 host is also proposed.
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78.55.Hx Other solid inorganic materials
71.20.Ps Other inorganic compounds
71.10.Li Excited states and pairing interactions in model systems

Localized surface plasmon resonance properties of two-layered gold nanowire: Effects of geometry, incidence angle, and polarization

Dajian Wu, Xiaojun Liu, and Bo Li

J. Appl. Phys. 109, 083540 (2011); http://dx.doi.org/10.1063/1.3580508 (7 pages) | Cited 5 times

Online Publication Date: 27 April 2011

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The extinction spectra of two-layered gold nanowires (TGNWs) have been calculated by using the vector wave function method. When the polarization direction of incident light is perpendicular to the incidence plane, it is found with increasing the incidence angle that the dipole resonance wavelength of TGNW shows a decrease, whereas the full-width at half-maximum (FWHM) for the dipole resonance peak decreases. With decreasing the shell thickness or increasing the dielectric constant of the inner core, the localized surface plasmon resonance (LSPR) of TGNW shows a distinct redshift, whereas the FWHM of the dipole peak increases. When the polarization direction of incident light is parallel to the incident plane, the LSPR in TGNW gradually appears with decreasing incidence angle and can be modulated by the geometry. We have ascribed the variations of the LSPR in TGNW to the competition between the variations of phase retardation and oscillation electrons.
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78.67.Uh Nanowires
77.22.Ch Permittivity (dielectric function)
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.22.Lp Collective excitations

Tuning energy levels in magnesium modified Alq3

Chien-Hung Lin, Chi-Ta Chou, Yu-Hung Chen, Meng-Hsiu Wu, Jiun-Haw Lee, Chih-I Wu, Surojit Chattopadhyay, Chin-Hsin J. Liu, Ker-Jar Song, Yian Tai, Juen-Kai Wang, Kuei-Hsien Chen, and Li-Chyong Chen

J. Appl. Phys. 109, 083541 (2011); http://dx.doi.org/10.1063/1.3573774 (3 pages) | Cited 2 times

Online Publication Date: 27 April 2011

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The low molecular weight tris-(8-hydroxyquinoline) aluminum (Alq3) has been modified with magnesium (Mg) incorporation that altered the nature of its opto-electronic characteristics. Ultraviolet photoelectron spectroscopy and optical absorption measurements yielded quantitative energy positions of the electronic states of the Mg:Alq3, its highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), and resultant shifts thereof due to the Mg incorporation. Consistent lowering of the HOMO and LUMO with Mg incorporation and a new distribution of the HOMO–LUMO separation are observed.
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71.35.Cc Intrinsic properties of excitons; optical absorption spectra
79.60.Fr Polymers; organic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Physical mechanism of quantum dot to quantum ring transformation upon capping process

Y. Y. Cao, X. L. Li, and G. W. Yang

J. Appl. Phys. 109, 083542 (2011); http://dx.doi.org/10.1063/1.3581961 (7 pages) | Cited 2 times

Online Publication Date: 27 April 2011

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A quantitative thermodynamic model has been established to investigate the evolution mechanism from quantum dot (QD) to quantum ring (QR) during the capping process. Taking the Ge/Si system as an example, we found that there is a critical concentration of Ge required in QDs in order for Si absorption by QDs to take place. When the Ge concentration of QDs decreases to that critical concentration during the Si capping process, Si atoms deposited on the surface of the QDs cannot be absorbed by the QDs and prefer to diffuse to the base side of the QDs. The accumulation of Si atoms on the base side of the QDs results in a great gradient of Ge composition on the surface of the QDs, which leads to the migration of Ge atoms from the top region of the QDs to the base side of the QDs. As a result, the QDs finally evolve into ringlike structures. Importantly, these theoretical results are in good agreement with experiments, which implies that our investigations not only offer general insight into the basic physics involved in QR formation during the capping process, but also provide useful information for experimenters.
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68.65.Hb Quantum dots (patterned in quantum wells)

Nanoscale silicon-on-insulator deformation induced by stressed liner structures

Conal E. Murray, A. Ying, S. M. Polvino, I. C. Noyan, M. Holt, and J. Maser

J. Appl. Phys. 109, 083543 (2011); http://dx.doi.org/10.1063/1.3579421 (4 pages) | Cited 5 times

Online Publication Date: 28 April 2011

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Rotation and strain fields were mapped across silicon-on-insulator (SOI) regions induced by overlying stressed Si3N4 features using x-ray nanobeam diffraction. The distribution in SOI tilt exhibited an antisymmetric distribution with a maximum magnitude of 7.9 milliradians, representing one of the first direct measurements of the lattice tilt conducted in situ within buried layers using a spot size of less than 100 nm. The measured rotation distribution corresponds to simulated values generated by boundary element method modeling, indicating that the strain transfer into the underlying SOI primarily induces elastic deformation.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
62.20.D- Elasticity
61.72.Dd Experimental determination of defects by diffraction and scattering

