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4 Aug 1997

Volume 71, Issue 5, pp. 563-724

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Ultrafast optical probes of excited states in poly(1,4-phenylene vinylene) and poly(2-fluoro-1,4-phenylene vinylene)

Hyo Soon Eom, Sae Chae Jeoung, Dungho Kim, J. I. Lee, H. K. Shim, C. M. Kim, C. S. Yoon, and K. S. Lim

Appl. Phys. Lett. 71, 563 (1997); http://dx.doi.org/10.1063/1.120285 (3 pages) | Cited 5 times

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We have investigated the photoexcitation dynamics of poly(1,4-phenylene vinylene) (PPV) and poly(2-fluoro-1,4-phenylene vinylene) (PFPV) by using the femtosecond transient absorption spectroscopic technique. It was demonstrated that photoinduced absorption originates from nonemissive indirect polaron pairs which are more efficiently produced in PFPV than in PPV. The decay processes of polaron pairs are mainly contributed by collisional annihilation and internal conversion by phonon emission in PFPV and PPV, respectively. The differences in photoexcitation dynamics between PPV and PFPV are believed to be caused by the strong electronegativity of substituted fluorine atom. © 1997 American Institute of Physics.
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78.47.-p Spectroscopy of solid state dynamics
71.38.-k Polarons and electron-phonon interactions
63.50.-x Vibrational states in disordered systems
78.55.Kz Solid organic materials

Stimulated luminescence from chromate ions adsorbed on disperse silica surfaces under intense ultraviolet excitation

Yuri D. Glinka

Appl. Phys. Lett. 71, 566 (1997); http://dx.doi.org/10.1063/1.119796 (3 pages) | Cited 1 time

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Luminescence measurements of chromate ions (CrO42−) adsorbed onto disperse silica (SiO2) surfaces under intense pulsed ultraviolet excitation (λexc = 354.7 or 266 nm, τp = 20 ns) were performed at room temperature. The luminescence spectra were collected for different values of the excitation intensity (I). The luminescence yield in the red band corresponding to emission from CrO42− ions varies linearly with I under 354.7 nm excitation. However, the intensity dependence of the luminescence yield under 266 nm excitation is characterized by a threshold (I ∼ 0.6×106 W cm−2), after which the intensity dependence of the luminescence yield is described by a power law with index n = 1.7 and then saturated at I ∼ 1.0×106 W cm−2. This feature of the excitation dynamics has been attributed to stimulated emission and considered by means of a four-level scheme. The cross section of stimulated emission was estimated as ∼ 3.3×10−21 cm2. © 1997 American Institute of Physics.
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78.45.+h Stimulated emission
78.55.Hx Other solid inorganic materials

“Blue” temperature-induced shift and band-tail emission in InGaN-based light sources

Petr G. Eliseev, Piotr Perlin, Jinhyun Lee, and Marek Osiński

Appl. Phys. Lett. 71, 569 (1997); http://dx.doi.org/10.1063/1.119797 (3 pages) | Cited 187 times

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Electro- and photoluminescence spectra of high-brightness light-emitting AlGaN/InGaN/GaN single-quantum-well structures are studied over a broad range of temperatures and pumping levels. Blue shift of the spectral peak position was observed along with an increase of temperature and current. An involvement of band-tail states in the radiative recombination was considered, and a quantitative description of the blue temperature-induced shift was proposed assuming a Gaussian shape of the band tail. © 1997 American Institute of Physics.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
81.05.Ea III-V semiconductors
71.20.Nr Semiconductor compounds
73.61.Ey III-V semiconductors
78.60.Fi Electroluminescence
73.20.At Surface states, band structure, electron density of states
85.60.Jb Light-emitting devices

Self-consistent multicomponent envelope function calculation of normal incidence second-harmonic generation in p-type doped symmetric quantum wells

Ansheng Liu

Appl. Phys. Lett. 71, 572 (1997); http://dx.doi.org/10.1063/1.120432 (3 pages)

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In a self-consistent multicomponent envelope function approach, we showed that χxxx(2) component of the second-harmonic susceptibility tensor of a symmetric p-type quantum well (QW) exists, because of the valence-band mixing at nonzero values of the in-plane wave vector. For a highly doped narrow GaAs/Al0.3Ga0.7As QW, our calculation indicates that at low temperatures a large value ( ∼ 10−7 m/V) of χxxx(2) can be achieved in the far-infrared frequency range without satisfying double resonance conditions. This value is comparable to the experimentally observed value of χzzz(2) in n-type asymmetric QW structures. © 1997 American Institute of Physics.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
78.66.Fd III-V semiconductors
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.30.Fs III-V and II-VI semiconductors

