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1 Aug 2008

Volume 104, Issue 3, Articles (03xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 104, 031101 (2008); http://dx.doi.org/10.1063/1.2959429 (11 pages)

B. S. Kang, H. T. Wang, F. Ren, and S. J. Pearton
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Surface relaxation effect on the distributions of energy and bulk stresses in the vicinity of Cu surface: An embedded-atom method study

Z. H. Sun, X. X. Wang, and H. A. Wu

J. Appl. Phys. 104, 033501 (2008); http://dx.doi.org/10.1063/1.2958330 (6 pages) | Cited 2 times

Online Publication Date: 1 August 2008

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Molecular statics simulations combined with an embedded-atom method potential were performed to calculate the distributions of energy and bulk stresses in crystalline and amorphous Cu slabs and to examine the effect of surface relaxation on the distributions of energy and bulk stresses in the surface region. The results reveal that a release of energy and bulk stresses in the surface region occurs upon surface relaxation. The profile of energy release upon surface relaxation of a crystalline Cu slab exhibits oscillatory damping from the topmost layer to the interior layers, while that of an amorphous Cu slab exhibits monotonic decreasing. A great diversity of patterns was observed in the profiles of bulk stresses released upon surface relaxation, which may exhibit anisotropy according to the symmetry of the surface considered. Both the profiles of energy release and bulk stress release exhibit surface-orientation dependence. The present results may provide useful information to analyze different phenomena at Cu surfaces.
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81.05.Bx Metals, semimetals, and alloys
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
68.35.Gy Mechanical properties; surface strains
68.35.bd Metals and alloys

Electrical, thermal, and elastic properties of the MAX-phase Ti2SC

T. H. Scabarozi, S. Amini, P. Finkel, O. D. Leaffer, J. E. Spanier, M. W. Barsoum, M. Drulis, H. Drulis, W. M. Tambussi, J. D. Hettinger, and S. E. Lofland

J. Appl. Phys. 104, 033502 (2008); http://dx.doi.org/10.1063/1.2959738 (5 pages) | Cited 11 times

Online Publication Date: 1 August 2008

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We report on the electronic, thermal, and elastic properties of the layered ternary, Ti2SC. Resistivity, Hall effect, and magnetoresistance were measured as a function of temperature between 2 and 300 K and at fields up to 9 T. The Hall coefficient is negative and roughly temperature independent. The transport results were analyzed within a two-band framework, with electrons as the dominant charge carrier. The room-temperature thermal conductivity ( ≈ 60 W/m K) is the highest of any MAX phase measured to date, with a substantial phonon contribution. The specific heat was measured from 2 to 300 K, yielding a Debye temperature of 765 K and in agreement with the Debye temperature of 745 K found from ultrasonic time-of-flight measurements. Young’s, shear, and bulk moduli from the latter measurements were 290, 125, and 145 GPa, respectively. The calculated values of the lattice parameters (a = 3.2051 Å and c = 11.2636 Å), and Young’s, shear, and bulk moduli (329, 138, and 179 GPa, respectively), based on the results of density functional theoretical simulations, compare favorably with measurements.
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72.20.My Galvanomagnetic and other magnetotransport effects
63.20.-e Phonons in crystal lattices
65.40.Ba Heat capacity
62.20.de Elastic moduli
81.40.Jj Elasticity and anelasticity, stress-strain relations
61.66.Fn Inorganic compounds

Direct observations of thermally induced structural changes in amorphous silicon carbide

Manabu Ishimaru, Akihiko Hirata, Muneyuki Naito, In-Tae Bae, Yanwen Zhang, and William J. Weber

J. Appl. Phys. 104, 033503 (2008); http://dx.doi.org/10.1063/1.2960342 (5 pages) | Cited 8 times

Online Publication Date: 1 August 2008

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Thermally induced structural relaxation in amorphous silicon carbide (SiC) has been examined by means of in situ transmission electron microscopy (TEM). The amorphous SiC was prepared by high-energy ion beam irradiation into a single crystalline 4H-SiC substrate. Cross-sectional TEM observations and electron energy-loss spectroscopy measurements revealed that thermal annealing induces a remarkable volume reduction, so-called densification, of amorphous SiC. From radial distribution function analyses using electron diffraction, notable changes associated with structural relaxation were observed in chemical short-range order. It was confirmed that the structural changes observed by the in situ TEM study agree qualitatively with those of the bulk material. On the basis of the alteration of chemical short-range order, we discuss the origin of thermally induced densification in amorphous SiC.
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64.70.kg Semiconductors
61.43.Dq Amorphous semiconductors, metals, and alloys
61.80.Jh Ion radiation effects
61.82.Fk Semiconductors
61.72.Cc Kinetics of defect formation and annealing
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
79.20.Uv Electron energy loss spectroscopy

