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3 Aug 1998

Volume 73, Issue 5, pp. 557-700

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The role of the solvent viscosity on the spatiotemporal instabilities of large aperture dye lasers

Oscar G. Calderón, I. Leyva, and J. M. Guerra

Appl. Phys. Lett. 73, 557 (1998); http://dx.doi.org/10.1063/1.121854 (3 pages) | Cited 2 times

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Local intensity fluctuations in a large aperture dye laser have been measured for several solvent viscosities. A decrease on the relative fluctuations amplitude as the solvent viscosity increases is found. From a theoretical approach, we show that the intensity fluctuations follow the same behavior as the molecular polarization orientation driven by the laser field. © 1998 American Institute of Physics.
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42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.55.Mv Dye lasers
66.20.-d Viscosity of liquids; diffusive momentum transport

Energy selective optically pumped stimulated emission from InGaN/GaN multiple quantum wells

T. J. Schmidt, Yong-Hoon Cho, G. H. Gainer, J. J. Song, S. Keller, U. K. Mishra, and S. P. DenBaars

Appl. Phys. Lett. 73, 560 (1998); http://dx.doi.org/10.1063/1.121855 (3 pages) | Cited 26 times

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Optically pumped stimulated emission (SE) from InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition has been systematically studied as a function of excitation photon energy (Eexc) to further understand the origin of SE in these structures. Optically pumped SE was observed for excitation photon energies well below that of the absorption edge of the MQWs, indicating the states responsible for the soft absorption edge in these structures can efficiently couple carriers with the gain region. “Mobility edge”-type behavior in the SE peak was observed as Eexc was varied. The effective mobility edge measured in these SE experiments lies ∼110 meV above the main spontaneous emission peak and ∼62 meV above the SE peak. Tuning the excitation energy below the mobility edge was found to be accompanied by a drastic increase in the SE threshold due to a decrease in the effective absorption cross section. The experimental results indicate that the SE peak observed here has the same microscopic origin as the spontaneous emission peak, i.e., radiative recombination of localized states. © 1998 American Institute of Physics.
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78.45.+h Stimulated emission
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
42.55.Px Semiconductor lasers; laser diodes

Epitaxial structuring of blue light-emitting p-phenylene oligomers

Tatsuo Mikami and Hisao Yanagi

Appl. Phys. Lett. 73, 563 (1998); http://dx.doi.org/10.1063/1.121856 (3 pages) | Cited 31 times

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Epitaxial structuring of blue light-emitting molecules, p-phenylene oligomers (p-nP, n = 3,4,5,6), was investigated in thin crystals vapor deposited on the (001) surface of a KCl single crystal. Their molecular orientation and morphology changed depending on the molecular length and substrate temperatures during deposition. As the length of p-nP was longer and the substrate temperature was lower, the orientation of the molecular axis changed from standing to lying on the KCl surface due to an increase of molecule–substrate interactions. p-terphenyl (3P) hardly adsorbed on the KCl (001) surface, but a portion of the standing molecules stuck along the step edge of KCl and formed a walllike structure. Long needlelike crystals of p-hexaphenyl (6P) epitaxially grew in two orthogonal KCl [110] directions taking lying orientation even at a higher substrate temperature. In these epitaxial needles, the transition dipoles parallel to the molecular axis gave rise to blue fluorescence sharply polarized along the KCl [110] directions. © 1998 American Institute of Physics.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
81.15.Kk Vapor phase epitaxy; growth from vapor phase
85.60.Jb Light-emitting devices
78.55.Kz Solid organic materials
78.66.Qn Polymers; organic compounds

Photon-spin controlled lasing oscillation in surface-emitting lasers

H. Ando, T. Sogawa, and H. Gotoh

Appl. Phys. Lett. 73, 566 (1998); http://dx.doi.org/10.1063/1.121857 (3 pages) | Cited 38 times

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We report on photon-spin controlled lasing oscillation in GaAs surface-emitting lasers at room temperature. We demonstrate experimentally that the partial electron-spin alignment, created by optically pumping the GaAs laser active media with circularly polarized pulses, drastically changes the polarization state of the lasing output, causing circularly polarized lasing emission. We discuss the laser polarization characteristics in relation to the measured electron-spin relaxation time. © 1998 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Hot carrier induced picosecond dynamics of a vertical cavity surface emitting laser: Influence of transverse effects

