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15 Oct 2007

Volume 102, Issue 8, Articles (08xxxx)

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J. Appl. Phys. 102, 081301 (2007); http://dx.doi.org/10.1063/1.2799091 (28 pages)

V. V. Afanas’ev and A. Stesmans
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Effect of thickness on the self-positioning of nanostructures

Y. Nishidate and G. P. Nikishkov

J. Appl. Phys. 102, 083501 (2007); http://dx.doi.org/10.1063/1.2795678 (5 pages) | Cited 3 times

Online Publication Date: 16 October 2007

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Atomic-scale modeling of self-positioning GaAs–InAs nanostructures is performed. Curvature radius values obtained by the atomic-scale finite element method are compared with those obtained by a continuum mechanics solution under plane strain conditions. Atomic-scale modeling and continuum mechanics solution predict the same curvature radius for structures with large thickness. However, atomic-scale modeling shows significant decrease of the curvature radius for structures with thickness less than 40 nm.
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81.07.Bc Nanocrystalline materials
61.46.Hk Nanocrystals
81.16.-c Methods of micro- and nanofabrication and processing
46.70.Lk Other structures

Ultrasonic characteristics of porous Pd42.5Cu30Ni7.5P20 glassy alloys

Mikio Fukuhara, Takeshi Wada, and Akihisa Inoue

J. Appl. Phys. 102, 083502 (2007); http://dx.doi.org/10.1063/1.2795684 (7 pages) | Cited 4 times

Online Publication Date: 16 October 2007

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Using ultrasonics, the acoustic characteristics of porous Pd42.5Cu30Ni7.5P20 glassy alloys were examined in terms of complex elasticity. Bulk modulus (K) and Lamè parameter (λ) decrease in a parabolic manner as porosity increases, while Young (E) and shear (G) moduli decrease linearly. Consequently, the order of K>λ>E>G, which is monopolistically characterized by nonporous glassy alloys, changes to the order of E>K>λ ≥ G at 58% porosity. The decrease in Poisson’s ratio and the increase in G/K ratio up to 58% porosity indicate crystallinelike characteristics that resist three-dimensional volume-nonpreserving deformation. An increase in porosity induces frequency convergence for longitudinal waves and an increment of the imaginary parts in the complex waves. The viscoelasticity of the porous glassy alloys is predominated by shear motion. In contrast to the continuous increment in dilational damping, the porosity-dependent increment in shear damping could be elucidated based on the accumulation of strains induced by the formation of pores.
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62.65.+k Acoustical properties of solids
61.43.Gt Powders, porous materials
61.43.Fs Glasses
62.20.D- Elasticity

A shock-induced phase transformation in a LiTaO3 crystal

Jun Li, Xianming Zhou, Wenjun Zhu, Jiabo Li, and Fuqian Jing

J. Appl. Phys. 102, 083503 (2007); http://dx.doi.org/10.1063/1.2794721 (5 pages) | Cited 5 times

Online Publication Date: 16 October 2007

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The high-pressure phase transformation behavior of LiTaO3 crystal has been studied by both Hugoniot measurements and first-principle calculations. We observe a discontinuity in shock velocity (D) versus particle velocity (UP) relation, a two-wave structure below 37.9 GPa, and a three-wave structure above 37.9 GPa. These data confirm that a shock-induced phase transformation of LiTaO3 occurs. The onset pressure of the phase transformation (37.9 GPa) defined by our new shock compression data is higher than the early shock wave value (19 GPa) reported by Stanton and Graham [ P. L. Stanton and R. A. Graham, J. Appl. Phys. 50, 6892 (1979) ]. A first-principle calculation of the zero degree isotherm for rhombohedral phase (R3c space group) is in good agreement with our low-pressure experimental data. The calculated zero degree isotherm for orthorhombic phase (Pbnm space group) is in concord with our high-pressure shock compression data.
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62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling

Annealing effects on the nanoscale indium and nitrogen distribution in Ga(NAs) and (GaIn)(NAs) quantum wells

