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20 Dec 2004

Volume 85, Issue 25, pp. 6083-6293

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 6281 (2004); http://dx.doi.org/10.1063/1.1834720 (3 pages)

M. P. Rao, M. F. Aimi, and N. C. MacDonald
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Persistent photocurrent spectroscopy of GaN metal–semiconductor–metal photodetectors on long time scale

B. Potì, A. Passaseo, M. Lomascolo, R. Cingolani, and M. De Vittorio

Appl. Phys. Lett. 85, 6083 (2004); http://dx.doi.org/10.1063/1.1840122 (3 pages) | Cited 8 times

Online Publication Date: 15 December 2004

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Some of the trapping mechanisms responsible for the persistent photocurrent effects in GaN metal–semiconductor–metal photodetectors have been studied on a time scale of several hours by analyzing the photocurrent decay kinetic as a function of temperature. The analysis of the Arrhenius plot of the decay kinetic on a long time scale shows two activation energies of about 140 meV and 1 eV. Such values are in good agreement with two slope changes observed in the room temperature photocurrent spectrum around 2.40 and 3.25 eV, in the GaN energy band gap. According to the Lucovsky theory the 140 meV activation energy was interpreted as due to the transition from a deep localized state to the conduction band edge, whereas the 1 eV activation energy was interpreted as due to the transition from the valence band edge to a deep localized state. Therefore, the persistent photocurrent on long time scale is primarily due to the presence of donor deep and acceptor deep states generated by gallium vacancies, gallium antisites, and carbon impurities.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
71.55.Eq III-V semiconductors
73.40.Sx Metal-semiconductor-metal structures
72.40.+w Photoconduction and photovoltaic effects
61.72.J- Point defects and defect clusters

Application of modified transmission line model to measure p-type GaN contact

N. C. Chen, C. Y. Tseng, A. P. Chiu, C. F. Shih, and P. H. Chang

Appl. Phys. Lett. 85, 6086 (2004); http://dx.doi.org/10.1063/1.1835993 (3 pages) | Cited 10 times

Online Publication Date: 15 December 2004

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This work presented a procedure for extending the modified transmission line model to measure non-ohmic contact. This method was applied to the p-type GaN contact with the resulting sheet resistance similar to that determined by the Hall measurement. The voltage–current density (V–J) curve obtained using this procedure was also similar to that by directly analyzing the current–voltage curve of a light-emitting diode. Both results revealed the validity of this procedure. Rather than yielding a specific contact resistance for an ohmic contact, this procedure yielded a V–J curve to describe the non-ohmic contact characteristics. Similarly, this procedure could also extend the linear transmission line model to the analysis of non-ohmic contacts.
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73.40.Cg Contact resistance, contact potential
73.40.Ns Metal-nonmetal contacts
73.61.Ey III-V semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects

Properties of defect modes in one-dimensional photonic crystals containing a defect layer with a negative refractive index

Kun-yuan Xu, Xiguang Zheng, and Wei-long She

Appl. Phys. Lett. 85, 6089 (2004); http://dx.doi.org/10.1063/1.1824175 (3 pages) | Cited 9 times

Online Publication Date: 15 December 2004

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The band structures of one-dimensional photonic crystals containing a defect layer with a negative refractive index are studied, showing that the defect modes possess three types of dispersion: positive, zero, and negative types. The zero and negative dispersion phenomena may be useful in the design of large incident angle filters and narrow frequency and sharp angular filters, respectively. The dispersion multiplicity of the defect modes can be understood by an approximate formula. Moreover, the splitting of one defect mode into multiple defect modes is observed in the band gap when the parameters of the defect layer vary. This phase transition phenomenon may be useful in the design of multiple channeled filters or filters with a rectangular profile. The condition for the critical point of the phase transition is also analyzed.
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42.70.Qs Photonic bandgap materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Direct optoelectronic generation and detection of sub-ps-electrical pulses on sub-mm-coaxial transmission lines

Tae-In Jeon and D. Grischkowsky

Appl. Phys. Lett. 85, 6092 (2004); http://dx.doi.org/10.1063/1.1839645 (3 pages) | Cited 25 times

