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12 Apr 2010

Volume 96, Issue 15, Articles (15xxxx)

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Appl. Phys. Lett. 96, 153701 (2010); http://dx.doi.org/10.1063/1.3385388 (3 pages)

Sinan Keten and Markus J. Buehler
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Nonlinear electrophoretic motion of dielectric microparticles in nematic liquid crystals

A. V. Ryzhkova, F. V. Podgornov, and W. Haase

Appl. Phys. Lett. 96, 151901 (2010); http://dx.doi.org/10.1063/1.3386570 (3 pages) | Cited 4 times

Online Publication Date: 12 April 2010

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The motion of dielectric microparticles induced by external ac electric field, E, was investigated in the cell with in-plane stripe electrodes and filled with homeotropically aligned nematic liquid crystal. It was demonstrated that the average velocity of the microparticles depended on the third power of the electric field strength (v = μ(3)E3). The experimental results were explained by the induced charge electrophoresis.
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82.45.Un Dielectric materials in electrochemistry
82.45.Wx Polymers and organic materials in electrochemistry
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order

Nature of deep center emissions in GaN

A. Sedhain, J. Li, J. Y. Lin, and H. X. Jiang

Appl. Phys. Lett. 96, 151902 (2010); http://dx.doi.org/10.1063/1.3389497 (3 pages) | Cited 15 times

Online Publication Date: 12 April 2010

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Photoluminescence (PL) emission spectroscopy was employed to probe the nature of deep center emissions in GaN. The room temperature PL spectrum measured in the infrared (IR) region revealed an emission band centered around 1.23 eV. Based on detailed analysis of both the IR and visible emission spectra, we suggest that this emission band is a band-to-impurity transition involving a deep level complex consisting of a gallium vacancy and an oxygen atom sitting on one of the neighboring nitrogen sites; the (VGa–ON)2− charge state of (VGa–ON)2−/1−. Two electronic structures, which arise due to two different configurations of (VGa–ON)2−/1−, with ON either along the c-axis (axial configuration) or in one of the three equivalent tetrahedral positions (basal configuration), were observed. Our result also provides explicit evidence that both the yellow luminescence band and the 1.23 eV emission line in GaN are related to a common deep center, which is believed to be (VGa–ON)2−/1−.
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78.55.Cr III-V semiconductors
78.30.Fs III-V and II-VI semiconductors
71.55.Eq III-V semiconductors
71.20.Nr Semiconductor compounds
78.40.Fy Semiconductors
61.72.jd Vacancies

Abnormal pressure-induced structural transformations of gallium nitride nanowires

Zhaohui Dong and Yang Song

Appl. Phys. Lett. 96, 151903 (2010); http://dx.doi.org/10.1063/1.3394009 (3 pages) | Cited 1 time

Online Publication Date: 12 April 2010

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One-dimensional nanostructured GaN was studied at pressures up to 65 GPa by in situ synchrotron x-ray diffraction. A wurtzite-to-rocksalt transformation was observed at ∼ 55 GPa, an onset pressure higher than that for bulk GaN, but lower than that for nanocrystalline GaN. Such transformation was found extremely incomplete even at the highest pressure but was facilitated by decompression. In addition, GaN nanowires exhibited drastically different volume responses to compression than did GaN in other forms. These unusual pressure behaviors of GaN nanowires are attributed to the interplay of several factors involving the intrinsic nanoproperties and the compression conditions.
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62.50.-p High-pressure effects in solids and liquids
64.70.kg Semiconductors
64.70.Nd Structural transitions in nanoscale materials
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

Photoluminescence spectroscopy and positron annihilation spectroscopy probe of alloying and annealing effects in nonpolar m-plane ZnMgO thin films

A. L. Yang (杨安丽), H. P. Song (宋华平), D. C. Liang (梁德春), H. Y. Wei (魏鸿源), X. L. Liu (刘祥林), P. Jin (金鹏), X. B. Qin (秦秀波), S. Y. Yang (杨少延), Q. S. Zhu (朱勤生), and Z. G. Wang (王占国)

Appl. Phys. Lett. 96, 151904 (2010); http://dx.doi.org/10.1063/1.3394012 (3 pages) | Cited 7 times

Online Publication Date: 12 April 2010

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Temperature-dependent photoluminescence characteristics of non-polar m-plane ZnO and ZnMgO alloy films grown by metal organic chemical vapor deposition have been studied. The enhancement in emission intensity caused by localized excitons in m-plane ZnMgO alloy films was directly observed and it can be further improved after annealing in nitrogen. The concentration of Zn vacancies in the films was increased by alloying with Mg, which was detected by positron annihilation spectroscopy. This result is very important to directly explain why undoped Zn1−xMgxO thin films can show p-type conduction by controlling Mg content, as discussed by Li et al. [Appl. Phys. Lett. 91, 232115 (2007)] .
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78.66.Hf II-VI semiconductors
61.72.Cc Kinetics of defect formation and annealing
78.55.Et II-VI semiconductors
78.70.Bj Positron annihilation
61.72.jd Vacancies
71.35.-y Excitons and related phenomena