Classical disordered ground states: Super-ideal gases and stealth and equi-luminous materials

Robert D. Batten, Frank H. Stillinger, and Salvatore Torquato

J. Appl. Phys. 104, 033504 (2008); http://dx.doi.org/10.1063/1.2961314 (12 pages) | Cited 12 times

Online Publication Date: 1 August 2008

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Using a collective coordinate numerical optimization procedure, we construct ground-state configurations of interacting particle systems in various space dimensions so that the scattering of radiation exactly matches a prescribed pattern for a set of wave vectors. We show that the constructed ground states are, counterintuitively, disordered (i.e., possess no long-range order) in the infinite-volume limit. We focus on three classes of configurations with unique radiation scattering characteristics: (i) “stealth” materials, which are transparent to incident radiation at certain wavelengths; (ii) “super-ideal” gases, which scatter radiation identically to that of an ensemble of ideal gas configurations for a selected set of wave vectors; and (iii) “equi-luminous” materials, which scatter radiation equally intensely for a selected set of wave vectors. We find that ground-state configurations have an increased tendency to contain clusters of particles as one increases the prescribed luminosity. Limitations and consequences of this procedure are detailed.
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51.10.+y Kinetic and transport theory of gases
02.50.-r Probability theory, stochastic processes, and statistics

The influences of substrate and metal properties on the magnetic response of metamaterials at terahertz region

Xiaoxiang Xia, Yimin Sun, Haifang Yang, Hui Feng, Li Wang, and Changzhi Gu

J. Appl. Phys. 104, 033505 (2008); http://dx.doi.org/10.1063/1.2961327 (5 pages) | Cited 2 times

Online Publication Date: 1 August 2008

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The metamaterials (MMs) with planar metal arrays of split ring resonators were fabricated on quartz and silicon substrates; the influences of substrate permittivity and metal conductance on the magnetic responses of in the terahertz region were investigated. The electromagnetic (EM) responses of the metamaterials were characterized by terahertz time-domain spectroscopy. The experimental result reveals that there is a 0.39 THz redshift in the transmission spectra for the magnetic response when the substrate changes from quartz to silicon, which is in consistence with the finite-difference time-domain (FDTD) simulation. Based on the simulation and experimental results, and taking into account the EM properties of substrate materials, a modified L-C model is presented to understand the MM system in terahertz domain.
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84.40.-x Radiowave and microwave (including millimeter wave) technology
84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
41.20.Jb Electromagnetic wave propagation; radiowave propagation
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Nucleation energetics during homogeneous solidification in elemental metallic liquids

Ramki Kalyanaraman

J. Appl. Phys. 104, 033506 (2008); http://dx.doi.org/10.1063/1.2961329 (7 pages) | Cited 1 time

Online Publication Date: 1 August 2008

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The solidification of a liquid by nucleation is an important first order phase transition process. It is known that in order for elemental liquids to solidify homogeneously, it is necessary to supercool the liquid to a characteristic temperature (TUC) below the thermodynamic melting point (TMP). Approximately 60 years ago Turnbull [J. Appl. Phys. 21, 1022 (1950) ] established the empirical rule that ΔT∗ = |TUC−TMP| is approximately given by 0.18 TMP for several elemental metallic liquids. We show here that the magnitude of ΔT∗ and TUC for the metals can be accurately predicted from classical nucleation theory (CNT) provided the excess volume resulting from the density difference between liquid and solid be accounted for. Specifically, the density change accompanying the formation of a microscopic nucleus of the solid from the liquid results in a volume change in the surrounding liquid. When this is included in the free energy calculations within CNT, the resulting predictions for ΔT∗ and TUC for several metals with TMP ranging from ∼ 200 to 2900 K are in very good agreement with experimental measurements. This theory also shows that there is a universal character in the minimum nucleation barrier energy and the critical radius. The minimum barrier energy occurs at temperature TN ∼ 0.27 TMP for all the elemental liquids investigated, while the temperature dependencies of the barrier energy and the critical radius appear identical when expressed as a function of the scaled temperature TUC/TMP.
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64.60.qj Studies of nucleation in specific substances
65.20.Jk Studies of thermodynamic properties of specific liquids
64.70.dj Melting of specific substances

Effect of off stoichiometry on Raman scattering from epitaxial and polycrystalline HfNx (0.85 ≤ x ≤ 1.50) grown on MgO(001)