Infrared near-field imaging of implanted semiconductors: Evidence of a pure dielectric contrast

A. Lahrech, R. Bachelot, P. Gleyzes, and A. C. Boccara

Appl. Phys. Lett. 71, 575 (1997); http://dx.doi.org/10.1063/1.119798 (3 pages) | Cited 42 times

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In this letter, we demonstrate the ability of our reflection mode scanning near-field optical microscope functioning in the mid-infrared to reveal infrared dielectric contrast in absence of any topographical contrast. This contrast is induced by local structures prepared by low energy boron implantation in silicon. © 1997 American Institute of Physics.
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81.05.Cy Elemental semiconductors
61.72.uf Ge and Si
07.79.Fc Near-field scanning optical microscopes
85.40.Ry Impurity doping, diffusion and ion implantation technology
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons

Noise performances in polycrystalline silicon thin-film transistors fabricated by excimer laser crystallization

R. Carluccio, A. Corradetti, G. Fortunato, C. Reita, P. Legagneux, F. Plais, and D. Pribat

Appl. Phys. Lett. 71, 578 (1997); http://dx.doi.org/10.1063/1.119799 (3 pages) | Cited 11 times

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A systematic study of the noise performances of polycrystalline silicon (polysilicon) thin-film transistors (TFTs) made by excimer laser crystallization is presented. The drain current spectral density of these devices shows an evident 1/f behavior and the origin of the noise was attributed to carrier number fluctuations. The flat-band voltage spectral density was found to be strongly correlated with the field-effect mobility, suggesting that the microscopic mechanism causing the carrier number fluctuations involves the localized states present at the grain boundaries. The noise level in the devices with the best electrical characteristics is comparable with that observed in c-Si metal–oxide–semiconductor field effect transistors, a major improvement if compared to polysilicon TFTs made by solid-phase crystallization. © 1997 American Institute of Physics.
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85.30.Tv Field effect devices
64.70.D- Solid-liquid transitions
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
72.70.+m Noise processes and phenomena
61.72.Mm Grain and twin boundaries
71.55.Cn Elemental semiconductors
42.62.-b Laser applications

Growth of high-quality GaAs/AlAs Bragg mirrors on patterned InP-based quantum well mesa structures

H. Gebretsadik, K. Kamath, K. K. Linder, X. Zhang, P. Bhattacharya, C. Caneau, and R. Bhat

Appl. Phys. Lett. 71, 581 (1997); http://dx.doi.org/10.1063/1.119800 (3 pages) | Cited 5 times

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We have investigated the regrowth of GaAs/AlAs quarter-wave Bragg reflectors on patterned mesa InP-based quantum well heterostructures that can be fabricated into 1.55 μm vertical cavity surface emitting lasers. It is seen from transmission electron and scanning electron microscopy that the multiple layer GaAs-based mirrors can be grown on InP-based heterostructure mesas of diameters 10–40 μm without noticeable propagation of defects into the reflector layers or the quantum well region below. At the same time the photoluminescence from the quantum wells after regrowth indicates that lasers can be fabricated. © 1997 American Institute of Physics.
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42.79.Bh Lenses, prisms and mirrors
81.05.Ea III-V semiconductors
42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
42.60.By Design of specific laser systems
42.82.-m Integrated optics
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Continuously tunable, 6–14 μm silver-gallium selenide optical parametric oscillator pumped at 1.57 μm

Suresh Chandra, Toomas H. Allik, Gary Catella, Richard Utano, and J. Andrew Hutchinson

Appl. Phys. Lett. 71, 584 (1997); http://dx.doi.org/10.1063/1.119920 (3 pages) | Cited 5 times

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An angle tuned silver gallium selenide (AgGaSe2) optical parametric oscillator (OPO), pumped by the fixed wavelength 1.57 μm output of a noncritically phase-matched KTiOPO4 OPO, yielded radiation continuously tunable from 6 to 14 μm. Energies of up to 1.2 mJ/pulse with bandwidths of ∼ 5 cm−1   (full width at half-maximum) were obtained using a 6.5×6.5×35.3  mm long, type I AgGaSe2 crystal. © 1997 American Institute of Physics.
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42.65.Yj Optical parametric oscillators and amplifiers
42.72.Ai Infrared sources

