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13 Nov 2006

Volume 89, Issue 20, Articles (20xxxx)

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Appl. Phys. Lett. 89, 202101 (2006); http://dx.doi.org/10.1063/1.2388049 (3 pages)

J. H. Lee, Zh. M. Wang, N. W. Strom, Yu. I. Mazur, and G. J. Salamo
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Structural and electrical properties of low pressure metalorganic chemical vapor deposition grown Eu2O3 films on Si(100)

M. P. Singh, K. Shalini, S. A. Shivashankar, G. C. Deepak, and N. Bhat

Appl. Phys. Lett. 89, 201901 (2006); http://dx.doi.org/10.1063/1.2388128 (3 pages) | Cited 1 time

Online Publication Date: 13 November 2006

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Structural and electrical properties of Eu2O3 films grown on Si(100) in 500–600 °C temperature range by low pressure metalorganic chemical vapor deposition are reported. As-grown films also possess the impurity Eu1−xO phase, which has been removed upon annealing in O2 ambient. Film’s morphology comprises uniform spherical mounds (40–60 nm). Electrical properties of the films, as examined by capacitance-voltage measurements, exhibit fixed oxide charges in the range of −1.5×1011 to −6.0×1010 cm−2 and dielectric constant in the range of 8–23. Annealing has resulted in drastic improvement of their electrical properties. Effect of oxygen nonstoichiometry on the film’s property is briefly discussed.
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68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
73.61.Ng Insulators
77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
81.40.Gh Other heat and thermomechanical treatments

Oxygen-assisted control of surface morphology in nonepitaxial sputter growth of Ag

J. M. Riveiro, P. S. Normile, J. P. Andrés, J. A. González, J. A. De Toro, T. Muñoz, and P. Muñiz

Appl. Phys. Lett. 89, 201902 (2006); http://dx.doi.org/10.1063/1.2388140 (3 pages) | Cited 3 times

Online Publication Date: 14 November 2006

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The surface roughness of Ag films (thickness of 50 nm) is drastically reduced from 2.4 to 1.0 nm by the presence of oxygen (pressure of 2×10−5 mbar) during sputter growth, without a change (undermining) in electrical transport properties nor a formation of any Ag oxide phase. It is concluded that oxygen plays the role of a surfactant. The growth scenario (nonepitaxial sputter deposition of relatively thick films) makes this observation complementary to previous studies reporting surfactant effects in epitaxial growth of Ag. The authors consider an application of this finding in the field of multilayers involving Ag metals.
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68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.15.Cd Deposition by sputtering
73.61.At Metal and metallic alloys

Depth-resolved optical studies of excitonic and phonon-assisted transitions in ZnO epilayers

Yong-Hoon Cho, Ji-Young Kim, Ho-Sang Kwack, Bong-Joon Kwon, Le Si Dang, Hang-Ju Ko, and Takafumi Yao

Appl. Phys. Lett. 89, 201903 (2006); http://dx.doi.org/10.1063/1.2388252 (3 pages) | Cited 15 times

Online Publication Date: 14 November 2006

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The authors report depth-resolved optical properties of excitonic and phonon-assisted transitions in ZnO epilayers by photoluminescence (PL) and cathodoluminescence. A weaker free exciton (FX) emission than its first longitudinal optical phonon replica (FX-1LO) is observed at elevated temperatures (T>150 K) for interior area, while a stronger FX than FX-1LO is seen at all temperatures for top surface area of the sample. The authors exclude out a possible self-absorption process by PL excited at back surface of the sample. Therefore, the authors conclude that the different intensity ratios of FX and FX-1LO depending on the sample depth are strongly associated with extrinsic features of ZnO.
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78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
71.35.-y Excitons and related phenomena
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.60.Hk Cathodoluminescence, ionoluminescence

Role of Ti in the reversible dehydrogenation of Ti-doped sodium alanate

Kewu Bai and Ping Wu

Appl. Phys. Lett. 89, 201904 (2006); http://dx.doi.org/10.1063/1.2388865 (3 pages) | Cited 4 times

Online Publication Date: 14 November 2006

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The role of Ti in the reversible dehydrogenation of NaAlH4 has been the subject of debate for many years. The authors resolve this controversy by calculating the phase stability diagram of Na–Al–H–Ti and defect formation enthalpy of Ti doped NaAlH4 from first principles. The calculations show that Ti substitutes Al in NaAlH4 and forms a defect pair with interstitial H under H-rich and Al-poor conditions. However, the defect pair is not stable under H-poor and Al-rich conditions resulting in the formation of a Ti–Al intermetallic compound. The doped Ti reduces the Al–H bonding via additional Al 2p and Ti 3d hybridizations. The results give a picture why and how doped Ti acts to enhance hydrogen storage related solid phase transitions.
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81.30.Dz Phase diagrams of other materials
84.60.-h Direct energy conversion and storage
61.72.J- Point defects and defect clusters
65.40.G- Other thermodynamical quantities
82.60.Cx Enthalpies of combustion, reaction, and formation
64.70.K- Solid-solid transitions

