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15 Dec 2005

Volume 98, Issue 12, Articles (12xxxx)

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Dielectric breakdown mechanisms in gate oxides

Salvatore Lombardo, James H. Stathis, Barry P. Linder, Kin Leong Pey, Felix Palumbo, and Chih Hang Tung

J. Appl. Phys. 98, 121301 (2005); http://dx.doi.org/10.1063/1.2147714 (36 pages) | Cited 75 times

Online Publication Date: 29 December 2005

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In this paper we review the subject of oxide breakdown (BD), focusing our attention on the case of the gate dielectrics of interest for current Si microelectronics, i.e., Si oxides or oxynitrides of thickness ranging from some tens of nanometers down to about 1 nm. The first part of the paper is devoted to a concise description of the subject concerning the kinetics of oxide degradation under high-voltage stress and the statistics of the time to BD. It is shown that, according to the present understanding, the BD event is due to a buildup in the oxide bulk of defects produced by the stress at high voltage. Defect concentration increases up to a critical value corresponding to the onset of one percolation path joining the gate and substrate across the oxide. This triggers the BD, which is therefore believed to be an intrinsic effect, not due to preexisting, extrinsic defects or processing errors. We next focus our attention on experimental studies concerning the kinetics of the final event of BD, during which the gate leakage increases above acceptable levels. In conditions of intrinsic BD, the leakage increase is due to the growth of damage within the oxide in localized regions. Observations concerning this damage are reviewed and discussed. The measurement of the current, voltage, and power dissipated during the BD transient are also reported and discussed in comparison with the data of structural damage. We then describe the current understanding concerning the dependence of the BD current transient on the conditions of electric field and voltage. In particular, as the oxide thickness and, as a consequence, the voltage levels used for accelerated reliability tests have decreased, the BD transient exhibits a marked change in behavior. As the stress voltage is decreased below a threshold value, the BD transient becomes slower. This recently discovered phenomenon has been termed progressive BD, i.e., a gradual growth of the BD spot and of the gate leakage, with a time scale that under operation conditions can be a large fraction of the total time to BD. We review the literature on this phenomenon, describing the current understanding concerning the dependence of the effect on voltage, temperature, oxide thickness, sample geometry, and its physical structure. We also discuss the possible relation to the so-called soft oxide BD mode and propose a simpler, more consistent terminology to describe different BD regimes. The last part of the paper is dedicated to exploratory studies, still at the early stages given the very recent subject, concerning the impact on the BD of materials for the metal-oxide-semiconductor gate stack and, in particular, metal gates.
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77.22.Jp Dielectric breakdown and space-charge effects
01.30.Rr Surveys and tutorial papers; resource letters
85.30.Tv Field effect devices
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Superprism phenomena in polymeric woodpile structures

Jesper Serbin and Min Gu

J. Appl. Phys. 98, 123101 (2005); http://dx.doi.org/10.1063/1.2149163 (5 pages) | Cited 2 times

Online Publication Date: 27 December 2005

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An analysis of the optical properties of photonic woodpile structures is presented. We demonstrate large superprism phenomena inside polymeric woodpile structures having a refractive index of less than n = 1.6. Due to the low contrast in refractive indices the structures investigated do not possess a complete photonic band gap. Nevertheless, their photonic band structures show strong anisotropy at frequencies slightly above the band gap in the (Γ-X) direction, leading to an extreme sensitivity to the angle and the frequency of the incident light in the propagation direction inside the crystal. Furthermore, if the woodpile structure is arranged in a prism-like shape, the transmitted beam outside the crystal shows a strong sensitivity to the frequency and angle of the incident light.
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42.70.Qs Photonic bandgap materials
42.25.Bs Wave propagation, transmission and absorption

Liquid-crystal diffraction gratings using polarization holography alignment techniques

Gregory P. Crawford, James N. Eakin, Marc D. Radcliffe, Andrew Callan-Jones, and Robert A. Pelcovits

J. Appl. Phys. 98, 123102 (2005); http://dx.doi.org/10.1063/1.2146075 (10 pages) | Cited 56 times

Online Publication Date: 27 December 2005

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A method of patterning surfaces for liquid-crystal alignment using a polarization holography exposure on a linear photopolymerizable polymer alignment layer is demonstrated. Three configurations are demonstrated which include registered planar-periodic surface boundary conditions on both surfaces (true polarization gratings), planar-periodic and uniform planary surface boundary conditions, and planar-periodic and homeotropic boundary conditions. Two-dimensional polarization gratings are also demonstrated by orientating planar-periodic alignment layers orthogonally. Passive polarization gratings are also demonstrated using reactive mesogens to capture the periodic order indefinitely. The underlying structure of the configuration is discussed, including the nature of their switching transition (threshold or thresholdless), for all three configurations. A simple phenomenological model is presented to describe the Freedericksz transition for the registered planar-periodic boundary condition case.
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42.40.Eq Holographic optical elements; holographic gratings
42.79.Kr Display devices, liquid-crystal devices
42.40.Kw Holographic interferometry; other holographic techniques
42.70.Jk Polymers and organics

