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14 Aug 2006

Volume 89, Issue 7, Articles (07xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 89, 073501 (2006); http://dx.doi.org/10.1063/1.2335838 (3 pages)

Ingrid Graz, Martin Kaltenbrunner, Christoph Keplinger, Reinhard Schwödiauer, Siegfried Bauer, Stéphanie P. Lacour, and Sigurd Wagner
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Work hardening ability of ductile Ti45Cu40Ni7.5Zr5Sn2.5 and Cu47.5Zr47.5Al5 bulk metallic glasses

K. B. Kim, J. Das, S. Venkataraman, S. Yi, and J. Eckert

Appl. Phys. Lett. 89, 071908 (2006); http://dx.doi.org/10.1063/1.2337534 (3 pages) | Cited 24 times

Online Publication Date: 16 August 2006

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Ductile Ti45Cu40Ni7.5Zr5Sn2.5 and Cu47.5Zr47.5Al5 bulk metallic glasses (BMGs) present different work hardening abilities under compression. Microstructural investigations reveal that nanoscale chemical heterogeneities occur throughout the samples. The morphology of the chemically heterogeneous domains in the as-cast Ti45Cu40Ni7.5Zr5Sn2.5 BMG is irregular and significantly interconnected. In contrast, the as-cast Cu47.5Zr47.5Al5 BMG exhibits a spherical morphology of the chemically heterogeneous regions. Furthermore, the distribution of the nanoscale chemical heterogeneity is macroscopically inhomogeneous throughout the material. These findings suggest that the different work hardening abilities of the Ti45Cu40Ni7.5Zr5Sn2.5 and Cu47.5Zr47.5Al5 BMGs possibly originate from the different morphologies and distributions of the chemically heterogeneous regions.
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61.43.Fs Glasses
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Melt dispersion mechanism for fast reaction of nanothermites

Valery I. Levitas, Blaine W. Asay, Steven F. Son, and Michelle Pantoya

Appl. Phys. Lett. 89, 071909 (2006); http://dx.doi.org/10.1063/1.2335362 (3 pages) | Cited 36 times

Online Publication Date: 16 August 2006

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An unexpected mechanism for fast oxidation of Al nanoparticles covered by a thin oxide shell (OS) is proposed. The volume change due to melting of Al induces pressures of 0.1–4 GPa and causes spallation of the OS. A subsequent unloading wave creates high tensile pressures resulting in dispersion of liquid Al clusters, oxidation of which is not limited by diffusion (in contrast to traditional mechanisms). Physical parameters controlling this process are determined. Methods to promote this melt dispersion mechanism, and consequently, improve efficiency of energetic nanothermites are discussed.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.40.Fp Shock wave initiated reactions, high-pressure chemistry
82.60.Qr Thermodynamics of nanoparticles
82.70.-y Disperse systems; complex fluids

Binary eutectic clusters and glass formation in ideal glass-forming liquids

Z. P. Lu, J. Shen, D. W. Xing, J. F. Sun, and C. T. Liu

Appl. Phys. Lett. 89, 071910 (2006); http://dx.doi.org/10.1063/1.2336597 (3 pages) | Cited 27 times

Online Publication Date: 16 August 2006

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In this letter, a physical concept of binary eutectic clusters in “ideal” glass-forming liquids is proposed based on the characteristics of most well-known bulk metallic glasses (BMGs). The authors approach also includes the treatment of binary eutectic clusters as basic units, which leads to the development of a simple but reliable method for designing BMGs more efficiently and effectively in these unique glass-forming liquids. As an example, bulk glass formers with superior glass-forming ability in the Zr–Ni–Cu–Al and Zr–Fe–Cu–Al systems were identified with the use of the strategy.
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64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
61.43.Fs Glasses
61.25.Mv Liquid metals and alloys
81.05.Kf Glasses (including metallic glasses)

Hexagonal diamond synthesis on h-GaN strained films

Abha Misra, Pawan K. Tyagi, Brajesh S. Yadav, P. Rai, D. S. Misra, Vivek Pancholi, and I. D. Samajdar

