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5 Jan 2004

Volume 84, Issue 1, pp. 1-157

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Appl. Phys. Lett. 84, 151 (2004); http://dx.doi.org/10.1063/1.1637950 (3 pages)

M. Feng, N. Holonyak, and W. Hafez
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Near-term aging and thermal behavior of polyfluorene in various aggregation states

H. Cheun, B. Tanto, W. Chunwaschirasiri, B. Larson, and M. J. Winokur

Appl. Phys. Lett. 84, 22 (2004); http://dx.doi.org/10.1063/1.1637431 (3 pages) | Cited 26 times

Online Publication Date: 30 December 2003

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Steady-stage photoluminescence and photoabsorption have been used to characterize the impact of aggregation in various poly(di-n-alkylfluorene) thin films. Highly aggregated films exhibit systematic changes in the photoluminescence while aging. These are indicative of a gradual increase in interchain interactions. Nonaggregated films do not exhibit this behavior. However the mesomorphic properties are altered and these films undergo irreversible changes on thermal cycling. All these properties appear to be correlated with changes in the local short-range molecular packing. © 2004 American Institute of Physics.
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61.41.+e Polymers, elastomers, and plastics
78.66.Qn Polymers; organic compounds
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
78.55.Kz Solid organic materials
78.40.Me Organic compounds and polymers

Time-resolved analysis of the white photoluminescence from SiO2 films after Si and C coimplantation

P. Pellegrino, A. Pérez-Rodriguez, B. Garrido, O. González-Varona, J. R. Morante, S. Marcinkevicius, A. Galeckas, and J. Linnros

Appl. Phys. Lett. 84, 25 (2004); http://dx.doi.org/10.1063/1.1634692 (3 pages) | Cited 17 times

Online Publication Date: 30 December 2003

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The analysis of the white photoluminescence (PL) from Si+ and C+ coimplanted SiO2 is reported as a function of the implanted dose. By both steady and time-resolved measurements, the presence of several components in the emission between 2 and 3.3 eV has been resolved. The decays of the PL transients are characterized by short lifetimes, below 2 ns. For the emission at 2.1–2.3 eV, photoluminescence decay transients have been measured, obtaining a fast relaxation component of about 50–70 ps, followed by a slower component of the order of 1 ns. These values contrast with the very slow behavior, characteristic for the light emission from Si nanocrystals, and make carbon-related emitting centers interesting for optoelectronic applications where fast switching behavior is important. © 2004 American Institute of Physics.
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78.55.Hx Other solid inorganic materials
78.47.-p Spectroscopy of solid state dynamics
78.66.Nk Insulators
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.up Other materials

Thermal stability and decomposition of the HfO2–Al2O3 laminate system

Hyo Sik Chang, Hyunsang Hwang, Mann-Ho Cho, Dae Won Moon, Seok Joo Doh, Jong Ho Lee, and Nae-In Lee

Appl. Phys. Lett. 84, 28 (2004); http://dx.doi.org/10.1063/1.1637955 (3 pages) | Cited 8 times

Online Publication Date: 30 December 2003

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The thermal stability of the HfO2–Al2O3 laminate gate stack grown by atomic layer chemical vapor deposition was investigated using medium-energy ion scattering spectroscopy and high-resolution x-ray photoelectron spectroscopy. The laminate structure was maintained up to 800 °C under ultrahigh vacuum conditions, while it was drastically degraded at 850 °C, resulting in silicide formation on the film surface. Dissociated oxygen in the Hf–Al-oxide preferentially diffuses out through the film and desorbing at the surface. Volatile SiO species and Al–O components desorb through the sample surface, while HfO2 contributes to Hf silicide formation on the film surface. © 2004 American Institute of Physics.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
79.60.Dp Adsorbed layers and thin films

Temperature memory effect of a nickel–titanium shape memory alloy

Yanjun Zheng, Lishan Cui, and Jan Schrooten

Appl. Phys. Lett. 84, 31 (2004); http://dx.doi.org/10.1063/1.1637958 (3 pages) | Cited 35 times

