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1 Aug 2005

Volume 87, Issue 5, Articles (05xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 87, 054101 (2005); http://dx.doi.org/10.1063/1.2006209 (3 pages)

Xiuqin Chen, S. Yang, M Hasegawa, K. Kawabe, and S. Motojima
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Liquid as a required catalyst phase for carbon single-walled nanotube growth

A. R. Harutyunyan, T. Tokune, and E. Mora

Appl. Phys. Lett. 87, 051919 (2005); http://dx.doi.org/10.1063/1.2005395 (3 pages) | Cited 36 times

Online Publication Date: 28 July 2005

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The evolution of the iron catalyst during carbon single-walled nanotube growth is studied using calorimetry, temperature-programmed oxidation and Raman measurements. Carbon-induced solid-liquid, and solid-liquid-solid phase transitions of the nanocatalyst during the synthesis were observed. We found that liquid phase is favored for the growth of nanotubes, while the solidification of the catalyst nearly terminates the growth. No growth was observed below the eutectic point, when the catalyst is in solid phase. Our results support a growth mechanism of single-walled carbon nanotubes on liquid catalyst particles.
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81.07.De Nanotubes
81.16.-c Methods of micro- and nanofabrication and processing
78.67.Ch Nanotubes
78.30.Hv Other nonmetallic inorganics
64.70.D- Solid-liquid transitions
81.30.Fb Solidification

Observation of two-photon-induced photoluminescence in ZnO microtubes

C. F. Zhang, Z. W. Dong, G. J. You, S. X. Qian, H. Deng, H. Gao, L. P. Yang, and Y. Li

Appl. Phys. Lett. 87, 051920 (2005); http://dx.doi.org/10.1063/1.1996848 (3 pages) | Cited 21 times

Online Publication Date: 29 July 2005

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The micrometer-sized ZnO tubes were fabricated by a hydrothermal method and characterized by scanning electron microscopy, high resolution transmission microscopy and micro-Raman spectroscopy. Photoluminescence (PL) emission from ZnO microtubes excited by light in the ultraviolet range from xenon lamp was measured. By using nanosecond laser beam at 527 nm to excite ZnO microtube, we observed two-photon induced PL emission with an intense peak at 3.15 eV and a weak peak at 2.65 eV, which was confirmed by the quadratic dependence of the emission intensity on the excitation power.
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78.55.Et II-VI semiconductors
78.30.Fs III-V and II-VI semiconductors
78.40.Fy Semiconductors
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Lp Transmission electron microscopy (TEM)

Fabrication of strained silicon on insulator by strain transfer process

Bo Jin, Xi Wang, Jing Chen, Xinli Cheng, and Zhijun Chen

Appl. Phys. Lett. 87, 051921 (2005); http://dx.doi.org/10.1063/1.1991987 (2 pages) | Cited 4 times

Online Publication Date: 29 July 2005

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The fabrication of ultrathin strained silicon layer directly on insulator is demonstrated. 50 nm strained silicon on insulator layers were fabricated by a method which includes four steps: Epitaxial growth of strained SiGe on ultrathin silicon on insulator (SOI) substrates, ion implantation, postannealing process, and etch-back process. Strain of the layer was observed by Raman spectroscopy. 0.72% tensile strain was maintained in the strained silicon layer even after removing the SiGe film. The strained layer was the result of strain equalization and transfer process between the SiGe film and top silicon layer.
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81.05.Cy Elemental semiconductors
68.55.A- Nucleation and growth
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
61.72.up Other materials
61.72.Cc Kinetics of defect formation and annealing
81.65.Cf Surface cleaning, etching, patterning
78.66.Db Elemental semiconductors and insulators
78.30.Am Elemental semiconductors and insulators
68.60.Bs Mechanical and acoustical properties

Ge diffusion in Ge metal oxide semiconductor with chemical vapor deposition HfO2 dielectric

N. Lu, W. Bai, A. Ramirez, C. Mouli, A. Ritenour, M. L. Lee, D. Antoniadis, and D. L. Kwong

Appl. Phys. Lett. 87, 051922 (2005); http://dx.doi.org/10.1063/1.2001757 (3 pages) | Cited 55 times

