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22 Jan 2001

Volume 78, Issue 4, pp. 393-559

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Helix unwinding of doped cholesteric liquid crystals

W. C. Yip and H. S. Kwok

Appl. Phys. Lett. 78, 425 (2001); http://dx.doi.org/10.1063/1.1342781 (3 pages) | Cited 5 times

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By introducing benzoyl–benzene into a cholesteric liquid crystal, the helix unwinding voltage was reduced. This reduction was roughly proportional to the concentration of the dopant and was present for driving frequencies across the audio spectrum. It is believed that this voltage reduction is primarily due to a perturbation of the intermolecular coupling in the liquid crystal mixture. It was found that so long as the long-range order was not destroyed, the helix unwinding voltage could be reduced by as much as 24% at 60 Hz. © 2001 American Institute of Physics.
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61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures
64.70.M- Transitions in liquid crystals

Raman-spectroscopic evaluation of the long-range order in Ba(B1/3′B2/3″)O3 ceramics

Roberto L. Moreira, Franklin M. Matinaga, and Anderson Dias

Appl. Phys. Lett. 78, 428 (2001); http://dx.doi.org/10.1063/1.1339254 (3 pages) | Cited 32 times

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Ordered Ba(B1/3′B2/3″)O3 ceramics are currently used as dielectric resonators for microwave and millimeter wave technologies. The degree of ordering is generally determined by x-ray techniques. In this work, we demonstrate that Raman spectroscopy can be used to evaluate the degree of long-range order of these materials. We also show how varying the degree of order of samples allows partial assignment of the optical vibrational modes. © 2001 American Institute of Physics.
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78.30.Hv Other nonmetallic inorganics
63.20.-e Phonons in crystal lattices

In situ observation of surface processes in InAs/GaAs(001) heteroepitaxy: The role of As on the growth mode

Akihiro Ohtake and Masashi Ozeki

Appl. Phys. Lett. 78, 431 (2001); http://dx.doi.org/10.1063/1.1342216 (3 pages) | Cited 10 times

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Surface processes of the growing thin films of InAs on GaAs(001) substrates have been studied as a function of substrate temperature and As to In flux ratio. They have been observed by reflection high-energy electron diffraction and total-reflection-angle x-ray spectroscopy in real time. At temperatures lower than ∼480 °C, InAs grows in a Stranski–Krastanov mode irrespective of the As/In flux ratio, while the growth mode of InAs strongly depends on the flux ratio above ∼500 °C. We have found that the sticking probability of In decreases as the As flux is decreased above ∼500 °C, which results in the changes in the growth mode of InAs. © 2001 American Institute of Physics.
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81.05.Ea III-V semiconductors
68.55.A- Nucleation and growth
68.35.B- Structure of clean surfaces (and surface reconstruction)
78.70.Ck X-ray scattering

Excitonic properties and band alignment in lattice-matched ZnCdSe/ZnMgSe multiple-quantum-well structures

B. Bonanni, E. Pelucchi, S. Rubini, D. Orani, A. Franciosi, A. Garulli, and A. Parisini

Appl. Phys. Lett. 78, 434 (2001); http://dx.doi.org/10.1063/1.1342043 (3 pages) | Cited 1 time

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Lattice-matched Zn0.85Cd0.15Se/Zn0.74Mg0.26Se multiple-quantum-well structures were obtained on GaAs(001) using graded-composition InyGa1−yAs layers to match the II–VI lattice parameter to the III–V substrate. Cross-sectional transmission electron microscopy studies show that the effect of the crosshatch pattern of the InyGa1−yAs surface is limited to long-period coherent undulations of quantum well and barrier layers. Optical measurements of the excitonic properties as a function of well thickness, complemented by self-consistent calculations of the transition energies, indicate good quantum confinement in the well, with a 68% conduction band contribution to the 0.482 eV band gap difference. © 2001 American Institute of Physics.
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78.67.De Quantum wells
78.55.Et II-VI semiconductors
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.21.Fg Quantum wells

Effects of the morphologies and structures of light-emitting layers on the performance of organic electroluminescent devices

D. S. Qin, D. C. Li, Y. Wang, J. D. Zhang, Z. Y. Xie, G. Wang, L. X. Wang, and D. H. Yan

Appl. Phys. Lett. 78, 437 (2001); http://dx.doi.org/10.1063/1.1342203 (3 pages) | Cited 23 times

