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1 May 2005

Volume 97, Issue 9, Articles (09xxxx)

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Quaternary InAlGaN-based high-efficiency ultraviolet light-emitting diodes

Hideki Hirayama

J. Appl. Phys. 97, 091101 (2005); http://dx.doi.org/10.1063/1.1899760 (19 pages) | Cited 94 times

Online Publication Date: 26 April 2005

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In order to realize 250–350-nm-band high-efficiency deep ultraviolet (UV) emitting devices using group-III-nitride materials, it is necessary to obtain high-efficiency UV emission from wide-band-gap (In)AlGaN. The use of the In-segregation effect, which has already been used for InGaN blue emitting devices, is quite effective for achieving high-efficiency deep UV emission. We have demonstrated high-efficiency UV emission from quaternary InAlGaN-based quantum wells in the wavelength range between 290 and 375 nm at room temperature (RT) using the In-segregation effect. Emission fluctuations in the submicron region due to In segregation were clearly observed for quaternary InAlGaN epitaxial layers. An internal quantum efficiency as high as 15% was estimated for a quaternary InAlGaN-based single quantum well at RT. Such high-efficiency UV emission can even be obtained on high threading-dislocation density buffer layers. A comparison of electroluminescence is made between light-emitting diodes (LEDs) with InAlGaN, AlGaN, and GaN active regions fabricated on SiC substrates with emission wavelengths between 340 and 360 nm. The emission intensity from the quaternary InAlGaN UV-LED was more than one order of magnitude higher than that from the AlGaN or GaN UV-LEDs under RT cw operation. We therefore fabricated 310–350-nm-band deep UV-LEDs with quaternary InAlGaN active regions. We achieved submilliwatt output power under RT pulsed operation for 308–314-nm LEDs. We also demonstrated a high output power of 7.4 mW from a 352-nm quaternary InAlGaN-based LED fabricated on a GaN substrate under RT cw operation. The maximum external quantum efficiency (EQE) of the 352-nm InAlGaN-based LED was higher than that obtained for an AlGaN-based LED with the same geometry. From these results, the advantages of the use of quaternary InAlGaN in 350-nm-band UV emitters were revealed.
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85.60.Jb Light-emitting devices
85.60.Gz Photodetectors (including infrared and CCD detectors)
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Surface relief grating formation in liquid-crystalline side-chain azopolymers by femtosecond pulse holography

Geon Joon Lee, Cha Hwan Oh, YoungPak Lee, In Ae Kang, and Yang Kyoo Han

J. Appl. Phys. 97, 093101 (2005); http://dx.doi.org/10.1063/1.1879080 (5 pages) | Cited 2 times

Online Publication Date: 14 April 2005

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Surface relief gratings were inscribed in the liquid-crystalline side-chain (LCSC) azopolymers using two-beam interference of near-infrared femtosecond laser pulses. The LCSC azopolymer films were prepared using acrylate-based homo- and copolymers with the azobenzene group in the side chain. The LC properties of the polymers were confirmed by differential scanning calorimetry and optical polarizing microscopy. By adjusting the writing conditions to the optimum fluence, highly efficient gratings were fabricated. This phenomenon is ascribed to a fact that the multiphoton absorption-induced photoisomerization facilitates the parallel ordering of LC mesogenic side groups and the grating formation. Even after the writing beams are removed, the grating relaxation hardly occurs. Such a very slow relaxation might be due to the collective behavior of aligned LC mesogenic side groups in the domain.
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42.40.Eq Holographic optical elements; holographic gratings
42.82.Cr Fabrication techniques; lithography, pattern transfer
42.70.Ln Holographic recording materials; optical storage media
42.70.Df Liquid crystals
42.79.Vb Optical storage systems, optical disks
42.62.-b Laser applications
61.30.Vx Polymer liquid crystals
82.50.Pt Multiphoton processes

Color tunable metal-cavity organic light-emitting diodes with fullerene layer

Sijin Han, Changjun Huang, and Zheng-Hong Lu

J. Appl. Phys. 97, 093102 (2005); http://dx.doi.org/10.1063/1.1887830 (5 pages) | Cited 25 times

Online Publication Date: 20 April 2005

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Three primary colors, red, green, and blue have been obtained from a single-emission layer organic light-emitting diode (OLED) through optical design using a half-wavelength all-metal-cavity device. Fullerene is used as an electron transport layer to further enhance the electrical performance of the cavity device and the optical tuning of the cavity OLED. This fullerene layer results in a ∼ 2 V driving voltage reduction and a ∼ 20% increase in power efficiency, as compared with traditional cavity OLED with Alq as the electron transport layer. The emissive spectra for the cavity OLEDs are well predicted by the Fabry–Perot cavity theory. The spectral narrowing and intensity enhancement at the resonance wavelength have been observed and are explained by the redistribution of optical-mode density inside the microcavity. Schemes to tune the emissive color by varying the cavity length through variations of indium tin oxide thickness, hole transport layer thickness, and electron transport thickness, individually or collectively, have been proposed and demonstrated.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.55.Kz Solid organic materials

