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27 Oct 2003

Volume 83, Issue 17, pp. 3447-3628

Issue Cover Spotlight Figure

Appl. Phys. Lett. 83, 3453 (2003); http://dx.doi.org/10.1063/1.1622431 (3 pages)

Giacomo Scalari, Stéphane Blaser, Lassaad Ajili, Jérôme Faist, Harvey Beere, Edmund Linfield, David Ritchie, and Giles Davies
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Donor–donor binding in semiconductors: Engineering shallow donor levels for ZnTe

A. Janotti, Su-Huai Wei, and S. B. Zhang

Appl. Phys. Lett. 83, 3522 (2003); http://dx.doi.org/10.1063/1.1622791 (3 pages) | Cited 7 times

Online Publication Date: 20 October 2003

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In the past, codoping by mixing donors with acceptors has been proposed to lower the dopant ionization energy. However, the level repulsion between donor and acceptor states is weak due to symmetry considerations. Here, we propose an innovative approach to lower the donor ionization energy by combining donor with donor. Using first-principles band structure method, we demonstrated this concept with n-type doping in ZnTe. For example, we find that the BrTe–SnZn pair has a binding energy of 0.9 eV and a shallow ϵ(+/0) donor level at 70 meV below the conduction-band minimum, compared to 240 meV for the isolated BrTe impurity. © 2003 American Institute of Physics.
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71.55.Gs II-VI semiconductors
71.15.Nc Total energy and cohesive energy calculations
71.15.-m Methods of electronic structure calculations
71.20.Nr Semiconductor compounds

On the nitrogen vacancy in GaN

D. C. Look, G. C. Farlow, P. J. Drevinsky, D. F. Bliss, and J. R. Sizelove

Appl. Phys. Lett. 83, 3525 (2003); http://dx.doi.org/10.1063/1.1623009 (3 pages) | Cited 30 times

Online Publication Date: 20 October 2003

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The dominant electrically active defect produced by 0.42 MeV electron irradiation in GaN is a 70 meV donor. Since only N-sublattice displacements can be produced at this energy, and since theory predicts that the N interstitial is a deep acceptor in n-type GaN, we argue that the 70 meV donor is most likely the isolated N vacancy. The background shallow donors, in the 24–26 meV range, actually decrease in concentration, probably due to interactions with mobile N interstitials that are produced by the irradiation. Thus, the recent assignment of a photoluminescence (PL) line as an exciton bound to a 25 meV N-vacancy donor is incompatible with our results. Moreover, we do not observe that PL line in our sample. © 2003 American Institute of Physics.
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71.55.Eq III-V semiconductors
61.72.J- Point defects and defect clusters
81.05.Ea III-V semiconductors
61.80.Fe Electron and positron radiation effects
78.55.Cr III-V semiconductors
61.82.Fk Semiconductors
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Influence of the doping concentration of Y1−yCayBa2Cu3O7−δ drain-source channels on the properties of superconducting field-effect devices

G. Yu. Logvenov, A. Sawa, C. W. Schneider, and J. Mannhart

Appl. Phys. Lett. 83, 3528 (2003); http://dx.doi.org/10.1063/1.1622780 (3 pages) | Cited 11 times

Online Publication Date: 20 October 2003

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Systematic electric-field-effect studies of the charge transport in doped YBa2Cu3O7−δ films were conducted. For overdoped drain-source channels, the normal-state resistance and the critical temperature Tc decrease with electric-field-induced enhancements of the hole concentration n. For underdoped channels, the resistance decreases, but Tc increases with n. For chemically optimally doped channels, the resistance decreases with n, however, Tc shifts are less pronounced compared with underdoped and overdoped films. The results verify that the superconducting properties of YBa2Cu3O7−δ can be controlled by electric fields, as predicted by the generic phase diagram of the cuprates. © 2003 American Institute of Physics.
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85.25.Qc Superconducting surface acoustic wave devices and other superconducting devices
74.72.-h Cuprate superconductors
74.62.Bf Effects of material synthesis, crystal structure, and chemical composition
74.25.Dw Superconductivity phase diagrams

