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27 Sep 2004

Volume 85, Issue 13, pp. 2451-2664

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 2619 (2004); http://dx.doi.org/10.1063/1.1802384 (3 pages)

R. Basu, N. P. Guisinger, M. E. Greene, and M. C. Hersam
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Dual-frequency liquid crystal gels with submillisecond response time

Yun-Hsing Fan, Hongwen Ren, Xiao Liang, Yi-Hsin Lin, and Shin-Tson Wu

Appl. Phys. Lett. 85, 2451 (2004); http://dx.doi.org/10.1063/1.1799249 (3 pages) | Cited 30 times

Online Publication Date: 28 September 2004

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Two types of gels using dual-frequency liquid crystal are demonstrated. The one using a homogeneous cell shows anisotropic scattering behavior while the other prepared using a cell without polyimide alignment layers exhibits isotropic scattering properties. Both liquid crystal gels are highly transparent in the voltage-off state. Light scattering occurs when a high frequency voltage is applied. The isotropic gel exhibits a high contrast ratio and submillisecond response time. Potential applications of these gels for switchable polarizer, telecom optical switch, and reflective displays are emphasized.
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61.30.Vx Polymer liquid crystals
82.70.Gg Gels and sols
78.35.+c Brillouin and Rayleigh scattering; other light scattering
42.70.Df Liquid crystals
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Use of extended and prepared reference objects in experimental Fourier transform x-ray holography

H. He, U. Weierstall, J. C. H. Spence, M. Howells, H. A. Padmore, S. Marchesini, and H. N. Chapman

Appl. Phys. Lett. 85, 2454 (2004); http://dx.doi.org/10.1063/1.1795360 (3 pages) | Cited 14 times

Online Publication Date: 28 September 2004

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The use of one or more gold nanoballs as reference objects for Fourier transform holography (FTH) is analyzed using experimental soft x-ray diffraction from objects consisting of separated clusters of these balls. The holograms are deconvoluted against ball reference objects to invert to images, in combination with a Wiener filter to control noise. A resolution of ∼30 nm, smaller than one ball, is obtained even if a large cluster of balls is used as the reference, giving the best resolution yet obtained by x-ray FTH. Methods of dealing with missing data due to a beamstop are discussed. Practical prepared objects which satisfy the FTH condition are suggested, and methods of forming them described.
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42.40.Lx Diffraction efficiency, resolution, and other hologram characteristics
07.85.-m X- and γ-ray instruments
42.30.Va Image forming and processing
42.30.Wb Image reconstruction; tomography
61.46.-w Structure of nanoscale materials

Generation of polarization-entangled photon pairs in 1550 nm band by a fiber-optic two-photon interferometer

Akio Yoshizawa and Hidemi Tsuchida

Appl. Phys. Lett. 85, 2457 (2004); http://dx.doi.org/10.1063/1.1799233 (3 pages) | Cited 9 times

Online Publication Date: 28 September 2004

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Polarization-entangled photon pairs in a band around 1550 nm are generated by use of a fiber-optic two-photon interferometer having two periodically poled lithium niobate waveguides. Coincidence measurements are adopted for feedback control, in order to suppress phase drifts in the state of entanglement. Two-photon interference with a visibility of ∼1 (after subtraction of accidental coincidences) is observed over an hour. The visibility including accidental coincidences is ∼0.77.
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42.81.Qb Fiber waveguides, couplers, and arrays
07.60.Ly Interferometers

Multiexcitonic two-state lasing in a CdSe nanocrystal laser

Y. Chan, J.-Michel Caruge, P. T. Snee, and M. G. Bawendi

Appl. Phys. Lett. 85, 2460 (2004); http://dx.doi.org/10.1063/1.1795368 (3 pages) | Cited 32 times

