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4 Jul 2005

Volume 87, Issue 1, Articles (01xxxx)

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Appl. Phys. Lett. 87, 013110 (2005); http://dx.doi.org/10.1063/1.1977187 (3 pages)

R. C. Wang, C. P. Liu, J. L. Huang, S.-J. Chen, Y.-K. Tseng, and S.-C. Kung
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Reduction of the internal electric field in wurtzite a-plane GaN self-assembled quantum dots

N. Garro, A. Cros, J. A. Budagosky, A. Cantarero, A. Vinattieri, M. Gurioli, S. Founta, H. Mariette, and B. Daudin

Appl. Phys. Lett. 87, 011101 (2005); http://dx.doi.org/10.1063/1.1977210 (3 pages) | Cited 21 times

Online Publication Date: 27 June 2005

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We present a study of the emission of GaN/AlN self-assembled quantum dots grown on a-plane 6H-SiC showing evidence of the suppression of the internal electric field. The strain in dots and barriers is determined by means of Raman scattering and the induced piezoelectric polarizations are estimated. These reveal a compensation of the spontaneous polarization and justify the lack of a quantum confined Stark effect found in the photoluminescence spectra. Strain effects and strong confinement are responsible for the partial depolarization of the emission and its energy dependence.
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73.63.Kv Quantum dots
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
78.67.Hc Quantum dots
78.30.Fs III-V and II-VI semiconductors
78.55.Cr III-V semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
77.65.Ly Strain-induced piezoelectric fields
77.22.Ej Polarization and depolarization

Switching mirror in the CdTe-based photonic crystal

Michael Cada

Appl. Phys. Lett. 87, 011102 (2005); http://dx.doi.org/10.1063/1.1954876 (3 pages) | Cited 1 time

Online Publication Date: 28 June 2005

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Theoretical investigations, modeling and design of a Bragg mirror (photonic crystal structure) in the CdTe-based material system are presented. It is shown that, in addition to the third-order nonlinearity (Kerr-like effect), the two-photon absorption and the resulting free-carrier refraction have to be included in the analysis and design. Inclusion of changes in the refractive index with the square of the light intensity (fifth-order nonlinearity) leads to desirable functional features of the switching mirror. The analysis provides insight into the complex nonlinear effects and serves as a tool for designing a nonlinear intra-cavity mirror for a novel pulsed laser.
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42.79.Bh Lenses, prisms and mirrors
42.15.Eq Optical system design
42.70.Qs Photonic bandgap materials
42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.25.Gy Edge and boundary effects; reflection and refraction
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Ultraviolet direct printing of rare-earth-doped polymer waveguide amplifiers

W. H. Wong, K. S. Chan, and E. Y. B. Pun

Appl. Phys. Lett. 87, 011103 (2005); http://dx.doi.org/10.1063/1.1968422 (3 pages) | Cited 11 times

Online Publication Date: 28 June 2005

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Polymer channel waveguide amplifier arrays were fabricated using ultraviolet direct printing methodology. The effects of different weight percentage (wt. %) combinations of rare-earth ions erbium (Er3+) and ytterbium (Yb3+) on the absorption spectrum, as well as the internal gain, were investigated. With an input signal power of ∼ −18 dBm, the highest internal gain obtained is ∼ 16.5 dB at a wavelength of 1533 nm for a 20 mm long multimode channel waveguide codoped with ∼ 1 wt. % of Er3+ and ∼ 10 wt. % of Yb3+ derivatives. The UV direct printing methodology opens a simple platform for the fabrication of dense and compact polymer waveguide laser arrays.
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42.86.+b Optical workshop techniques
42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.By Design of specific laser systems
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Active Q-switched distributed feedback erbium-doped fiber lasers

P. Pérez-Millán, J. L. Cruz, and M. V. Andrés

Appl. Phys. Lett. 87, 011104 (2005); http://dx.doi.org/10.1063/1.1990252 (3 pages) | Cited 11 times

