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28 Mar 2005

Volume 86, Issue 13, Articles (13xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 86, 131114 (2005); http://dx.doi.org/10.1063/1.1889243 (3 pages)

R. Chan, M. Feng, N. Holonyak, and G. Walter
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Pinch off of nanopipes under electron irradiation in GaN

F. Pailloux, J. Colin, J. F. Barbot, and J. Grilhé

Appl. Phys. Lett. 86, 131908 (2005); http://dx.doi.org/10.1063/1.1894591 (3 pages) | Cited 5 times

Online Publication Date: 21 March 2005

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Long hollow nanopipes in epitaxial GaN thin films have been found to evolve under electron irradiation into the so-called bamboolike structure. The transition from the single nanopipe into a chain of pinholes is described in terms of morphological instability driven by the differences in the surface energy of crystallographic planes.
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68.55.-a Thin film structure and morphology
61.46.-w Structure of nanoscale materials
61.80.Fe Electron and positron radiation effects
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.82.Fk Semiconductors
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Self-organized nanocolumnar structure in superhard TiB2 thin films

P. H. Mayrhofer, C. Mitterer, J. G. Wen, J. E. Greene, and I. Petrov

Appl. Phys. Lett. 86, 131909 (2005); http://dx.doi.org/10.1063/1.1887824 (3 pages) | Cited 22 times

Online Publication Date: 21 March 2005

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TiB2 thin films are well known for their high hardness which makes them useful for wear-resistant applications. Overstoichiometric TiB2 deposited at 300 °C by nonreactive sputtering has been shown to exhibit superhardness (H ≥ 40 GPa), while the hardness of their bulk stoichiometric counterparts is ∼ 25 GPa. We show, using high-resolution transmission electron microscopy, that overstoichiometric TiB2.4 layers have a complex self-organized columnar nanostructure. The ∼ 20 nm wide columns, encapsulated in excess B and oriented along 0001, consist of a bundle of ∼ 5 nm diameter TiB2 subcolumns separated by an ultrathin B-rich tissue phase. The nanocolumnar structure, which is thermally stable to postannealing temperatures up to 700 °C, inhibits nucleation and glide of dislocations during hardness indentation measurements, while the high cohesive strength of the B-rich tissue phase prevents grain-boundary sliding. The combination of these effects results in the observed superhardness of ∼ 60 GPa.
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68.37.Lp Transmission electron microscopy (TEM)
81.05.Mh Cermets, ceramic and refractory composites
68.60.-p Physical properties of thin films, nonelectronic
81.16.Dn Self-assembly

Temperature-dependent photoluminescence of nanocrystalline ZnO thin films grown on Si (100) substrates by the sol–gel process

Yang Zhang, Bixia Lin, Xiankai Sun, and Zhuxi Fu

Appl. Phys. Lett. 86, 131910 (2005); http://dx.doi.org/10.1063/1.1891288 (3 pages) | Cited 37 times

Online Publication Date: 22 March 2005

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Temperature-dependent photoluminescence (PL) of nanocrystalline ZnO thin films grown on Si (100) substrates using a sol–gel method has been investigated. From the PL spectra measured in 83–293 K, the excitonic emissions and their multiple-phonon replicas have been observed in ultraviolet region, and their origins have been identified. Moreover, it has been found that the temperature dependence of the free exciton peak position can be described by standard expression, and the thermal activation energy extracted from the temperature dependence of the free exciton peak intensity is about 101 meV.
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81.05.Dz II-VI semiconductors
78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
68.55.A- Nucleation and growth
63.20.K- Phonon interactions
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.55.-a Thin film structure and morphology
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
63.20.kk Phonon interactions with other quasiparticles

Suppression of hydrogen diffusion at the hydrogen-induced platelets in p-type Czochralski silicon

Y. L. Huang, Y. Ma, R. Job, and W. R. Fahrner

Appl. Phys. Lett. 86, 131911 (2005); http://dx.doi.org/10.1063/1.1896443 (3 pages) | Cited 4 times