Online Publication Date: 29 July 2005

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We report a study on Ge diffusion and its impact on the electrical properties of TaN/HfO2/Ge metal-oxide-semiconductor (MOS) device. It is found that Ge diffusion depends on the amount of GeO2 formed at the HfO2/Ge interface and can be retarded by surface nitridation. It is speculated that Ge diffusion is in the form of GeO or Ge-riched HfGeO. Effective suppression of Ge diffusion by NH3 nitridation has resulted in improved electrical properties of TaN/HfO2/Ge MOS device, including equivalent oxide thickness (EOT), leakage current, hysteresis, and interface state density. The degradation of leakage current after high temperature post metallization anneal (PMA) is found to be due to Ge diffusion.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
85.30.Tv Field effect devices
61.72.Cc Kinetics of defect formation and annealing
85.40.Ls Metallization, contacts, interconnects; device isolation
81.65.Lp Surface hardening: nitridation, carburization, carbonitridation
81.65.Rv Passivation

X-ray absorption and photoelectron spectroscopy studies on graphite and single-walled carbon nanotubes: Oxygen effect

M. Abbas, Z. Y. Wu, J. Zhong, K. Ibrahim, A. Fiori, S. Orlanducci, V. Sessa, M. L. Terranova, and Ivan Davoli

Appl. Phys. Lett. 87, 051923 (2005); http://dx.doi.org/10.1063/1.2006214 (3 pages) | Cited 16 times

Online Publication Date: 29 July 2005

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We have investigated the electronic states of highly oriented pyrolitic graphite and single-walled carbon nanotubes using x-ray absorption spectroscopy (XAS) before and after annealing treatment in ultrahigh vacuum, and observed that the small peak between π* and σ* features, which has been previously assigned to free-electron-like interlayer states, disappears after in situ annealing treatment, suggesting that the signal may be assigned to a surface contamination, especially oxygen contamination introduced by chemical processing or gas adsorption. Additional experiments by photoelectron spectroscopy as well as XAS methods, performed after aging in air, fully support this interpretation.
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81.05.U- Carbon/carbon-based materials
81.07.De Nanotubes
78.70.Dm X-ray absorption spectra
79.60.Jv Interfaces; heterostructures; nanostructures
71.20.Tx Fullerenes and related materials; intercalation compounds
81.40.Gh Other heat and thermomechanical treatments
81.40.Cd Solid solution hardening, precipitation hardening, and dispersion hardening; aging
81.65.-b Surface treatments
68.43.Mn Adsorption kinetics
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Aluminum arsenide cleaved-edge overgrown quantum wires

J. Moser, T. Zibold, D. Schuh, M. Bichler, F. Ertl, G. Abstreiter, M. Grayson, S. Roddaro, and V. Pellegrini

Appl. Phys. Lett. 87, 052101 (2005); http://dx.doi.org/10.1063/1.1994955 (3 pages) | Cited 11 times

Online Publication Date: 26 July 2005

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We report conductance measurements in quantum wires made of aluminum arsenide, a heavy-mass, multivalley one-dimensional (1D) system. Zero-bias conductance steps are observed as the electron density in the wire is lowered, with additional steps observable upon applying a finite dc bias. We attribute these steps to depopulation of successive 1D subbands. The quantum conductance is substantially reduced with respect to the anticipated value for a spin- and valley-degenerate 1D system. This reduction is consistent with disorder-induced, intrawire backscattering that suppresses the transmission of 1D modes. Calculations are presented to demonstrate the role of strain in the 1D states of this cleaved-edge structure.
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81.07.Vb Quantum wires
81.05.Ea III-V semiconductors
73.63.Nm Quantum wires
72.20.-i Conductivity phenomena in semiconductors and insulators

Electronic transport properties of pentacene single crystals upon exposure to air

Oana D. Jurchescu, Jacob Baas, and Thomas T. M. Palstra

Appl. Phys. Lett. 87, 052102 (2005); http://dx.doi.org/10.1063/1.2001130 (3 pages) | Cited 67 times

Online Publication Date: 26 July 2005

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We report the effect of air exposure on the electronic properties of pentacene single crystals. Air can diffuse reversibly in and out of the crystals and influences the physical properties. We discern two competing mechanisms that modulate the electronic transport. The presence of oxygen increases the hole conduction, as in dark four O2 molecules introduce one charge carrier. This effect is enhanced by the presence of visible light. Contrarily, water, present in ambient air, is incorporated in the crystal lattice and forms trapping sites for injected charges.
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72.80.Le Polymers; organic compounds (including organic semiconductors)
68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation
66.30.J- Diffusion of impurities
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Fk Semiconductors