K. Volz, T. Torunski, O. Rubel, W. Stolz, P. Kruse, D. Gerthsen, M. Schowalter, and A. Rosenauer

J. Appl. Phys. 102, 083504 (2007); http://dx.doi.org/10.1063/1.2794739 (6 pages) | Cited 2 times

Online Publication Date: 16 October 2007

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III∕V semiconductors containing dilute amounts of nitrogen are metastable and need to be thermally treated after growth to optimize optoelectronic properties. The influence of thermal annealing on the nitrogen depth profile in metal organic vapor phase epitaxygrown Ga(NAs)/GaAs as well as (GaIn)(NAs)/GaAs heterostructures is examined on a nanometer scale by combining several high resolution transmission electron microscopy techniques, also with Rutherford backscattering spectrometry. Annealing conditions, which are optimized for quaternary alloys with respect to photoluminescence intensity, do not result in element redistribution for the In containing material. Contrary to the quaternary material, the result of annealing the ternary Ga(NAs) is a pronounced pileup of the nitrogen profile without any out diffusion of nitrogen. These findings have important influence on device structures, which often contain Ga(NAs) barriers for strain-compensation purposes together with (GaIn)(NAs) active regions. In the light of metastability considerations for the ternary and quaternary alloy, one can conclude that the In contained in the quaternary material stabilizes the material and suppresses phase separation. Consequently (GaIn)(NAs) is more stable than its ternary counterpart Ga(NAs).
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61.72.S- Impurities in crystals
61.72.Cc Kinetics of defect formation and annealing
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
68.49.Sf Ion scattering from surfaces (charge transfer, sputtering, SIMS)

Optical patterning of silver nanoparticle Langmuir-Blodgett films

Hyunjun Kim, Hyunkwon Shin, Jungmin Ha, Myeongkyu Lee, and Ki-Soo Lim

J. Appl. Phys. 102, 083505 (2007); http://dx.doi.org/10.1063/1.2794971 (4 pages) | Cited 7 times

Online Publication Date: 17 October 2007

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We report on the optical patterning of Ag nanoparticle Langmuir-Blodgett (LB) films deposited on glass substrates. Ag nanoparticles could be quickly desorbed from the substrate without surface damage, when exposed to a nanosecond Nd:YAG laser pulse over threshold energy density. Well-aligned one-dimensional and two-dimensional patterns at the micrometer scale were fabricated in LB films by a simple spatial modulation of the laser beam. This parallel process enables the period and feature size of the pattern to be independently controlled, and may provide an effective method for nanoparticle pattering.
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81.16.Rf Micro- and nanoscale pattern formation
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
68.43.Mn Adsorption kinetics
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.62.-b Laser applications

Effects of the quantum dot ripening in high-coverage InAs/GaAs nanostructures

P. Frigeri, L. Nasi, M. Prezioso, L. Seravalli, G. Trevisi, E. Gombia, R. Mosca, F. Germini, C. Bocchi, and S. Franchi

J. Appl. Phys. 102, 083506 (2007); http://dx.doi.org/10.1063/1.2795661 (8 pages) | Cited 10 times

Online Publication Date: 17 October 2007

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We report a detailed study of InAs/GaAs quantum dot (QD) structures grown by molecular beam epitaxy with InAs coverages θ continuously graded from 1.5 to 2.9 ML. The effect of coverage on the properties of QD structures was investigated by combining atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence, capacitance-voltage, and deep level transient spectroscopy. In the 1.5–2.9 ML range small-sized coherent QDs are formed with diameters and densities that increase up to 15 nm and 2×1011 cm−2, respectively. For θ>2.4 ML large-sized QDs with diameters of 25 nm and densities ranging from 2×108 to 1.5×109 cm−2 coexist with small-sized QDs. We explain the occurrence of large-sized QDs as the inevitable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with θ, leading to the formation of V-shaped defects at the interface between QDs and upper confining layers that propagate toward the surface. Island relaxation substantially affects the properties of QD structures: (i) free carrier concentration is reduced near the QD plane, (ii) the QD photoluminescence intensity is significantly quenched, and (iii) deep levels show up with typical features related to extended structural defects.
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68.65.Hb Quantum dots (patterned in quantum wells)
82.53.Mj Femtosecond probing of semiconductor nanostructures
68.55.-a Thin film structure and morphology
68.37.Ps Atomic force microscopy (AFM)
68.37.Lp Transmission electron microscopy (TEM)
78.55.-m Photoluminescence, properties and materials