Three-dimensional optical modeling and optimizations of color filter liquid-crystal-on-silicon microdisplays

Baolong Zhang, Hoi-Sing Kwok, and Ho-Chi Huang

J. Appl. Phys. 98, 123103 (2005); http://dx.doi.org/10.1063/1.2149494 (9 pages) | Cited 15 times

Online Publication Date: 28 December 2005

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We have developed a three-dimensional (3D) optical modeling of small color pixels in color filter liquid-crystal-on-silicon (CF-LCOS) microdisplays. The 3D optical modeling includes a LC electromechanical analysis of color LC cells, a calculation of optical reflectance using the extended Jones matrix, and a standard RGB (sRGB) representation of the optical reflectance in the pixel array. The simulated optical reflectance agreed well with the experiments. With this 3D optical analysis as a tool, the relation of the lateral color fringing field with pixel size and thickness of color filter were studied. Minimizations of the fringing field in the CF-LCOS microdisplays were obtained by pixel arrangement, rubbing direction, and LC mode. The color purity of the CF-LCOS microdisplays could attain 63% National Television System Committee (NTSC) level for a typical pixel size of 15 μm. With an optimized LC mode, the color purity could still be maintained at 54% NTSC level even when the pixel size was reduced to 9 μm. This enabled the feasibility of the CF-LCOS microdisplays for very high-resolution display applications.
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42.79.Ci Filters, zone plates, and polarizers
42.79.Kr Display devices, liquid-crystal devices

Scattering model for understanding the optical behavior of powders embedded in a plastic matrix

L. Beluze, C. Varona, B. Viana, J. M. Tarascon, and M. Morcrette

J. Appl. Phys. 98, 123104 (2005); http://dx.doi.org/10.1063/1.2138797 (7 pages) | Cited 3 times

Online Publication Date: 28 December 2005

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Powders embedded in a plastic matrix could exhibit attractive optical properties as flexible emissivity modulator. The choice of the materials to obtain significant contrasts in reflectivity and consequently in emissivity is discussed using a two-flux model to simulate qualitatively the performance of high- and low-absorptive materials. For high-absorptive compounds, such as metallic powders, high reflectivity values can be obtained with a large particle size of a highly conductive metal. The reflectivity value for spherical copper particles at about 2 μm (about 70%) could be enhanced by 20% in the case of platelet-shaped particles. For low-absorptive compounds, reflection increases with decreasing absorption. For these compounds, larger particles do not lead to higher reflectivity values. An optimum size of the particles has to be found in order to obtain a maximum reflectivity value. Additional considerations on the electronic delocalization are also presented.
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78.66.-w Optical properties of specific thin films
61.43.Gt Powders, porous materials

Effect of distorted illumination waves on coherent diffraction microscopy

Yoshiki Kohmura, Yoshinori Nishino, Tetsuya Ishikawa, and Jianwei Miao

J. Appl. Phys. 98, 123105 (2005); http://dx.doi.org/10.1063/1.2149499 (6 pages) | Cited 6 times

Online Publication Date: 30 December 2005

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Coherent diffraction microscopy requires a well-defined illumination wave such as a plane wave on a specimen. Experimentally, a small pinhole or a focused beam is often used to reduce the illumination area but they unavoidably distort the waves. The distortion of the illumination wave causes artifacts in the phase retrieval of oversampled diffraction patterns. Using computer simulations, we searched for the conditions where strong artifacts arise by changing the Fresnel number, pinhole size, alignment error and photon statistics. The experimental setup with Fresnel number of around 1 and smaller than 1 realized a small reconstruction error when the pinhole radius is larger than a few times the specimen size. These conditions are suitable for the rotation of specimens for the three-dimensional (3D) observations. Such investigation will have an impact in the design of coherent diffraction microscopes for the 3D characterization of nanoscale materials and biological systems using the third generation synchrotron radiation and future x-ray free-electron lasers.
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07.85.Tt X-ray microscopes
07.85.Jy Diffractometers
61.05.cp X-ray diffraction
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Growth of ZnO thin films by ultraviolet pulsed-laser ablation: Study of plume dynamics

