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12 Feb 2007

Volume 90, Issue 7, Articles (07xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 90, 079903 (2007); http://dx.doi.org/10.1063/1.2692435 (3 pages)

Dong-Yol Yang, Sang Hu Park, Tae Woo Lim, Hong-Jin Kong, Shin Wook Yi, Hyun Kwan Yang, and Kwang-Sup Lee
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Effect of intense chirped pulses on the coherent phonon generation in Te

O. V. Misochko, T. Dekorsy, S. V. Andreev, V. O. Kompanets, Yu. A. Matveets, A. G. Stepanov, and S. V. Chekalin

Appl. Phys. Lett. 90, 071901 (2007); http://dx.doi.org/10.1063/1.2476306 (3 pages) | Cited 2 times

Online Publication Date: 12 February 2007

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The authors have studied the influence of chirped laser pulses on the coherent phonon generation in single crystal Te. They have shown that the pulse chirp affects the amplitude of coherent phonons with A1 symmetry in the case of intense excitation only. By varying the chirp of an intense exciting pulse, the authors demonstrated that negatively chirped pulses are almost twice more effective in the creation of lattice coherence than positively chirped pulses.
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78.47.-p Spectroscopy of solid state dynamics
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
63.20.-e Phonons in crystal lattices

Emission of unidentified energy from growing ice crystals

XiaoFeng Shen

Appl. Phys. Lett. 90, 071902 (2007); http://dx.doi.org/10.1063/1.2472536 (3 pages)

Online Publication Date: 12 February 2007

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Power-compensated differential scanning calorimeter (DSC) was used to quantitatively measure the thermodynamic and kinetic course in the phase transition of water and gallium. When the media between the samples and the temperature sensor were changed, the ratio of the thermal energy given off by freezing water to that absorbed by melting ice was different. A type of unidentified energy was emitted from the growing ice crystals in supercooled water. The emission came off with greater part of latent energy released from the freezing water. Kinetic analysis of the DSC data further confirmed the above conclusion.
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65.40.G- Other thermodynamical quantities
64.70.D- Solid-liquid transitions
82.60.Cx Enthalpies of combustion, reaction, and formation

Room temperature green light emission from nonpolar cubic InGaN/GaN multi-quantum-wells

Shunfeng Li, Jörg Schörmann, Donat J. As, and Klaus Lischka

Appl. Phys. Lett. 90, 071903 (2007); http://dx.doi.org/10.1063/1.2475564 (3 pages) | Cited 12 times

Online Publication Date: 12 February 2007

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Cubic InGaN/GaN multi-quantum-wells (MQWs) with high structural and optical quality are achieved by utilizing freestanding 3C-SiC (001) substrates and optimizing InGaN quantum well growth. Superlattice peaks up to fifth order are clearly resolved in x-ray diffraction. Bright green room temperature photoluminescence (PL) from c-InxGa1−xN/GaN MQWs (x = 0.16) is observed. The full width at half maximum of the PL emission is about 240 meV at 300 K. The PL intensity increases with well thickness, prooving that polarization fields which can limit the performance of the wurtzite III-nitride based devices are absent. The diffusion length of excess carriers is about 17 nm.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
68.65.Fg Quantum wells
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.63.Hs Quantum wells

Manipulation of strain relaxation in metamorphic heterostructures

I. Tångring, S. M. Wang, X. R. Zhu, A. Larsson, Z. H. Lai, and M. Sadeghi

Appl. Phys. Lett. 90, 071904 (2007); http://dx.doi.org/10.1063/1.2435609 (3 pages) | Cited 3 times

Online Publication Date: 12 February 2007

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The authors have discovered that high doping densities in an alloy graded InGaAs buffer have dramatic effects on strain relaxation dynamics and consequently surface and optical qualities in metamorphic heterostructures. Compared with undoped graded buffers, the use of Be doping significantly improves structural, surface, and optical qualities while the use of Si doping deteriorates all these properties. This discovery is significant for the realization of metamorphic optoelectronic devices.
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81.05.Ea III-V semiconductors
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
61.72.S- Impurities in crystals
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures

Phase diagrams for growing ordered heteroepitaxial quantum dots and quantum rings by surface prepatterning

P. Liu, Y. W. Zhang, and C. Lu

Appl. Phys. Lett. 90, 071905 (2007); http://dx.doi.org/10.1063/1.2560981 (3 pages) | Cited 4 times

