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10 May 2004

Volume 84, Issue 19, pp. 3723-3937

Issue Cover Spotlight Figure

Appl. Phys. Lett. 84, 3933 (2004); http://dx.doi.org/10.1063/1.1745103 (3 pages)

A. Cassinese, G. M. De Luca, A. Prigiobbo, M. Salluzzo, and R. Vaglio
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Fabrication of crystalline organic waveguides with an exceptionally large electro-optic coefficient

W. Geis, R. Sinta, W. Mowers, S. J. Deneault, M. F. Marchant, K. E. Krohn, S. J. Spector, D. R. Calawa, and T. M. Lyszczarz

Appl. Phys. Lett. 84, 3729 (2004); http://dx.doi.org/10.1063/1.1723697 (3 pages) | Cited 53 times

Online Publication Date: 29 April 2004

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Single-crystal optical waveguides of 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), an organic material with a large electro-optic coefficient, have been obtained. DAST decomposes at its melting temperature, making its growth from the melt difficult. However, graphoepitaxy allows for >1 mm s−1 growth, 1×105 times faster than conventional techniques, and produces crystals of the correct dimensions for optical waveguides, 1–15 μm on a side and 5–10 mm long. The crystals grow with the c-axis normal to the substrate, and with in-plane orientation determined by lithographic patterning. The electro-optic coefficient dn/dE is 600±300 pm V−1 at 1.55 μm wavelength. Optical losses are <10 dB cm−1. © 2004 American Institute of Physics.
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78.20.Jq Electro-optical effects
42.79.Gn Optical waveguides and couplers
81.10.Fq Growth from melts; zone melting and refining
64.70.D- Solid-liquid transitions
85.40.Hp Lithography, masks and pattern transfer

Electrodeless wet etching of GaN assisted with chopped ultraviolet light

Z. H. Hwang, J. M. Hwang, H. L. Hwang, and W. H. Hung

Appl. Phys. Lett. 84, 3759 (2004); http://dx.doi.org/10.1063/1.1737799 (3 pages) | Cited 11 times

Online Publication Date: 29 April 2004

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Electrodeless photoelectrochemical (PEC) etching of GaN was studied in a K2S2O8/KOH solution irradiated with ultraviolet (UV) light either continuously or periodically. The rate of recombination of electrons and holes at dislocation defects is greater than for crystalline GaN, resulting in a rough etched surface with hexagonal pyramids. The shorter the interval of UV irradiation, the smoother is the etched GaN surface; a chopped UV source thus serves to improve the morphology of the etched surface. A uniform and smooth GaN surface was obtained with a root-mean-square roughness 0.37 nm through electrodeless PEC etching in a solution (KOH 0.01 M, K2S2O8 0.05 M) with a chopper frequency 2500 Hz (i.e., duration of irradiation 0.2 ms). © 2004 American Institute of Physics.
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81.65.Cf Surface cleaning, etching, patterning
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.05.Ea III-V semiconductors
82.45.-h Electrochemistry and electrophoresis
82.50.-m Photochemistry
82.45.Jn Surface structure, reactivity and catalysis
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Heteroepitaxial growth of wurtzite InN films on Si(111) exhibiting strong near-infrared photoluminescence at room temperature

S. Gwo, C.-L. Wu, C.-H. Shen, W.-H. Chang, T. M. Hsu, J.-S. Wang, and J.-T. Hsu

Appl. Phys. Lett. 84, 3765 (2004); http://dx.doi.org/10.1063/1.1738183 (3 pages) | Cited 71 times

Online Publication Date: 29 April 2004

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High-quality InN epitaxial films have been grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering. At room temperature, these films exhibited strong near-infrared (0.6–0.9 eV) photoluminescence (PL). In addition to the optical absorption measurement of absorption edge and direct band nature, the PL signal was found to depend linearly on the excitation laser intensity over a wide intensity range. These results indicate that the observed PL is due to the emission of direct band-to-band recombination rather than the band-to-defect (or impurity) deep emission. © 2004 American Institute of Physics.
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81.05.Ea III-V semiconductors
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.30.Fs III-V and II-VI semiconductors
68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Improved quantum efficiency in nonpolar (11math0) AlGaN/GaN quantum wells grown on GaN prepared by lateral epitaxial overgrowth

T. Koida, S. F. Chichibu, T. Sota, M. D. Craven, B. A. Haskell, J. S. Speck, S. P. DenBaars, and S. Nakamura

Appl. Phys. Lett. 84, 3768 (2004); http://dx.doi.org/10.1063/1.1738185 (3 pages) | Cited 30 times