Nanoindentation of the a and c domains in a tetragonal BaTiO3 single crystal

Young-Bae Park, Matthew J. Dicken, Zhi-Hui Xu, and Xiaodong Li

J. Appl. Phys. 102, 083507 (2007); http://dx.doi.org/10.1063/1.2795664 (6 pages) | Cited 4 times

Online Publication Date: 17 October 2007

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Nanoindentation in conjunction with piezoresponse force microscopy was used to study domain switching and to measure the mechanical properties of individual ferroelectric domains in a tetragonal BaTiO3 single crystal. It was found that nanoindentation has induced local domain switching; the a and c domains of BaTiO3 have different elastic moduli but similar hardness. Nanoindentation modulus mapping on the a and c domains further confirmed such difference in elasticity. Finite element modeling was used to simulate the von Mises stress and plastic strain profiles of the indentations on both a and c domains, which introduces a much higher stress level than the critical value for domain nucleation.
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77.80.Fm Switching phenomena
77.80.Dj Domain structure; hysteresis
68.35.Gy Mechanical properties; surface strains
62.20.D- Elasticity
62.20.Qp Friction, tribology, and hardness

Aggregate structure and percolation behavior in polymer/carbon black conductive composites

Jizhao Liang and Quanquan Yang

J. Appl. Phys. 102, 083508 (2007); http://dx.doi.org/10.1063/1.2795674 (6 pages) | Cited 7 times

Online Publication Date: 17 October 2007

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The percolation behavior and mechanisms of electrical conductivity in carbon black filled polymer composites were analyzed. An aggregate structure model of the carbon black particles was proposed based on the effective media theory (EMT) and the equivalent sphere of carbon black aggregate acts as a basic cell to describe the nonlinear conductive behavior. Then, a formula describing the relationship between the conductivity and the volume fraction of carbon black particles as well as an expression estimating percolation threshold of the composites were derived. This aggregate structure model overcomes the disadvantage of the EMT which cannot explain the phenomenon that percolation threshold is lower than 1/3. Based on these expressions, the electrical conductivity and percolation threshold of several kinds of conductive polymeric composites were predicted and the results showed that the predictions were preferably consistent with the experimental data from the references.
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72.10.-d Theory of electronic transport; scattering mechanisms
72.80.Tm Composite materials

Activation energy and prefactor for surface electromigration and void drift in Cu interconnects

Z.-S. Choi, R. Mönig, and C. V. Thompson

J. Appl. Phys. 102, 083509 (2007); http://dx.doi.org/10.1063/1.2795663 (4 pages) | Cited 4 times

Online Publication Date: 17 October 2007

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Surface electromigration rates on oxide-free surfaces of unpassivated damascene Cu interconnect segments have been determined through electromigration testing under vacuum. Electromigration-induced voids grew at the cathode end of the segments due to a flux divergence at refractory-metal-lined vias to the lead lines below the test segment. Copper diffusivity on a clean Cu surface was determined by measuring the size of the voids as a function of time and test temperature at a fixed current. An activation energy of 0.45±0.11 eV and a prefactor of 3.35×10−12 m2/s were found for the product of the effective charge z* and the surface diffusivity Ds. This result is shown to be consistent with void drift rates measured in passivated interconnects.
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66.30.Qa Electromigration
61.72.Qq Microscopic defects (voids, inclusions, etc.)
68.35.Fx Diffusion; interface formation
85.40.Ls Metallization, contacts, interconnects; device isolation

Size effect on electronic sputtering of LiF thin films

Manvendra Kumar, S. A. Khan, Parasmani Rajput, F. Singh, A. Tripathi, D. K. Avasthi, and A. C. Pandey