O. D. Mücke and M. Wegener

Appl. Phys. Lett. 73, 569 (1998); http://dx.doi.org/10.1063/1.121858 (3 pages) | Cited 2 times

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The interplay of carrier heating effects and transverse variations of the carrier distribution functions has a significant influence on the ultrafast emission dynamics of semiconductor vertical cavity surface emitting lasers. This interplay can lead to dynamic overshooting and oscillations (here at 200 GHz) even for lasing on a single transverse and single longitudinal mode. A simple equivalent model is a cavity containing the gain medium and a saturable absorber. The numerical results can directly be compared with recently published experiments. © 1998 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
73.50.Fq High-field and nonlinear effects

Second harmonic generation in laser ablated zinc oxide thin films

H. Cao, J. Y. Wu, H. C. Ong, J. Y. Dai, and R. P. H. Chang

Appl. Phys. Lett. 73, 572 (1998); http://dx.doi.org/10.1063/1.121859 (3 pages) | Cited 79 times

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We have observed large second-order nonlinear optical response from zinc oxide (ZnO) thin films deposited on sapphire substrates by pulsed laser ablation. By comparing the second harmonic signal generated in a series of ZnO films with different crystallinity and thickness, we conclude that a significant part of the second harmonic signal is generated at the grain boundaries and interfaces. © 1998 American Institute of Physics.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.66.Hf II-VI semiconductors
61.72.Mm Grain and twin boundaries
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

1.35 W of stable single-frequency emission from an external-cavity tapered oscillator utilizing fiber Bragg grating feedback

A. K. Goyal, P. Gavrilovic, and H. Po

Appl. Phys. Lett. 73, 575 (1998); http://dx.doi.org/10.1063/1.121860 (3 pages) | Cited 6 times

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An external-cavity tapered diode laser utilizing fiber Bragg grating feedback was designed to oscillate on a single-longitudinal mode. Stable single-longitudinal mode operation was attained at powers as high as 1.35 W. The emission frequency tuned smoothly with current over the cavity’s free spectral range of 6 GHz. The power in the lasing mode was >30 dB above the background amplified spontaneous emission and the side-mode suppression ratio was >15 dB. No deterioration in the beam quality was measurable up to the highest output power. The coupling efficiency into a Gaussian resonator mode was >65% up to 1.35 W. © 1998 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Fc Modulation, tuning, and mode locking
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Optical temperature sensing using upconversion fluorescence emission in Er3+/Yb3+-codoped chalcogenide glass

P. V. dos Santos, M. T. de Araujo, A. S. Gouveia-Neto, J. A. Medeiros Neto, and A. S. B. Sombra

Appl. Phys. Lett. 73, 578 (1998); http://dx.doi.org/10.1063/1.121861 (3 pages) | Cited 43 times

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Optical temperature sensing using upconversion fluorescence emission in Er3+/Yb3+-codoped Ga2S3:La2O3 chalcogenide glass excited at 1.06 μm is reported. Temperature measurements in the region of 20–225 °C with a resolution of approximately 0.5 °C using excitation powers of a few tens of milliwatts were obtained. The temperature sensing mechanism is independent of variations in the excitation intensity, possible fluctuations of transmission, and utilizes a simple signal detection and processing system. The results also revealed that the glass host material plays an important role in the performance of the sensing system. © 1998 American Institute of Physics.
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07.20.Dt Thermometers
78.55.Hx Other solid inorganic materials

Distance measurements using a noisy white light interferometer

Moshe Ben-Chorin, Silvia Chuartzman, and Yehiam Prior

Appl. Phys. Lett. 73, 581 (1998); http://dx.doi.org/10.1063/1.121862 (3 pages) | Cited 1 time

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We describe a method for interferometric distance measurements in the presence of phase noise. The method is based on the beating between white light and a reference beam that travel along the same path through the interferometer. Since both the reference and the white light suffer the same phase noise, the envelope of the high frequency fringes is not affected by the noise. By measuring the signal variance, we recover the envelope while averaging out the high frequency fringes. We demonstrate the usefulness of the method for surface profilometers. © 1998 American Institute of Physics.
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06.30.Bp Spatial dimensions (e.g., position, lengths, volume, angles, and displacements)
07.60.Ly Interferometers

