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31 Dec 2001

Volume 79, Issue 27, pp. 4479-4603

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Carrier relaxation dynamics in an ultrafast all-optical modulator using an intersubband transition

T. Asano, S. Yoshizawa, and S. Noda

Appl. Phys. Lett. 79, 4509 (2001); http://dx.doi.org/10.1063/1.1427154 (3 pages) | Cited 12 times

Online Publication Date: 13 March 2002

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An improvement of an all-optical modulation scheme that utilizes an intersubband transition is proposed and investigated in connection with intersubband carrier relaxation dynamics. N-doped GaAs/AlGaAs quantum wells are pumped by an ultrashort (∼120 fs) intersubband-resonant pulse and probed by an interband-resonant white-light continuum. Ultrafast modulation speed of ∼0.4 ps is demonstrated, which is faster than that of the previous scheme by as much as a factor of 10. It is found that a large part of the carriers, which are excited from the first conduction subband to the second conduction subband, transfer to unknown trapping states at a relaxation time that is comparable to that of the pump–pulse duration. The existence of the carrier relaxation path via L valley subbands is strongly suggested as the cause. © 2001 American Institute of Physics.
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42.79.Hp Optical processors, correlators, and modulators
78.67.De Quantum wells

Fast switchable nematic emulsions with twisted director

G. Di Profio, J. Lanzo, F. P. Nicoletta, G. De Filpo, and G. Chidichimo

Appl. Phys. Lett. 79, 4512 (2001); http://dx.doi.org/10.1063/1.1428630 (3 pages) | Cited 9 times

Online Publication Date: 13 March 2002

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Switchable nematic emulsions, obtained by a fast cooling of isotropic mixtures of nematic liquid crystal and monomers, show very low saturation fields but rather long relaxation times because of the bipolar configuration adopted by nematic directors. Such inconvenience has prevented up to now use of switchable nematic emulsions as electro-optical devices. In order to obtain a good compromise between low saturation fields and fast relaxation times, we have experimentally studied the effect of the addition of a chiral dopant on the electro-optical properties of switchable nematic emulsions. The addition of chiral molecules results in a significant decrease of decay times, while the saturation fields are slightly affected. Results are in a rather good agreement with a phenomenological model, which takes into account the increase of elastic energy due to the induced twist deformation. © 2001 American Institute of Physics.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
82.70.Kj Emulsions and suspensions
78.20.Jq Electro-optical effects

Creep deformation and stress-induced structural disorder near Tg in a Zr55Al10Ni5Cu30 glassy alloy

H. Kato, A. Inoue, and H. S. Chen

Appl. Phys. Lett. 79, 4515 (2001); http://dx.doi.org/10.1063/1.1430269 (3 pages) | Cited 15 times

Online Publication Date: 13 March 2002

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Creep deformation under a constant applied load in a Zr55Al10Ni5Cu30 glassy alloy at the glass transition region is investigated. At an initial stress, σ0, less than a critical stress, σc = 80 MPa, the glass shows a Newtonian flow. When σ0 ≥ σc, the flow viscosity, η, initially decreases and attains a minimum. It then increases as the true stress, σ, decreases with further deformation. The initial decrease in η and the attendance of viscosity minimum are due to the stress-induced structural disorder and the structural equilibration with the applied stress, respectively. For stress, σ, less than the viscosity minimum stress, the stress dependence of viscosity, η(σ), curves all tend to merge together, and is fitted well with a master curve, η(σ), established previously for the steady-state flow under constant strain-rate experiments. These results render further support to the hypothesis of stress-induced structural disorder and the concept of fictive stress. © 2001 American Institute of Physics.
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81.40.Lm Deformation, plasticity, and creep
61.43.Fs Glasses
62.20.Hg Creep
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
81.05.Kf Glasses (including metallic glasses)
83.60.Df Nonlinear viscoelasticity

Evolution of self-assembled Ge/Si(211) islands

Margaret Floyd, Yangting Zhang, Jeff Drucker, David J. Smith, S. Tari, and S. Sivananthan

Appl. Phys. Lett. 79, 4518 (2001); http://dx.doi.org/10.1063/1.1428772 (3 pages) | Cited 2 times

Online Publication Date: 13 March 2002

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Atomic force microscopy (AFM) and transmission electron microscopy have been used to investigate Ge islands, grown by molecular-beam epitaxy on Si(211) substrates with a nominal 15 Å Ge coverage at temperatures between 600 and 700 °C. The majority of islands grown at all three temperatures had irregular hexagonal footprints. Intermediate-sized islands grown at 650 and 700 °C became elongated laterally in the 〈111〉 direction parallel to the direction of substrate surface step edges. AFM cross-sectional analysis indicated that small coherent clusters were bound by {111} and {113} facets, whereas the largest dislocated clusters were primarily bound by {111} and {001} facets. The upper size bound for coherent clusters increased with increasing growth temperature. © 2001 American Institute of Physics.
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68.55.A- Nucleation and growth
81.05.Cy Elemental semiconductors
81.16.Dn Self-assembly
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Lp Transmission electron microscopy (TEM)
68.37.Ps Atomic force microscopy (AFM)
68.55.-a Thin film structure and morphology

