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15 Apr 2012

Volume 111, Issue 8, Articles (08xxxx)

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J. Appl. Phys. 111, 084701 (2012); http://dx.doi.org/10.1063/1.3698319 (11 pages)

Xerxes Lopez-Yglesias, Jason M. Gamba, and Richard C. Flagan
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back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Multiple excitation process in deep-ultraviolet emission from AlGdN thin films pumped by an electron beam

Shinya Iwahashi, Naohiro Kishi, Shinya Kitayama, Takashi Kita, Yoshitaka Chigi, Tetsuro Nishimoto, Hiroyuki Tanaka, Mikihiro Kobayashi, Tsuguo Ishihara, and Hirokazu Izumi

J. Appl. Phys. 111, 083526 (2012); http://dx.doi.org/10.1063/1.4705416 (4 pages)

Online Publication Date: 25 April 2012

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We studied the deep-ultraviolet emission properties of Al0.999Gd0.001 N thin films pumped by an electron beam. The Al0.999Gd0.001 N thin films were grown on fused silica substrates using an ultra-pure reactive sputtering technique. The intra-orbital electron transition of the Gd3+ ions in Al0.999Gd0.001 N showed an extremely narrow luminescence line at 318 nm. We fabricated field-emission devices using an Al0.999Gd0.001 N phosphor thin film and analyzed the dependence of the device characteristics on the injected current and acceleration voltage. The maximum output power was 1.0 mW/cm2. The excitation cross section was of the order of 10−13 cm2 and was found to depend on the acceleration voltage. These results indicate that injected high-energy electrons multiply excite Gd3+ ion.
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81.15.Cd Deposition by sputtering
78.66.Fd III-V semiconductors
81.05.Ea III-V semiconductors
68.55.ag Semiconductors
78.55.Cr III-V semiconductors

Helium irradiation effects in polycrystalline Si, silica, and single crystal Si

K. J. Abrams, J. A. Hinks, C. J. Pawley, G. Greaves, J. A. van den Berg, D. Eyidi, M. B. Ward, and S. E. Donnelly

J. Appl. Phys. 111, 083527 (2012); http://dx.doi.org/10.1063/1.4705450 (6 pages)

Online Publication Date: 26 April 2012

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Transmission electron microscopy (TEM) has been used to investigate the effects of room temperature 6 keV helium ion irradiation of a thin (≈55 nm thick) tri-layer consisting of polycrystalline Si, silica, and single-crystal Si. The ion irradiation was carried out in situ within the TEM under conditions where approximately 24% of the incident ions came to rest in the specimen. This paper reports on the comparative development of irradiation-induced defects (primarily helium bubbles) in the polycrystalline Si and single-crystal Si under ion irradiation and provides direct measurement of a radiation-induced increase in the width of the polycrystalline layer and shrinkage of the silica layer. Analysis using TEM and electron energy-loss spectroscopy has led to the hypothesis that these result from helium-bubble-induced swelling of the silicon and radiation-induced viscoelastic flow processes in the silica under the influence of stresses applied by the swollen Si layers. The silicon and silica layers are sputtered as a result of the helium ion irradiation; however, this is estimated to be a relatively minor effect with swelling and stress-related viscoelastic flow being the dominant mechanisms of dimensional change.
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61.80.Jh Ion radiation effects
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
71.55.Cn Elemental semiconductors

Molecular dynamics simulation study on heat transport in monolayer graphene sheet with various geometries