Quasiphase matched second harmonic generation in ZnSe waveguide structures modulated by focused ion beam implantation

M. Kühnelt, T. Leichtner, S. Kaiser, B. Hahn, H. P. Wagner, D. Eisert, G. Bacher, and A. Forchel

Appl. Phys. Lett. 73, 584 (1998); http://dx.doi.org/10.1063/1.121863 (3 pages) | Cited 9 times

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Second harmonic generation is investigated in a ZnSe/ZnS0.22Se0.78 waveguide grown by metal organic vapor phase epitaxy on (001) GaAs substrate. Phase matching is achieved by a first order periodic modulation of the nonlinear susceptibility χ(2) within the waveguide structure. The modulation is generated by a focused ion beam implantation technique, resulting in a well confined damage of the crystalline structure. The observed phase matching wavelengths corresponding to various modulation periods between 1.6 and 1.9 μm agree with those predicted by calculations. © 1998 American Institute of Physics.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.79.Nv Optical frequency converters
81.15.Kk Vapor phase epitaxy; growth from vapor phase
78.66.Hf II-VI semiconductors
42.65.An Optical susceptibility, hyperpolarizability
42.65.Wi Nonlinear waveguides
42.79.Gn Optical waveguides and couplers

0.45 W diffraction-limited beam and single-frequency operation from antiguided phase-locked laser array with distributed feedback grating

M. P. Nesnidal, T. Earles, L. J. Mawst, D. Botez, and J. Buus

Appl. Phys. Lett. 73, 587 (1998); http://dx.doi.org/10.1063/1.121864 (3 pages) | Cited 7 times

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A second-order diffraction grating placed below the active region of a phase-locked resonant antiguided array selects the in-phase array mode in addition to its role as a single-longitudinal-mode selector. This type of array-mode discrimination relies on the fact that the resonant in-phase array mode has significantly better field overlap with the grating region than nonresonant array modes. Furthermore, it eliminates the need for a conventional array-mode discriminator: interelement loss; which can cause self-pulsations. Diffraction-limited beam and single-frequency operation is obtained to at least 0.45 W peak pulsed power from 20 element, InGaAs/InGaP/GaAs structures (λ = 0.97 μm) of 120-μm-wide aperture. Distributed-feedback operation is confirmed over the 20–40 °C temperature range. The results are in good agreement with theory. © 1998 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.79.Dj Gratings
42.60.Fc Modulation, tuning, and mode locking

Voltage-controlled lasing pixels for projection display

Joel A. Firehammer, Gregory P. Crawford, and Nabil M. Lawandy

Appl. Phys. Lett. 73, 590 (1998); http://dx.doi.org/10.1063/1.121865 (3 pages) | Cited 8 times

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A lasing pixel device that implements a low-voltage addressable spatially patterned variable loss element placed inside an optically pumped high-gain laser cavity is studied experimentally. A polymer dispersed liquid crystal (PDLC) was used as the loss element. It was found to have high output (600 lumens/cm2 at the device faceplate), very high on-screen contrast (1000:1), and narrow spectral linewidth (∼3 nm), which make it potentially useful for a nonscanned laser projection display. A grayscale model of pixel output versus element loss was developed and compared favorably to measurements. © 1998 American Institute of Physics.
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42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems
42.62.-b Laser applications

Growth and second harmonic generation characterization of Er3+ doped bulk periodically poled LiNbO3

V. Bermúdez, J. Capmany, J. García Solé, and E. Diéguez

Appl. Phys. Lett. 73, 593 (1998); http://dx.doi.org/10.1063/1.121866 (3 pages) | Cited 19 times

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Samples of Er3+ doped bulk periodically poled lithium niobate have been grown by the Czochralski method. The efficiency, tuning and thermal tolerances of infrared to green second harmonic generation have been measured, showing good accordance with a 4%–5% dispersion in the domain period length. © 1998 American Institute of Physics.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
81.10.Fq Growth from melts; zone melting and refining
42.70.Mp Nonlinear optical crystals

Size dependence of transverse mode structure in oxide-confined vertical-cavity laser diodes

