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15 Feb 2005

Volume 97, Issue 4, Articles (04xxxx)

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Vertically aligned carbon nanofibers and related structures: Controlled synthesis and directed assembly

A. V. Melechko, V. I. Merkulov, T. E. McKnight, M. A. Guillorn, K. L. Klein, D. H. Lowndes, and M. L. Simpson

J. Appl. Phys. 97, 041301 (2005); http://dx.doi.org/10.1063/1.1857591 (39 pages) | Cited 203 times

Online Publication Date: 3 February 2005

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The controlled synthesis of materials by methods that permit their assembly into functional nanoscale structures lies at the crux of the emerging field of nanotechnology. Although only one of several materials families is of interest, carbon-based nanostructured materials continue to attract a disproportionate share of research effort, in part because of their wide-ranging properties. Additionally, developments of the past decade in the controlled synthesis of carbon nanotubes and nanofibers have opened additional possibilities for their use as functional elements in numerous applications. Vertically aligned carbon nanofibers (VACNFs) are a subclass of carbon nanostructured materials that can be produced with a high degree of control using catalytic plasma-enhanced chemical-vapor deposition (C-PECVD). Using C-PECVD the location, diameter, length, shape, chemical composition, and orientation can be controlled during VACNF synthesis. Here we review the CVD and PECVD systems, growth control mechanisms, catalyst preparation, resultant carbon nanostructures, and VACNF properties. This is followed by a review of many of the application areas for carbon nanotubes and nanofibers including electron field-emission sources, electrochemical probes, functionalized sensor elements, scanning probe microscopy tips, nanoelectromechanical systems (NEMS), hydrogen and charge storage, and catalyst support. We end by noting gaps in the understanding of VACNF growth mechanisms and the challenges remaining in the development of methods for an even more comprehensive control of the carbon nanofiber synthesis process.
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81.05.U- Carbon/carbon-based materials
81.07.De Nanotubes
81.16.Hc Catalytic methods
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
79.70.+q Field emission, ionization, evaporation, and desorption
61.46.-w Structure of nanoscale materials
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
52.77.-j Plasma applications
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
85.35.Kt Nanotube devices
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Cell parameter determination of a twisted-nematic liquid crystal display by single-wavelength polarimetry

Vicente Durán, Jesús Lancis, Enrique Tajahuerce, and Zbigniew Jaroszewicz

J. Appl. Phys. 97, 043101 (2005); http://dx.doi.org/10.1063/1.1846142 (6 pages) | Cited 17 times

Online Publication Date: 20 January 2005

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Optical transmittance of a 90-twisted nematic liquid crystal cell inserted between crossed or parallel polarizers is neither sensitive to the molecular twist sense nor to the exchange between the extraordinary and the ordinary birefringence axis at the input face of the cell. In this article, the equivalence between a twisted-nematic liquid crystal cell and the combination of a retardation wave plate and a polarization rotator is fully exploited to understand the physical origin of these ambiguities. We determine, in a simple way, cell parameters by means of standard cell transmittance measurements when both crossed and parallel polarizers are rotated simultaneously. We show that this procedure, which is subjected to the aforementioned ambiguities, can be completed by additional polarimetric measurements. In particular, we demonstrate that the twist angle, twist sense, retardation, and the rubbing direction can be univocally determined by experimental measurement of the Stokes parameters of the light transmitted through the cell at one wavelength. Laboratory results corresponding to a commercial liquid crystal display panel are carried out.
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42.79.Kr Display devices, liquid-crystal devices
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.20.Fm Birefringence

Spatial homogeneity of optically switched semiconductor photonic crystals and of bulk semiconductors

Tijmen G. Euser and Willem L. Vos

J. Appl. Phys. 97, 043102 (2005); http://dx.doi.org/10.1063/1.1846949 (7 pages) | Cited 25 times

Online Publication Date: 20 January 2005

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In this paper we discuss free carrier generation by pulsed laser fields as a mechanism to switch the optical properties of semiconductor photonic crystals and bulk semiconductors on an ultrafast time scale. Requirements are set for the switching magnitude, the time-scale, the induced absorption, as well as the spatial homogeneity, in particular for silicon at λ = 1550 nm. Using a nonlinear absorption model, we calculate carrier depth profiles and define a homogeneity length lhom. Homogeneity length contours are visualized in a plane spanned by the linear and two-photon absorption coefficients. Such a generalized homogeneity plot allows us to find optimum switching conditions at pump frequencies near ν/c = 5000 cm−1 (λ = 2000 nm). We discuss the effect of scattering in photonic crystals on the homogeneity. We experimentally demonstrate a 10% refractive index switch in bulk silicon within 230 fs with a lateral homogeneity of more than 30 μm. Our results are relevant for switching of modulators in the absence of photonic crystals.
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42.70.Qs Photonic bandgap materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.65.Pc Optical bistability, multistability, and switching, including local field effects
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Optical characterization of polychromatic organic light emitting diodes