Appl. Phys. Lett. 89, 071911 (2006); http://dx.doi.org/10.1063/1.2218043 (3 pages) | Cited 7 times

Online Publication Date: 17 August 2006

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Chemical vapor deposited diamond films grown on strained gallium nitride-coated quartz substrate are found to display a dominantly hexagonal diamond phase. The phase identification is done using Raman spectroscopy and orientation imaging microscopy (OIM). The presence of a 1324.4 cm−1 band in the Raman spectra is attributed to a hexagonal diamond symmetry, but the unambiguous signature of the hexagonal phase is confirmed by OIM. A phase map of the sample clearly shows that 88% of the scanned sample area is hexagonal diamond.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.30.Am Elemental semiconductors and insulators
68.55.-a Thin film structure and morphology
68.60.Bs Mechanical and acoustical properties

Buckling of C60 whiskers

Koji Asaka, Ryoei Kato, Kun’ichi Miyazawa, and Tokushi Kizuka

Appl. Phys. Lett. 89, 071912 (2006); http://dx.doi.org/10.1063/1.2336590 (3 pages) | Cited 12 times

Online Publication Date: 17 August 2006

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The authors demonstrated the mechanics of materials for crystalline whiskers composed of C60 molecules; compressive deformation of the whiskers was observed by in situ transmission electron microscopy with simultaneous force measurement by means of an optical cantilever method, as used in atomic force microscopy. In response to compression along the long axis, the whiskers bent first elastically, then buckled. A whisker with 160 nm diameter fractured brittlely at a strain of 0.08. According to Euler’s formula, Young’s modulus of the whisker was estimated to be 32–54 GPa, which is 160%–650% of that of C60 bulk crystals.
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81.05.ub Fullerenes and related materials
68.70.+w Whiskers and dendrites (growth, structure, and nonelectronic properties)
81.40.Lm Deformation, plasticity, and creep
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.F- Deformation and plasticity

Low compressible noble metal carbides with rocksalt structure: Ab initio total energy calculations of the elastic stability

Chang-Zeng Fan, Song-Yan Zeng, Zai-Ji Zhan, Ri-Ping Liu, Wen-Kui Wang, Ping Zhang, and Yu-Gui Yao

Appl. Phys. Lett. 89, 071913 (2006); http://dx.doi.org/10.1063/1.2335571 (3 pages) | Cited 23 times

Online Publication Date: 17 August 2006

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The authors have systematically studied the mechanical stability of all noble metal carbides with a rocksalt structure by calculating their elastic constants within the density function theory scheme. It was found that only four carbides (RuC, PdC, AgC, and PtC) are mechanically stable. In particular, the authors have shown that RuC, PdC, and PtC have very high bulk modulus, which has been remarkably observed by the most recent experiment for the case of PtC. From the calculated density of states, it can be concluded that these compounds are metallic, like the conventional group IV and group V transition metal carbides.
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62.20.D- Elasticity
71.20.Ps Other inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Room temperature gas sensitivity of ultrathin SnO2 films prepared from Langmuir-Blodgett film precursors

Sipra Choudhury, C. A. Betty, K. G. Girija, and S. K. Kulshreshtha

Appl. Phys. Lett. 89, 071914 (2006); http://dx.doi.org/10.1063/1.2336725 (3 pages) | Cited 9 times

Online Publication Date: 17 August 2006

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The authors report the room temperature gas sensitivity of ultrathin SnO2 films prepared by thermal decomposition of multilayer Langmuir-Blodgett (LB) films. The SnO2 films, which are characterized by various spectroscopic techniques and electrical measurements, have been shown to sense ammonia gas at room temperature with fast response and recovery. The presence of a large number of surface states in the ultrathin SnO2 film is responsible for the room temperature gas sensitivity. Present results demonstrate the potential of LB technique to fabricate high quality ultrathin oxide films useful for sensor applications.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
73.61.Le Other inorganic semiconductors
81.05.Hd Other semiconductors
73.20.At Surface states, band structure, electron density of states

