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15 Jun 2012

Volume 111, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 111, 123510 (2012); http://dx.doi.org/10.1063/1.4729803 (4 pages)

Sz-Chin Steven Lin, Bernhard R. Tittmann, and Tony Jun Huang
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back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

On the oscillator strength in dilute nitride quantum wells on GaAs

K. Ryczko, G. Sęk, J. Misiewicz, F. Langer, S. Höfling, and M. Kamp

J. Appl. Phys. 111, 123503 (2012); http://dx.doi.org/10.1063/1.4729320 (6 pages)

Online Publication Date: 18 June 2012

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We have investigated theoretically two kinds of dilute-nitride-based quantum well structures, InGaAsN/GaAs and InGaAsN/GaAsN/GaAs, both able to emit at 1.3 μm. The ground state transition energy and its oscillator strength have been probed as a function of the material composition in the single particle as well as the excitonic approximations. The modification of the bandgap energy due to nitrogen incorporation has been taken into account by using a two-level repulsion model. We have shown that in spite of a decrease of the electron-hole wave functions overlap with the mole fraction of nitrogen, the overall transition intensity of the excitonic transition can increase significantly due to the strongly composition dependent mass of the exciton. The latter makes dilute nitride quantum wells good candidates for the polaritonic physics and Bose-Einstein condensation of exciton polaritons at telecommunication wavelengths. We have also demonstrated that the exact values of the band offsets are necessary to be known as they have a critical impact on the actual transition oscillator strengths in these quantum wells.
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73.21.Fg Quantum wells
71.20.Nr Semiconductor compounds
71.35.Lk Collective effects (Bose effects, phase space filling, and excitonic phase transitions)
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Electrospinning and optical characterization of organic rubrene nanofibers

Krishna P. Dhakal, Hyunsoo Lee, Jin Woo Lee, Jinsoo Joo, Martin Guthold, and Jeongyong Kim

J. Appl. Phys. 111, 123504 (2012); http://dx.doi.org/10.1063/1.4729537 (6 pages) | Cited 3 times

Online Publication Date: 18 June 2012

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We report on the preparation of continuous organic rubrene nanofibers using the electrospinning method. We added the minimal amount of poly (ethylene oxide) in the electro-spinning solution to provide the viscosity required for electrospinning. Optical characteristics such as absorption, photoluminescence, and Raman spectra all confirmed the successful formation of rubrene nanofibers. Confocal Raman spectra obtained from single rubrene nanofibers showed co-existence of the amorphous and the crystal phase of the rubrene molecule. We also demonstrated that our rubrene nanofibers can be used as efficient optical waveguides. Our result suggests that abundant fluorescent, continuous nanofibers of small molecule materials can be successfully prepared using electrospinning.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
78.30.Jw Organic compounds, polymers
66.20.-d Viscosity of liquids; diffusive momentum transport
78.55.Kz Solid organic materials

Ideal transparent conductors for full spectrum photovoltaics

Kin Man Yu, Marie A. Mayer, Derrick T. Speaks, Hongcai He, Ruying Zhao, L. Hsu, Samuel S. Mao, E. E. Haller, and Wladek Walukiewicz

J. Appl. Phys. 111, 123505 (2012); http://dx.doi.org/10.1063/1.4729563 (5 pages) | Cited 2 times

Online Publication Date: 18 June 2012

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In current technologies, state-of-the-art transparent conducting oxides exhibit good conductivity (∼5 × 103 S/cm) and transparency up to only λ ∼ 1000 nm, restricting the use of such thin films to photovoltaics that are not utilizing the infrared part of the solar spectrum. We have found that among metal oxides, high electron mobility CdO satisfies the essential requirements for a low resistance and high infrared transmission transparent contact. With appropriate intentional doping, we have achieved ideal uncompensated CdO with extremely high conductivity (>104 S/cm) and an excellent transmission window in the range from 400 to >1500 nm, making this material an ideal TCO for photovoltaics with low band gap absorbers.
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73.50.Pz Photoconduction and photovoltaic effects
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.30.Fs III-V and II-VI semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance
61.72.uj III-V and II-VI semiconductors
78.66.Hf II-VI semiconductors

Interplay between Sb flux and growth temperature during the formation of GaSb islands on GaP

