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23 Oct 2000

Volume 77, Issue 17, pp. 2629-2769

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Enhanced wavelength tuning of an InGaAsP-InP laser with a thermal-strain-magnifying trench

D. A. Cohen, B. Mason, J. Dolan, C. Burns, and L. A. Coldren

Appl. Phys. Lett. 77, 2629 (2000); http://dx.doi.org/10.1063/1.1319527 (3 pages) | Cited 4 times

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We have used temperature-dependent strain from differential thermal expansion to increase the temperature tuning rate, dλ/dT, of the modal wavelength of an InP-based laser. The effectiveness of the strain may be further enhanced with a deep trench etch beneath the laser waveguide. We have obtained a 50% increase in the tuning rate, without degradation of the threshold current, and a maximum increase of 86%. © 2000 American Institute of Physics.
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42.60.Fc Modulation, tuning, and mode locking
42.55.Px Semiconductor lasers; laser diodes
65.40.De Thermal expansion; thermomechanical effects
81.65.Cf Surface cleaning, etching, patterning
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Optically resolving dynamic processes in commercial liquid crystal cells

N. J. Smith and J. R. Sambles

Appl. Phys. Lett. 77, 2632 (2000); http://dx.doi.org/10.1063/1.1319533 (3 pages) | Cited 11 times

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The recently developed fully leaky guided mode technique is a powerful tool for accurately resolving the director profile in standard, commercially fabricated liquid crystal cells. By employing a convergent beam to simultaneously excite many fully leaky guided modes, angular dependent reflectivity and transmissivity data can be captured in a fraction of a millisecond with a charge coupled device array, thus enabling the switching dynamics of the liquid crystal cell to be resolved. A preliminary study of the director profile (optical tensor distribution) during the relaxation of a homogeneously aligned cell (antiparallel rubbed) filled with the nematic material ZLI-2293 is presented here. By resolving the director distribution as a function of time, the time constant for the relaxation process is obtained from which the effective rotational viscosity coefficient γ∗ is then evaluated. © 2000 American Institute of Physics.
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42.70.Df Liquid crystals
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
42.82.Et Waveguides, couplers, and arrays
42.79.Gn Optical waveguides and couplers
78.20.Jq Electro-optical effects
66.20.-d Viscosity of liquids; diffusive momentum transport

Efficient photovoltaic cells from semiconducting polymer heterojunctions

Samson A. Jenekhe and Shujian Yi

Appl. Phys. Lett. 77, 2635 (2000); http://dx.doi.org/10.1063/1.1320022 (3 pages) | Cited 91 times

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Solar cells made from spin-coated bilayer thin-film heterojunctions of poly(p-phenylene vinylene) and poly(benzimidazobenzophenanthroline ladder) were found to have photovoltaic charge collection efficiency as high as 49%. The power conversion efficiency varied from 1.4% under sunlight illumination to 2.0% at the peak wavelength. A space-charge region around the polymer/polymer interface, Ohmic contacts at the electrodes, and complementary absorption bands of the semiconducting polymers, play important roles in the efficient charge collection in the photocells. © 2000 American Institute of Physics.
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84.60.Jt Photoelectric conversion
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.61.Ph Polymers; organic compounds

Near-field fiber tip to handle high input power more than 150 mW

S. T. Jung, D. J. Shin, and Y. H. Lee

Appl. Phys. Lett. 77, 2638 (2000); http://dx.doi.org/10.1063/1.1320035 (3 pages) | Cited 4 times

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A high-power near-field fiber tip is proposed and demonstrated. This high-power tip can handle an optical power of greater than 150 mW injected into the fiber core, higher than any previous tip. The tip has a unique, thick, heat-conducting metal layer deposited by an electroplating process. The subwavelength aperture of the tip is fabricated by the controlled lapping of the end face with in situ optical monitoring. We also demonstrate using this tip to record on phase change and photoresist media. © 2000 American Institute of Physics.
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07.60.Vg Fiber-optic instruments
42.81.Wg Other fiber-optical devices
07.79.Fc Near-field scanning optical microscopes

Organic solid-state distributed feedback dye laser with a nonmorphological modification grating

Michifumi Nagawa, Musubu Ichikawa, Toshiki Koyama, Hirofusa Shirai, Yoshio Taniguchi, Akihito Hongo, Shinji Tsuji, and Yoshiaki Nakano