Groups III and V impurity solubilities in silicon due to laser, flash, and solid-phase-epitaxial-regrowth anneals

R. Duffy, T. Dao, Y. Tamminga, K. van der Tak, F. Roozeboom, and E. Augendre

Appl. Phys. Lett. 89, 071915 (2006); http://dx.doi.org/10.1063/1.2337081 (3 pages) | Cited 15 times

Online Publication Date: 17 August 2006

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In this work the authors studied impurity solubilities of groups III and V elements in silicon resulting from laser anneal, flash anneal, and solid-phase-epitaxial regrowth. Rutherford backscattering channeling analysis was used to determine substitutional impurity depth profiles generated from the difference between the random and aligned spectra. Despite the large difference in peak temperatures and times, the anneals produce similar results with maximum solubilities beating the maximum equilibrium values by one to two orders of magnitude depending on the impurity. The correlation between the metastable solubility and the equilibrium distribution coefficient allows a prediction of values for other impurities not extracted experimentally.
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64.75.-g Phase equilibria
61.72.S- Impurities in crystals
61.72.uf Ge and Si
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
81.15.Np Solid phase epitaxy; growth from solid phases
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis

Migration and redistribution of oxygen vacancy in barium titanate ceramics

L. Chen, X. M. Xiong, H. Meng, P. Lv, and J. X. Zhang

Appl. Phys. Lett. 89, 071916 (2006); http://dx.doi.org/10.1063/1.2337097 (3 pages) | Cited 15 times

Online Publication Date: 17 August 2006

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Degradation of barium titanate based multilayer capacitor mainly results from migration and redistribution of oxygen vacancy. For barium titanate ceramics, the authors observe an internal friction relaxation peak around 70 °C due to oxygen vacancy, and its relaxation strength differs greatly for specimen aged at 85 °C for 120 h and at 150 °C for 5 h. Two possible explanations are proposed, one based on symmetry-conforming short-range order while the other on the interaction between oxygen vacancy and domain wall during aging process. In any case, relaxation thermodynamics is a powerful tool to investigate the migration and redistribution of oxygen vacancy in barium titanate ceramics.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.Dj Domain structure; hysteresis
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Cd Solid solution hardening, precipitation hardening, and dispersion hardening; aging

Conformational flexibility and its effect on the vibrationally resolved absorption and fluorescence spectra of oligofluorenes

Jian Song, Wan Zhen Liang, Yi Zhao, and Jinlong Yang

Appl. Phys. Lett. 89, 071917 (2006); http://dx.doi.org/10.1063/1.2337530 (3 pages) | Cited 7 times

Online Publication Date: 18 August 2006

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The authors present a first-principles density-functional theory study on the electronic excitation of oligofluorenes. Their results indicate that the strong electron-nuclear coupling results in a dynamic structural distortion in the S1 state, which extends over eight monomers in the center of the long chain and contributes a relaxation energy of ∼ 0.23 eV. The field-induced density matrices evidence that the excitons localize in the regions where the structural alternations occur. The asymmetry of absorption and fluorescence spectra of oligofluorenes is significantly pronounced. The experimentally observed spectroscopic features are reasonably reproduced by properly taking into account the anharmonic torsional potentials without any adjustable parameters.
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42.70.Jk Polymers and organics
78.30.Jw Organic compounds, polymers
33.20.Ea Infrared spectra
33.50.Dq Fluorescence and phosphorescence spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis

Surface morphologies of homoepitaxial ZnO on Zn- and O-polar substrates by plasma assisted molecular beam epitaxy

Huaizhe Xu, K. Ohtani, M. Yamao, and H. Ohno

Appl. Phys. Lett. 89, 071918 (2006); http://dx.doi.org/10.1063/1.2337541 (3 pages) | Cited 21 times