Online Publication Date: 15 December 2004

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We report efficient direct optoelectronic generation of sub-ps-THz pulses on 50 Ω coaxial transmission lines with a 330 μm diameter solid copper outer tube filled with Teflon containing the 80 μm diameter inner conductor. The transmitted pulses after propagating as much as 105 mm were measured at the end of the line with an optoelectronic antenna having sub-ps-time resolution. We observed low-loss, single transverse electromagnetic mode propagation with very little group velocity dispersion. The experimentally derived values for the frequency-dependent absorption are consistent with the theoretical predictions.
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84.40.Az Waveguides, transmission lines, striplines
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Electrically switchable lasing from pyrromethene 597 embedded holographic-polymer dispersed liquid crystals

Rachel Jakubiak, Lalgudi V. Natarajan, Vincent Tondiglia, Guang S. He, Paras N. Prasad, Timothy J. Bunning, and Richard A. Vaia

Appl. Phys. Lett. 85, 6095 (2004); http://dx.doi.org/10.1063/1.1839282 (3 pages) | Cited 22 times

Online Publication Date: 15 December 2004

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One-dimensional photonic band gap (PBG) materials created from holographic polymer dispersed liquid crystals (H-PDLCs) provide enhanced light localization in an organic electro-optic device. Distributed feedback within the reflection notch of a H-PDLC grating narrowed the bandwidth of pyrromethene 597 fluorescence from 56 to 8.4 nm at a lasing threshold of 0.12 mJ cm−2, compared to 2.6 mJ cm−2 required to observe amplified spontaneous emission in a nonstructured, but comparable floodlit (PDLC) sample. Application of an electric field (10–40 V∕μm) continuously decreased the diffraction efficiency of the grating and the commensurate dynamic lasing intensity thus demonstrating electrically modulated gain from an optically pumped, all-organic PBG.
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42.70.Df Liquid crystals
42.70.Qs Photonic bandgap materials
42.55.Px Semiconductor lasers; laser diodes
42.55.Rz Doped-insulator lasers and other solid state lasers
42.55.Tv Photonic crystal lasers and coherent effects
42.40.Eq Holographic optical elements; holographic gratings
78.55.Kz Solid organic materials

Ion-exchanged waveguides in glass doped with PbS quantum dots

Jason M. Auxier, Michael M. Morrell, Brian R. West, Seppo Honkanen, Axel Schülzgen, Nasser Peyghambarian, Sabyasachi Sen, and Nicholas F. Borrelli

Appl. Phys. Lett. 85, 6098 (2004); http://dx.doi.org/10.1063/1.1839284 (3 pages) | Cited 5 times

Online Publication Date: 15 December 2004

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The lowest-loss (≲1 dB∕cm) ion-exchanged waveguides in glass doped with PbS quantum dots are presented. Near-field mode profile and refractive index profile using the refracted near-field technique were measured for these waveguides. We demonstrate that the optical properties of this glass remain unchanged during the ion-exchange process.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
42.79.Gn Optical waveguides and couplers
78.67.Hc Quantum dots
61.43.Fs Glasses
78.55.Qr Amorphous materials; glasses and other disordered solids
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.70.Ce Glasses, quartz

Microsphere whispering-gallery-mode laser using HgTe quantum dots

S. I. Shopova, G. Farca, A. T. Rosenberger, W. M. S. Wickramanayake, and N. A. Kotov

Appl. Phys. Lett. 85, 6101 (2004); http://dx.doi.org/10.1063/1.1841459 (3 pages) | Cited 32 times

Online Publication Date: 15 December 2004

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Ultralow-threshold continuous-wave lasing is achieved at room temperature in a fused-silica microsphere that is coated with HgTe quantum dots (colloidal nanoparticles). The 830 nm pump input and HgTe microlaser output are efficiently coupled into and out of whispering-gallery modes by tapered fibers. Lasing occurs at wavelengths ranging from 1240 to 1780 nm, depending on the size and composition of the quantum dots (HgCdTe is also used). A linear fit to the data determines the lowest observed threshold pump power to be 0±2 μW.
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42.55.Px Semiconductor lasers; laser diodes
78.67.Hc Quantum dots
78.55.Et II-VI semiconductors

Stimulated emission and optical gain in LaF3:Nd nanoparticle-doped polymer-based waveguides