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Organic electroluminescent devices with a structure of ITO/ploy (9-vinylcarbazole)/tris (8-hydroxyquinoline) aluminum (Alq3)/Mg:Ag are fabricated at different substrate temperatures (77, 298, and 438 K) during Alq3 deposition. It is found that the surface morphologies of Alq3 thin films greatly affect the I–V characteristics of the devices by the contact area between metal cathode and light-emitting layer. There is an increase in the luminous efficiency of the devices in the order 77 K<298 K<438 K. We attribute this trend to different structures of Alq3 thin films. © 2001 American Institute of Physics.
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85.60.Jb Light-emitting devices
42.70.Jk Polymers and organics
68.35.B- Structure of clean surfaces (and surface reconstruction)
78.60.Fi Electroluminescence

On “radiation-free” states in transient motion of dislocations

Xanthippi Markenscoff

Appl. Phys. Lett. 78, 440 (2001); http://dx.doi.org/10.1063/1.1339852 (3 pages) | Cited 3 times

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The existence of "radiation-free" states in transient motion of dislocations is investigated. By analyzing an edge dislocation jumping from rest to a supersonic speed, we see that the Mach fronts disappear at vd = mathc2 because at this dislocation velocity, the pole singularity in the complex plane is replaced by a removable one. However, there exist radiated stress waves propagating with velocity c2, as well as c1. Also, due to radiated longitudinal waves, there is a “force” required to be supplied to sustain such a motion. © 2001 American Institute of Physics.
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61.72.Bb Theories and models of crystal defects
66.30.Lw Diffusion of other defects

X-ray scattering from misfit dislocations in heteroepitaxial films: The case of Nb(110) on Al2O3

R. I. Barabash, W. Donner, and H. Dosch

Appl. Phys. Lett. 78, 443 (2001); http://dx.doi.org/10.1063/1.1342215 (3 pages) | Cited 12 times

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We apply the Krivoglaz theory of x-ray scattering to thin epitaxial films containing misfit dislocations and reanalyze the seemingly puzzling x-ray scattering phenomena observed in several heteroepitaxial films. We show that the two-line shape scattering distribution and its dependence upon film thickness and momentum transfer can be understood in natural way and on a quantitative level. Extended diffuse x-ray scattering maps have been obtained from Nb(110)/Al2O3(11math0) which are discussed within the framework of this theory disclose a particular dislocation network at the Nb–Al2O3 interface. © 2001 American Institute of Physics.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.05.cf X-ray scattering (including small-angle scattering)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Neutron transmutation of 10B isotope-doped diamond

K. Jagannadham, K. Verghese, and J. E. Butler

Appl. Phys. Lett. 78, 446 (2001); http://dx.doi.org/10.1063/1.1342207 (2 pages) | Cited 4 times

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Diamond samples grown by microwave plasma chemical vapor deposition and doped with 10B have been irradiated under thermal neutron flux of 1013 cm−2 s−1 for 76 h to examine transmutation of 10B to 7Li and the attendant lattice damage to diamond. To prevent graphitization and formation of diamond-like carbon, continuous cooling in water is provided during irradiation. Characterization of the diamond samples using Raman spectroscopy, photoluminescence spectroscopy, and secondary ion mass spectrometry showed that diamond remained crystalline without a major damage. Formation of vacancies due to neutron irradiation is inferred from photoluminescence spectroscopy. © 2001 American Institute of Physics.
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81.05.U- Carbon/carbon-based materials
61.80.Hg Neutron radiation effects
61.82.Fk Semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.up Other materials
85.40.Ry Impurity doping, diffusion and ion implantation technology
52.77.Dq Plasma-based ion implantation and deposition
61.80.Jh Ion radiation effects
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.30.Am Elemental semiconductors and insulators
78.66.Db Elemental semiconductors and insulators
78.55.Ap Elemental semiconductors
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Comparison of aluminum and sodium doped poly(vinylidene fluoride-trifluoroethylene) copolymers by x-ray photoemission spectroscopy

B. Xu, Jaewu Choi, C. N. Borca, P. A. Dowben, A. V. Sorokin, S. P. Palto, N. N. Petukhova, and S. G. Yudin

Appl. Phys. Lett. 78, 448 (2001); http://dx.doi.org/10.1063/1.1340858 (3 pages) | Cited 4 times

