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15 Jan 2006

Volume 99, Issue 2, Articles (02xxxx)

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Emission characteristics of near-stoichiometric Er/Yb-codoped LiNbO3 crystals

De-Long Zhang and E. Y. B. Pun

J. Appl. Phys. 99, 023101 (2006); http://dx.doi.org/10.1063/1.2159551 (11 pages) | Cited 6 times

Online Publication Date: 17 January 2006

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Polarized visible, near-infrared, and 980-nm up-conversion emission characteristics in near-stoichiometric Z-cut Er(0.6 mol %)/Yb(0.3 mol %)-codoped LiNbO3 crystals, prepared by vapor-transport-equilibration (VTE) technique, were investigated and discussed in comparison with corresponding as-grown crystal, thermally calcined ErNbO4 powder, as well as Z-cut only Er(2.0 mol %)-doped LiNbO3 crystal that was subjected to the same VTE procedure as one of the Er/Yb-codoped crystals. The VTE-induced crystalline phase transformation is discussed in combination with optical absorption results reported previously. The percentage of Er3+ ions present in the crystals as the ErNbO4 phase is evaluated based upon the optical absorption and the emission data at 1.5 μm. An empirical expression for evaluating the Li composition in a near-stoichiometric Er(/Yb)-doped LiNbO3 crystal from the narrowing percentage of the σ- or α-polarized 1530 nm emission peak is established based upon the emission spectra of singly Er-doped VTE LiNbO3 crystals. From the measured absorption band areas and the Judd-Ofelt theory, the influence of the VTE treatment on the Er3+ spectroscopic property was studied. The experimentally observed VTE effect on the emission intensity is conducted with the theoretical results of the transition probability. Both the theoretical and experimental results reveal that the VTE treatment results in the lengthening of the 1.5 μm lifetime as much as 15%. A mechanism that the VTE treatment induces the crystalline phase transformation and the reduction of both OH− and intrinsic defect contents is proposed to explain the VTE effect on the lifetime.
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78.55.Hx Other solid inorganic materials

Real-time tunability of chip-based light source enabled by microfluidic mixing

Brian Bilenberg, Torben Rasmussen, Søren Balslev, and Anders Kristensen

J. Appl. Phys. 99, 023102 (2006); http://dx.doi.org/10.1063/1.2163011 (5 pages) | Cited 11 times

Online Publication Date: 23 January 2006

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We demonstrate real-time tunability of a chip-based liquid light source enabled by microfluidic mixing. The mixer and light source are fabricated in SU-8 which is suitable for integration in SU-8-based laboratory-on-a-chip microsystems. The tunability of the light source is achieved by changing the concentration of rhodamine 6G dye inside two integrated vertical resonators, since both the refractive index and the gain profile are influenced by the dye concentration. The effect on the refractive index and the gain profile of rhodamine 6G in ethanol is investigated and the continuous tuning of the laser output wavelength is demonstrated using an ethanolic rhodamine 6G solution of 2×10−2 mol/l mixed with pure ethanol. This yields rhodamine 6G concentrations from 5×10−3 to 1.5×10−2 mol/l inside the laser resonators and a wavelength change of 10 nm with a response time of 110 s.
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42.55.Mv Dye lasers
42.60.Fc Modulation, tuning, and mode locking
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.By Design of specific laser systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Probing photonic crystals on silicon-on-insulator with Ge/Si self-assembled islands as an internal source

X. Li, P. Boucaud, X. Checoury, O. Kermarrec, Y. Campidelli, and D. Bensahel

J. Appl. Phys. 99, 023103 (2006); http://dx.doi.org/10.1063/1.2163007 (5 pages) | Cited 26 times

Online Publication Date: 24 January 2006

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We report the study of two-dimensional photonic crystals fabricated on silicon-on-insulator substrates. Ge/Si self-assembled islands are embedded as an active internal optical source inside the photonic crystals. We present a detailed analysis of photonic crystal microcavities and waveguides using the room-temperature Ge/Si island photoluminescence. The tunability of the microcavity resonant emission is demonstrated between 1.2 and 1.5 μm. We show that the microcavity photoluminescence is weakly dependent on the temperature. The polarized transmission properties of W1 single-line defect waveguides are investigated using the photoluminescence as an internal source. The transmission spectra are correlated to those given by two-dimensional finite-difference time-domain calculations.
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42.70.Qs Photonic bandgap materials
42.82.Et Waveguides, couplers, and arrays
78.55.Hx Other solid inorganic materials

Transient radiation responses of silica-based optical fibers: Influence of modified chemical-vapor deposition process parameters

