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6 Jan 2003

Volume 82, Issue 1, pp. 1-153

Issue Cover Spotlight Figure

Appl. Phys. Lett. 82, 118 (2003); http://dx.doi.org/10.1063/1.1535263 (3 pages)

Phillip Walsh, Andrey Omeltchenko, Rajiv K. Kalia, Aiichiro Nakano, Priya Vashishta, and Subhash Saini
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GaN-free transparent ultraviolet light-emitting diodes

Toshio Nishida, Naoki Kobayashi, and Tomoyuki Ban

Appl. Phys. Lett. 82, 1 (2003); http://dx.doi.org/10.1063/1.1533851 (3 pages) | Cited 59 times

Online Publication Date: 30 December 2002

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By introducing a GaN-free layer structure, we have transparent light-emitting diodes (LEDs) at the ultraviolet emission wavelength of 348–351 nm, which is shorter than the GaN band gap wavelength of 363 nm. The buffer layer consists of an AlGaN alloy directly grown on an AlN template layer on a sapphire substrate, and a short period alloy superlattice is adopted as p-type cladding and p-type contact layers. The transparency of the epitaxially grown layer structure is confirmed from transmission spectra. The output powers of the device are 1 mW at injection currents of 20 and 7 mW at 220 mA under room temperature continuous wave operation. The highest external quantum efficiency is 1.4%. This value is superior to that of an ultraviolet LED grown on a high-quality bulk-GaN substrate, where the performance was significantly deteriorated by light absorption into the GaN substrate. The results here indicate the importance of a transparent device structure free of GaN to improve the performance of ultraviolet LEDs in wavelength ranges shorter than 363 nm. © 2003 American Institute of Physics.
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85.60.Jb Light-emitting devices

Two-dimensional photonic crystal coupled-defect laser diode

Thomas D. Happ, Martin Kamp, Alfred Forchel, Jean-Louis Gentner, and L. Goldstein

Appl. Phys. Lett. 82, 4 (2003); http://dx.doi.org/10.1063/1.1527703 (3 pages) | Cited 64 times

Online Publication Date: 30 December 2002

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We demonstrate a two-dimensional photonic crystal defect laser diode based on a coupled cavity waveguide. The laser cavity is formed by 40 coupled hexagonal defect microcavities in a triangular lattice of air cylinders, which are etched into an InGaAsP/InP laser structure. The coupling of the individual cavity modes creates minibands within the photonic band gap. Stable single-mode lasing occurs on the first miniband mode with the lowest group velocity with side mode suppression greater than 40 dB. The lasers emit up to 2.6 mW at a 1.53 μm wavelength under continuous-wave operation at room temperature. © 2003 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
42.55.Sa Microcavity and microdisk lasers

Systematic design of antireflection coating for semi-infinite one-dimensional photonic crystals using Bloch wave expansion

Jun Ushida, Masatoshi Tokushima, Masayuki Shirane, and Hirohito Yamada

Appl. Phys. Lett. 82, 7 (2003); http://dx.doi.org/10.1063/1.1534936 (3 pages) | Cited 31 times

Online Publication Date: 30 December 2002

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We present a systematic method for designing a perfect antireflection coating (ARC) for a semi-infinite one-dimensional (1D) photonic crystal (PC) with an arbitrary unit cell. We use Bloch wave expansion and time reversal symmetry, which leads exactly to analytic formulas of structural parameters for the ARC and renormalized Fresnel coefficients of the PC. Surface immittance (admittance and impedance) matching plays an essential role in designing the ARCs of 1D PCs, which is shown together with a practical example. © 2003 American Institute of Physics.
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42.70.Qs Photonic bandgap materials
42.79.Wc Optical coatings
42.15.Eq Optical system design
42.25.Gy Edge and boundary effects; reflection and refraction

Isolated hexaphenyl nanofibers as optical waveguides

F. Balzer, V. G. Bordo, A. C. Simonsen, and H.-G. Rubahn

Appl. Phys. Lett. 82, 10 (2003); http://dx.doi.org/10.1063/1.1533845 (3 pages) | Cited 48 times