Online Publication Date: 28 September 2004

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We report simultaneous amplified spontaneous emission from two different multiexcitonic transitions −1Se–1S3∕2 and 1Pe–1P3∕2− of colloidal CdSe nanocrystals (NCs) stabilized in high volume fraction in titania matrices. Two-state lasing from both multiexcitonic transitions is achieved in a surface-emitting distributed feedback CdSe NC laser. Variable stripe length measurements show that the gain from the 1Pe–1P3∕2 transition is approximately twice that of the −1Se–1S3∕2 transition for 4.2 nm radius CdSe∕ZnS core-shell NCs.
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42.55.Px Semiconductor lasers; laser diodes
71.35.-y Excitons and related phenomena
81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
78.55.Et II-VI semiconductors

Terahertz emission from p-lnAs due to the instantaneous polarization

R. Adomavičius, A. Urbanowicz, G. Molis, A. Krotkus, and E. Šatkovskis

Appl. Phys. Lett. 85, 2463 (2004); http://dx.doi.org/10.1063/1.1795980 (3 pages) | Cited 12 times

Online Publication Date: 28 September 2004

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Terahertz radiation from differently doped n- and p-type InAs crystal surfaces was investigated by time-resolved measurement. Large increase of the emitted terahertz power has been observed for p-InAs samples with the p-doping levels of approximately 1016–1017 cm−3. This increase was explained by a large surface depletion layer and an electric-field-induced optical rectification effect in this layer.
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78.47.-p Spectroscopy of solid state dynamics
61.82.Fk Semiconductors
68.47.Fg Semiconductor surfaces
78.70.Gq Microwave and radio-frequency interactions

Resonant and thermal changes of refractive index in a heavily doped erbium fiber pumped at wavelength 980 nm

Yu. O. Barmenkov, A. V. Kir’yanov, and M. V. Andrés

Appl. Phys. Lett. 85, 2466 (2004); http://dx.doi.org/10.1063/1.1787151 (3 pages) | Cited 16 times

Online Publication Date: 28 September 2004

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We report a theoretical and experimental study of the refractive index variation in a heavily doped erbium silica fiber within the spectral range 1500–1580 nm under the pumping at the wavelength 980 nm. The two main contributions in the refractive index change are addressed—the resonant part determined by the saturation effect in the fiber and the thermal part stemming from the fiber heating due to the excited-state absorption and Stokes losses. We demonstrate that the thermal contribution in the resultant refractive index change is a notable value, which is the feature of erbium fibers with a high concentration of erbium ions.
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42.81.Dp Propagation, scattering, and losses; solitons
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption
42.62.-b Laser applications

An effective cathode structure for inverted top-emitting organic light-emitting devices

Chieh-Wei Chen, Chun-Liang Lin, and Chung-Chih Wu

Appl. Phys. Lett. 85, 2469 (2004); http://dx.doi.org/10.1063/1.1798392 (3 pages) | Cited 47 times

Online Publication Date: 28 September 2004

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Inverted top-emitting organic light-emitting devices (OLEDs) combine technical merits of top-emitting OLEDs and inverted OLEDs for active-matrix OLED displays. One major challenge in inverted top-emitting OLEDs, however, is to prepare a reflective bottom cathode capable of effective electron injection. In this letter, we report an effective cathode structure for enhancing the electron-injection capability of the bottom cathode in inverted top-emitting OLEDs. Such an approach does not involve handling reactive metals during fabrication and permits use of highly reflective materials such as Al and Ag as the bottom cathodes. Efficient inverted top-emitting devices employing such a cathode scheme have been demonstrated.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
79.70.+q Field emission, ionization, evaporation, and desorption

Focusing inside negative index materials

Jeffrey B. Brock, Andrew A. Houck, and Isaac L. Chuang

Appl. Phys. Lett. 85, 2472 (2004); http://dx.doi.org/10.1063/1.1798398 (3 pages) | Cited 7 times

Online Publication Date: 28 September 2004

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Two key theoretical predictions for flat negative index lenses are that thicker lenses should have both internal and external foci, and that these foci should move linearly with small deviations in frequency. Two-dimensional electric field profiles of microwave transmission through rectangular slabs of composite wire and split-ring resonator material are presented. By scanning inside and outside the material, both internal and external foci are revealed. The power concentration around the internal focus is shown to have a negative curvature, consistent with the theoretical predictions.
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41.20.Jb Electromagnetic wave propagation; radiowave propagation