Applying uniform reversible strain to epitaxial oxide films

M. D. Biegalski, K. Dörr, D. H. Kim, and H. M. Christen

Appl. Phys. Lett. 96, 151905 (2010); http://dx.doi.org/10.1063/1.3374323 (3 pages) | Cited 20 times

Online Publication Date: 13 April 2010

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We demonstrate using four-circle x-ray diffraction that the piezoelectric substrate of Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) induces uniform reversible in-plane strain to epitaxially-grown oxide films and bilayers. The biaxial in-plane strain depends linearly on the applied electrical voltage. Utilizing the reversible strain, the strain-dependent lattice structure and the Poisson number characterizing the elastic response is determined for 200 nm thick SrTiO3, LaScO3, and BiFeO3 films. The uniformity and reversibility of the strain provides access to the direct quantitative measurement of strain-dependent properties of epitaxial oxide films.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.dj Poisson's ratio
68.35.Gy Mechanical properties; surface strains
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Thermal conductivity and dynamic heat capacity across the metal-insulator transition in thin film VO2

Dong-Wook Oh, Changhyun Ko, Shriram Ramanathan, and David G. Cahill

Appl. Phys. Lett. 96, 151906 (2010); http://dx.doi.org/10.1063/1.3394016 (3 pages) | Cited 13 times

Online Publication Date: 13 April 2010

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The thermal properties of VO2 thin films, 90–440 nm thick, are measured by time-domain thermoreflectance (TDTR) across the metal-insulator transition (MIT). The thermal conductivity increases by as much as 60% in the metallic phase; this increase in conductivity is in good agreement with the expected electronic contribution to the thermal conductivity. For relatively thick layers, TDTR data are sensitive to the dynamic heat capacity and show a pronounced peak near the MIT temperature created by a contribution to the enthalpy from periodic transformations at the 10 MHz frequency of the thermal waves used in the experiment. The dynamic heat capacity increases as the amplitude ΔT of the thermal waves becomes comparable to the width of the MIT and reaches ≈30% of the bulk latent heat for ΔT ≈ 1.6 K.
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71.30.+h Metal-insulator transitions and other electronic transitions
05.70.Ce Thermodynamic functions and equations of state
65.40.Ba Heat capacity
78.20.N- Thermo-optic effects

Thermal stability comparison of TaN on HfO2 and Al2O3

Jinhee Kwon and Yves J. Chabal

Appl. Phys. Lett. 96, 151907 (2010); http://dx.doi.org/10.1063/1.3396189 (3 pages) | Cited 1 time

Online Publication Date: 14 April 2010

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Changes in gate stacks (TaN/high-κ/SiO2/Si) are investigated during thermal processing using in situ Fourier transform infrared spectroscopy. Ta–O bonds, present in the initial tantalum nitride films, are crystallized in contact with HfO2 at 700–800 °C and completely dissociate at 900 °C, resulting in a microcrystalline TaNx phase. The TaN remains amorphous, however, with Al2O3 as the underlying dielectric layer. A partial reduction into a metallic Ta occurs after dissociation of Ta–N and Ta–O bonds, and Al2O3 decomposes at 700 °C. Dissociated Al atoms diffuse into all the neighboring layers to form silicate and Ta- and N-bound Al.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
81.40.Gh Other heat and thermomechanical treatments
64.70.K- Solid-solid transitions
78.30.-j Infrared and Raman spectra

Electron-nuclear spin control in charged semiconductor quantum dots by electrical currents through micro-coils

Jungtaek Kim, J. Puls, Y. S. Chen, G. Bacher, and F. Henneberger

Appl. Phys. Lett. 96, 151908 (2010); http://dx.doi.org/10.1063/1.3373591 (3 pages) | Cited 2 times

Online Publication Date: 14 April 2010

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We have fabricated micrometer-sized single-turn coils on top of charged CdSe/ZnSe quantum dot heterostructures by lithographical techniques. Current injection creates magnetic fields in the some 10 mT range, strong enough to modulate the hyperfine interaction. The very low coil inductance allows for generation of fast field transients. We demonstrate local control of the resident electron spin as well as read-out of the nuclear spin state on the 10 ns time scale by electrical current pulses.
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73.21.La Quantum dots
81.07.Ta Quantum dots
81.16.Nd Micro- and nanolithography
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
71.70.Jp Nuclear states and interactions

Structural and antireflective characteristics of catalyst-free GaN nanostructures on GaN/sapphire template for solar cell applications

C. Y. Park, J. M. Lim, J. S. Yu, and Y. T. Lee

Appl. Phys. Lett. 96, 151909 (2010); http://dx.doi.org/10.1063/1.3386538 (3 pages) | Cited 3 times

Online Publication Date: 14 April 2010

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We report the structural and antireflective characteristics of catalyst-free GaN nanostructures on GaN/sapphire template for solar cell applications. The formation of GaN nanostructure depends on the growth temperature. At 530 °C, the nucleation of the initial seed due to the enhanced strain leads to the hexagonal closely packed nanorods. As the growth temperature decreases, the depression of atomic Ga transport changes the films into nanocolumns, and then the nanorods. The catalyst-free GaN nanorods have wurtzite structure and good single crystalline quality. The GaN nanorods exhibit a remarkably low reflectance over a wide-angle broadband spectrum, enhancing the antireflective property of GaN surface.
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68.55.ag Semiconductors
42.79.Wc Optical coatings
78.67.Qa Nanorods
78.30.Fs III-V and II-VI semiconductors
78.66.Fd III-V semiconductors
78.40.Fy Semiconductors