Appl. Phys. Lett. 77, 2641 (2000); http://dx.doi.org/10.1063/1.1320034 (3 pages) | Cited 21 times

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We report an organic solid-state distributed feedback laser consisting of an organic active layer and a Bragg grating without morphological change. The active layer consists of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran as a laser dye, tris(8hydroxyquinoline) aluminum as a host, and poly(methyl methacrylate) as a binder. The threshold and linewidth of the laser are 400 μJ/cm2 and 1.5 nm, respectively. With this laser, the Bragg grating exists out of the active layer and the grating surface is flat, which are very important for device fabrication and electric driving of the laser. © 2000 American Institute of Physics.
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42.55.Mv Dye lasers
42.60.By Design of specific laser systems
42.70.Hj Laser materials
42.55.Rz Doped-insulator lasers and other solid state lasers
42.79.Dj Gratings

Optical alignment and fixation of liquid crystals using azopolymer networks

Yue Zhao, Yanick Chénard, and Tigran V. Galstian

Appl. Phys. Lett. 77, 2644 (2000); http://dx.doi.org/10.1063/1.1320461 (3 pages) | Cited 2 times

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Anisotropic gels of a nematic liquid crystal (NLC) were prepared using optical alignment and fixation in the absence of surface orientation layers. The method consists of first making a homogeneous mixture that contains a reactive diacrylate monomer bearing azobenzene group, a nonreactive NLC and a photoinitiator, then irradiating the mixture by a laser at λ = 532 nm for the liquid crystal (LC) orientation induction. Subsequently, the mixture is exposed by a second laser at λ = 350 nm for photopolymerization giving rise to anisotropic azobenzene network. Infrared dichroism measurements show that a remnant long-range LC orientation is retained by the network. © 2000 American Institute of Physics.
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42.70.Df Liquid crystals
61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures
78.20.Fm Birefringence
78.30.C- Liquids
82.70.Gg Gels and sols
82.35.-x Polymers: properties; reactions; polymerization
82.50.-m Photochemistry
42.62.-b Laser applications

Distributed-feedback sol-gel dye laser tunable in the near ultraviolet

Xiao-lei Zhu and Dennis Lo

Appl. Phys. Lett. 77, 2647 (2000); http://dx.doi.org/10.1063/1.1320454 (3 pages) | Cited 12 times

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Laser emission in the near ultraviolet region was observed from a distributed-feedback solid-state dye laser based on Exalite 377E dye-doped sol-gel silica slabs. The output performance of the laser was studied in detail. Pumped at 308 nm, wavelength tuning from 367 to 383 nm with the emission linewidth of 80 pm has been achieved. Exalite 377E-doped sol-gel silica distributed feedback laser showed a slope efficiency of 4.4% and a pump energy threshold of approximately 100 μJ (15 mJ/cm2 in energy density). The variation of output energy as a function of the number of pump pulse at 0.5 Hz repetition rate was also investigated. © 2000 American Institute of Physics.
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42.55.Mv Dye lasers
42.60.Fc Modulation, tuning, and mode locking
42.70.Hj Laser materials
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Investigation of the sites of dark spots in organic light-emitting devices

Yoon-Fei Liew, Hany Aziz, Nan-Xing Hu, Hardy Sze-On Chan, Gu Xu, and Zoran Popovic

Appl. Phys. Lett. 77, 2650 (2000); http://dx.doi.org/10.1063/1.1320459 (3 pages) | Cited 60 times

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Poor environmental stability has been a major concern for organic light-emitting devices. Exposure to ambient conditions leads to the formation of nonemissive areas (dark spots) that result in a decrease in device luminescence. Although a number of mechanisms for the formation of dark spots have been proposed, the causes underlying their initiation, and their nucleation sites are still far from being clear. In this study, optical microscopy is used to investigate the sites of dark spots of devices in which the original cathodes are peeled off and replaced by newer cathodes. Results confirm that the growth of dark spots occurs primarily due to cathode delamination. The growth of dark spots is also associated with changes in the organic layers, especially at the organic/cathode interface. Results also suggest that the nucleation of dark spots takes place at the organic/cathode interface and originates during the deposition of the cathode. On the other hand, both the anode and the hole transport layer do not appear to play a role in the formation of dark spots. © 2000 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.66.Qn Polymers; organic compounds
73.20.Hb Impurity and defect levels; energy states of adsorbed species
71.55.Ht Other nonmetals