A. Klini, A. Manousaki, D. Anglos, and C. Fotakis

J. Appl. Phys. 98, 123301 (2005); http://dx.doi.org/10.1063/1.2149498 (8 pages) | Cited 12 times

Online Publication Date: 22 December 2005

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A comparative study is presented on the growth of ZnO thin films by ultraviolet pulsed-laser deposition using nanosecond and femtosecond pulses at 248 nm. Different experimental parameters were examined including substrate temperature, oxygen ambient pressure, and laser energy density at the target. At optimum conditions the films grown exhibited high optical transmittance and a preferential orientation along the c axis, both with nanosecond and femtosecond irradiations. However, those produced with the ultrashort laser pulses are composed of smaller dimension crystallites, with higher angular distribution on the substrate and higher roughness, suggesting different film growth mechanism. This was investigated by studying the plume dynamics of the ejected material accompanying laser ablation of the ZnO targets by means of spatially and time-resolved optical emission spectroscopy in order to obtain information on the nature of the ejected species and the distribution of their propagation velocities. Employing irradiation conditions, typically used in the deposition experiments, emission lines assigned to electronically excited neutral zinc atoms (Zn*) were observed both in the case of nanosecond and femtosecond pulses, while the latter additionally gave rise to emission attributable to zinc ions (Zn+*). The mean propagation velocities measured suggest the presence of highly energetic Zn cations in the case of femtosecond ablation, which are likely to be responsible for defect formation on the films, disturbing the smooth growth of crystallites that takes place under nanosecond irradiation, and giving rise to films composed of smaller crystallites with higher mosaicity and roughness.
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81.05.Dz II-VI semiconductors
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth
78.66.Hf II-VI semiconductors
78.47.-p Spectroscopy of solid state dynamics
68.55.-a Thin film structure and morphology

Compact soft x-ray source using Thomson scattering

Shigeru Kashiwagi, Ryunosuke Kuroda, Takashi Oshima, Fumio Nagasawa, Tomoaki Kobuki, Daisuke Ueyama, Yoshimasa Hama, Masakazu Washio, Kiminori Ushida, Hitoshi Hayano, and Junji Urakawa

J. Appl. Phys. 98, 123302 (2005); http://dx.doi.org/10.1063/1.2148619 (6 pages) | Cited 7 times

Online Publication Date: 28 December 2005

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A compact soft x-ray source using Thomson scattering, enabled by the combination of a picosecond laser and an electron rf gun, was developed aiming at biological studies such as those using an x-ray microscope. The x-ray source included both a photoinjector system and a picosecond laser system with a tabletop size of 2×2 m2. An infrared laser beam (λ0 = 1047 nm) was obtained from an all-solid-state mode-locked Nd:YLF laser system and injected into the photocathode of an accelerator system. A 4.2 MeV electron beam was generated from a laser-driven photocathode rf gun system. The residual laser beam was amplified up to about 4.2 mJ/pulse using a flash-lamp-pumped laser amplifier. Upon collision of the electron beam with the amplified laser beam, 300 eV soft x rays were generated by Thomson backscattering. The stable interaction between the two beams was achieved using the same seed laser pulse for irradiating the photocathode and the scattering process with laser photons.
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07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
07.85.Tt X-ray microscopes
29.25.Bx Electron sources
29.20.-c Accelerators
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Composition-dependent structural properties in ScGaN alloy films: A combined experimental and theoretical study

Costel Constantin, Muhammad B. Haider, David Ingram, Arthur R. Smith, Nancy Sandler, Kai Sun, and Pablo Ordejón

J. Appl. Phys. 98, 123501 (2005); http://dx.doi.org/10.1063/1.2140889 (8 pages) | Cited 8 times

Online Publication Date: 16 December 2005

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Experimental and theoretical results are presented regarding the incorporation of scandium into wurtzite GaN. Variation of the a and c lattice constants with Sc fraction in the low Sc concentration regime (0%–17%) are found that can be well explained by the predictions of first-principles theory. The calculations allow a statistical analysis of the variations of the bond lengths and bond angles as functions of Sc concentration. The results are compared to predictions from both a prior experimental study [Constantin et al., Phys. Rev. B 70, 193309 (2004)] and a prior theoretical study [Farrer and Bellaiche et al. Phys. Rev. B 66, 201203(R) (2002)] . It is found that the ScGaN lattice can be very well modeled as being wurtzitelike but with local lattice distortions arising from the incorporation of the Sc atoms. Effects of the addition of Sc on the stacking order for a large Sc fraction is also studied by high resolution transmission electron microscopy. The results show the existence of stacking faults, and induced stacking disorder. The explanation for the lattice constant variations is based on the effects of local lattice distortions and not related to the stacking faults.
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05.70.Fh Phase transitions: general studies
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.05.Bx Metals, semimetals, and alloys
81.05.Hd Other semiconductors
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Efficacy of single and double SiNx interlayers on defect reduction in GaN overlayers grown by organometallic vapor-phase epitaxy