Spiroconjugation-enhanced intermolecular charge transport

Chung-Chih Wu, Wei-Guang Liu, Wen-Yi Hung, Tsung-Li Liu, Yu-Ting Lin, Hao-Wu Lin, Ken-Tsung Wong, Yuh-Yih Chien, Ruei-Tang Chen, Tsung-Hsi Hung, Teng-Chih Chao, and You-Ming Chen

Appl. Phys. Lett. 87, 052103 (2005); http://dx.doi.org/10.1063/1.2001140 (3 pages) | Cited 19 times

Online Publication Date: 26 July 2005

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We report charge-transport and related physical properties of terfluorenes with various C9 substitutions. Results show that by introducing slight spiroconjugation between a core chromophore and conjugated substitutions through spiro-linking, the bulky substitution not only serves as a spatial hindrance to enhance morphological stability and emission efficiency in thin films, but also bridges and enhances intermolecular charge transport under certain situations. Interestingly, such characteristics are acquired without altering major electronic properties of core chromophores, rendering it a flexible molecular scheme for tuning characteristics of functional molecules to meet various demands of different applications.
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73.61.Ph Polymers; organic compounds

Screening of external electric field by photoinduced carriers in Bragg multiple quantum wells

V. I. Puller, L. I. Deych, M. V. Erementchouk, and A. A. Lisyansky

Appl. Phys. Lett. 87, 052104 (2005); http://dx.doi.org/10.1063/1.2001743 (3 pages)

Online Publication Date: 26 July 2005

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We study screening of external bias in a multiple-quantum-well structure by optically injected excess carriers. By solving self-consistently the Poisson equation and the equations for the electron and hole densities, we analyze how realization of different screening regimes depends on the applied bias, excitation power, temperature, and the parameters of the structure. Our calculations show the feasibility of using the proposed setup as an optically controlled electric switch in photonic circuits.
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73.63.Hs Quantum wells
73.21.Fg Quantum wells
73.50.Pz Photoconduction and photovoltaic effects

Field electron emission of Si nanotips with apexes of various compositions

J. C. She, K. Zhao, S. Z. Deng, J. Chen, and N. S. Xu

Appl. Phys. Lett. 87, 052105 (2005); http://dx.doi.org/10.1063/1.2001733 (3 pages) | Cited 9 times

Online Publication Date: 26 July 2005

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We report procedure with use of self-assembled silicon carbide (SiC) nanomasks for preparation of ultrahigh-density Si nanotips. Si nanotips with SiC apex may be firstly prepared in a CH4/H2 plasma treatment and in a subsequent H2 plasma etching a SiC apex may be converted into an amorphous silicon (a-Si) one with an additional function of sharpening the nanotips. A comparative study of the field electron emission from the Si nanotips with apexes of SiC, a-Si and pure Si is carried out, and shows that nanotips with a-Si apexes have not only the highest field enhancement factor but also the best emission uniformity. The physical origins for the above two improvements in field emission are discussed.
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81.07.Bc Nanocrystalline materials
81.05.Gc Amorphous semiconductors
81.05.Cy Elemental semiconductors
79.70.+q Field emission, ionization, evaporation, and desorption
61.46.-w Structure of nanoscale materials
61.43.Dq Amorphous semiconductors, metals, and alloys
52.77.Bn Etching and cleaning

High mobility two-dimensional electron system on hydrogen-passivated silicon(111) surfaces

K. Eng, R. N. McFarland, and B. E. Kane

Appl. Phys. Lett. 87, 052106 (2005); http://dx.doi.org/10.1063/1.2001734 (3 pages) | Cited 17 times

Online Publication Date: 26 July 2005

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We have fabricated and characterized a field-effect transistor in which an electric field is applied through an encapsulated vacuum cavity and induces a two-dimensional electron system on a hydrogen-passivated Si(111) surface. This vacuum cavity preserves the ambient sensitive surface and is created via room temperature contact bonding of two Si substrates. Hall measurements are made on the H–Si(111) surface prepared in aqueous ammonium fluoride solution. We obtain electron densities up to 6.5×1011 cm−2 and peak mobilities of ∼ 8000 cm2/V s at 4.2 K.
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85.30.Tv Field effect devices
81.65.Rv Passivation

Room-temperature plasma waves resonant detection of sub-terahertz radiation by nanometer field-effect transistor