Tuneability of amplified spontaneous emission through control of the thickness in organic-based waveguides

Eva M. Calzado, José M. Villalvilla, Pedro G. Boj, José A. Quintana, and María A. Díaz-García

J. Appl. Phys. 97, 093103 (2005); http://dx.doi.org/10.1063/1.1886891 (5 pages) | Cited 16 times

Online Publication Date: 21 April 2005

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The thickness dependence of amplified spontaneous emission (ASE) in optically pumped polystyrene films containing 15 wt % of the luminescent and hole-transporting organic molecule N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) is reported. It is observed that the position of ASE can be tuned between 404 and 417 nm by changing the film thickness from 100 to 200 nm. By measuring and modeling the waveguide modes, we demonstrate that the calculated cut-off thickness for the propagation of one mode determine the thickness below which ASE disappears although they do not perfectly correlate with the observed wavelength shifts. Results show that the position of ASE depends on the shape of the photoluminescence spectrum, that is composed of various vibronic peaks whose relative intensities change with film thickness due to the existence of the cut-off thickness. Finally, it is also observed that both the position and the threshold of ASE depend on the different confinement of the propagation modes due to thickness variations.
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42.79.Gn Optical waveguides and couplers
42.70.Jk Polymers and organics

Metallic photonic crystals with strong broadband absorption at optical frequencies over wide angular range

Georgios Veronis, Robert W. Dutton, and Shanhui Fan

J. Appl. Phys. 97, 093104 (2005); http://dx.doi.org/10.1063/1.1889248 (4 pages) | Cited 16 times

Online Publication Date: 22 April 2005

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We show theoretically that a finite two-dimensional square lattice of metallic cylinders in air can be designed to have almost 100% absorptance over a wide optical wavelength range and for a wide range of incidence angles. The broadband and wide-angle strong absorption is attributed to the presence of a large number of flat bands interacting with air bands and the greatly improved impedance matching between metallic photonic crystals and air. The frequency band of intense absorption is in the visible, ultraviolet, or near infrared, depending on the metallic material.
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42.70.Qs Photonic bandgap materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.40.Kc Metals, semimetals, and alloys
78.30.Er Solid metals and alloys

A photodriven dual-frequency addressable optical device

Gurumurthy Hegde, Geetha G. Nair, S. Krishna Prasad, and C. V. Yelamaggad

J. Appl. Phys. 97, 093105 (2005); http://dx.doi.org/10.1063/1.1891275 (6 pages) | Cited 11 times

Online Publication Date: 22 April 2005

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We propose a photonic switch employing a liquid-crystalline material. The material exhibits a change in the sign of the dielectric anisotropy switching from a positive to a negative value at a certain crossover frequency. By application of an electric field this phenomenon can be used to alter the orientation of the sample between two orthogonal directions leading to a large change in the optical transmission characteristics of the medium. Here we demonstrate that this feature can be realized by an unpolarized ultraviolet UV beam, owing to the photoisomerization of the constituent azobenzene molecules. Possible usage of this for optically driven display devices and image-storage applications are suggested.
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42.79.Kr Display devices, liquid-crystal devices
82.30.Qt Isomerization and rearrangement
82.50.Hp Processes caused by visible and UV light
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Route towards optimization of the response times of a pi-cell liquid-crystal mode

E. J. Acosta, M. J. Towler, and M. D. Tillin

J. Appl. Phys. 97, 093106 (2005); http://dx.doi.org/10.1063/1.1888047 (8 pages) | Cited 9 times

Online Publication Date: 22 April 2005

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A method to obtain and compare the relaxation times of pi-cells of equal brightness is presented. The method provides the means to investigate the effect of the device, drive, and material parameters on the relaxation times and hence provides a route to optimize the relaxation time without loss of brightness. The main results from the investigation are included.
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42.79.Kr Display devices, liquid-crystal devices
77.22.Gm Dielectric loss and relaxation
77.84.Nh Liquids, emulsions, and suspensions; liquid crystals
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Microstructural comparisons of ultrathin Cu films deposited by ion-beam and dc-magnetron sputtering