Online Publication Date: 22 March 2005

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Hydrogen diffusion in p-type Czochralski silicon is investigated by combined Raman spectroscope, scanning electron microscope, and spreading resistance probe measurements. Exposure of silicon wafers to rf hydrogen plasma results in the formation of platelets. The increase of hydrogenation duration leads to the growth of the platelets and the reduction of the hydrogen diffusivity. The large platelets grow faster than the small ones. The growth of the platelets is based on the capture of hydrogen. The dependence of the hydrogen diffusivity upon the average size of the platelets suggests that the indiffusion of hydrogen is suppressed by the platelets.
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66.30.J- Diffusion of impurities
78.30.Am Elemental semiconductors and insulators
52.77.-j Plasma applications

Liquid-crystal photoalignment by photosensitive fluorinated poly(arylene ether)

Xiang-Dan Li, Zhen-Xin Zhong, Seung Hee Lee, Gilson Ghang, and Myong-Hoon Lee

Appl. Phys. Lett. 86, 131912 (2005); http://dx.doi.org/10.1063/1.1894604 (3 pages) | Cited 7 times

Online Publication Date: 22 March 2005

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We report a liquid-crystal (LC) photoalignment material with a high photosensitivity based on the fluorinated poly(arylene ether) containing a chalcone unit in the main chain. The fluorinated poly(arylene ether) exhibited defect-free homogeneous alignment of LCs upon irradiation of linearly polarized UV light for 10 s. Spectroscopic analyses revealed that [2+2] cycloaddition between the chalcone moieties generated the surface anisotropy to induce an efficient alignment of LCs.
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61.30.Vx Polymer liquid crystals
64.70.M- Transitions in liquid crystals
61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Pv Polymers, organic compounds
42.70.Df Liquid crystals

Changes of magnetic domain structure induced by temperature-variation and electron-beam irradiation in Pr0.5Sr0.5CoO3

M. Uchida, R. Mahendiran, Y. Tomioka, Y. Matsui, K. Ishizuka, and Y. Tokura

Appl. Phys. Lett. 86, 131913 (2005); http://dx.doi.org/10.1063/1.1887816 (3 pages) | Cited 8 times

Online Publication Date: 22 March 2005

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In situ observation of magnetic domain structures of Pr0.5Sr0.5CoO3 was made by Lorentz electron microscopy to reveal an unusual magnetic behavior around 120 K below the Curie temperature (TC ∼ 220 K). The observations in a cooling run clearly showed that the magnetic domain structure changes below about 90 K and that the transformed domain structure remains unchanged down to 20 K. Furthermore, it was found that an electron-beam irradiation in an electron microscope induces a reversible transformation of the magnetic domain structure at 20 K. Possible mechanisms of such magnetic-domain structural changes are discussed.
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75.50.Dd Nonmetallic ferromagnetic materials
75.60.Ch Domain walls and domain structure
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
61.80.Fe Electron and positron radiation effects
61.82.Ms Insulators
68.37.Lp Transmission electron microscopy (TEM)

Photorefractivity of Hafnium-doped congruent lithium–niobate crystals

Luca Razzari, Paolo Minzioni, Ilaria Cristiani, Vittorio Degiorgio, and Edvard P. Kokanyan

Appl. Phys. Lett. 86, 131914 (2005); http://dx.doi.org/10.1063/1.1895478 (3 pages) | Cited 31 times

Online Publication Date: 23 March 2005

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Transient and steady-state measurements of photoinduced birefringence in Hf-doped congruent lithium–niobate crystals are presented. Through a systematic investigation of crystals with different dopant concentrations, we find that Hf doping induces a progressive decrease of photorefraction up to a threshold concentration of about 4 mol %. Such a decrease is correlated to a corresponding increase in photoconductivity. The interpretation of measurements has to take into account heating effects that may be attributed to nonlinear absorption processes.
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78.20.Fm Birefringence
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
72.40.+w Photoconduction and photovoltaic effects
61.72.S- Impurities in crystals
78.20.N- Thermo-optic effects
78.20.nb Photothermal effects