R. Dekker, D. J. W. Klunder, A. Borreman, M. B. J. Diemeer, K. Wörhoff, A. Driessen, J. W. Stouwdam, and F. C. J. M. van Veggel

Appl. Phys. Lett. 85, 6104 (2004); http://dx.doi.org/10.1063/1.1840110 (3 pages) | Cited 30 times

Online Publication Date: 15 December 2004

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We report experiments which show evidence that stimulated emission at 863 nm takes place in hybrid monomode Si3N4 waveguides where LaF3:Nd nanoparticle-doped polymethylmethacrylate (PMMA) was used as a top cladding material. Furthermore, optical gain at 1319 nm in LaF3:Nd nanoparticle dispersed PMMA (0.1 dB∕cm) and photodefinable epoxy (Microchem SU-8) multimode waveguides has been observed at pump powers below 10 mW. This class of composite materials based on polymers with dispersed nanoparticles shows promising properties for planar optical amplifiers. Simulation showed that optical gain in the order of 10 dB can be achieved at 100 mW pump power in a 20 cm long monomode waveguide.
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78.45.+h Stimulated emission
42.79.Gn Optical waveguides and couplers
42.70.Jk Polymers and organics
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
42.82.-m Integrated optics

The evolution of electroluminescence in Ge quantum-dot diodes with the fold number

Y. H. Peng, Chih-Hsiung Hsu, C. H. Kuan, C. W. Liu, P. S. Chen, M.-J. Tsai, and Y. W. Suen

Appl. Phys. Lett. 85, 6107 (2004); http://dx.doi.org/10.1063/1.1842371 (3 pages) | Cited 7 times

Online Publication Date: 15 December 2004

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The electroluminescence of the light-emitting diodes with five-, ten and 30-fold p-type Ge quantum dots grown on n+ Si substrates is studied. The enhanced integral electroluminescence intensity and blueshift of the 30-fold one at high temperature (>200 K) act contrary to those in five- and ten-fold ones. It is attributed to the emission in the higher-fold quantum dots enabled by the injected electrons diffusing the farther at the higher temperature. Transmission electron microscopy shows that the size of the Ge quantum dots and the Si component in them, both increase with increasing the fold number. Due to the strain-induced intermixing at the high-fold quantum dots, those dots hence have large band gap and result in the intensity increment and blueshift at the high temperature.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
78.60.Fi Electroluminescence
68.37.Lp Transmission electron microscopy (TEM)

Guiding 1.5 μm light in photonic crystals based on dielectric rods

Solomon Assefa, Peter T. Rakich, Peter Bienstman, Steven G. Johnson, Gale S. Petrich, John D. Joannopoulos, Leslie A. Kolodziejski, Erich P. Ippen, and Henry I. Smith

Appl. Phys. Lett. 85, 6110 (2004); http://dx.doi.org/10.1063/1.1840107 (3 pages) | Cited 25 times

Online Publication Date: 15 December 2004

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Photonic-crystal structures consisting of dielectric rods were designed, fabricated, and optically characterized. The combination of the high refractive-index-contrast GaAs∕AlxOy material system with electron-beam lithography enabled the fabrication of structures suitable for the optical propagation of 1.5 μm light. Experimental transmission spectra were obtained for structures consisting of a two-dimensional array of rods and line-defect waveguides. Optical measurements confirmed the presence of a photonic band gap, as well as band gap guidance in the line-defect waveguide. A two-stage coupling scheme facilitated efficient optical coupling into the line-defect waveguide.
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42.70.Qs Photonic bandgap materials
42.79.Gn Optical waveguides and couplers
42.82.Cr Fabrication techniques; lithography, pattern transfer

Demonstration of ultra-high-Q small mode volume toroid microcavities on a chip

T. J. Kippenberg, S. M. Spillane, and K. J. Vahala

Appl. Phys. Lett. 85, 6113 (2004); http://dx.doi.org/10.1063/1.1833556 (3 pages) | Cited 38 times