S. Girard, Y. Ouerdane, A. Boukenter, and J.-P. Meunier

J. Appl. Phys. 99, 023104 (2006); http://dx.doi.org/10.1063/1.2161826 (5 pages) | Cited 9 times

Online Publication Date: 26 January 2006

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We characterized the behaviors of eight prototype single-mode optical fibers, made by the modified chemical-vapor deposition process, under pulsed x-ray ( ∼ 1 MeV) irradiation. For this purpose, we measured the time-dependent changes (10−6–102 s) in the radiation-induced attenuation at 1.55 and 1.31 μm after exposure to an x-ray pulse. By using a dedicated set of prototype germanosilicate fibers with carefully designed process parameters, we show the predominant impact on their vulnerability of the two codopants (germanium and phosphorus) incorporated in their claddings ( ∼ 0.3 Wt %). Compared to these influences on the radiation-induced loss levels and recovery kinetics, the impacts of the preform deposition temperature and of the fiber drawing tension on the fiber radiation sensitivity are less important. However, our results show that lowering the standard preform deposition temperature from 2000 to 1600 °C and the drawing tension from 140 to 20 g slightly decreases the induced losses at both wavelengths. We propose some hypotheses on the radiation-induced defects and physical mechanisms at the origin of these influences.
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42.81.Cn Fiber testing and measurement of fiber parameters
42.81.Bm Fabrication, cladding, and splicing
42.81.Dp Propagation, scattering, and losses; solitons
61.80.Cb X-ray effects

Sum and difference frequency generation as diagnostics for leaky eigenmodes in two-dimensional photonic crystal waveguides

A. D. Bristow, J. P. Mondia, and H. M. van Driel

J. Appl. Phys. 99, 023105 (2006); http://dx.doi.org/10.1063/1.2161415 (6 pages) | Cited 3 times

Online Publication Date: 31 January 2006

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We experimentally demonstrate how sum frequency generation (SFG) and difference frequency generation (DFG) of 150 fs pulses can be enhanced and serve as diagnostics for leaky eigenmodes in a two-dimensional GaAs photonic crystal waveguide. SFG at 795 nm is obtained in reflection from s-polarized 1900 nm and p-polarized 1360 nm pulses, with both input beams coupled to leaky eigenmodes; the SFG is enhanced by >350× compared to that from an untextured GaAs surface. We are able to detect Drude induced subnanometer blueshifts of the SFG, corresponding to refractive index changes of ⩽ 10−3. DFG of 1360 nm light obtained in reflection from s-polarized 1900 nm and p-polarized 793 nm pulses displays an enhancement of >500× via three different leaky eigenmodes. As the 793 nm beam polarization is varied from p polarized to right and left circularly polarized, the DFG remains essentially linearly polarized but with a reduced, albeit different, intensity for right and left circularly polarized 795 nm pulses. Futhermore, the plane of polarization also rotates by different amounts for the left and right circularly polarized light, demonstrating interference of the components generated by s- and p-polarized 793 nm pulses. Overall, our results demonstrate how enhanced DFG and SFG from leaky eigenmodes can be used to characterize their properties more precisely than linear optical techniques.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.70.Qs Photonic bandgap materials
42.79.Gn Optical waveguides and couplers
42.87.-d Optical testing techniques
42.82.Et Waveguides, couplers, and arrays
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
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Backward branch of surface dust cyclotron wave in a semi-bounded magnetized dusty plasma

Won-Seok Chang, Dong-Man Chang, Iyoung Michelle Jung, and Young-Dae Jung

J. Appl. Phys. 99, 023301 (2006); http://dx.doi.org/10.1063/1.2165407 (5 pages) | Cited 3 times

Online Publication Date: 30 January 2006

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The surface dust cyclotron waves are investigated in semi-bounded magnetized dusty plasmas. The dispersion relation is obtained by the plasma dielectric function with the specular reflection boundary condition. The result shows the existence of the low-frequency backward wave branch in the surface dust cyclotron wave. The frequency of the wave is found to be decreased with the dust Debye wave number. It is also found that there is a minimum position of the scaled group velocity in the small-wave-number region. It is interesting to note that the group velocity increases below the minimum position and, however, decreases above the minimum position with increasing the dust Debye wave number.
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Wavelength dependence of combined local and carrier transport optical nonlinearities in a hetero n‐i‐p‐i structure

Alan R. Kost, Michael H. Jupina, Thomas C. Hasenberg, and Elsa M. Garmire

J. Appl. Phys. 99, 023501 (2006); http://dx.doi.org/10.1063/1.2161410 (4 pages)

Online Publication Date: 17 January 2006

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The combination of carrier transport-type optical nonlinearity and absorption saturation is examined for a hetero n‐i‐p‐i sample at wavelengths between 966 and 985 nm and for intensities up to 1000 W/cm2. Transmission changes are found to depend on the interplay between the shift, narrowing, and increasing peak height of an excitonic absorption resonance. Transmission changes previously attributed to absorption saturation are found to be due to carrier transport nonlinearity. Absorption saturation contributed to transmission change for intensities greater than 100 W/cm2, a larger value than previously reported; however saturation intensity still appears to have been reduced by charge separation and a correspondingly slower electron-hole relaxation rate.
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78.67.De Quantum wells
78.30.Fs III-V and II-VI semiconductors
73.21.Fg Quantum wells
42.65.-k Nonlinear optics