Online Publication Date: 18 August 2006

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Homoepitaxial ZnO layers are grown on Zn-polar (0001) and O-polar (000math) surfaces of single crystal ZnO substrates by plasma assisted molecular beam epitaxy. It is found that the growth conditions to obtain smooth surfaces are significantly different for the two surface polarities. For growth on Zn-polar surface, moderate temperature (650 °C) and highly O-rich condition (low Zn/O2) are required, while high temperature (1000–1050 °C) and Zn-rich condition (high Zn/O2 ratio) are essential for growth on O-polar surfaces.
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81.05.Dz II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.55.A- Nucleation and growth
68.47.Fg Semiconductor surfaces
52.77.-j Plasma applications

Local epitaxial growth of aluminum nitride and molybdenum thin films in fiber texture using aluminum nitride interlayer

Toshihiro Kamohara, Morito Akiyama, Naohiro Ueno, Kazuhiro Nonaka, and Noriyuki Kuwano

Appl. Phys. Lett. 89, 071919 (2006); http://dx.doi.org/10.1063/1.2337558 (3 pages) | Cited 5 times

Online Publication Date: 18 August 2006

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The authors have found the local epitaxial growth of aluminum nitride (AlN) and molybdenum (Mo) films in fiber texture, although the interface between the AlN and Mo films has different crystal symmetries. The local heteroepitaxial relationship is (0001)AlN[2mathmath0]‖(110)Mo[math11]‖(0001)AlN[2mathmath0]. The AlN films changes from nonequiaxed microstructures to equiaxed columnar structures. The authors think that the AlN interlayer is effective in decreasing the crystallization energy of the Mo electrode due to the coherent heteroepitaxial nucleation. It is interesting that the local heteroepitaxial relationship does not satisfy the criteria for heteroepitaxial growth.
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81.05.Ea III-V semiconductors
81.05.Bx Metals, semimetals, and alloys
81.15.Cd Deposition by sputtering
68.55.A- Nucleation and growth

Observation of secondary relaxation in a fragile Pd40Ni10Cu30P20 bulk metallic glass

Zuo Feng Zhao, Ping Wen, Wei Hua Wang, and C. H. Shek

Appl. Phys. Lett. 89, 071920 (2006); http://dx.doi.org/10.1063/1.2337876 (3 pages) | Cited 11 times

Online Publication Date: 18 August 2006

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The dynamic elastic moduli in the temperature domain of a Pd40Ni10Cu30P20 bulk metallic glass were measured with dynamic mechanical spectroscopy at frequencies less than 100 Hz. The primary relaxation is characterized by a single loss modulus peak and a rapid drop of storage modulus in the metallic supercooled liquid region. Below the conventional glass transition temperature, a pronounced shoulder in the loss modulus curves appears and the corresponding storage modulus decreases with increasing temperature. The phenomenon is common to glass formers of all types. The observations present the evidence of the existence of the slow β relaxation and affirm the universality of the separation of the primary and secondary relaxations in the supercooled liquid.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
62.20.D- Elasticity
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
61.43.Fs Glasses

Red-light-emitting diodes fabricated by near-ultraviolet InGaN chips with molybdate phosphors

Zhengliang Wang, Hongbin Liang, Jing Wang, Menglian Gong, and Qiang Su

Appl. Phys. Lett. 89, 071921 (2006); http://dx.doi.org/10.1063/1.2335579 (3 pages) | Cited 41 times

Online Publication Date: 18 August 2006

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Phosphors Na5La(MoO4)4:xEu3+ and NaEu(MoO4)2 were prepared with a solid-state reaction technique. Their photoluminescent properties were investigated at room temperature. Bright red-light-emitting diodes were fabricated by coating the phosphors onto near-ultraviolet/violet-emitting InGaN chips, respectively. The diodes prepared with phosphor Na5Eu(MoO4)4 show appropriate CIE chromaticity coordinates (x = 0.65,y = 0.34) and exhibit more intensive red emission than that prepared with NaEu(MoO4)2, indicating that Na5Eu(MoO4)4 may be applied as an excellent red component for near-ultraviolet InGaN-based white diodes.
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85.60.Jb Light-emitting devices
78.55.Hx Other solid inorganic materials