Online Publication Date: 28 June 2005

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This letter presents a distributed feedback fiber laser that operates in an actively controlled Q-switched regime. The laser is based on a Bragg grating made in an erbium-doped fiber. The grating has a defect induced by a magnetostrictive transducer that configures the distributed feedback laser structure. The phase shift generated by the defect can be dynamically modified by an electric current, permitting active Q-switching of the laser. The laser generates pulses of 75 ns duration and the repetition rate can be continuously adjusted from 0 to 10 kHz.
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42.55.Px Semiconductor lasers; laser diodes
42.55.Rz Doped-insulator lasers and other solid state lasers
42.55.Wd Fiber lasers
42.60.By Design of specific laser systems
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Observation of far-infrared emission from excited cytosine molecules

Y. C. Shen, P. C. Upadhya, E. H. Linfield, and A. G. Davies

Appl. Phys. Lett. 87, 011105 (2005); http://dx.doi.org/10.1063/1.1968412 (3 pages) | Cited 8 times

Online Publication Date: 29 June 2005

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We report a time-resolved investigation of the resonant absorption of far-infrared radiation and the subsequent vibrational relaxation processes in a sample of polycrystalline cytosine at 4 K, using terahertz time-domain spectroscopy. The subpicosecond time resolution achieved in our experiments corresponds to a near single-cycle of far-infrared radiation, and this enables us to observe the damped oscillations of the electric field produced by excited molecules as they decay. Furthermore, we show that the progressive absorption and subsequent emission of far-infrared radiation at the frequency of the corresponding vibrational mode can be followed directly as a function of time by means of time-partitioned Fourier transforms of the transmitted signal.
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33.20.Ea Infrared spectra
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment
31.50.Df Potential energy surfaces for excited electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants

Transflective spatial filter based on azo-dye-doped cholesteric liquid crystal films

Tsung-Hsien Lin and Andy Y.-G. Fuh

Appl. Phys. Lett. 87, 011106 (2005); http://dx.doi.org/10.1063/1.1990248 (3 pages) | Cited 2 times

Online Publication Date: 29 June 2005

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This work demonstrates the feasibility of exploiting the photoisomerization effect in azo-dye-doped cholesteric liquid crystal (DDCLC) films with a concomitant decline of the phase transition temperature from the cholesteric to an isotropic phase (TCh‐I) as a spatial filter. The fabrication depends on the fact that the various intensities of the diffracted orders are responsible for the various degrees of transparency associated with the photoisomerized DDCLC film. High- and low-pass images in the Fourier optical signal process can be simultaneously observed via reflected and transmitted signals, respectively. A simulation is also performed, and the results are consistent closely with experimental data.
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42.79.Ci Filters, zone plates, and polarizers
42.70.Df Liquid crystals
61.30.-v Liquid crystals
64.70.M- Transitions in liquid crystals
42.30.Kq Fourier optics
78.66.Qn Polymers; organic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
82.50.-m Photochemistry

Electro-optic phase modulator using metal-defined polymer optical waveguide

Seong-Ku Kim, W. Yuan, K. Geary, Yu-Chueh Hung, H. R. Fetterman, Dong-Gun Lee, C. Zhang, C. Wang, W. H. Steier, G.-C. Park, S.-J. Kang, and I. Oh

Appl. Phys. Lett. 87, 011107 (2005); http://dx.doi.org/10.1063/1.1991998 (3 pages) | Cited 4 times

Online Publication Date: 30 June 2005

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We demonstrate metal-defined electro-optic polymer phase modulators for the first time. A metal strip patterned on top of a three-layer slab waveguide causes a strain-induced refractive index change, resulting in lateral optical mode confinement within the core layer. The strain profile, refractive indexes, and optical mode shapes of the metal defined optical waveguide are obtained analytically. Electro-optic phase modulators fabricated using this technique have a driving voltage of 5.6 V, and ∼ 18 dB of extinction ratio at 1.55 μm wavelength. These results may be used in the complex design of integrated polymer optical circuits that require a simple and cheap fabrication process.
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42.79.Hp Optical processors, correlators, and modulators
42.82.Et Waveguides, couplers, and arrays
42.82.Cr Fabrication techniques; lithography, pattern transfer
42.79.Gn Optical waveguides and couplers
42.70.Jk Polymers and organics

