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24 Dec 2001

Volume 79, Issue 26, pp. 4271-4458

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Temperature dependence of stresses in GaN thin films grown on (0001) sapphire: Modeling of thermal stresses

J. Keckes, J. W. Gerlach, R. Averbeck, H. Riechert, S. Bader, B. Hahn, H.-J. Lugauer, A. Lell, V. Härle, A. Wenzel, and B. Rauschenbach

Appl. Phys. Lett. 79, 4307 (2001); http://dx.doi.org/10.1063/1.1427424 (3 pages) | Cited 7 times

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Residual stresses and unstressed lattice parameters are characterized in heteroepitaxial GaN thin films grown on (0001) sapphire using three different deposition techniques. X-ray diffraction measurements in the temperature range of 25–600 °C indicate a reversible change of stresses in the films from compressive to tensile state and vice versa. The thermal behavior of stresses in the samples prepared by different methods is comparable. The experimental results are consistent with the model of thermal stresses originating from the mismatch of thermal expansion coefficients of GaN and sapphire.© 2001 American Institute of Physics.
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68.60.Bs Mechanical and acoustical properties
68.55.-a Thin film structure and morphology
62.20.-x Mechanical properties of solids

Photoluminescence of transparent strontium–barium–niobate-doped silica nanocomposites

S. G. Lu, C. L. Mak, K. H. Wong, and K. W. Cheah

Appl. Phys. Lett. 79, 4310 (2001); http://dx.doi.org/10.1063/1.1427419 (3 pages) | Cited 8 times

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Optically transparent nanocomposites of strontium–barium–niobate (SBN) and silica were fabricated via a novel sol–gel method. The nanocomposites were annealed in temperatures of 600–800 °C to yield the nanometer-sized tetragonal tungsten–bronze phase of SBN crystallites. Photoluminescence studies at temperatures from 10 K to room temperature were performed. The undoped silica matrix showed a strong emission band at 3.2 eV and a weak emission band at 2.65 eV. They were noticeably suppressed in our SBN/SiO2 nanocomposites. Extra sharp peaks on top of the 3.2 eV band were seen. An additional emission band at 2.3 eV due to transitions within the NbO6 complex of SBN crystallites was also observed. This emission band showed a large blueshift with decreasing SBN crystallite size. This demonstrates that the photoluminescence spectra of ferroelectric-doped silica nanocomposites have a strong dependence on crystallite size. © 2001 American Institute of Physics.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.07.Bc Nanocrystalline materials
77.84.Lf Composite materials
78.55.Hx Other solid inorganic materials
61.46.-w Structure of nanoscale materials
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)

Does beryllium doping suppress the formation of Ga vacancies in nonstoichiometric GaAs layers grown at low temperatures?

J. Gebauer, R. Zhao, P. Specht, E. R. Weber, F. Börner, F. Redmann, and R. Krause-Rehberg

Appl. Phys. Lett. 79, 4313 (2001); http://dx.doi.org/10.1063/1.1427150 (3 pages) | Cited 5 times

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We investigate native defects in nonstoichiometric GaAs layers grown at low temperatures by molecular-beam epitaxy (LT-GaAs) doped with Be. Ga vacancies (VGa) are found by positron annihilation in all layers. The concentration of VGa is independent of the Be doping in contrast to the previous belief that it decreases. Simultaneously, the concentration of As antisite defects (AsGa) was measured by optical absorption. We find the same relationship between VGa and AsGa concentrations as reported earlier for undoped LT-GaAs. Thus, Be doping has no significant influence on the incorporation of native point defects in LT-GaAs. The thermal stability of such material must therefore be explained otherwise. © 2001 American Institute of Physics.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.J- Point defects and defect clusters
78.66.Fd III-V semiconductors
68.60.Dv Thermal stability; thermal effects
61.72.uj III-V and II-VI semiconductors
78.70.Bj Positron annihilation

Effect of dislocations on thermal conductivity of GaN layers

D. Kotchetkov, J. Zou, A. A. Balandin, D. I. Florescu, and Fred H. Pollak

Appl. Phys. Lett. 79, 4316 (2001); http://dx.doi.org/10.1063/1.1427153 (3 pages) | Cited 41 times

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We report calculation of the lattice thermal conductivity in wurtzite GaN. The proposed model is material specific and explicitly includes phonon relaxation on threading dislocations and impurities typical for GaN. We have found that a decrease of the dislocation density by two orders of magnitude in GaN leads to a corresponding increase of the thermal conductivity from 1.31 to 1.97 W/cm K. This theoretical prediction is in very good agreement with experimental data obtained from scanning thermal microscopy. The developed model can be used for thermal budget calculations in high-power density GaN devices. © 2001 American Institute of Physics.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Localized exciton dynamics in strained cubic In0.1Ga0.9N/GaN multiple quantum wells