M. Stoehr, H.-S. Seo, I. Petrov, and J. E. Greene

J. Appl. Phys. 104, 033507 (2008); http://dx.doi.org/10.1063/1.2961332 (7 pages) | Cited 1 time

Online Publication Date: 1 August 2008

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Group-IV HfNx transition-metal nitride layers, with 0.85 ≤ x ≤ 1.50, are grown on MgO(001) and analyzed by Raman spectroscopy. Samples with 0.85 ≤ x ≤ 1.20 are single-phase epitaxial NaCl-structure HfNx(001), while layers with higher N concentrations are two-phase mixtures containing N-rich inclusions. All samples exhibit both first- and second-order Raman scattering. The presence of first-order scattering indicates that Oh symmetry, a characteristic of the NaCl crystal structure, is broken even in stoichiometric HfN. As x decreases in understoichiometric HfNx samples, corresponding to a decrease in the number of valence electrons per unit cell from nine (x = 1) to eight (x = 0.75), the positions of the acoustic lines shift to higher frequencies. This provides an indirect probe of phonon anomalies in acoustic mode dispersion curves and signifies an increase in N vacancy concentration. The persistence of strong first-order acoustic scattering from overstoichiometric (x>1) samples indicates that adding more N does not substantially reduce the average defect density around Hf sites. The absence of a frequency shift in the acoustic modes of overstoichiometric HfNx, with 1.17 ≤ x ≤ 1.27, also shows that the local order around Hf sites does not change significantly, while the observed shift of the first-order optical modes to higher frequency with increasing x is characteristic of increasing lattice disorder adjacent to N sites. The Raman results, together with the nature of the variation in the HfNx lattice parameter, the electron carrier concentration, and the superconducting transition temperature as a function of x, can be explained by the presence of N vacancies for x<1, both N and Hf vacancies in stoichiometric HfN(001), and the combination of N vacancies and NHf antisite defects for x>1.
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74.78.-w Superconducting films and low-dimensional structures
74.70.-b Superconducting materials other than cuprates
74.25.Gz Optical properties
74.25.F- Transport properties
74.25.Kc Phonons
61.72.jd Vacancies

Optical pump-probe measurements of sound velocity and thermal conductivity of hydrogenated amorphous carbon films

J. L. Arlein, S. E. M. Palaich, B. C. Daly, P. Subramonium, and G. A. Antonelli

J. Appl. Phys. 104, 033508 (2008); http://dx.doi.org/10.1063/1.2963366 (6 pages) | Cited 8 times

Online Publication Date: 1 August 2008

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In this paper, we report the measurements of the thermal conductivity and longitudinal sound velocity of four types of hydrogenated amorphous carbon films. The measurements were made with an ultrafast optical pump-probe apparatus. The films were deposited at various temperatures by plasma enhanced chemical vapor deposition. The films are of a low density (1.0–1.4 g cm−3) and are found to have sound velocities that range from as low as 3 km/s to as high as 10 km/s. The thermal conductivities are between 0.6 and 1.4 W/mK which are low compared to amorphous SiO2 but are larger than published measurements of thin films of similar composition and density by a factor of 2–3.
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81.05.U- Carbon/carbon-based materials
81.05.Gc Amorphous semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.47.J- Ultrafast spectroscopy (<1 psec)
62.65.+k Acoustical properties of solids
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves

Competition between InP and In2O3 islands during the growth of InP on SrTiO3

G. Saint-Girons, P. Regreny, J. Cheng, G. Patriarche, L. Largeau, M. Gendry, G. Xu, Y. Robach, C. Botella, G. Grenet, and G. Hollinger

J. Appl. Phys. 104, 033509 (2008); http://dx.doi.org/10.1063/1.2963484 (4 pages) | Cited 2 times

Online Publication Date: 1 August 2008

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A study of the growth of InP islands on SrTiO3 (STO) substrates is presented. The nature and crystal orientation of the islands strongly depend on the growth temperature: below 410 °C, both InP and In2O3 islands coexist, while InP islands alone are formed above this temperature. InP islands are randomly oriented in the low growth temperature range and adopt an equilibrium orientation defined by [111]InP∥[001]STO in the growth direction and [110]InP∥[100]STO in the growth plane between 410 and 475 °C. This study highlights the complexity of the growth of InP on STO, which results from a combined influence of interface chemistry and crystallographic properties as well as of the nucleation kinetics.
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81.05.Ea III-V semiconductors
68.55.A- Nucleation and growth

Graphitization of nanocrystalline carbon microcoils synthesized by catalytic chemical vapor deposition

