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3 Oct 2005

Volume 87, Issue 14, Articles (14xxxx)

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Appl. Phys. Lett. 87, 143902 (2005); http://dx.doi.org/10.1063/1.2077839 (3 pages)

Kaustubh D. Bhalerao, Edward Eteshola, Matthew Keener, and Stephen C. Lee
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Influence of edge diffusion on the growth mode on vicinal surfaces

C. Ratsch, J. Garcia, and R. E. Caflisch

Appl. Phys. Lett. 87, 141901 (2005); http://dx.doi.org/10.1063/1.2077851 (3 pages) | Cited 3 times

Online Publication Date: 26 September 2005

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Growth on vicinal surfaces typically occurs via nucleation and growth of islands or via step flow. In this letter, we present computational results based on an island dynamics model that employs the level-set technique that study the transition between these two competing mechanisms. Our results demonstrate that the rate of diffusion of atoms along step edges is an additional parameter that controls the competition between these mechanisms. We make suggestions about how one might be able to exploit these results experimentally.
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81.15.Aa Theory and models of film growth
68.55.A- Nucleation and growth
66.30.Dn Theory of diffusion and ionic conduction in solids
68.35.Ct Interface structure and roughness

The strength of diamond

Mikhail I. Eremets, Ivan A. Trojan, Patience Gwaze, Joachim Huth, Reinhard Boehler, and Vladimir D. Blank

Appl. Phys. Lett. 87, 141902 (2005); http://dx.doi.org/10.1063/1.2061853 (3 pages) | Cited 23 times

Online Publication Date: 26 September 2005

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Diamond is the hardest known material; however, its mechanical stability, in particular the yield strength σy, is not known. We report reproducible generation of plastic deformation of diamond at room and low temperatures. We determined σy = 130–140 GPa by measuring the pressure distribution over the diamond anvils and by using the theory of elasticity. Yielding in diamond is accompanied by a strong luminescence with a characteristic spectrum. Our findings may be used to extend the static pressure limit beyond the current value of about 300 GPa(3 Megabars).
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
78.55.Hx Other solid inorganic materials
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.Qp Friction, tribology, and hardness

In situ transmission electron microscope observations of electromigration in copper lines at room temperature

C. N. Liao, K. C. Chen, W. W. Wu, and L. J. Chen

Appl. Phys. Lett. 87, 141903 (2005); http://dx.doi.org/10.1063/1.2081138 (3 pages) | Cited 12 times

Online Publication Date: 26 September 2005

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Atomic-scale electromigration (EM) in unpassivated copper metal lines was directly observed in ultrahigh vacuum at room temperature by transmission electron microscopy. It was found that copper atoms on a (211) crystal plane vanished directionally within half an hour when applying an electric current with a density of 2×106 A/cm2 through the tested Cu line. The EM-induced atomic migration appeared to be anisotropic, and the combination of {111} planes and ⟨110⟩ directions was suggested to be the easiest electromigration system for crystalline copper. EM-induced mass transport was also found to be responsible for the weakening (111) texture of the Cu lines after electric current stressing.
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66.30.Qa Electromigration
85.40.Ls Metallization, contacts, interconnects; device isolation
68.37.Lp Transmission electron microscopy (TEM)

Determination of crystallographic polarity of ZnO layers

H. Tampo, P. Fons, A. Yamada, K.-K. Kim, H. Shibata, K. Matsubara, S. Niki, H. Yoshikawa, and H. Kanie

Appl. Phys. Lett. 87, 141904 (2005); http://dx.doi.org/10.1063/1.2067689 (3 pages) | Cited 24 times

Online Publication Date: 26 September 2005

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The crystallographic polarity of ZnO epilayers was determined by x-ray diffraction (XRD) using anomalous dispersion near the Zn K edge. The method is not destructive and is straightforward to carry out using a typical XRD measurement system. The polarity difference between the Zn (0001) and O (000math) surfaces could be easily determined using a {0002} diffraction peak and the Bremstrahlung radiation from a Cu rotating anode source. By using the normalized pre- and post-Zn K-edge diffraction intensity ratios of the (0002) diffraction peak, Zn polar and O polar ZnO layers could always be distinguished but, the absolute value of the ratio was found to change with layer thickness. The absolute value of the ratio with layer thickness was found to have a linear dependence on layer thickness allowing determination of the polarity of (0001) ZnO epilayers with a single x-ray measurement and the known layer thickness in conjunction with standard data. Acid etching results confirmed the veracity of the polarity determination of the XRD measurement. To test the technique, Zn and O polar ZnO layers were grown by radical source molecular beam epitaxy (RS MBE) on MgO buffer layers on c-sapphire substrate and O polar ZnO layers were grown on a-plane substrates and measured using the x-ray technique with excellent agreement.
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68.55.-a Thin film structure and morphology
61.05.cp X-ray diffraction