Electro-optic characteristics and switching principle of a single-cell-gap transflective liquid-crystal display associated with in-plane rotation of liquid crystal driven by a fringe-field

J. H. Song, Y. J. Lim, M.-H. Lee, S. H. Lee, and S. T. Shin

Appl. Phys. Lett. 87, 011108 (2005); http://dx.doi.org/10.1063/1.1991981 (3 pages) | Cited 34 times

Online Publication Date: 30 June 2005

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A single-gap transflective liquid-crystal (LC) display associated with in-plane rotation of the LC director was designed. In the device, a fringe electric field drives to rotate a homogeneously aligned LC in plane to optimize polarization efficiency. Rotating degrees of the LC director, 22.5° and 45° were required in both reflective and transmissive regions, respectively. In the device, the cell gap was the same for both regions, and a dark state was irrespective of the cell retardation value at normal direction, which was highly important in massive fabrications. The switching principle and electro-optic characteristics of the device are reported herein.
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42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems
42.70.Df Liquid crystals
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order

Low-power, multiport, ultrafast, parametric switching in cascaded waveguide couplers

Roland Schiek, Robert Iwanow, George Stegeman, Thomas Pertsch, Falk Lederer, Yoo Hong Min, and Wolfgang Sohler

Appl. Phys. Lett. 87, 011109 (2005); http://dx.doi.org/10.1063/1.1991993 (3 pages)

Online Publication Date: 30 June 2005

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We explore an approach to multiport, phase-insensitive all-optical switching of milliwatt power signals based on linear coupling in cascaded directional couplers and on nonlinear parametric gain via the second-order nonlinearity. For the basic unit, a two-core coupler, we experimentally demonstrated ultrafast transparent switching of 1550 nm signals triggered by 5-ps-long control pulses at ≈ 775 nm with a peak power of 1.3 W. Theoretically, we show that this concept has the potential for terabit/s switching with a few picojoules control energy and without pulse degradation.
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42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.79.Gn Optical waveguides and couplers
42.82.Et Waveguides, couplers, and arrays
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

Tensile strained Ge p-i-n photodetectors on Si platform for C and L band telecommunications

Jifeng Liu, Douglas D. Cannon, Kazumi Wada, Yasuhiko Ishikawa, Samerkhae Jongthammanurak, David T. Danielson, Jurgen Michel, and Lionel C. Kimerling

Appl. Phys. Lett. 87, 011110 (2005); http://dx.doi.org/10.1063/1.1993749 (3 pages) | Cited 39 times

Online Publication Date: 1 July 2005

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We demonstrate a 0.25% tensile strained Ge p-i-n photodetector on Si platform that effectively covers both C and L bands in telecommunications. The direct band edge of the Ge film has been pushed from 1550 to 1623 nm with 0.25% tensile strain, enabling effective photon detection in the whole L band. The responsivities of the device at 1310, 1550, and 1620 nm are 600, 520, and 100 mA/W under 0 V bias, which can be further improved to 980, 810, and 150 mA/W with antireflection coating based on calculations. Therefore, the device covers the whole wavelength range used in telecommunications. The responsivities at 1310 and 1550 nm are comparable to InGaAs photodetectors currently used in telecommunications. In the spectrum range of 1300–1650 nm, maximum responsivity was already achieved at 0 V bias because carrier transit time is much shorter than carrier recombination life time, leading to ∼ 100% collection efficiency even at 0 V bias. This is a desirable feature for low voltage operation. The absorption coefficients of 0.25% tensile strained Ge in the L band have been derived to be nearly an order of magnitude higher than bulk Ge. The presented device is compatible with conventional Si processing, which enables monolithic integration with Si circuitry.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.60.Dw Photodiodes; phototransistors; photoresistors