Chemical bonding state analysis of silicon carbide layers in Mo/SiC/Si multilayer mirrors by soft x-ray emission and absorption spectroscopy

Yasuji Muramatsu, Hisataka Takenaka, Yuko Ueno, Eric M. Gullikson, and Rupert C. C. Perera

Appl. Phys. Lett. 77, 2653 (2000); http://dx.doi.org/10.1063/1.1318231 (3 pages) | Cited 7 times

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Soft x-ray emission and absorption spectra in the C K region of Mo/SiC/Si multilayer mirrors were measured using highly brilliant synchrotron radiation to identify the chemical bonding states of the buried silicon carbide layers. Comparison with the C 2p density of state (DOS) spectra, calculated by discrete variational-Xα molecular orbital calculations, of several SiC-based cluster models showed that the measured x-ray spectra approximately agreed with the calculated C 2p–DOS spectra of the c- and h-SiC-based SiCx models in which some silicon atoms were replaced by carbon atoms. The chemical bonding states of the silicon carbide layers in the Mo/SiC/Si multilayer mirrors were therefore estimated to be carbon-excessive silicon carbide. © 2000 American Institute of Physics.
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07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
42.79.Wc Optical coatings
42.79.Bh Lenses, prisms and mirrors
78.66.-w Optical properties of specific thin films
78.70.En X-ray emission spectra and fluorescence
61.50.Lt Crystal binding; cohesive energy
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

Analysis of spatial light modulation characteristics of C60

Sukhdev Roy, C. P. Singh, and K. P. J. Reddy

Appl. Phys. Lett. 77, 2656 (2000); http://dx.doi.org/10.1063/1.1320033 (3 pages) | Cited 6 times

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Spatial light modulation based on nonlinear absorption of light by excited-state molecules in C60–toluene solution is demonstrated. Amplitude modulation of probe laser beam transmissions at 747 and 885 nm, corresponding to the peak absorption of T1 and S1 states, by the modulation laser beam intensity induced population changes at 532 nm has been analyzed using the rate equation approach, for various values of the small-signal absorption coefficient. It is shown that for T1 and S1 states, modulation up to 100% and 94.5% can be achieved for a laser beam intensity of 800 W/cm2 and 1 kW/cm2 at 532 nm, respectively. © 2000 American Institute of Physics.
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78.66.Tr Fullerenes and related materials
42.79.Hp Optical processors, correlators, and modulators
42.30.Lr Modulation and optical transfer functions
78.20.Bh Theory, models, and numerical simulation
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The role of electrode material and polarization fatigue on electron emission from ferroelectric Pb(ZrxTi1−x)O3 cathodes

M. Angadi, O. Auciello, A. R. Krauss, and H. W. Gundel

Appl. Phys. Lett. 77, 2659 (2000); http://dx.doi.org/10.1063/1.1319532 (3 pages) | Cited 18 times

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Electron emission from bulk Pt/PZT/Pt and RuO2/PZT/RuO2 ferroelectric cathodes is reported. The total emission current (0.8 A) and polarization fatigue are high for ferroelectric cathodes with Pt electrodes, while emission current (0.18 A) and polarization fatigue are significantly lower for cathodes with RuO2 electrodes. The data correlate with earlier results from research on ferroelectric memories, which suggest that oxygen vacancies near the electrode/ferroelectric interface and through the bulk of the ferroelectric layer play a major role in controlling the polarization switching of PZT-based capacitors and consequently in controlling the electron emission process in PZT cathodes. The strong correlation between polarization fatigue and emission observed in the present investigation provides evidence for the role of polarization inversion on the electron emission phenomenon. In addition, the data provide evidence for the role that oxygen vacancies play in controlling electron emission from PZT cathodes. © 2000 American Institute of Physics.
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79.70.+q Field emission, ionization, evaporation, and desorption
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.Dj Domain structure; hysteresis
77.22.Ej Polarization and depolarization
77.80.Fm Switching phenomena
85.50.-n Dielectric, ferroelectric, and piezoelectric devices