Fluorescence enhancement of Er3+-doped sol–gel glass by aluminum codoping

Y. Zhou, Y. L. Lam, S. S. Wang, H. L. Liu, C. H. Kam, and Y. C. Chan

Appl. Phys. Lett. 71, 587 (1997); http://dx.doi.org/10.1063/1.119801 (3 pages) | Cited 18 times

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Erbium-doped silica glasses prepared by the sol–gel process are desirable for applications in high-power lasers and integrated optical devices. However, its fluorescence could be quenched due to the high hydroxyl (OH−) content. Although this OH− content can be greatly reduced through special gas treatment [Y. Zhou, S. S. Wang, H. L. Liu, Y. L. Lam, Y. C. Chan, and C. H. Kam, EEE J. 8, 109 (1996).], the fluorescence is still considered weak for practical applications. We have used sol–gel aluminum codoping to increase the solubility of the Er3+ ions in a silica glass host and, consequently, significantly enhanced the fluorescence intensity of Er3+-doped sol–gel glass to the extent suitable for practical usage. © 1997 American Institute of Physics.
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78.55.Hx Other solid inorganic materials
81.05.Kf Glasses (including metallic glasses)
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)

Controlling polarization of quantum-dot surface-emitting lasers by using structurally anisotropic self-assembled quantum dots

Hideaki Saito, Kenichi Nishi, Shigeo Sugou, and Yoshimasa Sugimoto

Appl. Phys. Lett. 71, 590 (1997); http://dx.doi.org/10.1063/1.119802 (3 pages) | Cited 39 times

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Polarization control is achieved in vertical-cavity surface-emitting lasers (VCSELs) by using structurally anisotropic self-assembled quantum dots (QDs) as active layers. The dot shape is long in the [011] direction on the (100) surface. The photoluminescence from the dots has a polarization dependence whose intensity along the [011] direction is 1.37 times stronger than that along the orthogonal [011] direction. The QD VCSEL operates at the wavelength of the QD ground state transition. Lasing emission shows polarization along the [011] direction and an orthogonal polarization suppression ratio of 18 dB. © 1997 American Institute of Physics.
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42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Free-space electro-optic sampling with a high-repetition-rate regenerative amplified laser

Z. G. Lu, P. Campbell, and X.-C. Zhang

Appl. Phys. Lett. 71, 593 (1997); http://dx.doi.org/10.1063/1.119803 (3 pages) | Cited 32 times

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The use of a high-repetition-rate (250 kHz) regenerative amplified laser with μJ pulse energy in a free-space THz beam electro-optic sampling system has produced a significant improvement in both the signal-to-noise ratio (>105) and absolute probe beam photomodulation depth (>19%). Focal plane images of the electric field distribution (strength and polarity) in dipole and quadrupole planar photoconductive emitters are presented. Preliminary results of real-time 2D THz images of moving objects have also been obtained. © 1997 American Institute of Physics.
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85.60.-q Optoelectronic devices
78.20.Jq Electro-optical effects
42.79.Hp Optical processors, correlators, and modulators
42.62.Eh Metrological applications; optical frequency synthesizers for precision spectroscopy
42.30.-d Imaging and optical processing

Dynamic flow and switching bistability in twisted nematic liquid crystal cells

Tie-Zheng Qian, Zhi-Liang Xie, Hoi-Sing Kwok, and Ping Sheng

Appl. Phys. Lett. 71, 596 (1997); http://dx.doi.org/10.1063/1.119804 (3 pages) | Cited 45 times

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We investigate the switching bistability based on the interaction between dynamic flow and director rotation in twisted nematic liquid crystal cells. Numerical calculation shows that there exists a general type of bistable twisted director configuration. Two specific cases are verified experimentally. © 1997 American Institute of Physics.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
47.65.-d Magnetohydrodynamics and electrohydrodynamics
78.20.Jq Electro-optical effects
83.80.Xz Liquid crystals: nematic, cholesteric, smectic, discotic, etc.
42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems

Surface roughness determination using the acousto-optic technique: Theory and experiment