Online Publication Date: 15 December 2004

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Optical microcavities confine light spatially and temporally and find application in a wide range of fundamental and applied studies. In many areas, the microcavity figure of merit is not only determined by photon lifetime (or the equivalent quality-factor, Q), but also by simultaneous achievement of small mode volume (V). Here we demonstrate ultra-high Q-factor small mode volume toroid microcavities on-a-chip, which exhibit a Q∕V factor of more than 106 (λ∕n)−3. These values are the highest reported to date for any chip-based microcavity. A corresponding Purcell factor in excess of 200 000 and a cavity finesse of >2.8×106 is achieved, demonstrating that toroid microcavities are promising candidates for studies of the Purcell effect, cavity QED or biochemical sensing.
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42.79.-e Optical elements, devices, and systems
07.60.-j Optical instruments and equipment

Coherent plasmons in InSb

Michael P. Hasselbeck, L. A. Schlie, and D. Stalnaker

Appl. Phys. Lett. 85, 6116 (2004); http://dx.doi.org/10.1063/1.1840118 (3 pages) | Cited 2 times

Online Publication Date: 15 December 2004

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Far-infrared electromagnetic radiation is emitted by coherent plasmon oscillations in bulk (111) InSb. The oscillations are excited by near-infrared ultrashort laser pulses and characterized as a function of temperature. The coherent plasmon frequency is determined by the intrinsic electron concentration and donor doping density. The amplitude of the oscillations decreases with increasing temperature due to a weakening of the photo-Dember starting mechanism.
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71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
78.55.Cr III-V semiconductors
78.47.-p Spectroscopy of solid state dynamics

A microfluidic 2×2 optical switch

Kyle Campbell, Alex Groisman, Uriel Levy, Lin Pang, Shayan Mookherjea, Demetri Psaltis, and Yeshaiahu Fainman

Appl. Phys. Lett. 85, 6119 (2004); http://dx.doi.org/10.1063/1.1839281 (3 pages) | Cited 26 times

Online Publication Date: 15 December 2004

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A 2×2 microfluidic-based optical switch is proposed and demonstrated. The switch is made of an optically clear silicon elastomer, Polydimethylsiloxane (PDMS), using soft lithography. It has insertion loss smaller than 1 dB and extinction ratio on the order of 20 dB. The device is switching between transmission (bypass) and reflection (exchange) modes within less than 20 ms.
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42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.82.Gw Other integrated-optical elements and systems

Operating characteristics of a traveling-wave semiconducting polymer optical amplifier

G. Heliotis, D. D. C. Bradley, M. Goossens, S. Richardson, G. A. Turnbull, and I. D. W. Samuel

Appl. Phys. Lett. 85, 6122 (2004); http://dx.doi.org/10.1063/1.1835555 (3 pages) | Cited 11 times

Online Publication Date: 15 December 2004

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We report a study of light amplification in dilute solutions of the semiconducting polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). The operating characteristics of the F8BT solution amplifier are investigated in detail, and we show that it provides a large optical gain (>40 dB∕cm) across a wide (>48 nm) spectral bandwidth in the green. The saturation characteristics of the amplifier are found to be in excellent agreement with theoretical predictions, allowing an estimate of the F8BT stimulated emission cross-section, σ. We find σ∼1.7×10−16 cm2, comparable with the reported values for established laser dyes and other high-gain conjugated polymers.
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42.55.Px Semiconductor lasers; laser diodes
42.70.Jk Polymers and organics
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption

Diffraction by an optical fractal grating

Bo Hou, Gu Xu, Weijia Wen, and George K. L. Wong

Appl. Phys. Lett. 85, 6125 (2004); http://dx.doi.org/10.1063/1.1840112 (3 pages) | Cited 8 times

Online Publication Date: 15 December 2004

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We report experimental and theoretical studies of Fraunhofer diffraction pattern of a 15-level H-fractal grating. The diffraction pattern was found to exhibit self-similarity. In particular, the diffracted light was found to be more intense at higher spatial frequencies than at lower frequencies, in stark contrast to the diffraction patterns of wire gratings and grid gratings. Using Fourier transform theory, we show that this unusual behavior comes from the structural coherence of the H-fractal, which makes it favorable to use higher-order diffraction spectra for larger dispersion. In addition, the fractal dimension of the grating is shown analytically and experimentally to be two.
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42.25.Fx Diffraction and scattering
42.79.Dj Gratings
42.30.Kq Fourier optics
42.25.Kb Coherence

Photoluminescence and polarized photodetection of single ZnO nanowires

Zhiyong Fan, Pai-chun Chang, Jia G. Lu, Erich C. Walter, Reginald M. Penner, Chien-hung Lin, and Henry P. Lee