F. Yun, Y. -T. Moon, Y. Fu, K. Zhu, Ü. Ozgür, H. Morkoç, C. K. Inoki, T. S. Kuan, Ashutosh Sagar, and R. M. Feenstra

J. Appl. Phys. 98, 123502 (2005); http://dx.doi.org/10.1063/1.2142074 (8 pages) | Cited 8 times

Online Publication Date: 16 December 2005

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We report on the growth of and evolution of defects in GaN epilayers having single- and double-layer SiNx nanoporous insertion layers. The SiNx was formed in situ in the growth chamber of an organometallic vapor-phase epitaxy system by simultaneous flow of diluted silane and ammonia. The GaN epilayers and SiNx interlayers were grown on 6H-SiC substrates using three different nucleation layers, namely, low-temperature GaN, high-temperature GaN, and high-temperature AlN nucleation layers. X-ray-diffraction rocking curves and cross-sectional and plan-view transmission electron microscope analyses indicated that a nanoporous SiNx layer can reduce the dislocations density in the GaN overgrown layer to ∼ 3×108 cm−2 range; below this level the defect blocking effect of SiNx would saturate. Therefore the insertion of a second SiNx layer becomes much less effective in reducing dislocations, although it continues to reduce the point defects, as suggested by time-resolved photoluminescence measurements. The insertion of SiNx interlayers was found to improve significantly the mechanical strength of the GaN epilayers resulting in a much lower crack line density.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Kk Vapor phase epitaxy; growth from vapor phase
61.43.Gt Powders, porous materials
78.55.Cr III-V semiconductors
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials
68.60.Bs Mechanical and acoustical properties

Dynamic response of binders; teflon, estane™ and Kel-F-800™

N. K. Bourne and G. T. Gray

J. Appl. Phys. 98, 123503 (2005); http://dx.doi.org/10.1063/1.2143118 (9 pages) | Cited 4 times

Online Publication Date: 20 December 2005

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It is important to understand the mechanical behavior of polymeric materials that have a range of industrial and defence applications. In particular, several are used as the binder phase in plastic bonded explosives and propellants. Binder materials used in energetics need to be understood first to ensure their long-term, high-temperature stability when cycled during service, and second to ensure that the accidental loading of such materials leaves the explosives crystals within safe to handle. This work presents experimental data on equation-of-state and shock-induced damage evolution of the polymers teflon, estane™ and Kel-F-800™. The tensile (spall) strength at high rate was quantified using real-time laser interferometric techniques on each polymer as a function of impact stress. Measurements were made of the lateral stress under impact using manganin gauges, which, when combined with longitudinal data, allowed investigation of the shear strength of each polymer. An increase of shear strength, determined in this manner, was noted for two polymers and comparisons are drawn with others investigated using the same technique. A discussion of the mechanical response of these polymers is presented.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
61.41.+e Polymers, elastomers, and plastics

Defect accumulation during channeled erbium implantation into GaN

Bert Pipeleers, Susan M. Hogg, and André Vantomme

J. Appl. Phys. 98, 123504 (2005); http://dx.doi.org/10.1063/1.2143120 (6 pages) | Cited 23 times

Online Publication Date: 20 December 2005

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Gallium nitride films epitaxially grown on sapphire, were irradiated at room temperature with 80 keV math or 170 keV math ions to fluences ranging from 1×1013 cm−2 to 1×1015 cm−2. The defects induced by ion implantation (as a result of the nuclear energy transfer) generate a perpendicular elastic strain in the hexagonal GaN lattice. The accumulation of lattice damage and lattice deformation were investigated for Er ions impinging along the GaN〈0001〉 axis, i.e., channeled implantation, and compared to random implantation, i.e., the conventional geometry in which the ion beam is tilted 10° off the GaN c axis. For this purpose, Rutherford backscattering and channeling spectrometry and high-resolution x-ray diffraction were used. The defect concentration and the maximum perpendicular strain exhibit the same increasing trend with the ion fluence. Three regimes can be distinguished for both implantation geometries, for low fluences (corresponding to a value below 1 displacement per atom in case of random implantation), the defect concentration remains low due to an effective dynamic annealing process. In the second fluence regime, the defect concentration rises sharply, which is characteristic for nucleation-limited amorphization and finally, a third regime is found where layer-by-layer amorphization of the implanted area starts from the surface. The onset of the steep increase in the case of implantations along the GaN c axis is found at a significantly higher erbium fluence compared to random implantation.
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81.05.Ea III-V semiconductors
81.05.Gc Amorphous semiconductors
61.72.uj III-V and II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
61.80.Jh Ion radiation effects
61.82.Fk Semiconductors
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
68.49.Sf Ion scattering from surfaces (charge transfer, sputtering, SIMS)
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis
61.72.Cc Kinetics of defect formation and annealing