W. L. Prater, E. L. Allen, W.-Y. Lee, M. F. Toney, A. Kellock, J. S. Daniels, J. A. Hedstrom, and T. Harrell

J. Appl. Phys. 97, 093301 (2005); http://dx.doi.org/10.1063/1.1886275 (9 pages) | Cited 3 times

Online Publication Date: 21 April 2005

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We report and contrast both the electrical resistance and the microstructure of copper thin films deposited in an oxygen-containing atmosphere by ion-beam and dc-magnetron sputtering. For films with thicknesses of 5 nm or less, the resistivity of the Cu films is minimized at oxygen concentrations ranging from 0.2% to 1% for dc-magnetron sputtering and 6%–10% for ion-beam sputtering. Films sputtered under both conditions show a similar decrease of interface roughness with increasing oxygen concentration, although the magnetron-deposited films are smoother. The dc-magnetron-produced films have higher resistivity, have smaller Cu grains, and contain a higher concentration of cuprous oxide particles. We discuss the mechanisms leading to the grain refinement and the consequent reduced resistivity in both types of films.
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68.55.-a Thin film structure and morphology
73.61.At Metal and metallic alloys
81.15.Cd Deposition by sputtering
81.40.Rs Electrical and magnetic properties related to treatment conditions
81.05.Bx Metals, semimetals, and alloys
81.15.Jj Ion and electron beam-assisted deposition; ion plating
68.55.A- Nucleation and growth
68.35.Ct Interface structure and roughness
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

Molecular-dynamics model of energetic fluorocarbon-ion bombardment on SiO2 I. Basic model and CF2+-ion etch characterization

V. V. Smirnov, A. V. Stengach, K. G. Gaynullin, V. A. Pavlovsky, S. Rauf, P. J. Stout, and P. L. G. Ventzek

J. Appl. Phys. 97, 093302 (2005); http://dx.doi.org/10.1063/1.1893205 (11 pages) | Cited 17 times

Online Publication Date: 25 April 2005

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A molecular-dynamics-based model has been developed to understand etching of amorphous SiO2, with and without a fluorocarbon reactive layer, by energetic fluorocarbon (CFx+) ions. The model includes a representation of the solid and a set of interatomic potentials required for the SiO2–CFx interaction system. Two- and three-body pseudopotentials have either been obtained from published literature or computed using ab initio techniques. The Stillinger–Weber potential construct is used to represent potentials in our model and particle trajectories are advanced using the velocity-Verlet algorithm. The model is validated by comparing computed bond lengths and energies with published experimental results. Computed yield for Ar+ ion sputtering of SiO2 is also compared with published data. In the computational results described in this article, the model SiO2 test structure (with a thin fluorocarbon reactive layer) is prepared by starting with α-quartz ([001] orientation) and bombarding it with 50-eV CF2+ ions. Energetic CF2+ ions with different energies and angles of impact are then bombarded on this test structure to determine ion etch characteristics. Results show that etch yield increases with ion energy for all angles of impact. Etch yield, however, exhibits a nonlinear dependence on angle of impact with a peak around 60°. This nonlinear behavior is attributed to the balance among fraction of incident ion energy deposited in the material, ion energy deposition depth, and direction of scattering during secondary interaction events. Si in the lattice is primarily etched by F atoms and the primary Si-containing etch by-products are SiFx and SiOxFy radicals. However, oxygen either leaves the test structure as atomic O or in combination with C. While fragments of the energetic incident ion retain a substantial fraction of incident ion energy on ejection from the surface, etch by-products that have their origin in test structure atoms only have a few eV of energy on exit. Etch results are sensitive to fluorocarbon layer characteristics and etch yields decrease as the fluorocarbon reactive layer thickens.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
81.65.Cf Surface cleaning, etching, patterning

Molecular-dynamics model of energetic fluorocarbon-ion bombardment on SiO2. II. CFx+ (x = 1, 2, 3) ion etch characterization

V. V. Smirnov, A. V. Stengach, K. G. Gaynullin, V. A. Pavlovsky, S. Rauf, P. Stout, and P. L. G. Ventzek

J. Appl. Phys. 97, 093303 (2005); http://dx.doi.org/10.1063/1.1893206 (10 pages) | Cited 5 times

Online Publication Date: 25 April 2005

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A molecular-dynamics-based model has been used to understand etching of SiO2, with and without a fluorocarbon-polymer layer, by energetic fluorocarbon (CFx+) ions. The test structures for computational experiments are prepared by starting with α-quartz ([001] orientation) and bombarding it with low-energy ions: Ar+ ion for amorphous and fluorocarbon ions for fluorocarbon-polymerized test structures. CF+, CF2+, and CF3+ ions with a range of energies and angles of impact are then bombarded on these test structures to characterize fluorocarbon-ion etching. Results show that aggregate Si and O etch yields increase with ion energy for all ions and all angles of impact. Etch yields, however, exhibit nonlinear dependence on angle of impact with a peak around 60°. This nonlinear behavior is attributed to the balance among the incident ion energy transfer fraction, depth of energy deposition, and cluster scattering direction during secondary scattering events. The Si etch yield increases going from CF+ to CF2+ and then decreases for CF3+. This etch yield dependence on the nature of ion is because the amount of F per ion increases but the energy per F atom decreases as one moves from CF+→CF2+→CF3+. Si and O etch yields decrease considerably without the presence of the fluorocarbon layer, emphasizing the importance of C and F in etching SiO2. Parent ion clusters are only observed in the etch effluent at low ion energy and near the grazing angle of incidence. Under other conditions, the incident ion fragments upon impact at the surface. If fragments of the incident ions make it out of the surface, they retain a substantial fraction of the incident ion energy and their angle of ejection from the surface is related to the ion angle of incidence. However, clusters that contain test structure atoms are ejected from the surface with only a few eV of energy, and their angular and energy distributions are relatively independent of incident ion characteristics.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
81.65.Cf Surface cleaning, etching, patterning
52.77.Bn Etching and cleaning
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
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Enhanced photoluminescence from porous silicon by hydrogen-plasma etching