Photoluminescence and multiphonon resonant Raman scattering in low-temperature grown ZnO nanostructures

Bhupendra Kumar, Hao Gong, Shue Yin Chow, Sudhiranjan Tripathy, and Younan Hua

Appl. Phys. Lett. 89, 071922 (2006); http://dx.doi.org/10.1063/1.2336997 (3 pages) | Cited 30 times

Online Publication Date: 18 August 2006

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The authors report on the optical properties of nanocrystalline ZnO grown at 200 °C by radio-frequency magnetron sputtering. The nanocrystalline nature of the films was confirmed by cross-sectional transmission electron microscopy. In these films, ZnO nanocrystals with an average size of about 3–5 nm were embedded in an amorphous matrix. The photoluminescence spectra from such nanostructured thin films show the near-band-edge emissions around 3.3 eV. A redshift of about 8–11 cm−1 is observed in the case of first-order longitudinal-optical (LO) phonon of ZnO in such nanostructures when compared to the LO phonon peak of bulk ZnO. The ultraviolet resonant Raman excitation at 77 K shows multiphonon LO modes up to eighth order.
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78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
78.30.Fs III-V and II-VI semiconductors
68.55.A- Nucleation and growth
61.46.-w Structure of nanoscale materials
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Optical determination of three-dimensional nanotrack profiles generated by single swift-heavy ion impacts in lithium niobate

J. Olivares, A. García-Navarro, G. García, A. Mýndez, and F. Agulló-López

Appl. Phys. Lett. 89, 071923 (2006); http://dx.doi.org/10.1063/1.2236221 (3 pages) | Cited 13 times

Online Publication Date: 18 August 2006

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Three-dimensional (3D) profiles of single nanotracks generated by a low impact density of Cl ions at 46 MeV have been determined by optical methods, using an effective-medium approach. The buried location of the maximum stopping power induces a surface optical waveguiding layer even at ultralow fluences (1011–1013 at./cm2) that allows to obtain the effective refractive index profiles (from dark-mode measurements). Combining the optical information with Rutherford backscattering spectroscopy/channeling experiments, the existence of a surrounding defective halo around the amorphous track core has been ascertained. The 3D profile of the halo has also been determined.
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81.16.-c Methods of micro- and nanofabrication and processing
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
42.79.Gn Optical waveguides and couplers
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis
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Spin-wave relaxation in diluted magnetic semiconductors within the self-consistent Green’s function approach

J. E. Bunder, Shih-Jye Sun, and Hsiu-Hau Lin

Appl. Phys. Lett. 89, 072101 (2006); http://dx.doi.org/10.1063/1.2335666 (3 pages) | Cited 4 times

Online Publication Date: 15 August 2006

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The authors employ a self-consistent Green’s function approach to investigate the spin-wave relaxation Γ(p) in diluted magnetic semiconductors. They find that the trend of the spin-wave relaxation strongly depends on the ratio of the itinerant and impurity spin densities. For density ratios in the Ruderman-Kittel-Kasuya-Yosida phase, Γ(p) decreases even though thermal fluctuations increase. On the other hand, in the strong coupling phase, an interesting peak structure appears. They discuss the implications of their numerical results for experiments.
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75.50.Pp Magnetic semiconductors
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.30.Ds Spin waves
75.30.Et Exchange and superexchange interactions

Manganese impurities in boron nitride

L. V. C. Assali, W. V. M. Machado, and J. F. Justo

Appl. Phys. Lett. 89, 072102 (2006); http://dx.doi.org/10.1063/1.2266930 (3 pages) | Cited 7 times

