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10 May 2004

Volume 84, Issue 19, pp. 3723-3937

Issue Cover Spotlight Figure

Appl. Phys. Lett. 84, 3933 (2004); http://dx.doi.org/10.1063/1.1745103 (3 pages)

A. Cassinese, G. M. De Luca, A. Prigiobbo, M. Salluzzo, and R. Vaglio
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Influence of vapors of volatile organic compounds on spectral shift of bending vibrations of freely suspended ferroelectric liquid-crystal films

Serguei V. Yablonskii, Kazuyuki Nakano, Masanori Ozaki, and Katsumi Yoshino

Appl. Phys. Lett. 84, 3723 (2004); http://dx.doi.org/10.1063/1.1697630 (3 pages)

Online Publication Date: 29 April 2004

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The influence of the vapors of volatile inert fluorocarbon compounds on the mechanical properties of freely suspended ferroelectric liquid-crystal films has been studied. It was shown that spectral shift of bending vibrations of freely suspended films depends on the mass variations of the associated ambient gas as well as on the adsorption of the vapors of volatile fluorocarbon compounds onto the film surface. It was also shown that adsorption layers similar to surfactants decrease the film surface tension by 11%–35% (4–12.5 dyn/cm). © 2004 American Institute of Physics.
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68.15.+e Liquid thin films
61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions
68.03.Cd Surface tension and related phenomena
68.43.Mn Adsorption kinetics
62.10.+s Mechanical properties of liquids
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Field-effect transistor based on KTaO3 perovskite

K. Ueno, I. H. Inoue, T. Yamada, H. Akoh, Y. Tokura, and H. Takagi

Appl. Phys. Lett. 84, 3726 (2004); http://dx.doi.org/10.1063/1.1703841 (3 pages) | Cited 22 times

Online Publication Date: 29 April 2004

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An n-channel accumulation-type field-effect transistor (FET) has been fabricated utilizing a KTaO3 single crystal as an active element and a sputtered amorphous Al2O3 film as a gate insulator. The device demonstrated an ON/OFF ratio of 104 and a field-effect mobility of 0.4 cm2/V s at room temperature, both of which are much better than those of the SrTiO3 FETs reported previously. The field-effect mobility was almost temperature independent down to 200 K. Our results indicate that the Al2O3/KTaO3 interface is worthy of further investigations as an alternative system of future oxide electronics. © 2004 American Institute of Physics.
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85.30.Tv Field effect devices
72.20.Fr Low-field transport and mobility; piezoresistance
85.30.De Semiconductor-device characterization, design, and modeling
72.80.Jc Other crystalline inorganic semiconductors
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Fabrication of crystalline organic waveguides with an exceptionally large electro-optic coefficient

W. Geis, R. Sinta, W. Mowers, S. J. Deneault, M. F. Marchant, K. E. Krohn, S. J. Spector, D. R. Calawa, and T. M. Lyszczarz

Appl. Phys. Lett. 84, 3729 (2004); http://dx.doi.org/10.1063/1.1723697 (3 pages) | Cited 53 times

Online Publication Date: 29 April 2004

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Single-crystal optical waveguides of 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), an organic material with a large electro-optic coefficient, have been obtained. DAST decomposes at its melting temperature, making its growth from the melt difficult. However, graphoepitaxy allows for >1 mm s−1 growth, 1×105 times faster than conventional techniques, and produces crystals of the correct dimensions for optical waveguides, 1–15 μm on a side and 5–10 mm long. The crystals grow with the c-axis normal to the substrate, and with in-plane orientation determined by lithographic patterning. The electro-optic coefficient dn/dE is 600±300 pm V−1 at 1.55 μm wavelength. Optical losses are <10 dB cm−1. © 2004 American Institute of Physics.
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78.20.Jq Electro-optical effects
42.79.Gn Optical waveguides and couplers
81.10.Fq Growth from melts; zone melting and refining
64.70.D- Solid-liquid transitions
85.40.Hp Lithography, masks and pattern transfer
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Highly resistive p-PbTe films with carrier concentration as low as 1014 cm−3

V. Sandomirsky, A. V. Butenko, I. G. Kolobov, A. Ronen, Y. Schlesinger, A. Yu. Sipatov, and V. V. Volubuev

Appl. Phys. Lett. 84, 3732 (2004); http://dx.doi.org/10.1063/1.1728319 (3 pages) | Cited 3 times