Energy-angle correlation of electrons accelerated by laser beam in vacuum

Z. Chen, Y. K. Ho, Y. J. Xie, S. Y. Zhang, Z. Yan, J. J. Xu, Y. Z. Lin, and J. F. Hua

Appl. Phys. Lett. 85, 2475 (2004); http://dx.doi.org/10.1063/1.1798391 (3 pages) | Cited 4 times

Online Publication Date: 28 September 2004

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The correlation between the outgoing energy and the scattering angle of electrons accelerated by a laser beam in vacuum has been investigated. Essentially, the single-valued function of the correlation, derived from classical electrodynamics Compton scattering for a plane wave, is broadened to a band. It means electrons with the same outgoing energy will have an angular spread. An equation to describe this correlation has been derived. Dependence of the spread width of scattering angle on laser beam parameters is examined, and physical explanations of these features are given. The results are found to be consistent with the simulation results for a proposed vacuum laser acceleration scheme: the capture and acceleration scenario.
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41.75.Jv Laser-driven acceleration
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
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Generation of atomic hydrogen during radio-frequency nitrogen plasma-assisted gas-source molecular-beam epitaxy of III-V dilute nitrides

A. Fotkatzikis, M.-A. Pinault, and A. Freundlich

Appl. Phys. Lett. 85, 2478 (2004); http://dx.doi.org/10.1063/1.1796537 (3 pages) | Cited 4 times

Online Publication Date: 28 September 2004

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The interaction of a typical gas-source molecular-beam epitaxy (GSMBE) environment with a radio-frequency (RF) nitrogen plasma source is investigated. In particular, a real-time in situ analysis of the evolution of the emission spectrum of an RF nitrogen plasma source, under high partial pressures of hydrogen (∼10−5 Torr), is presented. Hydrogen, emanating from the decomposition of hydride precursors in GSMBE, results in the appearance of a sharp emission peak at the region of 656 nm in the plasma spectrum, suggesting the generation of atomic hydrogen species in the nitrogen plasma cavity. The intensity of this peak is used for a qualitative evaluation of this interaction and its evolution as a function of the RF nitrogen plasma source conditions is investigated.
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81.05.Ea III-V semiconductors
68.55.A- Nucleation and growth
52.50.Dg Plasma sources
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
52.77.Dq Plasma-based ion implantation and deposition

On the potential distribution in Hall thrusters

Michael Keidar, Alec D. Gallimore, Yevgeny Raitses, and Iain D. Boyd

Appl. Phys. Lett. 85, 2481 (2004); http://dx.doi.org/10.1063/1.1797555 (3 pages) | Cited 14 times

Online Publication Date: 28 September 2004

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A model of the plasma flow in a Hall thruster channel is developed that takes into account the two-dimensional current conservation effect and relies on some experimental input parameters, such as magnetic field and electron temperature distribution. The model is an attempt to explain the experimentally found nonuniform potential distribution across the thruster channel. This effect is explained by the change of the electron mobility across a magnetic field due to the magnetic field gradient and due to the electron current along the magnetic field driven by the electron temperature gradient.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj Magnetohydrodynamic and fluid equation
52.25.Fi Transport properties
52.75.Di Ion and plasma propulsion
52.25.Ya Neutrals in plasmas
52.55.Dy General theory and basic studies of plasma lifetime, particle and heat loss, energy balance, field structure, etc.
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Evidence for generalized Kirchhoff’s law from angle-resolved electroluminescence of high efficiency silicon solar cells

L. Ferraioli, P. Maddalena, E. Massera, A. Parretta, M. A. Green, A. Wang, and J. Zhao

Appl. Phys. Lett. 85, 2484 (2004); http://dx.doi.org/10.1063/1.1795361 (3 pages) | Cited 2 times