J. Appl. Phys. 102, 083510 (2007); http://dx.doi.org/10.1063/1.2794694 (6 pages) | Cited 5 times

Online Publication Date: 17 October 2007

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Electronic sputtering in polycrystalline LiF thin film by 120 MeV Ag25+ is investigated. The sputter yields of Li and F for the different thicknesses (10–265 nm) of films are measured with online elastic recoil detection analysis technique. A reduction in sputter yield, from ∼ 2.3×106 to 2.2×104 atoms/ion, is observed with increase in the film thickness. The trend in the experimental results can be explained in terms of size effect in thin film following inelastic thermal spike model. The confinement of energy in the film having smaller grains and lower thickness results in higher temperature causing higher sputtering yield.
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61.80.Jh Ion radiation effects
68.55.-a Thin film structure and morphology
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.66.Bi Elemental solids
61.66.Dk Alloys
82.80.-d Chemical analysis and related physical methods of analysis

Molecular dynamics simulation of plastic deformation of nanotwinned copper

A. J. Cao and Y. G. Wei

J. Appl. Phys. 102, 083511 (2007); http://dx.doi.org/10.1063/1.2794884 (5 pages) | Cited 20 times

Online Publication Date: 17 October 2007

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The plastic deformation of polycrystalline Cu with ultrathin lamella twins has been studied using molecular dynamics simulations. The results of uniaxial tensile deformation simulation show that the abundance of twin boundaries provides obstacles to dislocation motion, which in consequence leads to a high strain hardening rate in the nanotwinned Cu. We also show that the twin lamellar spacing plays a vital role in controlling the strengthening effects, i.e., the thinner the thickness of the twin lamella, the harder the material. Additionally, twin boundaries can act as dislocation nucleation sites as they gradually lose coherency at large strain. These results indicate that controlled introduction of nanosized twins into metals can be an effective way of improving strength without suppression tensile ductility.
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81.05.Bx Metals, semimetals, and alloys
81.07.Bc Nanocrystalline materials
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
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.)

Atomic structure in Zr70Ni30 metallic glass

L. Yang, S. Yin, X. D. Wang, Q. P. Cao, J. Z. Jiang, K. Saksl, and H. Franz

J. Appl. Phys. 102, 083512 (2007); http://dx.doi.org/10.1063/1.2798386 (5 pages) | Cited 15 times

Online Publication Date: 17 October 2007

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Atomic structure of Zr70Ni30 metallic glass (MG) was investigated by reverse Monte Carlo simulation combining with x-ray diffraction and Ni and Zr K-edge extended x-ray absorption of fine structure measurements. Distributions of coordination number (CN) and Voronoi clusters were analyzed by Voronoi tessellation method. The average CN of atoms was obtained to be 11.4 together with the average CN of Zr and Ni atoms of about 11.8 and 10.6, respectively. It is found that Z11 Kasper polyhedron and distorted icosahedra are mainly favored structural units in Zr70Ni30 MG. The discrepancy in atomic structure between Zr70Ni30 MG and its corresponding crystalline (or quasicrystalline) phases can explain the fact that Zr70Ni30 MG does not transform to neither icosahedral nor fcc Zr2Ni phase during crystallization process.
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61.44.Br Quasicrystals
78.70.Dm X-ray absorption spectra
61.43.Dq Amorphous semiconductors, metals, and alloys
61.43.Fs Glasses

Influence of grain size on the tensile response of aluminum under plate-impact loading

P. B. Trivedi, J. R. Asay, Y. M. Gupta, and D. P. Field

J. Appl. Phys. 102, 083513 (2007); http://dx.doi.org/10.1063/1.2798497 (9 pages) | Cited 12 times

Online Publication Date: 17 October 2007

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Plate-impact experiments were performed to examine the influence of grain size on the dynamic tensile (or spall) behavior of shocked polycrystalline aluminum. Ultrapure and commercially pure 1050 aluminum plates were cold rolled to 80% strain and heat treated under predetermined conditions to produce recrystallized samples with average grain sizes varying between 49 and 453 μm. Well-characterized samples were subjected to plane wave loading at peak compressive stresses of 4 and 21 GPa, and free-surface velocity profiles were obtained using velocity interferometry. At 4 GPa, the observed pullback velocity, a characteristic feature of the spall response, was similar for different grain sizes of 1050 and ultrapure Al, suggesting that the preferential failure mode is intragranular. At 21 GPa, the spall response (i.e., the pullback velocity and the signal structure) depended on the alloy content; the pullback velocity of ultrapure Al increased with increase in grain size, while it remained constant for 1050 Al. In addition, the structure of pullback signals showed a well-defined change in slope for different grain size samples in ultrapure Al, while no such feature was observed for 1050 Al. For the grain sizes examined, the σHEL was nearly independent of the grain size for 1050 Al and beyond a certain grain size for ultrapure Al.
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81.05.Bx Metals, semimetals, and alloys
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