Generating positrons with femtosecond-laser pulses

C. Gahn, G. D. Tsakiris, G. Pretzler, K. J. Witte, C. Delfin, C.-G. Wahlström, and D. Habs

Appl. Phys. Lett. 77, 2662 (2000); http://dx.doi.org/10.1063/1.1319526 (3 pages) | Cited 73 times

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Utilizing a femtosecond table-top laser system, we have succeeded in converting via electron acceleration in a plasma channel, low-energy photons into antiparticles, namely positrons. The average intensity of this source of positrons is estimated to be equivalent to 2×108 Bq and it exhibits a very favorable scaling for higher laser intensities. The advent of positron production utilizing femtosecond laser pulses may be the forerunner to a table-top positron source appropriate for applications in material science, and fundamental physics research like positronium spectroscopy. © 2000 American Institute of Physics.
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07.77.Ka Charged-particle beam sources and detectors
41.75.Jv Laser-driven acceleration
52.38.-r Laser-plasma interactions
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
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Dislocation reduction in GaN films through selective island growth of InGaN

S. Keller, G. Parish, J. S. Speck, S. P. DenBaars, and U. K. Mishra

Appl. Phys. Lett. 77, 2665 (2000); http://dx.doi.org/10.1063/1.1319528 (3 pages) | Cited 1 time

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Dislocation reduction in GaN films grown on sapphire and silicon substrates was achieved by inserting thin InGaN layers grown in a selective island growth mode after partial passivation of the GaN surface with a submonolayer of silicon nitride. We show that this technique is most effective at reducing the pure edge dislocation density when it is high (i.e., >1010 cm−2) and less when the density is in the 108–109 cm−2 range. Thus, the structural quality of typically highly dislocated GaN on silicon films could be significantly improved, visible in a reduction of the (0002) full width at half maximum (FWHM) from 1300 arcsec for ordinary GaN on silicon to 800 arcsec for GaN films with silicon nitride/InGaN interlayers. In the case of GaN layers grown on sapphire (dislocation density ∼ 109 cm−2), the method resulted mainly in a reduction of the FWHM of the (10math2) and (20math1) diffraction peaks. © 2000 American Institute of Physics.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.65.Rv Passivation
81.05.Ea III-V semiconductors

Enhanced luminescence in InGaN multiple quantum wells with quaternary AlInGaN barriers

Jianping Zhang, J. Yang, G. Simin, M. Shatalov, M. Asif Khan, M. S. Shur, and R. Gaska

Appl. Phys. Lett. 77, 2668 (2000); http://dx.doi.org/10.1063/1.1319531 (3 pages) | Cited 50 times

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We report on the comparative photoluminescence studies of AlGaN/GaN, GaN/InGaN, and AlInGaN/InGaN multiple quantum well (MQW) structures. The study clearly shows the improvement in materials quality with the introduction of indium. Our results point out the localized state emission mechanism for GaN/InGaN structures and the quantum well emission mechanism for AlInGaN/InGaN structures. The introduction of indium is the dominant factor responsible for the observed differences in the photoluminescence spectra of these MQW structures. © 2000 American Institute of Physics.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

Scanning calorimeter for nanoliter-scale liquid samples

E. A. Olson, M. Yu. Efremov, A. T. Kwan, S. Lai, V. Petrova, F. Schiettekatte, J. T. Warren, M. Zhang, and L. H. Allen

Appl. Phys. Lett. 77, 2671 (2000); http://dx.doi.org/10.1063/1.1319506 (3 pages) | Cited 23 times

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We introduce a scanning calorimeter for use with a single solid or liquid sample with a volume down to a few nanoliters. Its use is demonstrated with the melting of 52 nL of indium, using heating rates from 100 to 1000 K/s. The heat of fusion was measured to within 5% of the bulk value, and the sensitivity of the measurement was ±7 μW. The heat of vaporization of water was measured in the scanning mode to be within ±23% of the bulk value by actively vaporizing water droplets from 2 to 100 nL in volume. Results within 25% were obtained for the heat of vaporization by using the calorimeter in a heat-conductive mode and measuring the passive evaporation of water. Temperature measurements over a period of 10 h had a standard deviation of 3 mK. © 2000 American Institute of Physics.
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07.20.Fw Calorimeters
64.70.F- Liquid-vapor transitions
64.70.D- Solid-liquid transitions
07.20.Dt Thermometers