Online Publication Date: 28 September 2004

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The angular distribution of infrared radiation, emitted by high efficiency single-crystalline silicon solar cells, was analyzed. Measurements were performed on cells with planar and inverted-pyramids surfaces, both showing integral emissions that approach the cosine function in the 0°–90° interval. Textured cell maintains the cosine distribution at the different wavelengths; planar device shows a distribution, which deviates from the cosine function at increasing wavelength. Correspondence between emission and absorption properties was demonstrated valid as a function of emission∕absorption angle. From the angular distribution of electroluminescence light, the devices absorption properties for incident light with directions different from the surface normal were estimated.
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84.60.Jt Photoelectric conversion
78.60.Fi Electroluminescence

Surface stress-driven instabilities of a free film

Konstantin G. Kornev and David J. Srolovitz

Appl. Phys. Lett. 85, 2487 (2004); http://dx.doi.org/10.1063/1.1795352 (3 pages) | Cited 2 times

Online Publication Date: 28 September 2004

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We examine the effect of surface stress on the stability of free unconstrained solid films. We first determine the general thermodynamic criteria for film stability. Then, we show that two different morphological instabilities are possible, depending on the sign of the surface stress. The first corresponds to a self-induced Euler buckling mode and the second to an accordionlike wrinkling of the film surfaces.
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68.60.Bs Mechanical and acoustical properties
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
68.35.Md Surface thermodynamics, surface energies

Electromigration-induced microstructure evolution in tin studied by synchrotron x-ray microdiffraction

Albert T. Wu, K. N. Tu, J. R. Lloyd, N. Tamura, B. C. Valek, and C. R. Kao

Appl. Phys. Lett. 85, 2490 (2004); http://dx.doi.org/10.1063/1.1795353 (3 pages) | Cited 19 times

Online Publication Date: 28 September 2004

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Under constant current electromigration, white tin exhibited a resistance drop of up to 10%. It has a body-centered-tetragonal structure, and the resistivity along the a and b axes is 35% smaller than along the c axis. Microstructure evolution under electromigration could be responsible for the resistance drop. Synchrotron radiation white beam x-ray microdiffraction was used to study this evolution. Grain-by-grain analysis was obtained from the diffracted Laue patterns about the changes of grain orientation before and after electromigration. We observed that high-resistance grains reorient with respect to the neighboring low-resistance grains, most likely by grain growth of the latter. A different mechanism of grain growth under electromigration from the normal grain growth is proposed and discussed.
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66.30.Qa Electromigration
61.72.Mm Grain and twin boundaries
66.30.Lw Diffusion of other defects

Observation of stacking faults in strained Si layers

S. W. Bedell, K. Fogel, D. K. Sadana, H. Chen, and A. Domenicucci

Appl. Phys. Lett. 85, 2493 (2004); http://dx.doi.org/10.1063/1.1795354 (3 pages) | Cited 13 times

Online Publication Date: 28 September 2004

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Defects in strained Si layers grown on relaxed SiGe layers were studied using chemical etching and transmission electron microscopy. Defect densities were measured in strained Si layers formed on SiGe buffer layers grown on bulk Si, as well as silicon–germanium-on-insulator substrates. It is found that, in addition to threading dislocations and dislocation pile ups, stacking faults are present in nearly all of the materials studied. The stacking faults are shown to originate in the relaxed SiGe alloy suggesting that they form during the relaxation of the SiGe layer.
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61.72.Nn Stacking faults and other planar or extended defects
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Interface disorder and inhomogeneous broadening of quantum well excitons: Do narrow lines always imply high-quality interfaces?