S. F. Chichibu, M. Sugiyama, T. Onuma, T. Kitamura, H. Nakanishi, T. Kuroda, A. Tackeuchi, T. Sota, Y. Ishida, and H. Okumura

Appl. Phys. Lett. 79, 4319 (2001); http://dx.doi.org/10.1063/1.1428404 (3 pages) | Cited 31 times

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Radiative and nonradiative recombination dynamics in strained cubic (c-) In0.1Ga0.9N/c-GaN multiple quantum wells were studied using temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy. In contrast to hexagonal InGaN quantum wells, low-excitation photoluminescence peak energy increased moderately with decreasing well thickness L and the PL lifetime did not strongly depend on L. The results clearly indicated that the piezoelectric field was not acting on the transition process. The TRPL signal was well fitted as a stretched exponential decay from 10 to 300 K, showing that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures such as In clusters. The localized states were considered to have two-dimensional density of states at 300 K (quantum disk size), since the radiative lifetime increased with increasing temperature above 150 K.© 2001 American Institute of Physics.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.67.De Quantum wells
78.55.Cr III-V semiconductors
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
73.20.At Surface states, band structure, electron density of states
78.47.-p Spectroscopy of solid state dynamics

Correlation between nucleation layer structure, dislocation density, and electrical resistivity for GaN films grown on a-plane sapphire by metalorganic vapor phase epitaxy

M. E. Twigg, D. D. Koleske, A. E. Wickenden, R. L. Henry, and S. C. Binari

Appl. Phys. Lett. 79, 4322 (2001); http://dx.doi.org/10.1063/1.1428769 (3 pages) | Cited 7 times

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Transmission electron microscopy has been used to study AlN nucleation layers (NLs) grown using metalorganic vapor phase epitaxy, in order to link the resistivity of unintentionally doped GaN films to NL microstructure. We observed that high-resistivity (HR) GaN films grew on large-grained NLs, which, in turn, grew on smooth sapphire surfaces. These HR films had a higher density of edge dislocations than screw and mixed dislocations. Low-resistivity GaN films were found to grow on fine-grained NLs that grew on rougher sapphire surfaces. These LR films had a density of edge dislocations close to the combined density of screw and mixed dislocations. The LR films in this study were also found to have a buried conductive layer that is detectable using capacitance–voltage measurements. © 2001 American Institute of Physics.
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68.55.A- Nucleation and growth
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.61.Ey III-V semiconductors
81.05.Ea III-V semiconductors
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.37.Lp Transmission electron microscopy (TEM)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Orientation control of liquid crystal droplets dispersed in a polymer matrix

F. P. Nicoletta, G. De Filpo, D. Cupelli, M. Macchione, and G. Chidichimo

Appl. Phys. Lett. 79, 4325 (2001); http://dx.doi.org/10.1063/1.1427750 (3 pages) | Cited 5 times

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The encapsulation of monomer molecules in liquid crystal droplets dispersed in a thermoplastic matrix provides a convenient method to control the orientation of liquid crystal directors. The imprint of droplet interfaces is obtained by photopolymerization of monomer molecules by means of a polymerization induced phase separation process performed in a magnetic field. In such a way, uniform orientations of liquid crystal directors can be achieved in cells without any surface treatment. The concentration of monomer molecules is an important parameter, which influences the final electro-optical properties of films. © 2001 American Institute of Physics.
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61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
64.75.-g Phase equilibria
78.20.Jq Electro-optical effects
82.35.-x Polymers: properties; reactions; polymerization

Silicon self-diffusion under extrinsic conditions

Ant Ural, P. B. Griffin, and J. D. Plummer

Appl. Phys. Lett. 79, 4328 (2001); http://dx.doi.org/10.1063/1.1425953 (3 pages) | Cited 4 times

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Self-diffusion in silicon is investigated under extrinsic carrier conditions by monitoring the diffusion of 30Si in isotopically enriched silicon layers with boron and phosphorus background doping. At 1000 °C, we find that the Si self-diffusion coefficient is slightly enhanced in both n- and p-type backgrounds. This is direct evidence of the existence of both negatively and positively charged native point defects in Si. We use a simple model involving three charge states to explain the data, which yield the relative contributions of these charge states to the overall self-diffusion coefficient and the locations of the deep levels they introduce in the band gap. © 2001 American Institute of Physics.
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66.30.H- Self-diffusion and ionic conduction in nonmetals
71.55.Cn Elemental semiconductors
81.05.Cy Elemental semiconductors
72.80.Cw Elemental semiconductors
73.61.Cw Elemental semiconductors

