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1 Nov 2007

Volume 102, Issue 9, Articles (09xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 102, 091101 (2007); http://dx.doi.org/10.1063/1.2804122 (21 pages)

Franky So, Benjamin Krummacher, Mathew K. Mathai, Dmitry Poplavskyy, Stelios A. Choulis, and Vi-En Choong
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Electron source concept for single-shot sub-100 fs electron diffraction in the 100 keV range

T. van Oudheusden, E. F. de Jong, S. B. van der Geer, W. P. E. M. Op ’t Root, O. J. Luiten, and B. J. Siwick

J. Appl. Phys. 102, 093501 (2007); http://dx.doi.org/10.1063/1.2801027 (8 pages) | Cited 45 times

Online Publication Date: 1 November 2007

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We present a method for producing sub-100 fs electron bunches that are suitable for single-shot ultrafast electron diffraction experiments in the 100 keV energy range. A combination of analytical estimates and state-of-the-art particle tracking simulations show that it is possible to create 100 keV, 0.1 pC, 30 fs electron bunches with a spot size smaller than 500 μm and a transverse coherence length of 3 nm, using established technologies in a table-top setup. The system operates in the space-charge dominated regime to produce energy-correlated bunches that are recompressed by radio-frequency techniques. With this approach we overcome the Coulomb expansion of the bunch, providing a single-shot, ultrafast electron diffraction source concept.
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07.77.Ka Charged-particle beam sources and detectors
07.78.+s Electron, positron, and ion microscopes; electron diffractometers
29.25.Bx Electron sources

Correlating exciton localization with compositional fluctuations in InGaN/GaN quantum wells grown on GaN planar surfaces and facets of GaN triangular prisms

S. Khatsevich, D. H. Rich, X. Zhang, and P. D. Dapkus

J. Appl. Phys. 102, 093502 (2007); http://dx.doi.org/10.1063/1.2802291 (8 pages) | Cited 2 times

Online Publication Date: 2 November 2007

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We have used spatially and temporally resolved cathodoluminescence (CL) to study the carrier recombination dynamics of InGaN quantum wells (QWs) grown on (0001)-oriented planar GaN and {1math01}-oriented facets of GaN triangular prisms prepared by lateral epitaxial overgrowth in a metal-organic chemical vapor deposition system. The effects of In migration during growth on the resulting QW thickness and composition were examined. We employed a variable temperature time-resolved CL imaging approach that enables a spatial correlation between regions of enhanced exciton localization, luminescence efficiency, and radiative lifetime with the aim of distinguishing between excitons localized in In-rich quantum dots and those in the surrounding Ga-rich QW regions.
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73.21.Fg Quantum wells
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.60.Hk Cathodoluminescence, ionoluminescence
78.67.De Quantum wells
68.47.Fg Semiconductor surfaces
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Crystallization kinetics and recording mechanism of a-Si/Ni bilayer for write-once blue-ray recording

Yung-Chiun Her, Sen-Tsun Jean, and Jyun-Lin Wu

J. Appl. Phys. 102, 093503 (2007); http://dx.doi.org/10.1063/1.2802992 (7 pages) | Cited 6 times

Online Publication Date: 2 November 2007

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For the a-Si/Ni bilayer with a thickness ratio of 4:1, the formation of NiSi and NiSi2 phases took place in the temperature range between 200 and 350 °C and only ∼ 9% of unreacted a-Si would crystallize to c-Si, which will not cause appreciable reflectivity change. As the thickness ratio of a-Si to Ni was increased to 20:1, the formation of NiSi2 phase and subsequent crystallization of a-Si mediated by NiSi2 precipitates were clearly observed. The crystallization temperature of a-Si in the a-Si/Ni bilayer with a thickness ratio of 20:1 was significantly reduced to around 350 °C, which was 130 °C lower than that in the a-Si/Cu bilayer. The activation energies for NiSi2 phase formation and crystallizations of a-Si for the a-Si/Ni bilayer with a thickness ratio of 20:1 were determined to be 1.12±0.09 and 2.19±0.08 eV, respectively. The crystallization behavior of the a-Si(20 nm)/Ni(1 nm) bilayer recording film under pulsed laser irradiation is similar to that under thermal annealing. During the recording process, the NiSi2 phase will precipitate first and serve as the nucleation sites for the following crystallization of the remaining amorphous Si. The maximum value of carrier to noise ratio for 3 T could reach 43 dB for the write-once blue-ray disk with layer structure of a-Si(20 nm)/Ni(1 nm), demonstrating a high potentiality for practical use.
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64.70.K- Solid-solid transitions
64.60.Q- Nucleation
64.75.-g Phase equilibria
81.30.Mh Solid-phase precipitation
81.15.Fg Pulsed laser ablation deposition