Tensile fracture criterion of metallic glass

R. T. Qu, J. Eckert, and Z. F. Zhang

J. Appl. Phys. 109, 083544 (2011); http://dx.doi.org/10.1063/1.3580285 (12 pages) | Cited 6 times

Online Publication Date: 28 April 2011

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The fracture criterion of metallic glass (MG) is still under debate so far. On the issue of describing the normal stress effect on the tensile fracture behaviors of MGs, there are two fracture criteria: the Mohr-Coulomb (M-C) criterion and the Ellipse criterion. In this study, the normal stress acting on the tensile fracture planes was controlled and varied in a wide range by introducing inclined notches with different inclination angles to a series of Zr-based MG specimens. Based on the experimental results, both the Ellipse criterion and the M-C criterion are compared and analyzed. The experimental results reveal that the Ellipse criterion gives a better prediction than the M-C criterion for the tensile fracture behavior of the investigated Zr-based MG in a wide normal stress range. Moreover, the pressure and normal stress effects were separately considered and some other criteria were further discussed. It is found that under tensile loading, the pressure effect is weak while the normal stress effect is strong. Combined with previously reported results for a variety of MGs, the Ellipse criterion is found to have the capability to describe all the different contributions of the normal stress effect and thus may be suitable as a unified tensile fracture criterion for all MGs with different fracture behaviors.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
62.20.mm Fracture
61.43.Fs Glasses

Structural phase transitions of barium halide nanocrystals in fluorozirconate glasses studied by Raman spectroscopy

C. Pfau, C. Bohley, P.-T. Miclea, and S. Schweizer

J. Appl. Phys. 109, 083545 (2011); http://dx.doi.org/10.1063/1.3580281 (6 pages) | Cited 4 times

Online Publication Date: 29 April 2011

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Rare-earth-doped fluorochlorozirconate (FCZ) and fluorobromozirconate (FBZ) glasses developed for fluorescence applications are analyzed, with particular attention paid to their phonon energy spectra. After thermal processing of as-made Eu-doped FCZ and FBZ glasses, Raman measurements show additional phonon bands at low phonon energies, indicating the formation of BaCl2 and BaBr2 nanocrystals, respectively. The phonon bands can be assigned to hexagonal, orthorhombic, or a mixture of both phases of BaCl2 and BaBr2 depending on the annealing conditions. Apart from line broadening, the Raman spectra of the orthorhombic nanocrystals agree well with those of BaCl2 and BaBr2 bulk crystals, while the metastable hexagonal phases of BaCl2 and BaBr2 are investigated only in appropriately treated FCZ and FBZ glasses, respectively. The experimental Raman spectra are compared to first principle studies of the phonon frequencies of the hexagonal and orthorhombic phases of both barium halides, showing good agreement.
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64.70.ph Nonmetallic glasses (silicates, oxides, selenides, etc.)
61.43.Fs Glasses
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
64.70.kj Glasses
63.50.Lm Glasses and amorphous solids
63.22.Kn Clusters and nanocrystals

Voids and Mn-rich inclusions in a (Ga,Mn)As ferromagnetic semiconductor investigated by transmission electron microscopy

A. Kovács, J. Sadowski, T. Kasama, J. Domagała, R. Mathieu, T. Dietl, and R. E. Dunin-Borkowski

J. Appl. Phys. 109, 083546 (2011); http://dx.doi.org/10.1063/1.3581108 (5 pages) | Cited 5 times

Online Publication Date: 29 April 2011

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Voids adjacent to cubic (ZnS-type) and hexagonal (NiAs-type) Mn-rich nanocrystals are characterized using aberration-corrected transmission electron microscopy in an annealed Ga0.995Mn0.005As magnetic semiconductor specimen grown by molecular beam epitaxy. Nanobeam electron diffraction measurements suggest that the nanocrystals exhibit deviations in lattice parameter as compared to bulk MnAs. After annealing at 903 K, the magnetic transition temperature of the specimen is likely to be dominated by the presence of cubic ferromagnetic nanocrystals. In situ annealing inside the electron microscope is used to study the nucleation, coalescence, and grain growth of individual nanocrystals.
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61.72.-y Defects and impurities in crystals; microstructure
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.75.-c Magnetic properties of nanostructures
61.46.-w Structure of nanoscale materials
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