Residual strain and texture in free-standing nanoscale Cu-Nb multilayers

C. C. Aydıner, D. W. Brown, A. Misra, N. A. Mara, Y.-C. Wang, J. J. Wall, and J. Almer

J. Appl. Phys. 102, 083514 (2007); http://dx.doi.org/10.1063/1.2794862 (10 pages) | Cited 8 times

Online Publication Date: 17 October 2007

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We investigate the residual strains in a free-standing Cu/Nb multilayer of 30 nm nominal layer thickness with synchrotron x-rays. This material system is characterized by columnar grains of Cu and Nb with incoherent interfaces and a sharp physical-vapor-deposition texture. High energy x-rays were used with an area detector along with multiple sample rotations to yield diffraction strain components in a very large number of directions. Due to the texture and the elastic anisotropy of constituents, observed diffraction strains cannot be derived from a single strain tensor (also known as linear sin2 ψ). Orientation-dependent diffraction strain modeling is utilized with a Vook–Witt micromechanical model. Obtained phase-resolved in-plane stress magnitudes are −515 MPa in Nb and +513 MPa in Cu, satisfying force equilibrium within experimental errors. The stresses of this magnitude will certainly influence the mechanical behavior of the multilayer upon further loading. The Vook–Witt model describes the Nb diffraction strains very well, and thereby provides information on the stress distribution in crystallites as a function of their orientation. On the other hand, the same level of agreement with the Vook–Witt model has not been achieved for Cu diffraction strains.
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68.65.Ac Multilayers
81.05.Bx Metals, semimetals, and alloys
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Photoluminescence and electromodulation spectroscopy characterization of a Zn0.81Be0.04Mg0.15Se mixed crystal

P. J. Huang, H. P. Hsu, Y. S. Huang, F. Firszt, S. Łęgowski, H. Męczyńska, A. Marasek, and K. K. Tiong

J. Appl. Phys. 102, 083515 (2007); http://dx.doi.org/10.1063/1.2796107 (5 pages) | Cited 3 times

Online Publication Date: 18 October 2007

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We present an optical characterization of a Bridgman-grown zinc-blende-type Zn0.81Be0.04Mg0.15Se mixed crystal in the near-band-edge interband transitions using temperature-dependent photoluminescence (PL), contactless electroreflectance (CER), and photoreflectance (PR) in the temperature range of 15–400 K. Typical PL spectrum at low temperature consists of an exciton line, an edge emission due to recombination of shallow donor-acceptor pairs, and a broad band related to recombination through deep level defects. Interband transitions, originating from the band edge and spin-orbital splitting critical points of the sample, have been observed in the CER/PR spectra. The peak positions of the excitonic emission lines in the PL spectra correspond quite well to the energies of the fundamental transitions determined from electromodulation data. The parameters that describe the temperature dependence of the fundamental and spin split-off band gaps, and the broadening function of the band edge exciton are evaluated and discussed.
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78.55.Et II-VI semiconductors
78.20.Jq Electro-optical effects
71.35.-y Excitons and related phenomena
71.55.Gs II-VI semiconductors
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
71.20.Nr Semiconductor compounds

Fabrication and electrical, photosensitive properties of p-poly(9,9-diethylfluorene)/n-silicon nanowire heterojunction

Yanzhao Cheng, Guojia Fang, Chun Li, Longyan Yuan, Lei Ai, Bingruo Chen, Xingzhong Zhao, Zhiyuan Chen, Weibin Bai, and Caimao Zhan