Recombination centers in as-grown and electron-irradiated ZnO substrates

N. T. Son, I. G. Ivanov, A. Kuznetsov, B. G. Svensson, Q. X. Zhao, M. Willander, N. Morishita, T. Ohshima, H. Itoh, J. Isoya, E. Janzén, and R. Yakimova

J. Appl. Phys. 102, 093504 (2007); http://dx.doi.org/10.1063/1.2802186 (5 pages) | Cited 7 times

Online Publication Date: 2 November 2007

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Optical detection of magnetic resonance (ODMR) was used to study defects in ZnO substrates irradiated with 3 MeV electrons at room temperature. The Zn vacancy and some other ODMR centers were detected. Among these, the Zn vacancy and two other centers, labeled as LU3 and LU4, were also commonly observed in different types of as-grown ZnO substrates. The LU3 and LU4 are related to intrinsic defects and act as dominating recombination centers in irradiated and as-grown ZnO.
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61.72.J- Point defects and defect clusters
61.80.Fe Electron and positron radiation effects
76.70.-r Magnetic double resonances and cross effects

Size-dependent excitation spectra and energy transfer in Tb3+-doped Y2O3 nanocrystalline

Qingyu Meng, Baojiu Chen, Wu Xu, Yanmin Yang, Xiaoxia Zhao, Weihua Di, Shaozhe Lu, Xiaojun Wang, Jiashi Sun, Lihong Cheng, Tao Yu, and Yong Peng

J. Appl. Phys. 102, 093505 (2007); http://dx.doi.org/10.1063/1.2803502 (6 pages) | Cited 18 times

Online Publication Date: 2 November 2007

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Nanocrystal Y2O3 powders with different grain sizes and various doping concentrations of Tb3+ were prepared by an autocombustion reaction. The size and surface effects on the 4f-5d transitions and energy transfers between Tb3+ ions were studied by using x-ray diffraction, transmission electron microscopy, fluorescent spectra, and luminescent decay. It was found that the excitation spectra are composed of two parts: one is the contribution from the Tb3+ at/near the nanoparticle surfaces; another is from the Tb3+ inside the nanoparticles. The study on the concentration quenching and luminescent decay indicated that the energy transfers depopulating the 5D3 and 5D4 level were assigned to the mechanisms of electric dipole-dipole and exchange interaction, respectively. The size confinement greatly affects the energy transfer quenching the emission from the 5D3 level, but slightly affects the energy transfer quenching the emission from the 5D4 level.
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32.50.+d Fluorescence, phosphorescence (including quenching)
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Crystallization kinetics and spectroscopic investigations on Tb3+ and Yb3+ codoped glass ceramics containing CaF2 nanocrystals

Lihui Huang, Guanshi Qin, Yusuke Arai, Rajan Jose, Takenobu Suzuki, Yasutake Ohishi, Tatsuya Yamashita, and Yusuke Akimoto