Q. Wang, C. Z. Gu, J. J. Li, Z. L. Wang, C. Y. Shi, P. Xu, K. Zhu, and Y. L. Liu

J. Appl. Phys. 97, 093501 (2005); http://dx.doi.org/10.1063/1.1877814 (4 pages) | Cited 6 times

Online Publication Date: 14 April 2005

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Porous silicon (PS) was etched by hydrogen plasma. On the surface a large number of silicon nanocone arrays and nanocrystallites were formed. It is found that the photoluminescence of the H-etched porous silicon is highly enhanced. Correspondingly, three emission centers including red, green, and blue emissions are shown to contribute to the enhanced photoluminescence of the H-etched PS, which originate from the recombination of trapped electrons with free holes due to SiO bonding at the surface of the silicon nanocrystallites, the quantum size confinement effect, and oxygen vacancy in the surface SiO2 layer, respectively. In particular, the increase of SiOx(x<2) formed on the surface of the H-etched porous silicon plays a very important role in enhancing the photoluminescence properties.
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81.05.Cy Elemental semiconductors
81.07.Bc Nanocrystalline materials
81.05.Rm Porous materials; granular materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Mb Porous materials
52.77.Bn Etching and cleaning
81.65.Cf Surface cleaning, etching, patterning
78.55.Ap Elemental semiconductors
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.25.+i Surface conductivity and carrier phenomena
73.63.Bd Nanocrystalline materials
61.72.J- Point defects and defect clusters

Characterization and evaluation of silicon carbide for high-velocity impact

Timothy J. Holmquist and Gordon R. Johnson

J. Appl. Phys. 97, 093502 (2005); http://dx.doi.org/10.1063/1.1881798 (12 pages) | Cited 10 times

Online Publication Date: 14 April 2005

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This article presents a characterization and evaluation of silicon carbide for high-velocity impact. This includes a wide range of loading conditions that produce large strains, high strain rates, and high pressures. Experimental data from the literature are used to determine constants for the Johnson–Holmquist–Beissel (JHB) constitutive model for brittle materials. A previous article by the authors presented a characterization of silicon carbide for high-velocity impact using an earlier version of the model (JH-1). The previous work provided good agreement with a broad range of experimental data with the exception of high-velocity penetration data. The current work uses the more recently developed JHB constitutive model, a target geometry that more closely matches the experimental design, and a computational technique that allows for target prestress. These recent developments (primarily the prestress) produce computed results that agree with all the experimental data, including the high-velocity penetration data. The computed results also provide a detailed analysis of the penetration process into a prestressed target and show why it is necessary to include the target prestress. A specific result is the ability to reproduce the nonsteady penetration rate that occurs in the prestressed target.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
68.35.Gy Mechanical properties; surface strains
81.70.Bt Mechanical testing, impact tests, static and dynamic loads
62.20.M- Structural failure of materials
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Microstructure of compositionally-graded (Ba1−xSrx)TiO3 thin films epitaxially grown on La0.5Sr0.5CoO3-covered (100) LaAlO3 substrates by pulsed laser deposition

Xinhua Zhu, Helen Lai-Wah Chan, Chung-Loong Choy, Kin-Hung Wong, and Dietrich Hesse

J. Appl. Phys. 97, 093503 (2005); http://dx.doi.org/10.1063/1.1882766 (5 pages) | Cited 3 times

Online Publication Date: 14 April 2005

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Compositionally-graded (Ba1−xSrx)TiO3 (BST) epitaxial thin films (with x decreasing from 0.25 to 0.0) were deposited by pulsed laser deposition on (100)LaAlO3 (LAO) single-crystal substrates covered with a conductive La0.5Sr0.5CoO3 (LSCO) layer as a bottom electrode. X-ray and electron diffraction patterns demonstrate that the entire graded film has a single-crystal cubic structure. The epitaxial relationship between BST, LSCO, and LAO can be described as (100)BST‖(100)LSCO‖(100)LAO; [001]BST‖[001]LSCO‖[001]LAO. Cross-sectional transmission electron microscopy (TEM) images reveal that both the BST films and the LSCO bottom electrodes have sharp interfaces and overall uniform thickness across the entire specimen, and that they grow with a columnar structure. Planar TEM images show that the graded films exhibit granular and/or polyhedral morphologies with an average grain size of 50 nm. High-resolution TEM images reveal aligned rectangular-shaped voids in the graded BST film, with length size of 12–17 nm, and width of 5–8 nm along the ⟨001⟩ direction in the (100) plane.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
68.55.-a Thin film structure and morphology
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth
61.72.Qq Microscopic defects (voids, inclusions, etc.)
77.22.Ch Permittivity (dielectric function)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.Nq Composition and phase identification
68.37.Lp Transmission electron microscopy (TEM)