Hui Bi, Kai-Chang Kou, Kostya (Ken) Ostrikov, and Jiao-Qiang Zhang

J. Appl. Phys. 104, 033510 (2008); http://dx.doi.org/10.1063/1.2963712 (6 pages) | Cited 4 times

Online Publication Date: 1 August 2008

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Graphitization, a common process involving the transformation of metastable nongraphitic carbon into graphite is one of the major present-day challenges for micro- and nanocarbons due to their unique structural character and highly unusual thermal activation. Here we report on the successful graphitization of nanocrystalline carbon microcoils prepared by catalytic chemical vapor deposition and post-treated in argon atmosphere at temperatures ∼ 2500 °C for 2 h. The morphology, microstructure, and thermal properties of the carbon microcoils are examined in detail. The graphitization mechanism is discussed by invoking a model of structural transformation of the carbon microcoils. The results reveal that after graphitization the carbon microcoils are prominently purified and feature a clear helical morphology, as well as a more regular and ordered microstructure. The interlayer spacing of the carbon microcoils decreases from 0.36 to 0.34 nm, whereas the mean crystal sizes in the c- and a-directions increase from 1.64 to 2.04 nm and from 3.86 to 7.21 nm, respectively. Thermal treatment also substantially improves the antioxidation properties of the microcoils by lifting the oxidation onset temperature from 550 to 672 °C. This process may be suitable for other nongraphitic micro- and nanomaterials.
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81.16.Hc Catalytic methods
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
64.70.Nd Structural transitions in nanoscale materials
61.46.-w Structure of nanoscale materials
64.70.K- Solid-solid transitions
81.16.Pr Micro- and nano-oxidation

MgO-codoping effects on the spectroscopic properties of Er3+-doped LiNbO3

Ai-Hua Li, Zhi-Ren Zheng, Liang Sun, Qiang Lü, Wei-Long Liu, Wen-Zhi Wu, Yan-Qiang Yang, and Tian-Quan Lü

J. Appl. Phys. 104, 033511 (2008); http://dx.doi.org/10.1063/1.2957060 (6 pages) | Cited 5 times

Online Publication Date: 1 August 2008

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Optical absorption, spectrally resolved polarized upconversion fluorescence, and time-resolved luminescence spectra are used to investigate the Er3+ spectroscopic properties in a series of congruent LiNbO3 crystals codoped with Er3+ (1 mol %) and MgO (X mol %, X = 0, 2, 4, 6, and 8). The absorption spectra indicate that the transition cross section of Er3+ ions decreases with increasing MgO concentration; it is explained as the improvement of the Er3+-site symmetries. A new Er3+ energy site with low energy, which appears in the long wavelength side of the polarized fluorescence spectra for the samples heavily codoped with MgO, is demonstrated in the time-resolved luminescence spectra. An important finding is that mild codoping with MgO clearly does not facilitate the formation of clustered sites, whereas heavy codoping clearly does facilitate it in Er3+-doped LiNbO3 crystals. These results are important to those who consider the material as a 1.5 μm laser or fiber optical cable.
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78.55.Hx Other solid inorganic materials
78.47.jd Time resolved luminescence
61.72.up Other materials

Phase-field simulation of thermal conductivity in porous polycrystalline microstructures

Paul C. Millett, Dieter Wolf, Tapan Desai, Srujan Rokkam, and Anter El-Azab

J. Appl. Phys. 104, 033512 (2008); http://dx.doi.org/10.1063/1.2964116 (6 pages) | Cited 8 times

Online Publication Date: 1 August 2008

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Mesoscale computer simulations are used to study the effective thermal conductivity of two-dimensional polycrystalline model microstructures containing finely dispersed stationary voids. The microstructural evolution is captured by phase-field modeling in which the competing mechanisms of curvature-driven grain-boundary (GB) migration and Zener pinning due to the void/grain-boundary interactions control the grain-growth kinetics. We investigate porosity fractions between 0% and 8% by systematically increasing the number of voids in the simulation cell. The temperature distribution throughout the microstructure at progressive instances in time is calculated by solving the solid-state heat-conduction equation. The thermal conductivity of each grid point is assigned a value according to the microstructural feature it represents (grain interiors, GBs, and voids) as determined by the phase-field order parameters. The effective conductivities of the microstructures are analyzed with respect to average grain size as well as porosity fraction, and good agreement with theoretical models is obtained.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
61.43.Gt Powders, porous materials
61.72.Mm Grain and twin boundaries
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Dislocation nucleation from a sharp corner in silicon