Effect of lattice mismatch on surface morphology of InAs quantum dots on (100) In1−xAlxAs/InP

B. H. Koo, T. Hanada, H. Makino, and T. Yao

Appl. Phys. Lett. 79, 4331 (2001); http://dx.doi.org/10.1063/1.1428763 (3 pages) | Cited 10 times

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We present the results of the formation of the InAs quantum dots (QDs) on the (100) In1−xAlxAs(InAlAs)/InP substrate by using relaxed InAlAs buffer layers with different compositions. Variations of surface morphology of InAs QDs as a function of InAs–InAlAs lattice mismatch have been evaluated by atomic force microscopy. When the lattice mismatch increases from 2.4% to 4.2%, the size of QDs decreases, and the density of QDs increases. Each of these dependences can be fitted to a power function of the misfit unless the Al diffusion, roughness of the buffer layer, and/or the ripening of small dots modify the size and density.© 2001 American Institute of Physics.
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68.65.Hb Quantum dots (patterned in quantum wells)
81.07.Ta Quantum dots
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)
68.35.Md Surface thermodynamics, surface energies

Flux dependence of oxygen-beam-induced ripple growth on silicon

Z. X. Liu and P. F. A. Alkemade

Appl. Phys. Lett. 79, 4334 (2001); http://dx.doi.org/10.1063/1.1428414 (3 pages) | Cited 11 times

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The influence of flux on the growth of ripples on Si(111) under bombardment of oblique, 1 keV O2+ beams was investigated. We found that a low flux leads to a significantly higher ripple growth rate per ion than a high flux. This effect is attributed to a reduction in the viscous flow of the amorphized material. At low fluxes, the viscous flow is reduced because there is more thermal annealing of the radiation damage. Current models of surface roughening remain valid if the flux dependence of the viscous flow is taken into account. © 2001 American Institute of Physics.
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79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
61.72.Cc Kinetics of defect formation and annealing
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.49.Sf Ion scattering from surfaces (charge transfer, sputtering, SIMS)

Electron work function: A parameter sensitive to the adhesion behavior of crystallographic surfaces

D. Y. Li and W. Li

Appl. Phys. Lett. 79, 4337 (2001); http://dx.doi.org/10.1063/1.1428766 (2 pages) | Cited 32 times

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The adhesive force (AF) and the electron work function (EWF) of different crystallographic planes of Cu were determined, with the aim of exploring the potential application of the Kelvin method in characterizing the adhesion of solid surfaces especially those in nano/microdevices. It was demonstrated that there was a close correlation between the EWF and AF, and both the parameters were dependent on the surface atomic arrangement. This study indicates that the EWF is a parameter that could be used to characterize the adhesion behavior of a surface. © 2001 American Institute of Physics.
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68.35.Np Adhesion
73.30.+y Surface double layers, Schottky barriers, and work functions
68.47.De Metallic surfaces
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)

Measurement of interstitial oxygen concentration in silicon below 1015 atoms/cm3

A. Sassella

Appl. Phys. Lett. 79, 4339 (2001); http://dx.doi.org/10.1063/1.1429293 (3 pages) | Cited 2 times

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The absorption coefficient of silicon at 1118 cm−1 due to the multiphonon contribution related to the combination of two transverse optical and one transverse acoustical phonons is measured at the liquid He temperature. The precise knowledge of this contribution, usually hidden by the absorption band of the unavoidable oxygen impurities, permits the quantitative measurement of interstitial oxygen concentration down to 3×1014 atoms/cm3. © 2001 American Institute of Physics.
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61.72.S- Impurities in crystals
61.72.J- Point defects and defect clusters
63.20.K- Phonon interactions
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.30.Am Elemental semiconductors and insulators

Direct observation of the coexistence of coherent and incoherent InAs self-assembled dots by x-ray scattering

A. Malachias, R. Magalhães-Paniago, B. R. A. Neves, W. N. Rodrigues, M. V. B. Moreira, H.-D. Pfannes, A. G. de Oliveira, S. Kycia, and T. H. Metzger

Appl. Phys. Lett. 79, 4342 (2001); http://dx.doi.org/10.1063/1.1427421 (3 pages) | Cited 9 times