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The chemical interaction between the simple metals, aluminum and sodium, and crystalline copolymer thin films of vinylidene fluoride (70%) with trifluoroethylene (30%), has been studied using x-ray photoemission spectroscopy. Aluminum and sodium metalize the polymer differently and different binding sites for the two metals can be inferred from the corresponding core level shifts. Aluminum leads to enhanced screening of final photoemission states associated with the polymer, while sodium doping strongly influences the fluorine, but perturbs the carbon backbone only slightly. © 2001 American Institute of Physics.
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61.41.+e Polymers, elastomers, and plastics
79.60.Fr Polymers; organic compounds
71.55.Jv Disordered structures; amorphous and glassy solids
71.20.Rv Polymers and organic compounds
71.15.Nc Total energy and cohesive energy calculations

Local structure of uncapped and Si-capped Ge/Si(100) self-assembled quantum dots

Alexander V. Kolobov, Hiroyuki Oyanagi, Karl Brunner, Peter Schittenhelm, Gerhard Abstreiter, and Kazunobu Tanaka

Appl. Phys. Lett. 78, 451 (2001); http://dx.doi.org/10.1063/1.1339841 (3 pages) | Cited 11 times

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Local structure of uncapped and Si-capped Ge quantum dots on Si(100) has been probed by extended x-ray absorption fine structure and x-ray absorption near-edge structure spectroscopy. It is found that the uncapped Ge dots are partially oxidized and partially alloyed with Si. The amount of Ge present in the Ge phase is found to be about 20–30%. In the Si-capped sample, Ge is found to be dissolved in silicon, the fraction of Ge atoms existing as a Ge phase being less than 10%. © 2001 American Institute of Physics.
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68.65.Hb Quantum dots (patterned in quantum wells)
61.05.cj X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.
78.70.Dm X-ray absorption spectra

Compliant effect of low-temperature Si buffer for SiGe growth

Y. H. Luo, J. Wan, R. L. Forrest, J. L. Liu, G. Jin, M. S. Goorsky, and K. L. Wang

Appl. Phys. Lett. 78, 454 (2001); http://dx.doi.org/10.1063/1.1337633 (3 pages) | Cited 19 times

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Relaxed SiGe attracted much interest due to the applications for strained Si/SiGe high electron mobility transistor, metal-oxide-semiconductor field-effect transistor, heterojunction bipolar transistor, and other devices. High-quality relaxed SiGe templates, especially those with a low threading dislocation density and smooth surface, are critical for device performance. In this work, SiGe films on low-temperature Si buffer layers were grown by solid-source molecular-beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, and photoluminescence spectroscopy. It was demonstrated that, with the proper growth temperature and Si buffer thickness, the low-temperature Si buffer became tensily strained and reduced the lattice mismatch between the SiGe and the Si buffer layer. This performance is similar to that of the compliant substrate: a thin substrate that shares the mismatch strain in heteroepitaxy. Due to the smaller mismatch, misfit dislocation and threading dislocation densities were lower. © 2001 American Institute of Physics.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.05.Hd Other semiconductors
68.55.A- Nucleation and growth
68.60.Bs Mechanical and acoustical properties
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
78.66.Li Other semiconductors
78.55.Hx Other solid inorganic materials

Nanofractography of shocked RDX explosive crystals with atomic force microscopy

J. Sharma, R. W. Armstrong, W. L. Elban, C. S. Coffey, and H. W. Sandusky

Appl. Phys. Lett. 78, 457 (2001); http://dx.doi.org/10.1063/1.1342046 (3 pages) | Cited 8 times

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Examination with atomic force microscopy has revealed apparent shear-type cleavage steps with heights as small as 0.05 nm, smaller than the size of cyclotrimethylenetrintramine (RDX) molecules, on the fracture surfaces of crystals that were subjected to aquarium shocks of 61.6 or 129 kbar, both greater than the pressure (38 kbar) required for the alpha-to-gamma phase transformation. The shocked centimeter size, originally transparent crystals became opaque and white from prolific fractures and internal cracks that are associated with their breakup into nanocrystallites of sizes extending from 500 down to 20 nm. The submolecular steps are related geometrically to the macroscale (K∥) fracture mechanics mode of shear fracturing that has obvious consequences at the nanoscale level for nonregistry between molecules across the crack surfaces. The results are of interest in relation to lattice trapping of crack fronts and as a contribution to the possibility of deformation-induced chemical decomposition/detonations. © 2001 American Institute of Physics.
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62.50.-p High-pressure effects in solids and liquids
82.40.Fp Shock wave initiated reactions, high-pressure chemistry
62.20.M- Structural failure of materials
68.35.B- Structure of clean surfaces (and surface reconstruction)
82.33.Vx Reactions in flames, combustion, and explosions
68.37.Ps Atomic force microscopy (AFM)
61.46.-w Structure of nanoscale materials
62.25.-g Mechanical properties of nanoscale systems