F. Teppe, W. Knap, D. Veksler, M. S. Shur, A. P. Dmitriev, V. Yu. Kachorovskii, and S. Rumyantsev

Appl. Phys. Lett. 87, 052107 (2005); http://dx.doi.org/10.1063/1.2005394 (3 pages) | Cited 54 times

Online Publication Date: 27 July 2005

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We report on room-temperature, resonant detection of 0.6 THz radiation by 250 nm gate length GaAs/AlGaAs heterostructure field-effect transistor. We show that the detection is strongly increased (and becomes resonant) when the drain current increases and the transistor is driven into the current saturation region. We interpret the results as due to resonant plasma wave detection that is enhanced by increasing the electron drift velocity.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.30.Tv Field effect devices

Three-dimensional atom mapping of dopants in Si nanostructures

Keith Thompson, John H. Booske, David J. Larson, and Thomas F. Kelly

Appl. Phys. Lett. 87, 052108 (2005); http://dx.doi.org/10.1063/1.2005368 (3 pages) | Cited 29 times

Online Publication Date: 27 July 2005

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Atom-probe tomography has successfully mapped three-dimensional (3D) dopant atom distributions in nanoscale volumes of Si subjected to various processing procedures. The 3D evolution of dopants, specifically effects such as dopant clustering and grain-boundary segregation, were analyzed in implanted polycrystalline Si gate contacts and implanted shallow junctions. A cluster of dimensions 2×7×8 nm3 and containing 264 B atoms, was identified at the intersection of three poly-Si grains, verifying that annealing highly overdoped thin poly-Si layers does not facilitate uniformly doped and highly conductive gate contact layers.
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61.72.uf Ge and Si
61.72.Mm Grain and twin boundaries
64.75.-g Phase equilibria
61.46.-w Structure of nanoscale materials
61.72.Cc Kinetics of defect formation and annealing
81.05.Cy Elemental semiconductors
81.07.Bc Nanocrystalline materials

Modulation of the work function of silicon gate electrode using thin TaN interlayers

H. N. Alshareef, H. C. Wen, H. R. Harris, K. Choi, H. F. Luan, P. Lysaght, P. Majhi, B. H. Lee, M. El-Bouanani, and V. Ukride

Appl. Phys. Lett. 87, 052109 (2005); http://dx.doi.org/10.1063/1.2006977 (3 pages) | Cited 4 times

Online Publication Date: 27 July 2005

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The impact of thin TaN layers (0.5–10 nm) on the effective work function of polycrystalline silicon (poly-Si)∕TaN stacks has been investigated. It is found that when the TaN layer is as thin as 0.5 nm, it can have a significant effect on the effective work function of poly-Si, and that n-type and p-type poly-Si behave differently. The observed results are explained by reactions between poly-Si and the TaN layer leading to the formation of TaxSiyNz at the poly-Si-gate dielectric interface. Electrical tests show minimal poly-Si depletion with the TaN layers, and gate leakage current and fixed charges that are comparable to conventional poly-Si electrodes. The results show that these stacked electrodes can be useful for nearly n-type effective work functions (4.2–4.3 eV).
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.30.+y Surface double layers, Schottky barriers, and work functions

W2B-based rectifying contacts to n-GaN

Rohit Khanna, S. J. Pearton, F. Ren, I. Kravchenko, C. J. Kao, and G. C. Chi

Appl. Phys. Lett. 87, 052110 (2005); http://dx.doi.org/10.1063/1.2007865 (3 pages) | Cited 11 times

Online Publication Date: 28 July 2005

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Schottky contact formation on n-GaN using a novel W2B/Ti/Au metallization scheme was studied using current-voltage, scanning electron microscopy and Auger electron spectroscopy measurements. A maximum barrier height of 0.55 eV was achieved on as-deposited samples, with a negative temperature coefficient of 8×10−4 eV/°C over the range 25–150 °C. There was also a negative temperature coefficient for the reverse breakdown of the Schottky diodes with W2B contacts. The barrier height was essentially independent of annealing temperature up to 500 °C and decreased thereafter due to the onset of metallurgical reactions with the GaN. The Ti began to out-diffuse to the surface at temperatures of >500 °C. The reverse current magnitude was larger than predicted by thermionic emission alone.
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85.40.Ls Metallization, contacts, interconnects; device isolation
73.30.+y Surface double layers, Schottky barriers, and work functions
85.30.Hi Surface barrier, boundary, and point contact devices
79.20.Fv Electron impact: Auger emission
61.72.Cc Kinetics of defect formation and annealing
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
79.40.+z Thermionic emission