Online Publication Date: 29 April 2004

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Radiative and nonradiative excitonic transitions in nonpolar (11math0) AlxGa1−xN/GaN multiple quantum wells (MQWs) grown on the GaN template prepared by lateral epitaxial overgrowth (LEO-GaN) were investigated. The structural advantages of using nonpolar orientations were confirmed by a moderate shift of the photoluminescence (PL) peak energy and negligible change in low-temperature PL lifetime with decreasing GaN well width, both of which are the results of eliminating quantum-confined Stark effects due to the polarization fields that exist in polar (0001) MQWs. Appearance of the correct in-plane light polarization and improved internal quantum efficiency for the PL peak in the MQWs on LEO-GaN were attributed to the reduction in densities of nonradiative defects and bound states. © 2004 American Institute of Physics.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
68.65.Fg Quantum wells
78.47.-p Spectroscopy of solid state dynamics
73.21.Fg Quantum wells

Molecular weight dependence of birefringence of thin films of the conjugated polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene]

Kaloian Koynov, Ayi Bahtiar, Taek Ahn, Christoph Bubeck, and Hans-Heinrich Hörhold

Appl. Phys. Lett. 84, 3792 (2004); http://dx.doi.org/10.1063/1.1739513 (3 pages) | Cited 16 times

Online Publication Date: 29 April 2004

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We prepared thin films of the conjugated model polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) by the spin coating of polymer samples whose weight-average molecular weight was varied from 104–106 g/mol. Waveguide prism coupling and polarized optical transmission and reflection spectroscopy were used to determine the refractive index, birefringence, and intrinsic absorption coefficient of each film. We show that these optical constants significantly depend on the molecular weight of the polymers. With decreasing molecular weight, the birefringence is strongly reduced and a nearly isotropic refractive index is approached. We conclude that the molecular weight primarily determines the extent of polymer chain alignment in the film plane. © 2004 American Institute of Physics.
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78.66.Qn Polymers; organic compounds
42.70.Jk Polymers and organics
78.20.Fm Birefringence
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.A- Nucleation and growth

Determination of density profile of ultrathin SiO2/Si3N4/SiO2/Si(001) multilayer structures using x-ray reflectivity technique

S. Banerjee, S. Ferrari, R. Piagge, and S. Spandoni

Appl. Phys. Lett. 84, 3798 (2004); http://dx.doi.org/10.1063/1.1739519 (3 pages) | Cited 7 times

Online Publication Date: 29 April 2004

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In this report, we present an analysis of a SiO2/Si3N4/SiO2/Si(001) ultrathin multilayer structure using the x-ray reflectivity technique. The trilayer was grown using low-pressure chemical vapor deposition with each layer having a nominal thickness of ∼50 Å. Here, we propose an approach to analyze ultrathin multilayer films using two analysis schemes in tandem, i.e., distorted wave Born approximation and exact recursive formalism for x-ray reflectivity. We observe that, when SiO2 is deposited on a Si substrate, the electron density is lower than when it is deposited on Si3N4. We also observe that the two interfaces of the nitride (Si3N4) film are different, i.e., the interfacial width with the lower SiO2 is larger than the width with the top SiO2 layer. © 2004 American Institute of Physics.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
61.05.cm X-ray reflectometry (surfaces, interfaces, films)

Broadband 1400 nm emission from Ni2+ in zinc—alumino—silicate glass

Takenobu Suzuki and Yasutake Ohishi

Appl. Phys. Lett. 84, 3804 (2004); http://dx.doi.org/10.1063/1.1741027 (3 pages) | Cited 44 times

Online Publication Date: 29 April 2004

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Broadband near-infrared emission from Ni2+ in zinc–alumino–silicate glass was observed at room temperature. The emission band had a peak at ∼1400 nm with a bandwidth more than 300 nm, and covered the O, E, S, C, and L bands (1260–1625 nm). The emission lifetime was more than 200 μs even at room temperature. The observed near-infrared emission could be attributed to the 3A2g(3F)→3T1g(3F) transition of Ni2+ in octahedral sites. This nickel-doped glass can be expected as an amplification medium for tunable lasers and broadband optical amplifiers for the wavelength division multiplexing transmission system applications. © 2004 American Institute of Physics.
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78.35.+c Brillouin and Rayleigh scattering; other light scattering
78.55.Qr Amorphous materials; glasses and other disordered solids

Room-temperature growth of a carbon nanofiber on the tip of conical carbon protrusions

Masaki Tanemura, T. Okita, H. Yamauchi, S. Tanemura, and R. Morishima

Appl. Phys. Lett. 84, 3831 (2004); http://dx.doi.org/10.1063/1.1745109 (3 pages) | Cited 36 times