Shock propagation through alumina observed at the mesoscale

N. K. Bourne

J. Appl. Phys. 99, 023502 (2006); http://dx.doi.org/10.1063/1.2161423 (7 pages) | Cited 2 times

Online Publication Date: 17 January 2006

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The shock response of 88% and 99.9% pure aluminas, chosen since they had been tested previously, was simulated at the mesoscale. Microstructures were investigated using electron microscopy and then digitized for use in simulation. These microstructural units were stacked to recover larger-scale structures generated randomly. These extended the length ranges in the simulations from the micrometer to the continuum scales. Modeling emphasized the inhomogeneity of the flow at this length scale and phenomena such as precursor decay were accentuated in the material with the greater impurities. The bulk composite behavior could be recovered from the properties of the individual phases by adopting simple expressions for the shock parameters. These behaviors include the profile of the stress histories and quantitative prediction of the Hugoniot elastic limits recovered using information derived purely on constituents.
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62.50.-p High-pressure effects in solids and liquids

Strain and its effect on optical properties of Al-N codoped ZnO films

H. P. He, F. Zhuge, Z. Z. Ye, L. P. Zhu, F. Z. Wang, B. H. Zhao, and J. Y. Huang

J. Appl. Phys. 99, 023503 (2006); http://dx.doi.org/10.1063/1.2161419 (5 pages) | Cited 34 times

Online Publication Date: 17 January 2006

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The dependence of lattice strain in Al-N codoped p-type ZnO films on Al content and growth temperature was investigated. With increasing Al content, the compressive strain in the film first increases and then decreases. We suggest that the strain decrease is due to the occupation of more substitutional sites by Al at relatively high Al content, which partially compensates the compressive strain. Reversion of conduction type at high Al content and high temperature was also observed. By studying the strain and electrical properties of the codoped films, we conclude that ZnO film should be grown at intermediate temperatures and with low Al content to achieve both good p-type conduction and reasonable crystal quality. The compressive strain results in increase of the optical band gap, and a linear relationship between them was obtained.
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71.55.Gs II-VI semiconductors
61.72.uj III-V and II-VI semiconductors
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Temperature dependence of mobility and carrier density in InN films

J. S. Thakur, R. Naik, V. M. Naik, D. Haddad, G. W. Auner, H. Lu, and W. J. Schaff

J. Appl. Phys. 99, 023504 (2006); http://dx.doi.org/10.1063/1.2158133 (5 pages) | Cited 20 times

Online Publication Date: 18 January 2006

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We investigate the temperature dependence of Hall mobility μ and carrier density Ne for thin InN films grown by molecular-beam epitaxy and plasma source molecular-beam epitaxy over three orders-of-magnitude difference in their carrier density: for the low-density film Ne = 5.8×1017/cm3 and for the high-density film Ne = 3.2×1020/cm3. In both the films, for temperature up to 300 K, a large temperature-independent concentration of carriers is observed. For higher temperatures, however, carrier density increases with temperature. The characteristic behavior of the mobility for the low-density film is different from that of the high-density film, particularly for temperatures less than 300 K. The low-density film shows a peak behavior in the mobility around 250 K in contrast to the temperature-independent mobility observed for the high-density film for T<300 K. We investigate theoretically the concentrations of donor, acceptor, and threading dislocations for both the films and also discussed various electron-scattering mechanisms which contribute to the mobility in these films.
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73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
73.61.Ey III-V semiconductors

Targeted manipulation of grain boundaries and triple junctions on thin films using lasers: A Potts model simulation

Mark T. Lusk, Moneesh Upmanyu, and Tyrone Vincent

J. Appl. Phys. 99, 023505 (2006); http://dx.doi.org/10.1063/1.2158141 (7 pages) | Cited 3 times

Online Publication Date: 18 January 2006

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A two-dimensional Potts model is used to simulate the motion of a laser spot on a polycrystalline thin film. The model is used to demonstrate the feasibility of reshaping individual grain boundaries by either dragging grain boundaries or by repositioning triple junctions.
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61.72.Mm Grain and twin boundaries
68.55.-a Thin film structure and morphology
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
42.62.-b Laser applications