David Troadec, André Moliton, Bernard Ratier, Rémy Antony, and Roger C. Hiorns

J. Appl. Phys. 97, 043103 (2005); http://dx.doi.org/10.1063/1.1835538 (7 pages)

Online Publication Date: 20 January 2005

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Luminance and quantum yields of organic light emitting diodes (OLEDs) are generally calculated by using a hypothetic monochromatic emission even though the actual external emission is lower than the internal emission because of internal reflection, and is polychromatic. For organic materials, we present the effects of the total internal reflection on quantum yields. Calculations for the luminance and the efficiency of an ideal pseudomonochromatic source are compared with those for a real polychromatic source. While we demonstrate that the calculation of the yield is practically unchanged whatever hypothesis is used, we do show, however, that by assuming that the source is pseudomonochromatic, the luminances of green and blue OLEDs are overvalued or undervalued by a factor of 4, respectively.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence

Dynamic response of optical-fiber modulator by using magnetic fluid as a cladding layer

J. J. Chieh, S. Y. Yang, Y. H. Chao, H. E. Horng, Chin-Yih Hong, and H. C. Yang

J. Appl. Phys. 97, 043104 (2005); http://dx.doi.org/10.1063/1.1849824 (4 pages) | Cited 7 times

Online Publication Date: 28 January 2005

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An in-line optical-fiber modulator that uses magnetic fluid as a cladding layer has been developed to control transmission loss generated from external magnetic fields. In this work, we investigate the instant variation in the transmission loss of an optical-fiber modulator after an external magnetic field has been turned on (referred to as the charging operation) or turned off (referred to as the discharging operation). Research showed that when the external field was turned on∕off, the transmission loss of the optical-fiber modulator did not change until after 10 ms. Successively, the transmission loss varied exponentially. Thus, two kinds of characteristic time were observed to describe quantitatively the instant variation in the transmission loss: retarding time and response time. It was found that both the retarding time in a charging∕discharging operation increased with the length of the magnetic-fluid cladding over a critical level, whereas the response time in a charging∕discharging operation was cladding length independent.
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42.79.Hp Optical processors, correlators, and modulators
42.81.Dp Propagation, scattering, and losses; solitons
42.81.Bm Fabrication, cladding, and splicing
75.50.Mm Magnetic liquids
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Characterization of helium discharge cleaning plasmas in ADITYA tokamak using collisional-radiative model code

Ram Prakash, P. Vasu, Vinay Kumar, R. Manchanda, M. B. Chowdhuri, and M. Goto

J. Appl. Phys. 97, 043301 (2005); http://dx.doi.org/10.1063/1.1847704 (7 pages) | Cited 4 times

Online Publication Date: 20 January 2005

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The wall conditioning of the ADITYA tokamak is usually done, by first producing an electron cyclotron resonance (ECR) plasma inside the vessel and then superimposing a pulsed ohmic discharge on the ECR background. Sometimes, helium gas is used as a working gas. In this article, the spectral line emissions of neutral helium for the two different plasmas—namely, the ECR and the pulsed discharge cleaning (PDC) plasmas—have been analyzed using a collisional-radiative (CR)-model code to estimate the electron density and temperature. We are able to match the experimentally obtained relative intensity ratios with those predicted by the model under the assumption of ionizing plasma condition if the possible effects of the metastable states are not ignored. This has been done by using the populations of two metastable levels (2 math and 2 math) as independent parameters in addition to the ground states of neutrals and ions in the CR model under a quasisteady-state approximation. It is further seen that, it is the metastables and not the recombination (including dielectronic) processes that lead to a better fit with experimental observations. The column density of neutrals inferred from this analysis implies that the emission from the PDC discharge emanates from a large region of the vessel, while in the ECR discharge, the plasma responsible for the emission is restricted to a narrow region. This is also borne out by experimental observation.
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52.55.Fa Tokamaks, spherical tokamaks
52.55.Pi Fusion products effects (e.g., alpha-particles, etc.), fast particle effects
52.50.Sw Plasma heating by microwaves; ECR, LH, collisional heating
52.50.Nr Plasma heating by DC fields; ohmic heating, arcs
52.25.Jm Ionization of plasmas
52.20.Fs Electron collisions
52.40.Hf Plasma-material interactions; boundary layer effects
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.80.Pi High-frequency and RF discharges
52.65.-y Plasma simulation