Online Publication Date: 30 December 2002

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Laser-supported, dipole-assisted self-assembly results in blue-light guiding nanostructures, namely single-crystalline nanofibers of hexaphenyl molecules. The nanofibers are up to 1 mm long, extremely well-aligned to each other and their cross sections can be tuned to span the range from nonguiding to guiding single optical modes at λ = 425.5 nm. An analytical theory for such organic waveguides can reproduce quantitatively the experimentally observed behavior. From the measured damping of propagating, vibrationally dressed excitons the imaginary part of the dielectric function of isolated nanoscaled organic aggregates is determined. © 2003 American Institute of Physics.
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42.79.Gn Optical waveguides and couplers
42.65.Wi Nonlinear waveguides
81.16.Dn Self-assembly
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
77.22.Ch Permittivity (dielectric function)

Method for the measurement of the K22 nematic elastic constant

E. P. Raynes, C. V. Brown, and J. F. Strömer

Appl. Phys. Lett. 82, 13 (2003); http://dx.doi.org/10.1063/1.1534942 (3 pages) | Cited 16 times

Online Publication Date: 30 December 2002

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A technique has been developed for the measurement of the K22 twist elastic constant in nematic liquid crystal materials. This involves the measurement of the Freedericksz transition voltages in untwisted linear and π-twist regions in a wedge cell geometry. The method avoids the need for the accurate determination of the cell thickness and cholesteric pitch and is far more straightforward to implement than other methods in the literature. The validity of this method is demonstrated for the well-characterized material E7. © 2003 American Institute of Physics.
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64.70.M- Transitions in liquid crystals
62.20.D- Elasticity

Electrical control of the structure and lasing in chiral photonic band-gap liquid crystals

Seiichi Furumi, Shiyoshi Yokoyama, Akira Otomo, and Shinro Mashiko

Appl. Phys. Lett. 82, 16 (2003); http://dx.doi.org/10.1063/1.1534613 (3 pages) | Cited 71 times

Online Publication Date: 30 December 2002

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This letter describes the electrical control of structure and lasing in the photonic bandgaps of cholesteric liquid crystals (CLCs). Photoexcitation of dye-doped CLC cells with a linearly polarized laser gives rise to laser emission at the edge(s) of the chiral photonic band gap. Applying voltages to the optically pumped CLC cells enables reversible switching of the laser action as a result of the structural changes in the chiral photonic band gap. © 2003 American Institute of Physics.
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42.70.Qs Photonic bandgap materials
42.70.Df Liquid crystals
42.55.Tv Photonic crystal lasers and coherent effects
42.70.Hj Laser materials
78.20.Jq Electro-optical effects

Distortions in Z-scan spectroscopy

Qiguang Yang, JaeTae Seo, Santiel Creekmore, Doyle Temple, Andy Mott, Namkung Min, KiPung Yoo, Sun Young Kim, and Sungsoo Jung

Appl. Phys. Lett. 82, 19 (2003); http://dx.doi.org/10.1063/1.1535264 (3 pages) | Cited 18 times

Online Publication Date: 30 December 2002

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The sample imperfection induced distortions in the closed Z-scan curve are analyzed. It is found that both the magnitudes of the peak and valley transmittances and the shape of the curve are distorted significantly by the inhomogeneous thickness, the nonuniform nonlinearity, and the curved surfaces of the sample. The results suggest that the I-scan method is a more practical choice for the measurement of the third-order nonlinear susceptibility of an inhomogeneous sample. © 2003 American Institute of Physics.  
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42.65.An Optical susceptibility, hyperpolarizability
07.60.Rd Visible and ultraviolet spectrometers
42.65.-k Nonlinear optics

Tunable electronic lens using a gradient polymer network liquid crystal

Hongwen Ren and Shin-Tson Wu

Appl. Phys. Lett. 82, 22 (2003); http://dx.doi.org/10.1063/1.1534915 (3 pages) | Cited 19 times

Online Publication Date: 30 December 2002

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Tunable electronic lenses using gradient polymer network liquid crystal (PNLC) cells were demonstrated. By changing the photomask pattern, both positive and negative lenses were fabricated. The advantages of such a PNLC lens are low operation voltage, large aperture size, and simple electrode design. To overcome the polarization dependence, stacking two orthogonal homogeneous PNLC cells is considered. © 2003 American Institute of Physics.
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42.79.Bh Lenses, prisms and mirrors
42.79.Kr Display devices, liquid-crystal devices
42.79.Ry Gradient-index (GRIN) devices
42.70.Df Liquid crystals

Shift multiplexing for holographic storage system using fiber bundle referencing

Jiasen Zhang, Shin Yoshikado, and Tadashi Aruga

Appl. Phys. Lett. 82, 25 (2003); http://dx.doi.org/10.1063/1.1535266 (3 pages) | Cited 25 times