Ajing Cao

J. Appl. Phys. 111, 083528 (2012); http://dx.doi.org/10.1063/1.4705510 (9 pages) | Cited 2 times

Online Publication Date: 26 April 2012

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Using non-equilibrium molecular dynamics (MD) simulations, we study heat transport in monolayer graphene sheet. We show that the thermal transport in monolayer graphene sheet exhibits a strong length dependence on thermal conductivity, reaching 2360 W/mK at 2.8 μm. By modeling a two-dimensional heat spread type of heat conduction mimicking the experimental probing using the excitation laser light focused on a graphene, the isotropic nature of heat flow in graphene is revealed, which is in support of recent experimental probing. The T−1 dependence of thermal conductivity is observed at temperatures above room temperature. A peak value at 300 K is observed with further decreasing T, in good agreement with that of carbon nanotubes reported experimentally. Thermal conductivity of graphene nanoribbons (GNRs) strongly depends on the ribbon width, which is attributed to arise from the surface phonon scattering. Furthermore, the nonlinear temperature profile is revealed for asymmetric GNRs. A fitting approach for the MD obtained temperature profile based upon the analytic solution is proposed to obtain the thermal conductivity of GNRs of asymmetric geometry. These findings shed light on tuning thermal properties of GNRs with geometry optimizations.
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65.80.Ck Thermal properties of graphene
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
61.48.Gh Structure of graphene
63.22.Rc Phonons in graphene

Optimizing the crystal environment through extended x-ray absorption fine structure to increase the luminescent lifetimes of Er3+ doped Y2O3 nanoparticles

James A. Dorman, Ju H. Choi, Gregory Kuzmanich, John R. Bargar, and Jane P. Chang

J. Appl. Phys. 111, 083529 (2012); http://dx.doi.org/10.1063/1.3702789 (8 pages)

Online Publication Date: 26 April 2012

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To predict and optimize luminescence efficiency of rare-earth ion doped (RE) nanophosphors, a relationship between the RE-concentration and the luminescent parameters is often obtained by Judd-Ofelt analysis, where the quality factor (χ = Ω4/Ω6) depends on the Er interactions with other RE elements in the second nearest neighboring shell. In this work, a detailed analysis of the local bonding environment by extended x-ray absorption fine structure (EXAFS) analyses is shown as effective as the Judd-Ofelt analysis to quantify the Er↔RE interaction in the second nearest neighboring shell (ρN = IREr↔RE2/IREr↔RE1). As the physical basis of ρN is consistent to that of χ, the EXAFS analysis becomes a viable alternative to replace Judd-Ofelt analysis to predict the optimum dopant concentration. This approach was corroborated based on analysis of Er3+:Y2O3 and core-shell Er3+:Y2O3|Y2O3 (5 nm shell) nanoparticles (NPs), with Er3+ concentrations up to 20 mol %. The ρN ratio from EXAFS analysis was shown to strongly correlate to the lifetimes extracted from the Judd-Ofelt analysis, both predicting the optimal dopant concentrations to be at 5 mol % and 2 mol % for the Er3+:Y2O3 and core-shell NPs, respectively. This confirms that EXAFS analysis can be used as a more time efficient method to achieve the same outcome typically obtained by Judd-Ofelt analysis, enabling the optimization of the luminescent lifetimes of RE doped nano-phosphors.
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78.55.Hx Other solid inorganic materials
78.70.Dm X-ray absorption spectra
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
61.72.up Other materials

Impact of the Ga/In ratio on the N incorporation into (In,Ga)(As,N) quantum dots

R. Gargallo-Caballero, A. Guzmán, J. M. Ulloa, A. Hierro, M. Hopkinson, E. Luna, and A. Trampert

J. Appl. Phys. 111, 083530 (2012); http://dx.doi.org/10.1063/1.4706559 (7 pages)

Online Publication Date: 26 April 2012

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In this work, we demonstrate the dependence of the nitrogen incorporation on the Ga/In content into (In,Ga)(As,N) quantum dots (QDs) grown on GaAs (100) by radio-frequency plasma assisted molecular beam epitaxy (MBE). Morphological analysis by atomic force microscopy and cross-sectional transmission electron microscopy, together with an estimation of the transition thickness, monitored in situ during the growth, predict a maximum in the N incorporation for 30% Ga content. This result is confirmed by photoluminescence measurements of the as-grown and post-growth annealed samples. We attribute this behavior to a trade off between two mechanisms depending on the Ga/In content: one related to the stability of the Ga-N bond, and the other related to the surface strain and/or In segregation.
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81.07.Ta Quantum dots
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.05.Ea III-V semiconductors

Deflection of a cantilever rectangular plate induced by surface stress with applications to surface stress measurement

