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8 Oct 2007

Volume 91, Issue 15, Articles (15xxxx)

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Appl. Phys. Lett. 91, 153101 (2007); http://dx.doi.org/10.1063/1.2793688 (3 pages)

B. J. Lee, K. Park, and Z. M. Zhang
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Dislocation reduction in gallium nitride films using scandium nitride interlayers

M. A. Moram, Y. Zhang, M. J. Kappers, Z. H. Barber, and C. J. Humphreys

Appl. Phys. Lett. 91, 152101 (2007); http://dx.doi.org/10.1063/1.2794009 (3 pages) | Cited 16 times

Online Publication Date: 8 October 2007

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We describe a method of reducing threading dislocation densities in 0001-oriented GaN from (5.0±0.5)×109 cm−2 to (3.1±0.4)×107 cm−2 (for coalesced films) or to below 5×106 cm−2 (for partially coalesced films) in a single step, without lithography. Lattice-matched, dislocation-blocking scandium nitride interlayers are deposited on a 500 nm GaN-on-sapphire template. Dislocation-free GaN islands grown on the ScN interlayer nucleate both on the interlayer and on tiny areas of the GaN template exposed through openings in the interlayer. However, some dislocations are generated above the interlayer during subsequent island coalescence.
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81.05.Ea III-V semiconductors
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

N–O related shallow donors in silicon: Stoichiometry investigations

H. E. Wagner, H. Ch. Alt, W. von Ammon, F. Bittersberger, A. Huber, and L. Koester

Appl. Phys. Lett. 91, 152102 (2007); http://dx.doi.org/10.1063/1.2795801 (3 pages) | Cited 5 times

Online Publication Date: 8 October 2007

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For clarification of the unknown chemical composition of the electrically active N–O defects in silicon, an ingot with variable oxygen content and fixed nitrogen concentration was investigated by infrared spectroscopy. Shallow donor spectra taken at different sample positions, i.e., oxygen concentrations, show a strong oxygen influence on the absorption of the different N–O species, allowing determination of the number of oxygen atoms for each species via the corresponding mass-action law. From that, the energetically deepest defect N–O-5 is associated with a NO configuration, whereas the strongest complex N–O-3 has NO2 composition. Further members of the shallow donor family contain three oxygen atoms.
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81.05.Cy Elemental semiconductors
81.05.Bx Metals, semimetals, and alloys
61.72.-y Defects and impurities in crystals; microstructure
61.66.Bi Elemental solids
61.66.Dk Alloys
71.55.Cn Elemental semiconductors
78.30.Am Elemental semiconductors and insulators

Polarity-dependent reversible resistance switching in Ge–Sb–Te phase-change thin films

Ramanathaswamy Pandian, Bart J. Kooi, George Palasantzas, Jeff T. M. De Hosson, and Andrew Pauza

Appl. Phys. Lett. 91, 152103 (2007); http://dx.doi.org/10.1063/1.2798242 (3 pages) | Cited 14 times

Online Publication Date: 8 October 2007

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In this paper, we demonstrate reversible resistance switching in a capacitorlike cell using a Ge–Sb–Te film that does not rely on amorphous-crystalline phase change. The polarity of the applied electric field switches the cell resistance between lower- and higher-resistance states, as was observed in current-voltage characteristics. Moreover, voltage pulses less than 1.25 V showed this switching within time scales of microseconds with more than 40% contrast between the resistance states. The latter are found to be nonvolatile for months. The switching could also be achieved at nanoscales with atomic force microscopy with a better resistance contrast of three orders of magnitude.
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73.61.At Metal and metallic alloys

Charge-retraction time-of-flight measurement for organic charge transport materials

Jason U. Wallace, Ralph H. Young, Ching W. Tang, and Shaw H. Chen

Appl. Phys. Lett. 91, 152104 (2007); http://dx.doi.org/10.1063/1.2798592 (3 pages) | Cited 3 times