Online Publication Date: 30 December 2002

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We propose a shift multiplexing method for volume holographic storage using fiber bundle referencing. The reference beam is guided by a fiber bundle and multiplexing can be implemented by shifting the recording medium for a small distance. No sidelobe or period has been found and the signal-to-noise ratio is high. Multiple images are stored in a crystal with a spatial separation of 3 μm between successive holograms. © 2003 American Institute of Physics.
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42.40.Pa Volume holograms
42.40.Ht Hologram recording and readout methods
42.79.Vb Optical storage systems, optical disks
42.70.Ln Holographic recording materials; optical storage media
42.81.Wg Other fiber-optical devices
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Reduction of stress at the initial stages of GaN growth on Si(111)

A. Dadgar, M. Poschenrieder, A. Reiher, J. Bläsing, J. Christen, A. Krtschil, T. Finger, T. Hempel, A. Diez, and A. Krost

Appl. Phys. Lett. 82, 28 (2003); http://dx.doi.org/10.1063/1.1534940 (3 pages) | Cited 34 times

Online Publication Date: 30 December 2002

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GaN growth on heterosubstrates usually leads to an initially high dislocation density at the substrate/seed layer interface. Due to the initial growth from small crystallites, tensile stress is generated at the coalescence boundaries during GaN growth. In addition, with tensile thermal stress this leads to cracking of GaN on Si and SiC substrates when cooling to room temperature. By partially masking the typically applied AlN seed layer on Si(111) with an in situ deposited SiN mask a reduction in tensile stress can be achieved for the subsequently grown GaN layer. Additionally, the 6 K GaN band edge photoluminescence is increased by about an order of magnitude and shifts by 21 meV, which can be attributed to a change in tensile stress of ∼ 0.8 GPa, in good agreement with x-ray diffractometry measurements. This improvement in material properties can be attributed to a reduction of grain boundaries by the growth of larger sized crystallites and lateral overgrowth of less defective GaN. © 2003 American Institute of Physics.
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68.60.Bs Mechanical and acoustical properties
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Observation of compositional pulling phenomenon in AlxGa1−xN (0.4<x<1.0) films grown on (0001) sapphire substrates

Yu-Li Tsai, Cheng-Liang Wang, Po-Hung Lin, Wei-Tsai Liao, and Jyh-Rong Gong

Appl. Phys. Lett. 82, 31 (2003); http://dx.doi.org/10.1063/1.1533850 (3 pages) | Cited 12 times

Online Publication Date: 30 December 2002

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High Al content AlxGa1−xN (0.4<x<1.0) films were grown at 1050 °C either on (0001) sapphire substrates or on AlN films by alternative supply of group III metalorganics (trimethylaluminum and trimethylgallium) and NH3 in an inductively heated quartz reactor operated at atmospheric pressure. θ–2θ x-ray diffraction data show a compositional separation in the AlxGa1−xN films grown on low-temperature (LT) aluminum nitride (AlN) buffer layer-coated (0001) sapphire substrates. Absorption spectroscopic measurements also reveal double cutoff edges in the AlxGa1−xN sample grown directly on a LT AlN-coated (0001) sapphire substrate while only one absorption edge having higher energy was observed in the AlxGa1−xN film grown on a 0.1-μm-thick high-temperature (HT) AlN intermediate layer, which was deposited on top of the (0001) sapphire substrate. Cross-sectional transmission electron microscopic observations show the bilayer nature of the AlxGa1−xN films grown directly on the LT AlN-coated sapphire substrate. Such a compositional pulling phenomenon in the AlxGa1−xN film without a HT AlN intermediate layer is attributed to the extensive biaxial strain caused by the large mismatch between the AlxGa1−xN film and the (0001) sapphire substrate. © 2003 American Institute of Physics.
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68.55.Nq Composition and phase identification
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.05.Ea III-V semiconductors

Dielectric loss in a C60 film observed by coupling with the external electromagnetic field of a surface acoustic wave

Yong Sun, Yuichi Yamasaki, Kenta Kirimoto, Tatsuro Miyasato, J. Keith Wigmore, Fuminori Moriyama, and Tsuyoshi Takase

Appl. Phys. Lett. 82, 34 (2003); http://dx.doi.org/10.1063/1.1534917 (3 pages) | Cited 3 times