Online Publication Date: 29 April 2004

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We propose here a model according to which a high density of semiconductor–insulator interface states can deplete practically the whole film volume, provided that the film thickness is of the order of Debye screening length. We demonstrated this experimentally by showing that thin p-PbTe films, thermally deposited on mica substrate, have an unusually low concentration of free holes, as low as 1014 cm−3 at 100 K, resulting in a very high value of resistance, Hall constant, and Seebeck coefficient, respectively. Such low concentration of free carriers allows an investigation of a whole series of phenomena in AIVBVI semiconductors, such as injection currents, injection electroluminescence, electronic memory phenomena, electric field effect control of thermopower, and more. © 2004 American Institute of Physics.
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72.20.Fr Low-field transport and mobility; piezoresistance
73.61.Le Other inorganic semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects
72.20.Pa Thermoelectric and thermomagnetic effects
78.66.Li Other semiconductors
78.60.Fi Electroluminescence

Radiative efficiency limits of solar cells with lateral band-gap fluctuations

U. Rau and J. H. Werner

Appl. Phys. Lett. 84, 3735 (2004); http://dx.doi.org/10.1063/1.1737071 (3 pages) | Cited 23 times

Online Publication Date: 29 April 2004

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The radiative recombination limit of photovoltaic power conversion under one sun terrestrial illumination is calculated for solar cells with lateral fluctuations of the band-gap energy. A simple analytical model quantifies the fluctuations by the standard deviation σEg from the mean band gap. The calculated maximum efficiency decreases by 1.7% (absolute) for σEg = 50 meV and by 6.1% for σEg = 100 meV with respect to a uniform band gap. © 2004 American Institute of Physics.
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84.60.Jt Photoelectric conversion
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
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Liquid-phase fabrication of patterned carbon nanotube field emission cathodes

S. J. Oh, J. Zhang, Y. Cheng, H. Shimoda, and O. Zhou

Appl. Phys. Lett. 84, 3738 (2004); http://dx.doi.org/10.1063/1.1737074 (3 pages) | Cited 52 times

Online Publication Date: 29 April 2004

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High-resolution electron field emission cathodes were fabricated at room temperature by a high throughput electrophoresis process using functionalized carbon nanotubes (CNTs). Well-defined and adherent CNT patterns with 20 μm feature size were achieved on a variety of substrates with fine control of the CNT morphology. The cathodes show uniform emission pattern across the entire surfaces and emission current with long-term stability. This room-temperature liquid-phase process is efficient and has no intrinsic limit on the deposition area. The emission characteristics of these cathodes compare favorably to those from other fabrication methods for CNT based field emission display applications. © 2004 American Institute of Physics.
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81.07.De Nanotubes
85.35.Kt Nanotube devices
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.55.A- Nucleation and growth
82.45.-h Electrochemistry and electrophoresis
82.45.Fk Electrodes
79.70.+q Field emission, ionization, evaporation, and desorption
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Effect of surface NH3 anneal on the physical and electrical properties of HfO2 films on Ge substrate

Nan Wu, Qingchun Zhang, Chunxiang Zhu, Chia Chin Yeo, S. J. Whang, D. S. H. Chan, M. F. Li, Byung Jin Cho, Albert Chin, Dim-Lee Kwong, A. Y. Du, C. H. Tung, and N. Balasubramanian

Appl. Phys. Lett. 84, 3741 (2004); http://dx.doi.org/10.1063/1.1737057 (3 pages) | Cited 57 times

Online Publication Date: 29 April 2004

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Metal-oxide-semiconductor capacitors were fabricated on germanium substrates by using metalorganic-chemical-vapor-deposited HfO2 as the dielectric and TaN as the metal gate electrode. It is demonstrated that a surface annealing step in NH3 ambient before the HfO2 deposition could result in significant improvement in both gate leakage current and the equivalent oxide thickness (EOT). It was possible to achieve a capacitor with an EOT of 10.5 Å and a leakage current of 5.02×10−5 A/cm2 at 1 V gate bias. X-ray photoelectron spectroscopy analysis indicates the formation of GeON during surface NH3 anneal. The presence of Ge was also detected within the HfO2 films. This may be due to Ge diffusion at the high temperature (∼400 °C) used in the chemical-vapor deposition process. © 2004 American Institute of Physics.
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77.55.-g Dielectric thin films
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.61.Ng Insulators
68.55.-a Thin film structure and morphology
66.30.Ny Chemical interdiffusion; diffusion barriers
84.32.Tt Capacitors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.40.Gh Other heat and thermomechanical treatments
79.60.Bm Clean metal, semiconductor, and insulator surfaces
79.60.Jv Interfaces; heterostructures; nanostructures
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Toward improved and tunable polymer field-effect transistors