R. Briers, O. Leroy, S. Devolder, M. Wevers, and P. De Meester

Appl. Phys. Lett. 71, 599 (1997); http://dx.doi.org/10.1063/1.119805 (3 pages)

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With the acousto-optic nondestructive testing technique, phase shifts between the incident and the reflected ultrasound are measured at critical angles of the investigated material. This letter is focusing theoretically as well as experimentally on the influence of a small surface roughness (some micrometers) of a thick material on the phase information. © 1997 American Institute of Physics.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects
62.65.+k Acoustical properties of solids
43.35.Sx Acoustooptical effects, optoacoustics, acoustical visualization, acoustical microscopy, and acoustical holography

Fast ambipolar carrier transport in smectic phases of phenylnaphthalene liquid crystal

Masahiro Funahashi and Jun-ichi Hanna

Appl. Phys. Lett. 71, 602 (1997); http://dx.doi.org/10.1063/1.119806 (3 pages) | Cited 69 times

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The carrier transport of a new calamitic liquid crystal, i.e., 2-(4′-octylphenyl)6-dodecyloxynaphthalene (8-PNP-O12) was investigated by the time-of-flight technique. The fast ambipolar electronic conduction was observed in two smectic phases of 8-PNP-O12. The carrier mobilities were determined to be 1.6×10−3 cm2/V⋅s in the lower temperature smectic phase (Sm1) between 79 and 100 °C and to be 2.5×10−4 cm2/V⋅s in the higher temperature phase (Sm2) between 100 and 121 °C, regardless of carrier signs, i.e., electron and hole. The slower ambipolar transport was observed in isotropic phase as well, whose mobility was 8×10−5 cm2/V⋅s. These mobilities were independent of applied electric fields and temperature. © 1997 American Institute of Physics.
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72.20.Fr Low-field transport and mobility; piezoresistance
61.30.-v Liquid crystals
72.80.Le Polymers; organic compounds (including organic semiconductors)

Arrhenius-like behavior in plasma reactions

N. Spiliopoulos, D. Mataras, and D. E. Rapakoulias

Appl. Phys. Lett. 71, 605 (1997); http://dx.doi.org/10.1063/1.119807 (3 pages) | Cited 5 times

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An Arrhenius-like behavior of the silane dissociation rate constant as a function of the power actually fed in a rf discharge system is reported. The rate constant is calculated using mass-spectrometric measurements of silane consumption for 50 and 75 mTorr discharges, while the power consumed in the discharge is determined by using detailed voltage and current measurements. The slope of the natural logarithm of the dissociation rate constant as a function of the inverse of the discharge power increases with pressure. This is attributed to a change in the shape and/or the mean energy of the electron energy distribution function, while the linearity indicates an analogous increase of the active electron concentration with increasing discharge power, excluding a significant change in the shape of the electron energy distribution function. © 1997 American Institute of Physics.
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82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
52.80.Pi High-frequency and RF discharges
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
82.20.Pm Rate constants, reaction cross sections, and activation energies
52.25.-b Plasma properties

Sheath dynamics observed in a 13.56 MHz-driven plasma

C. M. O. Mahony, R. Al Wazzan, and W. G. Graham

Appl. Phys. Lett. 71, 608 (1997); http://dx.doi.org/10.1063/1.119808 (3 pages) | Cited 20 times

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We have established, through time correlated plasma emission and electrode and plasma potential measurements, that the near electrode emission observed in asymmetric capacitively coupled 13.56 MHz-driven hydrogen plasmas is caused by field reversal that leads to sheath collapse. Near-electrode emission has now been observed in Ar and He. The field reversal appears to be due to collision-induced electron drag. © 1997 American Institute of Physics.
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52.40.Hf Plasma-material interactions; boundary layer effects
52.70.Ds Electric and magnetic measurements
52.80.Pi High-frequency and RF discharges

Nanoscale metal/self-assembled monolayer/metal heterostructures

C. Zhou, M. R. Deshpande, M. A. Reed, L. Jones, and J. M. Tour

Appl. Phys. Lett. 71, 611 (1997); http://dx.doi.org/10.1063/1.120195 (3 pages) | Cited 238 times