Use of patterned laser liftoff process and electroplating nickel layer for the fabrication of vertical-structured GaN-based light-emitting diodes

Shui-Jinn Wang, Kai-Ming Uang, Shiue-Lung Chen, Yu-Cheng Yang, Shu-Cheng Chang, Tron-Min Chen, Chao-Hsuing Chen, and Bor-Wen Liou

Appl. Phys. Lett. 87, 011111 (2005); http://dx.doi.org/10.1063/1.1993757 (3 pages) | Cited 43 times

Online Publication Date: 1 July 2005

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The fabrication process and performance characteristics of a vertical-structured GaN-based light-emitting diode (VM-LED) employing nickel electroplating and patterned laser liftoff techniques are presented. As compared to regular LED, the forward voltage drop of the VM-LED at 20–80 mA is about 10%–21% lower, while the light output power (Lop) is more than twice in magnitude. Especially, the Lop exhibits no saturation or degradation at an injection current up to 520 mA which is about 4.3 times higher than that of the regular one. Substantial improvements in the VM-LEDs performances are mainly attributed to the use of metallic substrate which results in less current crowding, larger effective area, and higher thermal conductivity.
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85.60.Jb Light-emitting devices
42.62.-b Laser applications
81.15.Pq Electrodeposition, electroplating
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Effects of focal volume and spatial inhomogeneity on uncertainty in single-aerosol laser-induced breakdown spectroscopy measurements

G. A. Lithgow and S. G. Buckley

Appl. Phys. Lett. 87, 011501 (2005); http://dx.doi.org/10.1063/1.1984086 (3 pages) | Cited 6 times

Online Publication Date: 27 June 2005

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Particle location with respect to the plasma volume and the focal volume of the collection optics is identified as an important source of uncertainty in single-aerosol laser-induced breakdown spectroscopy (LIBS) measurements. Two sets of optics were used to image different regions of a LIBS plasma and simultaneously collect spectra from single particles. Strong variation and lack of correlation between the two measurement channels suggest that emission from a single particle is localized within the plasma. Measurement precision and detection efficiency were improved by imaging a larger plasma region, thus integrating the spatially distributed signal.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
82.80.Dx Analytical methods involving electronic spectroscopy
82.70.Rr Aerosols and foams
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)

Measurements of dioxygen fluoride (O2F) in an atmospheric pressure plasma jet

Yongho Kim, Jaeyoung Park, Louis A. Rosocha, Hilary L. Teslow, and Hans W. Herrmann

Appl. Phys. Lett. 87, 011502 (2005); http://dx.doi.org/10.1063/1.1968420 (3 pages) | Cited 3 times

Online Publication Date: 28 June 2005

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When etching tantalum with a coaxial-cylinder-type atmospheric pressure plasma jet (APPJ) in a He/CF4/O2 gas mixture, we found that the etch rates vary little with the distance between the plasma region and the substrate. Etch rates decrease by less than a factor of 3 when the distance increases from 0.3 to 20.3 cm in a closed tube that carries the APPJ effluent. These results indicate that reactive species may last up to 40 ms in the effluent. Ultraviolet absorption spectroscopy was employed to measure possible reactive species, specifically dioxygen fluoride (O2F) and ozone (O3), in a parallel-plate type APPJ. O2F was found to be a significantly long-lived species, lasting up to 10 ms in the effluent with a constant density of about 1015 cm−3. Using a measured O3 density profile, the concentration of atomic fluorine (F) was estimated to be about 1013 cm−3, which is two orders of magnitude lower than that of O2F at a distance of 1 cm away from the APPJ exit. In summary, F atoms produced inside the APPJ combine with O2 molecules, producing O2F molecules, which can live long enough to reach and possibly etch metal surfaces.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.77.Bn Etching and cleaning
52.50.Dg Plasma sources
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)

Transient observation of extended x-ray absorption fine structure in laser-melted Si by using femtosecond laser-produced-plasma soft x ray