Remarkable solute trapping within rapidly growing dendrites

H. P. Wang, W. J. Yao, and B. Wei

Appl. Phys. Lett. 89, 201905 (2006); http://dx.doi.org/10.1063/1.2387971 (3 pages) | Cited 8 times

Online Publication Date: 15 November 2006

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Solute microsegregation always takes place during dendritic crystal growth. Although this may be reduced with the increase of crystal growth velocity, the realization of segregationless dendritic growth is quite difficult. Here the authors present the results of remarkable solute trapping within the rapidly growing dendrites of highly undercooled liquid Ni–5 wt %Si alloy. The dendrites grow at a velocity of 15 m/s at the maximum experimental undercooling of 304 K. Such a high growth velocity results in the pronounced solute trapping and almost segregationless solidification. Furthermore, a model is proposed to describe the correlation between dendritic growth velocity and undercooling. It agrees well with the experimental results in the whole undercooling regime and provides a reasonable prediction for the dendritic growth trend under extremely great undercooling conditions.
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81.05.Bx Metals, semimetals, and alloys
81.30.Fb Solidification
68.70.+w Whiskers and dendrites (growth, structure, and nonelectronic properties)

Brewster angle phenomenon in two-dimensional metallic photonic crystals and its application to polarization beam splitting

Xinhua Hu and Kai-Ming Ho

Appl. Phys. Lett. 89, 201906 (2006); http://dx.doi.org/10.1063/1.2387981 (3 pages)

Online Publication Date: 15 November 2006

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The authors show the Brewster angle phenomenon of p waves in two-dimensional metallic photonic crystals of rectangular lattices with effective medium theory and accurate multiple-scattering calculations. The Brewster angle can be tuned over a wide range by varying the structure parameters. Based on this phenomenon, the authors propose a polarization beam splitter which can completely separate the p and s waves at an incident angle of 45°±10° in a very wide wavelength range (1 ∼ 500 μm).
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42.70.Qs Photonic bandgap materials
42.79.Fm Reflectors, beam splitters, and deflectors
42.25.Ja Polarization

Visible band-gap ZnCdO heterostructures grown by molecular beam epitaxy

S. Sadofev, S. Blumstengel, J. Cui, J. Puls, S. Rogaschewski, P. Schäfer, and F. Henneberger

Appl. Phys. Lett. 89, 201907 (2006); http://dx.doi.org/10.1063/1.2388250 (3 pages) | Cited 59 times

Online Publication Date: 15 November 2006

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Single-phase ZnCdO alloys with a band gap extending from the violet to yellow spectral range are fabricated by molecular beam epitaxy using extremely low growth temperatures in conjunction with O-rich growth conditions. The Cd concentration can be systematically adjusted via the Cd/Zn beam pressure ratio. Despite growth temperatures as low as 150 °C, layer-by-layer growth is accomplished allowing for the preparation of ZnCdO/ZnO quantum well structures. Both epilayers and quantum wells exhibit strong band-gap-related emission at room temperature in the whole composition range.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.67.De Quantum wells
78.40.Fy Semiconductors
81.07.St Quantum wells
71.20.Nr Semiconductor compounds
68.55.A- Nucleation and growth

Mechanism for Ohmic contact formation on Si3N4 passivated AlGaN/GaN high-electron-mobility transistors

B. Van Daele, G. Van Tendeloo, J. Derluyn, P. Shrivastava, A. Lorenz, M. R. Leys, and M. Germain

Appl. Phys. Lett. 89, 201908 (2006); http://dx.doi.org/10.1063/1.2388889 (3 pages) | Cited 2 times

Online Publication Date: 15 November 2006

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Recent experiments have shown that in situ passivation by Si3N4 of AlGaN/GaN high-electron-mobility transistors results in improved electrical characteristics. Transmission electron microscopy techniques have been applied to study the metal contact formation on top of passivated AlGaN/GaN structures. Contrary to unpassivated AlGaN/GaN, the AlGaN top layer is not consumed by a typical Au/Mo/Al/Ti metal stack. Instead, a thin AlN interface layer is formed, being the key factor in the Ohmic contact formation. The formation of this AlN is believed to be due to extraction of N atoms out of the AlGaN. The resulting N vacancies, electrical donors, create a conducting channel through the AlGaN.
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85.30.Tv Field effect devices