J. Appl. Phys. 102, 083516 (2007); http://dx.doi.org/10.1063/1.2798494 (6 pages) | Cited 8 times

Online Publication Date: 18 October 2007

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P-poly(9, 9-diethylfluorene)/n-silicon nanowire (SiNW) heterojunctions were prepared by spin coating p-type conductive organic polymer on as-prepared n-SiNWs, which were fabricated by electroless metal deposition method. This hybrid nanowire p-n heterojunction shows good rectifying behavior with turn-on voltage of about 1 V at room temperature. Temperature-dependent current-voltage properties in the range of 253–413 K were investigated and they follow the standard p-n heterojunction electron transport mechanism. The hybrid nanowire heterojunction shows good photovoltaic properties and good sensitivity toward visible light, and a responsivity of 3.23 mA/W under a reverse bias of 10 V was obtained. These results present potential applications in the future nanoelectronic and photonic devices based on organic and inorganic p-n heterojunction arrays.
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81.07.Pr Organic-inorganic hybrid nanostructures
81.07.Vb Quantum wires
81.15.Pq Electrodeposition, electroplating
68.65.La Quantum wires (patterned in quantum wells)
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
72.40.+w Photoconduction and photovoltaic effects

Synthesis and in situ characterization of low-resistivity TaNx films by remote plasma atomic layer deposition

E. Langereis, H. C. M. Knoops, A. J. M. Mackus, F. Roozeboom, M. C. M. van de Sanden, and W. M. M. Kessels

J. Appl. Phys. 102, 083517 (2007); http://dx.doi.org/10.1063/1.2798598 (11 pages) | Cited 24 times

Online Publication Date: 18 October 2007

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Remote plasma atomic layer deposition (ALD) of TaNx films from Ta[N(CH3)2]5 and H2, H2-N2, and NH3 plasmas is reported. From film analysis by in situ spectroscopic ellipsometry and various ex situ techniques, data on growth rate, atomic composition, mass density, TaNx microstructure, and resistivity are presented for films deposited at substrate temperatures between 150 and 250 °C. It is established that cubic TaNx films with a high mass density (12.1 g cm−3) and low electrical resistivity (380 μΩ cm) can be deposited using a H2 plasma with the density and resistivity of the films improving with plasma exposure time. H2-N2 and NH3 plasmas resulted in N-rich Ta3N5 films with a high resistivity. It is demonstrated that the different TaNx phases can be distinguished in situ by spectroscopic ellipsometry on the basis of their dielectric function with the magnitude of the Drude absorption yielding information on the resistivity of the films. In addition, the saturation of the ALD surface reactions can be determined by monitoring the plasma emission, as revealed by optical emission spectroscopy.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.A- Nucleation and growth
52.77.Dq Plasma-based ion implantation and deposition
73.61.Ng Insulators
77.55.-g Dielectric thin films

Growth-temperature optimization for low-carrier-density In0.75Ga0.25As-based high electron mobility transistors on InP

P. J. Simmonds, H. E. Beere, D. A. Ritchie, and S. N. Holmes

J. Appl. Phys. 102, 083518 (2007); http://dx.doi.org/10.1063/1.2798873 (4 pages) | Cited 7 times

Online Publication Date: 18 October 2007

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Two-dimensional electron gases (2DEGs) were formed in undoped In0.75Al0.25As/In0.75Ga0.25As/In0.75Al0.25As quantum wells. The optimal growth temperature for this structure is 410 °C, with peak 2DEG electron mobility and density values of μ = 221 000 cm2/V s and n = 1.36×1011 cm−2 at 1.5 K. This electron mobility is equal to the highest previously published for these undoped structures but with a factor of 2 reduction in n. This has been achieved through the use of a significantly thinner InAlAs graded buffer, which supports the theory that this is the source of the 2DEG electrons. For n>1.6×1011 cm−2, μ is shown to be 10% higher for transport parallel to the [011] crystal axis. This is a direct result of anisotropic surface morphology.
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85.30.Tv Field effect devices
72.20.Fr Low-field transport and mobility; piezoresistance
73.63.Hs Quantum wells

Stresses in textured and polycrystalline cubic films by Raman spectroscopy: Application to diamond