Appl. Phys. Lett. 85, 6128 (2004); http://dx.doi.org/10.1063/1.1841453 (3 pages) | Cited 120 times

Online Publication Date: 15 December 2004

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Single crystal ZnO nanowires are synthesized and configured as field-effect transistors. Photoluminescence and photoconductivity measurements show defect-related deep electronic states giving rise to green-red emission and absorption. Photocurrent temporal response shows that current decay time is significantly prolonged in vacuum due to a slower oxygen chemisorption process. The photoconductivity of ZnO nanowires is strongly polarization dependent. Collectively, these results demonstrate that ZnO nanowire is a remarkable optoelectronic material for nanoscale device applications.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Et II-VI semiconductors
73.50.Pz Photoconduction and photovoltaic effects
73.20.Hb Impurity and defect levels; energy states of adsorbed species
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
68.43.Mn Adsorption kinetics
61.46.-w Structure of nanoscale materials

In-rich In1−xGaxN films by metalorganic vapor phase epitaxy

Chin-An Chang, Chuan-Feng Shih, Nai-Chuan Chen, T. Y. Lin, and Kuo-Shiun Liu

Appl. Phys. Lett. 85, 6131 (2004); http://dx.doi.org/10.1063/1.1842375 (3 pages) | Cited 17 times

Online Publication Date: 15 December 2004

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Single crystalline In1−xGaxN films containing high In content (70%–100%) were grown by metalorganic vapor phase epitaxy. A linear relation was observed between the lattice constants and gas phase Ga∕In ratios. The surface morphology changed from pyramid for InN to more planar ones for the InGaN alloys with increasing Ga content. The electron mobility decreased rapidly from 1200 cm2∕V s for InN to less than 100 cm2∕V s for In0.7Ga0.3N, with a carrier concentration of low- 1019 cm−3 for all the as-grown films. Using photoluminescence a single emission peak was observed at 1.4–1.6 μm for the In-rich InGaN with decreasing wavelengths up to below 20% of Ga. Two peaks were observed for the In0.80Ga0.20N, however, indicating possible phase separation. The x-ray photoelectron spectroscopic measurement showed shifts to higher binding energies for both In and Ga with increasing Ga content. The estimated alloy composition, however, depended sensitively on the sputtering conditions of the samples.
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81.05.Ea III-V semiconductors
68.55.A- Nucleation and growth
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)
78.66.Fd III-V semiconductors
73.61.Ey III-V semiconductors
78.55.Cr III-V semiconductors
73.50.Dn Low-field transport and mobility; piezoresistance
79.60.Bm Clean metal, semiconductor, and insulator surfaces
79.60.Dp Adsorbed layers and thin films
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Mechanisms of target poisoning during magnetron sputtering as investigated by real-time in situ analysis and collisional computer simulation

D. Güttler, B. Abendroth, R. Grötzschel, W. Möller, and D. Depla

Appl. Phys. Lett. 85, 6134 (2004); http://dx.doi.org/10.1063/1.1835002 (3 pages) | Cited 17 times

Online Publication Date: 15 December 2004

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The evolution of reactive gas uptake at the target surface has been investigated by real-time in situ diagnostics during magnetron sputtering. Using a planar circular dc magnetron for reactive sputter deposition of TiN from a Ti target in an argon/nitrogen gas mixture, the target uptake of nitrogen was determined at varying gas flows of nitrogen using the 14N(d,α)12C nuclear reaction, directly demonstrating the target “poisoning” effect. The expected hysteresis behavior at increasing/decreasing nitrogen gas flow is confirmed. Within the precision of the measurement, the nitrogen content remains unaltered after switching off the magnetron, indicating the absence of a significant mobile fraction of nitrogen in the target. The maximum amount of retained nitrogen significantly exceeds one adsorbed monolayer, which is attributed to nitrogen ion implantation and recoil implantation of adsorbed nitrogen. This is quantitatively reproduced by TRIDYN collisional computer simulations.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Cd Deposition by sputtering

Observation of self-sputtering in energetic condensation of metal ions

André Anders

Appl. Phys. Lett. 85, 6137 (2004); http://dx.doi.org/10.1063/1.1840115 (3 pages) | Cited 10 times