Katsuya Oguri, Yasuaki Okano, Tadashi Nishikawa, and Hidetoshi Nakano

Appl. Phys. Lett. 87, 011503 (2005); http://dx.doi.org/10.1063/1.1989445 (3 pages) | Cited 9 times

Online Publication Date: 1 July 2005

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We have demonstrated the time-resolved measurement of the extended x-ray absorption fine structure (EXAFS) in laser-melted Si foil by using a pump-probe absorption spectroscopy system that utilizes a femtosecond laser-produced-plasma soft x-ray source. By using 100-fs laser irradiation, we observed the transient change in the Si L-edge EXAFS, that is, a slight shortening of its oscillation period and a decrease in its oscillation amplitude. This result suggests that the Si-Si atomic distance expanded and structural disordering occurred as a result of the production of liquid Si.
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78.70.Dm X-ray absorption spectra
78.47.-p Spectroscopy of solid state dynamics
78.40.Fy Semiconductors
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Fk Semiconductors
42.62.-b Laser applications
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
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Patterning of hydrogenated microcrystalline silicon growth by magnetic field

A. Fejfar, J. Stuchlík, T. Mates, M. Ledinský, S. Honda, and J. Kočka

Appl. Phys. Lett. 87, 011901 (2005); http://dx.doi.org/10.1063/1.1984102 (3 pages) | Cited 2 times

Online Publication Date: 27 June 2005

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A way of influencing growth of silicon films by magnetic field is demonstrated. Permanent magnet(s) placed under the substrate influenced the discharge in a mixture of silane and hydrogen and led to formation of microcrystalline regions in otherwise amorphous film. The pattern of microcrystalline regions varied with the orientation of the magnetic field. Microscopic study by atomic force microscopy and by micro-Raman spectroscopy revealed that the microcrystalline regions resulted from a higher density of crystalline grain nuclei, increased at the locations where the magnetron effect could be expected. This phenomenon could be used to study the transition between amorphous and microcrystalline growth. Moreover, we suggest it as a kind of “magnetic lithography” for the preparation of predefined microcrystalline patterns in otherwise amorphous silicon films.
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81.05.Cy Elemental semiconductors
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.66.Db Elemental semiconductors and insulators
78.30.Am Elemental semiconductors and insulators
52.77.Dq Plasma-based ion implantation and deposition
68.37.Ps Atomic force microscopy (AFM)
68.35.B- Structure of clean surfaces (and surface reconstruction)

Study of fluorine behavior in silicon by selective point defect injection

M. N. Kham, H. A. W. El Mubarek, J. M. Bonar, and P. Ashburn

Appl. Phys. Lett. 87, 011902 (2005); http://dx.doi.org/10.1063/1.1984094 (3 pages) | Cited 4 times

Online Publication Date: 27 June 2005

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This letter reports a point defect injection study of 185 keV 2.3×1015 cm−2 fluorine implanted silicon. After an inert anneal at 1000 °C, fluorine peaks are seen at depths of 0.3Rp and Rp and a shoulder between 0.5–0.7Rp. The shallow peak (at 0.3Rp) is significantly smaller under interstitial injection than under both inert and vacancy injection conditions. For a longer anneal under interstitial injection, both the shallow peak and the shoulder are eliminated. These results support earlier work suggesting that the shallow fluorine peak is due to vacancy-fluorine clusters which are responsible for suppression of boron thermal diffusion in silicon. The elimination of the shallow fluorine peak and the shoulder is explained by the annihilation of vacancies in the clusters with injected interstitials.
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81.05.Cy Elemental semiconductors
61.72.uf Ge and Si
61.72.Cc Kinetics of defect formation and annealing
61.72.J- Point defects and defect clusters

Optical study of disorder and defects in hydrogenated amorphous silicon carbon alloys

Th. Nguyen-Tran, V. Suendo, and P. Roca i Cabarrocas

Appl. Phys. Lett. 87, 011903 (2005); http://dx.doi.org/10.1063/1.1968413 (3 pages) | Cited 4 times