Ludvig Edman, James Swensen, Daniel Moses, and Alan J. Heeger

Appl. Phys. Lett. 84, 3744 (2004); http://dx.doi.org/10.1063/1.1737483 (3 pages) | Cited 13 times

Online Publication Date: 29 April 2004

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We report an electrochemical method to improve charge injection in thin-film field-effect transistors fabricated with semiconducting polymers. By having ions, which are mobile only at elevated temperatures, in the active material [a mixture of a soluble poly(para-phenylene vinylene) copolymer, a crown ether and a LiCF3SO3 salt] we create electric double layers at the drain/source Au electrode interfaces by applying a low voltage (V = 2 V) at T = 85 °C for a short time (t ∼ 1–5 min). After cooling to room temperature under applied voltage, we demonstrate significantly improved transistor performance. In addition, we present evidence of reversible electrochemical doping in this active material. © 2004 American Institute of Physics.
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85.30.Tv Field effect devices
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Imaging phonons in a fcc Pu–Ga alloy by thermal diffuse x-ray scattering

Joe Wong, M. Wall, A. J. Schwartz, R. Xu, M. Holt, Hawoong Hong, P. Zschack, and T.-C. Chiang

Appl. Phys. Lett. 84, 3747 (2004); http://dx.doi.org/10.1063/1.1737482 (3 pages) | Cited 10 times

Online Publication Date: 29 April 2004

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X-ray thermal diffuse scattering intensity patterns from phonons in a fcc δ-Pu–Ga alloy have been recorded using an 18 keV undulator x-ray beam with a beam diameter of 25 μm. The results are consistent with patterns calculated using the Born–von Karman force constant model of lattice dynamics, and support the pronounced softening of the transverse acoustic branch along the [111] direction observed from inelastic x-ray scattering measurements. This work demonstrates the feasibility of using a “large-grain, small beam” approach to study lattice properties, such as phonon dispersion curves, of materials not readily available in the form of large single crystals. © 2004 American Institute of Physics.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.70.Ck X-ray scattering
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Hybrid near-field scanning optical microscopy tips for live cell measurements

Luka K. Kapkiai, David Moore-Nichols, Jonathan Carnell, Jeffrey R. Krogmeier, and Robert C. Dunn

Appl. Phys. Lett. 84, 3750 (2004); http://dx.doi.org/10.1063/1.1737464 (3 pages) | Cited 7 times

Online Publication Date: 29 April 2004

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We report a near-field scanning optical microscopy (NSOM) probe that enables high-resolution imaging of living cells under physiological buffered conditions. The hybrid design combines a conventional fiber optic near-field probe with a standard atomic force microscopy cantilever. Imaging of fluorescent latex spheres suspended in an acetate matrix demonstrates the subdiffraction limited fluorescence and topography capabilities of the tips. The reduced spring constant of the hybrid tip is also shown to be amenable to measurements on living cells. NSOM fluorescence and topography measurements on living human arterial smooth muscle cells under buffered conditions are demonstrated. © 2004 American Institute of Physics.
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87.64.mt Near-field scanning
87.80.-y Biophysical techniques (research methods)
07.79.Fc Near-field scanning optical microscopes
87.64.Dz Scanning tunneling and atomic force microscopy
87.19.Ff Muscles
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Comparison of predicted ferromagnetic tendencies of Mn substituting the Ga site in III–V’s and in I–III–VI2 chalcopyrite semiconductors

Yu-Jun Zhao, Priya Mahadevan, and Alex Zunger

Appl. Phys. Lett. 84, 3753 (2004); http://dx.doi.org/10.1063/1.1737466 (3 pages) | Cited 10 times