S. El Kazzi, L. Desplanque, X. Wallart, Y. Wang, and P. Ruterana

J. Appl. Phys. 111, 123506 (2012); http://dx.doi.org/10.1063/1.4729548 (5 pages) | Cited 3 times

Online Publication Date: 19 June 2012

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We investigate the influence of the Sb flux on the growth of GaSb islands on a highly mismatched (001) GaP substrate. Between low and medium Sb flux values, standard kinetics drives the GaSb island formation and their relaxation is progressively favored by 90° misfit dislocations at the GaSb/GaP interface. However, under high Sb flux, the GaSb islands are elongated in the [110] direction and their density decreases. Further experiments varying the growth temperature at fixed Sb flux confirm this finding. We relate this observation to an enhancement of Ga diffusion when the effective Sb flux on the surface is increased. This behavior is qualitatively explained by the large cohesive energy of Sb-Sb bonds present on the surface, which impede the Ga adatom incorporation.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.ag Semiconductors
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.05.Ea III-V semiconductors
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)

Optical analysis of doped ZnO thin films using nonparabolic conduction-band parameters

J. S. Kim, J.-h. Jeong, J. K. Park, Y. J. Baik, I. H. Kim, T.-Y. Seong, and W. M. Kim

J. Appl. Phys. 111, 123507 (2012); http://dx.doi.org/10.1063/1.4729571 (9 pages) | Cited 4 times

Online Publication Date: 19 June 2012

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The optical properties of impurity doped ZnO thin films were analyzed by taking into account the nonparabolicity in the conduction-band and the optically determined carrier concentration and mobility were correlated with those measured by Hall measurement. The Drude parameters obtained by applying a simple Drude model combined with the Lorentz oscillator model for the optical transmittance and reflectance spectrum were analyzed by using the carrier density dependent bare band effective mass determined by the first-order nonparabolicity approximation. The squared plasma energy multiplied by the carrier density dependent effective mass yielded fairly linear relationship with respect to the carrier concentration in wide carrier density range of 1019 − 1021 cm−3, verifying the applicability of the nonparabolicity parameter for various types of impurity doped ZnO thin films. The correlation between the optical and Hall analyses was examined by taking the ratios of optical to Hall measurements for carrier density, mobility, and resistivity by introducing a parameter, Rdl, which represents the ratio of the resistances to electron transport from the inside of the lattice and from the crystallographic defects. For both the carrier concentration and mobility, the ratios of optical to Hall measurements were shown to exhibit a monotonically decreasing function of Rdl, indicating that the parameter Rdl could be used as a yardstick in correlating the optically determined carrier density and mobility with those measured by Hall analysis.
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81.05.Dz II-VI semiconductors
78.66.Hf II-VI semiconductors
73.61.Ga II-VI semiconductors
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
71.20.Nr Semiconductor compounds
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor

Formation and evolution of oxygen-vacancy clusters in lead and tin doped silicon

C. A. Londos, D. Aliprantis, E. N. Sgourou, A. Chroneos, and P. Pochet

J. Appl. Phys. 111, 123508 (2012); http://dx.doi.org/10.1063/1.4729573 (6 pages) | Cited 5 times

Online Publication Date: 19 June 2012

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Infrared spectroscopy (IR) measurements were used to investigate the effect of lead (Pb), tin (Sn), and (Pb, Sn) codoping on electron radiation-induced defects in silicon (Si). The study was mainly focused on oxygen-vacancy (VOn) clusters and in particular their formation and evolution upon annealing. It was determined that Pb causes a larger reduction in the production of the VO defect than Sn. In (Pb, Sn) co-doped Si isochronal anneals revealed that the evolution of VO increases substantially at ∼170 °C. This is attributed to the release of V from the SnV pair. Interestingly, in the corresponding evolution curves of VO in the Sn- and the Pb-doped samples, this inverse annealing stage is also present for the former while it is not present for the latter. This is attributed to the formation of PbV pairs that do not dissociate below 280 °C. The partial capture of V by Sn in co-doped samples is rationalized through the higher compressive local strain around Pb atoms that leads to a retardation of vacancy diffusion. The conversion of VO to the VO2 defect is substantially reduced in the Pb-doped sample. The evolution curves of VO and VO2 clusters in the isovalent doped Si samples hint the production of VO2 from other mechanisms (i.e., besides VO + Oi → VO2). For larger VOn clusters (n = 3,4), the signals are very weak in the Pb-doped sample, whereas for n ≥ 5, they are not present in the spectra. Conversely, bands related with the VO5 and VOnCs defects are present in the spectra of the Sn-doped and (Pb, Sn) codoped Si.
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61.72.jd Vacancies
61.72.Cc Kinetics of defect formation and annealing
78.30.Am Elemental semiconductors and insulators