Satoshi Izumi and Sidney Yip

J. Appl. Phys. 104, 033513 (2008); http://dx.doi.org/10.1063/1.2963487 (4 pages) | Cited 16 times

Online Publication Date: 4 August 2008

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By combining molecular dynamics simulation with reaction pathway sampling, we have observed the nucleation of a three-dimensional dislocation loop from a sharp corner in silicon and investigated the shear stress dependence of the activation energy and saddle-point configuration. The nucleated shuffle-set half-loop consisted of two 60° segments and one screw segment, each lying along a Peierls valley. The half-hexagonal shape is in good agreement with experiments at low temperature. Under high stress (90%–95% of athermal shear stress), the dislocation embryo is far from perfect, with half-size Burgers vector (about 0.2 nm) and a 0.4–0.7 nm radius forming a diffuse core region. A consequence is that the Rice–Thompson theory gives incorrect predictions regarding the activation energy and saddle-point configuration. With decreasing applied stress (less than 70%), the embryo approaches that of a perfect dislocation.
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61.72.Bb Theories and models of crystal defects

Full-spectrum phonon relaxation times in crystalline Si from molecular dynamics simulations

Hong Zhao and Jonathan B. Freund

J. Appl. Phys. 104, 033514 (2008); http://dx.doi.org/10.1063/1.2963721 (6 pages) | Cited 6 times

Online Publication Date: 4 August 2008

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The lattice vibrations of a periodic silicon crystal are simulated by a molecular dynamics simulation. The history of atomic displacements and velocities obtained is used to compute the amplitudes of all phonon modes supported by the lattice. The phonon amplitude autocorrelation coefficients are found to decay exponentially in time, in agreement with single-mode relaxation time models for phonon scattering. However, the relaxation times extracted from the correlation curves are found to differ significantly from the empirical formulas currently used in models for microscale thermal transport, suggesting that improved relaxation time models are needed for accurate predictions in complex small-scale heat transfer systems.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Bandgap renormalization in titania modified nanostructured tungsten oxide thin films prepared by pulsed laser deposition technique for solar cell applications

K. J. Lethy, D. Beena, V. P. Mahadevan Pillai, and V. Ganesan

J. Appl. Phys. 104, 033515 (2008); http://dx.doi.org/10.1063/1.2953070 (12 pages) | Cited 13 times

Online Publication Date: 5 August 2008

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Pure and titania modified WO3 films are prepared using pulsed laser ablation technique in an oxygen ambient of 0.12 mbar at a substrate temperature of 873 K. Titania incorporation effects on the microstructure, optical, and electrical properties on the tungsten oxide films are systematically investigated using techniques such as x-ray diffraction, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, micro-Raman spectroscopy, and UV-visible absorption spectroscopy measurements. The resistivity measurements of the pure and titania modified WO3 films are done at room temperature. The variation of resistivity with temperature for the range of 170–450 K is also investigated. The microstructural analysis indicates that titania addition strongly perturbs the tungsten oxide lattice and suppresses the grain growth. Optical measurements revealed a bandgap renormalization in tungsten oxide films for higher titania concentrations. Bandgap values of the films decrease from 3.17 eV for pure WO3 to 2.7 eV for 10 wt % TiO2 modified WO3 films. The shifting of photoresponse of WO3 films to visible range by titania incorporation throw light on the feasibility of their application in visible-light-driven photocatalysis and in solar cells. All the films show semiconducting behavior in their temperature dependence of resistivity.
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81.05.Hd Other semiconductors
68.55.ag Semiconductors
81.15.Fg Pulsed laser ablation deposition
78.66.Li Other semiconductors
73.61.Le Other inorganic semiconductors
84.60.Jt Photoelectric conversion

Adsorption of oxygen atoms on the Mg3Nd(001) surface

Na Wang, Wei-Bing Zhang, Wen Li, Xiao-Qin Zeng, Wen-Jiang Ding, and Bi-Yu Tang

J. Appl. Phys. 104, 033516 (2008); http://dx.doi.org/10.1063/1.2961312 (5 pages)

Online Publication Date: 5 August 2008

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Based on the density functional theory, the stability of three possible Mg3Nd(001) surfaces is investigated, and the structural and electronic properties of O adsorption on the most stable Mg3Nd(001) surface are studied. The calculated results show that the most favorable adsorption site is the (2Nd+Mg) hollow site, and the adsorption energy decreases as the coverage increases. At low coverage O atoms prefer Nd atoms to Mg atoms as nearest neighbors. When the coverage increases to a full monolayer, some O atoms sink into the alloy surface and stay between Nd and Mg atoms. The electronic structures of the adsorption system show that for all coverage the interaction between O and the alloy surface mainly arises from the hybridization of O 2p states and Nd 5d states, while the interaction between Mg 3s states and O 2p states becomes noticeable at the high coverage. The present study reveals that during the initial oxidation stage of the alloy, Nd atoms can get a priority of oxidation, followed by Nd rich oxide film.
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68.43.Mn Adsorption kinetics
73.20.At Surface states, band structure, electron density of states
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Boron segregation in crystallized MgO/amorphous-Co40Fe40B20 thin films