Band-gap control optical Kerr effect and light self-focusing of semiconducting materials CdxHg1−xGa2S4

W.-D. Cheng, S.-P. Huang, D.-S. Wu, X.-D. Li, Y.-Z. Lan, F.-F. Li, J. Shen, H. Zhang, and Y.-J. Gong

Appl. Phys. Lett. 87, 141905 (2005); http://dx.doi.org/10.1063/1.2072787 (3 pages) | Cited 1 time

Online Publication Date: 26 September 2005

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We report the dispersion of the optical Kerr effect and the wavelength-dependent critical power of light self-focusing for semiconductors CdxHg1−xGa2S4 in view of the calculated results from the density functional theory combined with an anharmonic oscillator model. It is shown that the third-order optical properties are controlled by the band gaps or cation contents of CdxHg1−xGa2S4, and it is indicated that the optical Kerr effect decreases and the critical power of light self-focusing increases as the x contents and band gaps increase for CdxHg1−xGa2S4.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Bulk nanostructured alloys prepared by flux melting and melt solidification

T. D. Shen, R. B. Schwarz, and X. Zhang

Appl. Phys. Lett. 87, 141906 (2005); http://dx.doi.org/10.1063/1.2056610 (3 pages) | Cited 5 times

Online Publication Date: 26 September 2005

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We have prepared bulk nanostructured Ag60Cu40 alloys by a flux-melting and melt-solidification technique. The flux purifies the melts, leading to a large undercooling and nanometer-sized microstructure. The as-prepared alloys are composed of nanolayered Ag and Cu within micrometer-sized grains. The bulk nanostructured alloys have an ultimate tensile strength of approximately 560 MPa, similar yield strength in tension and compression, elongation of 7% in tension, strain hardening exponent of 0.1, and relatively high mechanical and thermal stability up to 400 °C.
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81.40.Lm Deformation, plasticity, and creep
81.05.Bx Metals, semimetals, and alloys
81.07.Bc Nanocrystalline materials
62.20.F- Deformation and plasticity
64.70.D- Solid-liquid transitions
61.46.-w Structure of nanoscale materials
81.30.Fb Solidification

Dual layer selective emitter

Donald L. Chubb and David S. Wolford

Appl. Phys. Lett. 87, 141907 (2005); http://dx.doi.org/10.1063/1.2077853 (3 pages) | Cited 2 times

Online Publication Date: 28 September 2005

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A selective emitter consisting of two layers separated by a vacuum is analyzed. The bottom layer consists of a selective emitting material such as a rare earth containing crystal on a metal substrate. The top layer, which blocks long wavelength radiation, is a window such as sapphire with a deposited metal film. As a result of reduced long wavelength emission, the theoretical analysis shows that the emitter efficiency can be increased by nearly a factor of 2.
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84.60.Jt Photoelectric conversion

Characterization and reduction of twist in Ge on insulator produced by localized liquid phase epitaxy

Douglas J. Tweet, Jong Jan Lee, Jer-Shen Maa, and Sheng Teng Hsu

Appl. Phys. Lett. 87, 141908 (2005); http://dx.doi.org/10.1063/1.2077860 (3 pages) | Cited 17 times

Online Publication Date: 28 September 2005

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Conditions for producing high-quality localized Ge-on-insulator film stacks on Si substrates by liquid phase epitaxy are discussed. In particular, we have found that the resulting Ge crystal planes have a tendency to exhibit a twist about the long axis of the crystal. If the wafer is heated much above the Ge melting temperature, this twist can be quite large ( ∼ 10°). The twist can be minimized by heating to just above the melting temperature and by using thicker Ge films. In spite of this twist, the Ge regions on top of the insulating Si3N4 are observed to be free of dislocations and stacking faults.
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61.72.Nn Stacking faults and other planar or extended defects
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)

Photoluminescence characteristics of coupled CdSe/ZnS quantum dots on self-assembled silica nanospheres

K. S. Kang, H. L. Ju, W. H. Han, J. H. Lee, J. G. Choi, and D. W. Boo

Appl. Phys. Lett. 87, 141909 (2005); http://dx.doi.org/10.1063/1.2081125 (3 pages) | Cited 2 times