Online Publication Date: 29 April 2004

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We report density-functional calculations of the ferromagnetic (FM) stabilization energy δ = EFM−EAFM for differently oriented Mn pairs in III–V’s (GaN, GaP, GaAs) and chalcopyrite (CuGaS2, CuGaSe2, CuGaTe2) semiconductors. Ferromagnetism is found to be the universal ground state (δ<0) in all cases. The order of FM stability in III–V’s is GaN>GaP>GaAs, whereas in chalcopyrites it is CuGaS2>CuGaSe2>CuGaTe2. Considering both groups, the order is GaN→GaP→GaAs→CuGaS2→CuGaSe2→GaSb ≈ CuGaTe2. The stronger FM stabilization in III–V’s is attributed to the stronger covalent coupling between the Mn 3d and the anion p orbitals. In contrast to expectations based on Ruderman–Kittel–(Kasuya)–Yosida, (i) all Mn–Mn pair separations show FM, with no FM to antiferromagnetic oscillations and, (ii) FM is orientationally dependent, with 〈110〉 Mn–Mn pairs being the most FM. © 2004 American Institute of Physics.
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71.20.Nr Semiconductor compounds
75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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X-ray photoelectron spectroscopy study on SiO2/Si interface structures formed by three kinds of atomic oxygen at 300 °C

M. Shioji, T. Shiraishi, K. Takahashi, H. Nohira, K. Azuma, Y. Nakata, Y. Takata, S. Shin, K. Kobayashi, and T. Hattori

Appl. Phys. Lett. 84, 3756 (2004); http://dx.doi.org/10.1063/1.1737793 (3 pages) | Cited 8 times

Online Publication Date: 29 April 2004

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Using the high-brilliant synchrotron radiation at SPring-8 we have studied the SiO2/Si interface structures, the interface state densities, and the uniformities of ∼1-nm-thick oxide films formed by three kinds of atomic oxygen at 300 °C by measuring Si 2p photoelectron spectra at the photon energy of 1050 eV and the energy loss spectra of O 1s photoelectrons at the photon energy of 714 eV. Among silicon oxide films studied here the abrupt compositional transition at SiO2/Si interface, the smallest deviation in interface state density, the interface state density comparable to that for thermal oxide formed in dry oxygen at 950 °C, and the highest uniformity was obtained with oxide film formed in krypton-mixed oxygen (Kr:O2 = 97:3) plasma. © 2004 American Institute of Physics.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
68.35.Ct Interface structure and roughness
79.60.-i Photoemission and photoelectron spectra
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
79.20.Uv Electron energy loss spectroscopy
68.49.Jk Electron scattering from surfaces
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Electrodeless wet etching of GaN assisted with chopped ultraviolet light

Z. H. Hwang, J. M. Hwang, H. L. Hwang, and W. H. Hung

Appl. Phys. Lett. 84, 3759 (2004); http://dx.doi.org/10.1063/1.1737799 (3 pages) | Cited 11 times

Online Publication Date: 29 April 2004

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Electrodeless photoelectrochemical (PEC) etching of GaN was studied in a K2S2O8/KOH solution irradiated with ultraviolet (UV) light either continuously or periodically. The rate of recombination of electrons and holes at dislocation defects is greater than for crystalline GaN, resulting in a rough etched surface with hexagonal pyramids. The shorter the interval of UV irradiation, the smoother is the etched GaN surface; a chopped UV source thus serves to improve the morphology of the etched surface. A uniform and smooth GaN surface was obtained with a root-mean-square roughness 0.37 nm through electrodeless PEC etching in a solution (KOH 0.01 M, K2S2O8 0.05 M) with a chopper frequency 2500 Hz (i.e., duration of irradiation 0.2 ms). © 2004 American Institute of Physics.
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81.65.Cf Surface cleaning, etching, patterning
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.05.Ea III-V semiconductors
82.45.-h Electrochemistry and electrophoresis
82.50.-m Photochemistry
82.45.Jn Surface structure, reactivity and catalysis
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
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Quantum key distribution over 122 km of standard telecom fiber

C. Gobby, Z. L. Yuan, and A. J. Shields

Appl. Phys. Lett. 84, 3762 (2004); http://dx.doi.org/10.1063/1.1738173 (3 pages) | Cited 29 times

Online Publication Date: 29 April 2004

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We report a demonstration of quantum key distribution over a standard telecom fiber exceeding 100 km in length. Through careful optimization of the interferometer and single photon detector, we achieve a quantum bit error ratio of 8.9% for a 122 km link, allowing a secure shared key to be formed after error correction and privacy amplification. Key formation rates of up to 1.9 kbit/s are achieved depending upon fiber length. We discuss the factors limiting the maximum fiber length in quantum cryptography. © 2004 American Institute of Physics.
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84.40.Ua Telecommunications: signal transmission and processing; communication satellites
42.81.-i Fiber optics
03.67.Dd Quantum cryptography and communication security
03.67.Pp Quantum error correction and other methods for protection against decoherence
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Heteroepitaxial growth of wurtzite InN films on Si(111) exhibiting strong near-infrared photoluminescence at room temperature