Online Publication Date: 28 June 2005

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We have studied the defect density and disorder in hydrogenated amorphous silicon carbon alloys produced by rf glow discharge of silane-methane-hydrogen mixtures, by combining spectroscopic ellipsometry and photothermal deflection spectroscopy measurements. Increasing the methane flow rate leads to a widening of the optical gap and to an increase of the apparent disorder, deduced from the standard analysis of the exponential absorption edge; the so-called Urbach energy. Interestingly, the subgap absorption decreases with increasing methane flow rate. This points towards a lower density of defects with increasing carbon content and is in contrast with the increased disorder. This apparent contradiction results from the presence of three absorption bands within the gap of this material, as reported by [ Ivashchenko et al., J. Phys.: Condens. Matter 14, 1799 (2002) ], and which make unreliable the standard analysis of the disorder in silicon carbon alloys.
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78.40.Fy Semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.66.Jg Amorphous semiconductors; glasses
82.80.Kq Energy-conversion spectro-analytical methods (e.g., photoacoustic, photothermal, and optogalvanic spectroscopic methods)

Electronic properties of N- and C-doped TiO2

Jung-Yup Lee, Jaewon Park, and Jun-Hyung Cho

Appl. Phys. Lett. 87, 011904 (2005); http://dx.doi.org/10.1063/1.1991982 (3 pages) | Cited 57 times

Online Publication Date: 29 June 2005

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We present first-principles density-functional calculations for the electronic properties of nitrogen(N)-doped as well as carbon(C)-doped titanium dioxide (TiO2). We find that the bands originating from N (C) 2p states appear in the band gap of TiO2, but the mixing of N (C) with O 2p states is too weak to produce a significant band-gap narrowing. Our results are consistent with several recent experimental data of N-doped TiO2, where the absorption of visible light is due to isolated N 2p states above the valence-band maximum of TiO2 rather than due to a band-gap narrowing.
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71.20.Ps Other inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
78.40.Ha Other nonmetallic inorganics

Surface oxide reduction and bilayer molecular assembly of a thiol-terminated organosilane on Cu

P. G. Ganesan, A. Kumar, and G. Ramanath

Appl. Phys. Lett. 87, 011905 (2005); http://dx.doi.org/10.1063/1.1968414 (3 pages) | Cited 10 times

Online Publication Date: 29 June 2005

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We demonstrate the use of thiol-terminated organosilane to reduce the surface oxide and form a protective layer on Cu surfaces. The thiol termini of mercapto-propyl-trimethoxy-silane molecules reduce the copper oxide, and release disulfide- and sulfonate-terminated silanes. Unreacted mercaptosilanes and disulfides then assemble on the clean Cu surface forming a monolayer via chemisorption. The outward pointing methoxy groups react with other methoxysilane termini of sulfonated- and unreacted organosilanes, forming a molecular bilayer with Si–O–Si linkages between the two layers. These findings open up new possibilities for surface cleaning and passivating Cu interconnects with molecular nanolayers, and minimize surface-scattering-induced conductivity decrease in nanometer-thick Cu lines, without destructively etching the surface Cu oxide.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
68.43.Mn Adsorption kinetics
81.65.-b Surface treatments
82.30.-b Specific chemical reactions; reaction mechanisms

Coherent epitaxial growth and superhardness effects of c‐TiN/h‐TiB2 nanomultilayers

Fanghua Mei, Nan Shao, Lun Wei, Yunshan Dong, and Geyang Li

Appl. Phys. Lett. 87, 011906 (2005); http://dx.doi.org/10.1063/1.1951047 (3 pages) | Cited 10 times