Large room-temperature spin-dependent tunneling magnetoresistance in polycrystalline Fe3O4 films

Hui Liu, E. Y. Jiang, H. L. Bai, R. K. Zheng, H. L. Wei, and X. X. Zhang

Appl. Phys. Lett. 83, 3531 (2003); http://dx.doi.org/10.1063/1.1622440 (3 pages) | Cited 54 times

Online Publication Date: 20 October 2003

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Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. Transmission electron microscopy images show that the films consist of quite uniform Fe3O4 grains well separated by grain boundaries. It was found that the tunneling of spin-polarized electrons across the antiferromagnetic coupled grain boundaries dominates the transport properties of the films. Magnetoresistance (MR) { = [ρ(H)−ρ(0)]/ρ(0)} shows linear and quadratic magnetic-field dependence in the low-field range when the field is applied parallel and perpendicular to film plane, which is similar to the behaviors observed in the epitaxial Fe3O4 films consisting of a large fraction of antiferromagnetic antiphase domain boundaries. At 300 K, the size of the MR reaches −7.4% under a 50-kOe magnetic field, which is a very large MR for polycrystalline Fe3O4 films. © 2003 American Institute of Physics.
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75.47.Pq Other materials
75.70.Ak Magnetic properties of monolayers and thin films
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.50.Ee Antiferromagnetics
75.47.De Giant magnetoresistance
61.72.Mm Grain and twin boundaries
81.15.Cd Deposition by sputtering
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Spin blockade in ferromagnetic nanocontacts

M. Ye. Zhuravlev, E. Y. Tsymbal, S. S. Jaswal, A. V. Vedyayev, and B. Dieny

Appl. Phys. Lett. 83, 3534 (2003); http://dx.doi.org/10.1063/1.1622986 (3 pages) | Cited 19 times

Online Publication Date: 20 October 2003

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Using a free-electron model and a linear response theory we investigate spin-dependent electronic transport in magnetic nanocontacts in the ballistic regime of conduction. We emphasize the fact that in atomic-size ferromagnetic contacts it is possible to achieve the conductance value of e2/h, which implies a fully spin-polarized electric current. We explore some consequences of this phenomenon. In particular, we show that the presence of a nonmagnetic region in the nanocontact separating two ferromagnetic electrodes can lead to a spin blockade resulting in very large values of magnetoresistance. © 2003 American Institute of Physics.
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73.63.Rt Nanoscale contacts
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.-m Magnetotransport phenomena; materials for magnetotransport
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
73.50.Fq High-field and nonlinear effects
73.23.Ad Ballistic transport
72.25.Mk Spin transport through interfaces
71.15.-m Methods of electronic structure calculations

Large anomalous enhancement of perpendicular exchange bias by introduction of a nonmagnetic spacer between the ferromagnetic and antiferromagnetic layers

F. Garcia, J. Sort, B. Rodmacq, S. Auffret, and B. Dieny

Appl. Phys. Lett. 83, 3537 (2003); http://dx.doi.org/10.1063/1.1619562 (3 pages) | Cited 60 times

Online Publication Date: 20 October 2003

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In (Pt/Co)n/FeMn multilayers, the magnitude of exchange bias, HE, can be considerably enhanced by placing an ultrathin nonmagnetic Pt spacer between the multilayer (ML) and the antiferromagnetic (AFM) layer. The bias is maximum for a spacer layer thickness, t, of a few angstroms and it decreases progressively as t is further increased. This bias enhancement is accompanied by an increase of coercivity, HC. This behavior is due to the role of the Pt spacer in enhancing the perpendicular effective anisotropy of the last Co layer in the ML, which has the effect of increasing the net ferromagnetic (FM)/AFM spin projection, thus leading to the HE and HC enhancements. The decrease of HE and HC for thicker spacer layers is due to the limited range of the FM–AFM proximity effect. © 2003 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
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Characteristics of LaAlO3 gate dielectrics on Si grown by metalorganic chemical vapor deposition