T. Gries, L. Vandenbulcke, P. Simon, and A. Canizares

J. Appl. Phys. 102, 083519 (2007); http://dx.doi.org/10.1063/1.2798940 (11 pages) | Cited 2 times

Online Publication Date: 18 October 2007

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The determination of the stress/strain level in cubic films is carried out here by polarized Raman spectroscopy. The Voigt-Reuss-Hill averaging technique has been used for calculating the phonon deformation potentials in textured cubic films, especially here in diamond. With the corresponding elastic constants known, the resolution of the secular equation has allowed us to calculate the strain/stress configuration from the shifts and splitting of textured diamond polycrystalline films. For isotropic polycrystalline films, the results obtained by averaging over the different textures compare very well to the results obtained with the complete averaging procedure of Anastassakis [J. Appl. Phys. 86, 249 (1999)]. With both averaging methods, the stress/strain level in polycrystalline films are evaluated with a fairly good accuracy, either for biaxial isotropic in-plane stresses due to thermal expansion mismatch between the film and the titanium alloy substrates or for anisotropic stresses induced by permanent deformations of titanium tensile specimens. The difference between these averaging procedures and a method using the addition of the contributions of different textures is shown for evaluating the strains/stresses in diamond films. A slightly more accurate method is obtained because of the different influences of the various textures on the Raman shifts. However, the textured polycrystalline state being averaged with a method that results in an effective hexagonal symmetry, this approach is still insufficient to take into account the influence of the in-plane orientation of each crystallite relative to strong anisotropic biaxial stresses, an influence which is shown experimentally and theoretically for one texture, along the [110] direction of growth.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations

Pairwise cobalt doping of boron carbides with cobaltocene

A. Yu. Ignatov, Ya. B. Losovyj, L. Carlson, D. LaGraffe, J. I. Brand, and P. A. Dowben

J. Appl. Phys. 102, 083520 (2007); http://dx.doi.org/10.1063/1.2799053 (6 pages) | Cited 9 times

Online Publication Date: 18 October 2007

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We have performed Co K-edge x-ray absorption fine structure and x-ray absorption near edge structure measurements of Co-doped plasma enhanced chemical vapor phase deposition (PECVD) grown “C2B10Hx” semiconducting boron carbides, using cobaltocene. Cobalt does not dope PECVD grown boron carbides as a random fragment of the cobaltocene source gas. The Co atoms are fivefold boron coordinated (R = 2.10±0.02 Å) and are chemically bonded to the icosahedral cages of B10CHx or B9C2Hy. Pairwise Co doping occurs, with the cobalt atoms favoring sites some 5.28±0.02 Å apart.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.uf Ge and Si

Atomic layer deposition of TiO2 from tetrakis-dimethyl-amido titanium or Ti isopropoxide precursors and H2O

Qi Xie, Yu-Long Jiang, Christophe Detavernier, Davy Deduytsche, Roland L. Van Meirhaeghe, Guo-Ping Ru, Bing-Zong Li, and Xin-Ping Qu

J. Appl. Phys. 102, 083521 (2007); http://dx.doi.org/10.1063/1.2798384 (6 pages) | Cited 32 times

Online Publication Date: 18 October 2007

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Atomic layer deposition (ALD) of TiO2 thin films using Ti isopropoxide and tetrakis-dimethyl-amido titanium (TDMAT) as two kinds of Ti precursors and water as another reactant was investigated. TiO2 films with high purity can be grown in a self-limited ALD growth mode by using either Ti isopropoxide or TDMAT as Ti precursors. Different growth behaviors as a function of deposition temperature were observed. A typical growth rate curve-increased growth rate per cycle (GPC) with increasing temperatures was observed for the TiO2 film deposited by Ti isopropoxide and H2O, while surprisingly high GPC was observed at low temperatures for the TiO2 film deposited by TDMAT and H2O. An energetic model was proposed to explain the different growth behaviors with different precursors. Density functional theory (DFT) calculation was made. The GPC in the low temperature region is determined by the reaction energy barrier. From the experimental results and DFT calculation, we found that the intermediate product stability after the ligand exchange is determined by the desorption behavior, which has a huge effect on the width of the ALD process window.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.43.Mn Adsorption kinetics
68.55.A- Nucleation and growth

Lateral heat flow method for thickness independent determination of thermal diffusivity

Nilesh Tralshawala, Donald R. Howard, Bryon Knight, Yuri Plotnikov, and Harry I. Ringermacher