Online Publication Date: 1 July 2005

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TiN/TiB2 nanomultilayers with different TiB2 layer thicknesses were deposited by the multitarget magnetron sputtering method. Studies show that because of the template effects of the cubic TiN layer, the normally amorphous TiB2 layer crystallizes into a compact hexagonal structure when its thickness is less than 2.9 nm. As a result, the multilayers form a c‐TiN/h‐TiB2 coherent epitaxial structure with the orientation relationship of {111}TiN//{0001}TiB2, 〈110〉TiN//〈11math0〉TiB2. Correspondingly, the multilayers show a significant hardness enhancement with a maximum hardness of 46.9 GPa. Further increase in TiB2 layer thickness leads to the formation of amorphous TiB2 that blocks the coherent growth of the films, and thus the hardness of the multilayers decreases gradually.
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81.07.Bc Nanocrystalline materials
68.65.Ac Multilayers
68.35.Gy Mechanical properties; surface strains
81.15.Cd Deposition by sputtering
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.Qp Friction, tribology, and hardness
64.70.K- Solid-solid transitions
61.50.Ks Crystallographic aspects of phase transformations; pressure effects

Controlled erbium incorporation and photoluminescence of Er-doped Y2O3

Trinh Tu Van and Jane P. Chang

Appl. Phys. Lett. 87, 011907 (2005); http://dx.doi.org/10.1063/1.1984082 (3 pages) | Cited 24 times

Online Publication Date: 1 July 2005

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A high concentration of erbium doping was achieved in Y2O3 thin films on Si (100) by depositing Y2O3 alternatively with Er2O3 using radical-enhanced atomic layer deposition (ALD). Specifically, the erbium doping level was controlled by varying the ratio of Y2O3:Er2O3 cycles during deposition, and a 10:5 ratio yielded ∼ 9 at. % erbium incorporation in Y2O3, confirmed by the compositional analysis using x-ray photoelectron spectroscopy. Room-temperature photoluminescence was observed in a 320-Å Er-doped (9 at. %) Y2O3 film deposited at 350 °C. This result is very promising, since the film was fairly thin and no annealing at high temperature was needed to activate the erbium ions. This suggests that radical-enhanced ALD was able to preserve the optically active trivalent state of the erbium ion from its precursor state. The effective absorption cross section for Er3+ ions incorporated in Y2O3 was estimated to be on the order of 10−18 cm2, about three orders of magnitude larger than the direct optical absorption cross section reported for Er3+ ions in a stoichiometric SiO2 host. These results validate Y2O3 as a promising Er3+ host material and demonstrate that radical-enhanced ALD is a viable technique for synthesizing these materials.
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81.05.-t Specific materials: fabrication, treatment, testing, and analysis
68.55.A- Nucleation and growth
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.72.up Other materials
78.66.Nk Insulators
78.55.Hx Other solid inorganic materials
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
79.60.Dp Adsorbed layers and thin films
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Influence of plasmons on terahertz conductivity measurements

Han-Kwang Nienhuys and Villy Sundström

Appl. Phys. Lett. 87, 012101 (2005); http://dx.doi.org/10.1063/1.1977213 (3 pages) | Cited 11 times

Online Publication Date: 27 June 2005

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Time-domain terahertz spectroscopy allows measuring the complex conductivity spectrum of materials at frequencies on the order of 1 THz. Typically, terahertz (THz) studies produce conductivity spectra that are different from those predicted by the classical Drude model, especially in nanostructured materials. We claim that plasmon resonances in particles that are small compared to the THz wavelength cause these deviations. This is supported by measurements on photoexcited silicon, in bulk as well as in micron-sized particles. In the latter, the behavior is vastly different and strongly dependent on charge carrier concentration.
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72.30.+q High-frequency effects; plasma effects
72.80.Cw Elemental semiconductors
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons

Role of near-surface states in ohmic-Schottky conversion of Au contacts to ZnO

H. L. Mosbacker, Y. M. Strzhemechny, B. D. White, P. E. Smith, D. C. Look, D. C. Reynolds, C. W. Litton, and L. J. Brillson

Appl. Phys. Lett. 87, 012102 (2005); http://dx.doi.org/10.1063/1.1984089 (3 pages) | Cited 67 times