C. Y. You, T. Ohkubo, Y. K. Takahashi, and K. Hono

J. Appl. Phys. 104, 033517 (2008); http://dx.doi.org/10.1063/1.2963709 (6 pages) | Cited 17 times

Online Publication Date: 5 August 2008

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We have investigated the crystallization process of an amorphous Co40Fe40B20 ribbon and a MgO/amorphous-Co40Fe40B20 thin film to simulate the segregation of B in the magnetic tunneling junctions that use CoFeB ferromagnetic electrodes and a MgO tunneling barrier. Both types of samples show the same crystallization sequence: a primary crystallization of bcc-FeCo from an amorphous precursor followed by boride crystallization at a higher annealing temperature. Three-dimensional atom probe analysis has shown that very little boron is dissolved in the bcc-FeCo crystals ( ∼ 1 at. %). A transmission electron microscopy observation of an annealed MgO/Co40Fe40B20 bilayer thin film has shown clear evidence of MgO serving as heterogeneous nucleation sites for bcc-FeCo grains. Energy filtering transmission electron microscopy has shown strong boron segregation at the MgO/FeCo interface after the crystallization of the MgO/amorphous-Co40Fe40B20 bilayer film.
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75.70.Ak Magnetic properties of monolayers and thin films
81.40.Gh Other heat and thermomechanical treatments
75.50.Bb Fe and its alloys
64.70.K- Solid-solid transitions
64.75.St Phase separation and segregation in thin films

Surface crystallography and electronic structure of potassium yttrium tungstate

V. V. Atuchin, L. D. Pokrovsky, O. Yu. Khyzhun, A. K. Sinelnichenko, and C. V. Ramana

J. Appl. Phys. 104, 033518 (2008); http://dx.doi.org/10.1063/1.2963957 (6 pages) | Cited 15 times

Online Publication Date: 5 August 2008

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Structural and electronic characteristics of KY(WO4)2 (KYW) (010) crystal surfaces have been studied using reflection high-energy electron diffraction (RHEED) and x-ray photoelectron spectroscopy (XPS). The results indicate that the crystal structure and chemical composition of the mechanically polished pristine surface is stoichiometrically well maintained as expected for KYW crystals. Combined measurements of RHEED and XPS as a function of 1.5 keV Ar+ ion irradiation of the KYW (010) surfaces indicate amorphization, partial loss of potassium atoms, and partial transformation of chemical valence state of tungsten from W6+ to a lower valence state, W0 state predominantly, which induces electronic states at the top of valence band.
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73.20.At Surface states, band structure, electron density of states
61.66.Fn Inorganic compounds
79.60.Bm Clean metal, semiconductor, and insulator surfaces
64.70.K- Solid-solid transitions
61.80.Jh Ion radiation effects

Electron paramagnetic resonance and optical spectroscopy of Er-doped β-Ga2O3

J. Vincent, O. Guillot-Noël, L. Binet, P. Aschehoug, Y. Le Du, F. Beaudoux, and P. Goldner

J. Appl. Phys. 104, 033519 (2008); http://dx.doi.org/10.1063/1.2948936 (6 pages) | Cited 1 time

Online Publication Date: 6 August 2008

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Conducting β-Ga2O3 single crystals doped with Er3+ were grown using the floating zone method. Electron paramagnetic resonance (EPR) showed that conduction electrons can coexist with the Er3+ dopant. Optical and EPR characterizations of samples nominally doped with 0.5% and 1.5% were performed at low temperature showing that erbium substitution into β-Ga2O3 can only be achieved effectively at the lower concentration because of the appearance of an erbium gallium garnet phase when the erbium concentration is increased. Despite the existence of two cationic sites in β-Ga2O3, EPR measurements demonstrate that Er incorporation occurs at a single crystallographic position. Optical spectroscopy of 0.5% doped samples of the 1.5 μm transition allowed us to determine some crystal field levels of the 4I15/2 and 4I13/2 multiplets. A lifetime of about 12 ms was found for the 1.5 μm emission, indicating a high quantum yield for the 4I13/2 multiplet.
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76.30.Pk Conduction electrons
78.40.Ha Other nonmetallic inorganics
61.72.up Other materials
71.70.Ch Crystal and ligand fields
81.10.Fq Growth from melts; zone melting and refining
81.05.Hd Other semiconductors