Ai-Dong Li, Qi-Yue Shao, Hui-Qin Ling, Jin-Bo Cheng, Di Wu, Zhi-Guo Liu, Nai-Ben Ming, Cathy Wang, Hong-Wei Zhou, and Bich-Yen Nguyen

Appl. Phys. Lett. 83, 3540 (2003); http://dx.doi.org/10.1063/1.1622794 (3 pages) | Cited 27 times

Online Publication Date: 20 October 2003

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Amorphous LaAlO3 (LAO) gate dielectric thin films have been deposited on Si substrates using La(dpm)3 and Al(acac)3 sources by low-pressure metalorganic chemical vapor deposition. The growth mechanism, interfacial structure, and electrical properties have been investigated by various techniques. The ultrathin films show smaller roughness of ∼0.3 nm, larger band gap of 6.47 eV, and good thermal stability. The growth follows a chemical dynamic control mechanism. High-resolution transmission electron microscopy confirms there exists no interfacial layer, or only thinner ones, between LAO and Si. X-ray photoelectron spectroscopy analyses reveal that the thinner interfacial layer is compositionally graded La–Al–Si–O silicate and Al element is deficient in the interfacial layer. The reliable value of equivalent oxide thickness around 1.2 nm of LAO/Si has been achieved. © 2003 American Institute of Physics.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.-a Thin film structure and morphology
68.35.Ct Interface structure and roughness
68.60.Dv Thermal stability; thermal effects
73.61.Ng Insulators
68.37.Lp Transmission electron microscopy (TEM)
79.60.Jv Interfaces; heterostructures; nanostructures
71.20.Ps Other inorganic compounds

Carbonate formation during post-deposition ambient exposure of high-k dielectrics

Theodosia Gougousi, Dong Niu, Robert W. Ashcraft, and Gregory N. Parsons

Appl. Phys. Lett. 83, 3543 (2003); http://dx.doi.org/10.1063/1.1623316 (3 pages) | Cited 20 times

Online Publication Date: 20 October 2003

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When thick films of group-III (La, Y)- and group-IV (Hf, Zr)-based high-k dielectrics are exposed to ambient for several months, Fourier transform infrared spectroscopy shows formation of carbonate species in the film bulk, likely due to reaction with atmospheric CO2. Group-III-based films show signs of carbonate feature growth within 10 min of air exposure, especially in films processed at relatively low temperatures (<600 °C). Carbonate formation is verified also for group-IV-based films, but at a significantly reduced concentration. Post-exposure annealing can reduce the carbonate observed in the IR spectra. However, post-exposure annealing likely does not remove carbon contamination, and it results in interface silicon oxide growth. The observed reactions of high-k films with the ambient may impose significant constraints on the post-deposition handling of high-k films. © 2003 American Institute of Physics.
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77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
78.66.Nk Insulators
78.30.Hv Other nonmetallic inorganics
68.35.Dv Composition, segregation; defects and impurities
61.72.Cc Kinetics of defect formation and annealing

La–silicate gate dielectrics fabricated by solid phase reaction between La metal and SiO2 underlayers

Heiji Watanabe, Nobuyuki Ikarashi, and Fuminori Ito

Appl. Phys. Lett. 83, 3546 (2003); http://dx.doi.org/10.1063/1.1622107 (3 pages) | Cited 23 times

Online Publication Date: 20 October 2003

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La-based high-k gate dielectrics were fabricated by reoxidation of thin La layers deposited on SiO2 underlayers. Interface reaction that causes metal diffusion through the oxide underlayer increases permittivity of the oxide and forms high-quality La–silicate films. We successfully fabricated ultrathin La–silicates of equivalent oxide thickness ranging from 0.75 to 1.6 nm with low-leakage current by controlling the interface solid phase reaction. We characterized degradation in the silicate film caused by electrical stressing and demonstrated the effectiveness of high-temperature annealing to improve the reliability of silicate dielectrics. Moreover, it was found that water absorption during exposure to air causes positive fixed charge in the silicate (flatband voltage shift), but degradation can be annealed out at relatively low temperatures. © 2003 American Institute of Physics.
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77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
61.72.Cc Kinetics of defect formation and annealing
68.35.Fx Diffusion; interface formation
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Instability and dynamics of thin slipping films