On line-ratio analysis for helium–argon microwave discharges

I. Koleva, A. Shivarova, K. Makasheva, and H. Schlüter

J. Appl. Phys. 97, 043302 (2005); http://dx.doi.org/10.1063/1.1841467 (8 pages) | Cited 6 times

Online Publication Date: 20 January 2005

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Regarding spectroscopy diagnostics via line-intensity ratio of high-density diffusion-controlled discharges in helium–argon gas mixtures, the study presents a collisional-radiative model of the discharge. In order to show variations of excited-state population densities and line-intensity ratios related to changes of plasma density and electron temperature, the model is applied to surface-wave-sustained discharges, since the axial structure of these discharges displays self-consistent variations of the discharge characteristics. The comparison with the corona model shows the necessity of developing a collisional-radiative model. Possibilities for simultaneous determination—based on the collisional-radiative model—of electron temperature and plasma density from measurements of line-intensity ratios are outlined.
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52.80.Pi High-frequency and RF discharges
52.65.-y Plasma simulation
52.25.Fi Transport properties
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions

Effect of ion streaming on particle–particle interactions in a dusty plasma

Vivek Vyas and Mark J. Kushner

J. Appl. Phys. 97, 043303 (2005); http://dx.doi.org/10.1063/1.1841468 (7 pages) | Cited 3 times

Online Publication Date: 20 January 2005

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Dust particles in low-temperature, low-pressure plasmas form Coulomb crystals and display collective behavior under select conditions. The trajectories of ions can be perturbed as they pass by negatively charged dust particles and, in some cases, will converge beyond the particle. This process, called ion streaming, produces a positive potential in the wakefield of the particle that can be large enough to perturb interparticle dynamics. In this paper, we discuss results from a three-dimensional model for dust particle transport in plasma processing reactors with which we investigated the effects of ion streaming on particle–particle interactions. When including the wakefield potential produced by ion streaming, dust particles can form vertically correlated pairs when trapped in electrical potential wells. The ion-streaming force was found to be significant only over a select range of pressures and for given combinations of particle sizes and mass densities. The formation of vertically correlated pairs critically depends on the shape of the potential well. Wakefield forces can also affect the order of multilayer lattices by producing vertical correlations between particles in adjacent layers.
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52.27.Lw Dusty or complex plasmas; plasma crystals
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.65.Rr Particle-in-cell method
52.25.Fi Transport properties
52.58.-c Other confinement methods
52.25.Vy Impurities in plasmas
52.80.-s Electric discharges

Pd/H system in H2 plasmas

F. Di Pascasio, D. Gozzi, B. Panella, and C. Trionfetti

J. Appl. Phys. 97, 043304 (2005); http://dx.doi.org/10.1063/1.1844607 (11 pages) | Cited 1 time

Online Publication Date: 21 January 2005

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The absorption of hydrogen by Pd wire samples has been studied at 299 K and in the H2 pressure range from 18 to 1800 mbar under both tensile stress and tensile stress under cold H2 plasma. Plasma is generated in a corona discharge regime by applying a dc high voltage between a Pd wire and a cylindrical counterelectrode. Positive and negative coronas have been performed. Three independent experimental quantities have been simultaneously measured: ratio x = nH/nPd, with n the number of atoms; ratio R/R0, with R the four-probe resistance of the wire and R0 the resistance when x = 0; and ratio Δl/l0, with Δl the length change of the wire and l0 its length when x = 0. From the thermodynamic analysis of data, the absorption standard chemical potential, Δμabs0, and the excess chemical potential, Δμexc, of hydrogen atoms in Pd have been obtained. At constant T, H2 pressure, and tensile stress, the x value under cold plasma is always higher.
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82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
52.25.Kn Thermodynamics of plasmas
52.80.Hc Glow; corona
68.43.-h Chemisorption/physisorption: adsorbates on surfaces