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We present the investigation of novel metal/organic monolayer/metal heterostructure diodes. Our technique provides well-defined, stable, and reproducible metallic contacts to a self-assembled monolayer of 4-thioacetylbiphenyl with nanoscale area. Electronic transport measurements show a prominent rectifying behavior arising from the asymmetry of the molecular heterostructure. Variable-temperature measurements reveal that thermal emission of electrons over a barrier of 0.22 eV dominates for electron injection from Ti into the organic layer while the transport for electron injection from Au into the organic layer satisfies the formula for hopping conduction. © 1997 American Institute of Physics.
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73.40.Rw Metal-insulator-metal structures
73.40.Ei Rectification
73.50.Dn Low-field transport and mobility; piezoresistance

Diffusional narrowing of Ge on Si(100) coherent island quantum dot size distributions

Jeff Drucker and Sergio Chaparro

Appl. Phys. Lett. 71, 614 (1997); http://dx.doi.org/10.1063/1.119809 (3 pages) | Cited 26 times

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The normalized width=standard deviation of island radius/mean island radius (σr/〈r〉) of molecular beam epitaxy grown Ge on Si(100) coherent island quantum dot size distributions is analyzed for various deposition conditions. It is found that this width decreases as substrate temperature increases independent of deposition flux. This result is interpreted in the context of models which suppose that the energy barrier for edge atom detachment decreases with island size. The faster diffusion kinetics at higher growth temperatures allow these detached edge atoms to more rapidly find the smaller islands producing sharper island size distributions. © 1997 American Institute of Physics.
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68.55.-a Thin film structure and morphology
68.35.Ct Interface structure and roughness
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Fx Diffusion; interface formation
81.05.Cy Elemental semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Transport in gold cluster structures defined by electron-beam lithography

L. Clarke, M. N. Wybourne, Mingdi Yan, S. X. Cai, and J. F. W. Keana

Appl. Phys. Lett. 71, 617 (1997); http://dx.doi.org/10.1063/1.120568 (3 pages) | Cited 20 times

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The near-room temperature current-voltage (I-V) characteristics of small structures made from the metal-cluster material Au55[P(C6H5)3]12Cl6 were studied. It is shown that these electron-beam defined structures have highly nonlinear characteristics with features, including a threshold voltage and scaling behavior, which are consistent with Coulomb charging of individual Au55 cores in a disordered array. Applied radio frequency signals introduce plateaus in the I-V characteristics, which demonstrates the presence of coherent tunneling in these cluster systems. © 1997 American Institute of Physics.
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72.30.+q High-frequency effects; plasma effects
73.40.Gk Tunneling
85.40.Hp Lithography, masks and pattern transfer

Morphology definition by disclinations and dislocations in a mesostructured silicate crystal

J. Feng, Q. Huo, P. M. Petroff, and G. D. Stucky

Appl. Phys. Lett. 71, 620 (1997); http://dx.doi.org/10.1063/1.119810 (3 pages) | Cited 8 times

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Curved surface morphologies resulting from disclinations are unusual in inorganic phases because the relatively large lattice energies generally permit only translational dislocations. In this letter, we show, however, that disclinations can be readily observed and identified in composite organic/inorganic phases. Disclinations and dislocations in MCM-41, a two-dimensional hexagonal (p6mm) mesophase crystal synthesized using silicate and surfactant precursors, were investigated by using transmission electron microscopy. We observed two types of dislocations and two types of disclinations. All dislocations observed are of the smallest Burgers vector a (the basis vector). Dislocation dipoles and disclination quadrupoles appear more frequently than isolated ones. © 1997 American Institute of Physics.
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61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Lp Transmission electron microscopy (TEM)

Chromium-containing metallic fibers confined within carbon nanotubes: Possibility of template-mediated crystal growth

Fumio Okuyama and Isao Ogasawara

Appl. Phys. Lett. 71, 623 (1997); http://dx.doi.org/10.1063/1.119811 (3 pages) | Cited 16 times

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So-called “anode activation” is shown to entail the growth of metallic fibers with nanometric dimensions. These fibers grow from dendritic deposits comprised of carbon and chromium, and thus chromium is a major component. They are confined inside thin-wall carbon nanotubes, with their diameter defined by the inner diameter of the host nanotube. This growth behavior reflects a template-mediated type of fiber growth. © 1997 American Institute of Physics.
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81.20.-n Methods of materials synthesis and materials processing
61.48.-c Structure of fullerenes and related hollow and planar molecular structures