Hexagonal close-packed Ni nanostructures grown on the (001) surface of MgO

W. Tian, H. P. Sun, X. Q. Pan, J. H. Yu, M. Yeadon, C. B. Boothroyd, Y. P. Feng, R. A. Lukaszew, and R. Clarke

Appl. Phys. Lett. 86, 131915 (2005); http://dx.doi.org/10.1063/1.1890472 (3 pages) | Cited 22 times

Online Publication Date: 23 March 2005

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We report the in situ microscopy observation of an unnatural phase of Ni, a highly strained hexagonal close-packed (hcp) form which we believe is stabilized by heteroepitaxial growth on the (001) face of MgO. We find that the nanosized hcp nickel islands transform into the normal face-centered cubic structure when the size of the islands exceeds a critical value (about 2.5 nm thick with a lateral size of ∼ 5 nm). The structural transition proceeds via a martensitic change in the stacking sequence of the close-packed planes. The formation of hcp Ni nanostructures with an unusually large crystallographic c/a ratio ( ∼ 6% larger than ideal hcp) is very interesting for spintronic and recording applications where large uniaxial anisotropies are desirable.
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81.07.Bc Nanocrystalline materials
81.05.Bx Metals, semimetals, and alloys
68.55.A- Nucleation and growth
61.46.-w Structure of nanoscale materials
81.16.-c Methods of micro- and nanofabrication and processing
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
64.70.Nd Structural transitions in nanoscale materials
64.70.K- Solid-solid transitions
68.55.-a Thin film structure and morphology
81.30.Kf Martensitic transformations
75.50.Ss Magnetic recording materials

Zinc blende GaAs films grown on wurtzite GaN∕sapphire templates

V. V. Chaldyshev, B. Nielsen, E. E. Mendez, Yu. G. Musikhin, N. A. Bert, Zh. Ma, and Todd Holden

Appl. Phys. Lett. 86, 131916 (2005); http://dx.doi.org/10.1063/1.1875759 (3 pages) | Cited 1 time

Online Publication Date: 23 March 2005

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1-μm-thick zinc-blende GaAs (111) films were grown by molecular-beam epitaxy on wurtzite GaN∕sapphire (0001) templates. In spite of a ∼ 20% lattice mismatch, epitaxial growth was realized, so that the GaAs films showed good adhesion and their surface had a larger mirror-like area with an average surface roughness of 10 nm. Transmission electron microscopy revealed a flat and abrupt epitaxial GaAs∕GaN interface with some nanocavities and a large number of dislocations. Reasonably good crystalline quality of the GaAs films was confirmed by Raman characterization. Spectroscopic ellipsometry showed sharp interference fringes and characteristic parameters in the range of 0.75–5.3 eV. Photoluminescence study revealed extended band tails and dominance of non-radiative carrier recombination.
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81.05.Ea III-V semiconductors
68.55.A- Nucleation and growth
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.-a Thin film structure and morphology
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.35.B- Structure of clean surfaces (and surface reconstruction)
78.30.Fs III-V and II-VI semiconductors
68.35.Ct Interface structure and roughness
68.35.Np Adhesion
68.37.Ps Atomic force microscopy (AFM)
68.37.Lp Transmission electron microscopy (TEM)

Electroreflectance studies of InAs quantum dots with InxGa1−xAs capping layer grown by metalorganic chemical vapor deposition

W.-H. Chang, Hsiang-Yu Chen, H.-S. Chang, W.-Y. Chen, T. M. Hsu, T.-P. Hsieh, J.-I. Chyi, and N.-T. Yeh

Appl. Phys. Lett. 86, 131917 (2005); http://dx.doi.org/10.1063/1.1894613 (3 pages) | Cited 13 times

Online Publication Date: 23 March 2005

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Electroreflectance spectroscopy was used to study the effect of InxGa1−xAs capping layer on InAs quantum dots grown by metalorganic chemical vapor deposition. The optical transitions of the quantum dots and the InxGa1−xAs capping layer were well resolved. The energy shifts in the InxGa1−xAs capping layer show a different trend as compared to a series of referent InxGa1−xAs quantum wells. These results support the concept of strain-driven alloy decomposition during the InxGa1−xAs layer overgrowth.
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81.07.Ta Quantum dots
81.05.Ea III-V semiconductors
78.67.Hc Quantum dots
78.20.Jq Electro-optical effects
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Phase-graded deposition of diamond-like carbon on nanotips by near-field induced chemical vapor deposition