Two-dimensional simulation of a direct-current microhollow cathode discharge

Prashanth S. Kothnur and Laxminarayan L. Raja

J. Appl. Phys. 97, 043305 (2005); http://dx.doi.org/10.1063/1.1849816 (12 pages) | Cited 29 times

Online Publication Date: 21 January 2005

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Microhollow cathode discharges (MHCD’s) are miniature direct-current discharges that operate at elevated pressures (several tens to hundreds of Torr) with electrode dimensions in the 10–100-μm range. MHCD’s have been proposed for a number of applications based on their unique characteristics such as presence of intense excimer radiation and significant gas heating within the submillimeter discharge volume. A two-dimensional, self-consistent fluid model of a helium MHCD in the high-pressure (several hundreds of Torr), high-current ( ∼ 1 mA) operating regime is presented in this study. Results indicate that the MHCD operates in an abnormal glow discharge mode with charged and excited metastable species with densities of ∼ 1020 m−3, electron temperatures of approximately tens of eV, and gas temperatures of hundreds of Kelvin above room temperature. Significant discharge activity exists outside of the hollow region. The discharge volume and intensity increases with increasing current and becomes more confined with increasing pressures. Most predictions presented in this paper are in qualitative and quantitative agreement with experimental data for MHCD’s under similar conditions.
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52.80.Hc Glow; corona
52.65.-y Plasma simulation
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
52.25.Fi Transport properties
52.20.Fs Electron collisions

Modeling of and experiments on dust particle levitation in the sheath of a radio frequency plasma reactor

Heru Setyawan, Manabu Shimada, Yutaka Hayashi, Kikuo Okuyama, and Sugeng Winardi

J. Appl. Phys. 97, 043306 (2005); http://dx.doi.org/10.1063/1.1852696 (6 pages) | Cited 4 times

Online Publication Date: 26 January 2005

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The equilibrium and trapping of dust particles in a plasma sheath are investigated, both experimentally and theoretically. A self-consistent sheath model including input power as one of the model parameters is proposed, to predict the equilibrium position of particle trap. The electron temperature and density are estimated from the observed current and power (I-P) characteristics using the sheath model developed. Direct comparisons are made between the measured equilibrium position and the predicted equilibrium position. The equilibrium position moves closer to the electrode with increasing rf power and particle size. The position is apparently related to the sheath thickness, which decreases with increasing rf power. The model can correctly predict the experimentally observed trend in the equilibrium position of particle trap. It is found that the particle charge becomes positive when the particle gets closer to the electrode, due to the dominant influence of ion currents to the particle surface.
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52.25.Vy Impurities in plasmas
52.40.Kh Plasma sheaths
52.80.Pi High-frequency and RF discharges
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Interaction of wide-band-gap single crystals with 248-nm excimer laser irradiation. X. Laser-induced near-surface absorption in single-crystal NaCl

K. H. Nwe, S. C. Langford, J. T. Dickinson, and W. P. Hess

J. Appl. Phys. 97, 043501 (2005); http://dx.doi.org/10.1063/1.1847696 (5 pages) | Cited 5 times

Online Publication Date: 20 January 2005

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Ultraviolet laser-induced desorption of neutral atoms and molecules from nominally transparent, ionic materials can yield particle velocities consistent with surface temperatures of a few thousand kelvin even in the absence of visible surface damage. The origin of the laser absorption required for this surface heating has been often overlooked. In this work, we report simultaneous neutral emission and laser transmission measurements on single-crystal NaCl exposed to 248-nm excimer laser radiation. As much as 20% of the incident radiation at 248 nm must be absorbed in the near-surface region to account for the observed particle velocities. We show that the laser absorption grows from low values over several pulses and saturates at values sufficient to account for the surface temperatures required to explain the observed particle velocity distributions. The growth of absorption in these early pulses is accompanied by a corresponding increase in the emission intensities. The diffuse reflectance spectra acquired after exposure suggest that near-surface V-type centers are responsible for most of the absorption at 248 nm in single-crystal NaCl.
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79.20.Ds Laser-beam impact phenomena
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Ms Insulators
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
68.43.Mn Adsorption kinetics
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Interaction of wide-band-gap single crystals with 248-nm excimer laser radiation. XI. The effect of water vapor and temperature on laser desorption of neutral atoms from sodium chloride

K. H. Nwe, S. C. Langford, J. T. Dickinson, and W. P. Hess

J. Appl. Phys. 97, 043502 (2005); http://dx.doi.org/10.1063/1.1847697 (10 pages) | Cited 4 times

Online Publication Date: 20 January 2005

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We show that low partial pressures of water vapor (10−5 Pa) dramatically increase the intensity of neutral Na and Cl emissions from cleaved, single-crystal NaCl during pulsed laser irradiation at 248 nm (KrF excimer). The time-of-flight distributions of these emissions are consistent with thermal desorption from laser-heated surfaces. Significantly, introducing water vapor lowers the particle velocities and thus the effective surface temperature during emission. Transmission measurements confirm that laser absorption is reduced in the presence of water vapor. The Arrhenius analysis of the emission intensities and effective temperatures show reduced activation energies in the presence of water vapor, which more than compensate for the vapor-induced reduction in laser absorption and surface temperature. Atomic force and scanning electron microscopy of the irradiated surfaces show evidence for accelerated monolayer-scale erosion in the presence of water vapor. A mechanism for the effect of water on these emission and erosion processes is proposed and discussed.
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79.20.Ds Laser-beam impact phenomena
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
68.43.Mn Adsorption kinetics
79.70.+q Field emission, ionization, evaporation, and desorption
68.37.Ps Atomic force microscopy (AFM)
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)