Online Publication Date: 15 December 2004

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The condensation of energetic metal ions on a surface may cause self-sputtering even in the absence of substrate bias. Charge-state-averaged self-sputtering yields were determined for both zirconium and gold ions generated by a cathodic vacuum arc. Films were deposited on differently biased substrates exposed to streaming Zr and Au vacuum arc plasma. The self-sputtering yields for both metals were estimated to be about 0.05 in the absence of bias, and exceeding 0.5 when bias reached −50 V. These surprisingly high values can be reconciled with binary collision theory and molecular dynamics calculations taking the high kinetic and potential energy of vacuum arc ions into account.
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79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
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Direct observations of dislocation half-loops inserted from the surface of the GeSi heteroepitaxial film

Yu. B. Bolkhovityanov, A. S. Deryabin, A. K. Gutakovskii, M. A. Revenko, and L. V. Sokolov

Appl. Phys. Lett. 85, 6140 (2004); http://dx.doi.org/10.1063/1.1839271 (3 pages) | Cited 6 times

Online Publication Date: 15 December 2004

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The initial stage of relaxation of mechanical stresses in the Ge0.32Si0.68∕Si(001) heterostructure grown by low-temperature (300 °C) molecular-beam epitaxy is studied by means of transmission electron microscopy. Dislocation half-loops propagating from the film surface and generating misfit dislocations during expansion are visualized.
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81.05.Hd Other semiconductors
81.05.Cy Elemental semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.60.Bs Mechanical and acoustical properties
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
68.37.Lp Transmission electron microscopy (TEM)
61.72.Cc Kinetics of defect formation and annealing
68.55.-a Thin film structure and morphology

Atomic arrangement at the Au∕p-GaN interface in low-resistance contacts

H. Omiya, F. A. Ponce, H. Marui, S. Tanaka, and T. Mukai

Appl. Phys. Lett. 85, 6143 (2004); http://dx.doi.org/10.1063/1.1840105 (3 pages) | Cited 10 times

Online Publication Date: 15 December 2004

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The atomic arrangement of Ni∕Au contacts on p-type GaN has been studied by transmission electron microscopy (TEM). The initial Au∕Ni∕GaN structure transforms upon annealing at 400–600 °C into Ni∕Au∕GaN. The Au layer consists of thin platelets with uniform thickness. High-resolution TEM reveals an atomically sharp interface between GaN and Au, with no intermediate phases present. The epitaxial relationship between the Au layer and the GaN film is (111)Au∥(0002)GaN, and [1math0]Au∥[11math0]GaN. Analysis of TEM images shows that Au is directly in contact with Ga atoms, with no evidence of presence of Ni. The interface separation corresponds to covalent Ga and metallic Au, with a bond length of ∼2.5 math. This corresponds to an atomically abrupt transition between covalently bonded Ga and metallic bonded Au.
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68.35.Ct Interface structure and roughness
71.55.Eq III-V semiconductors
68.55.-a Thin film structure and morphology
61.72.Cc Kinetics of defect formation and annealing
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.Qq Microscopic defects (voids, inclusions, etc.)
61.72.Mm Grain and twin boundaries
68.37.Lp Transmission electron microscopy (TEM)

Epitaxial thin film growth of Ca2RuO4+δ by pulsed laser deposition

X. Wang, Y. Xin, P. A. Stampe, R. J. Kennedy, and J. P. Zheng

Appl. Phys. Lett. 85, 6146 (2004); http://dx.doi.org/10.1063/1.1841451 (3 pages) | Cited 2 times

Online Publication Date: 15 December 2004

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Epitaxial Ca2RuO4+δ thin films have been grown on (001) LaAlO3 substrate by pulsed laser deposition. X-ray diffraction and transmission electron microscopy studies show that the films are single crystal with good structural quality. The films are determined to have quasitetragonal structure with a=5.352(8) math, c=12.20(9) math and epitaxially grown on the substrate with in-plane tensile strains. C plane of the film is parallel to the substrate surface and its 〈100〉 is along 〈110〉 of the pseudocubic LaAlO3 cell. Resistivity versus temperature measurement reveals that the thin film has metallic-like behavior with low resistivity (<0.002 Ω cm) and no metal-to-insulator transition between 2 and 300 K, different from its bulk material property.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
75.50.Ee Antiferromagnetics
81.15.Np Solid phase epitaxy; growth from solid phases
81.15.Fg Pulsed laser ablation deposition
72.60.+g Mixed conductivity and conductivity transitions
73.61.Ng Insulators
68.55.A- Nucleation and growth
68.37.Lp Transmission electron microscopy (TEM)
68.55.-a Thin film structure and morphology