Nanocrystalline Ba0.97Ca0.03SO4:Eu for ion beams dosimetry

S. P. Lochab, D. Kanjilal, Numan Salah, Sami S. Habib, Jyoti Lochab, Ranju Ranjan, V. E. Aleynikov, A. A. Rupasov, and A. Pandey

J. Appl. Phys. 104, 033520 (2008); http://dx.doi.org/10.1063/1.2955459 (4 pages) | Cited 5 times

Online Publication Date: 6 August 2008

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Nanoparticles of Ba0.97Ca0.03SO4:Eu have been irradiated by 48 MeV Li3+, 75 MeV C6+, and 90 MeV O7+ ion beams with a fluence range of 1×109−1×1013 ion/cm2. The thermoluminescence (TL) glow curves along with the response curves of this nanophosphor have been investigated and compared with those of the corresponding microcrystalline samples. TL glow curve of the nanophosphor exposed to γ-rays has also been included in the letter with the aim of reporting some of the comparative measurements. The glow curves of the ion-beam irradiated nanomaterials are similar in their shapes to those induced by gamma rays, with a shift in the peak positions to the higher temperature side by around 30 K. The TL intensity of the ion-beam irradiated nanomaterials is found to decrease, while going from low to high atomic number (Z) ions (i.e., Li3+→O7+). Similar trend was also observed in the linearity of the TL response curves. The response curve of Li ion irradiated nanomaterials is sublinear in the whole range of studied fluences. C ion has also a more or less similar response, while the O ion has a sublinear response over a shorter range of 1×109−1×1012 ion/cm2, and above this range the response saturates. These results for the nanomaterials are much better than that of the corresponding microcrystalline sample irradiated by Li3+ ions. The curve was sublinear up to the fluence 1×1010 ion/cm2 and then saturates at higher fluences. The wider sublinear TL response of the nanocrystalline Ba0.97Ca0.03SO4:Eu makes it superior to its corresponding microform and thus makes it a suitable candidate as a dosimeter to be used for detecting the doses of ions, especially the Li and C ions for their application in cancer radiotherapy and radiation biology.
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61.80.Jh Ion radiation effects
78.60.Kn Thermoluminescence

Optical properties of thermally annealed hafnium oxide and their correlation with structural change

Jun-Woo Park, Do-Kyu Lee, D. Lim, Hosun Lee, and Suk-Ho Choi

J. Appl. Phys. 104, 033521 (2008); http://dx.doi.org/10.1063/1.2961326 (5 pages) | Cited 9 times

Online Publication Date: 6 August 2008

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We studied the optical properties of hafnium oxide as its structure changed. The shoulderlike feature that appears in crystalline HfO2 near the energy trough of the optical absorption edge after thermal anneal was found to have a direct correlation with the crystallization of the film. A luminescence feature near 4.1 eV could also be observed for some of the crystallized hafnium oxide films, but it did not have a direct correlation with crystallization or the shoulderlike feature. Our experimental results do not support the recently proposed self-trapped exciton (STE) model, where the shoulderlike absorption and the UV luminescence features are both attributed to STE. Simultaneous measurements of structural and optical properties of thermally annealed hafnium oxide films revealed that the shoulderlike feature is not related to defects, but rather is intrinsic to crystallized hafnium oxides. We discuss some possible origins of the shoulderlike feature based on our experimental results. Nevertheless, the UV luminescence feature seems to be related to oxygen defects in crystalline hafnium oxide.
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78.66.Nk Insulators
77.55.-g Dielectric thin films
81.40.Gh Other heat and thermomechanical treatments
78.55.Hx Other solid inorganic materials
71.35.-y Excitons and related phenomena

Effects of rapid thermal annealing on optical properties of p-doped and undoped InAs/InGaAs dots-in-a-well structures