Online Publication Date: 8 October 2007

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This letter describes an all-electrical technique, charge-retraction time-of-flight (CR-TOF), to measure charge carrier mobility through an organic layer. Carriers are injected and accumulated at a blocking interface, then retracted. The retraction current transient is nearly indistinguishable from a traditional time-of-flight photocurrent. The CR-TOF technique is validated by measurement of the hole mobility of two well-known compounds, 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine and 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl, utilizing 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene as a hole-blocking layer. A sample layer thickness of less than 300 nm can be used for the measurement.
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72.20.Fr Low-field transport and mobility; piezoresistance
73.61.Ph Polymers; organic compounds

Strong dependence of transport properties of metal-semiconductor-metal graphene ribbons on their geometrical features

Antonis N. Andriotis, Ernst Richter, and Madhu Menon

Appl. Phys. Lett. 91, 152105 (2007); http://dx.doi.org/10.1063/1.2798593 (3 pages) | Cited 12 times

Online Publication Date: 8 October 2007

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The conducting properties of various finite graphene ribbons and their junctions are investigated. These were found to depend strongly on their size and geometric features. Small arm-chair ribbons, when connected through tapered geometries, are found to exhibit very narrow conducting windows. No conductance is found through small armchair zigzag armchair ribbon junctions when connected via pentagon-heptagon defect lines. However, the conducting windows become broader as the size of the ribbons increases.
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73.40.Sx Metal-semiconductor-metal structures
73.61.Wp Fullerenes and related materials

Band engineering in Al0.5Ga0.5N/GaN superlattice by modulating Mg dopant

Jinchai Li and Junyong Kang

Appl. Phys. Lett. 91, 152106 (2007); http://dx.doi.org/10.1063/1.2798589 (3 pages) | Cited 5 times

Online Publication Date: 10 October 2007

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The electronic structures of Mg modulation-doped and undoped Al0.5Ga0.5N/GaN superlattices (SLs) are investigated by using first-principles density function theory. The layer-projected densities of states indicate that the band alignment is changed from type I to type II and the band bending due to polarization is reduced significantly by modulating Mg dopant in AlGaN layer. It is further confirmed by the calculations of the partial charge density profiles and the valence band offsets where the valence-band maximum of AlGaN in Mg modulation-doped SL is located above that of GaN. The strong hybridization between N and Mg orbitals plays an important role on the upward shifts of the valence band edges.
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73.21.Cd Superlattices

The origin of electron injection improvement in organic light-emitting devices with an organic oxide/rubrene electron injection layer

Kwanghee Cho, Sang Wan Cho, Chung-Nam Whang, Kwangho Jeong, Seong Jun Kang, and Yeonjin Yi

Appl. Phys. Lett. 91, 152107 (2007); http://dx.doi.org/10.1063/1.2798588 (3 pages) | Cited 1 time

Online Publication Date: 11 October 2007

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The electronic structure of tris(8-hydroquinoline) aluminum (Alq3)/rubrene/poly(ethylene glycol) dimethyl ether (PEGDE)/Al interfaces was studied using in situ ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). The UPS and XPS spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interfaces. When a PEGDE/rubrene double layer was inserted between Al and Alq3, the electron injection barrier height was greatly reduced compared to the interface without PEGDE/rubrene or with a single insertion layer of either PEGDE or rubrene.
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85.60.Jb Light-emitting devices
79.60.Jv Interfaces; heterostructures; nanostructures
71.20.-b Electron density of states and band structure of crystalline solids

Determination of InN–GaN heterostructure band offsets from internal photoemission measurements

Zahid Hasan Mahmood, A. P. Shah, Abdul Kadir, M. R. Gokhale, Sandip Ghosh, Arnab Bhattacharya, and B. M. Arora

Appl. Phys. Lett. 91, 152108 (2007); http://dx.doi.org/10.1063/1.2794788 (3 pages) | Cited 8 times