Ashutosh Sharma and Kajari Kargupta

Appl. Phys. Lett. 83, 3549 (2003); http://dx.doi.org/10.1063/1.1618376 (3 pages) | Cited 16 times

Online Publication Date: 20 October 2003

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The linear stability analysis of the full Navier–Stokes equations shows that the surface instability and dynamics of thin liquid films are profoundly altered by the presence of slippage on the substrate. For example, the exponents for the length scale (λm∝h0n; h0 is film thickness) and time scale of instability (tr∝h0m) change nonmonotonically with slippage [for van der Waals force induced instability, n ∊ (1.25,2), m ∊ (3,6)]. Slippage always encourages faster rupture and can greatly reduce the number density of holes for moderate to strong slip. Thus, any interpretation of thin film experiments, including determination of intermolecular forces from the length and time scales, needs to account for the possibility of slippage. © 2003 American Institute of Physics.
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68.15.+e Liquid thin films

Enhanced field emission from printed carbon nanotubes by mechanical surface modification

T. J. Vink, M. Gillies, J. C. Kriege, and H. W. J. J. van de Laar

Appl. Phys. Lett. 83, 3552 (2003); http://dx.doi.org/10.1063/1.1622789 (3 pages) | Cited 80 times

Online Publication Date: 20 October 2003

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A method is presented where the morphology of screen printed carbon nanotube pastes is modified using an adhesive tape. In this way, the organic matrix material is preferentially removed leaving an optimal emitter surface of sparsely distributed and well-aligned carbon nanotubes. From these emitter surfaces, homogeneous emission was observed with emitter site densities of at least 104 emitters cm−2 and extracted current densities over 500 mA cm−2. © 2003 American Institute of Physics.
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79.70.+q Field emission, ionization, evaporation, and desorption
81.65.-b Surface treatments
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.07.De Nanotubes
61.46.-w Structure of nanoscale materials

Quantum-dot optical temperature probes

Glen W. Walker, Vikram C. Sundar, Christina M. Rudzinski, Aetna W. Wun, Moungi G. Bawendi, and Daniel G. Nocera

Appl. Phys. Lett. 83, 3555 (2003); http://dx.doi.org/10.1063/1.1620686 (3 pages) | Cited 79 times

Online Publication Date: 20 October 2003

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The steady-state photoluminescence (PL) properties of cadmium selenide quantum dots (QDs) with a zinc sulfide overlayer [(CdSe)ZnS] can be strongly dependent on temperature in the range from 100 to 315 K. The PL intensity from 50 to 55 Å (CdSe)ZnS QDs in poly(lauryl methacrylate) matrices increases by a factor of ∼ 5 when the temperature is decreased from 315 to 100 K, and the peak of the emission band is blueshifted by 20 nm over the same range. The change in PL intensity is appreciable, linear, and reversible (−1.3% per °C) for temperatures close to ambient conditions. These properties of (CdSe)ZnS dots are retained in a variety of matrices including polymer and sol–gel films, and they are independent of excitation wavelength above the band gap. The significant temperature dependence of the luminescence combined with its insensitivity to oxygen quenching establishes (CdSe)ZnS dots as optical temperature indicators for temperature-sensitive coatings. © 2003 American Institute of Physics.
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07.20.Dt Thermometers
78.67.Hc Quantum dots
78.55.Et II-VI semiconductors

Effect of germanium concentration and tunnel oxide thickness on nanocrystal formation and charge storage/retention characteristics of a trilayer memory structure