Semiconductor-metal transition of titanium sesquioxide nanopowder

Weiwei Yan, Ming Fang, Mao Liu, Shenghong Kang, Ruining Wang, Lide Zhang, and Ling Liu

J. Appl. Phys. 111, 123509 (2012); http://dx.doi.org/10.1063/1.4729801 (6 pages)

Online Publication Date: 19 June 2012

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Titanium sesquioxide (Ti2O3) nanopowders have been successfully synthesized using TiCl4 assisted hydrogen reduction method from P25 (TiO2) powders at 970 °C. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy reveal its high purity. The temperature dependent XRD, resistance, and infrared absorption investigations exhibit Z-shaped curves (cell parameter a, resistance, and infrared transmittance) indicating the semiconductor-metal transition (SMT) in the range of 135–220 °C. With prolonging the annealing time, the starting temperature of SMT is found to move toward the low-temperature side, and this was also discussed. The c/a ratio alteration is considered to result in the modulation of property. Besides, the results are verified comparing with the theoretical calculation of band structure.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
71.30.+h Metal-insulator transitions and other electronic transitions
72.60.+g Mixed conductivity and conductivity transitions
78.30.-j Infrared and Raman spectra
79.60.-i Photoemission and photoelectron spectra

Design of acoustic beam aperture modifier using gradient-index phononic crystals

Sz-Chin Steven Lin, Bernhard R. Tittmann, and Tony Jun Huang

J. Appl. Phys. 111, 123510 (2012); http://dx.doi.org/10.1063/1.4729803 (4 pages) | Cited 1 time

Online Publication Date: 19 June 2012

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This article reports the design concept of a novel acoustic beam aperture modifier using butt-jointed gradient-index phononic crystals (GRIN PCs) consisting of steel cylinders embedded in a homogeneous epoxy background. By gradually tuning the period of a GRIN PC, the propagating direction of acoustic waves can be continuously bent to follow a sinusoidal trajectory in the structure. The aperture of an acoustic beam can therefore be shrunk or expanded through change of the gradient refractive index profiles of the butt-jointed GRIN PCs. Our computational results elucidate the effectiveness of the proposed acoustic beam aperture modifier. Such an acoustic device can be fabricated through a simple process and will be valuable in applications, such as biomedical imaging and surgery, nondestructive evaluation, communication, and acoustic absorbers.
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43.38.-p Transduction; acoustical devices for the generation and reproduction of sound
43.38.Zp Acoustooptic and photoacoustic transducers
43.40.-r Structural acoustics and vibration

Mesoscale simulation of shock wave propagation in discrete Ni/Al powder mixtures

Ryan A. Austin, David L. McDowell, and David J. Benson

J. Appl. Phys. 111, 123511 (2012); http://dx.doi.org/10.1063/1.4729304 (9 pages)

Online Publication Date: 20 June 2012

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A numerical model is developed to simulate shock wave propagation in discrete Ni/Al powder mixtures. The model is used to investigate the particle-level deformation, heating, and mixing of two distinct Ni/Al powders, as mixing intensity dictates whether or not shock ignition is achieved in these reactive material systems. The main innovations of this work are (1) use of a rate-dependent, dislocation-based model of particle flow stress in the shock simulations and (2) quantitative analysis of the Ni/Al interfaces that are formed during wave propagation. An experimental powder, which is composed of micron-scale spherical Ni and Al particles, is simulated to validate the numerical model. An additional powder, composed of smaller particles, is simulated to investigate the effects of particle size on constituent deformation and mixing under shock wave loading. The simulations indicate that a reduction in particle size leads to increased Ni/Al interface temperature and dislocation density, as well as increased stress-sensitivity of Ni/Al interface formation. Finally, it is shown that accounting for the rate-dependence of particle flow stress likely yields improved accuracy in predicted flow morphologies, especially at intermediate stress wave amplitudes.
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62.50.Ef Shock wave effects in solids and liquids
64.75.Ef Mixing