Online Publication Date: 30 December 2002

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The dielectric loss in C60 films was studied by a noncontacting technique utilizing the external electric fields associated with surface acoustic waves (SAW) on a piezoelectric crystal. A sharp increase in loss was observed at temperatures below 220 K together with other structure not found with standard SAW measurements. We believe that these features are due to induced current in C60, causing joule loss, and to the formation of localized dipole moments by charge transfer between adsorbed O2 and C60 molecules, giving rise to thermally activated relaxation. © 2003 American Institute of Physics.
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77.22.Gm Dielectric loss and relaxation
61.48.-c Structure of fullerenes and related hollow and planar molecular structures
77.55.-g Dielectric thin films
68.35.Iv Acoustical properties

Intersubband absorption in n-doped InAs/AlSb multiple-quantum-well structures

K. Ohtani, N. Matsumoto, H. Sakuma, and H. Ohno

Appl. Phys. Lett. 82, 37 (2003); http://dx.doi.org/10.1063/1.1534939 (3 pages) | Cited 4 times

Online Publication Date: 30 December 2002

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Well-thickness dependence of intersubband absorption energies is investigated in n-doped InAs/AlSb multiple-quantum-well structures grown by molecular-beam epitaxy. Decreasing the InAs well thickness from 18 to 9 monolayers, the absorption peak shifts from 3.31 μm (375 meV) to 1.98 μm (627 meV). These absorption energies are found to be larger than those of the corresponding spatially indirect band gap between the electron ground state in the InAs well and the heavy-hole ground state in the AlSb barrier. Intersubband transition energies are calculated self-consistently using multiband k⋅p calculation combined with Poisson equation, and compared with the experimental results. © 2003 American Institute of Physics.
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78.67.De Quantum wells
78.40.Fy Semiconductors
73.21.Fg Quantum wells
73.20.At Surface states, band structure, electron density of states
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.15.-m Methods of electronic structure calculations

Observation of defect complexes containing Ga vacancies in GaAsN

J. Toivonen, T. Hakkarainen, M. Sopanen, H. Lipsanen, J. Oila, and K. Saarinen

Appl. Phys. Lett. 82, 40 (2003); http://dx.doi.org/10.1063/1.1533843 (3 pages) | Cited 38 times

Online Publication Date: 30 December 2002

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Positron annihilation spectroscopy was used to study GaAsN/GaAs epilayers. GaAsN layers were found to contain Ga vacancies in defect complexes. The density of the vacancy complexes increases rapidly to the order of 1018 cm−3 with increasing N composition and decreases after annealing at 700 °C. The anticorrelation of the vacancy concentration and the integrated photoluminescence intensity suggests that the Ga vacancy complexes act as nonradiative recombination centers. © 2003 American Institute of Physics.
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61.72.J- Point defects and defect clusters
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
78.70.Bj Positron annihilation
78.66.Fd III-V semiconductors
61.72.Cc Kinetics of defect formation and annealing
78.55.Cr III-V semiconductors

Strain relaxation during the layer by layer growth of cubic CdSe onto ZnSe

O. de Melo, C. Vargas-Hernández, and I. Hernández-Calderón

Appl. Phys. Lett. 82, 43 (2003); http://dx.doi.org/10.1063/1.1534941 (3 pages) | Cited 7 times

Online Publication Date: 30 December 2002

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A detailed reflection high-energy electron diffraction analysis shows relevant features of the lattice parameter relaxation of CdSe thin films grown in a layer-by-layer mode onto ZnSe. In situ investigations of different azimuths show a clear lattice parameter oscillation in the [110] azimuth. The lattice parameter has a minimum value (similar to that of ZnSe) during Se exposure steps, and a higher and increasing lattice parameter during Cd exposure steps. The behavior is ascribed to the formation of CdSe islands during Cd exposure steps. The cumulative effect in CdSe exposure steps is considered to be a consequence of a decrease in the island size with the number of cycles. Actual plastic deformation does occur after 5 ML. © 2003 American Institute of Physics.
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68.65.Fg Quantum wells
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.A- Nucleation and growth
68.60.Bs Mechanical and acoustical properties

Diamond formation in cubic silicon carbide

B. Pécz, H. Weishart, V. Heera, and L. Tóth

Appl. Phys. Lett. 82, 46 (2003); http://dx.doi.org/10.1063/1.1534611 (3 pages) | Cited 6 times