Impurity doping effect on thermal stability of InGaN/GaN multiple quantum-well structures

Kazuhide Kusakabe, Tomoaki Hara, and Kazuhiro Ohkawa

J. Appl. Phys. 97, 043503 (2005); http://dx.doi.org/10.1063/1.1846944 (3 pages) | Cited 3 times

Online Publication Date: 20 January 2005

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Thermal stability of InGaN/GaN multiple quantum-well (MQW) structures grown by metalorganic vapor phase epitaxy on (0001) sapphire substrates was investigated. Samples were annealed under atmospheric nitrogen ambient at 1000 °C after growth. The thermal stability of MQW structures was estimated by high-resolution x-ray diffraction. It was found that thermal annealing degraded MQW periodicity in an undoped sample. This was due to the thermal diffusion of indium atoms via Ga vacancies in the GaN barrier region. It was also found that both Si doping and Mg doping improved the thermal stability of MQW structures. This mechanism was considered that Si and Mg, which were incorporated into column-III sites, prevented formation of the Ga vacancies. Thus, the thermal diffusion of indium atoms was suppressed. Room temperature photoluminescence (PL) from the Si-doped MQWs retained intense emission after annealing, while the undoped and Mg-doped MQWs showed degradation of PL intensities after annealing. It was, therefore, found that Si was a desirable dopant to promote the thermal stability of InGaN/GaN MQWs.
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81.07.St Quantum wells
81.05.Ea III-V semiconductors
61.72.uj III-V and II-VI semiconductors
68.65.Fg Quantum wells
78.67.De Quantum wells
66.30.J- Diffusion of impurities
61.72.Cc Kinetics of defect formation and annealing
78.55.Cr III-V semiconductors
61.72.J- Point defects and defect clusters

Photoluminescence of mechanically polished ZnO

D. W. Hamby, D. A. Lucca, and M. J. Klopfstein

J. Appl. Phys. 97, 043504 (2005); http://dx.doi.org/10.1063/1.1840102 (8 pages) | Cited 14 times

Online Publication Date: 20 January 2005

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The effects of mechanical polishing on the photoluminescence (PL) from each polar face of wurtzite-structure ZnO are presented. Differences observed for the 4.2 K PL of a mechanically polished surface when compared to that of a chemomechanically polished surface include broadened bound-exciton peaks, hot-exciton luminescence, and a donor-acceptor pair peak at 3.2108 eV. Analysis of this donor-acceptor pair peak results in estimated donor and acceptor ionization energies of 52±10 and 230±10 meV, respectively, with a mean separation distance between pairs of approximately 3–4 nm. The donors and acceptors are attributed to point defects introduced by dislocation motion during the polishing process and identified as octahedral Zn interstitials and Zn vacancies, respectively.
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81.05.Dz II-VI semiconductors
78.55.Et II-VI semiconductors
81.65.Ps Polishing, grinding, surface finishing
61.72.J- Point defects and defect clusters
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
71.55.Gs II-VI semiconductors
61.72.Yx Interaction between different crystal defects; gettering effect
71.35.Ee Electron-hole drops and electron-hole plasma

Optical properties of Er3+ and Tm3+ ions in a tellurite glass

Hiroki Yamauchi, Ganapathy Senthil Murugan, and Yasutake Ohishi

J. Appl. Phys. 97, 043505 (2005); http://dx.doi.org/10.1063/1.1830088 (8 pages) | Cited 21 times

Online Publication Date: 20 January 2005

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Optical spectroscopic properties of Er3+ and Tm3+ ions in a tellurite glass, TeO2-BaO-SrO-Nb2O5 (TBSN), were investigated for C- and S-band optical amplifications. On the basis of consideration for cross relaxation among Tm3+ ions and multiphonon relaxation, it was shown that the presence of Nb2O5, which was a source of the highest energy phonon, contributed to the relaxation processes in TBSN glasses.
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78.40.Pg Disordered solids
78.35.+c Brillouin and Rayleigh scattering; other light scattering
63.50.-x Vibrational states in disordered systems
61.43.Fs Glasses
71.23.Cq Amorphous semiconductors, metallic glasses, glasses