J. Shi, Y. F. Lu, X. Y. Chen, R. S. Cherukuri, K. K. Mendu, H. Wang, and N. Batta

Appl. Phys. Lett. 86, 131918 (2005); http://dx.doi.org/10.1063/1.1887840 (3 pages) | Cited 3 times

Online Publication Date: 23 March 2005

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Diamond-like carbon (DLC) films were deposited on tungsten (W) tips under the KrF excimer laser in a laser chemical vapor deposition (LCVD) chamber. Raman spectroscopy showed that the deposited DLC films were phase-graded along the tips from the apexes. The DLC films were more diamondlike at or near the tip apexes. From numerical simulation, there is a strongly confined and enhanced optical field at the tip apexes. The simulation also indicates that there is an optical-field gradient from tip apexes to tip bodies. Therefore, the variations in the phases of deposited DLC films were attributed to the corresponding variations in local optical intensities along the tips. Hence, optical local near field was confirmed to be responsible to the DLC deposition.
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81.05.U- Carbon/carbon-based materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Fg Pulsed laser ablation deposition
78.66.Nk Insulators
78.30.Hv Other nonmetallic inorganics
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology

Ordering of Si0.55Ge0.45 islands on vicinal Si(001) substrates: Interplay between kinetic step bunching and strain-driven island growth

H. Lichtenberger, M. Mühlberger, and F. Schäffler

Appl. Phys. Lett. 86, 131919 (2005); http://dx.doi.org/10.1063/1.1896425 (3 pages) | Cited 16 times

Online Publication Date: 24 March 2005

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We synchronized the kinetic and the strain-driven growth instabilities in the SiGe/Si(001) heterosystem to implement a two-stage self-organization scheme for the fabrication of long-range-ordered SiGe islands. In the first step, a homoepitaxial Si buffer is grown under optimized step-bunching conditions on a 4° miscut Si(001) substrate. In the second step, SiGe is deposited under conditions that yield three-dimensional island growth in registry with the underlying step-bunching template. By varying the deposition temperature of the SiGe layer, the nucleation phase of the islands could be resolved: At 4° miscut along [110] the slope of the step-bunching areas is most favorable for their disintegration into trains of adjacent (math05) and (0math5) facets upon pseudomorphic overgrowth with SiGe. This strain-driven step meandering instability is a precursor of the SiGe islands, which evolve during further coarsening. At high enough deposition temperatures, we find face-centered, rectangular island ordering that is entirely based on self-organization.
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81.05.Hd Other semiconductors
81.15.Np Solid phase epitaxy; growth from solid phases
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)

Ion-irradiation-induced porosity in GaSb

S. M. Kluth, J. D. Fitz Gerald, and M. C. Ridgway

Appl. Phys. Lett. 86, 131920 (2005); http://dx.doi.org/10.1063/1.1896428 (3 pages) | Cited 11 times

Online Publication Date: 24 March 2005

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Porosity in GaSb induced by math ion irradiation has been investigated as a function of implant dose and temperature. Initially pores form in the implanted material which become elongated as they increase in size. With increasing implant dose, the structure continues to evolve into plates and finally a network of nanoscale rods. Swelling to 25 times the original implanted layer thickness has been observed. The temperature dependence of the minimum feature size has been established. The crystalline-to-amorphous and continuous-to-porous transformations proceed simultaneously. We suggest the latter results from the precipitation of interstitials at extended crystalline defects in preference to Frenkel pair recombination as potentially related to anomalous diffusion in GaSb.
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61.72.uj III-V and II-VI semiconductors
61.43.Gt Powders, porous materials
61.80.Jh Ion radiation effects
61.82.Fk Semiconductors
61.82.Rx Nanocrystalline materials
61.46.-w Structure of nanoscale materials
61.72.J- Point defects and defect clusters
61.72.Nn Stacking faults and other planar or extended defects