Determination of coverage in passivated porous silicon by Brillouin spectroscopy

H. J. Fan, M. H. Kuok, S. C. Ng, R. Boukherroub, and D. J. Lockwood

Appl. Phys. Lett. 79, 4521 (2001); http://dx.doi.org/10.1063/1.1428628 (3 pages) | Cited 3 times

Online Publication Date: 13 March 2002

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Brillouin scattering has been performed to probe surface acoustic waves in porous silicon films that have been chemically modified with various surface passivators. The surface Rayleigh mode and two film acoustic modes are observed. Unlike the Rayleigh mode, the two film modes are found to exhibit velocity dispersion. The effect of passivation is manifested as a shift of Brillouin peaks to lower frequencies relative to those of the freshly prepared porous silicon samples. The coverage of the surface area of the pores of the entire porous layers by the respective chemical passivators has been estimated from the phase velocity of one of the film modes. © 2001 American Institute of Physics.
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78.66.Db Elemental semiconductors and insulators
78.35.+c Brillouin and Rayleigh scattering; other light scattering
81.65.Rv Passivation
81.05.Rm Porous materials; granular materials
68.35.Iv Acoustical properties
81.05.Cy Elemental semiconductors

Incorporation-related structural issues for beryllium doping during growth of GaN by rf-plasma molecular-beam epitaxy

A. J. Ptak, Lijun Wang, N. C. Giles, T. H. Myers, L. T. Romano, C. Tian, R. A. Hockett, S. Mitha, and P. Van Lierde

Appl. Phys. Lett. 79, 4524 (2001); http://dx.doi.org/10.1063/1.1429290 (3 pages) | Cited 11 times

Online Publication Date: 13 March 2002

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Beryllium incorporation was studied for both Ga-polarity and N-polarity GaN using a series of Be step-doped epitaxial layers. Dopant concentration profiles indicated that surface polarity-related incorporation differences are not pronounced for Be. Significant surface accumulation of Be occurs during growth with surface accumulations approaching a monolayer for heavier doping levels. Transmission electron microscopy studies indicate the surface layer of Be has a significant effect on the microstructure, particularly for near monolayer coverage. © 2001 American Institute of Physics.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.05.Ea III-V semiconductors
61.72.uj III-V and II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Stability of carbon nanotubes under electric field studied by scanning electron microscopy

Yi Wei, Chenggang Xie, Kenneth A. Dean, and Bernard F. Coll

Appl. Phys. Lett. 79, 4527 (2001); http://dx.doi.org/10.1063/1.1429300 (3 pages) | Cited 53 times

Online Publication Date: 13 March 2002

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The influence of an applied electric field on carbon nanotubes protruding from a surface was investigated in situ using a high-resolution scanning electron microscopy. Under the applied electric field, the nanotubes flexed to orient themselves parallel to the electric field lines. For moderate field strengths below the electron field emission threshold, the flexed nanotubes relaxed back to their original shapes after the electric field was removed. However, when high electron field emission currents were extracted from the nanotubes, they were permanently deformed, leaving them aligned to the electric field direction after the electric field was removed. For high currents, the length of the carbon nanotubes were found to be shortened after field emission lasted for a period of time. © 2001 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
79.70.+q Field emission, ionization, evaporation, and desorption

High-quality, faceted cubic boron nitride films grown by chemical vapor deposition

W. J. Zhang, X. Jiang, and S. Matsumoto

Appl. Phys. Lett. 79, 4530 (2001); http://dx.doi.org/10.1063/1.1428762 (3 pages) | Cited 19 times

Online Publication Date: 13 March 2002

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Thick cubic boron nitride (cBN) films showing clear crystal facets were achieved by chemical vapor deposition. The films show the highest crystallinity of cBN films ever achieved from gas phase. Clear evidence for the growth via a chemical route is obtained. A growth mechanism is suggested, in which fluorine preferentially etches hBN and stabilizes the cBN surface. Ion bombardment of proper energy activates the cBN surface bonded with fluorine so as to enhance the bonding probability of nitrogen-containing species on the F-stabilized B (111) surface. © 2001 American Institute of Physics.
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68.55.A- Nucleation and growth
81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
52.77.Dq Plasma-based ion implantation and deposition
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Lp Transmission electron microscopy (TEM)
68.37.Ps Atomic force microscopy (AFM)
78.30.Fs III-V and II-VI semiconductors
78.66.Fd III-V semiconductors
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