S. P. Hegarty, G. Huyet, J. G. McInerney, K. D. Choquette, K. M. Geib, and H. Q. Hou

Appl. Phys. Lett. 73, 596 (1998); http://dx.doi.org/10.1063/1.121867 (3 pages) | Cited 11 times

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We analyze the transverse mode structure of selectively oxidized vertical-cavity surface-emitting semiconductor lasers as a function of the aperture size and injection current. Measurements of the spectral splittings, the near and far field sizes and the response to pulsed operation demonstrate that a parasitic thermal lens can be a dominant factor in determining the transverse resonator properties of small (<3 μm) lasers. © 1998 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.Fc Modulation, tuning, and mode locking

Influence of roughness and interdiffusion in zone plate structures on the diffraction efficiency described by coupled wave theory

G. Schneider

Appl. Phys. Lett. 73, 599 (1998); http://dx.doi.org/10.1063/1.121868 (3 pages) | Cited 9 times

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Diffractive transmission x-ray optics, e.g., zone plates and gratings, are well suited for x-ray microscopy and spectroscopy. X-ray optics for use at short wavelengths require high zone aspect ratios which can be obtained by alternately depositing two materials with different x-ray scattering properties on a microwire. The roughness and the diffusion inherent to this process influence the diffraction efficiency of these optics. Using coupled wave theory, it is shown that roughness and interdiffusion increase the structure height required for optimal diffraction efficiency compared to ideally smooth structure profiles. In addition, the maximum obtainable diffraction efficiency decreases with increasing root-mean-square roughness and interdiffusion. © 1998 American Institute of Physics.  
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42.79.Ci Filters, zone plates, and polarizers
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Fx Diffusion; interface formation
41.50.+h X-ray beams and x-ray optics
07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
68.35.Ct Interface structure and roughness
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Room temperature operation of 8–12 μm InSbBi infrared photodetectors on GaAs substrates

J. J. Lee, J. D. Kim, and M. Razeghi

Appl. Phys. Lett. 73, 602 (1998); http://dx.doi.org/10.1063/1.121869 (3 pages) | Cited 8 times

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We report the room temperature operation of 8–12 μm InSbBi long-wavelength infrared photodetectors. The InSbBi/InSb heterostructures were grown on semi-insulating GaAs (001) substrates by low pressure metalorganic chemical vapor deposition. The voltage responsivity at 10.6 μm was about 1.9 mV/W at room temperature and the corresponding Johnson noise limited detectivity was estimated to be about 1.2×106 cmHz1/2/W. The carrier lifetime derived from the voltage dependent responsivity measurements was about 0.7 ns. © 1998 American Institute of Physics.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

Analysis of terrace size fluctuations of B-induced stripe structures on the Si (001) surface

Ken Fujita and Masakazu Ichikawa

Appl. Phys. Lett. 73, 605 (1998); http://dx.doi.org/10.1063/1.121870 (3 pages) | Cited 6 times

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Terrace size fluctuations of B-induced stripe structures on Si (001) were analyzed by using scanning tunneling microscopy. The terrace size was varied independently of the substrate temperature by supplying diborane (B2H6). Histograms of the terrace size were fitted by Gaussian distributions and the standard deviation was proportional to the square root of the mean terrace size. These results indicate that the B-induced stripe structures arise from the competition between the stress anisotropy and step energy on Si (001), and mean that the surface anisotropy is almost independent of the B concentration. © 1998 American Institute of Physics.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Md Surface thermodynamics, surface energies

Enhancement of electrical and structural properties of GaN layers grown on vicinal-cut, a-plane sapphire substrates

M. Fatemi, A. E. Wickenden, D. D. Koleske, M. E. Twigg, J. A. Freitas, R. L. Henry, and R. J. Gorman

Appl. Phys. Lett. 73, 608 (1998); http://dx.doi.org/10.1063/1.121871 (3 pages) | Cited 20 times