I. V. Ponomarev, L. I. Deych, and A. A. Lisyansky

Appl. Phys. Lett. 85, 2496 (2004); http://dx.doi.org/10.1063/1.1793341 (3 pages) | Cited 7 times

Online Publication Date: 28 September 2004

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It is a commonly assumed that narrow lines in absorption or luminescence of quantum well excitons at low temperatures indicates high quality of quantum well interfaces. We show, that at least for narrow quantum wells, this is not always the case. Correlations between morphological fluctuations of two interfaces confining a quantum well, which were neglected in previous studies of exciton line shape, strongly suppress an inhomogeneous broadening due to interface disorder.
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73.21.Fg Quantum wells
78.67.De Quantum wells
78.55.Hx Other solid inorganic materials
68.35.Ct Interface structure and roughness
68.65.Fg Quantum wells

Tensely strained silicon on SiGe produced by strain transfer

D. Buca, B. Holländer, H. Trinkaus, S. Mantl, R. Carius, R. Loo, M. Caymax, and H. Schaefer

Appl. Phys. Lett. 85, 2499 (2004); http://dx.doi.org/10.1063/1.1790593 (3 pages) | Cited 16 times

Online Publication Date: 28 September 2004

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An approach for the controlled formation of thin strained silicon layers based on strain transfer in an epitaxial Si∕SiGe∕Si(100) heterostructure during the relaxation of the SiGe layer is established. He+ ion implantation and annealing is employed to initiate the relaxation process. The strain transfer between the two epilayers is explained as an inverse strain relaxation which we modeled in terms of the propagation of the dislocations through the layers. Effcient strain buildup in the Si top layer strongly depends on the Si top layer thickness and on the relaxation degree of the SiGe buffer. 100% strain transfer was observed up to a critical thickness of the strained silicon layer of 8  nm for a 150  nm relaxed Si0.74Ge0.26 buffer.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology
61.72.uf Ge and Si
68.60.Bs Mechanical and acoustical properties
68.47.Fg Semiconductor surfaces
61.72.Cc Kinetics of defect formation and annealing
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity

In situ observation of electromigration-induced void migration in dual-damascene Cu interconnect structures

A. V. Vairagar, S. G. Mhaisalkar, Ahila Krishnamoorthy, K. N. Tu, A. M. Gusak, Moritz Andreas Meyer, and Ehrenfried Zschech

Appl. Phys. Lett. 85, 2502 (2004); http://dx.doi.org/10.1063/1.1795978 (3 pages) | Cited 26 times

Online Publication Date: 28 September 2004

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In situ electromigration experiments were carried out to study electromigration-induced failure in the upper and lower layers in dual-damascene Cu test structures. The observations revealed electromigration-induced void movement along the Cu/dielectric cap interface. It supports the premise that Cu∕Si3N4 interface acts as the dominant electromigration path. However, the observed void nucleation occurs in the Cu∕Si3N4 interface at locations which are far from the cathode, and void movement along the Cu∕Si3N4 interface in opposite direction of electron flow eventually causes void agglomeration at the via in the cathode end. The different electromigration behaviors of the upper and lower layer dual-damascene structures are discussed.
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66.30.Qa Electromigration
61.72.Qq Microscopic defects (voids, inclusions, etc.)
68.35.Ct Interface structure and roughness
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)

Surface relief gratings on polymer dispersed liquid crystals by polarization holography

A. Mazzulla, P. Pagliusi, C. Provenzano, G. Russo, G. Carbone, and G. Cipparrone

Appl. Phys. Lett. 85, 2505 (2004); http://dx.doi.org/10.1063/1.1793355 (3 pages) | Cited 13 times

Online Publication Date: 28 September 2004

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We report the observation of surface relief gratings (SRGs) on polymer dispersed liquid crystal films after polarization holographic recording, demonstrating the formation of SRGs in systems without azo compounds, where photoisomerization and chromophore reorientation processes do not occur. Permanent SRGs, several hundred nanometers deep, are recorded on the surface of a polymeric material containing oriented liquid crystal droplets. The results suggest that SRG growth under uniform intensity irradiation is not exclusively related to the photoisomerization, but is a more general phenomenon which can involve different photoinduced chemical and physical mechanisms sensitive to the light polarization state. These effects contribute to the formation of anisotropic structures during the recording process.
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61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems
68.15.+e Liquid thin films
42.70.Df Liquid crystals
42.70.Jk Polymers and organics
42.40.Eq Holographic optical elements; holographic gratings
82.50.-m Photochemistry
42.25.Ja Polarization
82.30.Qt Isomerization and rearrangement