J. Appl. Phys. 102, 093506 (2007); http://dx.doi.org/10.1063/1.2802289 (8 pages) | Cited 17 times

Online Publication Date: 5 November 2007

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Transparent Tb3+ and Yb3+ codoped oxyfluoride glass ceramics containing CaF2 nanocrystals were prepared by melt quenching and subsequent heat treatment. Crystallization kinetics of CaF2 nanocrystals was investigated by differential scanning calorimetric method. The average apparent activation energy Ea of the crystallization was ∼ 498 kJ/mol. Moreover, the value of the Avrami exponent n was 1.01. These results suggest that the crystallization mechanism of CaF2 is a diffusion controlled growth process of needles and plates of finite long dimensions. X-ray diffraction patterns and transmission electron microscopy image confirmed the CaF2 nanocrystals in the glass ceramic. Ultraviolet (UV) and visible emission spectra of the as-made glass and the glass ceramic with an excitation of a 974 nm laser diode were recorded at room temperature. An intense UV emission at 381 nm was observed in the glass ceramic. The origin of the enhancement of the emission at 381 nm was investigated using spectroscopic technique and Judd-Ofelt analysis. The enhancement of the emission at 381 nm could be attributed to the change of the ligand field of Tb3+ ions due to the incorporation of some Tb3+ and Yb3+ ions into CaF2 nanocrystals in the glass ceramic.
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64.70.K- Solid-solid transitions
61.46.Hk Nanocrystals
81.16.-c Methods of micro- and nanofabrication and processing
81.05.Pj Glass-based composites, vitroceramics
66.30.H- Self-diffusion and ionic conduction in nonmetals
78.55.Hx Other solid inorganic materials

Photoluminescence-free photoreflectance spectra using dual frequency modulation

J. Plaza, D. Ghita, J. L. Castaño, and B. J. Garcia

J. Appl. Phys. 102, 093507 (2007); http://dx.doi.org/10.1063/1.2802991 (5 pages)

Online Publication Date: 5 November 2007

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Photoreflectance (PR) spectra are usually obtained by measuring the relative change on the reflectivity of a semiconducting sample induced by a chopped laser beam. The laser beam can also produce photoluminescence (PL) emission at the sample surface which, detected at its same frequency, could appear as an offset distorting the PR spectrum. This parasitic and intrinsically noisy PL signal, not easily discriminated electronically nor optically filtered, can become the dominant part of the PR spectrum at low sample temperatures, hiding spectrum features under its associated noise, or even avoiding data acquisition. An alternative method for PL signal discrimination is proposed in this work, using a different chopping frequency for each light beam: PL and reflected signals will appear each one at its own chopping frequency, while PR signal will be detected at its frequency sum, allowing signal separation by frequency. Both experimental setups are compared using a highly luminescent quantum well structure at low temperatures. While the standard setup suffers the PL limitation, the proposed method overcomes this constraint, allowing good quality spectra to be measured at temperatures as low as 12 K.
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07.60.Rd Visible and ultraviolet spectrometers
42.60.Fc Modulation, tuning, and mode locking
78.67.De Quantum wells

Grain size effect on the switching current in soft ferroelectric lead zirconate titanate

Talal M. Kamel and Gijsbertus de With

J. Appl. Phys. 102, 093508 (2007); http://dx.doi.org/10.1063/1.2803878 (3 pages)

Online Publication Date: 5 November 2007

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Recently, we reported on the appearance of a double peak in the switching current during the reverse poling. In the present paper, the switching current measurements have been carried out on a soft lead zirconate titanate as a function of grain size. While in small grains only a small single switching peak is observed, large grains, however, showed double peak switching, as commonly observed in this material. The pyroelectric coefficient curves show a consistent trend with the switching curves. This behavior is attributed to non-180° domain switching during the reversed poling case as a result of residual stresses developed during forward poling.
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77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
77.70.+a Pyroelectric and electrocaloric effects
77.80.Fm Switching phenomena
77.80.Dj Domain structure; hysteresis
77.22.Ej Polarization and depolarization

Micro-photoluminescence characterizations of GaInAsP/InP single quantum wires fabricated by dry etching and regrowth

Hirotake Itoh, Masahiro Yoshita, Hidefumi Akiyama, Dhanorm Plumwongrot, Takeo Maruyama, and Shigehisa Arai