J. Appl. Phys. 102, 083522 (2007); http://dx.doi.org/10.1063/1.2798581 (5 pages)

Online Publication Date: 19 October 2007

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A pulsed transient thermography method is described where a high-intensity light pulse is used to heat a long, uniform stripe on the surface of a plate. A high spatial resolution, high frame rate focal plane array infrared camera is used to monitor surface temperature. We explain the theoretical model and data analysis framework used to experimentally determine all three thermal diffusivity components from the temperature measurements. The analysis does not require any fitting to the temperature profile and is based on the creation of thermal time-of-flight (tof) images from the temperature data and the relationship between tof and the distance from the stripe edge. The in-plane components of thermal diffusivity are obtained without the need for thickness information. Experimental validation of this procedure was carried out using anisotropic carbon fiber reinforced polymer composites.
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44.90.+c Other topics in heat transfer (restricted to new topics in section 44)
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves

Optical characterization of niobium-doped rhenium disulphide single crystals

D. O. Dumcenco, Y. S. Huang, C. H. Liang, and K. K. Tiong

J. Appl. Phys. 102, 083523 (2007); http://dx.doi.org/10.1063/1.2798923 (7 pages) | Cited 5 times

Online Publication Date: 19 October 2007

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In this study, the optical properties of niobium-doped rhenium disulphide (ReS2:Nb) single crystals have been reported. The doping effects of the material were characterized by polarization-dependent transmittance, photoluminescence (PL), and piezoreflectance (PzR) measurements in the temperature range of 10–300 K. The indirect energy gap of ReS2:Nb shows a slight redshift with respect to the undoped sample. The low temperature PL spectra reveal two near direct band-edge excitonic peaks as well as two additional prominent features at higher energy side. The results agree well with that of the PzR investigation of the sample. In comparison with the undoped ReS2, the excitonic transition energies remain practically unchanged, while the broadening parameter of the excitonic transition features slightly increases due to impurity scattering. In addition, the parameters that describe the temperature variations of the band-edge excitonic transitions were evaluated and discussed.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.55.Hx Other solid inorganic materials
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects
61.72.S- Impurities in crystals
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials

Temperature dependence of the elastic and vibronic behavior of Si, Ge, and diamond crystals

Mingxia Gu, Yichun Zhou, Likun Pan, Zhuo Sun, Shanzhong Wang, and Chang Q. Sun

J. Appl. Phys. 102, 083524 (2007); http://dx.doi.org/10.1063/1.2798941 (4 pages) | Cited 9 times

Online Publication Date: 19 October 2007

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The thermally induced softening of the elastic and vibronic identities in crystals and their correlations have long been a puzzle. Analytical solutions have been developed, showing that the detectable elastic and vibronic properties could be related directly to the bonding parameters, such as bond length and strength, and their response to the temperature change. Reproduction of measured T-dependent Young’s modulus and Raman shift of Si, Ge, and diamond reveals that the thermally driven softening of the elasticity and the optical Raman frequency arises from bond expansion and vibration, with derived information about the atomic cohesive energy and clarification of their interdependence.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.30.Am Elemental semiconductors and insulators

Effect of intentional bulk contamination with iron on multicrystalline silicon solar cell properties

S. Dubois, O. Palais, P. J. Ribeyron, N. Enjalbert, M. Pasquinelli, and S. Martinuzzi

J. Appl. Phys. 102, 083525 (2007); http://dx.doi.org/10.1063/1.2799057 (7 pages) | Cited 17 times

Online Publication Date: 19 October 2007

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The influence of intentional iron bulk contamination on the performances of boron doped p-type multicrystalline silicon solar cells was investigated. Solar cells were made from iron contaminated wafers, with an initial dissolved iron concentration 100 times higher than that of standard wafers. Nevertheless, the conversion efficiency of these cells was not impacted by this intentional contamination. We showed that this tolerance toward iron was due to the efficiency of the gettering and hydrogenation effects, complementary in this material. While phosphorus diffusion (extracting more than 99% of the iron from the bulk) is slightly limited in regions of high dislocation density, hydrogen diffuses through the whole thickness of the wafer and passivates defects and remaining impurities, with its diffusion being faster along extended defects
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84.60.Jt Photoelectric conversion
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