Optimized leaky mode spectroscopy with a single planar film

M. Leitz, R. P. Podgorsek, H. Franke, and J. Woods

Appl. Phys. Lett. 77, 2674 (2000); http://dx.doi.org/10.1063/1.1319512 (3 pages) | Cited 6 times

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A transparent dielectric layer coated on a high refractive index prism forms a planar leaky waveguide. The leaky mode spectrum of this layer strongly depends on the imaginary part of the layer refractive index κ. As an example the leaky mode spectrum of a doped polymer layer is studied theoretically and experimentally. For κ = 0 no modes are detectable. In the range of κ ≈ 10−4, so-called “optimized leaky modes,” can be observed while, in the range κ ≥ 10−3, broad resonances with reduced reflectivity can be observed. The imaginary part of the polymer refractive index was adjusted by doping with the dye malachit green with an absorption maximum at λ = 620 nm. © 2000 American Institute of Physics.
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78.66.Qn Polymers; organic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
42.70.Jk Polymers and organics
42.79.Gn Optical waveguides and couplers

Optical strain characteristics of holographically formed polymer-dispersed liquid crystal films

Darran R. Cairns, Christopher C. Bowley, Sorasak Danworaphong, Adam K. Fontecchio, Gregory P. Crawford, Le Li, and Sadag M. Faris

Appl. Phys. Lett. 77, 2677 (2000); http://dx.doi.org/10.1063/1.1319510 (3 pages) | Cited 22 times

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The optical strain characteristics of a reflective holographic-polymer dispersed liquid crystal are investigated. A spectral blueshift and induced polarization dependence are observed to occur with increasing strain. The spectral dependence on strain is explained by the Poisson contraction of the material and the polarization dependence is largely attributed to elongation of liquid crystal droplets. The material has potential use as an optical strain gauge. © 2000 American Institute of Physics.
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42.70.Df Liquid crystals
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects
78.40.Dw Liquids
42.40.My Applications
42.40.Pa Volume holograms
61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures
07.10.Pz Instruments for strain, force, and torque

Quantitative long-range-order measurement and disordering efficiency estimation in ion-irradiated bulk Ni3Al using cross-sectional conventional transmission electron microscopy

P. de Almeida, R. Schäublin, A. Almazouzi, M. Victoria, and M. Döbeli

Appl. Phys. Lett. 77, 2680 (2000); http://dx.doi.org/10.1063/1.1320037 (3 pages) | Cited 2 times

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Irradiation-induced disordering of bulk Ni3Al under 6 MeV Ni2+ irradiation is studied at room temperature. A self-consistent long-range-order quantification method for ion-irradiated intermetallics is presented using cross-sectional sample preparation and conventional transmission electron microscopy in selected area diffraction depth profiling mode. This method, which does not require the full assessment of dynamic diffraction image simulation information, is unique to precisely measure long-range-order as a function of depth or dose for a single total fluence. The disordering efficiency constant ϵ is estimated in the range 10.0±0.7/dpa for a total fluence of 1014 cm−2. © 2000 American Institute of Physics.
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61.82.Bg Metals and alloys
61.80.Jh Ion radiation effects
61.50.Ks Crystallographic aspects of phase transformations; pressure effects
61.66.Dk Alloys

Boron diffusion in silicon in the presence of other species

Hong-Jyh Li, Puneet Kohli, Swaroop Ganguly, Taras A. Kirichenko, Sanjay Banerjee, and Peter Zeitzoff

Appl. Phys. Lett. 77, 2683 (2000); http://dx.doi.org/10.1063/1.1320019 (3 pages) | Cited 7 times

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We model B diffusion in the category of equilibrium diffusivity in the presence of other species using ab initio calculations. The result shows that in the presence of other atoms X (X=F, N, C, Al, Ga, In, and Ge), the migration energy for B along the migration pathway is larger than the case if X=Si (0.33 eV). This suggests the reduction of B diffusion. If another B occupies that position (X=B), then a smaller migration energy is observed and enhanced diffusion is expected. The simulation results are consistent with experiments. The reason for the migration energy difference can be understood in terms of electronic and strain effects. Estimates of the enthalpy of bond formation agree with the simulation except for the low-enthalpy cases where the strain effect caused by atom-size misfit dominates. © 2000 American Institute of Physics.
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66.30.J- Diffusion of impurities
61.72.uf Ge and Si
85.40.Ry Impurity doping, diffusion and ion implantation technology