Microwave heating of conductive powder materials

K. I. Rybakov, V. E. Semenov, S. V. Egorov, A. G. Eremeev, I. V. Plotnikov, and Yu. V. Bykov

J. Appl. Phys. 99, 023506 (2006); http://dx.doi.org/10.1063/1.2159078 (9 pages) | Cited 32 times

Online Publication Date: 18 January 2006

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In recent years, a considerable interest has been drawn to microwave heating of powder metals and other electrically conductive materials. In this paper a consistent formulation describing the absorption of microwaves in electrically conductive materials under different microwave heating conditions is developed. A special case when conductive powder particles are surrounded by insulating oxide layers is investigated in detail using the effective-medium approximation. The conditions giving rise to skin effect governed, volumetric, and localized microwave heating are analyzed. Experimental observations of different microwave heating regimes in silicon, iron, and copper powder compacts are in general agreement with the theoretical model.
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81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
72.30.+q High-frequency effects; plasma effects
81.40.Gh Other heat and thermomechanical treatments

Rapid thermal annealing of hot wire chemical-vapor-deposited a-Si:H films: The effect of the film hydrogen content on the crystallization kinetics, surface morphology, and grain growth

A. H. Mahan, B. Roy, R. C. Reedy, D. W. Readey, and D. S. Ginley

J. Appl. Phys. 99, 023507 (2006); http://dx.doi.org/10.1063/1.2159550 (9 pages) | Cited 10 times

Online Publication Date: 18 January 2006

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The ability to crystallize thin amorphous Si layers into large grain Si can lead to significant improvements in Si solar cells and thin-film transistors. Here we report on the effect of the hydrogen content in as-grown films on the crystallization kinetics, surface morphology, and grain growth for hot wire chemical-vapor-deposited a-Si:H films crystallized by rapid thermal annealing (RTA). At RTA temperatures >750 °C for high-hydrogen-content films, we observe the explosive evolution of hydrogen, with a resultant destruction of the film. Little or no damage is observed for films containing low hydrogen content. At a lower RTA temperature (600 °C), the films remain intact with similar morphologies. At this same lower RTA temperature, both the incubation time and crystallization time decrease, and the grain size as measured by x-ray diffraction increases with decreasing hydrogen film content. Measurements of the crystallization time versus H evolution time indicate that the vast majority of the hydrogen must evolve from both films before crystallization commences. To examine the relationship between hydrogen evolution and crystallization, a two-step annealing process was utilized. For the high hydrogen content films, the final grain size increases if a large portion of the hydrogen is driven out at temperatures well below the crystallization temperature.
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81.40.Gh Other heat and thermomechanical treatments
68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.72.Cc Kinetics of defect formation and annealing
61.43.Dq Amorphous semiconductors, metals, and alloys
81.05.Gc Amorphous semiconductors

Nucleation and growth study of atomic layer deposited HfO2 gate dielectrics resulting in improved scaling and electron mobility

P. D. Kirsch, M. A. Quevedo-Lopez, H. -J. Li, Y. Senzaki, J. J. Peterson, S. C. Song, S. A. Krishnan, N. Moumen, J. Barnett, G. Bersuker, P. Y. Hung, B. H. Lee, T. Lafford, Q. Wang, D. Gay, et al.

J. Appl. Phys. 99, 023508 (2006); http://dx.doi.org/10.1063/1.2161819 (8 pages) | Cited 47 times

Online Publication Date: 19 January 2006

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HfO2 films have been grown with two atomic layer deposition (ALD) chemistries: (a) tetrakis(ethylmethylamino)hafnium (TEMAHf)+O3 and (b) HfCl4+H2O. The resulting films were studied as a function of ALD cycle number on Si(100) surfaces prepared with chemical oxide, HF last, and NH3 annealing. TEMAHf+O3 growth is independent of surface preparation, while HfCl4+H2O shows a surface dependence. Rutherford backscattering shows that HfCl4+H2O coverage per cycle is l3% of a monolayer on chemical oxide while TEMAHf+O3 coverage per cycle is 23% of a monolayer independent of surface. Low energy ion scattering, x-ray reflectivity, and x-ray photoelectron spectroscopy were used to understand film continuity, density, and chemical bonding. TEMAHf+O3 ALD shows continuous films, density >9 g/cm3, and bulk Hf–O bonding after 15 cycles [physical thickness (Tphys) = 1.2±0.2 nm] even on H-terminated Si(100). Conversely, on H-terminated Si(100), HfCl4+H2O requires 50 cycles (Tphys ∼ 3 nm) for continuous films and bulk Hf–O bonding. TEMAHf+O3 ALD was implemented in HfO2/TiN transistor gate stacks, over the range 1.2 nm ⩽ Tphys ⩽ 3.3 nm. Electrical results are consistent with material analysis suggesting that at Tphys = 1.2 nm HfO2 properties begin to deviate from thick film properties. At Tphys = 1.2 nm, electrical thickness scaling slows, gate current density begins to deviate from scaling trendlines, and no hard dielectric breakdown occurs. Most importantly, n-channel transistors show improvement in peak and high field electron mobility as Tphys scales from 3.3 to 1.2 nm. This improvement may be attributed to reduced charge trapping and Coulomb scattering in thinner films. Scaled HfO2 enables 1 nm equivalent oxide thickness and 82% of universal SiO2 mobility.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.55.-g Dielectric thin films
73.50.Dn Low-field transport and mobility; piezoresistance
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.A- Nucleation and growth