Interface structures and periodic film distortions induced by substrate-surface steps in Gd-doped ceria thin-film growth

D. X. Huang, C. L. Chen, and A. J. Jacobson

J. Appl. Phys. 97, 043506 (2005); http://dx.doi.org/10.1063/1.1845576 (5 pages) | Cited 3 times

Online Publication Date: 21 January 2005

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Gadolinium-doped ceria (Ce0.8Gd0.2O2−δ) thin films were grown on single-crystal (001) LaAlO3 (LAO) substrates by a pulsed laser ablation. The transmission electron microscope observation reveals a unique type of periodic film distortion along the film∕substrate interface. Each distorted film area is associated with a few substrate-surface steps and the spacing between these distorted areas is about 50 μm. The distortion starts at the substrate-surface steps and extends into the film along one of the {111} planes at the step-forward direction. The {111} planar defects induced by the nearby steps can interact with each other to form a planar defect network. The structure of the (001) LAO surface, the structure of the film∕substrate interface, and the mechanism of the formation of these {111} planar defects have been analyzed using a high-resolution electron microscopy. Structural models for these planar defects and their interaction are suggested.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology
61.72.Mm Grain and twin boundaries
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.35.Ct Interface structure and roughness
68.37.Lp Transmission electron microscopy (TEM)

Optical absorption and emission in the defect-chalcopyrite semiconductor CdGa2Te4

Shunji Ozaki, Kei-Ichi Muto, Hisatoshi Nagata, and Sadao Adachi

J. Appl. Phys. 97, 043507 (2005); http://dx.doi.org/10.1063/1.1845582 (7 pages) | Cited 2 times

Online Publication Date: 21 January 2005

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Optical-absorption and photoluminescence (PL) spectra have been measured on the defect-chalcopyrite-type semiconductor CdGa2Te4 in the 0.9–1.5‐eV photon-energy range at temperatures between 11 and 300 K. The temperature dependence of the direct-gap energy of CdGa2Te4 has been determined from the optical-absorption spectra and fit using the Varshni equation and an analytical four-parameter expression developed for the explanation of the band-gap shrinkage effect in semiconductors. The PL spectra show an asymmetric emission band peaking at ∼ 1.326 eV and a symmetric emission band at ∼ 1.175 eV at T = 11 K, which are attributed to donor-acceptor-pair recombination between exponentially tailed or Gaussian-like donor states and acceptor levels, respectively. A multiple-exponential fit analysis of the PL emission suggests acceptor levels of 50 and 86 meV and a deep donor level of 190 meV, together with an unidentified shallow level of 9 meV. An energy-band scheme has been proposed for the explanation of PL emission observed in CdGa2Te4.
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78.55.Hx Other solid inorganic materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.55.-i Impurity and defect levels
71.20.Nr Semiconductor compounds
72.80.Jc Other crystalline inorganic semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
63.20.K- Phonon interactions

Dopant penetration studies through Hf silicate

M. A. Quevedo-Lopez, M. R. Visokay, J. J. Chambers, M. J. Bevan, A. LiFatou, L. Colombo, M. J. Kim, B. E. Gnade, and R. M. Wallace

J. Appl. Phys. 97, 043508 (2005); http://dx.doi.org/10.1063/1.1846138 (15 pages) | Cited 10 times

Online Publication Date: 21 January 2005

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We present a study of the penetration of B, P, and As through Hf silicate (HfSixOy) and the effect of N incorporation in Hf silicate (HfSixOyNz) on dopant penetration from doped polycrystalline silicon capping layers. The extent of penetration through Hf silicate was found to be dependent upon the thermal annealing budget for each dopant investigated as follows: B(T ≥ 950 °C/60 s), P(T ≥ 1000 °C/20 s), and As (T ≥ 1050 °C/60 s). We propose that the enhanced diffusion observed for these dopants in HfSixOy, compared with that of SiO2 films, is related to grain boundary formation resulting from HfSixOy film crystallization. We also find that, as in the case of SiO2, N incorporation inhibits dopant (B, P, and As) diffusion through the Hf silicate and thus penetration into the underlying Si substrate. Only B penetration is clearly observed through HfSiON films for anneals at 1050 °C for durations of 10 s or longer. The calculated B diffusivity through the HfSixOyNz layer is D0 = 5.2×10−15 cm2/s.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
66.30.J- Diffusion of impurities
66.30.Dn Theory of diffusion and ionic conduction in solids
61.72.up Other materials
61.72.Cc Kinetics of defect formation and annealing
61.72.S- Impurities in crystals
61.72.Mm Grain and twin boundaries
81.65.Cf Surface cleaning, etching, patterning
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology
77.55.-g Dielectric thin films