Oxidation kinetics in SrTiO3 homoepitaxy on SrTiO3(001)

X. D. Zhu, Weidong Si, X. X. Xi, and Qidu Jiang

Appl. Phys. Lett. 78, 460 (2001); http://dx.doi.org/10.1063/1.1338497 (3 pages) | Cited 14 times

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Using an oblique-incidence optical reflectivity difference technique, we investigated kinetic processes in SrTiO3 homoepitaxy on SrTiO3(001) under pulsed-laser deposition conditions. Depending upon growth temperature and oxygen ambient pressure, we found that the oxidation of an as-grown SrTiO3 monolayer may take a much longer time to complete than the recrystallization of the monolayer. The oxidation reaction was found to be characterized by an effective activation energy barrier of 1.35 eV and a large preexponential factor. © 2001 American Institute of Physics.
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81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth
78.66.Nk Insulators
81.65.Mq Oxidation
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
82.20.Pm Rate constants, reaction cross sections, and activation energies

Chemical ordering in AlGaN alloys grown by molecular beam epitaxy

E. Iliopoulos, K. F. Ludwig, T. D. Moustakas, and S. N. G. Chu

Appl. Phys. Lett. 78, 463 (2001); http://dx.doi.org/10.1063/1.1341222 (3 pages) | Cited 23 times

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Aluminum gallium nitride alloys were grown by molecular beam epitaxy and their film composition, structure, and microstructure were investigated by Rutherford backscattering spectroscopy, atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It was found that the ratio of group-III to group-V fluxes influences the relative incorporation of gallium and aluminum in the films. The transmission electron microscopy and x-ray diffraction studies revealed the existence of three types of spontaneously formed superlattice structures with periodicities of 2, 7, and 12 ML. While the 2 ML ordering is preferred under group-V rich conditions of growth, the 7 and 12 ML orderings were observed under group-III rich conditions of growth. © 2001 American Institute of Physics.
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81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.Nq Composition and phase identification
61.66.Bi Elemental solids
61.66.Dk Alloys

Magnetic susceptibility and microstructure of hydrogenated amorphous silicon measured by nuclear magnetic resonance on a single thin film

Jonathan Baugh, Daxing Han, Alfred Kleinhammes, and Yue Wu

Appl. Phys. Lett. 78, 466 (2001); http://dx.doi.org/10.1063/1.1341217 (3 pages) | Cited 12 times

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A nuclear magnetic resonance technique for precisely measuring the bulk magnetic susceptibility of micron-thick hydrogenated amorphous silicon (a-Si:H) film is introduced. The large disorder-induced diamagnetic enhancement exhibited by a-Si:H is shown to provide a sensitive bulk measurement for detecting variations in structural order in a-Si:H films. Furthermore, this approach is shown to be effective in revealing the details of microstructure in a-Si:H, including the presence of microstructural anisotropy. © 2001 American Institute of Physics.
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75.20.Ck Nonmetals
68.55.-a Thin film structure and morphology
76.60.-k Nuclear magnetic resonance and relaxation
75.70.Ak Magnetic properties of monolayers and thin films
61.43.Dq Amorphous semiconductors, metals, and alloys

Strip inverse-configuration photopyroelectric technique to measure the thermal conductivity of bulk samples

Srirang Manohar and S. Asokan

Appl. Phys. Lett. 78, 469 (2001); http://dx.doi.org/10.1063/1.1342782 (3 pages) | Cited 1 time

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The strip inverse-configuration photopyroelectric technique to measure the thermal conductivity of bulk samples of metals, semiconductors, and insulators is presented. It utilizes a narrow (500 μm wide) strip of a 9 μm thick pyroelectric sensor in an inverse configuration. A mathematical model is outlined that yields an expression for the output voltage of the pyroelectric sensor following heat dissipation into the sample, in a frequency range where the proposed measurement is possible. Experimentally, the sensor was operated in an “isothermal-voltage” mode to identify the frequency regime within which the measurement of the thermal conductivity of representative samples was possible after suitable calibration of the system. © 2001 American Institute of Physics.
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07.20.-n Thermal instruments and apparatus
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
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