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In this letter, grazing incidence x-ray scattering is employed as a method to identify relaxed islands in an ensemble of partially coherent self-assembled InAs quantum dots. A simple model of strained pyramidal islands enables the association of the local lattice parameter of an island to its lateral size. A comparison between the island side length and its strain state allows the identification of coherent and incoherent nanostructures, revealing the size–strain interplay during growth. © 2001 American Institute of Physics.
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68.65.Hb Quantum dots (patterned in quantum wells)
78.70.Ck X-ray scattering

Observation of orientation dependence of electrorheology effect in ferroelectric liquid crystal using shear horizontal wave

M. Inoue, K. Yoshino, H. Moritake, and K. Toda

Appl. Phys. Lett. 79, 4345 (2001); http://dx.doi.org/10.1063/1.1424472 (3 pages) | Cited 1 time

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The electrorheology (ER) effect of ferroelectric liquid crystals is observed under shear-horizontal (SH) wave propagation. The ER effect is strongly dependent on the orientation direction of the liquid-crystal molecule. The apparent viscosity is evaluated through the numerical analysis result on the propagation characteristics of a SH wave in a trilayer structure including a ferroelectric liquid-crystal (Chisso, CS1014) layer between two glass plates. The ER effect is examined by the relation between the orientation of smectic layers and the propagation direction of the SH wave. © 2001 American Institute of Physics.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
77.80.-e Ferroelectricity and antiferroelectricity
77.84.Nh Liquids, emulsions, and suspensions; liquid crystals
02.60.-x Numerical approximation and analysis
66.20.-d Viscosity of liquids; diffusive momentum transport
62.30.+d Mechanical and elastic waves; vibrations

Nitrogen bonding structure in carbon nitride thin films studied by soft x-ray spectroscopy

Niklas Hellgren, Jinghua Guo, Conny Såthe, Akane Agui, Joseph Nordgren, Yi Luo, Hans Ågren, and Jan-Eric Sundgren

Appl. Phys. Lett. 79, 4348 (2001); http://dx.doi.org/10.1063/1.1428108 (3 pages) | Cited 49 times

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Soft x-ray absorption (SXAS) and emission (SXES) spectroscopies were applied to study the nitrogen bonding structure in magnetron sputtered CNx thin films. By comparing with calculated spectra of N in different model systems, N in three main bonding environments can be identified: (i) C�N bonds, with a sharp SXAS peak at 399.5 eV, (ii) pyridine-like N (i.e., N bonded to two C atoms), with an x-ray absorption resonance at ∼398.5 eV, and (iii) N substituted in graphite, possibly with one sp3 carbon as a neighbor (SXAS energy ∼401 eV). These bondings are present in all CNx films analyzed; however, as shown earlier, the relative intensities between the peaks may vary with the growth conditions. Differences in the coordination of the nearest or second nearest C neighbors only cause slight changes in the peak positions and spectrum shape. © 2001 American Institute of Physics.
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68.55.-a Thin film structure and morphology
78.70.Dm X-ray absorption spectra
78.70.En X-ray emission spectra and fluorescence
61.50.Lt Crystal binding; cohesive energy

Lattice parameters of wurtzite Al1−xSixN ternary alloys

Yoshitaka Taniyasu, Makoto Kasu, and Naoki Kobayashi

Appl. Phys. Lett. 79, 4351 (2001); http://dx.doi.org/10.1063/1.1428768 (3 pages) | Cited 8 times

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Lattice constants and bond lengths of wurtzite Al1−xSixN ternary alloys (0 ⩽ x ⩽ 0.08) were determined by reciprocal lattice maps around Al1−xSixN (0002) and (11–24) reflections. The measured lattice constants obtained directly from as-grown Al1−xSixN layers were scattered because they include the factor of residual strain. Therefore, the lattice constants in the strain-free case were calculated from the measured lattice constants taking the residual strain into account. We found that the a-axis and c-axis lattice constants of the strain-free Al1−xSixN linearly decreased with the Si content as a0 = 3.1113−0.1412x (Å) and c0 = 4.9814−0.2299x (Å). Further, we obtained the bond length as d0 = 1.86818−0.0862x (Å). The bond length is nearly equal to the interpolation between the Al–N bond and the Si–N bond. © 2001 American Institute of Physics.
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61.66.Fn Inorganic compounds
68.55.-a Thin film structure and morphology
61.50.Lt Crystal binding; cohesive energy
68.60.Bs Mechanical and acoustical properties
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