Role of shear stiffening in reducing hydrogenation in intermetallic compounds

I. Jacob, M. Bereznitsky, O. Yeheskel, and R. G. Leisure

Appl. Phys. Lett. 89, 201909 (2006); http://dx.doi.org/10.1063/1.2390635 (3 pages) | Cited 3 times

Online Publication Date: 16 November 2006

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The authors report on a pronounced increase in the elastic shear modulus of a wide class of binary intermetallics upon pseudobinary substitution. They further claim that the aforementioned increase in the rigidity of these compounds is the mechanism underlying the well documented, sharp decline of hydrogen absorption in these compounds. They suggest that a clamping effect occurs in the bulk due to the high shear modulus, preventing the formation of dislocations and thus inhibits the hydrogenation. Identifying this mechanism suggests that, in order to reach good hydrogen absorbers, effort should be invested in designing compounds where shear stiffening does not occur.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
84.60.-h Direct energy conversion and storage

Structural basis for the fast phase change of Ge2Sb2Te5: Ring statistics analogy between the crystal and amorphous states

Shinji Kohara, Kenichi Kato, Shigeru Kimura, Hitoshi Tanaka, Takeshi Usuki, Kentaro Suzuya, Hiroshi Tanaka, Yutaka Moritomo, Toshiyuki Matsunaga, Noboru Yamada, Yoshihito Tanaka, Hiroyoshi Suematsu, and Masaki Takata

Appl. Phys. Lett. 89, 201910 (2006); http://dx.doi.org/10.1063/1.2387870 (3 pages) | Cited 70 times

Online Publication Date: 17 November 2006

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The three-dimensional atomic configuration of amorphous Ge2Sb2Te5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge2Sb2Te5 can be regarded as “even-numbered ring structure,” because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge–Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge2Sb2Te5 is the key for the fast crystallization speed of the material.
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61.43.Dq Amorphous semiconductors, metals, and alloys
61.43.Bn Structural modeling: serial-addition models, computer simulation
64.70.K- Solid-solid transitions

Generation of misfit dislocations by basal-plane slip in InGaN/GaN heterostructures

R. Liu, J. Mei, S. Srinivasan, F. A. Ponce, H. Omiya, Y. Narukawa, and T. Mukai

Appl. Phys. Lett. 89, 201911 (2006); http://dx.doi.org/10.1063/1.2388895 (3 pages) | Cited 21 times

Online Publication Date: 17 November 2006

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The authors have observed that for InxGa1−xN epitaxial layers grown on bulk GaN substrates exhibit slip on the basal plane, when in the presence of free surfaces that intercept the heterointerface and for indium compositions x ≥ 0.07. This leads to almost complete relaxation of the local misfit strain by generation of radial-shape dislocation half loops. For x ≥ 0.17, generation of straight misfit dislocations by glide on the secondary 〈11math3〉 {11math2} slip system is observed, in addition to the radial-shape half loops at surface pits. These two mechanisms act independently with no observed interaction between them, leading to the conclusion that slip on the basal plane occurs first during the growth process. The secondary slip system is activated later and involves a significantly higher critical stress energy.
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61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Protective layer formation during oxidation of Cu3Au(100) using hyperthermal O2 molecular beam

Michio Okada, Michihiro Hashinokuchi, Masayuki Fukuoka, Toshio Kasai, Kousuke Moritani, and Yuden Teraoka

Appl. Phys. Lett. 89, 201912 (2006); http://dx.doi.org/10.1063/1.2390672 (3 pages) | Cited 9 times

Online Publication Date: 17 November 2006

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Oxidation of Cu3Au(100) using a hyperthermal O2 molecular beam (HOMB) was investigated by x-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident energy dependence of the O-uptake curve, it was determined that the dissociative adsorption of O2 implies a higher activation barrier and therefore less reactivity compared to Cu, owing to the Au alloying. The dissociative adsorption progresses with the Cu segregation on the surface. No prominent growth of Cu2O even for 2 eV HOMB suggests that the Au alloying of Cu can serve as a protective layer against further oxidation into the bulk.
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81.05.Bx Metals, semimetals, and alloys
81.65.Mq Oxidation
79.60.Bm Clean metal, semiconductor, and insulator surfaces
68.43.Mn Adsorption kinetics
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
64.75.-g Phase equilibria
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