Annealing effect on the structural and magnetic properties of La0.7Sr0.3MnO3 films

Tong Li, Bo Wang, Hongyun Dai, Yongsheng Du, Hui Yan, and Yanpin Liu

J. Appl. Phys. 98, 123505 (2005); http://dx.doi.org/10.1063/1.2148621 (3 pages) | Cited 13 times

Online Publication Date: 21 December 2005

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The effects of annealing on the structural and magnetic properties of La0.7Sr0.3MnO3 (LSMO) films sputtered on the (100) LaAlO3 substrate at 600 °C, have been studied by using x-ray diffraction, Raman spectroscopy, and superconducting quantum interference device magnetometers. After annealing at high temperatures, a lattice contraction attributed to the oxygen absorption was observed. The procedure was accompanied by the lattice ordering and the vanishing of Jahn-Teller (JT) distortion evidenced by Raman spectra. As a result, both the magnetic moment and Curie temperature (Tc) were enhanced and the coercivity was dramatically reduced in the annealed LSMO films.
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68.55.A- Nucleation and growth
61.50.−f

Transition from amorphous to crystalline beta phase in co-sputtered FeSi2 films as a function of temperature

M. Milosavljević, G. Shao, M. A. Lourenço, R. M. Gwilliam, K. P. Homewood, S. P. Edwards, R. Valizadeh, and J. S. Colligon

J. Appl. Phys. 98, 123506 (2005); http://dx.doi.org/10.1063/1.2148629 (6 pages) | Cited 9 times

Online Publication Date: 21 December 2005

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A study of the stability of amorphous FeSi2 films and their transition to a crystalline phase as a function of deposition or annealing temperature is presented. Stoichiometric FeSi2 films, 300–400 nm thick, were deposited on (100) Si substrates by co-sputtering of Fe and Si. It was found that the films grow in an amorphous form for the substrate temperature ranging from room temperature to 200 °C, while from 300–700 °C, they grow in form of a crystalline β‐FeSi2 phase. In a postdeposition 30 min heat treatments, the layers retain the amorphous structure up to 400 °C, transforming to the crystalline β phase at 500–700 °C. The results are discussed in the frame of the existing models, and compared to those found in the literature. It is shown that in as-deposited films, the growth is controlled by surface diffusion, the crystalline layers growing in a columnar structure strongly correlated to the Si substrate. Postdeposition treatments induce a random crystallization controlled by bulk diffusion, the resulting structure not being influenced by the substrate. The results of this work contribute to a better understanding of the processes involved in a transition of amorphous FeSi2 films to a crystalline phase, and provide a basis to determine the processing parameters in potential applications of this promising semiconducting material.
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68.37.−d
64.70.K- Solid-solid transitions

Ultrasonic microspectroscopy of congruent LiNbO3 crystals

Jun-ichi Kushibiki, Yuji Ohashi, and Junji Hirohashi

J. Appl. Phys. 98, 123507 (2005); http://dx.doi.org/10.1063/1.2146076 (8 pages) | Cited 1 time

Online Publication Date: 21 December 2005

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Experimental procedures for evaluating the compositional homogeneity of LiNbO3 single crystals were developed using the line-focus-beam ultrasonic material characterization system, and the true congruent composition was determined. First, the relationships among leaky surface acoustic wave (LSAW) velocities (VLSAW), chemical compositions, Curie temperatures, densities, and lattice constants were investigated for crystal evaluation, using X-, Y-, and Z-cut substrates prepared from three LiNbO3 crystals grown along the crystallographic Z axis with different Li2O contents of 48.0, 48.5, and 49.0 mol %. We selected VLSAW for Z-cut Y-axis propagating (ZY) LiNbO3 with the highest sensitivity to chemical composition changes [0.0253 Li2O mol %/(m/s)], and also VLSAW for Y-cut X-axis propagating (YX) LiNbO3 [0.0464 Li2O mol %/(m/s)] that was advantageous for detailed evaluation of distributions in the chemical composition along the pulling direction as well as the diameter direction.The congruent composition was estimated to be 48.477 Li2O mol %. Next, the homogeneities of the three above-mentioned crystals and a commercially available LiNbO3 crystal with a nominally congruent composition were evaluated using data for YX-LiNbO3 specimens. Consequently, compositional changes were observed clearly as VLSAW changes. The commercial crystal specimen had a gradient of −0.0046 (m/s)/mm from top to bottom. The VLSAW variations for the entire examined region (60×36 mm2) exhibited a maximum change of 0.55 m/s, corresponding to the chemical composition change of 0.025 Li2O mol %. The relationships between the chemical compositions and the acoustical physical constants (the elastic, piezoelectric, and dielectric constants, and density) were also provided.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
43.35.Sx
68.35.Iv Acoustical properties
77.22.Ch Permittivity (dielectric function)
77.65.Bn Piezoelectric and electrostrictive constants
77.65.Dq Acoustoelectric effects and surface acoustic waves (SAW) in piezoelectrics
77.80.B- Phase transitions and Curie point
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity

Depth dependence of defect density and stress in GaN grown on SiC

N. Faleev, H. Temkin, I. Ahmad, M. Holtz, and Yu. Melnik

J. Appl. Phys. 98, 123508 (2005); http://dx.doi.org/10.1063/1.2141651 (7 pages) | Cited 12 times

Online Publication Date: 21 December 2005

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We report high resolution x-ray diffraction studies of the relaxation of elastic strain in GaN grown on SiC(0001). The GaN layers were grown with thickness ranging from 0.29 to 30 μm. High level of residual elastic strain was found in thin (0.29 to 0.73 μm thick) GaN layers. This correlates with low density of threading screw dislocations of 1-2×107 cm−2, observed in a surface layer formed over a defective nucleation layer. Stress was found to be very close to what is expected from thermal expansion mismatch between the GaN and SiC. A model based on generation and diffusion of point defects accounts for these observations.
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61.05.cp X-ray diffraction
61.72.Dd Experimental determination of defects by diffraction and scattering
68.55.−a

Mechanism of a remarkable enhancement in the light emission from nanocrystalline porous silicon annealed in high-pressure water vapor

Bernard Gelloz and Nobuyoshi Koshida

J. Appl. Phys. 98, 123509 (2005); http://dx.doi.org/10.1063/1.2147847 (7 pages) | Cited 20 times

Online Publication Date: 22 December 2005

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To clarify the effect of surface passivation on the optical properties of nanocrystalline porous silicon (PS), the photoluminescence (PL) characteristics of PS have been investigated by employing a high-pressure water vapor annealing (HWA). PS samples with various porosities were prepared on (100)-oriented p-type (4 Ω cm) single-crystalline silicon wafers by electrochemical anodization. Some samples were then electrochemically oxidized. The HWA treatment was then applied to the prepared PS samples at 0.5–3 MPa and 200–300 °C for 2–3 h. The PL intensities, spectra, and dynamics after HWA were measured in relation to surface analyses by Fourier-transform-infrared (FTIR) spectroscopy. It is shown that the HWA treatment leads to a drastic enhancement in both the PL efficiency and stability. Under the optimum condition, the PS sample exhibits an extremely high external quantum efficiency of 23% at room temperature. According to the FTIR spectra analyses, silicon nanocrystallites in HWA-treated PS are covered with a high-quality SiO2 tissue. The PL decays are found to be longer than those of as-prepared PS, and become closer to a single-exponential behavior near the PL peak wavelength. The observed high efficiency and stability of PL emission from HWA-treated PS is attributed to (i) suppression of nonradiative surface defect density, (ii) uniform passivation by unstrained thin oxides, and (iii) strong localization of excitons in silicon nanocrystals. This low-temperature treatment is very useful for obtaining highly efficient and stable luminescent PS and devices.
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78.55.Mb Porous materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.40.Vw Pressure treatment
81.65.Mq Oxidation

Thermoelectric properties of the CoSi1−xGex alloys

Y. K. Kuo, K. M. Sivakumar, S. J. Huang, and C. S. Lue

J. Appl. Phys. 98, 123510 (2005); http://dx.doi.org/10.1063/1.2149185 (5 pages) | Cited 14 times

Online Publication Date: 23 December 2005

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The effects of Ge partial substitution for Si on the thermoelectric properties of CoSi1−xGex alloys were investigated by means of thermal and electrical transport measurements. Electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements were performed on a series of CoSi1−xGex alloys with x varying from 0 to 0.15. A substantial decrease in electrical resistivity and lattice thermal conductivity was noticed with increasing substitution level, whereas the Seebeck coefficient shows a weak variation with respect to Ge concentration. The thermoelectric efficiency was found to be enhanced by an order of magnitude in CoSi with Ge substitution. These observations were interpreted on the basis of the changes in the electronic band structure induced by Ge substitution.
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72.15.Jf Thermoelectric and thermomagnetic effects
72.15.Eb Electrical and thermal conduction in crystalline metals and alloys

Relationship between the physical and structural properties of NbzSiyNx thin films deposited by dc reactive magnetron sputtering