Online Publication Date: 30 December 2002

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High-dose carbon implantation (3×1017 and 1×1018 ions/cm2) into cubic SiC on Si was carried out at elevated temperatures (600 to 1200 °C) and different dose rates (1×1013 to 1.5×1014 cm−2 s−1). Transmission electron microscopy revealed the formation of either graphite or diamond precipitates, depending on the implantation parameters. In all cases, the diamond grains were epitaxial to the SiC lattice, while the graphite was textured. The minimum temperature for diamond formation was 900 °C, while graphite formed at 600 °C. The synthesized phase depends as well on the dose rate; graphite was formed at 900 °C with a high dose rate. Obviously, a critical temperature for diamond formation exists and increases with increasing dose rate. This behavior is explained by the competition between the accumulation and dynamic annealing of radiation defects in the SiC lattice, which acts as a template for diamond nucleation. Diamond grains with diameters as large as 10 nm have been observed after implantation at 1200 °C. © 2003 American Institute of Physics.
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61.82.Fk Semiconductors
64.75.-g Phase equilibria
61.80.Jh Ion radiation effects
81.30.Mh Solid-phase precipitation
61.72.up Other materials
81.05.Hd Other semiconductors

Sharp photoluminescence of CdSeS nanocrystals embedded in silica glass

Y. S. Wang, P. Sun, Y. H. Wang, R. Z. Wang, D. Zheng, and Y. L. Li

Appl. Phys. Lett. 82, 49 (2003); http://dx.doi.org/10.1063/1.1526173 (3 pages) | Cited 7 times

Online Publication Date: 30 December 2002

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CdSeS nanocrystals were grown by annealing silica glass with supersaturated Cd, Se, S at temperatures of 550–800 °C. The linear absorption spectra, photoluminescence (PL) spectra, and photoluminescence excitation spectra were measured. Sharp PL peaks superposed on broadened PL spectra were observed at room temperature in the samples grown at temperatures of 650–800 °C. The sharp PL peak energy was independent of the excited energy and nanocrystal growth conditions. The origin of sharp luminescence is discussed. © 2003 American Institute of Physics.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.67.Hc Quantum dots
78.55.Et II-VI semiconductors

Properties of optical phonons in cubic InxGa1−xN

K. Torii, N. Usukura, A. Nakamura, T. Sota, S. F. Chichibu, T. Kitamura, and H. Okumura

Appl. Phys. Lett. 82, 52 (2003); http://dx.doi.org/10.1063/1.1535273 (3 pages) | Cited 1 time

Online Publication Date: 30 December 2002

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Infrared reflectance spectroscopy was carried out to study the properties of zone center optical phonons in ternary cubic InxGa1−xN free from phase separation. Raman spectra were also measured as auxiliary measures. Optical phonon behavior was confirmed to be type I. It is demonstrated that the behavior arises from both the small mass ratio of Ga to In atoms and the rather strong ionicity of Ga–N and In–N bonds. © 2003 American Institute of Physics.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.30.Fs III-V and II-VI semiconductors

Selective enhancement of blue electroluminescence from GaN:Tm

D. S. Lee and A. J. Steckl

Appl. Phys. Lett. 82, 55 (2003); http://dx.doi.org/10.1063/1.1534414 (3 pages) | Cited 6 times

Online Publication Date: 30 December 2002

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Selective enhancement of electroluminescent emission from high-energy transitions in Tm-doped GaN has been observed to be a strong function of GaN growth temperature. GaN:Tm thin films have been grown by molecular beam epitaxy at temperatures from 100 to 700 °C. At low growth temperatures (100–200 °C) the low energy (infrared-801 nm) transition dominates, while at higher growth temperatures (400–700 °C) the high energy (blue-477 nm) transition dominates. For films grown at low temperatures the main emission excitation mechanism is impact excitation, while for films grown at higher temperatures (⩾600 °C) the main excitation mechanism appears to be lattice impact ionization followed by energy transfer to Tm ions. © 2003 American Institute of Physics.
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78.60.Fi Electroluminescence
78.66.Fd III-V semiconductors

Surface structure and anchoring properties of modified self-assembled monolayers

João G. Fonseca, Jean Hommet, and Yves Galerne

Appl. Phys. Lett. 82, 58 (2003); http://dx.doi.org/10.1063/1.1535265 (3 pages) | Cited 5 times

