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2 May 2011

Volume 98, Issue 18, Articles (18xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 98, 183101 (2011); http://dx.doi.org/10.1063/1.3584006 (3 pages)

Tuba Oznuluer, Ercag Pince, Emre O. Polat, Osman Balci, Omer Salihoglu, and Coskun Kocabas
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Ultimate photovoltage in perovskite oxide heterostructures with critical film thickness

Cong Wang, Kui-juan Jin, Rui-qiang Zhao, Hui-bin Lu, Hai-zhong Guo, Chen Ge, Meng He, Can Wang, and Guo-zhen Yang

Appl. Phys. Lett. 98, 181101 (2011); http://dx.doi.org/10.1063/1.3586250 (3 pages) | Cited 2 times

Online Publication Date: 2 May 2011

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One order larger photovoltage is obtained with critical thicknesses of La0.9Sr0.1MnO3 films in both kinds of heterostructures of La0.9Sr0.1MnO3/SrTiO3 (0.8 wt % Nb-doped) and La0.9Sr0.1MnO3/Si fabricated at various oxygen pressures. Our self-consistent calculation reveals that the critical thickness of the La0.9Sr0.1MnO3 film with the ultimate value of photovoltage is just the thickness of the depletion layer of La0.9Sr0.1MnO3 in both heterojunctions, respectively.
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73.50.Pz Photoconduction and photovoltaic effects
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.ag Semiconductors
FREE

Room temperature quantum cascade lasers with 27% wall plug efficiency

Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi

Appl. Phys. Lett. 98, 181102 (2011); http://dx.doi.org/10.1063/1.3586773 (3 pages) | Cited 32 times

Online Publication Date: 2 May 2011

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Using the recently proposed shallow-well design, we demonstrate InP based quantum cascade lasers (QCLs) emitting around 4.9 μm with 27% and 21% wall plug efficiencies in room temperature (298 K) pulsed and continuous wave (cw) operations, respectively. The laser core consists of 40 QCL-stages. The highest cw efficiency is obtained from a buried-ridge device with a ridge width of 8 μm and a cavity length of 5 mm. The front and back facets are antireflection and high-reflection coated, respectively. The maximum single facet cw power at room temperature amounts to 5.1 W.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.15.Eq Optical system design
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.79.Wc Optical coatings

Self-frequency-doubling of ultrafast laser inscribed neodymium doped yttrium aluminum borate waveguides

Ningning Dong, J. Martínez de Mendivil, E. Cantelar, G. Lifante, J. Vázquez de Aldana, G. A. Torchia, Feng Chen, and Daniel Jaque

Appl. Phys. Lett. 98, 181103 (2011); http://dx.doi.org/10.1063/1.3584852 (3 pages) | Cited 5 times

Online Publication Date: 3 May 2011

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Buried channel optical waveguides, supporting orthogonal polarizations, have been fabricated in a neodymium doped yttrium aluminum borate nonlinear laser crystal by ultrafast laser inscription following the so-called “double line” approach. Confocal fluorescence and second harmonic imaging experiments have revealed that the original fluorescence and nonlinear properties have been not deteriorated by the waveguide inscription procedure. Preliminary laser experiments have shown the ability of the fabricated structures for green laser light generation under 808 nm optical pumping by self-frequency-doubling of the 1.06 μm laser line of neodymium ions.
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42.79.Gn Optical waveguides and couplers
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.62.-b Laser applications

Green GaInN photonic-crystal light-emitting diodes with small surface recombination effect

Hitoshi Kitagawa, Masayuki Fujita, Toshihide Suto, Takashi Asano, and Susumu Noda

Appl. Phys. Lett. 98, 181104 (2011); http://dx.doi.org/10.1063/1.3576927 (3 pages) | Cited 5 times

Online Publication Date: 3 May 2011

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We have fabricated green GaInN light-emitting diodes (LEDs) containing two-dimensional photonic crystals (PCs). The PC structure is comprised of air holes that penetrate through the active layer. The observed emission intensity at room temperature was enhanced by a factor of ∼ 3 when a PC was introduced, which is close to the theoretical enhancement calculated without taking into account surface recombination at the air-hole edges. The carrier lifetime changes little when a PC is incorporated due to the low surface recombination. In contrast, the carrier lifetime in blue-emitting LEDs decreased by a factor of 4 when a PC was introduced. The surface-recombination velocity in green-emitting devices is estimated to be 3×102 cm/s, an order of magnitude smaller than in blue-emitting devices. This is due to the strong carrier localization effect in green-emitting GaInN.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes

Lead silicate glass microsphere resonators with absorption-limited Q

Pengfei Wang, Ganapathy Senthil Murugan, Timothy Lee, Xian Feng, Yuliya Semenova, Qiang Wu, Wei Loh, Gilberto Brambilla, James S. Wilkinson, and Gerald Farrell

Appl. Phys. Lett. 98, 181105 (2011); http://dx.doi.org/10.1063/1.3586771 (3 pages) | Cited 2 times

Online Publication Date: 3 May 2011

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We report the fabrication and characterization of a lead-silicate glass microsphere resonator. We show that at the wavelengths near 1555 nm high Q modes can be efficiently excited from a 109 μm diameter lead-silicate glass microsphere via evanescent coupling using a tapered silica fiber with a waist diameter of 2 μm. Resonances with Q-factors as high as 0.9×107 were observed. This is very close to the theoretical material-limited Q-factor and is the highest Q-factor reported so far from a nonlinear glass microsphere.
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42.65.-k Nonlinear optics
42.81.Qb Fiber waveguides, couplers, and arrays
42.70.Ce Glasses, quartz
42.86.+b Optical workshop techniques

2.4 W room temperature continuous wave operation of distributed feedback quantum cascade lasers

Q. Y. Lu, Y. Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi

Appl. Phys. Lett. 98, 181106 (2011); http://dx.doi.org/10.1063/1.3588412 (3 pages) | Cited 19 times

Online Publication Date: 4 May 2011

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We demonstrate high power continuous-wave room-temperature operation surface-grating distributed feedback quantum cascade lasers at 4.8 μm. High power single mode operation benefits from a combination of high-reflection and antireflection coatings. Maximum single-facet continuous-wave output power of 2.4 W and peak wall plug efficiency of 10% from one facet is obtained at 298 K. Single mode operation with a side mode suppression ratio of 30 dB and single-lobed far field without beam steering is observed.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Terahertz emission from InAs/GaAs quantum dot based photoconductive devices

N. S. Daghestani, M. A. Cataluna, G. Berry, G. Ross, and M. J. Rose

Appl. Phys. Lett. 98, 181107 (2011); http://dx.doi.org/10.1063/1.3586774 (3 pages)

Online Publication Date: 4 May 2011

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We report terahertz (THz) generation from InAs/GaAs quantum-dot based photoconductive antennae with femtosecond optical excitation at 800 nm, with an estimated infrared-to-THz conversion efficiency of ∼ 0.9×10−5. The quantum dots act as recombination centers for carriers generated in the GaAs layers within the structure. Photoreflective pump-probe measurements reveal a decrease in the carrier lifetime when a lateral voltage is applied. These antennae displayed resilience to Joule heating when operated at a field strength of 46 MV/m. The breakdown field of the devices was 48 MV/m, which is comparable to the breakdown field of bulk GaAs.
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85.60.Bt Optoelectronic device characterization, design, and modeling
84.40.Ba Antennas: theory, components and accessories
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Anisotropic structural and optical properties of a-plane (11math0) AlInN nearly-lattice-matched to GaN

Masihhur R. Laskar, Tapas Ganguli, A. A. Rahman, Ashish Arora, Nirupam Hatui, M. R. Gokhale, Sandip Ghosh, and Arnab Bhattacharya

Appl. Phys. Lett. 98, 181108 (2011); http://dx.doi.org/10.1063/1.3583457 (3 pages) | Cited 3 times

Online Publication Date: 5 May 2011

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We report epitaxial growth of a-plane (11math0) AlInN layers nearly-lattice-matched to GaN. Unlike for c-plane oriented epilayers, a-plane Al1−xInxN cannot be simultaneously lattice-matched to GaN in both in-plane directions. We study the influence of temperature on indium incorporation and obtain nearly-lattice-matched Al0.81In0.19N at a growth temperature of 760 °C. We outline a procedure to check in-plane lattice mismatch using high-resolution x-ray diffraction, and evaluate the strain and critical thickness. Polarization-resolved optical transmission measurements of the Al0.81In0.19N epilayer reveal a difference in band gap of ∼ 140 meV between (electric field) E∥c [0001]-axis and E⊥c conditions with room-temperature photoluminescence peaked at 3.38eV strongly polarized with E∥c, in good agreement with strain-dependent band-structure calculations.
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81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.55.ag Semiconductors
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Far field subwavelength focusing using optical eigenmodes