V. Ho, L. W. Teo, W. K. Choi, W. K. Chim, M. S. Tay, D. A. Antoniadis, E. A. Fitzgerald, A. Y. Du, C. H. Tung, R. Liu, and A. T. S. Wee

Appl. Phys. Lett. 83, 3558 (2003); http://dx.doi.org/10.1063/1.1615840 (3 pages) | Cited 13 times

Online Publication Date: 20 October 2003

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The effect of germanium concentration and the rapid thermal oxide (RTO) layer thickness on the nanocrystal formation and charge storage/retention capability of a trilayer metal–insulator–semiconductor device was studied. We found that the RTO and the capping oxide layers were not totally effective in confining the Ge nanocrystals in the middle layer when a pure Ge middle layer was used for the formation of nanocrystals. From the transmission electron microscopy and secondary ion mass spectroscopy results, a significant diffusion of Ge atoms through the RTO and into the silicon substrate was observed when the RTO layer thickness was reduced to 2.5 nm. This resulted in no (or very few) nanocrystals formed in the system. For devices with a Ge+SiO2 cosputtered middle layer (i.e., lower Ge concentration), even though a higher charge storage capability was obtained from devices with a thinner RTO layer, the charge retention capability was poorer as compared to devices with a thicker RTO layer. © 2003 American Institute of Physics.
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85.30.Tv Field effect devices
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
61.46.-w Structure of nanoscale materials

Structure and growth of monoclinic Mo2S3 nanorods

R. C. Che, N. Bai, and L.-M. Peng

Appl. Phys. Lett. 83, 3561 (2003); http://dx.doi.org/10.1063/1.1623005 (3 pages) | Cited 3 times

Online Publication Date: 20 October 2003

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We report the synthesis of sulfide nanostructures with trivalent molybdenum, i.e., crystalline Mo2S3 nanorods via a solid-gas reaction between porous Al2O3 impregnated with MoO3 and H2S gas. We show that the introduction of additional 8% H2 gas results in MoS2 nanotubes of the same size. A growth model is proposed for the formation of Mo2S3 nanorods, and the effect of H2 is discussed and demonstrated. © 2003 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
81.10.Bk Growth from vapor

Photoinduced conductivity changes in carbon nanotube transistors

Moonsub Shim and Giles P. Siddons

Appl. Phys. Lett. 83, 3564 (2003); http://dx.doi.org/10.1063/1.1622450 (3 pages) | Cited 30 times

Online Publication Date: 20 October 2003

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Photoinduced conductivity changes in single-walled carbon nanotube transistors have been examined. Low-intensity ultraviolet light significantly reduces the p-channel conductance while simultaneously increasing the n-channel conductance. A combination of optical absorption and electron transport measurements reveals that these changes occur without variations in dopant concentrations. Possible sites of oxygen photodesorption and its implications on the observed electronic properties of nanotubes are considered. © 2003 American Institute of Physics.
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85.35.Kt Nanotube devices
73.63.Fg Nanotubes
85.30.Tv Field effect devices
68.43.Tj Photon stimulated desorption
79.20.La Photon- and electron-stimulated desorption

Determination of carrier density in Te-doped Bi nanowires

Yu-Ming Lin and M. S. Dresselhaus

Appl. Phys. Lett. 83, 3567 (2003); http://dx.doi.org/10.1063/1.1614443 (3 pages) | Cited 4 times

Online Publication Date: 20 October 2003

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A promising approach is presented to characterize the carrier density in Te-doped Bi nanowires from their temperature-dependent resistance measurements, based on the comparison of the scattering rates due to charged impurity scattering and due to other scattering mechanisms that are independent of the carrier density. The result shows that the Te doping efficiency δe is only about 10%–15% for Te-doped Bi nanowires synthesized in an alumina template by molten-metal pressure injection. This analysis technique can be extended to other nanowire systems to provide valuable information regarding the carrier concentration and the Fermi energy for use in controlling and optimizing nanowire properties for specific applications. © 2003 American Institute of Physics.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.80.Cw Elemental semiconductors
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)