Two-dimensional colloidal crystals formed by thermophoresis and convection

Stefan Duhr and Dieter Braun

Appl. Phys. Lett. 86, 131921 (2005); http://dx.doi.org/10.1063/1.1888036 (3 pages) | Cited 20 times

Online Publication Date: 24 March 2005

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Temperature gradients can trap micrometer-sized particles into two-dimensional crystals. We form colloidal crystals from otherwise repellent 2 μm polystyrene beads in diverse thermal convection settings. Our experiments indicate that the accumulation is driven by particle thermophoresis. Particles move along the temperature gradient and are pushed out of the warm liquid to a cold wall. We find reduced accumulation for decreased surface temperature gradients and enhanced salt concentrations. Moreover, thermophoretic fluid dynamics calculations predict flat accumulation profiles with 107-fold enhanced concentrations that are consistent with our experiments. The accumulated crystals could be used as molecular sieves for microfluidic biotechnological applications. A natural environment for similar accumulations are pores of rock near hydrothermal vents.
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82.70.Dd Colloids
66.10.C- Diffusion and thermal diffusion
47.27.T- Turbulent transport processes

Phase stability tuning in the NbxZr1−xN thin-film system for large stacking fault density and enhanced mechanical strength

T. Joelsson, L. Hultman, H. W. Hugosson, and J. M. Molina-Aldareguia

Appl. Phys. Lett. 86, 131922 (2005); http://dx.doi.org/10.1063/1.1884743 (3 pages) | Cited 16 times

Online Publication Date: 25 March 2005

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The phase stability of hexagonal WC-structure and cubic NaCl-structure 4d transition metal nitrides was calculated using first-principles density functional theory. It is predicted that there is a multiphase or polytypic region for the 4d transition metal nitrides with a valence electron concentration around 9.5 to 9.7 per formula unit. For verification, epitaxial NbxZr1−xN (0 ⩽ x ⩽ 1) was grown by reactive magnetron sputter deposition on MgO(001) substrates and analyzed with transmission electron microscopy (TEM) and x-ray diffraction. The defects observed in the films were threading dislocations due to nucleation and growth on the lattice-mismatched substrate and planar defects (stacking faults) parallel to the substrate surface. The highest defect density was found at the x = 0.5 composition. The nanoindentation hardness of the films varied between 21 GPa for the binary nitrides, and 26 GPa for Nb0.5Zr0.5N. Unlike the cubic binary nitrides, no slip on the preferred 〈1math0〉{110} slip system was observed. The increase in hardness is attributed to the increase in defect density at x = 0.5, as the defects act as obstacles for dislocation glide during deformation. The findings present routes for the design of wear-resistant nitride coatings by phase stability tuning.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.Nn Stacking faults and other planar or extended defects
68.60.Bs Mechanical and acoustical properties
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
68.55.-a Thin film structure and morphology
68.55.A- Nucleation and growth
68.55.Nq Composition and phase identification
62.20.Qp Friction, tribology, and hardness
68.35.Gy Mechanical properties; surface strains
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Lm Deformation, plasticity, and creep
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.F- Deformation and plasticity
62.20.D- Elasticity
62.20.M- Structural failure of materials
68.37.Lp Transmission electron microscopy (TEM)

Impact of growth conditions on vacancy-type defects in silicon–germanium structures grown by molecular-beam epitaxy

Kareem M. Shoukri, Yaser M. Haddara, A. P. Knights, and P. G. Coleman

Appl. Phys. Lett. 86, 131923 (2005); http://dx.doi.org/10.1063/1.1897826 (3 pages) | Cited 2 times