On the fragility of Nb-Ni-based and Zr-based bulk metallic glasses

L. Shadowspeaker and R. Busch

Appl. Phys. Lett. 85, 2508 (2004); http://dx.doi.org/10.1063/1.1796523 (3 pages) | Cited 41 times

Online Publication Date: 28 September 2004

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The heating rate dependencies of the glass transition temperature of the Ni65Nb35, Ni60Nb35Sn5, Ni59.35Nb34.45Sn6.2, Ni60(Nb40Ta60)34Sn6, and Ni57Fe3Nb35Sn5 metallic glass-forming alloys were investigated with a differential scanning calorimeter (DSC). The relaxation time for each DSC experiment was plotted versus inverse temperature, and a Vogel-Fulcher-Tamman (VFT)-type relation was fitted to the data. The fragilities of the alloys were characterized with the fragility parameter, D*, and the VFT temperature, T0, which are the fit parameters from the VFT relation. It was found that for the binary alloy D*=6.2, for the ternary alloys D*=11.0, and that for the quaternary alloys D* was between 16.4 and 19.0. These D* increase monotonically as the number of components in the alloy is increased. It was also found that the fragilities of Zr-based alloys show a similar trend.
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81.05.Kf Glasses (including metallic glasses)
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition

Carrier removal in lattice-mismatched InGaP solar cells under 1-MeV-electron irradiation

N. J. Ekins-Daukes, H. S. Lee, T. Sasaki, M. Yamaguchi, A. Khan, T. Takamoto, T. Agui, K. Kamimura, M. Kaneiwa, M. Imaizumi, T. Ohshima, and T. Kamiya

Appl. Phys. Lett. 85, 2511 (2004); http://dx.doi.org/10.1063/1.1794371 (3 pages) | Cited 3 times

Online Publication Date: 28 September 2004

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Radiation-induced majority carrier removal is investigated from n+∕p− lattice-mismatched In0.56Ga0.44P solar cells under 1-MeV-electron irradiation. The change in carrier concentration in the 1×1017 cm−3p− base layer is determined using standard capacitance–voltage techniques and found to proceed at a rate Rc=1.3 cm−1, in agreement with that observed in lattice-matched InGaP. However, the observation of an increased short-circuit current and short-wavelength quantum efficiency over the unirradiated values at electron fluence levels in excess of 3×1015 cm−2, allows the carrier concentration from the n+ emitter layer to be measured. By modeling the quantum efficiency of these solar cells, it is shown that the main photoresponse from these lattice-mismatched solar cells is due to drift transport, making the spectral response highly sensitive to changes in the width of the depletion region. Using this technique, the carrier concentration in the 2×1018 cm−3 n+ emitter layer is found to be reduced to 1×1018 cm−3 after exposure to an electron fluence of 3×1015 cm−2.
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84.60.Jt Photoelectric conversion
61.80.Fe Electron and positron radiation effects
61.82.Fk Semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Fabrication of high-quality strain-relaxed thin SiGe layers on ion-implanted Si substrates

K. Sawano, S. Koh, Y. Shiraki, Y. Ozawa, T. Hattori, J. Yamanaka, K. Suzuki, K. Arimoto, K. Nakagawa, and N. Usami

Appl. Phys. Lett. 85, 2514 (2004); http://dx.doi.org/10.1063/1.1794353 (3 pages) | Cited 15 times

Online Publication Date: 28 September 2004

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We fabricated high-quality strain-relaxed thin SiGe layers by Ar ion implantation into Si substrates before epitaxial growth. The surface of 100-nm-thick Si0.8Ge0.2 layers, the relaxation ratio of which was more than 80%, was found to be very smooth, with a rms roughness of 0.34 nm. Cross-sectional transmission electron microscopy analysis confirmed that strain-relieving dislocations were effectively generated due to the ion-implantation-induced defects and confined in the vicinity of the heterointerface, resulting in a dislocation-free SiGe surface. Moreover, in-plane strain-field fluctuation was found to be largely reduced by this ion implantation method.
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68.55.A- Nucleation and growth
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.uf Ge and Si
71.55.Cn Elemental semiconductors
68.55.-a Thin film structure and morphology
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
68.60.Bs Mechanical and acoustical properties
68.37.Lp Transmission electron microscopy (TEM)