Binding energy of parallel carbon nanotubes

B. Chen, M. Gao, J. M. Zuo, S. Qu, B. Liu, and Y. Huang

Appl. Phys. Lett. 83, 3570 (2003); http://dx.doi.org/10.1063/1.1623013 (2 pages) | Cited 29 times

Online Publication Date: 20 October 2003

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We study the interaction between two separate carbon nanotubes that bond together by examing their geometry in transmission electron microscopy (TEM). The TEM image, in conjunction with a continuum mechanics model interpretation, provides an estimate of 0.36 nN binding energy between the two parallel carbon nanotubes. This result agrees well with the calculated binding energy using an atomistic model. © 2003 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
81.07.De Nanotubes
68.37.Lp Transmission electron microscopy (TEM)
73.22.-f Electronic structure of nanoscale materials and related systems
61.50.Lt Crystal binding; cohesive energy
71.15.Nc Total energy and cohesive energy calculations

Ultrafast intraband spectroscopy of electron capture and relaxation in InAs/GaAs quantum dots

T. Müller, F. F. Schrey, G. Strasser, and K. Unterrainer

Appl. Phys. Lett. 83, 3572 (2003); http://dx.doi.org/10.1063/1.1622432 (3 pages) | Cited 42 times

Online Publication Date: 20 October 2003

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The electron capture and relaxation dynamics in self-assembled InAs/GaAs quantum dots (QDs) is investigated by means of interband-pump–intraband-probe spectroscopy. By tuning femtosecond infrared pulses into resonance with intraband transitions between confined QD states and the wetting layer continuum, the electron population of the QD ground and first excited states is determined as a function of time delay after the interband pump. Our experiments indicate that the most efficient relaxation pathway into the QD ground state is the stepwise relaxation through the excited states of the dot. The capture time at room temperature decreases from 2.8 to 1.5 ps with increasing excitation density above a certain threshold, and changes only slightly at low excitation densities. At low temperature (T = 5 K), we determine a longer capture time of 4.7 ps. © 2003 American Institute of Physics.
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78.67.Hc Quantum dots
73.21.La Quantum dots
78.47.-p Spectroscopy of solid state dynamics
78.30.Fs III-V and II-VI semiconductors
78.55.Cr III-V semiconductors

Tuning the properties of magnetic CdMnTe quantum dots

S. Mackowski, H. E. Jackson, L. M. Smith, J. Kossut, G. Karczewski, and W. Heiss

Appl. Phys. Lett. 83, 3575 (2003); http://dx.doi.org/10.1063/1.1622438 (3 pages) | Cited 21 times

Online Publication Date: 20 October 2003

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We show that CdMnTe self-assembled quantum dots (QDs) can be formed by depositing a submonolayer of Mn ions over a ZnTe surface prior to deposition of the CdTe dot layer. Single-dot emission lines and strongly polarized QD photoluminescence (PL) in an applied magnetic field confirm the presence of Mn in individual QDs. The width of PL lines of the single CdMnTe dots is 3 meV due to magnetic moment fluctuations (MMFs) of the Mn ions. After rapid thermal annealing, the emission lines of individual magnetic QDs narrow significantly to 0.25 meV, showing that effect of MMFs is strongly reduced, most probably due to an increase in the average QD size. These results suggest a way to tune the spin properties of magnetic QDs. © 2003 American Institute of Physics.
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78.67.Hc Quantum dots
73.63.Kv Quantum dots
73.21.La Quantum dots
75.50.Pp Magnetic semiconductors
75.75.-c Magnetic properties of nanostructures
78.55.Et II-VI semiconductors
75.30.Cr Saturation moments and magnetic susceptibilities

Thermal emission of electrons from selected s-shell configurations in InAs/GaAs quantum dots

O. Engström, M. Malmkvist, Y. Fu, H. Ö. Olafsson, and E. Ö. Sveinbjörnsson

Appl. Phys. Lett. 83, 3578 (2003); http://dx.doi.org/10.1063/1.1622437 (3 pages) | Cited 27 times