J. Appl. Phys. 102, 093509 (2007); http://dx.doi.org/10.1063/1.2804107 (5 pages) | Cited 2 times

Online Publication Date: 5 November 2007

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We characterized the micro-photoluminescence (micro-PL) of GaInAsP/InP single quantum wires (Q-wires) at 6 K. The Q-wires had 6–39-nm lateral widths and a 6-nm vertical thickness and were fabricated by dry etching and regrowth. Micro-PL spectra were clearly observed for all single Q-wires. The spectra revealed a high degree of uniformity along the Q-wires. The PL peak energy showed a systematic blueshift with the reduction of the Q-wire lateral width. The blueshift was attributed to the lateral quantum confinement and strain from a lateral direction, and was 100 meV in a 6-nm-wide Q-wire. Systematic analysis on the PL widths of the Q-wires showed that the PL spectra were broadened by fluctuations of 3 nm in the lateral width of the Q-wire after the dry etching and the regrowth in addition to the broadening caused by the fluctuation in the vertical thickness during the initial film growth and that caused by ion bombardment through a Ti metal mask in the dry etching process. The decreased PL intensities in narrow Q-wires were also analyzed, and they were attributed to decreased PL quantum yield because of damage to the etched sidewalls of the Q-wires during dry etching, and to the absorption cross-sections of the excitation light.
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78.67.Lt Quantum wires
81.07.Vb Quantum wires
78.55.Cr III-V semiconductors
81.65.Cf Surface cleaning, etching, patterning

Defect behavior induced by helium cluster growth in titanium crystals

Jun Wang, Qing Hou, Tieying Sun, Xinggui Long, Xingchun Wu, and Shunzhong Luo

J. Appl. Phys. 102, 093510 (2007); http://dx.doi.org/10.1063/1.2804110 (4 pages) | Cited 6 times

Online Publication Date: 5 November 2007

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The growth of helium cluster in titanium crystals is simulated in great detailed approach using molecular dynamics. We observe that, as the helium cluster grows, defects around the cluster are formed and the local pressure increases. However, at certain point in the growth process, the defects are found to rapidly escape as a whole from the helium cluster, accompanied by the relief of local high pressure and the recovery of Ti crystal structure around the helium cluster.
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61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling
62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions

Toward the jamming threshold of sphere packings: Tunneled crystals

S. Torquato and F. H. Stillinger

J. Appl. Phys. 102, 093511 (2007); http://dx.doi.org/10.1063/1.2802184 (8 pages) | Cited 18 times

Online Publication Date: 6 November 2007

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We have discovered a family of three-dimensional crystal sphere packings that are strictly jammed (i.e., mechanically stable) and yet possess an anomalously low density. This family constitutes an uncountably infinite number of crystal packings that are subpackings of the densest crystal packings and are characterized by a high concentration of self-avoiding “tunnels” (chains of vacancies) that permeate the structures. The fundamental geometric characteristics of these tunneled crystals command interest in their own right and are described here in some detail. These include the lattice vectors (that specify the packing configurations), coordination structure, Voronoi cells, and density fluctuations. The tunneled crystals are not only candidate structures for achieving the jamming threshold (lowest-density rigid packing), but may have substantially broader significance for condensed matter physics and materials science.
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61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling
61.72.J- Point defects and defect clusters

Characterization of recompressed spall in copper gas gun targets

R. Becker, M. M. LeBlanc, and J. U. Cazamias

J. Appl. Phys. 102, 093512 (2007); http://dx.doi.org/10.1063/1.2802589 (8 pages)

Online Publication Date: 6 November 2007

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Complementary experiments and simulations are conducted to characterize the microstructure and mechanisms involved in recompression of spalled ductile metals. Soft capture experiments performed on copper targets in a gas gun include a dense secondary plate spaced behind the customary flyer to recompress the voids in the wake of the spall induced by the flyer. Control experiments are run without the secondary plate to obtain spall damage without recompression. The simulations feature explicit representation of void nucleating particles in a narrow strip of material spanning the flyer package and target. Analysis of the spall closure in the simulations reveals the void collapse mechanisms and the origin of features observed experimentally. The experiments and simulations show little trace of the prior voids, and a thin ribbon of highly strained material is the only readily observable remnant of the spall surface.
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81.40.Lm Deformation, plasticity, and creep
61.72.-y Defects and impurities in crystals; microstructure
62.20.F- Deformation and plasticity
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Growth stresses in α-Cr2O3 thermal oxide films determined by in situ high temperature Raman spectroscopy