Online Publication Date: 11 October 2007

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Band discontinuities at the InN–GaN heterointerface are experimentally determined from internal photoemission spectroscopy measurements on n+ InN on GaN epilayers. The photocurrent shows two threshold energies, one at 1.624 eV and the other at 2.527 eV. From these, we obtain the band offsets ΔEv = 0.85 eV and ΔEc = 1.82 eV.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
79.60.Jv Interfaces; heterostructures; nanostructures
68.55.-a Thin film structure and morphology
71.20.Nr Semiconductor compounds
79.60.Bm Clean metal, semiconductor, and insulator surfaces

Investigation of magnetic and electronic coupling between two (Ga,Mn)As layers in (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junctions

Z. Ge, Y. Y. Zhou, Y.-J. Cho, X. Liu, J. K. Furdyna, and M. Dobrowolska

Appl. Phys. Lett. 91, 152109 (2007); http://dx.doi.org/10.1063/1.2799238 (3 pages) | Cited 11 times

Online Publication Date: 12 October 2007

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The coupling between the two magnetic layers in a series of (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junctions with different nonmagnetic spacer thicknesses tNM were studied by magnetization and planar Hall effect (PHE) measurements. The PHE data indicate that the magnetization reversals of the two layers are strongly correlated when the tNM is less than 3 nm and are independent when tNM is larger than 15 nm. From the results, it is concluded that considerable redistribution of hole wave functions plays a major role for small tNM. The PHE results for the sample with 6 nm spacer also suggest an antiferromagnetic interlayer exchange coupling.
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75.50.Pp Magnetic semiconductors
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Jk Magnetization reversal mechanisms

Possible efficient p-type doping of AlN using Be: An ab initio study

R. Q. Wu, L. Shen, M. Yang, Z. D. Sha, Y. Q. Cai, Y. P. Feng, Z. G. Huang, and Q. Y. Wu

Appl. Phys. Lett. 91, 152110 (2007); http://dx.doi.org/10.1063/1.2799241 (3 pages) | Cited 4 times

Online Publication Date: 12 October 2007

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Spin density functional theory based ab initio study is carried out to investigate the feasibility of fabricating p-type AlN using Be as an efficient dopant. It is found that substitutional BeAl is an acceptor with an activation energy of 0.34 eV. To overcome the low solubility of direct incorporation of Be into AlN and self-compensation from Be interstitials, we propose a hydrogen-assisted growth scheme which improves the solubility and suppresses interstitials. Oxygen is also found to be an effective codopant to activate Be in AlN. Our results suggest the possibility of improving p-type conductivity of AlN by Be doping.
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61.72.uj III-V and II-VI semiconductors
61.72.J- Point defects and defect clusters
61.72.Bb Theories and models of crystal defects

High efficiency double heterojunction polymer photovoltaic cells using highly ordered TiO2 nanotube arrays

Gopal K. Mor, Karthik Shankar, Maggie Paulose, Oomman K. Varghese, and Craig A. Grimes

Appl. Phys. Lett. 91, 152111 (2007); http://dx.doi.org/10.1063/1.2799257 (3 pages) | Cited 86 times

Online Publication Date: 12 October 2007

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Vertically oriented TiO2 nanotube arrays formed by anodization offer a highly ordered material architecture for efficient charge generation and collection in photoelectrochemical devices. A blend of regioregular poly(3-hexylthiophene) and a methanofullerene (phenyl C71-butyric acid methyl ester) was infiltrated into transparent TiO2 nanotube films. The heterojunction poly(3-hexylthiophene) (P3HT)-([6,6]-phenyl-C71-butyric acid methyl ester) and P3HT-TiO2 interfaces both result in charge separation. The resulting solid state solar cells show a short-circuit current density of 12.4 mA/cm2, 641 mV open circuit potential, and a 0.51 fill factor, yielding power conversion efficiencies of 4.1% under AM 1.5 sun.
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84.60.Jt Photoelectric conversion
85.35.Kt Nanotube devices
85.65.+h Molecular electronic devices
82.47.Jk Photoelectrochemical cells, photoelectrochromic and other hybrid electrochemical energy storage devices
82.45.Yz Nanostructured materials in electrochemistry
82.45.Wx Polymers and organic materials in electrochemistry
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