Online Publication Date: 28 September 2005

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The photoluminescence (PL) characteristics of the self-assembled silica nanospheres containing coupled CdSe/ZnS core-shell quantum dots (QDs) on the surface of the sphere were studied. The degree of QD-coupling and the distribution of the coupled QDs were controlled by generating spatially separated QD-binding sites on the surface of the spheres and varying the QD concentration during the attachment process. When the sizes of QD spots on the surface probed by atomic force microscopy gradually increased, the band gap energy shifted to lower energy via strong Förster energy transfer between nearby QDs, and when UV-exposure time increased, the band gap energy shifted to slightly higher energy, accompanied by a large enhancement in the emission intensity. Moreover, the Bragg diffraction of the self-assembled crystal structure of the spheres affected the PL characteristics of QDs by shifting the entire emission spectra toward longer wavelength as the incident angles were increased.
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78.55.Et II-VI semiconductors
78.67.Hc Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
81.16.Dn Self-assembly
71.20.Nr Semiconductor compounds

Observation of the lamellar phase in a Zr-free Sm(Co0.45Fe0.15Cu0.4)5 alloy

Y. Zhang, A. M. Gabay, and G. C. Hadjipanayis

Appl. Phys. Lett. 87, 141910 (2005); http://dx.doi.org/10.1063/1.2081120 (3 pages) | Cited 1 time

Online Publication Date: 29 September 2005

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A lamellar phase is reported in a Zr-free Sm(Co0.45Fe0.15Cu0.4)5 alloy after it had been solutionized and aged at 650 °C for 50 h. The lamellar phase has a composition of Sm(Co,Fe,Cu)3 with a Be3Nb structure type, which is similar to that in the fully heat-treated Sm(Co,Cu,Fe,Zr)z 2:17 magnets containing Zr. This observation is contrary to the traditional thinking that associates the lamellar phase to the presence of Zr. This present result provides additional information that can be used to elaborate the origin of coercivity in precipitation Sm-Co magnets.
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75.50.Bb Fe and its alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.40.Cd Solid solution hardening, precipitation hardening, and dispersion hardening; aging
81.40.Gh Other heat and thermomechanical treatments
81.30.Mh Solid-phase precipitation

Strain relaxation in InAs/InGaAs quantum dots investigated by photoluminescence and capacitance-voltage profiling

J. F. Chen, R. S. Hsiao, Y. P. Chen, J. S. Wang, and J. Y. Chi

Appl. Phys. Lett. 87, 141911 (2005); http://dx.doi.org/10.1063/1.2081132 (3 pages) | Cited 13 times

Online Publication Date: 29 September 2005

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We present detailed studies of the onset of strain relaxation in InAs/InGaAs quantum dots. We show that the ground-state photoluminescence (PL) emission redshifts with increasing the InAs coverage before relaxation and blueshifts when relaxation occurs. PL spectra of the relaxed samples show two predominant families of dots with very different temperature-dependent efficiency. By comparison we show that the dots emitting at long wavelength are degraded by relaxation while the dots emitting at short wavelength remain coherently strained. Consequently, the PL spectra are dominated by the dots emitting at short wavelength, leading to the observed blueshift. This result suggests that the relaxation does not occur uniformly. In addition, we show that the relaxation occurs in the dot bottom interface.
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78.67.Hc Quantum dots
73.61.Ey III-V semiconductors
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
73.63.Kv Quantum dots

Fracture toughness of nanostructured silicon carbide

M. Ippolito, A. Mattoni, L. Colombo, and F. Cleri

Appl. Phys. Lett. 87, 141912 (2005); http://dx.doi.org/10.1063/1.2081135 (3 pages) | Cited 6 times

Online Publication Date: 29 September 2005

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By using atomistic simulations, we derive a constitutive equation for a microfractured β-SiC matrix containing hard or soft inclusions. The proposed equation is shown to follow the Eshelby theory for elastic inclusions, and appears to hold for any crack tip-inclusion distance and for a wide range of values of matrix-inclusion elastic mismatch. A comparison of the atomistic results with existing continuum elasticity models points out the weaknesses of some commonly adopted simplifying assumptions.
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81.05.Mh Cermets, ceramic and refractory composites
81.07.Bc Nanocrystalline materials
62.25.-g Mechanical properties of nanoscale systems
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.M- Structural failure of materials
62.20.D- Elasticity

High-quality GaNAs/GaAs quantum wells with light emission up to 1.44 μm grown by molecular-beam epitaxy

S. M. Wang, Q. F. Gu, Y. Q. Wei, M. Sadeghi, A. Larsson, Q. X. Zhao, X. D. Wang, C. H. Ma, and Z. G. Xing

Appl. Phys. Lett. 87, 141913 (2005); http://dx.doi.org/10.1063/1.2084337 (3 pages) | Cited 6 times