M. Kemdehoundja, J. L. Grosseau-Poussard, J. F. Dinhut, and B. Panicaud

J. Appl. Phys. 102, 093513 (2007); http://dx.doi.org/10.1063/1.2805638 (5 pages) | Cited 10 times

Online Publication Date: 7 November 2007

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Growth stresses have been investigated in relation with the microstructure in the case of α-Cr2O3 growing oxide films on NiCr30 alloy. The equibiaxial growth stresses have been measured thanks to a technique coupling Raman spectroscopy and in situ high temperature oxidation of the NiCr30 alloy in the temperature range (700−900 °C). It is established that the compressive growth stress in such oxide films can reach more than 2 GPa, before additional thermal stress arises on cooling. Moreover, the growth stress kinetics—subsequent establishment and relaxation—are highly microstructure sensitive: in particular, as the oxidation temperature rises, the chromia mean grain size also increases, and it consequently retards the occurrence of the creep relaxation phenomena which needs an additional stress level to start.
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68.60.Bs Mechanical and acoustical properties
78.30.Hv Other nonmetallic inorganics
81.40.Lm Deformation, plasticity, and creep
62.20.Hg Creep

Vacuum ultraviolet spectroscopic properties of rare earth (RE = Ce,Tb,Eu,Tm,Sm)-doped hexagonal KCaGd(PO4)2 phosphate

Z. J. Zhang, J. L. Yuan, C. J. Duan, D. B. Xiong, H. H. Chen, J. T. Zhao, G. B. Zhang, and C. S. Shi

J. Appl. Phys. 102, 093514 (2007); http://dx.doi.org/10.1063/1.2800172 (7 pages) | Cited 11 times

Online Publication Date: 7 November 2007

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Hexagonal KCaGd(PO4)2:RE3+ (RE = Ce,Tb,Eu,Tm,Sm) were synthesized by coprecipitation method and their vacuum ultraviolet–ultraviolet (VUV-UV) spectroscopic properties were investigated. The bands at about 165 nm in the VUV excitation spectra are attributed to the host lattice absorptions. For Ce3+-doped samples, the bands at 207, 256, 275, and 320 nm are assigned to the 4f-5d transitions of Ce3+ in KCaGd(PO4)2. For Tb3+-doped sample, the bands at 203 and 222 nm are related to the 4f-5d spin-allowed transitions. For Eu3+-doped sample, the O2−–Eu3+ charge-transfer band (CTB) at 229 nm is observed, and the fine emission spectrum of Eu3+ indicates that Eu3+ ions prefer to occupy Gd3+ or Ca2+ sites in the host lattice. For Tm3+- and Sm3+-doped samples, the O2−–Tm3+ and O2−–Sm3+ CTBs are observed to be at 176 and 186 nm, respectively. From the standpoints of the absorption band, color purity, and luminescent intensity, Tb3+-doped KCaGd(PO4)2 is a potential candidate for 172 nm excited green plasma display phosphors.
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78.40.Ha Other nonmetallic inorganics
78.55.Hx Other solid inorganic materials
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions

Correlation between microstructure and optical properties of ZnO nanoparticles synthesized by ball milling