Dynamic smoothing of nanocomposite films

Y. T. Pei, A. A. Turkin, C. Q. Chen, K. P. Shaha, D. Vainshtein, and J. Th. M. De Hosson

Appl. Phys. Lett. 96, 151910 (2010); http://dx.doi.org/10.1063/1.3394019 (3 pages) | Cited 2 times

Online Publication Date: 14 April 2010

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In contrast to the commonly observed dynamic roughening in film growth we have observed dynamic smoothing in the growth of diamondlike-carbon nanocomposite (TiC/a-C) films up to 1.5 μm thickness. Analytical and numerical simulations, based on the Edwards–Wilkinson model and the Mullins model, visualize the effect of the diffusivity parameters and the noise strength on the interface evolution of dynamic smoothing. The prediction is in a good agreement with the measured roughness evolution. High-resolution transmission electron microscopy shows that the formation of an amorphous front layer 2 nm thick excludes possible influence of nanocrystallites on the dynamic growth behavior of the nanocomposite film.
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81.05.uj Diamond/nanocarbon composites
68.55.-a Thin film structure and morphology
81.16.-c Methods of micro- and nanofabrication and processing
68.37.Og High-resolution transmission electron microscopy (HRTEM)
68.35.Ct Interface structure and roughness
66.30.-h Diffusion in solids
61.46.-w Structure of nanoscale materials

Theoretical investigation of optical polarization properties in Al-rich AlGaN quantum wells with various substrate orientations

A. Atsushi Yamaguchi

Appl. Phys. Lett. 96, 151911 (2010); http://dx.doi.org/10.1063/1.3409121 (3 pages) | Cited 13 times

Online Publication Date: 15 April 2010

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The optical polarization properties of Al-rich AlGaN thin quantum wells on AlN substrates with various substrate orientations are numerically calculated using a 6×6 k⋅p Hamiltonian. The calculation results show that the predominant polarization direction abruptly switches from the in-plane direction to the c-axis direction at an Al composition of ∼ 0.76 in quantum wells on c-plane substrates. It is also shown that the polarization characteristics drastically change by small inclination of the substrates due to valence band mixing effects. It is found that the use of the vicinal substrates as well as semipolar and nonpolar substrates could be beneficial in improving optical device performance.
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78.67.De Quantum wells
71.20.Nr Semiconductor compounds
78.66.Fd III-V semiconductors

Optimization of silica–silver–gold layered nanoshell for large near-field enhancement

D. J. Wu and X. J. Liu

Appl. Phys. Lett. 96, 151912 (2010); http://dx.doi.org/10.1063/1.3398032 (3 pages) | Cited 7 times

Online Publication Date: 15 April 2010

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The near-field enhancement of silica–silver–gold (SSG) nanoshells has been investigated by using Mie theory. If the thickness of Au shell is smaller than 5 nm, the maximum of near-field for SSG increases first and then decreases with increasing Ag layer thickness. Thus, the near-field for SSG can be modified by Ag layer and can possess more excellent near-field enhancement. If the thickness of Au shell is larger than 10 nm, the maximum of near-field of SSG nanoshell decreases with increasing Ag layer thickness. We further discuss the influence of Au shell thickness on the near-field of SSG nanoshell.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.30.-j Infrared and Raman spectra
78.40.-q Absorption and reflection spectra: visible and ultraviolet
61.46.-w Structure of nanoscale materials

Resonant photoluminescent charging of epitaxial graphene

S. D. Lounis, D. A. Siegel, R. Broesler, C. G. Hwang, E. E. Haller, and A. Lanzara

Appl. Phys. Lett. 96, 151913 (2010); http://dx.doi.org/10.1063/1.3396201 (3 pages) | Cited 2 times

Online Publication Date: 16 April 2010

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Photoluminescence spectroscopy (PL) and angle resolved photoemission spectroscopy have been used to study the interaction between epitaxially grown graphene and the silicon carbide (SiC) substrate. We report evidence of an anomalous temperature dependent shift in the Fermi energy with a maximum at 65 K. A similarly anomalous onset of the photoluminescence spectrum is also observed at this temperature. These results are explained by the formation of a Schottky barrier at the graphene/SiC interface, which is also responsible for large electron doping of epitaxially grown graphene films. Moreover, we identify a strong resonance between the sharp no-phonon peaks of the D1 photoluminescence spectrum in SiC and the electronic transition across the barrier, suggesting that epitaxial graphene might be also used to selectively detect photons of precise energies.
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78.66.Tr Fullerenes and related materials
73.30.+y Surface double layers, Schottky barriers, and work functions
78.55.Hx Other solid inorganic materials
73.22.Pr Electronic structure of graphene
68.55.-a Thin film structure and morphology
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