Conversion of basal plane dislocations to threading edge dislocations in 4H-SiC epilayers by high temperature annealing

Xuan Zhang and Hidekazu Tsuchida

J. Appl. Phys. 111, 123512 (2012); http://dx.doi.org/10.1063/1.4729326 (8 pages)

Online Publication Date: 20 June 2012

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Conversion of basal plane dislocations (BPDs) to threading edge dislocations (TEDs) is found in 4H-SiC epilayers after being annealed simply at high temperatures. Grazing incidence reflection synchrotron x-ray topography for the dislocations in the epilayers before and after annealing confirmed that some of the BPDs in the epilayers had converted to TEDs from the epilayer surface by the annealing. Observations on the dislocation behavior during annealing are explained in detail, and the mechanism of BPD conversion is discussed. It is argued that the conversion proceeds through the cross slip of constricted BPD segments towards the surface on the prismatic plane driven by the image force as well as TED glide driven by the line tension. Certain kinetic processes during annealing may facilitate the formation of constriction.
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81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Precipitation of Pt nanocrystallites from BaO-TiO2-GeO2 remelted glass

Hirokazu Masai, Katsumi Hamaguchi, Yoshiyuki Suzuki, Kenichiro Iwasaki, Rie Ihara, Yoshihiro Takahashi, and Takumi Fujiwara

J. Appl. Phys. 111, 123513 (2012); http://dx.doi.org/10.1063/1.4729488 (5 pages) | Cited 1 time

Online Publication Date: 20 June 2012

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We have examined the correlation between the thermal and optical properties of 30BaO-15TiO2-55GeO2 glass and its mode of preparation, i.e., the different temperatures at which the glass melts were held in a molten state (melt temperature). The changes in the absorption coefficients and the refractive indices, which increased with the increasing melt temperature, were reversible, and temperature-dependent structures were formed in the glass. In the remelted glass with short remelting duration, a black coloration and the precipitation of Pt nanocrystallites were observed. The precipitated Pt nanocrystallites acted as nucleation sites to induce crystallization by heat treatment at 10 °C below the glass transition temperature.
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81.05.Kf Glasses (including metallic glasses)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
64.70.ph Nonmetallic glasses (silicates, oxides, selenides, etc.)
64.70.dg Crystallization of specific substances
78.40.Pg Disordered solids
81.40.Gh Other heat and thermomechanical treatments

Compressibility and strength of nanocrystalline tungsten boride under compression to 60 GPa

Haini Dong, Susannah M. Dorfman, Ying Chen, Haikuo Wang, Jianghua Wang, Jiaqian Qin, Duanwei He, and Thomas S. Duffy

J. Appl. Phys. 111, 123514 (2012); http://dx.doi.org/10.1063/1.4728208 (5 pages)

Online Publication Date: 21 June 2012

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The compression behavior and stress state of nanocrystalline tungsten boride (WB) were investigated using radial x-ray diffraction (RXRD) in a diamond-anvil cell under non-hydrostatic compression up to 60.4 GPa. The compression properties and stress state are analyzed using lattice strain theory. Experiments were conducted at beamline X17C of the National Synchrotron Light Source. The radial x-ray diffraction data yield a bulk modulus that is qualitatively consistent with density functional theory calculations and demonstrate that WB is a highly incompressible material. A maximum differential stress, t, of about 14 GPa can be supported by nanocrystalline WB at the highest pressure. This corresponds to about 5% of the shear modulus, G, which is smaller than the values of t/G (∼8%–10%) observed for BC2N, B6O, TiB2, and γ-Si3N4 at high pressures. Thus, while WB is highly incompressible, its strength is relatively low at high pressures compared to other hard ceramics.
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62.50.Ef Shock wave effects in solids and liquids
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli

Atomic structure of Ca40+XMg25Cu35−X metallic glasses

O. N. Senkov, Y. Q. Cheng, D. B. Miracle, E. R. Barney, A. C. Hannon, and C. F. Woodward

J. Appl. Phys. 111, 123515 (2012); http://dx.doi.org/10.1063/1.4729450 (19 pages) | Cited 4 times

Online Publication Date: 21 June 2012

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The atomic structures of four Ca40+XMg25Cu35−X (X = 0, 5, 10, and 20 at. %) ternary metallic glasses have been determined using a synergistic combination of neutron diffraction, ab initio molecular dynamics (MD) simulation, and constrained reverse Monte Carlo modeling. It is described as close-packing of efficiently packed Cu-centered clusters that have Ca, Mg, and Cu atoms in the first coordination shell. The close-packed arrangement of the clusters provides a characteristic medium range order in these alloys. An average coordination number (CN) of 10 (with about 5–7 Ca, 2–3 Mg, and 1–2 Cu atoms) is most common for the Cu-centered clusters. The average coordination numbers around Mg and Ca are 12–13 (∼6–8 Ca, 3 Mg, and 1–4 Cu) and 13–15 (7–9 Ca, 3–4 Mg, and 2–5 Cu), respectively, and they are composition dependent. Strong interaction of Cu with Mg and Ca results in pair bond shortening. Icosahedral short range order does not dominate in these amorphous alloys, although polytetrahedral packing and five-fold bond configurations resulting in pentagonal bi-pyramids have been found to be the most common nearest atom configurations.
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61.43.Fs Glasses
61.43.Bn Structural modeling: serial-addition models, computer simulation

Bandgaps in phononic strip waveguides

N. Aravantinos-Zafiris and M. M. Sigalas

J. Appl. Phys. 111, 123516 (2012); http://dx.doi.org/10.1063/1.4729568 (7 pages)

Online Publication Date: 21 June 2012

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In this work, we numerically examine the usage of a sonic strip waveguide having a phononic crystal. This structure could have important applications in acoustic circuits. The well known finite difference time domain (FDTD) method was used for the simulations. Several different matrix materials were considered for the waveguide structure. All three polarizations of the input pulse where examined and the effects of all the geometric parameters of the waveguide structure have been considered in this study.
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43.58.-e Acoustical measurements and instrumentation
02.70.Bf Finite-difference methods

Oxidation kinetics of nanoscale copper films studied by terahertz transmission spectroscopy

Gopika K. P. Ramanandan, Gopakumar Ramakrishnan, and Paul C. M. Planken

J. Appl. Phys. 111, 123517 (2012); http://dx.doi.org/10.1063/1.4729808 (6 pages)

Online Publication Date: 22 June 2012

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Terahertz (THz) transmission spectroscopy is used to measure the oxidation kinetics of copper thin films evaporated on silicon substrates. The transmission of broadband THz pulses from 1 to 7 THz through the copper film is measured while it gets oxidized at an elevated temperature in ambient air. The change in the transmitted THz electric field is correlated with the growth of the cuprous oxide layer and the decrease in thickness of the copper layer. Oxidation curves were obtained for heating temperatures of 120–150 °C and were found to follow a parabolic rate law. Using the Arrhenius equation, we calculate an activation energy for diffusion of 0.55 eV. By measuring the THz transmission through unoxidized copper layers of several thicknesses, we also measured the optical properties of thin copper films around the percolation threshold thickness of 7 nm. Around the percolation transition, the optical properties of freshly deposited copper thin films are very different from that of copper layers of the same thickness remaining after partial oxidation of thick copper films.
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81.65.Mq Oxidation
68.35.Fx Diffusion; interface formation
78.66.Bz Metals and metallic alloys
78.70.Gq Microwave and radio-frequency interactions
81.40.Gh Other heat and thermomechanical treatments

Origin of temperature plateaus in laser-heated diamond anvil cell experiments

Zachary M. Geballe and Raymond Jeanloz

J. Appl. Phys. 111, 123518 (2012); http://dx.doi.org/10.1063/1.4729905 (15 pages) | Cited 2 times

Online Publication Date: 25 June 2012

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Many high-pressure high-temperature studies using laser-heated diamond cells have documented plateaus in the increase of temperature with increasing laser power or with time. By modeling heat transfer in typical laser-heated diamond anvil cell experiments, we demonstrate that latent heat due to melting or other phase transformation is unlikely to be the source of observed plateaus in any previously published studies, regardless of whether pulsed or continuous lasers were used. Rather, large increases (∼10-fold) in thermal conductivity can explain some of the plateaus, and modest increases in reflectivity (tens of percent) can explain any or all of them. Modeling also shows that the sub-microsecond timescale of heating employed in recent pulsed heating experiments is fast enough compared to heat transport into and through typical insulations, but too slow compared to heat transport into metallic laser absorbers themselves to allow the detection of a large plateau due to latent heat of fusion. Four new designs are suggested for future experiments that could use the simple observation of a latent heat-induced plateau to provide reliable high-pressure melting data.
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62.50.-p High-pressure effects in solids and liquids
64.70.dj Melting of specific substances
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
42.62.-b Laser applications
81.40.Gh Other heat and thermomechanical treatments