Jörg Baumgartl, Sebastian Kosmeier, Michael Mazilu, Edward T. F. Rogers, Nikolay I. Zheludev, and Kishan Dholakia

Appl. Phys. Lett. 98, 181109 (2011); http://dx.doi.org/10.1063/1.3587636 (3 pages) | Cited 6 times

Online Publication Date: 6 May 2011

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We report the focusing of light to generate a subdiffractive, subwavelength focal spot of full width half maximum 222 nm at an operating wavelength of 633 nm using an optical eigenmode approach. Crucially, the spot is created in the focal plane of a microscope objective thus yielding a practical working distance for applications. The optical eigenmode approach is implemented using an optimal superposition of Bessel beams on a spatial light modulator. The effects of partial coherence are also discussed. This far field method is a key advance toward the generation of subdiffractive optical features for imaging and lithographic purposes.
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42.79.-e Optical elements, devices, and systems
02.10.Ud Linear algebra

Fast self-heating in GaN-based laser diodes

W. G. Scheibenzuber and U. T. Schwarz

Appl. Phys. Lett. 98, 181110 (2011); http://dx.doi.org/10.1063/1.3587810 (3 pages) | Cited 3 times

Online Publication Date: 6 May 2011

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We study the time evolution of the internal temperature of GaN-based laser diodes in pulsed operation using time resolved spectroscopy. Time dependent emission spectra are compared to continuous-wave measurements at different temperatures to relate changes in the longitudinal mode spectrum to the internal temperature. From the different shift in emission center and longitudinal modes, two subsystems are identified which heat up on different time scales: the charge carrier plasma and the crystal lattice. While the lattice takes several microseconds to reach thermal equilibrium, the plasma heats up within 20 ns after the onset of the electrical pulse. This behavior is attributed to the small heat capacity of the charge carrier plasma compared to the crystal lattice.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Strain compensation in AlInN/GaN multilayers on GaN substrates: Application to the realization of defect-free Bragg reflectors

Gatien Cosendey, Jean-François Carlin, Nils A. K. Kaufmann, Raphaël Butté, and Nicolas Grandjean

Appl. Phys. Lett. 98, 181111 (2011); http://dx.doi.org/10.1063/1.3586767 (3 pages) | Cited 11 times

Online Publication Date: 6 May 2011

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We report on the growth conditions of AlInN layers grown on free-standing GaN substrates. We found that an average indium content of ∼ 20% is needed to obtain defect-free AlInN/GaN multilayers. This is larger than the commonly accepted value of 18% for lattice-matched condition. A model where tensile strain at the GaN/AlInN interface is induced by indium surface segregation occurring in AlInN layers is proposed to explain this discrepancy. A high In/Al flux ratio is shown to reduce this effect and allowed obtaining a defect-free AlInN/GaN Bragg reflector with a peak reflectivity of 99.6% suitable for vertical cavity light emitting devices.
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68.35.Gy Mechanical properties; surface strains
68.65.Ac Multilayers
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
68.35.Dv Composition, segregation; defects and impurities
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
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Coulomb versus drag effects in a photoelectron bunch for extreme ultraviolet laser spectroscopy

Davide Bleiner and Jürg Balmer

Appl. Phys. Lett. 98, 181501 (2011); http://dx.doi.org/10.1063/1.3586768 (3 pages)

Online Publication Date: 3 May 2011

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Coulomb effects in an extreme ultraviolet laser-induced photoelectron bunch for spectroscopy were investigated computationally. Post-photoemission alteration of the trajectory, due to Coulombic interactions, hampers an accurate angle-resolved photoelectron spectroscopy. Simulations however demonstrated that operation in the redundant-population regime produces a drag-confined bunch, detached from a boundary sheath that experiences losses. Drastic reduction in the photoelectron density can loosen bunch-confinement and proportionally increase the boundary losses. Our simulations led to a critical number of photoelectrons per spot size equal to 1.5×104 e−/mm2.
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07.60.Rd Visible and ultraviolet spectrometers
79.60.-i Photoemission and photoelectron spectra
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Electronic structure of GaInN semiconductors investigated by x-ray absorption spectroscopy

Q. X. Guo, H. Senda, K. Saito, T. Tanaka, M. Nishio, J. Ding, T. X. Fan, D. Zhang, X. Q. Wang, S. T. Liu, B. Shen, and R. Ohtani