R. Sanjinés, M. Benkahoul, C. S. Sandu, P. E. Schmid, and F. Lévy

J. Appl. Phys. 98, 123511 (2005); http://dx.doi.org/10.1063/1.2149488 (6 pages) | Cited 14 times

Online Publication Date: 23 December 2005

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The optical and electrical properties of NbzSiyNx thin films deposited by dc reactive magnetron sputtering have been investigated as a function of the Si content (CSi). Optical properties were studied by both specular reflectivity and spectroscopic ellipsometry. Electrical resistivity was measured by the van der Pauw method at room temperature and as a function of the temperature down to 10 K. Both the optical and electrical properties of NbzSiyNx films are closely related with the chemical composition and microstructure evolution caused by Si addition. For CSi up to 4 at. % the Si atoms are soluble in the lattice of the NbN crystallites. In this compositional regime, the optical and electrical properties show little dependence on the Si content. Between 4 and 7 at. % the surplus of Si atoms segregates at the grain boundaries, builds an insulating SiNx layer, and originates important modifications in the optical and electrical properties of these films. Further increase of CSi leads to the formation of nanocomposite structures. The electrical properties of these films are well described by the grain-boundary scattering model with low probability for electrons to cross the grain boundary. The appearance of the intragranular-insulating SiNx layer and the reduction of the grain size are noticed in the dielectric function mainly as a strong damping of the plasma oscillation.
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74.78.-w Superconducting films and low-dimensional structures
74.25.F- Transport properties
74.25.Gz Optical properties
81.15.Cd Deposition by sputtering
64.75.-g Phase equilibria
61.72.Mm Grain and twin boundaries
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Interfacial layer formations between Cu(In,Ga)Se2 and InxSy layers

D. Abou-Ras, G. Kostorz, A. Strohm, H.-W. Schock, and A. N. Tiwari

J. Appl. Phys. 98, 123512 (2005); http://dx.doi.org/10.1063/1.2149166 (7 pages) | Cited 9 times

Online Publication Date: 27 December 2005

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Cu(In,Ga)Se2 (CIGS) thin-film solar cells with InxSy buffer layers deposited by physical vapor deposition yield efficiencies of up to 14.8%. For substrate temperatures during the InxSy deposition ranging from 23 to 200 °C, air annealing of the completed solar cells leads to an improvement of the photovoltaic performance. However, at substrate temperatures of 300 °C, the efficiencies are practically zero, and air annealing does not improve this value. To understand the effects of substrate temperature and air annealing on the CIGS/InxSy interfaces of the solar cells, these interfaces have been studied by means of bright-field and high-resolution transmission electron microscopy, selected-area electron diffraction (SAED), and energy-dispersive x-ray spectrometry (EDX). It is shown that air annealing leads to a substantial Cu depletion on the CIGS side of the CIGS/InxSy interface, probably inducing the formation of a compositionally graded interface between the buffer and CIGS. For the 300 °C sample, CuIn5S8 formed instead of InxSy, as identified by means of SAED and EDX. The large density of vacancies and defects in the spinel-type cubic crystal structure of CuIn5S8 provides a large number of recombination centers at the heterojunction of the solar cell, thus deteriorating considerably its photovoltaic performance.
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84.60.Jt Photoelectric conversion
68.35.Ct Interface structure and roughness
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.72.Cc Kinetics of defect formation and annealing
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
61.72.J- Point defects and defect clusters

Structure, twinning behavior, and interface composition of epitaxial Si(111) films on hex-Pr2O3(0001)/Si(111) support systems

T. Schroeder, P. Zaumseil, G. Weidner, G. Lupina, Ch. Wenger, H.-J. Müssig, and P. Storck

J. Appl. Phys. 98, 123513 (2005); http://dx.doi.org/10.1063/1.2149186 (6 pages) | Cited 15 times

Online Publication Date: 27 December 2005

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The structure of epitaxial Si overlayers on a hexagonal Pr2O3(0001)/Si(111) substrate system was investigated by a combination of x-ray reflectivity, specular x-ray diffraction, off-specular grazing incidence x-ray diffraction, and transmission electron microscopy. The Pr2O3 film grows on the Si(111) substrate in the (0001)-oriented hexagonal phase matching the in-plane symmetry by aligning the [10math0] oxide along the bulk [01math] Si direction. The hexagonal Pr2O3(0001) surface induces the growth of [111]-oriented cubic-Si epilayers exhibiting a microstructure which is composed of two types of domains. The ABC-stacked domains preserve the crystal orientation of the substrate, while the CBA-stacked domains are rotated by 180°. A depth profile of the chemical composition of the epi-Si/Pr2O3/Si(111) material stack was recorded by combining ion-beam sputtering techniques with x-ray photoelectron spectroscopy.
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68.55.-a Thin film structure and morphology
61.72.Mm Grain and twin boundaries
68.37.Lp Transmission electron microscopy (TEM)
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Study of Cu diffusion in porous dielectrics using secondary-ion-mass spectrometry

Oscar R. Rodriguez, William N. Gill, Joel L. Plawsky, Ting. Y. Tsui, and Stephan Grunow