Properties inheritance in metallic glasses

Wei Hua Wang (汪卫华)

J. Appl. Phys. 111, 123519 (2012); http://dx.doi.org/10.1063/1.4730441 (8 pages) | Cited 1 time

Online Publication Date: 25 June 2012

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It is widely accepted that the structural features of a metallic glass inherits from its liquid state. The survey of the elastic, plastic, mechanical and physical properties, and glass transition of various metallic glasses indicates that metallic glasses can inherit their properties from their solvent components (or base components). It is found that the elastic properties are mainly determined by the weakest solvent-solvent bonds in metallic glasses, and the plastic events and glass transition prefer to be initiated in the regions with high density of weak solvent-solvent bonds and propagate toward the regions with less density of solvent-solvent bonds in metallic glasses. Some physical properties of the metallic glasses are found to be determined by the electronic structure of the solvent components. The properties inheritance allows new understanding of the glassy structure as well as the properties and structure relationship in metallic glasses, and also indicates the hierarchical atomic bands and inhomogeneous microstructure in metallic glasses which is helpful for understanding the mechanisms of plastic and elastic deformations and glass transition in metallic glasses.
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61.43.Fs Glasses
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
62.20.de Elastic moduli
62.20.fq Plasticity and superplasticity
64.70.pe Metallic glasses

Charged exciton creation with two-color optical excitation method and analysis of initialization process of electron spin qubit in quantum dots

Hideki Gotoh, Haruki Sanada, Hiroshi Yamaguchi, and Tetsuomi Sogawa

J. Appl. Phys. 111, 123520 (2012); http://dx.doi.org/10.1063/1.4730602 (6 pages)

Online Publication Date: 25 June 2012

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An optical pumping method for creating charged excitons in semiconductor quantum dots is studied using a micro-photoluminescence (PL) technique with a two-color excitation method. This method employs two laser sources whose energies are in the resonant and non-resonant barrier excitation conditions, respectively; one laser creates excitons, the other creates electrons. The two-color excitation exploits an asymmetric energy band structure for conduction and valence bands, which induces negatively charged excitons in single quantum dots. The PL spectra of a single quantum dot clearly vary from exciton-originated PL to charged exciton-originated PL as the excitation conditions are changed. This excitation method is applied to the initialization process of an electron spin qubit without a magnetic field. Experimental results are analyzed theoretically based on the use of a density matrix method for the initialization. The analysis reveals that the electron spin in quantum dots is sufficiently polarized for subsequent quantum computation processes. These results provide a simple and effective way of implementing quantum computing with spin qubits.
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71.35.Pq Charged excitons (trions)
73.21.La Quantum dots
78.55.Cr III-V semiconductors
78.67.Hc Quantum dots
03.67.Lx Quantum computation architectures and implementations

Brillouin scattering from porous silicon-based optical Bragg mirrors

L. C. Parsons and G. T. Andrews

J. Appl. Phys. 111, 123521 (2012); http://dx.doi.org/10.1063/1.4730617 (9 pages) | Cited 1 time

Online Publication Date: 25 June 2012

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Brillouin light scattering experiments were done on porous silicon-based optical Bragg mirrors with modulation wavelengths of ∼100 nm. By using a combination of pseudo-reflection and backscattering geometries, phonon dispersion curves along the superlattice modulation axis were mapped. Excellent agreement is obtained with the bulk acoustic mode band structure calculated using a one-dimensional elastic continuum model. In addition to zone-folding of the bulk longitudinal mode dispersion curve, the samples are marked by a surface-localized acoustic mode at the superlattice-air interface. The frequency of this mode lies near the upper edge of a phononic band gap centered at ∼16 GHz. These results, along with optical reflectance data showing visible-range photonic band gaps, reveal that these samples are one-dimensional hypersonic phononic-photonic crystals.
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42.79.Bh Lenses, prisms and mirrors
63.22.Np Layered systems
42.65.Es Stimulated Brillouin and Rayleigh scattering
42.70.Qs Photonic bandgap materials