Correlation of structural, chemical, and magnetic properties in annealed Ti/Ni multilayers

Pramod Bhatt, Anupam Sharma, and S. M. Chaudhari

J. Appl. Phys. 97, 043509 (2005); http://dx.doi.org/10.1063/1.1835561 (8 pages) | Cited 4 times

Online Publication Date: 21 January 2005

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Ti/Ni multilayer samples have been synthesized on float glass substrates using an electron-beam evaporation technique under ultrahigh vacuum conditions at room temperature. Grazing incidence x-ray diffraction (GIXRD) and grazing incidence x-ray reflectivity (GIXRR) techniques were used, respectively, to study structural modifications and to determine corresponding changes in microstructural parameters, such as individual layer thickness, interface roughness, and electron density due to annealing treatment. In addition to this, the chemical nature of the surface and interfaces of these samples were also studied using a depth profile x-ray photoelectron spectroscopy (XPS) technique. The GIXRD measurement show clear amorphization of the as-deposited multilayer sample annealed in the temperature range of 300–400 °C. The corresponding GIXRR measurement indicates the formation of a sufficiently thick layer of Ti–Ni at interfaces converting the Ti/Ni bilayer into a Ti/Ti–Ni/Ni trilayer multilayer structure. The precipitation of the Ti–Ni alloy phase at the interface in the case of samples annealed at 400 °C has been confirmed by XPS measurements. The magnetization behavior investigated using the magneto-optical Kerr effect technique clearly shows well the saturation magnetization behavior for all samples annealed up to 300 °C, while the sample annealed at 400 °C does not show saturation magnetization. The corresponding coercivity value (Hc) is also found to be changed drastically from 15.5 to 0.6 Oe. This observed magnetization behavior is discussed and correlated with structural and chemical changes in the multilayer structure.
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81.05.Bx Metals, semimetals, and alloys
68.65.Ac Multilayers
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.35.Ct Interface structure and roughness
81.40.Gh Other heat and thermomechanical treatments
75.50.Cc Other ferromagnetic metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.20.Ls Magneto-optical effects
61.43.Dq Amorphous semiconductors, metals, and alloys
68.37.Xy Scanning Auger microscopy, photoelectron microscopy
79.60.Bm Clean metal, semiconductor, and insulator surfaces
79.60.Jv Interfaces; heterostructures; nanostructures

Force-displacement relationships for spherical inclusions in finite elastic media

David C. Lin, Noshir A. Langrana, and Bernard Yurke

J. Appl. Phys. 97, 043510 (2005); http://dx.doi.org/10.1063/1.1847698 (4 pages) | Cited 5 times

Online Publication Date: 21 January 2005

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Here we present derivations of the force-displacement relationship for a rigid spherical inclusion embedded in homogeneous, isotropic, linear solids. Formulas are given for both the case of perfect interfacial bonding (no slip) and the case of a sliding contact (slip) between the medium and the inclusion. The formulas are applicable to both compressible and incompressible solids and are applicable for elastic media of finite extent. The results allow the determination of the elastic modulus of the medium from force-displacement measurements on the inclusion provided Poisson’s ratio is known. We find that, when the size of the medium is much larger than that of the inclusion, the inclusion displacement in response to an applied force is 50% larger when slip is present.
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46.25.Cc Theoretical studies
46.80.+j Measurement methods and techniques in continuum mechanics of solids
46.35.+z Viscoelasticity, plasticity, viscoplasticity

Stark effect in the optical absorption in quantum wires

Johnson Lee and Harold N. Spector

J. Appl. Phys. 97, 043511 (2005); http://dx.doi.org/10.1063/1.1849430 (6 pages) | Cited 8 times

Online Publication Date: 21 January 2005

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We have calculated the effect of a transverse electric field on the ground and the first few excited states of the electrons confined in a quantum wire and used these results to examine the effect of the electric field on the intersubband optical absorption in such quantum wires. The electric field removes the degeneracies between several of the excited states in the wire, which leads to peaks in the intersubband optical absorption of the wire. The application of the electric field leads to a Stark shift of the electron energies, which is quadratic in the field at low fields but becomes almost linear in the field at high fields. The electric field shifts the peaks in a manner which depends upon the polarization of the optical field with respect to the applied electric field. We have also investigated the range of wire radii and electric fields at which the infinite well model is valid in a material where the height of the confining potential barrier is finite. We have found that for small wire radii and strong electric fields, the infinite well model breaks down.
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73.21.Hb Quantum wires
78.67.Lt Quantum wires
78.20.Jq Electro-optical effects
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Optimization techniques for the estimation of the thickness and the optical parameters of thin films using reflectance data