Online Publication Date: 15 August 2006

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The authors carried an ab initio investigation on the properties of Mn impurities in BN, presenting results on stability, spin states, magnetic moment, hyperfine parameters, and formation and transition energies. Substitutional Mn in the cation site in BN was compared to that in GaN and GaAs. Due to the small BN lattice parameter, the Mn impurity is energetically more favorable in a divacancy site, as compared to the cation site. Although the Mn-divacancy complex has a high spin configuration, with a prevailing Mn d character, the transition energies lie deep in the gap, preventing a hole-mediated ferromagnetism.
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75.50.Pp Magnetic semiconductors
75.30.Hx Magnetic impurity interactions
75.30.Cr Saturation moments and magnetic susceptibilities
71.55.Eq III-V semiconductors
61.72.J- Point defects and defect clusters
61.66.Fn Inorganic compounds

Carrier storage time of milliseconds at room temperature in self-organized quantum dots

A. Marent, M. Geller, D. Bimberg, A. P. Vasi’ev, E. S. Semenova, A. E. Zhukov, and V. M. Ustinov

Appl. Phys. Lett. 89, 072103 (2006); http://dx.doi.org/10.1063/1.2337000 (3 pages) | Cited 10 times

Online Publication Date: 15 August 2006

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Thermally activated hole emission from differently charged InAs/GaAs quantum dots (QDs) was investigated by using deep level transient spectroscopy. In a sample with an additional AlGaAs barrier below the QD layer, a thermal activation energy of 560 meV for hole emission from the QD ground states over the AlGaAs barrier is obtained. This large activation energy leads to a hole storage time at room temperature of about 5 ms, which is in the order of magnitude of a typical dynamic random access memory (DRAM) refresh time.
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73.21.La Quantum dots
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

ZnO tetrapod Schottky photodiodes

Marcus C. Newton, Steven Firth, and Paul A. Warburton

Appl. Phys. Lett. 89, 072104 (2006); http://dx.doi.org/10.1063/1.2335949 (3 pages) | Cited 36 times

Online Publication Date: 16 August 2006

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The fabrication of an ultraviolet photodiode employing a single ZnO tetrapod nanocrystal is reported. This diode structure is prepared by depositing W and Pt electrodes to form Ohmic and Schottky contacts, respectively. Dark current-voltage measurements show rectifying behavior. The properties of the metal-semiconductor interface are studied with above and below band gap illumination. It is found that with increasing UV excitation the device converts from a rectifying to an Ohmic behavior. This effect is attributed to a flattening of the energy bands due to the migration of photogenerated carriers within the space charge region at the metal-semiconductor interface.
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85.60.Dw Photodiodes; phototransistors; photoresistors
85.30.Hi Surface barrier, boundary, and point contact devices
85.30.Kk Junction diodes

Electrical and structural investigations of Ag-based Ohmic contacts for InAlAs/InGaAs/InP high electron mobility transistors

Weifeng Zhao, Liang Wang, and Ilesanmi Adesida

Appl. Phys. Lett. 89, 072105 (2006); http://dx.doi.org/10.1063/1.2337102 (3 pages) | Cited 2 times

Online Publication Date: 16 August 2006

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Ge/Ag/Ni and AuGe/Ni/Au Ohmic contacts on InAlAs/InGaAs/InP high electron mobility transistors with excellent contact resistance of 0.07 Ω mm were obtained after annealing at 425 and 265 °C, respectively. The Ag-based contacts have a large processing window of >130 °C. Structural analyses confirm that Ag and Au protrusions created during annealing effectively linked the two-dimensional electron gas layer with the metal contacts to produce excellent Ohmic characteristics. The formation of liquid AuGe eutectic phase in AuGe/Ni/Au at 300 °C is believed to cause overannealing. The eutectic temperature of Ag–Ge is ∼ 300 °C higher leading to a higher optimum annealing temperature and a wider processing window for the Ge/Ag/Ni contacts.
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85.30.Tv Field effect devices

Electrical-stress-induced conductivity increase in SrTiO3 films

S. Karg, G. I. Meijer, D. Widmer, and J. G. Bednorz

Appl. Phys. Lett. 89, 072106 (2006); http://dx.doi.org/10.1063/1.2335802 (3 pages) | Cited 17 times