Fast grain boundary diffusion and rate-limiting surface exchange reactions in polycrystalline materials

W. Preis and W. Sitte

J. Appl. Phys. 97, 093504 (2005); http://dx.doi.org/10.1063/1.1882770 (10 pages) | Cited 6 times

Online Publication Date: 14 April 2005

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Analytical solutions to the diffusion equations for fast grain boundary diffusion in polycrystalline solids of finite thickness bounded by two parallel surface planes are presented. The grain boundary diffusion coefficient may be higher by several orders of magnitude than the bulk (volume) diffusion coefficient. The microstructure of the polycrystalline sample has been modeled by means of an isolated grain boundary as well as an array of parallel grain boundaries. The surface concentration has been assumed to vary continuously during the diffusion process, as the surface exchange reactions of the diffusing species are considered to be fairly slow and equilibrium between the surface and the constant diffusion source (gas phase) is not attained. Two different cases for surface diffusion at the interface polycrystalline sample/diffusion source are taken into account, viz. negligible and fast surface diffusion. The pertinent analytical solutions allow the calculation of two-dimensional diffusion profiles in thin films and average concentration profiles for semi-infinite diffusion systems. In addition, the time dependence of the total amount of diffusing species exchanged between the diffusion source and the solid sample has been calculated. The theoretical results are interpreted in terms of Harrison’s classification of diffusion regimes. The parallel grain boundary model enables the calculation of diffusion profiles for both type-A and type-B kinetics. In the case of type-A kinetics expressions for the effective diffusion coefficient as well as the effective surface exchange coefficient are proposed, which depend on the corresponding bulk values, the microstructure of the specimen, and the respective surface condition.
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66.30.Lw Diffusion of other defects
61.72.Mm Grain and twin boundaries
66.30.Dn Theory of diffusion and ionic conduction in solids
68.35.Fx Diffusion; interface formation
61.72.Bb Theories and models of crystal defects
68.35.B- Structure of clean surfaces (and surface reconstruction)

Film growth model of atomic layer deposition for multicomponent thin films

Jin-Hyock Kim, Ja-Yong Kim, and Sang-Won Kang

J. Appl. Phys. 97, 093505 (2005); http://dx.doi.org/10.1063/1.1883728 (5 pages) | Cited 18 times

Online Publication Date: 14 April 2005

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Atomic layer deposition (ALD) has become an essential technique for fabricating nano-scale thin films in the microelectronics industry, and its applications have been extended to multicomponent thin films, as well as to single metal oxide and nitride films. A mathematical film growth model for ALD is proposed to predict the deposition characteristics of multicomponent thin films grown mainly in the transient regime, where the film thickness varies nonlinearly with the number of cycles. The nonlinear behavior of the growth rate and the composition of multicomponent thin films deposited by ALD depend on the precursor used and adsorbing surface. Hence, the equations to describe the change of surface coverage with precursor adsorption and the surface reaction are derived. The area reduction ratio is introduced as a parameter related to the number of adsorbed precursor molecules per unit area. The proposed model was applied to the deposition of Sr–Ti–O thin films to confirm its validity. SrO and TiO2 films were grown separately to investigate their ALD characteristics and to extract model parameters. As a result, it was shown that the thickness and composition of Sr–Ti–O films follow the trend predicted by the proposed model.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Aa Theory and models of film growth
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
68.55.Nq Composition and phase identification
68.43.Mn Adsorption kinetics
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Crystal structure and atomic arrangement of the metastable Ge2Sb2Te5 thin films deposited on SiO2/Si substrates by sputtering method