S. Gwo, C.-L. Wu, C.-H. Shen, W.-H. Chang, T. M. Hsu, J.-S. Wang, and J.-T. Hsu

Appl. Phys. Lett. 84, 3765 (2004); http://dx.doi.org/10.1063/1.1738183 (3 pages) | Cited 71 times

Online Publication Date: 29 April 2004

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High-quality InN epitaxial films have been grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering. At room temperature, these films exhibited strong near-infrared (0.6–0.9 eV) photoluminescence (PL). In addition to the optical absorption measurement of absorption edge and direct band nature, the PL signal was found to depend linearly on the excitation laser intensity over a wide intensity range. These results indicate that the observed PL is due to the emission of direct band-to-band recombination rather than the band-to-defect (or impurity) deep emission. © 2004 American Institute of Physics.
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81.05.Ea III-V semiconductors
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.30.Fs III-V and II-VI semiconductors
68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Improved quantum efficiency in nonpolar (11math0) AlGaN/GaN quantum wells grown on GaN prepared by lateral epitaxial overgrowth

T. Koida, S. F. Chichibu, T. Sota, M. D. Craven, B. A. Haskell, J. S. Speck, S. P. DenBaars, and S. Nakamura

Appl. Phys. Lett. 84, 3768 (2004); http://dx.doi.org/10.1063/1.1738185 (3 pages) | Cited 30 times

Online Publication Date: 29 April 2004

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Radiative and nonradiative excitonic transitions in nonpolar (11math0) AlxGa1−xN/GaN multiple quantum wells (MQWs) grown on the GaN template prepared by lateral epitaxial overgrowth (LEO-GaN) were investigated. The structural advantages of using nonpolar orientations were confirmed by a moderate shift of the photoluminescence (PL) peak energy and negligible change in low-temperature PL lifetime with decreasing GaN well width, both of which are the results of eliminating quantum-confined Stark effects due to the polarization fields that exist in polar (0001) MQWs. Appearance of the correct in-plane light polarization and improved internal quantum efficiency for the PL peak in the MQWs on LEO-GaN were attributed to the reduction in densities of nonradiative defects and bound states. © 2004 American Institute of Physics.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
68.65.Fg Quantum wells
78.47.-p Spectroscopy of solid state dynamics
73.21.Fg Quantum wells
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Ferro- and piezoelectric properties of polar-axis-oriented CaBi4Ti4O15 films

Kazumi Kato, Desheng Fu, Kazuyuki Suzuki, Kiyotaka Tanaka, Kaori Nishizawa, and Takeshi Miki

Appl. Phys. Lett. 84, 3771 (2004); http://dx.doi.org/10.1063/1.1738521 (3 pages) | Cited 26 times

Online Publication Date: 29 April 2004

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Polar-axis-oriented CaBi4Ti4O15 (CBTi144) films were fabricated on Pt foils using a complex metal alkoxide solution. The 500-nm-thick film showed the columnar structure and consisted of well-developed grains. The a/b-axis orientation of the ferroelectric films is considered to be associated with the preferred orientation of Pt foil. The film showed good ferro- and piezoelectric properties. The Pr and Ec were 25 μC/cm2 and 306 kV/cm, respectively, at an applied voltage of 115 V. The d33 was characterized as 30 pm/V by piezoresponse force microscopy. The values were twice as large as those of the CBTi144 thin film with random orientation. The polar-axis-oriented CBTi144 films would open up possibilities for devices as Pb-free piezoelectric materials. © 2004 American Institute of Physics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
77.65.-j Piezoelectricity and electromechanical effects
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)
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Two-dimensional computational model for electrochemical micromachining with ultrashort voltage pulses

Jason A. Kenney, Gyeong S. Hwang, and Woonsup Shin

Appl. Phys. Lett. 84, 3774 (2004); http://dx.doi.org/10.1063/1.1738937 (3 pages) | Cited 14 times