Online Publication Date: 17 August 2006

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The charge transport of polycrystalline SrTiO3 films doped with Cr is investigated by means of temperature- and electric-field-dependent current measurements. In particular, the effect of electrical stress on the conduction mechanism is analyzed to understand the forming process of samples exhibiting resistance-switching phenomena. The temperature dependence of the conductivity can be described with the variable-range hopping model. The electrical-stress-induced conductivity increase of SrTiO3:Cr films is accompanied by a change of the temperature dependence corresponding to a significant increase of the density of localized states.
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73.61.Ng Insulators
73.40.Rw Metal-insulator-metal structures

Time-resolved photoluminescence studies of carrier diffusion in GaN

S. M. Olaizola, W. H. Fan, S. A. Hashemizadeh, J.-P. R. Wells, D. J. Mowbray, M. S. Skolnick, A. M. Fox, and P. J. Parbrook

Appl. Phys. Lett. 89, 072107 (2006); http://dx.doi.org/10.1063/1.2335957 (3 pages) | Cited 1 time

Online Publication Date: 17 August 2006

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The authors have studied the diffusion of carriers in GaN by photoluminescence and pump-probe spectroscopy in an InGaN/GaN single quantum well sample at 300 K. The sample was designed with the quantum well buried beneath a thick (240 nm) GaN layer. When pumping above the GaN band gap, the carriers that are excited in the GaN layer diffuse to the InGaN quantum well before recombining radiatively. The hole diffusion coefficient was determined from the rise time of the quantum well photoluminescence, together with pump-probe experiments. A value of 2.0±0.4 cm2/s is found for the diffusion coefficient.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
82.53.Mj Femtosecond probing of semiconductor nanostructures
73.63.Hs Quantum wells
73.21.Fg Quantum wells
78.47.-p Spectroscopy of solid state dynamics

Dependency of organic phototransistor properties on the dielectric layers

Yan Hu, Guifang Dong, Chen Liu, Liduo Wang, and Yong Qiu

Appl. Phys. Lett. 89, 072108 (2006); http://dx.doi.org/10.1063/1.2336722 (3 pages) | Cited 20 times

Online Publication Date: 17 August 2006

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Organic phototransistors with pentacene semiconductor and Ta2O5 or polymethyl methacrylate (PMMA) dielectric layer have been investigated. It was found that the phototransistor properties strongly depend on the dielectric layer. Under a broadband light with 10 mW/cm2, the sensitivity of the Ta2O5 based transistor is much higher than that of the PMMA based transistor. For Ta2O5 based transistor, the photosensitivity (the ratio of photocurrent to dark current) and the threshold voltage shift are 4000 and 13.5 V, respectively. While for PMMA based transistor, the corresponding values are only 0.5 and 2.9 V, respectively. That large difference is attributed to the electron trapping ability of Ta2O5.
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85.60.Dw Photodiodes; phototransistors; photoresistors

Transport properties of the thermoelectric layered cobalt oxide Pb–Sr–Co–O single crystals

W. Kobayashi and I. Terasaki

Appl. Phys. Lett. 89, 072109 (2006); http://dx.doi.org/10.1063/1.2337080 (3 pages) | Cited 6 times

Online Publication Date: 17 August 2006

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The authors have synthesized single-crystal samples of the layered cobalt oxide Pb–Sr–Co–O and measured resistivity, thermopower, and Hall coefficient from 4.2 to 300 K. The resistivity, the thermopower, and the Hall coefficient are 13 mΩ cm, 150 μV/K, and 1.6×10−2 cm3/C at 300 K, respectively, which are typical values for a misfit-layered cobalt oxide. The thermopower and resistivity are associated with spin density wave, and a close relationship between the pseudogap temperature and the thermopower is suggested for all the misfit-layered cobalt oxides.
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72.20.Pa Thermoelectric and thermomagnetic effects
72.20.My Galvanomagnetic and other magnetotransport effects
72.80.Sk Insulators
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