Online Publication Date: 27 June 2005

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A conversion from ohmic to rectifying behavior is observed for Au contacts on atomically ordered polar ZnO surfaces following remote, room-temperature oxygen plasma treatment. This transition is accompanied by reduction of the “green” deep level cathodoluminescence emission, suppression of the hydrogen donor-bound exciton photoluminescence and a ∼ 0.75 eV increase in n-type band bending observed via x-ray photoemission. These results demonstrate that the contact type conversion involves more than one mechanism, specifically, removal of the adsorbate-induced accumulation layer plus lowered tunneling due to reduction of near-surface donor density and defect-assisted hopping transport.
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81.05.Dz II-VI semiconductors
73.20.At Surface states, band structure, electron density of states
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.30.+y Surface double layers, Schottky barriers, and work functions
71.55.Gs II-VI semiconductors
73.40.Ns Metal-nonmetal contacts
73.40.Ei Rectification
72.20.Ee Mobility edges; hopping transport
71.35.-y Excitons and related phenomena
52.77.-j Plasma applications
81.65.-b Surface treatments
78.55.Et II-VI semiconductors
79.60.Bm Clean metal, semiconductor, and insulator surfaces
78.60.Hk Cathodoluminescence, ionoluminescence

Direct experimental evidence of hybridization of Pb states with O 2p states in ferroelectric perovskite oxides

J. C. Jan, H. M. Tsai, C. W. Pao, J. W. Chiou, K. Asokan, K. P. Krishna Kumar, W. F. Pong, Y. H. Tang, M.-H. Tsai, S. Y. Kuo, and W. F. Hsieh

Appl. Phys. Lett. 87, 012103 (2005); http://dx.doi.org/10.1063/1.1988984 (3 pages) | Cited 7 times

Online Publication Date: 28 June 2005

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This work presents the O K- and Ti L3,2-edge x-ray absorption near-edge structure (XANES) spectra of PbxSr1−xTiO3 (PxSTO) and BaxSr1−xTiO3 (BxSTO) compounds with various Pb and Ba concentrations. The result provides direct evidence that the Pb–O bonding strongly affects O 2p–Ti 3d hybridization in the TiO6 octahedron of PxSTO. In contrast, the Ba–O bonding does not substantially affect O 2p–Ti 3d hybridization in BxSTO. The Ti L3-edge XANES spectra show the splitting of the eg band for PxSTO with x≳0.5, which provides an evidence of Pb-induced tetragonal distortion in the TiO6 octahedron. In contrast, eg band splitting is absent in BxSTO.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
71.20.Ps Other inorganic compounds
78.70.Dm X-ray absorption spectra
77.80.-e Ferroelectricity and antiferroelectricity

Surface roughness and dislocation distribution in compositionally graded relaxed SiGe buffer layer with inserted-strained Si layers

Tae-Sik Yoon, Jian Liu, Atif M. Noori, Mark S. Goorsky, and Ya-Hong Xie

Appl. Phys. Lett. 87, 012104 (2005); http://dx.doi.org/10.1063/1.1988986 (3 pages) | Cited 3 times

Online Publication Date: 28 June 2005

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We investigate the surface roughness and dislocation distribution of compositionally graded relaxed SiGe buffer layers by inserting two tensile-strained Si layers. The 20 nm thick strained Si layers, less than the critical thickness for dislocation formation, are inserted at 10 and 20% Ge content regions of the 1 μm thick graded SiGe layer with a final Ge content of 30%. The surface immediately after growing the second strained Si layer on SiGe with 20% Ge content is found to be flat with about 1.1 nm root-mean-square roughness. However, the crosshatched surface with the 7.8 nm roughness develops during subsequent SiGe growths, which is slightly less than the 10.3 nm value for SiGe without inserted Si layers. Another important issue of consideration is that inserting the strained Si layers leads to increased interaction among dislocations as shown by cross-sectional transmission electron microscopy. This study explores the possibility of using strained layers for achieving flat surfaces and illustrates the need for optimization when using this approach.
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81.05.Cy Elemental semiconductors
81.05.Hd Other semiconductors
68.47.Fg Semiconductor surfaces
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.55.-a Thin film structure and morphology
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.72.Lk Linear defects: dislocations, disclinations
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