P. K. Giri, S. Bhattacharyya, Dilip K. Singh, R. Kesavamoorthy, B. K. Panigrahi, and K. G. M. Nair

J. Appl. Phys. 102, 093515 (2007); http://dx.doi.org/10.1063/1.2804012 (8 pages) | Cited 19 times

Online Publication Date: 7 November 2007

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Zinc oxide (ZnO) nanoparticles (NPs) in the size range ∼ 7–35 nm are synthesized by ball-milling technique, and microstructural and optical properties of the NPs are studied using varieties of techniques. Results from ball-milled NPs are compared with those of the commercially available ZnO nanopowder. X-ray diffraction pattern of the milled NPs indicates lattice strain in the NPs. High-resolution transmission electron microscopy analysis reveal severe lattice distortion and reduction in lattice spacing in some of the NPs. Optical absorption spectra of milled NPs show enhanced absorption peaked at 368 nm, which is blueshifted with reference to starting ZnO powder. Room-temperature photoluminescence spectra show five peaks consisting of ultraviolet and visible bands, and relative intensity of these peaks drastically changes with increasing milling time. Raman spectra of milled powders show redshift and broadening of the Raman modes of ZnO, and a new Raman mode evolve in the milled NPs. A correlation between the microstructure and optical properties of ZnO NPs is made on the basis of these results. Our results clearly demonstrate that commercially available ZnO nanopowders do not exhibit nanosize effects due to relatively large size of the ZnO NPs. Implications of these results are discussed.
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81.20.Wk Machining, milling
81.07.Wx Nanopowders
81.07.Bc Nanocrystalline materials
81.05.Dz II-VI semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Et II-VI semiconductors
78.30.Fs III-V and II-VI semiconductors

Electromigration behavior of 60 nm dual damascene Cu interconnects

Jung Woo Pyun, Won-Chong Baek, Lijuan Zhang, Jay Im, Paul S. Ho, Larry Smith, and Gregory Smith

J. Appl. Phys. 102, 093516 (2007); http://dx.doi.org/10.1063/1.2805425 (5 pages) | Cited 2 times

Online Publication Date: 8 November 2007

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Electromigration (EM) reliability was investigated for Cu fine lines fabricated using a SiON trench filling process down to 60 nm in linewidth. EM was observed to be dominated by intrinsic failures due to void formation in the line trench. The lifetimes of 60 nm lines were longer than those of 125 nm lines with the standard damascene structure which can be attributed to a distinct via/metal-1 configuration with less process-induced defects at the via interface. The line scaling effect on EM reliability was investigated using three linewidths: 60, 110, and 185 nm. EM lifetimes were found to be similar for different linewidths, consistent with intrinsic failures caused by void formation in the line trench driven by interfacial mass transport. Statistical, multilinked EM test structures demonstrated a monomodal failure distribution for the fine lines, indicating suppression of processing-induced extrinsic defects although processing control on line dimension and geometry remained an issue. The activation energies were found to be around 0.80 eV for both fine lines and standard lines, indicating that interface diffusion dominates mass transport in Cu lines.
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66.30.Qa Electromigration
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Optical bandgap and photoconductance of electrospun tin oxide nanofibers

Yu Wang, Idalia Ramos, and Jorge J. Santiago-Avilés

J. Appl. Phys. 102, 093517 (2007); http://dx.doi.org/10.1063/1.2800261 (5 pages) | Cited 5 times

Online Publication Date: 12 November 2007

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Optical and photoconductive properties of transparent SnO2 nanofibers, made from C22H44O4Sn via electrospinning and metallorganic decomposition, were investigated using Fourier transform infrared and ultraviolet (UV)/visible spectrometry and the two-probe method. Their optical bandgap was determined from their UV absorption edge to be 3.95–4.08 eV. Their conductance responds strongly to UV light for a wavelength of 254 nm: in air its steady-state on-to-off ratios are 1.31–1.56 (rise) and 1.25–1.33 (fall); its 90% rise and fall times are 76–96 and 71–111 s, respectively. In a vacuum of about 10−4 torr, its on-to-off ratios are higher than 35.6 (rise) and 3.4 (fall), respectively, and its 90% rise and fall times are longer than 3×104 s.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
73.63.Bd Nanocrystalline materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
72.40.+w Photoconduction and photovoltaic effects
78.30.Hv Other nonmetallic inorganics
78.40.Ha Other nonmetallic inorganics

Computer simulation and experimental study of elastic properties of amorphous Cu-Zr alloys