Online Publication Date: 29 April 2004

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Glassy carbon was Ar+-ion bombarded with a simultaneous Mo supply under ultrahigh vacuum conditions using a microprotrusion fabrication system that consists of a differentially pumped ion gun and a seed-material supply source. Conical protrusions were formed by sputtering with a seed supply, and carbon nanofibers (CNFs) grew on the tips even at room temperature. The length of CNFs reached up to ∼10 μm, and their diameter was almost uniform (50 nm) in the growth direction. The short CNFs aligned in the ion beam direction, whereas the long ones were non-aligned. The CNF growth on a glassy carbon surface was ascribed to the enhanced surface texturing and to the massive redeposition of C atoms onto cones, both of which are specific to the oblique ion bombardment: The former would lead to an increase in the number of possible nucleation sites for the CNF growth, and the C atoms arising from the latter process would migrate toward the conical tips, thus forming CNFs. © 2004 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
81.05.U- Carbon/carbon-based materials
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.43.Er Other amorphous solids
61.80.Jh Ion radiation effects
64.60.Q- Nucleation
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.07.-b Nanoscale materials and structures: fabrication and characterization

Blue-light emission from sputtered Ti:SiO2 films without annealing

Osamu Hanaizumi, Kazutaka Ono, Yuichi Ogawa, Toshiaki Matsumoto, Hidehiko Yoda, and Kazuo Shiraishi

Appl. Phys. Lett. 84, 3843 (2004); http://dx.doi.org/10.1063/1.1745113 (3 pages) | Cited 3 times

Online Publication Date: 29 April 2004

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Blue-light emission from Ti:SiO2 sputtered films was observed at room temperature without annealing and it could be seen by the naked eye. The peaks of photoluminescence spectra were located at 3.03–3.05 eV and full width at half maximum ranged from 0.38–0.40 eV, which were almost the same in samples having different energies of absorption edge. SiOx layers may contribute to emission, which are interfacial regions between Ti particles and surrounding SiO2 medium, and the size of Ti particles may affect the efficiency of emission.© 2004 American Institute of Physics.
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78.66.Nk Insulators
78.55.Hx Other solid inorganic materials
68.55.A- Nucleation and growth

Observation of nonequilibrium longitudinal optical phonons in InN and its implications

W. Liang, K. T. Tsen, D. K. Ferry, Hai Lu, and William J. Schaff

Appl. Phys. Lett. 84, 3849 (2004); http://dx.doi.org/10.1063/1.1748838 (3 pages) | Cited 2 times

Online Publication Date: 29 April 2004

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Nonequilibrium longitudinal optical phonons in a high quality, single crystal wurtzite structure InN sample have been studied by picosecond Raman spectroscopy. Our experimental results demonstrate that the band gap of InN cannot be around 1.89 eV; but are consistent with a band gap of about 0.8 eV. In addition, they disprove the idea that 0.8 eV luminescence observed recently in InN is due to deep level radiative emission in InN. © 2004 American Institute of Physics.
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78.30.Fs III-V and II-VI semiconductors
78.66.Fd III-V semiconductors
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.55.Cr III-V semiconductors
78.47.-p Spectroscopy of solid state dynamics

Electroluminescence as internal light source for measurement of the photonic strength of random porous GaP

A. F. van Driel, D. Vanmaekelbergh, and J. J. Kelly

Appl. Phys. Lett. 84, 3852 (2004); http://dx.doi.org/10.1063/1.1748839 (3 pages) | Cited 5 times

Online Publication Date: 29 April 2004

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During porous etching of GaP, electroluminescence ranging from the ultraviolet to the near-infrared is generated at the interface of the porous and the nonporous layer. This is used to measure the wavelength-dependent transmission of light through porous layers in a wide thickness range. Two types of porous structures, characterized by different pore sizes, were studied. The transmission of the emitted light gives valuable information about wavelength-dependent diffusion of light through porous GaP. © 2004 American Institute of Physics.
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81.05.Ea III-V semiconductors
81.05.Rm Porous materials; granular materials
78.60.Fi Electroluminescence
81.65.Cf Surface cleaning, etching, patterning
78.40.Ha Other nonmetallic inorganics
78.30.Fs III-V and II-VI semiconductors

Emission from the higher-order excitons in ZnO films grown by laser molecular-beam epitaxy

A. Tsukazaki, A. Ohtomo, M. Kawasaki, T. Makino, C. H. Chia, Y. Segawa, and H. Koinuma

Appl. Phys. Lett. 84, 3858 (2004); http://dx.doi.org/10.1063/1.1748847 (3 pages) | Cited 17 times