Xianwei Zeng, Jiaquan Deng, and Xi Luo

J. Appl. Phys. 111, 083531 (2012); http://dx.doi.org/10.1063/1.4706562 (8 pages)

Online Publication Date: 26 April 2012

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Surface stress plays important roles in the fabrication and applications of thin-film substrate systems. Bending test of cantilever microbeams has been commonly applied to characterize the surface stress. Stoney’s equation, ideally valid for completely unconstrained plates, is typically used to convert the measured deflection to a surface stress. To assess the validity of Stoney’s equation for the more complicated case of a plate with a clamped end, an analytical solution has been obtained in this study for the deflection of a cantilever rectangular plate due to surface stresses at its upper and lower surfaces. The analytical solution is given by the summation of single Fourier cosine series in the length and the width directions of the plate and a lower order polynomial. Numerical results for the deflection, slope, and curvature for the midpoint of the free end are presented for cantilever plates with aspect ratios ranging from 0.1 to 10 and for different Poisson’s ratios. In most practical measurements of surface stress, the aspect ratio is greater than one and the maximum percentage errors of Stoney’s equation for the deflection, slope, and curvature for the midpoint of the free end are 16%, 16%, and 10%, respectively. The present analytical solution based on Fourier cosine series with the first two leading terms can provide a significant improvement over Stoney’s equation. The maximum percentage errors for the deflection, slope, and curvature for the midpoint of the free end are reduced to 3%, 2%, and 3%, respectively.
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46.70.De Beams, plates, and shells
46.80.+j Measurement methods and techniques in continuum mechanics of solids
02.10.De Algebraic structures and number theory
46.25.Cc Theoretical studies

Characterization and luminescence properties of AlON:Eu2+ phosphor for white-emitting-diode illumination

F. Zhang, S. Chen, J. F. Chen, H. L. Zhang, J. Li, X. J. Liu, and S. W. Wang

J. Appl. Phys. 111, 083532 (2012); http://dx.doi.org/10.1063/1.4705404 (4 pages)

Online Publication Date: 27 April 2012

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AlON phosphors with different Eu2+ doping concentrations were synthesized by solid-state reaction method. Two Eu2+ doping sites were confirmed by temperature-dependent emission intensity, band shift and biexponential decay studies. High internal quantum efficiency and tunable chromaticity coordinates make AlON:Eu2+ phosphor a suitable candidate for white light-emitting diodes application.
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85.60.Jb Light-emitting devices

Effect of gold composition on the orientations of oxide nuclei during the early stage oxidation of Cu-Au alloys

Langli Luo, Yihong Kang, Judith C. Yang, and Guangwen Zhou

J. Appl. Phys. 111, 083533 (2012); http://dx.doi.org/10.1063/1.4707929 (9 pages)

Online Publication Date: 27 April 2012

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In situ environmental transmission electron microscopy is employed to study the effect of Au composition in Cu-Au alloys on the orientations of oxide islands during the initial-stage oxidation of Cu-Au(100) alloys. An orientation transition from nucleating epitaxial Cu2O islands to randomly oriented oxide islands is observed upon increasing the oxygen gas pressure. By increasing the Au composition in the Cu-Au alloys, both the oxide nucleation time and saturation density of oxide islands increase, but the critical oxygen pressure leading to nucleating randomly oriented Cu2O islands decreases. It is shown by a kinetic model that such a dependence of the critical oxygen pressure on the alloy composition is related to its effect on two competing processes, the oxide-alloy structure match and the effective collision of oxygen atoms, in determining the overall nucleation rate of oxide islands during the oxidation.
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81.05.Bx Metals, semimetals, and alloys
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
81.65.Mq Oxidation
64.60.qj Studies of nucleation in specific substances
64.70.kd Metals and alloys

GaP heteroepitaxy on Si(001): Correlation of Si-surface structure, GaP growth conditions, and Si-III/V interface structure