Y. J. Park, J. Y. Lee, M. S. Youm, Y. T. Kim, and H. S. Lee

J. Appl. Phys. 97, 093506 (2005); http://dx.doi.org/10.1063/1.1877821 (6 pages) | Cited 27 times

Online Publication Date: 18 April 2005

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The Ge2Sb2Te5 thin films deposited by a sputtering method on SiO2/Si substrates were annealed through a rapid thermal annealing process and performed a high-resolution transmission electron microscopy study in order to investigate the atomic arrangement of the metastable Ge2Sb2Te5. The metastable rocksalt structure having face-centered-cubic lattice was confirmed by high-resolution transmission electron microscopy images and simulated images on the directions of 〈100〉, 〈110〉, and 〈211〉 zone axes. According to the position of Ge and Sb in the metastable rocksalt structure, the atomic distribution alters when observed in different direction and this causes change in the charge-density distribution, resulting in different images in a high-resolution transmission electron microscopy. It is expected that as the crystallization proceeds, the Ge and Sb atoms tend to position themselves on a specific plane. From this aspect, the ordered structure model of the metastable Ge2Sb2Te5 was proposed by varying the position of the Ge and Sb atoms.
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68.55.-a Thin film structure and morphology
61.72.Cc Kinetics of defect formation and annealing
68.55.Nq Composition and phase identification
64.70.K- Solid-solid transitions
61.43.Fs Glasses
68.37.Lp Transmission electron microscopy (TEM)

Quantitative analysis of damage in an octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazonic-based composite explosive subjected to a linear thermal gradient

Paul D. Peterson, Joseph T. Mang, and Blaine W. Asay

J. Appl. Phys. 97, 093507 (2005); http://dx.doi.org/10.1063/1.1879072 (7 pages) | Cited 4 times

Online Publication Date: 18 April 2005

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The microstructure within a slowly heated, consolidated explosive will be influenced by both physical changes and chemical reactions prior to thermal ignition. Thermal expansion, exothermic decomposition, endothermic phase change, and increased binder viscosity play significant roles in the cook-off to detonation. To further explore the details of this intricate cook-off process, we have conducted a series of experiments in which a carefully controlled temperature gradient has been applied along a cylinder of PBX 9501 [94.9/2.5/2.5/0.1-wt % octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)/Estane 5703/a eutectic mixture of bis(2,2 dinitropropyl) acetal and bis(2,2-dinitropropyl) formal [abbreviated BDNPA-F]/Irganox] and maintained for a specified amount of time. After heating and subsequent cooling of the PBX 9501, the sample morphology has been probed with polarized light microscopy and small-angle x-ray scattering. Using these techniques we have quantitatively characterized the particle morphology, porosity, and chemical state of the explosive as a function of position, and therefore thermal treatment. Results of the analyses clearly show that thermal damage in PBX 9501 can be classified into two separate temperature regimes—an initial low-temperature regime (155–174 °C) dominated by the endothermic β-δ crystalline phase change, thermal expansion, and Ostwald ripening, and a high-temperature regime (175–210 °C) dominated by exothermic chemical decomposition. The results further show the complex interplay between the evolving sample morphology and the chemical reactions leading to a potential thermal self-ignition in the explosive.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
82.33.Vx Reactions in flames, combustion, and explosions
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
78.70.Ck X-ray scattering
81.40.Gh Other heat and thermomechanical treatments
64.70.K- Solid-solid transitions
65.40.De Thermal expansion; thermomechanical effects
61.43.Gt Powders, porous materials

X-ray induced absorption in fused silica containing various amounts of OH

Nobu Kuzuu and Hideharu Horikoshi

J. Appl. Phys. 97, 093508 (2005); http://dx.doi.org/10.1063/1.1883307 (8 pages) | Cited 1 time

Online Publication Date: 18 April 2005

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Characteristics of x-ray-induced absorption spectra in various types of synthetic fused silica (SFS) containing 0.1–77×1018 cm−3 of OH were studied up to an irradiation time of 6 h. Induced absorption in the SFS spectra of irradiation with x ray from a Rh target with a dosage of 2×104 C kg−1 h−1 was reproduced by six Gaussian absorption bands, at 3.8, 4.8, 5.0, 5.4, 5.8, and 6.5 eV, respectively. Intensities of these absorption bands increased with power of irradiation time. The intensities of the 5.8, 5.4, and 5.0 eV bands decreased with increasing OH content at an irradiation time shorter than 2 h. At an irradiation time longer than 2 h, on the other hand, the intensities of each absorption band had a minimum at an OH content of ≈ 5×1018 cm−3, and increased with increasing OH content at higher OH content. The power of the time dependence had a minimum at an OH content of ≈ 5×1017 cm−3, and increased with increasing OH content for higher OH-containing samples.
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42.70.Ce Glasses, quartz
78.70.Dm X-ray absorption spectra
61.80.Cb X-ray effects
61.82.Ms Insulators

Investigation of the optical and electronic properties of Ge2Sb2Te5 phase change material in its amorphous, cubic, and hexagonal phases

Bong-Sub Lee, John R. Abelson, Stephen G. Bishop, Dae-Hwan Kang, Byung-ki Cheong, and Ki-Bum Kim