M. I. Mendelev, D. K. Rehbein, R. T. Ott, M. J. Kramer, and D. J. Sordelet

J. Appl. Phys. 102, 093518 (2007); http://dx.doi.org/10.1063/1.2805655 (8 pages) | Cited 22 times

Online Publication Date: 13 November 2007

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Molecular-dynamics simulations were performed to determine the elastic constants of CuxZr100−x (33.3 ≤ x ≤ 64.5) metallic glasses at room temperature. The accuracy of the interatomic potentials used to obtain the model glass structures was tested by comparing to the total structure factors obtained from high-energy synchrotron x-ray diffraction and, more importantly, to acoustic velocities measured from melt spun ribbons. Both the simulated and measured acoustic velocities increased at comparable rates with increasing Cu concentration, but the former underestimated the latter by about 10%. Young’s moduli of the simulated models were determined by combining the ultrasonic data with densities that were obtained from simulations. In addition, the compositional dependence of Poisson’s ratio, shear modulus, and bulk modulus for this series of simulated metallic glasses was determined. Examination of partial-pair correlations deduced from simulated glass structures shows a correlation between higher bulk moduli in Cu-rich compositions and concomitant changes in Zr-Zr nearest neighbors, which exhibit a stronger sensitivity to an imposed hydrostatic stress than do Cu-Cu or Cu-Zr nearest-neighbor distances.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
61.43.Fs Glasses
62.65.+k Acoustical properties of solids

Crystalline phases in the GeSb2Te4 alloy system: Phase transitions and elastic properties

T. Blachowicz, M. G. Beghi, G. Güntherodt, B. Beschoten, H. Dieker, and M. Wuttig

J. Appl. Phys. 102, 093519 (2007); http://dx.doi.org/10.1063/1.2809355 (7 pages) | Cited 7 times

Online Publication Date: 13 November 2007

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Chalcogenide alloys are attracting considerable interest as phase-change materials. Their elastic properties are investigated here. A film of stoichiometric GeSb2Te4 is amorphous as deposited; under annealing at increasing temperatures it crystallizes into a cubic phase and then into a hexagonal phase. The elastic properties of the three phases have been investigated nondestructively by Brillouin light scattering. Measurements detect frequency dispersion relations of Rayleigh and Sezawa acoustic modes. All the elastic moduli can be derived: they increase, together with mass density, upon transition from the amorphous to the cubic and to the hexagonal phase. The measured moduli provide a benchmark for computational predictions of film properties.
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61.43.Fs Glasses
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Gh Other heat and thermomechanical treatments

Glide and multiplication of basal plane dislocations during 4H‐SiC homoepitaxy

X. Zhang, M. Skowronski, K. X. Liu, R. E. Stahlbush, J. J. Sumakeris, M. J. Paisley, and M. J. O’Loughlin

J. Appl. Phys. 102, 093520 (2007); http://dx.doi.org/10.1063/1.2809343 (8 pages) | Cited 13 times

Online Publication Date: 14 November 2007

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Basal plane dislocations (BPDs) are an important category of extended defects in SiC epilayers. They act as nucleation sites for single layer Shockley-type stacking faults which account for the degradation of the bipolar devices operating under forward bias. It is well documented that most of the BPDs in the SiC epilayers propagate from the substrates. However, two characteristic types of BPDs were suggested to be due to either nucleation or multiplication during epitaxy, including interfacial dislocations and short BPD arrays connected to the epilayer surface by threading segments. Combining molten KOH etching, plan-view transmission x-ray topography, and photoluminescence mapping, both types are determined to be two parts of one defect produced by the sideway glide of a BPD under the influence of shear stress. During the glide, the down-step end of the BPD frequently produces a series of short BPD segments at the moving growth front. These BPD segments will grow into an array of dislocation half loops. At the same time, the sideway glide of the BPD in the epilayer leaves an edge-type BPD segment at the epilayer∕substrate interface, which is the interfacial dislocation. The defect morphology provides the evidence of significant level of shear stresses present in SiC homoepitaxy of typical power device structures. The magnitude of such stresses is estimated.
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61.72.Qq Microscopic defects (voids, inclusions, etc.)
78.55.Et II-VI semiconductors
68.37.Yz X-ray microscopy
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