Quantum dots and etch-induced depletion of a silicon two-dimensional electron gas

L. J. Klein, K. L. M. Lewis, K. A. Slinker, Srijit Goswami, D. W. van der Weide, R. H. Blick, P. M. Mooney, J. O. Chu, S. N. Coppersmith, Mark Friesen, and M. A. Eriksson

J. Appl. Phys. 99, 023509 (2006); http://dx.doi.org/10.1063/1.2159074 (7 pages) | Cited 8 times

Online Publication Date: 19 January 2006

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The controlled depletion of electrons in semiconductors is the basis for numerous devices. Reactive-ion etching provides an effective technique for fabricating both classical and quantum devices. However, Fermi-level pinning must be carefully considered in the development of small devices, such as quantum dots. Because of depletion, the electrical size of the device is reduced in comparison with its physical dimension. To investigate this issue in modulation-doped silicon single-electron transistors, we fabricate several types of devices in silicon-germanium heterostructures using two different etches, CF4 and SF6. We estimate the depletion width associated with each etch by two methods: (i) conductance measurements in etched wires of decreasing thickness (to determine the onset of depletion), and (ii) capacitance measurements of quantum dots (to estimate the size of the active region). We find that the SF6 etch causes a much smaller depletion width, making it more suitable for device fabrication.
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73.63.Kv Quantum dots
73.21.La Quantum dots
81.65.Cf Surface cleaning, etching, patterning
85.35.Gv Single electron devices
73.23.Hk Coulomb blockade; single-electron tunneling

Nonlinear anisotropic description for the thermomechanical response of shocked single crystals: Inelastic deformation

J. M. Winey and Y. M. Gupta

J. Appl. Phys. 99, 023510 (2006); http://dx.doi.org/10.1063/1.2161414 (11 pages) | Cited 11 times

Online Publication Date: 19 January 2006

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A nonlinear anisotropic continuum framework for describing the thermoelastic-plastic response of single crystals shocked along arbitrary orientations has been developed. Our modeling approach incorporates nonlinear elasticity, crystal plasticity, and thermodynamic consistency within an incremental tensor formulation. Crystal plasticity was incorporated by considering dislocation motion along specified slip planes. The theoretical developments presented here are sufficiently general to also accommodate other types of inelastic deformation mechanisms. As representative applications of the theoretical developments, numerical simulations of shock wave propagation in lithium fluoride (LiF) and copper single crystals are presented and compared to wave profile data for several crystal orientations. Simulations of shock wave propagation along low-symmetry directions, where data are not available, are also presented to examine the propagation of quasilongitudinal and quasishear waves in crystals undergoing elastic-plastic deformation. Temperature calculations for the shocked single crystals are discussed.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
62.50.-p High-pressure effects in solids and liquids

Effect of polymer concentration on stabilized large-tilt-angle flexoelectro-optic switching

B. J. Broughton, M. J. Clarke, S. M. Morris, A. E. Blatch, and H. J. Coles

J. Appl. Phys. 99, 023511 (2006); http://dx.doi.org/10.1063/1.2161824 (6 pages) | Cited 9 times

Online Publication Date: 20 January 2006

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In this letter, the uniform lying helix (ULH) liquid crystal texture, required for the flexoelectro-optic effect, is polymer stabilized by the addition of a small percentage of reactive mesogen to a high-tilt-angle (ϕ>60°) bimesogenic chiral nematic host. The electro-optic response is measured for a range of reactive mesogen concentration mixtures, and compared to the large-tilt-angle switch of the pure chiral nematic mixture. The optimum concentration of reactive mesogen, which is found to provide ample stabilization of the texture with minimal impact on the electro-optic response, is found to be approximately 3%. Our results indicate that polymer stabilization of the ULH texture using a very low concentration of reactive mesogen is a reliable way of ruggedizing flexoelectro-optic devices without interfering significantly with the electro-optics of the effect, negating the need for complicated surface alignment patterns or surface-only polymerization. The polymer stabilization is shown to reduce the temperature dependence of the flexoelectro-optic response due to “pinning” of the chiral nematic helical pitch. This is a restriction of the characteristic thermochromic behavior of the chiral nematic. Furthermore, selection of the temperature at which the sample is ultraviolet cured allows the tilt angle to be optimized for the entire chiral nematic temperature range. The response time, however, remains more sensitive to operating temperature than curing temperature. This allows the sample to be cured at low temperature and operated at high temperature, providing simultaneous optimization of these two previously antagonistic performance aspects.
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42.70.Df Liquid crystals
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
77.65.-j Piezoelectricity and electromechanical effects
78.20.Jq Electro-optical effects
82.35.-x Polymers: properties; reactions; polymerization
78.20.N- Thermo-optic effects
78.20.nb Photothermal effects