Electrical characterization of band gap states in C-doped TiO2 films

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

Appl. Phys. Lett. 87, 052111 (2005); http://dx.doi.org/10.1063/1.2008376 (3 pages) | Cited 30 times

Online Publication Date: 28 July 2005

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We report on band gap states in C-doped TiO2 films that were prepared by oxidative annealing of sputtered TiC films at 550 °C in flowing O2 gas. Deep-level optical spectroscopy measurements revealed three deep levels located at ∼ 0.86, ∼ 1.30, and ∼ 2.34 eV below the conduction band. The first level is probably attributable to the intrinsic nature of TiO2, whereas the latter two levels are newly introduced by the C-doping. In particular, the pronounced 2.34 eV band contributes to band gap narrowing by mixing with the O 2p valence band. Additionally, the 0.86 and 1.30 eV levels can be active as an efficient generation-recombination center.
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71.55.Ht Other nonmetals
61.72.S- Impurities in crystals
73.61.Le Other inorganic semiconductors
71.20.Nr Semiconductor compounds
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.72.Cc Kinetics of defect formation and annealing

Metalorganic chemical vapor deposition of atomically flat SrRuO3 films on stepped SrTiO3 substrates

Akihiro Sumi, Kenji Takahashi, Shintaro Yokoyama, Hitoshi Morioka, Hiroshi Funakubo, and Mamoru Yoshimoto

Appl. Phys. Lett. 87, 052112 (2005); http://dx.doi.org/10.1063/1.2006989 (3 pages) | Cited 8 times

Online Publication Date: 28 July 2005

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Epitaxial SrRuO3 films with thicknesses of 50–80 nm were systematically grown at 750 °C on (100)SrTiO3 substrates by metalorganic chemical vapor deposition with different supply rates of the Sr and Ru source gases. Stoichiometric films with a low resistivity of 240–260 μΩ cm can be grown on polished (100)SrTiO3 substrates over a wide range of source gas supply rates. However, the surface flatness of the deposited film was very sensitive to changes the input source gas supply rate. SrRuO3 films having step and trace structured surfaces were grown on polished and atomically flat SrTiO3 substrates under an optimized input gas supply rate. A height of 7–8 nm with a terrace width of 500–1000 nm were obtained on polished SrTiO3 substrates, but a single unit cell height of about 0.4 nm was obtained with a 200–300 nm terrace width on atomically flat SrTiO3 substrates. This opens the possibility of the mass production of atomically flat conductive perovskite layers.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
73.61.Ng Insulators
68.35.B- Structure of clean surfaces (and surface reconstruction)
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Antiferromagnetism in bulk Zn1−xCoxO magnetic semiconductors prepared by the coprecipitation technique

M. Bouloudenine, N. Viart, S. Colis, J. Kortus, and A. Dinia

Appl. Phys. Lett. 87, 052501 (2005); http://dx.doi.org/10.1063/1.2001739 (3 pages) | Cited 42 times

Online Publication Date: 25 July 2005

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Polycrystalline Zn1−xCoxO diluted magnetic semiconductors have been prepared by coprecipitation technique in the concentration range 0 ⩽ x ⩽ 0.1. Structure, composition analysis, and optical absorption measurements revealed that cobalt is incorporated into the lattice, as Co2+ substituting Zn2+ ions, forming a solid solution with wurtzite structure instead of Co precipitates. Room- and low-temperature magnetization measurements reveal a paramagnetic behavior for the Co-doped ZnO samples with a paramagnetic Co amount smaller than the nominal concentration. χT versus T evidenced that the remaining Co is antiferromagnetically coupled through oxygen. This is further supported by a simple model that shows that as the Co concentration increases the amount of nearest neighbors Co atoms increases thus giving antiferromagnetic coupling and reducing the paramagnetic contribution.
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75.50.Pp Magnetic semiconductors
75.30.Gw Magnetic anisotropy
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Lattice distortion and polarization switching in calcium copper titanate

Sung-Yoon Chung

Appl. Phys. Lett. 87, 052901 (2005); http://dx.doi.org/10.1063/1.2001758 (3 pages) | Cited 33 times