High-resolution transmission electron microscopic analysis of porous silicon∕silicon interface

R. J. Martín-Palma, L. Pascual, A. Landa, P. Herrero, and J. M. Martínez-Duart

Appl. Phys. Lett. 85, 2517 (2004); http://dx.doi.org/10.1063/1.1797558 (3 pages) | Cited 15 times

Online Publication Date: 28 September 2004

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From high-resolution transmission electron microscopy, a strong contrast was observed in the interface between porous silicon and the silicon substrate, which was associated with the presence of high stress. It was determined that stress in the porous silicon∕Si interface is caused by dislocations and that lattice matching occurs through pairs of edge-dislocations. In addition, a high density of dislocations was also observed in the neighborhood of the Si nanocrystals that compose porous silicon. From the experimental results, a mechanism for the formation of porous silicon is proposed.
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81.05.Cy Elemental semiconductors
81.05.Rm Porous materials; granular materials
61.46.-w Structure of nanoscale materials
68.35.Ct Interface structure and roughness
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.37.Lp Transmission electron microscopy (TEM)

Liquid crystal photoalignment material based on chloromethylated polyimide

Zhen-Xin Zhong, Xiangdan Li, Seung Hee Lee, and Myong-Hoon Lee

Appl. Phys. Lett. 85, 2520 (2004); http://dx.doi.org/10.1063/1.1797560 (3 pages) | Cited 10 times

Online Publication Date: 28 September 2004

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We report a liquid crystal photoalignment material with high photosensitivity and excellent thermal stability. The chloromethylated aromatic polyimide exhibited defect-free homogeneous alignment of liquid crystals upon irradiation of polarized deep ultraviolet (UV) for 50 s. The aligning ability of the film was retained up to 210°C, and the cell containing liquid crystals could be stored at 85°C for more than 14 days without any deterioration. FT-IR and UV–vis spectra confirmed that the alignment was induced by photodecomposition of polyimide, drastically accelerated by the introduction of chloromethyl side group.
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61.30.Vx Polymer liquid crystals
61.25.H- Macromolecular and polymers solutions; polymer melts
61.82.Pv Polymers, organic compounds
82.50.Hp Processes caused by visible and UV light
78.40.Dw Liquids
78.30.C- Liquids

Attenuation contrast between biomolecular and inorganic materials at terahertz frequencies

T. L. J. Chan, J. E. Bjarnason, A. W. M. Lee, M. A. Celis, and E. R. Brown

Appl. Phys. Lett. 85, 2523 (2004); http://dx.doi.org/10.1063/1.1794858 (3 pages) | Cited 23 times

Online Publication Date: 28 September 2004

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Show Abstract
Wideband photomixing spectroscopy is used in the present work to contrast the transmission spectra of macromolecules commonly found in biomaterials such as potato starch, wheat flour and cornstarch, and proteins (Cytoplex™), and micromolecules such as sucrose, and inorganic materials such as sodium bicarbonate, and calcium sulfate. Powdered samples were measured at 0.1–0.5 THz frequencies. A significant difference in attenuation is found between these samples. At 300 GHz starch shows an absorption coefficient of ∼6 cm−1 whereas Cytoplex shows 1–3 cm−1, while inorganic micromolecules have ∼1 cm−1. The absorption in starch increases rapidly with frequency tending to follow a power law α=fn with n typically between 1.5 and 2.0. In contrast, protein materials display a slower dependence on frequency with n between 1.0 and 1.5, and simple molecules show the least n among all three categories. The difference between these ubiquitous macromolecular and micromolecular materials is explained in terms of water content and molecular structure.
Show PACS
87.14.E- Proteins
87.15.B- Structure of biomolecules
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