Size dependent carrier thermal escape and transfer in bimodally distributed self assembled InAs/GaAs quantum dots

G. Muñoz-Matutano, I. Suárez, J. Canet-Ferrer, B. Alén, D. Rivas, L. Seravalli, G. Trevisi, P. Frigeri, and J. Martínez-Pastor

J. Appl. Phys. 111, 123522 (2012); http://dx.doi.org/10.1063/1.4729315 (8 pages) | Cited 1 time

Online Publication Date: 25 June 2012

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We have investigated the temperature dependent recombination dynamics in two bimodally distributed InAs self assembled quantum dots samples. A rate equations model has been implemented to investigate the thermally activated carrier escape mechanism which changes from exciton-like to uncorrelated electron and hole pairs as the quantum dot size varies. For the smaller dots, we find a hot exciton thermal escape process. We evaluated the thermal transfer process between quantum dots by the quantum dot density and carrier escape properties of both samples.
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73.21.La Quantum dots
73.22.Lp Collective excitations
73.63.Kv Quantum dots
81.16.Dn Self-assembly
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Valence band tail states in disordered superlattices embedded in wide parabolic AlGaAs well

L. Fernandes dos Santos, Yu. A. Pusep, A. K. Bakarov, and A. I. Toropov

J. Appl. Phys. 111, 123523 (2012); http://dx.doi.org/10.1063/1.4730769 (6 pages)

Online Publication Date: 26 June 2012

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Optical properties of intentionally disordered multiple quantum well (QW) system embedded in a wide AlGaAs parabolic well were investigated by photoluminescence (PL) measurements as functions of the laser excitation power and the temperature. The characterization of the carriers localized in the individual wells was allowed due to the artificial disorder that caused spectral separation of the photoluminescence lines emitted by different wells. We observed that the photoluminescence peak intensity from each quantum well shifted to high energy as the excitation power was increased. This blue-shift is associated with the filling of localized states in the valence band tail. We also found that the dependence of the peak intensity on the temperature is very sensitive to the excitation power. The temperature dependence of the photoluminescence peak energy from each QW was well fitted using a model that takes into account the thermal redistribution of the localized carriers. Our results demonstrate that the band tails in the studied structures are caused by alloy potential fluctuations and the band tail states dominate the emission from the peripheral wells.
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73.21.Cd Superlattices
73.21.Fg Quantum wells
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

The effects of group-I elements co-doping with Mn in ZnO dilute magnetic semiconductor

Liqiang Zhang, Yinzhu Zhang, Zhizhen Ye, Jianguo Lu, Bin Lu, and Bo He

J. Appl. Phys. 111, 123524 (2012); http://dx.doi.org/10.1063/1.4729530 (4 pages) | Cited 1 time

Online Publication Date: 26 June 2012

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Mn-Li codoped ZnO (Zn(Mn,Li)O), Mn-Na codoped ZnO (Zn(Mn,Na)O), and Mn-K codoped ZnO (Zn(Mn,K)O) thin films were deposited on quartz substrates by pulsed laser deposition. The doping effects of group-I elements (e.g., Li, Na, and K) on the structural, magnetic, and optical properties of the Mn doped ZnO (ZnMnO) films were discussed. X-ray diffraction and K-edge x-ray absorption near-edge structure measurements revealed that all the films showed a hexagonal wurtzite ZnO structure, and no other clusters, precipitates, or second phases were detected. Zn(Mn,Na)O and Zn(Mn,Li)O films showed a weak p-type conductivity, while the Zn(Mn,K)O film appeared a highly resistivity. The saturation magnetization of Zn(Mn,Na)O and Zn(Mn,Li)O films was 1.2 and 0.18 μB/Mn, respectively. The hole-related defects, induced by doping with a low content of Li or Na, contributed to the room temperature ferromagnetism in the ZnMnO system.
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61.72.uj III-V and II-VI semiconductors
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.15.Fg Pulsed laser ablation deposition
68.55.ag Semiconductors
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Pp Magnetic semiconductors