Online Publication Date: 25 March 2005

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Silicon–germanium layers of either 200 nm or 250 nm have been grown via molecular-beam epitaxy (MBE) on p-type (001) silicon substrates. Each sample was prepared using a unique combination of buffer-layer type, buffer-layer growth temperature, and layer Ge content. Vacancy-type defects have been identified using beam-based positron annihilation. These results, combined with those from previous work, indicate the size and concentration of defects in MBE grown SiGe layers depend strongly upon the buffer-layer growth temperature (T). For T>500 °C vacancy point defect concentrations are below the detectable limit of the measurement. As T is decreased to 300 °C, vacancy clusters form in the buffer layer and point defects appear in the SiGe film, even for a SiGe growth temperature of 500 °C.
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81.05.Hd Other semiconductors
68.55.A- Nucleation and growth
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.J- Point defects and defect clusters
78.70.Bj Positron annihilation
68.55.-a Thin film structure and morphology
68.55.Nq Composition and phase identification

Performance and reproducibility enhancement of HgCdTe molecular beam epitaxy growth on CdZnTe substrates using interfacial HgTe/CdTe superlattice layers

Yong Chang, Jun Zhao, Hisham Abad, Christoph H. Grein, Sivalingam Sivananthan, Toshihiro Aoki, and David J. Smith

Appl. Phys. Lett. 86, 131924 (2005); http://dx.doi.org/10.1063/1.1890471 (3 pages) | Cited 13 times

Online Publication Date: 25 March 2005

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Interfacial layers including HgTe/CdTe superlattices (SLs) were introduced during the molecular-beam epitaxy growth of HgCdTe on CdZnTe (211)B substrates. Transmission-electron-microscopic observations show that the SLs smooth out the substrates’ surface roughness during growth, and can also bend or block threading dislocations in a way that prevents their propagation from the substrate into the functional HgCdTe epilayers. An average etch pit density value in the low-105 cm−2 range was reproducibly achieved in long wavelength HgCdTe samples, with the best value being 4×104 cm−2. Photoconductive decay lifetime measurements give values approaching theoretical limits, as determined by the intrinsic radiative and Auger recombination mechanisms. The use of such interfacial layers thus leads to enhanced growth yields and material properties.
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81.05.Dz II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.A- Nucleation and growth
68.65.Cd Superlattices
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.55.-a Thin film structure and morphology
73.50.Pz Photoconduction and photovoltaic effects
79.60.Jv Interfaces; heterostructures; nanostructures
68.37.Lp Transmission electron microscopy (TEM)
68.55.Nq Composition and phase identification

Nitrogen and indium dependence of the band offsets in InGaAsN quantum wells

Massimo Galluppi, Lutz Geelhaar, and Henning Riechert

Appl. Phys. Lett. 86, 131925 (2005); http://dx.doi.org/10.1063/1.1898441 (3 pages) | Cited 13 times

Online Publication Date: 25 March 2005

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The band offsets of InGaAsN single quantum wells with varying nitrogen and indium content were quantitatively determined by surface photovoltage measurements. The experimental data directly show the different effect of nitrogen on the valence and on the conduction band states. While the conduction band offset strongly increases with increasing nitrogen concentration, the valence band offset is only weakly affected. In contrast, indium influences the valence and the conduction band states in the same way: both the valence and conduction band offsets increase with increasing indium content. In particular, the conduction band offset varies with In content as in N-free InGaAs quantum wells.
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73.21.Fg Quantum wells
73.20.At Surface states, band structure, electron density of states
72.40.+w Photoconduction and photovoltaic effects

Fabrication of single nickel-nitrogen defects in diamond by chemical vapor deposition

J. R. Rabeau, Y. L. Chin, S. Prawer, F. Jelezko, T. Gaebel, and J. Wrachtrup

Appl. Phys. Lett. 86, 131926 (2005); http://dx.doi.org/10.1063/1.1896088 (3 pages) | Cited 39 times

Online Publication Date: 25 March 2005

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Fabrication of single nickel-nitrogen (NE8) defect centers in diamond by chemical vapor deposition is demonstrated. Under continuous-wave 745 nm laser excitation single defects were induced to emit single photon pulses at 797 nm with a linewidth of 1.5 nm at room temperature. Photon antibunching of single centers was demonstrated using a Hanbury–Brown and Twiss interferometer. Confocal images revealed approximately 106 optically active sites/cm2 in the synthesized films. The controlled fabrication of an NE8 based single photon source in synthetic diamond is important for fiber based quantum cryptography, and potentially linear optics quantum computing.
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81.05.U- Carbon/carbon-based materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.J- Point defects and defect clusters

Mn-rich clusters in GaN: Hexagonal or cubic symmetry?