A. Beyer, J. Ohlmann, S. Liebich, H. Heim, G. Witte, W. Stolz, and K. Volz

J. Appl. Phys. 111, 083534 (2012); http://dx.doi.org/10.1063/1.4706573 (6 pages) | Cited 1 time

Online Publication Date: 30 April 2012

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GaP-layers on Si(001) can serve as pseudo-substrates for a variety of novel optoelectronic devices. The quality of the GaP nucleation layer is a crucial parameter for the performance of such devices. Especially, anti-phase domains (APDs) evolving at mono-atomic steps on the Si-surface can affect the quality of a layer adversely. The size, shape, and possible charge of the APDs and their boundaries depend on the polarity of the surrounding crystal. The observed polarity of the GaP is caused by the A-type double step configuration of the Si-surface reconstruction prior to GaP growth and the prevalent binding of Ga to Si under optimized growth conditions. The polarity of the GaP-layer and hence the atomic configuration at the Si-III/V interface can be changed by altering the growth conditions. With this knowledge, defect-free GaP/Si(001) templates for III/V device integration on Si-substrates can be grown.
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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
68.35.bg Semiconductors
68.55.ag Semiconductors
68.35.Ct Interface structure and roughness

Structural characterization of Ti-15Mo alloy used as biomaterial by Rietveld method

José Roberto Severino Martins, Jr. and Carlos Roberto Grandini

J. Appl. Phys. 111, 083535 (2012); http://dx.doi.org/10.1063/1.4707920 (8 pages)

Online Publication Date: 30 April 2012

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The biochemical and mechanical behavior of titanium alloys has been studied extensively for a variety of applications in the aerospace and biomedical fields. In the literature, there are studies that relate the microstructure and the phases of the material with its properties; however, there is little information that quantifies each phase and relates this to its properties. In addition, little has been done to analyze the effects of oxygen and heat treatment on the alloy’s structure. In this paper, the effect of doping with oxygen and the effect of heat treatments on structural properties of Ti-15Mo alloy used as biomaterials is examined using scanning electron microscopy, x-ray diffraction, and diffractogram analysis using the Rietveld method.
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87.85.J- Biomaterials
81.40.Gh Other heat and thermomechanical treatments
81.40.Lm Deformation, plasticity, and creep
61.72.up Other materials
62.20.F- Deformation and plasticity

Structural and optical characterization of two-dimensional arrays of Si nanocrystals embedded in SiO2 for photovoltaic applications

S. Gardelis, A. G. Nassiopoulou, P. Manousiadis, Silvia Milita, A. Gkanatsiou, N. Frangis, and Ch. B. Lioutas

J. Appl. Phys. 111, 083536 (2012); http://dx.doi.org/10.1063/1.4707939 (9 pages) | Cited 4 times

Online Publication Date: 30 April 2012

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We report on the structural and optical characterization of two-dimensional arrays of silicon nanocrystals (SiNCs) suitable for photovoltaic applications. Single and multiple SiNC layers were grown on quartz by low pressure chemical vapor deposition of Si and subsequent thermal oxidation steps. The single SiNC layers consisted of one SiNC layer embedded in two silicon dioxide (SiO2) layers, whereas the multi-layered structure consisted of five SiNC layers of equal thickness separated by SiO2 layers. SiNC layers with thicknesses ranging from 2 to 25 nm were investigated. A thorough structural characterization of the films was carried out by combining grazing incidence x-ray diffraction, x-ray reflectivity, and transmission electron microscopy (TEM). Both XRD and TEM measurements revealed that the SiNC layers were polycrystalline in nature and composed of SiNCs, separated by grain boundaries, with their vertical size equal to the SiNC layer and their lateral size characterized by a narrow size distribution. The high resolution TEM (HRTEM) images showed that oxidation of the SiNC layers proceeded by consumption of Si from their top surface, without any detectable oxidation at the grain boundaries. Only in the case of the thinnest investigated SiNC layer (2 nm), the SiNCs were well separated by SiO2 tunnel barriers. From transmission and reflection optical measurements, energy band gaps of the SiNCs were estimated. These results were correlated with the sizes of the SiNCs obtained by HRTEM. A shift of the estimated band gaps with decreasing SiNC size was observed. This was consistent with quantum size effects in the SiNCs. The film containing the smallest SiNCs (2 nm in the growth direction), besides a significant shift of the absorption edge to higher energies, showed light emission at room temperature which is due to radiative recombination of photo-generated carriers in localized SiNCs separated by SiO2 tunnel barriers.
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78.66.Db Elemental semiconductors and insulators
81.07.Bc Nanocrystalline materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.Mm Grain and twin boundaries
68.65.Ac Multilayers
78.55.Ap Elemental semiconductors
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