Online Publication Date: 13 February 2007

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Three-dimensional computer simulations are performed to obtain the phase diagrams for heteroepitaxially grown ordered quantum dots and quantum rings by surface prepatterning. Concave patterning in a squared array may lead to the formation of ordered dots, whereas convex patterning in a squared array may initially lead to the formation of ordered quantum rings, then a transition into quantum dots with further growth. The evolution of the surface chemical potential during growth explains the formation of the ordered surface structures.
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81.30.Dz Phase diagrams of other materials
68.65.Hb Quantum dots (patterned in quantum wells)
68.35.B- Structure of clean surfaces (and surface reconstruction)

Fabrication of deeply undercut GaN-based microdisk structures on silicon platforms

S. Vicknesh, S. Tripathy, Vivian K. X. Lin, L. S. Wang, and S. J. Chua

Appl. Phys. Lett. 90, 071906 (2007); http://dx.doi.org/10.1063/1.2472558 (3 pages) | Cited 5 times

Online Publication Date: 13 February 2007

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The authors demonstrate the use of a dry releasing technique to achieve deeply undercut GaN-based microdisk structures supported by silicon platforms. Varying dimensions of microdisk structures on silicon posts with large air gaps are fabricated by a XeF2-based dry etching of the underlying silicon material. The residual stress variation in these microdisks is studied by high spectral resolution micro-Raman mapping. Such a fabrication technique may effectively improve the light extraction efficiency from GaN-based microdisk light emitting diodes on silicon substrates.
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81.05.Ea III-V semiconductors
81.65.Cf Surface cleaning, etching, patterning
42.82.Cr Fabrication techniques; lithography, pattern transfer
78.30.Fs III-V and II-VI semiconductors
62.20.-x Mechanical properties of solids

Exciton spin splitting in ultrathin InAs layers

Zheng Sun, Z. Y. Xu, Yang Ji, B. Q. Sun, B. R. Wang, S. S. Huang, and H. Q. Ni

Appl. Phys. Lett. 90, 071907 (2007); http://dx.doi.org/10.1063/1.2539600 (3 pages) | Cited 4 times

Online Publication Date: 14 February 2007

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Unique spin splitting behaviors in ultrathin InAs layers, which show very different spin splitting characteristics between the InAs monolayer (ML) and submonolayer (SML) have been observed. While distinct spin splitting is observed in an InAs ML, no visible spin splitting is found in a 1/3 ML InAs SML. In addition, the spin relaxation time in the 1/3 ML InAs is found to be much longer than that in the 1 ML sample. These results are in good agreement with the theoretical prediction that the interexcitonic exchange interaction plays a dominant role in energy splitting, while the intraexciton exchange interaction controls the spin relaxation.
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71.35.-y Excitons and related phenomena
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
71.70.Gm Exchange interactions
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
78.47.-p Spectroscopy of solid state dynamics

Mechanism for strong yellow emission of Y3Al5O12:Ce3+ phosphor under electron irradiation for the application to field emission backlight units

Ho Seong Jang, Jong Hyuk Kang, Yu-Ho Won, Sora Lee, and Duk Young Jeon

Appl. Phys. Lett. 90, 071908 (2007); http://dx.doi.org/10.1063/1.2643064 (3 pages) | Cited 7 times

Online Publication Date: 14 February 2007

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This letter reports a strong yellow emission of Y3Al5O12:Ce3+ (YAG:Ce) mixed with ZnS:Ag,Cl under electron excitation. The penetration depths of electron of 1 keV and photon of 2.7 eV in YAG:Ce were estimated to be approximately 1450 Å and 4.65 mm, respectively. Deeper penetration of blue light from ZnS:Ag,Cl helps to excite a larger number of Ce3+ in a mixture (ZnS:Ag,Cl+YAG:Ce), and YAG:Ce showed strong yellow emission via both cathodoluminescence and photoluminescence. The mixture showed the brightness of 120.5% compared to R,G,B phosphor mixture. This mixture of two phosphors was applied to a carbon nanotube field emission backlight unit.
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78.60.Hk Cathodoluminescence, ionoluminescence
78.55.-m Photoluminescence, properties and materials
79.70.+q Field emission, ionization, evaporation, and desorption

Cooling-rate induced softening in a Zr50Cu50 bulk metallic glass

Y. Liu, H. Bei, C. T. Liu, and E. P. George

Appl. Phys. Lett. 90, 071909 (2007); http://dx.doi.org/10.1063/1.2678909 (3 pages) | Cited 23 times