Appl. Phys. Lett. 98, 181901 (2011); http://dx.doi.org/10.1063/1.3583461 (3 pages) | Cited 1 time

Online Publication Date: 2 May 2011

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We have investigated the electronic structure of GaInN semiconductors by performing x-ray absorption near-edge fine structure (XANES) spectroscopy at Ga K-edge and self-consistent-field real-space multiple-scattering theory calculations. It was demonstrated that the nondestructive Ga K-edge XANES spectra can be used as the fingerprints of structure and composition for GaInN. The theoretical calculations gave a reasonable reproduction of the experimental spectral features. The results revealed that the combination of the experimental XANES and the theoretical calculations is a powerful tool for studying the electronic structure of GaInN semiconductors.
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71.20.Nr Semiconductor compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
78.70.Dm X-ray absorption spectra

Surface roughness reduction in nanocrystalline Cu thin films by electrical stressing treatment

Tsung-Cheng Chan, Kuan-Chia Chen, and Chien-Neng Liao

Appl. Phys. Lett. 98, 181902 (2011); http://dx.doi.org/10.1063/1.3586772 (3 pages) | Cited 2 times

Online Publication Date: 2 May 2011

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Electromigration-induced surface morphological evolution of nanocrystalline Cu thin films is reported. When applying a high-density current (106 A/cm2), the Cu films showed reduced surface roughness from 7.5 to 1.4 nm after electrical stressing at the temperature below 100 °C. It is suggested that preferential surface diffusion on Cu(111) planes leads to thinning of extruded grains in the electrically stressed Cu film, as evidenced by the weakening (111) texture of the Cu film after electrical stressing. The electrical stressing process shall help reduce roughness of Cu metallization after post thermal treatment.
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68.35.bd Metals and alloys
68.35.Fx Diffusion; interface formation
66.30.Qa Electromigration
66.30.Pa Diffusion in nanoscale solids
81.07.Bc Nanocrystalline materials
81.40.Gh Other heat and thermomechanical treatments

Epitaxial graphene prepared by chemical vapor deposition on single crystal thin iridium films on sapphire

Chi Vo-Van, Amina Kimouche, Antoine Reserbat-Plantey, Olivier Fruchart, Pascale Bayle-Guillemaud, Nedjma Bendiab, and Johann Coraux

Appl. Phys. Lett. 98, 181903 (2011); http://dx.doi.org/10.1063/1.3585126 (3 pages) | Cited 4 times

Online Publication Date: 3 May 2011

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Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of defects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e., much higher than on metal thin films used so far.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.Mm Grain and twin boundaries
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
81.05.ue Graphene
78.30.Na Fullerenes and related materials
68.55.ap Fullerenes

Atomic scale characterization of GaInN/GaN multiple quantum wells in V-shaped pits

Shigetaka Tomiya, Yuya Kanitani, Shinji Tanaka, Tadakatsu Ohkubo, and Kazuhiro Hono

Appl. Phys. Lett. 98, 181904 (2011); http://dx.doi.org/10.1063/1.3585118 (3 pages) | Cited 2 times

Online Publication Date: 4 May 2011

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High-angle annular dark field scanning transmission electron microscopy and atom probe tomography of the V-shaped pits in GaInN/GaN multiple quantum wells have revealed that a quantum well structure exists at the slope region of the V-shaped pits. Their thickness and In concentrations were found to be much lower compared to those of the flat region. This suggests that threading dislocations in the V-shaped pits act as energy barriers for the lateral transport of charge carries and that the pit center may not work properly for vertical transport because of the collapse of well-defined quantum well structures.
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73.63.Hs Quantum wells
68.65.Fg Quantum wells
73.61.Ey III-V semiconductors
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

In-depth resolved Raman scattering analysis for the identification of secondary phases: Characterization of Cu2ZnSnS4 layers for solar cell applications

X. Fontané, L. Calvo-Barrio, V. Izquierdo-Roca, E. Saucedo, A. Pérez-Rodriguez, J. R. Morante, D. M. Berg, P. J. Dale, and S. Siebentritt

Appl. Phys. Lett. 98, 181905 (2011); http://dx.doi.org/10.1063/1.3587614 (3 pages) | Cited 25 times