Online Publication Date: 26 July 2005

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It is shown that CaCu3Ti4O12 grains in a polycrystalline specimen have domains and comparatively thick domain walls. The orientations of the domains within a grain were observed to be slightly tilted toward each other, suggesting the presence of lattice distortions. Multiple reflection spots in the electron diffraction pattern and misfit dislocations were also found in the domain walls. Kelvin probe force microscopy showed the variation in the surface potential of a grain by external electric fields, demonstrating the polarization of dipole moments. The influence of the local distortions on the formation of dipole moments is discussed in terms of the deviation from the cubic symmetry due to the small misfit between domains in a single grain of CaCu3Ti4O12 polycrystals.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
61.72.Lk Linear defects: dislocations, disclinations
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
77.22.Ej Polarization and depolarization
77.80.Dj Domain structure; hysteresis

Effects of applied electric field during postannealing on the tunable properties of (Ba,Sr)TiO3 thin films

Yidong Xia, Jinbo Cheng, Bai Pan, Di Wu, Xiangkang Meng, and Zhiguo Liu

Appl. Phys. Lett. 87, 052902 (2005); http://dx.doi.org/10.1063/1.2006982 (3 pages) | Cited 5 times

Online Publication Date: 27 July 2005

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The impact of postannealing in electric field on the structure, tunability, and dielectric behavior of rf magnetron sputtering derived (Ba,Sr)TiO3 films has been studied. It has been demonstrated that postannealing in the proper electric field can increase the dielectric constant and the tunability remarkably and destroy the symmetry of capacitance-voltage characteristics of the films. The increased out-of-plane lattice constant and the appearance of the hysteresis loops in the electric-annealed films indicated the formation of small polar regions with tetragonal structure, which are responsible for the increased dielectric constant and tunability. It was proposed that the segregation of Ti3+ ions caused by electric annealing could induce the formation of BaTiO3-like regions, which are ferroelectric at room temperature.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
73.61.Ng Insulators
77.22.Ch Permittivity (dielectric function)
81.40.Gh Other heat and thermomechanical treatments
77.80.Dj Domain structure; hysteresis
68.55.-a Thin film structure and morphology
68.55.A- Nucleation and growth
81.15.Cd Deposition by sputtering
61.66.Fn Inorganic compounds

Polarization of strained BaTiO3/SrTiO3 artificial superlattice: First-principles study

Leejun Kim, Juho Kim, Donggeun Jung, Jaichan Lee, and Umesh V. Waghmare

Appl. Phys. Lett. 87, 052903 (2005); http://dx.doi.org/10.1063/1.2006216 (3 pages) | Cited 19 times

Online Publication Date: 29 July 2005

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We performed first-principles calculation to investigate the effect of epitaxial strain on lattice instabilities and polarization behavior of BaTiO3/SrTiO3 artificial lattice with very short stacking period, i.e., BaTiO3 1 unit cell/SrTiO3 1 unit cell (BTO/STO). The structural analysis of BTO∕STO artificial superlattice under in-plane compressive state showed enhanced stability of the tetragonal phase. On the other hand, the stability of monoclinic phase was enhanced when the BTO∕STO was in the in-plane tensile state. The phase transition from tetragonal to the monoclinic phase occurs at the misfit strain of −0.25%. As the misfit strain of BTO∕STO superlattice increases from −0.25% to −1.5% (in-plane compressive state), the tetragonal superlattice exhibits an increasing polarization along the [001] direction. In the monoclinic phase, the polarization of the superlattice rotates progressively toward [110] direction with increasing the misfit strain, and the magnitude of the polarization simultaneously increases with the rotation. The first-principles study shows that the phase stability and polarization vector is sensitively influenced by the lattice misfit strain.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Ej Polarization and depolarization
64.70.K- Solid-solid transitions
68.65.Cd Superlattices
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PbS quantum dot electroabsorption modulation across the extended communications band 1200–1700 nm

Ethan J. D. Klem, Larissa Levina, and Edward H. Sargent

Appl. Phys. Lett. 87, 053101 (2005); http://dx.doi.org/10.1063/1.2001737 (3 pages) | Cited 10 times

Online Publication Date: 25 July 2005

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We report electric field-induced modulation of absorption in PbS nanocrystal quantum dots across the spectral region 600–2000 nm, encompassing the entire telecommunications band. The maxima in the electroabsorption spectra correspond with the positions of the first excitonic peak, confirming the predominance of excitonic broadening as the basis for the observed effect. We estimate the change in dipole moment to be on the order of 20 D in 7 nm diameter PbS nanocrystals compared to previously-reported ∼ 100 D in CdSe.
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78.67.Hc Quantum dots
78.20.Jq Electro-optical effects
71.35.-y Excitons and related phenomena
61.46.-w Structure of nanoscale materials
42.79.Hp Optical processors, correlators, and modulators