Electron-beam-induced surface quasimelting of Co granular nanowires

Seung H. Huh and Atsushi Nakajima

Appl. Phys. Lett. 85, 6149 (2004); http://dx.doi.org/10.1063/1.1839278 (3 pages) | Cited 5 times

Online Publication Date: 15 December 2004

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Electron-beam-induced surface quasimelting of Co granular nanowires (GNWs) of 6–10 nm in diameter and ∼30 nm in length has been studied using a high-resolution electron microscope. From the time evolution of electron microscope images as a function of the irradiation time, it has been revealed that the morphological change in the Co GNW is quite different from that in a Co nanocluster (NC) in terms of structures and phase transitions. For example, the allotropic β→α transition, inhibited in the Co NC, can proceed in the Co GNW through the lattice softening of fcc (111) facets without dimensional collapse.
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81.07.Bc Nanocrystalline materials
61.80.Fe Electron and positron radiation effects
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
61.82.Bg Metals and alloys
64.70.K- Solid-solid transitions
61.46.-w Structure of nanoscale materials

Tailoring the Ti∕4H–SiC Schottky barrier by ion irradiation

Fabrizio Roccaforte, Corrado Bongiorno, Francesco La Via, and Vito Raineri

Appl. Phys. Lett. 85, 6152 (2004); http://dx.doi.org/10.1063/1.1841476 (3 pages) | Cited 4 times

Online Publication Date: 15 December 2004

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The effects of ion irradiation on the Ti∕4H–SiC Schottky barrier are discussed. The Ti∕SiC interfacial region was modified by irradiating Schottky diodes with 8 MeV Si+4 ions at fluences between 1×109 and 1×1012 ions∕cm2. By increasing the ion fluence, an increase of the Schottky barrier ΦB occurs, from the value of 1.05 eV after preparation to the value of 1.21 eV after irradiation at a fluence of 1×1012 ions∕cm2, without substantial changes in the ideality factor (n=1.09). Along with the barrier height increase, a decrease of the leakage current of about two orders of magnitude was observed after irradiation. The results were interpreted in terms of the structural and electrical modification of the interfacial region.
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61.80.Jh Ion radiation effects
73.40.Ns Metal-nonmetal contacts
61.82.Fk Semiconductors
61.82.Bg Metals and alloys
85.30.Kk Junction diodes
73.30.+y Surface double layers, Schottky barriers, and work functions
85.30.Hi Surface barrier, boundary, and point contact devices

Photoemission study of energy-band alignment for RuOx∕HfO2∕Si system

Q. Li, S. J. Wang, K. B. Li, A. C. H. Huan, J. W. Chai, J. S. Pan, and C. K. Ong

Appl. Phys. Lett. 85, 6155 (2004); http://dx.doi.org/10.1063/1.1839287 (3 pages) | Cited 22 times

Online Publication Date: 15 December 2004

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Conductive oxides RuOx as alternative electrode on high-κ HfO2 gate dielectric have been fabricated by ultrahigh-vacuum sputtering and subsequently oxidized using oxygen plasma. The energy-band alignment for the RuOx∕HfO2∕Si system and the oxidation-state dependence of barrier height for RuOx contacting to HfO2 dielectrics has been analyzed by x-ray photoemission spectroscopy. The valence- and conduction-band offsets of HfO2∕Si are determined to be 3.05±0.1 and 1.48±0.1 eV, respectively. The barrier heights for the RuOx contacting to HfO2 are oxidation-state dependent, in the range of 1.95–2.73 eV.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.55.-g Dielectric thin films
73.20.At Surface states, band structure, electron density of states
81.15.Cd Deposition by sputtering
81.65.Mq Oxidation
79.60.Bm Clean metal, semiconductor, and insulator surfaces
79.60.Dp Adsorbed layers and thin films
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.61.Cw Elemental semiconductors
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