Hugoniot and strength behavior of silicon carbide

T. J. Vogler, W. D. Reinhart, L. C. Chhabildas, and D. P. Dandekar

J. Appl. Phys. 99, 023512 (2006); http://dx.doi.org/10.1063/1.2159084 (15 pages) | Cited 11 times

Online Publication Date: 20 January 2006

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The shock behavior of two varieties of the ceramic silicon carbide was investigated through a series of time-resolved plate impact experiments reaching stresses of over 140 GPa. The Hugoniot data obtained are consistent for the two varieties tested as well as with most data from the literature. Through the use of reshock and release configurations, reloading and unloading responses for the material were found. Analysis of these responses provides a measure of the ceramic’s strength behavior as quantified by the shear stress and the strength in the Hugoniot state. While previous strength measurements were limited to stresses of 20–25 GPa, measurements were made to 105 GPa in the current study. The initial unloading response is found to be elastic to stresses as high as 105 GPa, the level at which a solid-to-solid phase transformation is observed. While the unloading response lies significantly below the Hugoniot, the reloading response essentially follows it. This differs significantly from previous results for B4C and Al2O3. The strength of the material increases by about 50% at stresses of 50–75 GPa before falling off somewhat as the phase transformation is approached. Thus, the strength behavior of SiC in planar impact experiments could be characterized as metal-like in character. The previously reported phase transformation at ∼ 105 GPa was readily detected by the reshock technique, but it initially eluded detection with traditional shock experiments. This illustrates the utility of the reshock technique for identifying phase transformations. The transformation in SiC was found to occur at about 104 GPa with an associated volume change of about 9%.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
64.70.K- Solid-solid transitions
62.50.-p High-pressure effects in solids and liquids

Optical properties of AlGaN/GaN multiple quantum well structure by using a high-temperature AlN buffer on sapphire substrate

J. Bai, T. Wang, P. Comming, P. J. Parbrook, J. P. R. David, and A. G. Cullis

J. Appl. Phys. 99, 023513 (2006); http://dx.doi.org/10.1063/1.2161941 (4 pages) | Cited 14 times

Online Publication Date: 20 January 2006

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We demonstrated highly improved optical properties of the AlGaN/GaN multiple quantum well (MQW) structure grown on a GaN layer with significant reduction of dislocations achieved using an AlN buffer directly grown on a sapphire substrate at high temperature, in comparison with the MQW grown using a conventional two-step technique. An enhancement of room-temperature photoluminescence intensity by up to one order of magnitude has been observed, compared with the conventional MQW. Transmission electron microscopy measurements indicated that the dislocation density of the GaN grown using the AlN buffer technique was dramatically reduced. Furthermore, the significantly improved optical properties of this MQW using this AlN buffer technique were also demonstrated by increasing the thickness of the GaN layer underneath, whose crystal quality can be improved by increasing thickness, confirmed by detailed x-ray measurements. Since the technique can significantly improve the optical efficiency in the ultraviolet (UV) spectral region without involvement of any ex situ patterning process, it should be highlighted in the development of high performance UV light-emitting diodes, currently a highly regarded research area.
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78.67.De Quantum wells
68.65.Fg Quantum wells
78.55.Cr III-V semiconductors
68.55.-a Thin film structure and morphology
68.37.Lp Transmission electron microscopy (TEM)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Structural characterization of a-plane Zn1−xCdxO (0 ⩽ x ⩽ 0.085) thin films grown by metal-organic vapor phase epitaxy

J. Zúñiga-Pérez, V. Muñoz-Sanjosé, M. Lorenz, G. Benndorf, S. Heitsch, D. Spemann, and M. Grundmann

J. Appl. Phys. 99, 023514 (2006); http://dx.doi.org/10.1063/1.2163014 (6 pages) | Cited 29 times

Online Publication Date: 23 January 2006

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Zn1−xCdxO(11math0) films have been grown on (01math2) sapphire (r–plane) substrates by metal-organic vapor phase epitaxy. A 800-nm-thick ZnO buffer, deposited prior to the alloy growth, helps to prevent the formation of pure CdO. A maximum uniform Cd incorporation of 8.5 at. % has been determined by Rutherford backscattering spectrometry. Higher Cd contents lead to the coexistence of Zn1−xCdxO alloys of different compositions within the same film. The near band-edge photoluminescence emission shifts gradually to lower energies as Cd is incorporated and reaches 2.93 eV for the highest Cd concentration (8.5 at. %). The lattice deformation, due to Cd incorporation, has been described using a new reference frame in which the lattice distortions are directly related to the a-plane surface structure. Cd introduction does not affect the c lattice parameter but expands the lattice along the two perpendicular directions, [11math0] and [math100], resulting in a quadratic volume increase.
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68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
78.55.Et II-VI semiconductors