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We report the observation of significant enhancement in the electrical and crystalline properties of GaN layers grown on vicinally cut, a-plane sapphire substrates. Room-temperature Hall mobility and x-ray rocking curve data show a nearly twofold improvement, independent of the processing conditions, for layers grown on substrates having vicinal angles of 1.5° compared to on-axis substrates. Transmission electron microscopy shows reduced edge dislocation density and better alignment of the grains in layers grown on vicinally cut substrates. Preliminary photoluminescence measurements also indicate pronounced differences in the yellow band spectra between the on-axis and off-axis cut substrates. These findings contrast the relatively modest improvements observed in layers grown on c-plane substrates with vicinal angles as high as 10°.
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68.55.-a Thin film structure and morphology
73.61.Ey III-V semiconductors
72.80.Ey III-V and II-VI semiconductors
73.50.Dn Low-field transport and mobility; piezoresistance
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Contrast mechanisms in scanning ion microscope imaging for metals

Y. Sakai, T. Yamada, T. Suzuki, T. Sato, H. Itoh, and T. Ichinokawa

Appl. Phys. Lett. 73, 611 (1998); http://dx.doi.org/10.1063/1.121872 (3 pages) | Cited 12 times

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The contrast of the secondary electron images in scanning ion microscopy (SIM) is compared with that in scanning electron microscopy (SEM) with ultrahigh vacuum for Al, Cu, Ag, and Au metals deposited on the Si(100) clean surface. The order of the secondary electron yields as a function of the atomic number (Z2) for ion bombardment is opposite to that for electron bombardment. The brightness of the secondary electron images observed by a focused Ga+ ion beam at 30 keV decreases with increasing Z2, while that by the electron beam increases with Z2. On the other hand, the order of the total secondary ion yields in SIM increases with Z2. The secondary electron image observed by a focused Ar+-ion beam at 3 keV shows the similar contrast to that of the Ga+-ion beam. The different Z2 dependence of the secondary electron yields between SEM and SIM was quantitatively confirmed by the total secondary electron spectra and is discussed based on the range profile below the surface, and it is concluded that the decrease of the secondary electron yields in SIM is attributed to the increase of the reflected ions at the surface with increasing Z2. © 1998 American Institute of Physics.
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68.37.Vj Field emission and field-ion microscopy
07.78.+s Electron, positron, and ion microscopes; electron diffractometers
79.20.Hx Electron impact: secondary emission

Relationships between hardness, elastic modulus, and the work of indentation

Yang-Tse Cheng and Che-Min Cheng

Appl. Phys. Lett. 73, 614 (1998); http://dx.doi.org/10.1063/1.121873 (3 pages) | Cited 153 times

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The work done during indentation is examined using dimensional analysis and finite element calculations for conical indentation in elastic-plastic solids with work hardening. An approximate relationship between the ratio of hardness to elastic modulus and the ratio of irreversible work to total work in indentation is found. Consequently, the ratio of hardness to elastic modulus may be obtained directly from measuring the work of indentation. Together with a well-known relationship between elastic modulus, initial unloading slope, and contact area, a new method is then suggested for estimating the hardness and modulus of solids using instrumented indentation with conical or pyramidal indenters. © 1998 American Institute of Physics.
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46.55.+d Tribology and mechanical contacts
62.20.Qp Friction, tribology, and hardness
68.35.Gy Mechanical properties; surface strains
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
62.20.D- Elasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations

Hard hydrogenated carbon films with low stress

R. G. Lacerda and F. C. Marques

Appl. Phys. Lett. 73, 617 (1998); http://dx.doi.org/10.1063/1.121874 (3 pages) | Cited 48 times

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Analysis of hard a-C:H films with low stress prepared by methane plasma decomposition is reported. Films with hardness as high as 14 GPa and stress as low as 0.5 GPa were obtained. These films have a high Raman Id/Ig ratio (∼1.0), and small Tauc’s band gap (∼0.4 eV). This letter also supplies strong evidence that the subimplantation deposition model, used to explain the formation of ta-C and ta-C:H films, is also valid for a-C:H films deposited by methane plasma decomposition. It is proposed that the rigidity of the films is basically provided by a matrix of dispersed cross-linked sp2 sites, in addition to the contribution of the sp3 sites. © 1998 American Institute of Physics.
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61.43.Er Other amorphous solids
62.20.Qp Friction, tribology, and hardness
68.35.Gy Mechanical properties; surface strains
68.60.Bs Mechanical and acoustical properties
68.55.-a Thin film structure and morphology
52.77.Bn Etching and cleaning
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.40.Pq Friction, lubrication, and wear
78.66.-w Optical properties of specific thin films
78.40.Pg Disordered solids
71.23.-k Electronic structure of disordered solids