Online Publication Date: 5 May 2011

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This work reports the in-depth resolved Raman scattering analysis with different excitation wavelengths of Cu2ZnSnS4 layers. Secondary phases constitute a central problem in this material, particularly since they cannot be distinguished by x-ray diffraction. Raman spectra measured with 325 nm excitation light after sputtering the layers to different depths show peaks that are not detectable by excitation in the visible. These are identified with Cu3SnS4 modes at the surface region while spectra measured close to the back region show peaks from ZnS and MoS2. Observation of ZnS is enhanced by resonant excitation conditions achieved when working with UV excitation.
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78.30.Hv Other nonmetallic inorganics
78.66.Li Other semiconductors
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
88.40.H- Solar cells (photovoltaics)

Determination of Clausius–Mossotti factors and surface capacitances for colloidal particles

T. Honegger, K. Berton, E. Picard, and D. Peyrade

Appl. Phys. Lett. 98, 181906 (2011); http://dx.doi.org/10.1063/1.3583441 (3 pages) | Cited 6 times

Online Publication Date: 5 May 2011

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We propose a method to experimentally determine the Clausius–Mossotti factors and surface capacitances of colloidal particles. This two-step method is based on the following: (i) a precise positioning of particles on activated electrodes according to the applied frequency of an electric field and (ii) particles velocities measurements from a pure dielectrophoretic regime to build the Clausius–Mossotti factor. It confirms previous literature methods and measures the surface capacitance values for a wide range of particles such as polystyrene, silica, and gold whose diameters are at least 200 nm.
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82.45.Un Dielectric materials in electrochemistry
82.70.Dd Colloids
77.84.Nh Liquids, emulsions, and suspensions; liquid crystals

Improved diamond surfaces following lift-off and plasma treatments as observed by x-ray absorption spectroscopy

Alastair Stacey, Virginia S. Drumm, Barbara A. Fairchild, Kumar Ganesan, Sergey Rubanov, Rafi Kalish, Bruce C. C. Cowie, Steven Prawer, and Alon Hoffman

Appl. Phys. Lett. 98, 181907 (2011); http://dx.doi.org/10.1063/1.3585106 (3 pages) | Cited 6 times

Online Publication Date: 5 May 2011

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We have investigated the nature of the residual damage in diamond crystals following the ion implantation/graphitization “lift-off” process, using near-edge x-ray absorption fine structure spectroscopy and transmission electron microscopy. A defective but crystalline interface is found, which displays dense pre-edge unoccupied states and an almost complete loss of the core-level C 1s exciton signature. This residual crystalline damage is resistant to standard chemical etching, however a hydrogen plasma treatment is found to completely recover a pristine diamond surface. Analysis and removal of residual ion-induced damage is considered crucial to the performance of many diamond device architectures.
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52.77.Dq Plasma-based ion implantation and deposition
81.05.ug Diamond
78.70.Dm X-ray absorption spectra
61.05.cj X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.
61.72.-y Defects and impurities in crystals; microstructure
81.65.Cf Surface cleaning, etching, patterning

Carrier recombination processes in Mg-doped N-polar InN films

Daichi Imai, Yoshihiro Ishitani, Masayuki Fujiwara, Kazuhide Kusakabe, Xinqian Wang, and Akihiko Yoshikawa

Appl. Phys. Lett. 98, 181908 (2011); http://dx.doi.org/10.1063/1.3586775 (3 pages) | Cited 4 times

Online Publication Date: 5 May 2011

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We investigate the nonradiative carrier recombination (NR) process in Mg-doped p-InN films having lower photoluminescence (PL) intensity than n-InN films. The NR activation energy in the p-type films is found to be in a range of 9–15 meV, which is smaller than that in n-InN films (40–65 meV). We also investigate the effect of the greater mean free path of minority carriers in p-InN. At room temperature the collision rate of minority carriers with NR centers within the radiative lifetime in p-InN is found to be three orders of magnitude greater than that in n-InN.
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73.61.Ey III-V semiconductors
78.66.Fd III-V semiconductors
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
78.55.Cr III-V semiconductors

Nanoscale rotational deformation in solids at high stresses

I. A. Ovid’ko and A. G. Sheinerman

Appl. Phys. Lett. 98, 181909 (2011); http://dx.doi.org/10.1063/1.3587637 (3 pages) | Cited 3 times