J. Appl. Phys. 98, 123514 (2005); http://dx.doi.org/10.1063/1.2149501 (6 pages) | Cited 4 times

Online Publication Date: 27 December 2005

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Secondary-ion-mass spectrometry measurements were used to study Cu diffusion in porous silica. The total concentration of Cu+ decreases with increasing porosity of the dielectric. This behavior is the combined result of both the chemistry and the morphology of the dielectric. The injection of Cu is triggered by outgassing of hydroxyl and water-related species from the dielectric; furthermore, the reduced available cross-sectional area of solid for diffusion, due to porosity, leads to reduced diffusion through the porous film. This suggests that surface diffusion does not play an important role in this process. The Cu+ concentration at the Cu/dielectric interface is on the order of 1023 at./m3, but decreases with time and exponentially with porosity, which suggests the occurrence of a chemical reaction at the interface. A model of molecular diffusion and ion drift that considers the porosity of the film is developed and the results are consistent with the experimental data.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
66.30.Ny Chemical interdiffusion; diffusion barriers
61.43.Gt Powders, porous materials
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
77.55.-g Dielectric thin films

Theory of ultrafast optical manipulation of electron spins in quantum wells

Jinshuang Jin and Xin-Qi Li

J. Appl. Phys. 98, 123515 (2005); http://dx.doi.org/10.1063/1.2140868 (6 pages) | Cited 2 times

Online Publication Date: 27 December 2005

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Based on a multiparticle-state stimulated Raman adiabatic passage approach, a comprehensive theoretical study of the ultrafast optical manipulation of electron spins in quantum wells is presented. In addition to corroborating experimental findings [Gupta et al., Science 292, 2458 (2001)] , we improve the expression for the optical-pulse-induced effective magnetic field, in comparison with the one obtained via the conventional single-particle ac Stark shift. Further study of the effect of hole-spin relaxation reveals that, while the coherent optical manipulation of electron spin in undoped quantum wells would deteriorate in the presence of relatively fast hole-spin relaxation, the coherent control in doped systems can be quite robust against decoherence. The implications of the present results on quantum dots will also be discussed.
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78.47.-p Spectroscopy of solid state dynamics
78.67.−n

Influence of interfaces on the storage of ion-implanted He in multilayered metallic composites

T. Höchbauer, A. Misra, K. Hattar, and R. G. Hoagland

J. Appl. Phys. 98, 123516 (2005); http://dx.doi.org/10.1063/1.2149168 (7 pages) | Cited 42 times

Online Publication Date: 27 December 2005

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We studied ion beam mixing and He accumulation in Cu/Nb multilayer thin films after 33 keV He implantation at room temperature to a dose of 1.5×1017 atoms/cm2. Multilayered thin films consisting of alternating Cu and Nb layers were produced by magnetron sputtering. Two types of samples, one with an individual layer thickness of 4 nm and another with 40 nm were examined. The Cu/Nb samples were analyzed in the as-deposited state, after He ion implantation, as well as after post-implantation annealing. The ion beam mixing of the interface structure was monitored by Rutherford backscattering spectrometry and cross-section transmission electron microscopy imaging. Elastic recoil detection analysis was performed to examine the helium concentration depth distribution. Scanning electron microscopy was employed to investigate He blister formation upon annealing. A comparison of the results deduced from the methods listed above reveals a very high morphological stability of the nanolayered structure. The nanolayered structure of the Cu/Nb multilayer thin films is retained. He bubbles were observed to reside within the layers but more so at the Cu/Nb incoherent interfaces.
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81.05.Bx Metals, semimetals, and alloys
61.72.up Other materials
61.80.Jh Ion radiation effects
61.72.S- Impurities in crystals
68.35.Ct Interface structure and roughness
61.82.Bg Metals and alloys
61.72.Qq Microscopic defects (voids, inclusions, etc.)
68.37.Lp Transmission electron microscopy (TEM)
68.49.Sf Ion scattering from surfaces (charge transfer, sputtering, SIMS)
81.15.Cd Deposition by sputtering
81.40.Gh Other heat and thermomechanical treatments

Modeling nematic liquid crystals in the neighborhood of edges

Hans Desmet, Kristiaan Neyts, and Roel Baets

J. Appl. Phys. 98, 123517 (2005); http://dx.doi.org/10.1063/1.2143122 (6 pages) | Cited 12 times

Online Publication Date: 27 December 2005

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We present a method to simplify simulation techniques for determining liquid-crystal director orientation in geometries with sharply defined edges and for surfaces with strong anchoring without preferred direction. We show analytically that close to such edges, the liquid crystal can be described by just a few parameters. After examining the solutions, it is explained how these results can avoid the numerical problems that arise when simulating such configurations.
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61.30.-v Liquid crystals
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