Online Publication Date: 20 October 2003

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The thermal emission of electrons from self-assembled InAs/GaAs quantum dots, prepared by molecular-beam epitaxy, with an average base/height size of 20 nm/11 nm in Schottky diodes has been investigated using deep level transient spectroscopy (DLTS). By applying an appropriate set of voltage pulses across the Schottky diode, the two different s-electron configurations have been investigated separately. This avoids the problem of interference between overlapping peaks in DLTS data. We find that a difference in activation energy for the thermal electron emission between the two configurations agrees with expected variation in electron energy levels due to the size distribution of the quantum dots. © 2003 American Institute of Physics.
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79.70.+q Field emission, ionization, evaporation, and desorption
73.63.Kv Quantum dots
73.21.La Quantum dots

Tailoring structure and electrical properties of carbon nanotubes using kilo-electron-volt ions

B. Q. Wei, J. D’Arcy-Gall, P. M. Ajayan, and G. Ramanath

Appl. Phys. Lett. 83, 3581 (2003); http://dx.doi.org/10.1063/1.1622781 (3 pages) | Cited 32 times

Online Publication Date: 20 October 2003

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We report the effects of 30 and 50 keV Ga+ ion irradiation on the structure and electrical properties of arc-evaporated multiwalled carbon nanotubes (MWNTs). For 50 keV ions with doses of ∼ 1013 ions/cm2 the outer shells of the MWNTs remain intact, while the inner layers reorganize into highly ordered pillbox-like ∼5-nm-diam nanocompartments of varying lengths between 2 and 20 nm. Increasing the dose to ∼ 1014 ions cm−2 results in the gradual disordering of the graphitic shells and destroys the nanocapsules, while at doses of 1015 ions cm−2 the graphitic shells collapse into the hollow, resulting in the formation a homogenous amorphous rod. Irradiating nanotubes with 30 keV ions yields similar results, but at higher doses. Irradiated nanotubes exhibit a decrease in electron activation energy from 194 to 112 meV, while the semiconducting behavior is essentially preserved for ion doses up to 5×1015 ions cm−2. Ion irradiation could be a useful tool to locally modify nanotube structure and tailor properties for device applications. © 2003 American Institute of Physics.
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73.63.Fg Nanotubes
61.46.-w Structure of nanoscale materials
61.80.Jh Ion radiation effects
81.07.De Nanotubes
61.82.Fk Semiconductors

Synthesis, structure, and photoluminescence of very thin and wide alpha silicon nitride (α-Si3N4) single-crystalline nanobelts

Long-Wei Yin, Yoshio Bando, Ying-Chun Zhu, and Yu-Bao Li

Appl. Phys. Lett. 83, 3584 (2003); http://dx.doi.org/10.1063/1.1623940 (3 pages) | Cited 42 times

Online Publication Date: 20 October 2003

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Large quantities of very thin and wide single-crystal alpha silicon nitride (α-Si3N4) nanobelts were synthesized by a vapor-solid thermal reaction between ammonia and silicon monoxide (SiO) without using any added catalyst. Scanning electron microscopy, high-resolution electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction were used to characterize the formed nanobelts. The single-crystal α-Si3N4 nanobelts are about 800–1200 nm in width, 20–35 nm in thickness and about several tens to several hundreds of micrometers in length. The nanobelts are perfect in structure. The nanobelts grow along [011] and [100] direction. Intense visible photoluminescence (PL) occurring on the wide and thin nanobelts over a broad spectrum ranging from 420 to 750 nm was observed. The visible PL emission is related to the inherently imperfect Si and N dangling bonds in the α-Si3N4 structure. © 2003 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.07.Bc Nanocrystalline materials
78.55.Hx Other solid inorganic materials
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Lp Transmission electron microscopy (TEM)