S. D. Ventura, E. G. Birgin, J. M. Martínez, and I. Chambouleyron

J. Appl. Phys. 97, 043512 (2005); http://dx.doi.org/10.1063/1.1849431 (12 pages) | Cited 8 times

Online Publication Date: 21 January 2005

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The present work considers the problem of estimating the thickness and the optical constants (extinction coefficient and refractive index) of thin films from the spectrum of normal reflectance R. This is an ill-conditioned highly underdetermined inverse problem. The estimation is done in the spectral range where the film is not opaque. The idea behind the choice of this particular spectral range is to compare the film characteristics retrieved from transmittance T and from reflectance data. In the first part of the paper a compact formula for R is deduced. The approach to deconvolute the R data is to use well-known information on the dependence of the optical constants on photon energy of semiconductors and dielectrics and to formulate the estimation as a nonlinear optimization problem. Previous publications of the group on the subject provide the guidelines for designing the new procedures. The consistency of the approach is tested with computer-generated thin films and also with measured R and T spectral data of an a-Si:H film deposited onto glass. The algorithms can handle satisfactorily the problem of a poor photometric accuracy in reflectance data, as well as a partial linearity of the detector response. The results on gedanken films and on a-Si:H indicate a very good agreement between expected and retrieved values.
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78.66.Db Elemental semiconductors and insulators
78.66.Jg Amorphous semiconductors; glasses
68.55.-a Thin film structure and morphology
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.20.Bh Theory, models, and numerical simulation
78.30.Am Elemental semiconductors and insulators
78.40.Fy Semiconductors

Conditions for self-assembly of quantum fortresses and analysis of their possible use as quantum cellular automata

T. E. Vandervelde, R. M. Kalas, P. Kumar, T. Kobayashi, T. L. Pernell, and J. C. Bean

J. Appl. Phys. 97, 043513 (2005); http://dx.doi.org/10.1063/1.1844620 (9 pages) | Cited 1 time

Online Publication Date: 24 January 2005

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In this study, we detail the conditions that result in the generation of self-assembled quantum fortresses (QFs), in SiGe/Si. A QF consists of four quantum dots (QDs) clustered around a central square pit, one QD per side. This structure strongly resembles the proposed quantum cellular automata (QCA) unit cell—the basis for a computer architecture. We map the growth conditions (epilayer thickness and Ge concentration) under which self-assembly of strain-stabilized QFs and their precursors occur. Additionally, we characterize how QFs change in height, width, and internal size scales within this parameter space. From this information, we develop a phenomenological model for why QFs form based upon changes in lattice spacing. We then calculate how QFs of the observed shapes and sizes would function as QCAs based on a Hubbard-type Hamiltonian model. This analysis reveals that self-assembled QFs grown at 550 °C, a rate of 1 Å/s, a SiGe alloy composition of 37–40%, and a thickness of 15–35 nm could be used as QCAs.
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81.07.Ta Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Vacancy-type defects in Si-doped InN grown by plasma-assisted molecular-beam epitaxy probed using monoenergetic positron beams

A. Uedono, S. F. Chichibu, M. Higashiwaki, T. Matsui, T. Ohdaira, and R. Suzuki

J. Appl. Phys. 97, 043514 (2005); http://dx.doi.org/10.1063/1.1845575 (5 pages) | Cited 11 times

Online Publication Date: 24 January 2005

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High-quality InN layers grown on sapphire substrates by plasma-assisted molecular-beam epitaxy were characterized using monoenergetic positron beams. The carrier concentrations of the films were controlled by Si doping (2.1×1018 to 1.4×1019 cm−3), and the highest obtained Hall mobility was 1300 cm2 V−1 s−1. The Doppler broadening spectra of the annihilation radiation and the lifetime spectra of positrons were measured as a function of the incident positron energy for undoped and Si-doped InN films. The line-shape parameter S increased with increasing carrier concentration, suggesting the introduction of vacancy-type defects by a Fermi-level effect. The major defect species were varied with carrier concentration, and its species were identified as In vacancies (VIn) or their related defects.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.J- Point defects and defect clusters
61.82.Fk Semiconductors
61.80.Fe Electron and positron radiation effects
73.61.Ey III-V semiconductors
78.66.Fd III-V semiconductors
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
78.70.Bj Positron annihilation
73.20.At Surface states, band structure, electron density of states
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