Vibrational dynamics of force microscopy: Effect of tip dimensions

Oliver B. Wright and Norihiko Nishiguchi

Appl. Phys. Lett. 71, 626 (1997); http://dx.doi.org/10.1063/1.120547 (3 pages) | Cited 22 times

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The dynamics of a vibrating cantilever with an attached tip in contact with a solid is treated analytically. The tip length is shown to be crucial in determining the resonant response. The finite tip size changes the boundary conditions for the flexural motion, rendering the cantilever-tip-sample combination more rigid and implicating both the normal and lateral stiffnesses of the sample in the analysis. This is confirmed in an experiment with a silica sample, a sapphire tip, and a silicon cantilever. The theory has implications in the field of quantitative analysis with atomic ac force microscopy. © 1997 American Institute of Physics.
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07.79.Lh Atomic force microscopes
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
68.37.Uv Near-field scanning microscopy and spectroscopy
68.35.B- Structure of clean surfaces (and surface reconstruction)
87.64.Dz Scanning tunneling and atomic force microscopy

Silicon incorporation into chemical vapor deposition diamond: A role of oxygen

Isao Sakaguchi, Mikka Nishitani-Gamo, Kian Ping Loh, Hajime Haneda, Shunichi Hishita, and Toshihiro Ando

Appl. Phys. Lett. 71, 629 (1997); http://dx.doi.org/10.1063/1.119812 (3 pages) | Cited 12 times

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The suppression effect of oxygen on Si incorporation in homoepitaxial diamond films grown by microwave assisted chemical vapor deposition is investigated by secondary ion mass spectrometry. The Si depth profile in the multilayered diamond films continuously synthesized with different oxygen addition shows that Si incorporation decreases with increasing oxygen addition into the plasma. A change in the interfacial composition at the quartz glass due to oxygen-promoted surface chemistry may be the origin of this suppression effect. © 1997 American Institute of Physics.
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81.05.Cy Elemental semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.up Other materials
85.40.Ry Impurity doping, diffusion and ion implantation technology
81.15.Kk Vapor phase epitaxy; growth from vapor phase
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Metallic conductivity of amorphous carbon films under high pressure

Somnath Bhattacharyya and S. V. Subramanyam

Appl. Phys. Lett. 71, 632 (1997); http://dx.doi.org/10.1063/1.120425 (3 pages) | Cited 12 times

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Amorphous carbon films are prepared by plasma-assisted chemical vapor deposition. Resistivity of the films is measured from 300 down to 8 K showing a negative temperature coefficient of resistivity. An increase of room temperature conductivity from 102 S cm−1 to a value of about 104 S cm−1 is found at a pressure of 2 GPa. At a fixed pressure of 0.5 GPa, the films show a positive temperature coefficient of conductivity in the range from 300 to 200 K, followed by a very weak dependence of temperature down to 15 K. At a pressure of 2 GPa a positive temperature coefficient of resistivity is observed in the range between 300 and 15 K. The metallic behavior of the carbon films under high pressure is explained using electronic structure. © 1997 American Institute of Physics.
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73.61.At Metal and metallic alloys
62.50.-p High-pressure effects in solids and liquids
73.50.Dn Low-field transport and mobility; piezoresistance

Room temperature intrinsic optical transition in GaN epilayers: The band-to-band versus excitonic transitions

M. Smith, J. Y. Lin, H. X. Jiang, and M. Asif Khan

Appl. Phys. Lett. 71, 635 (1997); http://dx.doi.org/10.1063/1.119813 (3 pages) | Cited 36 times

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The mechanism of room-temperature (RT) intrinsic optical transition in high quality and purity GaN epilayer grown by metalorganic chemical vapor deposition (MOCVD) has been investigated. Our results show that the band-to-band instead of excitonic transition is the dominant transition in MOCVD grown GaN epilayer at RT. This conclusion is supported by the observation of the excitation intensity dependence of the photoluminescence emission peak position and by a model calculation. The band-to-band transition energy at RT at the limit of low carrier concentration has been determined to be 3.429 eV. Since the band-to-band transition is the dominant optical transition at RT, it thus suggests that the electron-hole plasma is most likely responsible for gain in GaN blue lasers similar to the case in other III–V semiconductor lasers. © 1997 American Institute of Physics.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.35.Ee Electron-hole drops and electron-hole plasma
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
72.30.+q High-frequency effects; plasma effects
72.20.Fr Low-field transport and mobility; piezoresistance
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