Optical and magnetic properties of Cr-doped ZnS nanocrystallites

Xiaoling Zeng, Jiye Zhang, and Feng Huang

J. Appl. Phys. 111, 123525 (2012); http://dx.doi.org/10.1063/1.4729877 (7 pages)

Online Publication Date: 27 June 2012

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Cr-doped ZnS (molar Cr:(Cr + Zn) between 0.51% and 19.69%) nanocrystallites have been prepared through co-precipitation method. The x-ray diffraction, transmission electron microscopy, and selected area electron diffraction results show that all the samples are in sphalerite structure with average particle size about 3 nm. No impurity phase relating to Cr element is found in all the samples. X-ray photoelectron spectroscopy spectra reveal that Cr was incorporated into ZnS lattice as Cr3+. It can be seen in the UV-visible absorption spectra that, besides the intrinsic band-gap absorption of ZnS below 370 nm, there are another two absorption bands (at 425 nm and 595 nm, respectively) in the visible light range, which are the characteristic bands of Cr3+. Photoluminescence spectroscopy was also used to characterize corresponding luminescence properties of the nanocrystallites. The band-edge emission in photoluminescence spectroscopy exhibits blue-shift as the concentration of Cr increases, and several emission peaks concerned with surface states and zinc vacancies were found in the wavelength range of 400 nm ∼ 500 nm. It is confirmed that Cr doping will bring about a considerable amount of zinc vacancies. Magnetic measurements indicated that all the samples are paramagnetic and the calculated effective magnetic moments  μeff of Cr3+ were close to the theoretical value of 3.87 μB. Accordingly, it seems that zinc vacancies give no contribution to the overall magnetic response of the samples.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
75.30.Cr Saturation moments and magnetic susceptibilities
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.40.Ha Other nonmetallic inorganics
78.55.Et II-VI semiconductors
75.50.Tt Fine-particle systems; nanocrystalline materials

Focus shift photothermal method for thermal diffusivity mapping

N. Mingolo and O. E. Martínez

J. Appl. Phys. 111, 123526 (2012); http://dx.doi.org/10.1063/1.4730636 (7 pages) | Cited 1 time

Online Publication Date: 27 June 2012

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In this work, we present a photothermal technique that measures at microscopic spatial scales the curvature of the surface of the sample due to the thermal expansion produced by a focused modulated laser beam. A modulated pump beam heats the sample surface and a probe beam delivered through the same optical fiber and slightly defocused probes the curvature, by analyzing the power collected by the same optical fiber. The collected power depends on the defocusing produced by the curvature of the surface induced by the thermal expansion. The phase delay is directly related to the thermal diffusivity of the sample at the impinging location. The scheme is based on the use of photonic technology borrowed from the optical communications field. In this way, the setup results extremely robust, both lasers (pump and probe) are inherently collinear, as they emerge from the same single mode fiber, and the detection is made through the same optical fiber.
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66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
42.55.Wd Fiber lasers
42.79.Sz Optical communication systems, multiplexers, and demultiplexers
65.40.De Thermal expansion; thermomechanical effects
66.30.Xj Thermal diffusivity
79.20.Ds Laser-beam impact phenomena

Optical characterization of ZnO nanopillars on Si and macroporous periodic Si structure

M. V. Castro Meira, A. Ferreira da Silva, G. Baldissera, C. Persson, J. A. Freitas, Jr., N. Gutman, A. Sa’ar, O. Nur, and M. Willander

J. Appl. Phys. 111, 123527 (2012); http://dx.doi.org/10.1063/1.4729260 (5 pages)

Online Publication Date: 28 June 2012

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ZnO nanopillars were successfully grown using both the vapor-liquid-solid and the aqueous chemical growth methods on different substrates, such as quartz, n-, and p-type non-porous Si wafer (flat) and microporous periodic Si structure (MPSiS). Scanning electron microscopy was employed to compare sample morphologies. The absorption was calculated employing the GW0 method, based on the local density approximation, and with the projector augmented wave approach. Experiment and theory show a reasonable agreement when the shape of the optical absorption is considered. The measured absorption of ZnO nanopillars, on different substrates, is lower than that observed for ZnO films on quartz substrate, in the energy gap spectral range. A strong effect of MPSiS substrates on ZnO nanopillar properties is observed. The photoluminescence technique was also employed as an optical characterization.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
81.16.-c Methods of micro- and nanofabrication and processing
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Nr Semiconductor compounds
78.30.Fs III-V and II-VI semiconductors
78.40.Fy Semiconductors
78.55.Et II-VI semiconductors
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