Light-induced long-range hydrogen motion in hydrogenated amorphous silicon at room temperature

Hyeonsik M. Cheong, Se-Hee Lee, Brent P. Nelson, Angelo Mascarenhas, and Satyen K. Deb

Appl. Phys. Lett. 77, 2686 (2000); http://dx.doi.org/10.1063/1.1320015 (3 pages) | Cited 11 times

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We present evidence for long-range hydrogen motion in hydrogenated amorphous silicon (a-Si:H) under room-temperature illumination, by monitoring the changes in the Raman spectrum of an a-WO3 overlayer with illumination. We observe that illumination causes hydrogen to diffuse out of the a-Si:H layer into the a-WO3 layer. This hydrogen motion is observed to saturate after about 30 min when the a-Si:H is illuminated with 15 W/cm2 of the 514.5 nm laser line at room temperature. The amount of hydrogen that diffuses out of the a-Si:H layer is estimated semiquantitatively to be approximately 9×10−4 at. %. © 2000 American Institute of Physics.
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66.30.J- Diffusion of impurities
61.82.Fk Semiconductors
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
68.35.Fx Diffusion; interface formation
78.30.-j Infrared and Raman spectra

Photochemical manipulation of discotic liquid crystal alignment by a poly(vinyl cinnamate) thin film

Seiichi Furumi, Kunihiro Ichimura, Hiroaki Sata, and Yosuke Nishiura

Appl. Phys. Lett. 77, 2689 (2000); http://dx.doi.org/10.1063/1.1320017 (3 pages) | Cited 14 times

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This letter describes the surface-assisted orientational photocontrol of discotic liquid crystals (DLCs) using thin films of a conventional photoresist, poly(vinyl cinnamate). Oblique illumination of a thin film of the polymer with nonpolarized UV light gave rise to inclined orientation of cinnamoyl moieties toward the light propagation through their direction-selective photochemical reactions. The DLCs were aligned on the photoirradiated film in a tilted hybrid manner, whereas the orientational direction of DLC molecules corresponds to the light propagation as a result of three-dimensional orientation of the cinnamoyl residues. © 2000 American Institute of Physics.
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61.30.-v Liquid crystals
82.50.-m Photochemistry
82.35.-x Polymers: properties; reactions; polymerization

Compositional mapping of the argon–methane–hydrogen system for polycrystalline to nanocrystalline diamond film growth in a hot-filament chemical vapor deposition system

T. Lin, G. Y. Yu, A. T. S. Wee, Z. X. Shen, and Kian Ping Loh

Appl. Phys. Lett. 77, 2692 (2000); http://dx.doi.org/10.1063/1.1320039 (3 pages) | Cited 41 times

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The transition in diamond crystal morphology from well-faceted microcrystalline to nanocrystalline phases as a function of increasing argon (Ar) concentration has been studied in a hot-filament chemical vapor deposition (HFCVD) system. The range of Ar concentration in a Ar–methane–hydrogen mixture that permits well-faceted diamond growth is up to 90%. At a concentration of 95.5%, a marked transition into nanocrystalline diamond phase could be observed. Although no graphitic phases were observed by transmission electron microscope and x-ray diffraction up to 95.5% Ar concentration, Raman analyses revealed a proportionate increase in amorphous carbon content with an increase in Ar concentration. The addition of Ar appears to create an electron-rich gas environment because a plasma can be readily discharged by applying a bias to the sample. Analysis of the plasma by optical emission spectroscopy revealed a linear correlation between Ar addition and the occurrence of C2 dimers in the plasma. Our results identified the narrow Ar-methane-hydrogen composition window that is effective for the growth of thick and smooth nanocrystalline diamond in HFCVD. © 2000 American Institute of Physics.
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81.05.ub Fullerenes and related materials
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
61.46.-w Structure of nanoscale materials
78.66.Db Elemental semiconductors and insulators
78.30.Am Elemental semiconductors and insulators
81.05.Cy Elemental semiconductors
81.07.-b Nanoscale materials and structures: fabrication and characterization
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Electron beam and optical depth profiling of quasibulk GaN