Q. Cao, S. F. Yoon, C. Y. Liu, and C. Z. Tong

J. Appl. Phys. 104, 033522 (2008); http://dx.doi.org/10.1063/1.2963689 (6 pages) | Cited 7 times

Online Publication Date: 6 August 2008

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Postgrowth rapid thermal annealing was used to investigate the intermixing and structural changes in p-doped and undoped InAs/In0.1Ga0.9As dots-in-a-well (DWELL) structures grown by molecular beam epitaxy. Interdiffusion of In and Ga atoms caused by thermal annealing was proven from photoluminescence (PL) measurements, where blueshifts of the energy peaks were observed. The results show that p-doped quantum dot (QD) structures are more resistant to intermixing with higher thermal energy onset, and the reason is explained as the suppressed Ga diffusion resulted from the Be dopant. Rapid quenching of the integrated PL intensity at high temperature was observed in both undoped and p-doped DWELL QDs. Good agreement was obtained by fitting the integrated PL profile using two nonradiative recombination mechanisms, resulting in two activation energies that correspond to loss of carriers to nonradiative centers.
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78.67.De Quantum wells
78.67.Hc Quantum dots
61.72.Cc Kinetics of defect formation and annealing
78.55.Cr III-V semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
68.65.La Quantum wires (patterned in quantum wells)

Raman study of self-assembled InAs/InP quantum wire stacks with varying spacer thickness

T. Angelova, A. Cros, A. Cantarero, D. Fuster, Y. González, and L. González

J. Appl. Phys. 104, 033523 (2008); http://dx.doi.org/10.1063/1.2963703 (6 pages) | Cited 3 times

Online Publication Date: 6 August 2008

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Self-assembled InAs/InP (001) quantum wire stacks have been investigated by means of Raman scattering. The characteristics of the observed vibrational modes show clear evidence of confinement and atomic intermixing between As and P atoms from the wire and the spacer. The change in the intermixing with spacer layer thickness and growth temperature is investigated. Likewise, the effect of annealing on the exchange of As and P atoms is also studied. Resonance effects in confined and interface phonons are discussed for excitation in the vicinity of the InAs E1 critical point. Finally, the energy of the interface modes is related to the structural characteristics of the wires by comparing the experimental data with a lattice dynamic calculation based on the dielectric continuum model.
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78.67.Lt Quantum wires
78.30.Fs III-V and II-VI semiconductors
81.16.Dn Self-assembly
68.65.La Quantum wires (patterned in quantum wells)

Acoustic modes of finite length homogeneous and layered cylindrical shells: Single and multiwall carbon nanotubes

Guangyan Li, G. A. Lamberton, Jr., and J. R. Gladden

J. Appl. Phys. 104, 033524 (2008); http://dx.doi.org/10.1063/1.2965187 (7 pages) | Cited 5 times

Online Publication Date: 6 August 2008

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We present a numerical study of the normal modes of vibration of both homogeneous and heterogeneous finite length cylindrical shells of arbitrary wall thickness with applications toward single and multiwall carbon nanotubes in the continuum limit. The method is checked by comparison of computed and measured resonance spectra for a machined aluminum cylindrical shell. The dependence of the natural frequencies of various radial modes with the length and radius of single wall tubes is investigated and compared to atomistic models and Raman spectroscopy data. The radial dependence for the radial breathing mode and four harmonics of the squash mode are found to be well fitted by power laws and agree with analytical solutions in the thin wall limit. A general model for an elastically heterogeneous layered cylindrical shell is applied to multiwall tubes with graphene sheets and gaps between the sheets represented by two different materials. The frequency dependence on length and diameter is investigated for tubes composed of two to four concentric shells.
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63.22.Gh Nanotubes and nanowires
61.46.Fg Nanotubes
78.30.Na Fullerenes and related materials

High pressure torsion of amorphous Cu60Zr30Ti10 alloy

Sándor Hóbor, Ádám Révész, Péter J. Szabó, Alex P. Zhilyaev, Viktória Kovács Kis, János L. Lábár, and Zsolt Kovács

J. Appl. Phys. 104, 033525 (2008); http://dx.doi.org/10.1063/1.2964115 (8 pages) | Cited 10 times

Online Publication Date: 7 August 2008

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High pressure torsion was applied to produce a disk-shaped specimen of Cu60Zr30Ti10 composition. The dependence of the morphology, microstructure, and thermal behavior on the applied shear strain was monitored by scanning and transmission electron microscopies, synchrotron x-ray diffraction, and calorimetry. The disk consists of a gradient microstructure ranging from large homogeneous blocks (about 20 μm) to finely dispersed nanocrystals (about 20 nm) of two stable hexagonal phases and continuously decreasing amorphous content with increasing strain. The evolution of such microstructure was interpreted by using a model based on heat conduction generated by the extensive shear deformation.
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81.40.Lm Deformation, plasticity, and creep
61.43.Dq Amorphous semiconductors, metals, and alloys
62.50.-p High-pressure effects in solids and liquids
65.40.Ba Heat capacity
62.20.fq Plasticity and superplasticity
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