Online Publication Date: 29 September 2005

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High-quality GaNAs/GaAs quantum wells with high substitutional N concentrations, grown by molecular-beam epitaxy, are demonstrated using a reduced growth rate in a range of 0.125–1 μm/h. No phase separation is observed and the GaNAs well thickness is limited by the critical thickness. Strong room-temperature photoluminescence with a record long wavelength of 1.44 μm is obtained from an 18-nm-thick GaN0.06As0.94/GaAs quantum well.
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81.07.St Quantum wells
81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.55.Cr III-V semiconductors
78.67.De Quantum wells

Metal-adlayer-stabilized ZnO(0001) surfaces: Toward a new growth mode for oxides

J. E. Northrup and J. Neugebauer

Appl. Phys. Lett. 87, 141914 (2005); http://dx.doi.org/10.1063/1.2077862 (3 pages) | Cited 7 times

Online Publication Date: 30 September 2005

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First-principles total energy calculations were performed for clean and In-rich ZnO(0001) surfaces. The calculations indicate that Zn adlayers themselves are not energetically favorable, but that In can form a stable adlayer in Zn-rich conditions. This result opens the possibility that In may serve as a surfactant for the growth of ZnO.
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73.20.At Surface states, band structure, electron density of states
71.15.Nc Total energy and cohesive energy calculations

Dislocation-induced nonuniform interfacial reactions of Ti/Al/Mo/Au ohmic contacts on AlGaN/GaN heterostructure

Liang Wang, Fitih M. Mohammed, and Ilesanmi Adesida

Appl. Phys. Lett. 87, 141915 (2005); http://dx.doi.org/10.1063/1.2081136 (3 pages) | Cited 25 times

Online Publication Date: 30 September 2005

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Transmission electron microscopy (TEM) is utilized to elucidate the postannealing interfacial microstructure of Ti/Al/Mo/Au metallization with AlGaN/GaN heterostructures to gain insight into the formation mechanism of low-resistance ohmic contacts. The reaction between the metal and the AlGaN layer does not proceed uniformly. Localized penetration through the AlGaN layer beyond the two-dimensional electron gas (2DEG) is observed while partial consumption of the AlGaN layer is noted in other areas. Analytical TEM analyses confirm that the main reaction product is TiN. A correlation between the appearance of TiN islands and threading dislocations is observed. Threading dislocations serve as short-circuit diffusion channels, and are responsible for the nonuniform reaction. TiN islands have a large total area of intimate contact with the 2DEG, and since no tunneling of electron through the AlGaN is required, a low-resistance ohmic contact is obtained.
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68.35.Fx Diffusion; interface formation
68.35.Ct Interface structure and roughness
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.37.Lp Transmission electron microscopy (TEM)
73.40.Ns Metal-nonmetal contacts
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
61.72.Cc Kinetics of defect formation and annealing
85.40.Ls Metallization, contacts, interconnects; device isolation

Adiabatic shear banding instability in bulk metallic glasses

L. H. Dai, M. Yan, L. F. Liu, and Y. L. Bai

Appl. Phys. Lett. 87, 141916 (2005); http://dx.doi.org/10.1063/1.2067691 (3 pages) | Cited 20 times

Online Publication Date: 30 September 2005

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In this letter, a linear instability analysis was performed to highlight the mechanism of formation of adiabatic shear banding instabilities in bulk metallic glasses (BMGs). It is found that this instability is determined by the free volume coalescence-diffusion Deborah number. The most important findings are that both free volume coalescence softening and adiabatic heating softening exert an influence on the formation of adiabatic shear banding instability in BMGs, and higher strain rate promotes the growth of instability. These results are of particular significance in understanding the mechanism of formation of shear bands in BMGs.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Theoretical study of environmental dependence of oxygen vacancy formation in CeO2

Yong Jiang, James B. Adams, Mark van Schilfgaarde, Renu Sharma, and Peter A. Crozier

Appl. Phys. Lett. 87, 141917 (2005); http://dx.doi.org/10.1063/1.2084324 (3 pages) | Cited 18 times

Online Publication Date: 30 September 2005

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Density functional methods were used to study the environmental dependence of O vacancy formation in CeO2. It was found that an O vacancy in the 2+ charged state has the lowest formation energy for a wide range of Fermi energies (EF) from 0 to ∼ 1.9 eV, while a neutral vacancy becomes the most stable at higher EF values. The O vacancy formation energy can be strongly affected by temperature (T) and the partial pressure of oxygen (PO2). The effect of T and PO2 on the equilibrium compositions of reduced ceria (CeO2−x) was also calculated and showed qualitative agreement with gravimetric experiments.
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61.72.J- Point defects and defect clusters
71.20.Ps Other inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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