G. Martínez-Criado, A. Somogyi, S. Ramos, J. Campo, R. Tucoulou, M. Salome, J. Susini, M. Hermann, M. Eickhoff, and M. Stutzmann

Appl. Phys. Lett. 86, 131927 (2005); http://dx.doi.org/10.1063/1.1886908 (3 pages) | Cited 32 times

Online Publication Date: 25 March 2005

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In this study, we report the application of synchrotron radiation microprobe to the analysis of Mn-rich clusters in GaN. From the Mn and Ga fluorescence line intensity ratio, an average Mn concentration of 11% was deduced. A combination of fluorescence mapping with spectroscopic techniques enabled us to examine not only the cluster elemental nature but also their crystallographic orientation on the submicron scale. The strong polarization-dependent x-ray absorption near-edge structure features showed the preservation of the hexagonal symmetry in both cluster-free and Mn-rich regions. However, from the x-ray absorption data taken inside the clusters, a preferential disorder was observed in the direction parallel to the crystal growth rather than perpendicular to it.
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61.46.-w Structure of nanoscale materials
78.70.Dm X-ray absorption spectra
61.72.S- Impurities in crystals
75.50.Pp Magnetic semiconductors
78.55.Cr III-V semiconductors
61.50.-f Structure of bulk crystals
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Low-voltage pentacene thin-film transistors with Ta2O5 gate insulators and their reversible light-induced threshold voltage shift

Yan Liang, Guifang Dong, Yan Hu, Liduo Wang, and Yong Qiu

Appl. Phys. Lett. 86, 132101 (2005); http://dx.doi.org/10.1063/1.1896099 (3 pages) | Cited 13 times

Online Publication Date: 21 March 2005

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We have fabricated pentacene thin-film transistors using Ta2O5 films prepared by magnetron reactive sputtering as gate insulators. These transistors exhibit good electrical characteristics at an operating voltage as low as 5 V, with a field-effect mobility of 0.32 cm2/Vs, an on∕off ratio of 104, and a subthreshold slope of 0.5 V/decade. We have also investigated the optical properties of these transistors and observed a reversible light-induced threshold voltage shift. Under illumination, the threshold voltage shifts towards the positive direction while the field-effect mobility and on∕off ratio remain almost unchanged. In the dark, however, the threshold voltage can slowly be restored to its original state. At a gate voltage of −5 V, the transistors show a broadband responsivity of 3.7 A/W after illumination at 60 μW/cm2 for 10 min.
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85.30.Tv Field effect devices
81.15.Cd Deposition by sputtering
73.50.Dn Low-field transport and mobility; piezoresistance
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
77.55.-g Dielectric thin films

Resonance-tunneling-assisted emission enhancement in green light-emitting diodes with nanocraters formed in InGaN/GaN quantum-well active layers

Jae Ho Song, G. Hugh Song, Jhang W. Lee, Young-Woo Ok, Tae-Yeon Seong, Oleg Laboutin, Paul Deluca, and H. K. Choi

Appl. Phys. Lett. 86, 132102 (2005); http://dx.doi.org/10.1063/1.1890475 (3 pages) | Cited 1 time

Online Publication Date: 21 March 2005

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Data are presented on the resonant tunneling-related abrupt redshift observed in the temperature-dependent electroluminescence spectra of high-brightness InGaN/GaN multi-quantum-well green light-emitting diodes (LEDs). It is found that the redshift arises mostly between 120 and 150 K, and brighter LEDs yield larger redshifts. These results are well explained by the proposed nanocrater model which comprises a Ga-rich quantum barrier surrounding the In-rich quantum-dot-like localized state. Intensity analysis manifests that the resonant tunneling from the quantum-well to the nanocrater-shaped localized states induces such an abrupt energy shift and enhances the room-temperature emission.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
78.60.Fi Electroluminescence
73.40.Gk Tunneling