Online Publication Date: 30 December 2003

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An incomplete transformation cycle induces a kinetic stop in the following complete transformation cycle in shape memory alloys. Therefore, the kinetic stop can be regarded as a memory of the previous arrest temperature. Herein, we show that the temperature memory effect of a nickel–titanium shape memory alloy can be expanded to be operational in a very wide temperature range by prestraining and constraining, which may be exploited for various practical applications. © 2004 American Institute of Physics.
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81.30.Kf Martensitic transformations
64.70.K- Solid-solid transitions
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep

Structural stability of BeH2 at high pressures

P. Vajeeston, P. Ravindran, A. Kjekshus, and H. Fjellvåg

Appl. Phys. Lett. 84, 34 (2004); http://dx.doi.org/10.1063/1.1637967 (3 pages) | Cited 17 times

Online Publication Date: 30 December 2003

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The electronic structure and structural stability of BeH2 are studied using first-principles density-functional calculation. The calculated structural parameters for α-BeH2 at the equilibrium volume are in very good agreement with experiments. At higher pressures α-BeH2 successively undergoes four structural transitions: (i) α- to β-BeH2 at 7.07 GPa; (ii) β- to γ-BeH2 at 51.41 GPa; (iii) γ- to δ-BeH2 at 86.56 GPa; and (iv) δ- to ϵ-BeH2 at 97.55 GPa [an effective two-phase (γ and δ) region is found at 73.71–86.56 GPa]. Density of states studies reveal that BeH2 remains insulating up to 100 GPa whereupon anomalous changes are seen in the band-gap region with increasing pressure. © 2004 American Institute of Physics.
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62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Ps Other inorganic compounds

CaAl-based bulk metallic glasses with high thermal stability

F. Q. Guo, S. J. Poon, and G. J. Shiflet

Appl. Phys. Lett. 84, 37 (2004); http://dx.doi.org/10.1063/1.1637940 (3 pages) | Cited 63 times

Online Publication Date: 30 December 2003

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We report that binary Ca–Al alloys can be readily cast into amorphous rods of 1 mm in diameter. Upon further alloying to depress the liquidus temperature, the amorphous rod diameter is increased to 3 mm. The high glass transition temperature Tg ∼ 210 °C and crystallization onset temperature Tonset ∼ 240 °C or higher observed are attributed to the covalent bonding trend noted in Ca–Al alloys that exhibit complex network structures. Along with a high microhardness value of 200–235 DPH (diamond pyramid hardness) and low mass densities of 2 gm/cm3, these thermally stable light-metal alloys are recognized as potential structural amorphous metals. © 2004 American Institute of Physics.
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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
62.20.Qp Friction, tribology, and hardness
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
61.43.Fs Glasses
68.35.Gy Mechanical properties; surface strains

Fast electro-optic switching in nematic liquid crystals

Anca-Luiza Alexe-Ionescu, Andrei Th. Ionescu, Emil S. Barna, Valentin Barna, and Nicola Scaramuzza

Appl. Phys. Lett. 84, 40 (2004); http://dx.doi.org/10.1063/1.1637961 (3 pages) | Cited 1 time

Online Publication Date: 30 December 2003

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The switching between on and off states of nematic liquid crystal pixels can be controlled with the well-known electro-optic effect. This effect however presents a fast response at switching on the electric field but a slow response at switching it off. Here we show a suitable choice of materials, cell geometries, surface preparations, and time dependence of the applied voltage that leads to a switch off response as fast as that at switch on. This is due to the particular conductivity mechanism in polymers with aromatic rings leading to rectifying properties when deposited on top of indium tin oxide surface. A transient negative charge at polymer–liquid crystal interface favors a faster and stronger planar orientation of the nematic molecules diminishing drastically the switch off time and increasing the contrast ratio. These facts, interesting enough from a fundamental point of view, could also lead to important technological consequences. © 2004 American Institute of Physics.
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42.65.Pc Optical bistability, multistability, and switching, including local field effects
78.20.Jq Electro-optical effects
78.66.Qn Polymers; organic compounds
42.70.Df Liquid crystals
42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
42.79.Kr Display devices, liquid-crystal devices
61.30.Vx Polymer liquid crystals