Online Publication Date: 6 May 2011

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A special physical mode of plastic flow and nanograin formation in nanocrystalline and polycrystalline solids deformed at high stresses is suggested and theoretically described. The mode represents the nanoscale rotational deformation (NRD) occurring through the collective events of ideal nanoscale shear in solids. We calculated its stress and energy characteristics. It is found that NRD can effectively occur in nanocrystalline and polycrystalline solids during dynamic loading.
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46.35.+z Viscoelasticity, plasticity, viscoplasticity
62.25.-g Mechanical properties of nanoscale systems

Photo- and field-induced charge-separation and phosphorescence quenching in organometallic complex Ir(ppy)3

Mohan Singh Mehata and Nobuhiro Ohta

Appl. Phys. Lett. 98, 181910 (2011); http://dx.doi.org/10.1063/1.3582916 (3 pages) | Cited 2 times

Online Publication Date: 6 May 2011

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Electric field effects on absorption and photoluminescence (PL) spectra of organometallic phosphorescent emitter Ir(ppy)3, {tris[2-phenylpyridinato-C2,N] iridium (III)} doped in a film of polymethyl methacrylate (PMMA) have been confirmed at temperatures in the range of 40–295 K. Field-induced quenching of PL observed for Ir(ppy)3 is attributed to the decrease both of emitting state population and of the lifetime of PL. The quenching is independent of excitation energy as well as temperature. Field-assisted charge separation or dissociation of electron-hole (e-h) pair produced by photoexcitation may decrease the population of the emitting state. The Stark shifts on absorption and PL spectra have also been analyzed.
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78.55.Kz Solid organic materials
78.20.Jq Electro-optical effects
78.66.Qn Polymers; organic compounds
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect

Negative thermal expansion in ZnF2

Tapan Chatterji, Mohamed Zbiri, and Thomas C. Hansen

Appl. Phys. Lett. 98, 181911 (2011); http://dx.doi.org/10.1063/1.3588414 (3 pages) | Cited 3 times

Online Publication Date: 6 May 2011

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We have investigated temperature dependence of the lattice parameters and the unit cell volume of ZnF2 by neutron diffraction and have discovered negative thermal expansion (NTE) at low temperature. To understand why this simple compound exhibits NTE we performed first principle calculations. These calculations reproduce qualitatively the experimental temperature dependence of volume.
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65.40.De Thermal expansion; thermomechanical effects
61.66.-f Structure of specific crystalline solids

Temperature-dependent shifts of ultraviolet multipeak emissions for Mn-doped ZnO nanowires

Feihong Jiang and Jun Zhang

Appl. Phys. Lett. 98, 181912 (2011); http://dx.doi.org/10.1063/1.3586253 (3 pages) | Cited 1 time

Online Publication Date: 6 May 2011

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Mn-doped ZnO nanowires have been fabricated through high-temperature vapor-solid deposition process. The temperature-dependent photoluminescence properties of Mn-doped ZnO nanowires under 10 to 300 K have been investigated. The results show that the prepared samples have intensive multipeak emissions in the ultraviolet (UV) region (about 3.4–3.0 eV) at low temperature. The analyses of Gaussian-fitted the UV band from 10 to 300 K reveal that all UV spectra can be well fitted by four Gaussian peaks. With increasing temperature, the four UV bands show different temperature dependences and have obvious redshift. The origins of the intensive UV multipeak emissions can be attributed to the excitonic transition in Mn-doped ZnO nanowires under low temperature. The analysis results indicate that the crystal quality and UV luminescence efficiency of ZnO nanowires are improved with Mn doping.
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78.67.Uh Nanowires
73.21.Hb Quantum wires
78.55.Et II-VI semiconductors
81.05.Dz II-VI semiconductors
71.35.-y Excitons and related phenomena
78.40.Fy Semiconductors

AX centers in II-VI semiconductors: Hybrid functional calculations

Koushik Biswas and Mao-Hua Du

Appl. Phys. Lett. 98, 181913 (2011); http://dx.doi.org/10.1063/1.3583661 (3 pages)

Online Publication Date: 6 May 2011

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Hybrid functional calculations predict significantly enhanced stability of AX centers against shallow acceptors in selected II-VI semiconductors (ZnO, ZnS, and ZnSe), as compared to the calculations based on local density approximation and generalized gradient approximation. The results agree well with the experimental observations on the p-type doping of ZnS and ZnSe. The improved description of the AX centers by hybrid functional calculations is due to the correction of the valence band maximum of the semiconductor.
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71.20.Nr Semiconductor compounds
71.15.-m Methods of electronic structure calculations
61.72.uj III-V and II-VI semiconductors
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