Local anisotropy in strained manganite thin films

N. M. Souza-Neto, A. Y. Ramos, H. C. N. Tolentino, E. Favre-Nicolin, and L. Ranno

Appl. Phys. Lett. 83, 3587 (2003); http://dx.doi.org/10.1063/1.1623936 (3 pages) | Cited 2 times

Online Publication Date: 20 October 2003

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We report on an angular resolved x-ray absorption spectroscopy study of the local atomic structure around the manganese ions in La0.7Sr0.3MnO3 thin films epitaxially grown on tensile and compressive substrates. Ab initio calculations provide strong support to the analysis of the experimental data and make possible the unambiguous derivation of a model of local distortion around the manganese atoms, without modification of the tilt angle Mn–O–Mn, among the octahedra. This distortion, tending to localize the charge carriers, is the driving parameter in the modifications of the magnetic and transport properties observed in thin films with respect to bulk systems. © 2003 American Institute of Physics.
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78.70.Dm X-ray absorption spectra
68.55.-a Thin film structure and morphology
61.66.Fn Inorganic compounds
68.60.Bs Mechanical and acoustical properties
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep

Titanium metal quantum-dot composite induced by subplantation

J. P. Zhao, D. X. Huang, A. J. Jacobson, and J. W. Rabalais

Appl. Phys. Lett. 83, 3590 (2003); http://dx.doi.org/10.1063/1.1622784 (3 pages) | Cited 4 times

Online Publication Date: 20 October 2003

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Crystalline titanium nanodots have been formed in the subsurface layer of single-crystal SiO2, i.e., a Ti-based metal quantum dot composite, by subplantation of 9 keV Ti+ ions. Transmission electron microscopy images show that the Ti nanodots have a single, uniform size distribution of ∼3–4 nm, they are single crystals of mainly the Ti bcc β-phase, and their position in the subsurface is controllable through the ion energy. The unique features of subplantation for promoting the precipitation/clustering of crystalline Ti nanodots are discussed. These results confirm previous findings based on the linear optical properties of Ti in SiO2. © 2003 American Institute of Physics.
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68.65.Hb Quantum dots (patterned in quantum wells)
81.16.-c Methods of micro- and nanofabrication and processing
81.07.Ta Quantum dots
78.67.Hc Quantum dots
61.46.-w Structure of nanoscale materials
61.72.up Other materials
61.80.Jh Ion radiation effects
61.82.Ms Insulators
85.40.Ry Impurity doping, diffusion and ion implantation technology
68.37.Lp Transmission electron microscopy (TEM)
64.75.-g Phase equilibria
81.30.Mh Solid-phase precipitation

Selective formation of ZnO nanodots on nanopatterned substrates by metalorganic chemical vapor deposition

Sang-Woo Kim, Teruhisa Kotani, Masaya Ueda, Shizuo Fujita, and Shigeo Fujita

Appl. Phys. Lett. 83, 3593 (2003); http://dx.doi.org/10.1063/1.1622795 (3 pages) | Cited 15 times

Online Publication Date: 20 October 2003

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Selective formation of ZnO nanodots was accomplished by metalorganic chemical vapor deposition on nanopatterned SiO2/Si substrates. Self-organized ZnO nanodots were selectively formed in nanopatterned lines of Si created by etching of SiO2 with focused ion beam (FIB), whereas any nanodots were hardly observed on the SiO2 surface in the vicinity of the FIB-sputtered Si areas. The mechanism of the selective formation of ZnO nanodots on FIB-nanopatterned lines is mainly attributed to the effective migration of Zn adatoms diffusing on the SiO2 surface into the Si lines followed by the nucleation at surface atomic steps and kinks created by Ga+ ion sputtering. Cathodoluminescence measurements confirmed that the emission originated from the selectively grown ZnO nanodots. © 2003 American Institute of Physics.
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81.07.Bc Nanocrystalline materials
61.46.-w Structure of nanoscale materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.05.Dz II-VI semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
52.77.Bn Etching and cleaning
81.65.Cf Surface cleaning, etching, patterning
68.35.Fx Diffusion; interface formation
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