J. Appl. Phys. 97, 093509 (2005); http://dx.doi.org/10.1063/1.1884248 (8 pages) | Cited 45 times

Online Publication Date: 18 April 2005

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Ge–Sb–Te alloys are widely used for data recording based on the rapid and reversible amorphous-to-crystalline phase transformation that is accompanied by increases in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gaps and electronic transport properties of these phases have persisted because of inappropriate interpretation of reported data and the lack of definitive analytical studies. In this paper we characterize the most widely used composition, Ge2Sb2Te5, in its amorphous, face-centered-cubic, and hexagonal phases, and explain the origins of inconsistent or unphysical results in previous reports. The optical absorption in all of these phases follows the relationship αhν∝(hν−Egopt)2, which corresponds to the optical transitions in most amorphous semiconductors as proposed by Tauc, Grigorovici, and Vancu [ Tauc et al., Phys. Status Solidi 15, 627 (1966) ], and to those in indirect-gap crystalline semiconductors. The optical band gaps of the amorphous, face-centered-cubic, and hexagonal phases are 0.7, 0.5, and 0.5 eV, respectively. The subgap absorption in the amorphous phase shows an exponential decay with an Urbach slope of 81 meV. We measured the photoconductivity of amorphous Ge2Sb2Te5 and determined a mobility-lifetime product of 3×10−9 cm2/V. The spectral photoconductivity shows a threshold at about 0.7 eV, in agreement with our analysis of the optical band gap. The face-centered-cubic and hexagonal phases are highly conductive and do not show freeze-out; even at 5 K the density of free carriers remains at 1019–1020 cm−3, so these are degenerate semiconductors in which the Fermi level resides inside a band. In the hexagonal phase, the effect of free electrons on the Hall coefficient is significant at high temperatures. When the Hall data are fitted using the two-carrier analysis, the hole mobility is found to decrease slowly with temperature, as expected. The considerations discussed in this paper can be readily applied to study related chalcogenide materials.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
72.80.Ng Disordered solids
72.40.+w Photoconduction and photovoltaic effects
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
71.20.Nr Semiconductor compounds
72.20.Fr Low-field transport and mobility; piezoresistance
72.20.My Galvanomagnetic and other magnetotransport effects

Temperature dependence of Nd3+↔Yb3+ energy transfer in the YAl3(BO3)4 nonlinear laser crystal

M. O. Ramirez, D. Jaque, L. E. Bausá, I. R. Martín, F. Lahoz, E. Cavalli, A. Speghini, and M. Bettinelli

J. Appl. Phys. 97, 093510 (2005); http://dx.doi.org/10.1063/1.1886887 (8 pages) | Cited 10 times

Online Publication Date: 18 April 2005

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The temperature dependence of the Nd3+→Yb3+ energy-transfer rate in the YAl3(BO3)4 nonlinear laser crystal has been investigated from the analysis of fluorescence decay curves recorded in the 10–600 K range. Three different regimes, independent on the dopant concentration, have been observed in the thermal behavior of the Nd3+→Yb3+ energy-transfer rate. By comparing experimental results with theoretical predictions based on the Dexter model [ J. Chem. Phys. 21, 836 (1953) ], the origin of these different regimes has been explained. In addition, the influence of temperature and of both Nd3+ and Yb3+ concentrations on the Nd3+←Yb3+ energy back-transfer rate has been also investigated, concluding that it is a migration-assisted energy-transfer process. Finally, the populations of both Nd3+ and Yb3+ metastable states achieved after continuous-wave Nd3+ excitation have been calculated and measured and results have been explained in terms of the thermal behavior of both forward- and back-transfer rates.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.70.Hj Laser materials
42.70.Mp Nonlinear optical crystals
61.72.S- Impurities in crystals
78.55.Hx Other solid inorganic materials
78.40.Ha Other nonmetallic inorganics
42.60.Pk Continuous operation

Coupling between the exciton and cavity modes in a GaAs∕GaAlAs asymmetric microcavity structure

D. Y. Lin, Y. H. Chou, Y. S. Huang, and K. K. Tiong

J. Appl. Phys. 97, 093511 (2005); http://dx.doi.org/10.1063/1.1883303 (4 pages)

Online Publication Date: 19 April 2005

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The coupling effect between the cavity mode and excitonic states in a GaAs∕GaAlAs asymmetric microcavity structure was studied via angle and temperature dependent reflectance and piezoreflectance (PzR) measurements in the angular range 10°<θinc<60° and temperature range 20 K<T<300 K. The so-called cavity mode (CM) consists of photonic waves confined in the microcavity, and shifts to higher energy with increasing angle or decreasing temperature, whereas the angle independent excitonic states of the quantum well structure shift to higher energy with decreasing temperature. By varying the angle of incidence of the probe beam and temperature, the coupling strength between the excitonic transitions and CM can be tuned. The related PzR features have shown significant enhancement when the cavity mode matches up with the excitonic transitions. A detailed experimental study of the resonance enhancement effect between the excitonic transitions and CM is presented.
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73.21.Fg Quantum wells
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects
78.67.De Quantum wells

Depth analysis of the in-plane lattice constants in compressively strained La0.67Ca0.33MnO3 thin films