Online Publication Date: 29 April 2004

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We have developed a computational model to simulate electrochemical micromachining of conducting substrates with ultrashort voltage pulses. This theoretical approach integrates (i) a circuit model to describe charging and discharging of electrochemical double layers and electric field variation in electrolytes and (ii) the level set method to simulate feature profile evolution during electrochemical etching. Our simulation results of transient current responses and etch profile evolution are qualitatively in agreement with experimental observations. From our simulations, we find that the resolution of etched features is a strong function of the substrate double layer capacity which may be controlled by electrolyte concentration and pulse duration. © 2004 American Institute of Physics.
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82.45.Fk Electrodes
82.45.Aa Electrochemical synthesis
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
82.45.Gj Electrolytes
82.20.Wt Computational modeling; simulation
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
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Dynamics of polarization modes in photonic crystals based on arrays of vertical-cavity surface-emitting lasers

Gilles Guerrero, Dmitri L. Boiko, and Eli Kapon

Appl. Phys. Lett. 84, 3777 (2004); http://dx.doi.org/10.1063/1.1739269 (3 pages) | Cited 1 time

Online Publication Date: 29 April 2004

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Polarization dynamics in coherent arrays of vertical-cavity surface-emitting lasers are studied experimentally. The two degenerate polarization lasing modes expected in infinitely large arrays are observed at different frequencies in finite-size structures. The frequency splitting increases with decreasing array size and reaches 60 GHz in 2×2 arrays. This splitting is attributed to the coupling of the two polarization states through depolarization and scattering effects at the boundary of the array. Small arrays lase in the two-frequency regime, whereas large ones oscillate in the single-frequency domain. © 2004 American Institute of Physics.
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42.70.Qs Photonic bandgap materials
42.55.Tv Photonic crystal lasers and coherent effects

Characterization of phototransistor internal gain in metamorphic high-electron-mobility transistors

Hyo-Soon Kang, Chang-Soon Choi, Woo-Young Choi, Dae-Hyun Kim, and Kwang-Seok Seo

Appl. Phys. Lett. 84, 3780 (2004); http://dx.doi.org/10.1063/1.1739278 (3 pages) | Cited 19 times

Online Publication Date: 29 April 2004

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We characterize the phototransistor internal gain of metamorphic high-electron-mobility transistors (mHEMTs). When the mHEMT operates as a phototransistor, it has internal gain provided by the photovoltaic effect. To determine this internal gain, photoresponse characteristics dominated by the photoconductive effect as well as the photovoltaic effect are investigated. When the device is turned off, it acts as a photoconductor, and by calculating photoconductor gain, the primary photodetected power can be determined, which indicates the absorbed optical power. The ratio between this and the photodetected power due to the photovoltaic effect represents phototransistor internal gain. It is demonstrated that the phototransistor internal gain is function of optical modulation frequency. © 2004 American Institute of Physics.
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85.60.Dw Photodiodes; phototransistors; photoresistors
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Synthesis and characterization of two-layer-structured ZnO p-n homojunctions by ultrasonic spray pyrolysis

J. M. Bian, X. M. Li, C. Y. Zhang, L. D. Chen, and Q. Yao

Appl. Phys. Lett. 84, 3783 (2004); http://dx.doi.org/10.1063/1.1739280 (3 pages) | Cited 36 times

Online Publication Date: 29 April 2004

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The two-layer-structured ZnO p-n homojunctions were prepared on single-crystal Si (100) substrate by depositing undoped n-type ZnO film on N–In codoped p-type ZnO film using ultrasonic spray pyrolysis. The crystal structure and morphology of the obtained ZnO homojunctions were examined by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The two-layer structure was confirmed by secondary ion mass spectroscopy depth profile analysis. The current–voltage (I–V) characteristics derived from the undoped ZnO/N–In codoped ZnO two-layer structure clearly show the rectifying characteristics of typical p-n junctions. © 2004 American Institute of Physics.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
81.15.Rs Spray coating techniques
68.55.-a Thin film structure and morphology
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Control of the spin polarization of photoelectrons/photoions using short laser pulses

Takashi Nakajima

Appl. Phys. Lett. 84, 3786 (2004); http://dx.doi.org/10.1063/1.1739281 (3 pages) | Cited 11 times