Well-defined excited states of self-assembled InAs/InAlGaAs quantum dots on InP (001)

Jin Soo Kim, Jin Hong Lee, Sung Ui Hong, Ho-Sang Kwack, Byung Seok Choi, and Dae Kon Oh

Appl. Phys. Lett. 87, 053102 (2005); http://dx.doi.org/10.1063/1.2005385 (3 pages) | Cited 6 times

Online Publication Date: 25 July 2005

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Self-assembled InAs/InAlGaAs quantum dots (QDs) in an InAlGaAs matrix on InP (001) substrates were grown by the alternate growth method (AGQD), where an InAs layer with a thickness of 1 monolayer (ML) and an InAlGaAs layer with a thickness of 1 ML were alternately deposited. Cross-sectional transmission electron microscopy images indicated that the aspect ratio (height/width) for the AGQDs was ∼ 0.25, which was higher than ∼ 0.10 of conventionally grown InAs QDs. The photoluminescence (PL) peak position for the ground states of the AGQDs was 1.485 μm with a linewidth broadening of 42 meV at room temperature, while the PL linewidth for the conventionally grown QDs was 85 meV. And the peaks for the excited-state transitions were also clearly observed from the excitation-power dependent PL. This is the first observation on the well-defined excited-state transitions from the InP-based InAs QDs, even though there were several reports on the features of the excited states.
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81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
73.21.La Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.55.Cr III-V semiconductors
78.67.Hc Quantum dots
68.37.Lp Transmission electron microscopy (TEM)

ZnO symmetric nanosheets integrated with nanowalls

Ruey-Chi Wang, Chuan-Pu Liu, Jow-Lay Huang, and Shu-Jen Chen

Appl. Phys. Lett. 87, 053103 (2005); http://dx.doi.org/10.1063/1.2005386 (3 pages) | Cited 33 times

Online Publication Date: 25 July 2005

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Diverse ZnO integrated nanostructures, constructed by epitaxial nanowalls and symmetric single-crystalline nanosheets, were successfully synthesized via a strain-assisted self-catalyzed process at a low temperature of 500 °C. The nanostructures started with the growth of ZnO nanowires, nucleated on a rugged ZnO single-crystalline film via a strain-assisted self-catalyzed growth mechanism. The nanowalls were then formed by the interconnection of the nanowires. Finally, the nanosheets were grown from the edges of the nanowalls. The growth mechanisms were supported by direct experimental evidence. Room-temperature cathodoluminance spectra show a relatively strong and sharp ultraviolet emission as well as a weak and broad green emission. The integrated nanostructure may be applied to develop self-inclusive nanoelectronics.
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81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
81.16.Hc Catalytic methods
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
78.40.Fy Semiconductors
78.60.Hk Cathodoluminescence, ionoluminescence
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Mechanism of nanoparticle agglomeration during the combustion synthesis

Igor S. Altman, Igor E. Agranovski, and Mansoo Choi

Appl. Phys. Lett. 87, 053104 (2005); http://dx.doi.org/10.1063/1.2005387 (3 pages) | Cited 10 times

Online Publication Date: 25 July 2005

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The mechanism of agglomeration of nanoparticles generated during combustion synthesis is discussed. This is based on the analysis of the transmission electron microscope images of probes collected at different heights. Although direct temperature measurements were not available, the qualitative temperature dependence of the particle formation streamlines is taken into account. It is demonstrated that agglomeration of the MgO nanoparticles, which are formed during a Mg particle combustion, occurs as the result of bonding the mature nanoparticles by the supercritical clusters existing in the system. Accumulation of these supercritical clusters in the flame has been revealed and their nature has been explained in our recent paper [ I.S. Altman, I.E. Agranovski, and M. Choi, Phys. Rev E 70, 062603 (2004) ]. Also, some inspection of the previously published experimental data on the nanoparticle generation shows that the similar supercritical clusters may exist in another flame reactor generating titania nanopaprticles. If this is the case, the cluster-based process of nanoparticle bonding we suggest can be considered to be general.
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82.33.Vx Reactions in flames, combustion, and explosions
61.46.-w Structure of nanoscale materials
68.37.Lp Transmission electron microscopy (TEM)
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