Grain-boundary grooving by surface diffusion with asymmetric and strongly anisotropic surface energies

Donghong Min and Harris Wong

J. Appl. Phys. 99, 023515 (2006); http://dx.doi.org/10.1063/1.2159082 (16 pages) | Cited 6 times

Online Publication Date: 23 January 2006

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Grain-boundary migration controls grain growth, which is important in material processing and synthesis. When a vertical grain boundary ends at a horizontal free surface, a groove forms at the tip to reduce the combined grain-boundary and surface energies. The groove affects the migration of the grain boundary, and its effect must be understood. This work studies grain-boundary grooving by capillarity-driven surface diffusion with asymmetric and strongly anisotropic surface energies. The surface energies are described by the delta-function facet model that holds for temperatures above the roughening temperature of the bicrystal. Since the asymmetric anisotropy does not introduce a length scale, the asymmetric groove still grows with time t as t1/4. Thus, the nonlinear partial differential equation that governs grooving is reduced by a self-similar transformation to an ordinary differential equation, which is then solved numerically by shooting methods. We vary systematically the crystallographic orientations of the bicrystal and the normalized grain-boundary energy. We find that the self-similar groove profile is faceted and asymmetric for most bicrystals. However, a few asymmetric bicrystals can also yield smooth and symmetric grooves. Some bicrystals may even form ridges instead of grooves. We also find that the asymmetric surface energies tilt the grain-boundary tip sideways, which induces the grain boundary to migrate. We show that anisotropic groove profiles measured in SrTiO3 and Ni can be well fitted by our model.
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61.72.Mm Grain and twin boundaries
68.35.Fx Diffusion; interface formation
68.35.Md Surface thermodynamics, surface energies

In situ measurements of cobalt thin-film surface roughening upon annealing

Jorge Espinosa, Hongtao Shi, and D. Lederman

J. Appl. Phys. 99, 023516 (2006); http://dx.doi.org/10.1063/1.2159553 (6 pages) | Cited 5 times

Online Publication Date: 24 January 2006

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Atomic force and scanning tunneling microscopies were employed to investigate the surface roughness of 5.0- and 8.0‐nm-thick Co films as a function of annealing temperature. Films were epitaxially grown on (110) Al2O3 at 315 °C via molecular-beam epitaxy. The as-grown films had a surface roughness average of approximately 0.7 nm. Images were acquired in situ at temperatures ranging from room temperature to 600 °C. For the 5.0 nm films, surface roughening abruptly starts at a critical temperature T0 = 459 °C and increases depending on the final annealing temperature and annealing time. After waiting long periods of time, so that the film is in quasiequilibrium, the roughness increases with increasing temperature T with a power-law dependence (T/T0−1)0.5. This sudden roughening process is irreversible and possibly related to the hcp-fcc martensitic phase transition in cobalt, which occurs at 415 °C in bulk, or due to bulk defect diffusion. The autocorrelation function of the images above T0 shows that the in-plane island size during roughening is larger in a direction perpendicular to the sapphire terraces. The 8.0‐nm-thick film sample revealed the formation of irregularly shaped pits with a lateral size of ∼ 350 nm at temperatures greater than ∼ 550 °C.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
68.55.-a Thin film structure and morphology
64.70.K- Solid-solid transitions
66.30.Lw Diffusion of other defects
61.72.Cc Kinetics of defect formation and annealing
81.30.Kf Martensitic transformations

Investigation of indium distribution in InGaAs/GaAs quantum dot stacks using high-resolution x-ray diffraction and Raman scattering

Yu. I. Mazur, Zh. M. Wang, G. J. Salamo, V. V. Strelchuk, V. P. Kladko, V. F. Machulin, M. Ya. Valakh, and M. O. Manasreh

J. Appl. Phys. 99, 023517 (2006); http://dx.doi.org/10.1063/1.2163009 (10 pages) | Cited 7 times

Online Publication Date: 24 January 2006

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Using high-resolution x-ray diffraction (HRXRD), Raman scattering, photoluminescence, and atomic-force microcopy, we investigated InxGa1−xAs/GaAs quantum dot (QD) stacks grown by molecular-beam epitaxy with nominal In contents of 0.30 and 0.35. The analysis of x-ray-diffraction rocking curves using symmetrical (004), asymmetrical (113), and quasiforbidden (002) reflections within the framework of dynamical theory allowed us to determine the average values of strains parallel and perpendicular to the growth direction. We also measured nonuniform In profiles in the InxGa1−xAs layers along the growth direction. This observation confirms the important role of surface segregation of In atoms and interdiffusion of Ga atoms from GaAs layers in the formation of InxGa1−xAs QDs. Both HRXRD and Raman scattering in InxGa1−xAs/GaAs-stacked QD structures demonstrate that the InGaAs inserts in these structures can be modeled effectively as sublayers with two different compositions: sufficiently thick InxGa1−xAs sublayer with the In concentration lower than the nominal one, which includes the thin layer of InGaAs islands with the In concentration much higher than the nominal one.
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81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
78.55.Cr III-V semiconductors
78.30.Fs III-V and II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