Online Publication Date: 15 February 2007

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Contrary to the cooling-rate induced hardening observed in crystalline metals, the authors report here an unexpected surface softening in rapidly solidified Zr50Cu50 bulk metallic glass. A soft layer ∼ 500 μm thick was detected near the surface with both hardness and elastic modulus increasing from the surface to the interior. To understand the reason for this, a correlation between cooling rate and defect concentration was derived. Defect concentration was found to increase as the cooling rate increased, suggesting that surface softening may be the result of freezing-in of excess defects, induced by a faster cooling rate near the surface compared to the interior.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.Qp Friction, tribology, and hardness
62.20.D- Elasticity
68.35.Gy Mechanical properties; surface strains
81.30.Fb Solidification

Intersubband absorption in CdSe/ZnxCdyMg1−x−ySe self-assembled quantum dot multilayers

A. Shen, H. Lu, W. Charles, I. Yokomizo, M. C. Tamargo, K. J. Franz, C. Gmachl, S. K. Zhang, X. Zhou, R. R. Alfano, and H. C. Liu

Appl. Phys. Lett. 90, 071910 (2007); http://dx.doi.org/10.1063/1.2679783 (3 pages) | Cited 4 times

Online Publication Date: 15 February 2007

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The authors report the observation of intersubband absorption in multilayers of CdSe/ZnxCdyMg1−x−ySe self-assembled quantum dots. The samples were grown by molecular beam epitaxy on InP substrates. For samples with the CdSe dot layers doped with Cl and with the deposited CdSe equivalent layer thickness between 5.2 and 6.9 ML, peak absorption between 2.5 and 3.5 μm was observed. These materials are promising for intersubband devices operating in the mid- and near-infrared ranges.
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78.67.Hc Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
78.30.-j Infrared and Raman spectra
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Shock compression response of nanoiron powder compact

Chengda Dai, Daniel Eakins, Naresh Thadhani, and J. Ping Liu

Appl. Phys. Lett. 90, 071911 (2007); http://dx.doi.org/10.1063/1.2695522 (3 pages) | Cited 6 times

Online Publication Date: 15 February 2007

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The shock compression response of nano-Fe powder ( ∼ 25 nm) pressed to ∼ 35% theoretical maximum density was determined based on shock stress and wave velocity measurements using piezoelectric stress gauges. The obtained data show a discontinuity in shock wave velocity plotted against particle velocity and an inflexion in specific volume from compression to expansion with increasing shock stress. It is found that the Hugoniot of 25 nm Fe powder cannot be fully described using analytical models that are otherwise capable of predicting the Hugoniot of micron-sized powder or highly porous materials.
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62.50.-p High-pressure effects in solids and liquids
64.30.-t Equations of state of specific substances
62.25.-g Mechanical properties of nanoscale systems
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Electrohydrogenation of MgH2-thin films

A. Borgschulte, J. H. Rector, H. Schreuders, B. Dam, and R. Griessen

Appl. Phys. Lett. 90, 071912 (2007); http://dx.doi.org/10.1063/1.2695626 (3 pages) | Cited 11 times

Online Publication Date: 15 February 2007

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Mg-metal hydride systems are potential hydrogen storage media and might be used in hydrogen switchable mirrors. However, absorption kinetics are too slow even with appropriate catalysts. The authors demonstrate the enhancement of the formation of MgH2 by an electric current, thus enabling the hydrogenation of Mg without an external heat source. The effect is explained by the local heating of the Mg together with an electromigration of H− ions. The electrically supported hydrogen uptake might also be a possibility to enhance the hydrogen uptake rate of complex hydrides due to a similar electronic structure.
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84.60.-h Direct energy conversion and storage
82.45.Jn Surface structure, reactivity and catalysis
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
66.30.Qa Electromigration

Annealing of InGaAsN quantum wells in hydrogen

L. Geelhaar, M. Galluppi, R. Averbeck, G. Jaschke, and H. Riechert

Appl. Phys. Lett. 90, 071913 (2007); http://dx.doi.org/10.1063/1.2695645 (3 pages) | Cited 1 time

Online Publication Date: 15 February 2007

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In0.37Ga0.63As0.983N0.017 quantum wells grown by molecular beam epitaxy at different temperatures were annealed under a comprehensive variety of conditions either in Ar or in H2. A significantly higher luminescence efficiency (i.e., room temperature photoluminescence intensity) can be obtained for annealing in H2. Thus, there is an additional chemical effect beyond the mere thermal effect of annealing. At the same time, band gap and localization of charge carriers are not influenced. Hence, the electronic structure is not affected by the H2 treatment. Indirect experiments suggest that hydrogen is reversibly incorporated into the samples and can be removed by mild annealing in Ar.
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61.72.Cc Kinetics of defect formation and annealing
78.67.De Quantum wells
78.55.Cr III-V semiconductors
73.21.Fg Quantum wells
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