M. Petit, M. Rajeswari, A. Biswas, R. L. Greene, T. Venkatesan, and L. J. Martínez-Miranda

J. Appl. Phys. 97, 093512 (2005); http://dx.doi.org/10.1063/1.1884761 (6 pages) | Cited 3 times

Online Publication Date: 19 April 2005

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We have performed depth profile analyses of the lattice parameters in epitaxial thin films of La0.67Ca0.33MnO3 to understand the evolution of strain relaxation processes in these materials. The analyses were done using grazing incidence x-ray scattering on films of different thicknesses on (100)-oriented LaAlO3 with a lattice mismatch of −2%. We find that such films exhibit two and sometimes up to three in-plane strained lattice constants, corresponding to a slight orthorhombic distortion of the crystal, as well as near-surface lattice relaxation. As a function of film thickness, we find that the strain is in the same order of magnitude up to a thickness of 70–80 nm, after which the strain begins to relax towards the bulk value of the lattice parameters. In addition, we find a depth behavior in the strain which is consistent with the formation of nonuniformly strained islands in the sample.
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68.55.-a Thin film structure and morphology
81.40.Jj Elasticity and anelasticity, stress-strain relations
68.60.Bs Mechanical and acoustical properties
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
62.20.D- Elasticity
78.70.Ck X-ray scattering

Determination of in-depth density profiles of multilayer structures

M. J. H. Kessels, F. Bijkerk, F. D. Tichelaar, and J. Verhoeven

J. Appl. Phys. 97, 093513 (2005); http://dx.doi.org/10.1063/1.1882773 (8 pages) | Cited 8 times

Online Publication Date: 20 April 2005

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We developed and demonstrate an analysis method in which we calibrate the intensity scale of cross-sectional transmission electron microscopy (TEM) using Cu Kα reflectometry. This results in quantitative in-depth density profiles of multilayer structures. Only three free parameters are needed to obtain the calibrated profiles, corresponding to three TEM image intensity levels. Additionally, the optical indices of the two multilayer materials used and the assumption that the layers are laterally homogeneous are used in the model. The power and the general usefulness of the method is demonstrated using experimental data of W/Si and Mo/Si multilayer systems with sharp interfaces as well as multilayers of which the interfaces were deliberately intermixed.
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68.65.Ac Multilayers
68.35.Ct Interface structure and roughness
68.37.Lp Transmission electron microscopy (TEM)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Lattice preamorphization by ion irradiation: Fluence dependence of the electronic stopping power threshold for amorphization

F. Agulló-López, G. García, and J. Olivares

J. Appl. Phys. 97, 093514 (2005); http://dx.doi.org/10.1063/1.1896444 (8 pages) | Cited 20 times

Online Publication Date: 20 April 2005

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A thermal-spike model has been applied to characterize the damage structure of the latent tracks generated by high-energy ion irradiations on LiNbO3 through electron excitation mechanisms. It applies to ions having electronic stopping powers both below and above the threshold value for lattice amorphization. The model allows to estimate the defect concentrations in the heavily damaged (preamorphized) regions that have not reached the threshold for amorphization. They include the halo and tail surrounding the core of a latent track. The existence of the preamorphized regions accounts for a synergy between successive irradiations and predicts a dependence of the amorphization threshold on previous irradiation fluence. The predicted dependence is in accordance with irradiation experiments using N (4.53 MeV), O (5.00 MeV), F (5.13 MeV), and Si (5 and 7.5 MeV). For electronic stopping powers above the threshold value the model describes the generation of homogeneous amorphous layers and predicts the propagation of the amorphization front with fluence. A theoretical expression, describing this propagation, has been obtained that is in reasonable agreement with silicon irradiation experiments at 5 and 7.5 MeV. The accordance is improved by including in a simple phenomenological way the velocity effect on the threshold. At the highest fluences (or depths) a significant discrepancy appears that may be attributed to the contribution of the nuclear collision damage.
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61.80.Jh Ion radiation effects
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
61.43.Bn Structural modeling: serial-addition models, computer simulation

Flexoelectric switching in a zenithally bistable nematic device

L. A. Parry-Jones and S. J. Elston

J. Appl. Phys. 97, 093515 (2005); http://dx.doi.org/10.1063/1.1885184 (7 pages) | Cited 22 times

Online Publication Date: 20 April 2005

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A Q-tensor method is used to model a grating-aligned zenithally bistable nematic device in two dimensions. The flexoelectrically driven switching between the two ground states is shown to occur through the creation and annihilation of pairs of defects, and the sign and magnitude of the voltage thresholds demonstrated to depend on the relative dielectric permittivity of the grating material.
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42.79.Kr Display devices, liquid-crystal devices
77.65.Bn Piezoelectric and electrostrictive constants
77.22.Ch Permittivity (dielectric function)
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