High-gain photorefractive reflection gratings in layered photoconductive polymers

O-Pil Kwon, Germano Montemezzani, Peter Günter, and Suck-Hyun Lee

Appl. Phys. Lett. 84, 43 (2004); http://dx.doi.org/10.1063/1.1638900 (3 pages) | Cited 12 times

Online Publication Date: 30 December 2003

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Large two-wave mixing gain in reflection grating geometry is obtained in layered organic photorefractive polymers doped with the chromophore piperidinodicyanostyrene. With an applied field of 60 V/μm and for a grating spacing of 0.205 μm, one measures a two-wave mixing gain coefficient of 104 cm−1 and a diffraction efficiency of 4.8% in 100-μm-thick samples. Our photorefractive and electrochemical investigations suggest that the excellent material performance in reflection grating geometry is associated with an increased effective number of trapping sites. © 2004 American Institute of Physics.
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42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.40.Eq Holographic optical elements; holographic gratings
72.80.Le Polymers; organic compounds (including organic semiconductors)

Observation of backflow in the switch-on dynamics of a hybrid aligned nematic

S. A. Jewell and J. R. Sambles

Appl. Phys. Lett. 84, 46 (2004); http://dx.doi.org/10.1063/1.1638903 (3 pages) | Cited 9 times

Online Publication Date: 30 December 2003

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The optical convergent-beam technique is used to measure, in 0.3 ms steps, the response of the director in a 4.6-μm-thick ZLI-2293 filled hybrid aligned nematic cell when a 10 kHz, 7 Vrms ac voltage is applied to the cell. The total time taken for the reorientation process is 2.4 ms, with backflow observed during the first 1.5 ms after the application of the voltage. The measured director profiles show excellent agreement with theoretical profiles produced from the Leslie–Eriksen–Parodi theory using typical values for the viscosity coefficients. Fluid velocity profiles within the cell are also modeled. © 2004 American Institute of Physics.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
83.80.Xz Liquid crystals: nematic, cholesteric, smectic, discotic, etc.

Carrier dynamics of high-efficiency green light emission in graded-indium-content InGaN/GaN quantum wells: An important role of effective carrier transfer

Yuanping Sun, Yong-Hoon Cho, E.-K. Suh, H. J. Lee, R. J. Choi, and Y. B. Hahn

Appl. Phys. Lett. 84, 49 (2004); http://dx.doi.org/10.1063/1.1637959 (3 pages) | Cited 15 times

Online Publication Date: 30 December 2003

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Optical properties and carrier dynamics of high-efficiency green-light-emitting InGaN/GaN multiple quantum wells (MQWs) with graded-In-content were studied by photoluminescence (PL), PL excitation, and time-resolved PL techniques. Two separated InGaN-related peaks were clearly found in PL spectra due to strong phase separation in the well of the graded-In-content InGaN MQWs. The integrated intensity of the main InGaN green emission ( ∼ 510 nm) decreased by only about a factor of 7 with increasing temperature from 10 to 300 K, indicating strong carrier localization and high quantum efficiency. Strong carrier transfer from low-In-content region with weak carrier localization to high-In-content part with strong carrier localization was observed by time-resolved PL. Therefore, we conclude that the effective carrier transfer from weakly to strongly localized states plays an important role to enhance brightness and quantum efficiency in the green-light-emitting InGaN MQWs with graded-In content. © 2004 American Institute of Physics.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
73.63.Hs Quantum wells
68.65.Fg Quantum wells
78.47.-p Spectroscopy of solid state dynamics

Optical properties of metallic nanoparticles in Ni-ion-implanted α-Al2O3 single crystals

X. Xiang, X. T. Zu, S. Zhu, and L. M. Wang

Appl. Phys. Lett. 84, 52 (2004); http://dx.doi.org/10.1063/1.1636817 (3 pages) | Cited 20 times