Online Publication Date: 29 April 2004

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We present a generic pump-probe scheme to control spin polarization of photoelectrons/photoions by short laser pulses. By coherently exciting fine structure manifolds of a multi-valence-electron system by the pump laser, a superposition of fine structure states is created. Since each fine structure state can be further decomposed into a superposition of various spin states of valence electrons, each spin component evolves differently in time. This means that varying the time delay between the pump and probe lasers leads to the control of spin states. Specific theoretical results are presented for two-valence-electron atoms, in particular for Mg, which demonstrate that not only the degree of spin polarization but also its sign can be manipulated through time delay. Since the underline physics is rather general and transparent, the presented idea may be potentially applied to nanostructures such as quantum wells and quantum dots. © 2004 American Institute of Physics.
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79.60.-i Photoemission and photoelectron spectra
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
32.80.Fb Photoionization of atoms and ions
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High mobility of pentacene field-effect transistors with polyimide gate dielectric layers

Yusaku Kato, Shingo Iba, Ryohei Teramoto, Tsuyoshi Sekitani, Takao Someya, Hiroshi Kawaguchi, and Takayasu Sakurai

Appl. Phys. Lett. 84, 3789 (2004); http://dx.doi.org/10.1063/1.1739508 (3 pages) | Cited 80 times

Online Publication Date: 29 April 2004

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Polyimide gate dielectric layers cured at 180 °C have been employed to fabricate high-quality pentacene field-effect transistors on polyethylenenaphthalate-based films. The surface roughness (root-mean square) of gate dielectric layers characterized by atomic force microscopy is only 0.2 nm, while that of the base film is 1 nm. The transistors with pentacene channel layers deposited on 990 nm polyimide gate dielectric layers attain the on/off ratio of 106 and mobility of 0.3 cm2/V s. Furthermore, by decreasing the thickness of polyimide gate dielectric layers down to 540 nm, the mobility is enhanced up to 1 cm2/V s. © 2004 American Institute of Physics.
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85.30.Tv Field effect devices
73.61.Ph Polymers; organic compounds
77.55.-g Dielectric thin films
68.37.Ps Atomic force microscopy (AFM)
73.50.Dn Low-field transport and mobility; piezoresistance
85.65.+h Molecular electronic devices
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Molecular weight dependence of birefringence of thin films of the conjugated polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene]

Kaloian Koynov, Ayi Bahtiar, Taek Ahn, Christoph Bubeck, and Hans-Heinrich Hörhold

Appl. Phys. Lett. 84, 3792 (2004); http://dx.doi.org/10.1063/1.1739513 (3 pages) | Cited 16 times

Online Publication Date: 29 April 2004

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We prepared thin films of the conjugated model polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) by the spin coating of polymer samples whose weight-average molecular weight was varied from 104–106 g/mol. Waveguide prism coupling and polarized optical transmission and reflection spectroscopy were used to determine the refractive index, birefringence, and intrinsic absorption coefficient of each film. We show that these optical constants significantly depend on the molecular weight of the polymers. With decreasing molecular weight, the birefringence is strongly reduced and a nearly isotropic refractive index is approached. We conclude that the molecular weight primarily determines the extent of polymer chain alignment in the film plane. © 2004 American Institute of Physics.
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78.66.Qn Polymers; organic compounds
42.70.Jk Polymers and organics
78.20.Fm Birefringence
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.A- Nucleation and growth
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Microwave radiation force and torque on a disk resonator excited by a circularly polarized plane wave

S. Makarov and S. Kulkarni

Appl. Phys. Lett. 84, 3795 (2004); http://dx.doi.org/10.1063/1.1739517 (3 pages)

Online Publication Date: 29 April 2004

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A numerical simulation method [S. Makarov and S. Kulkarni, Appl. Phys. Lett. 84, 1600 (2004)] is used in order to determine the radiation force and radiation torque on a parallel-plate disk resonator, whose size is comparable to wavelength. The method is based on the MOM solution of the electric-field integral equation, accurate calculation of the near field, and removal of the self-interaction terms responsible for the pinch effect. The local force/torque distribution at the normal incidence of a circularly polarized plane wave is found. It is observed that, at the resonance, the individual disks are subject to unexpectedly large local force densities, despite the fact that the net radiation force on the resonator remains very small. On the other hand, the total axial torque on the disk resonator also increases at the resonance. © 2004 American Institute of Physics.
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84.40.-x Radiowave and microwave (including millimeter wave) technology
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
37.10.Vz Mechanical effects of light on atoms, molecules, and ions
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