X-ray-diffraction study of crystalline Si nanocluster formation in annealed silicon-rich silicon oxides

D. Comedi, O. H. Y. Zalloum, E. A. Irving, J. Wojcik, T. Roschuk, M. J. Flynn, and P. Mascher

J. Appl. Phys. 99, 023518 (2006); http://dx.doi.org/10.1063/1.2162989 (8 pages) | Cited 18 times

Online Publication Date: 25 January 2006

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The formation and subsequent growth of crystalline silicon nanoclusters (Si-ncs) in annealed silicon-rich silicon oxides (SRSOs) were studied by glancing angle x-ray diffraction. SRSO samples with Si concentrations (y) of 0.40, 0.42, and 0.45 were grown by inductively coupled plasma-enhanced chemical-vapor deposition (PECVD). Samples with y = 0.42 grown by electron-cyclotron-resonance PECVD were also studied. Annealing treatments were performed at temperatures (T) of 900, 1000, and 1100 °C for times (t) between 0.5 and 3 h in flowing Ar. As-grown SRSO films did not present signs of Si clusters (amorphous or crystalline); however, (111), (220), and (311) Bragg peaks corresponding to c‐Si were clearly seen after annealing at 900 °C for the y = 0.45 sample, but only barely seen for the y = 0.42 and undetected for the y = 0.40 samples. For T = 1000 °C, all studied SRSO samples clearly showed the c‐Si diffraction peaks, which became narrower with increasing t and T. From the width of the Si (111) peaks, the mean size of Si-ncs and their dependence on T and t was determined. Activation energies were deduced from the T dependence by fitting the results to two growth models of Si precipitates in an a‐SiO2 matrix reported in the literature. The activation energies qualitatively agree with values deduced from transmission electron microscopy studies of annealed SRSO reported in the literature. However, they are significantly lower than Si diffusion activation energies available in the literature for SiO2 with low excess Si. A broad feature is also observed in the x-ray diffractograms for as-grown samples with low y, which shifts to the peak position corresponding to a‐SiO2 with increasing T. This behavior is explained by the formation of a well-defined a‐SiO2 phase with increasing T, where mixed Si–O4−nSin (n = 1,2,3) tetrahedra in the as-grown alloy are gradually converted into Si–O4 and Si–Si4 as phase separation of Si and SiO2 proceeds. From the measured Si (111) peak positions, small Si-ncs are found to be tensilely strained by as much as ∼ 0.8%. This effect becomes insignificant as Si-ncs become larger with increasing y or T.
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61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
81.07.Bc Nanocrystalline materials
81.40.Gh Other heat and thermomechanical treatments
81.16.-c Methods of micro- and nanofabrication and processing
64.75.-g Phase equilibria
81.30.Mh Solid-phase precipitation

Thermal stability of TiO2, ZrO2, or HfO2 on Si(100) by photoelectron emission microscopy

M. C. Zeman, C. C. Fulton, G. Lucovsky, R. J. Nemanich, and W.-C. Yang

J. Appl. Phys. 99, 023519 (2006); http://dx.doi.org/10.1063/1.2163984 (6 pages) | Cited 10 times

Online Publication Date: 25 January 2006

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The thermal stability of thin films (3 nm) of transition-metal (TM) oxides (TiO2, ZrO2, and HfO2) grown on ultrathin ( ∼ 0.5 nm) SiO2 buffer layers on Si(100) surfaces was investigated with ultraviolet photoelectron emission microscopy (UV-PEEM). The decomposition of the TM oxides was observed in the PEEM during ultrahigh-vacuum annealing at temperatures of ∼ 870, ∼ 900, and ∼ 1000 °C for the TiO2, ZrO2, and HfO2, respectively. Following the decomposition reaction, atomic force microscopy measurements of the annealed surfaces revealed a high density of islands in the decomposed regions. The degradation of the TM oxide films is attributed to a reaction occurring at defects at the TM oxide/SiO2/Si interfaces, which forms SiO species. Once a portion of the interfacial SiO2 layer is desorbed as a result of this reaction, Si from the substrate can diffuse into contact with the TM oxide layer, resulting in the formation of a TM silicide and the evolution of SiO. This process continues until the entire TM oxide layer is consumed and only silicide islands remain.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
79.60.Bm Clean metal, semiconductor, and insulator surfaces
68.37.Xy Scanning Auger microscopy, photoelectron microscopy
68.37.Ps Atomic force microscopy (AFM)
77.55.-g Dielectric thin films
61.72.Cc Kinetics of defect formation and annealing
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