Online Publication Date: 29 April 2004

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Epitaxial ZnO thin films were grown by laser molecular-beam epitaxy on lattice-matched ScAlMgO4 substrates following the deposition and annealing of suitable buffer layers. The samples were characterized by low-temperature photoluminescence (PL), absorption, and reflectivity measurements. PL from higher order (n = 2) excitons (A exciton) was observed at temperatures lower than 40 K. The absorption spectrum contained lines and the reflection spectrum exhibited anomalies that were assigned to the excited-states (n = 2,3) of A and B excitons. The optical quality could be improved dramatically by using annealed ZnO or MgZnO buffer layers. © 2004 American Institute of Physics.
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71.35.Aa Frenkel excitons and self-trapped excitons
78.66.Hf II-VI semiconductors

Diffusion and segregation effects in doped manganite/titanate heterostructures

J. Simon, T. Walther, W. Mader, J. Klein, D. Reisinger, L. Alff, and R. Gross

Appl. Phys. Lett. 84, 3882 (2004); http://dx.doi.org/10.1063/1.1738930 (3 pages) | Cited 13 times

Online Publication Date: 29 April 2004

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For the properties of devices based on oxide multilayers, interfaces play a crucial role. For ferromagnetic tunnel junctions the sign of the spin polarization depends on the barrier material and the interface termination. Here, we present a transmission electron microscopy study of heteroepitaxial trilayer systems consisting of a SrTiO3 insulating barrier sandwiched between La2/3D1/3MnO3 (D = Ca or Ba) electrodes grown on SrTiO3 substrates. Intermixing during growth depends on the earth alkaline ion: Ca segregates to the surface of the growing epitaxial thin film, leading to a Ca rich interface layer between the bottom electrode and the barrier. In contrast, no Ba segregation could be detected in the well lattice-matched Ba-doped film. In addition, a significant diffusion of both Ca and Ba into the SrTiO3 tunneling barrier is found, whereas the diffusion of Mn is negligibly small. Our results explain the different tunneling magnetoresistance in various manganite based tunnel junctions. © 2004 American Institute of Physics.
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73.43.Qt Magnetoresistance
68.35.B- Structure of clean surfaces (and surface reconstruction)
75.47.Lx Magnetic oxides

Preparation and phase transformation of highly ordered TiO2 nanodot arrays on sapphire substrates

Po-Lin Chen, Cheng-Tzu Kuo, Fu-Ming Pan, and Tzeng-Guang Tsai

Appl. Phys. Lett. 84, 3888 (2004); http://dx.doi.org/10.1063/1.1738941 (3 pages) | Cited 19 times

Online Publication Date: 29 April 2004

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Ordered nanodot arrays of titanium oxide (TiO2) were prepared from an epitaxial Al/TiN bilayered film on a sapphire substrate by electrochemical anodization of the TiN layer using a nanoporous anodic aluminum oxide (AAO) film as the template. The nanodots with an average diameter of about 60 nm can faithfully duplicate the size, shape, and hexagonal pore pattern of the AAO nanopores. The phase development of the isolated TiO2 nanodots is very much different from TiO2 thin films and powders. After high temperature annealing, the nanodots are polycrystalline and consist of a mixed phase of anatase and rutile instead of single rutile phase. We expect that TiO2 nanodots with a single phase of anatase can be realized as long as the size of the nanodots is smaller than the critical nuclei size for rutile formation. © 2004 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
81.07.Wx Nanopowders
81.20.-n Methods of materials synthesis and materials processing
64.70.K- Solid-solid transitions
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
82.45.Aa Electrochemical synthesis
81.40.Gh Other heat and thermomechanical treatments

Amorphization of silicon carbide by carbon displacement

R. Devanathan, F. Gao, and W. J. Weber

Appl. Phys. Lett. 84, 3909 (2004); http://dx.doi.org/10.1063/1.1739515 (3 pages) | Cited 12 times

Online Publication Date: 29 April 2004

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We have used molecular dynamics simulations to examine the possibility of amorphizing silicon carbide (SiC) by exclusively displacing C atoms. At a defect generation corresponding to 0.2 displacements per atom, the enthalpy surpasses the level of melt-quenched SiC, the density decreases by about 15%, and the radial distribution function shows a lack of long-range order. Prior to amorphization, the surviving defects are mainly C Frenkel pairs (67%), but Si Frenkel pairs (18%) and antisite defects (15%) are also present. The results indicate that SiC can be amorphized by C sublattice displacements. Chemical short-range disorder, arising mainly from Frenkel pair production, plays a significant role in the amorphization. © 2004 American Institute of Physics.
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61.43.Bn Structural modeling: serial-addition models, computer simulation
81.40.Gh Other heat and thermomechanical treatments
61.72.J- Point defects and defect clusters
65.40.G- Other thermodynamical quantities
61.43.-j Disordered solids
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