Two-dimensional ordering of self-assembled Ge islands on vicinal Si(001) surfaces with regular ripples

Jian-hong Zhu, K. Brunner, and G. Abstreiter

Appl. Phys. Lett. 73, 620 (1998); http://dx.doi.org/10.1063/1.121875 (3 pages) | Cited 60 times

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Two-dimensional ordering is achieved in a single layer of self-assembled Ge islands fabricated by molecular beam epitaxy on vicinal Si(001) surfaces with regular ripples caused by step bunching. The ripples with a typical period of about 120 nm lead to the long-range lineup of the Ge islands along their directions, while the strong repulsive interaction between the dense Ge islands determines their relative arrangement on different step bunches of a ripple. The ordering pattern can be tuned by the Ge coverage and the direction of the ripples. The ordering also helps to improve the size homogeneity of the Ge islands. © 1998 American Institute of Physics.
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81.05.Cy Elemental semiconductors
68.55.-a Thin film structure and morphology
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.B- Structure of clean surfaces (and surface reconstruction)
64.70.K- Solid-solid transitions

Photoacoustic phase transition of the ceramic BaTiO3

R. Castañeda-Guzmán, M. Villagrán-Muniz, J. M. Saniger-Blesa, and O. Pérez-Martínez

Appl. Phys. Lett. 73, 623 (1998); http://dx.doi.org/10.1063/1.121876 (3 pages) | Cited 11 times

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Photoacoustic signals from the ceramic BaTiO3 are analyzed, as a function of temperature close to structural phase transition from tetragonal to cubic at 110 °C. The amplitude of the signals depends on the changes of the thermoelastic properties in the ceramic with temperature. The Curie temperature value for the phase change was reproducible and was found to be 109±6 °C. A comparative analysis of this result with the differential thermal analysis and permittivity techniques is performed. Two changes of sign in the photoacoustic signal are observed, associated with the thermal expansion coefficient behavior in this material near the transition. © 1998 American Institute of Physics.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
77.80.B- Phase transitions and Curie point
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
77.22.Ch Permittivity (dielectric function)
65.40.De Thermal expansion; thermomechanical effects
62.65.+k Acoustical properties of solids
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects
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The Ge–C local mode in epitaxial GeC and Ge-rich GeSiC alloys

W. H. Weber, B.-K. Yang, and M. Krishnamurthy

Appl. Phys. Lett. 73, 626 (1998); http://dx.doi.org/10.1063/1.121877 (3 pages) | Cited 16 times

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The Raman signature of the local Ge–C mode for substitutional C is identified as a narrow line (8 cm−1 full width at half maximum) near 530 cm−1 in alloy films of Ge1−yCy (0 ⩽ y ⩽ 0.07) grown on Ge (100) substrates by low-temperature (200 °C) molecular beam epitaxy. The intensity of the Ge–C line relative to the c-Ge line suggests that only a small fraction of the nominal C is in substitutional sites. In ternary alloys of Ge1−x−ySixCy with x = 0.1 and 0.2 and y = 0.03, the Ge–C mode disappears, suggesting a strong bias towards C bonding with Si as opposed to Ge. In Ge1−xSnx films the Ge–Sn mode is seen at 263 cm−1. © 1998 American Institute of Physics.
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63.20.Pw Localized modes
78.30.Hv Other nonmetallic inorganics
78.66.Li Other semiconductors
68.55.-a Thin film structure and morphology

High brightness and efficiency blue light-emitting polymer diodes

A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu, and E. P. Woo

Appl. Phys. Lett. 73, 629 (1998); http://dx.doi.org/10.1063/1.121878 (3 pages) | Cited 133 times

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Efficient blue electroluminescence, peaked at 436 nm, is demonstrated from polymer light-emitting diodes operating at high brightness. A dioctyl-substituted polyfluorene was used as the emissive layer in combination with a polymeric triphenyldiamine hole transport layer. The luminance reaches 600 cd/m2 at a current density of 150 mA/cm2 for a bias voltage of 20 V, corresponding to an efficiency of 0.25 cd/A and a luminosity of 0.04 lm/W. These values are optimized at a critical emissive layer thickness. © 1998 American Institute of Physics.
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85.60.Jb Light-emitting devices
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