Impact of interface structure on Schottky-barrier height for Ni/ZrO2(001) interfaces

Y. F. Dong, S. J. Wang, J. W. Chai, Y. P. Feng, and A. C. H. Huan

Appl. Phys. Lett. 86, 132103 (2005); http://dx.doi.org/10.1063/1.1891285 (3 pages) | Cited 24 times

Online Publication Date: 21 March 2005

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The Schottky-barrier heights for the Ni and ZrO2 interfaces have been determined using the x-ray photoemission method. Depending on the surface treatment of ZrO2, the variation of Schottky-barrier heights at Ni/ZrO2 interfaces was found as large as 0.76±0.10 eV. The p-type Schottky-barrier height for the oxygen-rich (oxygen-deficient) interface was measured to be 2.60 eV (3.36 eV). First-principles calculations provide a microscopic explanation of such variation, which was attributed to the different interface dipole formed by interfacial Ni–O, Ni–Zr bonds, or oxygen vacancies.
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73.40.Ns Metal-nonmetal contacts
73.30.+y Surface double layers, Schottky barriers, and work functions
68.35.Ct Interface structure and roughness
79.60.Jv Interfaces; heterostructures; nanostructures
61.72.J- Point defects and defect clusters
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
73.20.At Surface states, band structure, electron density of states

Deep-level optical spectroscopy investigation of N-doped TiO2 films

Yoshitaka Nakano, Takeshi Morikawa, Takeshi Ohwaki, and Yasunori Taga

Appl. Phys. Lett. 86, 132104 (2005); http://dx.doi.org/10.1063/1.1896450 (3 pages) | Cited 72 times

Online Publication Date: 24 March 2005

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N-doped TiO2 films were deposited on n+-GaN/Al2O3 substrates by reactive magnetron sputtering and subsequently crystallized by annealing at 550 °C in flowing N2 gas. The N-doping concentration was ∼ 8.8%, as determined from x-ray photoelectron spectroscopy measurements. Deep-level optical spectroscopy measurements revealed two characteristic deep levels located at ∼ 1.18 and ∼ 2.48 eV below the conduction band. The 1.18 eV level is probably attributable to the O vacancy state and can be active as an efficient generation-recombination center. Additionally, the 2.48 eV band is newly introduced by the N doping and contributes to band-gap narrowing by mixing with the O 2p valence band.
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81.05.Hd Other semiconductors
61.72.up Other materials
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Cd Deposition by sputtering
61.72.Cc Kinetics of defect formation and annealing
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
73.20.At Surface states, band structure, electron density of states
79.60.Bm Clean metal, semiconductor, and insulator surfaces
61.72.J- Point defects and defect clusters
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
61.72.S- Impurities in crystals
73.61.Le Other inorganic semiconductors

Strain relaxation and oxygen superstructure modulation in epitaxial Sr4Fe6O13±δ films

J. Santiso, J. A. Pardo, C. Solís, G. Garcia, A. Figueras, M. D. Rossell, and G. Van Tendeloo

Appl. Phys. Lett. 86, 132105 (2005); http://dx.doi.org/10.1063/1.1886264 (3 pages) | Cited 2 times

Online Publication Date: 25 March 2005

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The compressive strain induced in epitaxial Sr4Fe6O13±δ (SFO) films of different thicknesses grown on SrTiO3 substrates is partially released by the modulation of the incommensurate oxygen superstructure along the a-axis parallel to the substrate [q = αam*, superspace group Xmmm(α00)0s0]. The modulation α value varies proportionally to the in-plane a-parameter in a continuous range from 0.41, for fully strained thin films of about 20–30 nm, to 0.44 for partially-relaxed thicker films of about 280 nm. This mechanism is responsible for the observed slow relaxation of the cell structure upon the film thickness increase in comparison with an equilibrium misfit dislocation-mediated relaxation.
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68.60.Bs Mechanical and acoustical properties
68.55.-a Thin film structure and morphology
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
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