L. Chernyak, A. Osinsky, G. Nootz, A. Schulte, J. Jasinski, M. Benamara, Z. Liliental-Weber, D. C. Look, and R. J. Molnar

Appl. Phys. Lett. 77, 2695 (2000); http://dx.doi.org/10.1063/1.1319530 (3 pages) | Cited 30 times

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Electron beam and optical depth profiling of thick (5.5–64 μm) quasibulk n-type GaN samples, grown by hydride vapor-phase epitaxy, were carried out using electron beam induced current (EBIC), microphotoluminescence (PL), and transmission electron microscopy (TEM). The minority carrier diffusion length, L, was found to increase linearly from 0.25 μm, at a distance of about 5 μm from the GaN/sapphire interface, to 0.63 μm at the GaN surface, for a 36-μm-thick sample. The increase in L was accompanied by a corresponding increase in PL band-to-band radiative transition intensity as a function of distance from the GaN/sapphire interface. We attribute the latter changes in PL intensity and minority carrier diffusion length to a reduced carrier mobility and lifetime at the interface, due to scattering at threading dislocations. The results of EBIC and PL measurements are in good agreement with the values for dislocation density obtained using TEM. © 2000 American Institute of Physics.
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73.61.Ey III-V semiconductors
78.66.Fd III-V semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
78.55.Cr III-V semiconductors
81.05.Ea III-V semiconductors
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Size-dependent electrical behavior of spatially inhomogeneous barrier height regions on silicon

Robert C. Rossi, Ming X. Tan, and Nathan S. Lewis

Appl. Phys. Lett. 77, 2698 (2000); http://dx.doi.org/10.1063/1.1319534 (3 pages) | Cited 13 times

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A series of ordered, periodic arrays of low barrier height n-Si/Ni nanometer-scale contacts interspersed among high barrier height n-Si/liquid contacts were prepared by evaporating Ni through bilayers of close-packed latex spheres deposited on n-Si. By varying the diameter of the spheres from 174 to 1530 nm, geometrically self-similar Si/Ni structures were produced having triangular Si/Ni features ranging from approximately 100 to 800 nm on a side. The resulting Si surfaces were used as electrodes in methanolic electrochemical cells containing LiClO4 and 1,1′-dimethylferrocene+/o. The dark current density–voltage properties of the resulting mixed barrier height contacts were strongly dependent on the size of the low barrier height contact regions even though the fraction of the Si surface covered by Ni remained constant. © 2000 American Institute of Physics.
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73.40.Ns Metal-nonmetal contacts
73.40.Mr Semiconductor-electrolyte contacts
82.47.-a Applied electrochemistry
81.07.-b Nanoscale materials and structures: fabrication and characterization
81.16.-c Methods of micro- and nanofabrication and processing
85.35.-p Nanoelectronic devices

Transparent p-type semiconductor: LaCuOS layered oxysulfide

K. Ueda, S. Inoue, S. Hirose, H. Kawazoe, and H. Hosono

Appl. Phys. Lett. 77, 2701 (2000); http://dx.doi.org/10.1063/1.1319507 (3 pages) | Cited 90 times

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La1−xSrxCuOS (x = 0, 0.05) thin films prepared by radio-frequency sputtering were found to have high optical transmission ( ≥ 70%) at the visible and near-infrared wavelengths and an energy gap of about 3.1 eV. The dc electrical conductivities of x = 0 and 0.05 thin films at room temperature were 1.2×10−2 and 2.6×10−1 S cm−1, respectively. The Seebeck coefficients of these samples were positive, indicating that p-type electrical conduction is dominant in these materials. A sharp photoluminescence peak, probably originating from an interband transition, was observed at the optical absorption edge. The present study demonstrates that LaCuOS is a promising transparent p-type semiconductor for optoelectronic applications. Moreover, our material design, based on chemical modulation of the valence band, was successfully extended to oxysulfide systems. © 2000 American Institute of Physics.
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73.61.Le Other inorganic semiconductors
78.66.Li Other semiconductors
71.20.Nr Semiconductor compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
68.55.-a Thin film structure and morphology
81.15.Cd Deposition by sputtering
78.40.Fy Semiconductors
78.30.Hv Other nonmetallic inorganics
73.50.Lw Thermoelectric effects
78.55.Hx Other solid inorganic materials
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