Online Publication Date: 30 December 2003

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64 keV Ni ion implantation was performed at room temperature up to a dose of 1×1017 cm−2 in α-Al2O3 single crystals. The charge states, structure, and optical properties of metallic embedded Ni nanoparticles were studied by using x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and optical spectroscopy, respectively. XPS analysis showed that implanted Ni ions are mainly in charge state of metallic Ni0. Nanoparticles distributed from the surface to 30 nm below the surface were observed in a high-angle annular dark-field image. The size of nanoparticles ranges from 1 to 5 nm in diameter. A high-resolution electron microscopy image indicated the Ni-implanted area had been entirely amorphized. A new broad absorption band centered at 400 nm appeared in the optical absorption spectrum of the as-implanted crystal, due to surface plasma resonance of Ni nanoparticles. We did not find any emission band in the as-implanted sample under a Xe lamp excitation wavelength of 250–430 nm in a spectrophotometer. © 2004 American Institute of Physics.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
61.72.up Other materials
61.46.-w Structure of nanoscale materials
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Spectral investigation for phosphorescent polymer light-emitting devices with doubly doped phosphorescent dyes

Fangzhong Shen, Hong Xia, Chengbo Zhang, Dong Lin, Xiaodong Liu, and Yuguang Ma

Appl. Phys. Lett. 84, 55 (2004); http://dx.doi.org/10.1063/1.1629795 (3 pages) | Cited 13 times

Online Publication Date: 30 December 2003

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The photoluminescence and electroluminescence (EL) spectral characteristics of two phosphorescent dyes, green fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3] and red tris-(4,7-diphenyl-1,10-phenanthroline)rhenium dications [Ru(4,7-Ph2-phen)3]2+, doubly doped poly(N-vinylcarbazol) (PVK) films were investigated. The efficient energy transfers from PVK host to both Ir(ppy)3 and [Ru(4,7-Ph2-phen)3]2+ dopants were observed in photoexcitation and EL devices. The intensity ratio (Ri) of red emission versus green emission as function of doping ratio (Rc) of [Ru(4,7-Ph2-phen)3]2+ and Ir(ppy)3 in PVK host is found according to a power-law function with the slopes of 2.28±0.2 for PL which obey the power-law with Ri–Rc2 predicated by Förster energy transfer model. EL spectra show a driving voltage depentent power law of Ri–Rc1.53±0.14 at low voltage and Ri–Rc1.16±0.08 at high voltage, respectively. The shift in the power law of Förster energy transfer is likely due to additional emission mechanisms such as carrier trapping and triplet exciton saturation at high current density. © 2004 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.66.Qn Polymers; organic compounds
78.60.Fi Electroluminescence
78.55.Kz Solid organic materials
71.35.Cc Intrinsic properties of excitons; optical absorption spectra

Localization versus field effects in single InGaN quantum wells

A. Bell, J. Christen, F. Bertram, F. A. Ponce, H. Marui, and S. Tanaka

Appl. Phys. Lett. 84, 58 (2004); http://dx.doi.org/10.1063/1.1638880 (3 pages) | Cited 17 times

Online Publication Date: 30 December 2003

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The optical properties of InxGa1−xN quantum wells (x = 0.13) have been studied by cathodoluminescence (CL) spectroscopy. A blueshift of the quantum well emission is observed with increasing excitation density, which can be explained by considering (a) band filling of in-plane potential fluctuations caused by compositional inhomogeneities, or (b) screening of piezoelectric fields inside the well. We have used time-resolved CL spectroscopy to distinguish between the two effects. The onset and decay of the relaxation and recombination kinetics are measured by using rectangular excitation pulses with ultrafast on and off switching and with pulse lengths sufficiently long to ensure excitation into quasi-steady-state conditions. For well widths of Lz ⩽ 6 nm, a redshift is observed after the electron beam is switched on and a further redshift occurs after the electron beam is switched off. For Lz ≥ 8 nm, a blueshift is observed after the electron beam is switched on and a redshift is observed after the electron beam is switched off. We attribute the different behaviors to the dominance of localization effects for Lz ⩽ 6 nm and the dominance of field effects for Lz ≥ 8 nm. © 2004 American Institute of Physics.
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78.67.De Quantum wells
78.60.Hk Cathodoluminescence, ionoluminescence
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