Online Publication Date: 30 December 2002

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The surface structure of plasma-modified self-assembled monolayers is studied by x-ray photoelectron spectroscopy. The measurements show that the oxygen postplasma gas produces gradual chemical modifications in a pure polyethylene-type monolayer. Different oxygenated functions are observed and quantitatively, they are measured to be denser at shorter distances from the plasma source. Used as liquid crystal alignment layers, these surfaces present a fast transition between the homeotropic and planar anchorings. The average distance between the oxygen atoms on the organic layer is found to be ∼ 6–10 Å at the anchoring transition. © 2003 American Institute of Physics.
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68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
68.37.Xy Scanning Auger microscopy, photoelectron microscopy
68.35.Md Surface thermodynamics, surface energies
82.35.Gh Polymers on surfaces; adhesion
61.30.-v Liquid crystals
79.60.Fr Polymers; organic compounds
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
52.77.-j Plasma applications
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Effect of an indium-tin-oxide overlayer on transparent Ni/Au ohmic contact on p-type GaN

Soo Young Kim, Ho Won Jang, and Jong-Lam Lee

Appl. Phys. Lett. 82, 61 (2003); http://dx.doi.org/10.1063/1.1534630 (3 pages) | Cited 19 times

Online Publication Date: 30 December 2002

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We report a low-resistant, thermally stable, and transparent ohmic contact on p-type GaN using an indium-tin-oxide (ITO) overlayer on Ni/Au contact. Ni (20 Å)/Au (30 Å)/ITO (600 Å) contact with preannealing at 500 °C before ITO deposition showed lower contact resistivity by one order of magnitude than the contact without the preannealing. The preannealing produced NiO, acting in the role of diffusion barrier for outdiffusion of N and Ga atoms and indiffusion of In during the subsequent post-annealing. Thus, the formation of Au–In solid solution was effectively suppressed, resulting in the decrease of contact resistivity and enhancement in thermal stability. © 2003 American Institute of Physics.
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73.40.Ns Metal-nonmetal contacts
73.40.Cg Contact resistance, contact potential
68.35.Fx Diffusion; interface formation
68.60.Dv Thermal stability; thermal effects
73.61.Ey III-V semiconductors
73.61.Le Other inorganic semiconductors

Time response in tunneling to a pn junction

Jeong Young Park and R. J. Phaneuf

Appl. Phys. Lett. 82, 64 (2003); http://dx.doi.org/10.1063/1.1533849 (3 pages) | Cited 6 times

Online Publication Date: 30 December 2002

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We report on the variation in the response time in scanning tunneling microscopy (STM) measurements across a Si pn junction, and its effect on STM images of such devices. The response time of the tip height subsequent to a stepwise change in the voltage at a fixed demanded current varies by almost two orders of magnitude across the junction, with the slowest response of nearly 80 ms corresponding to the tip–sample junction in strong inversion. Measurements at decreasing separation show an increase in response time up to a saturation point, consistent with screening due to the inversion charge. The slow response in regions where the tip–sample junction is in inversion explains observations of anomalously deep features in STM topography images and scan direction-dependent features in STM conductance maps. © 2003 American Institute of Physics.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Hopping conduction in Mn-doped ZnO

Jiaping Han, Mingrong Shen, Wenwu Cao, A. M. R. Senos, and P. Q. Mantas

Appl. Phys. Lett. 82, 67 (2003); http://dx.doi.org/10.1063/1.1535262 (3 pages) | Cited 24 times

Online Publication Date: 30 December 2002

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The dc and ac conductivities of Mn-doped ZnO were investigated at temperatures from 10 to 100 K. The temperature dependence of the dc conductivity from 10 to 100 K shows an abrupt change at ∼ 18 K, manifesting a much lower activation energy for conduction below 18 K. From 10 to 18 K, the ac conductivity, σac(ω), varies as σac(ω) = Aωs in the frequency range from 102 to 106 Hz with s in the range of 0.6–1. The dc and ac conductivity observations suggest that the dominant conduction mechanism at temperatures between 10 to 18 K in these samples is a hopping conduction. © 2003 American Institute of Physics.
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72.20.Ee Mobility edges; hopping transport
72.80.Ey III-V and II-VI semiconductors

Conjugated organic molecules on metal versus polymer electrodes: Demonstration of a key energy level alignment mechanism

N. Koch, A. Kahn, J. Ghijsen, J.-J. Pireaux, J. Schwartz, R. L. Johnson, and A. Elschner

Appl. Phys. Lett. 82, 70 (2003); http://dx.doi.org/10.1063/1.1532102 (3 pages) | Cited 233 times

Online Publication Date: 30 December 2002

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Ultraviolet photoemission spectroscopy is used to determine the energy level alignment at interfaces between three electroactive conjugated organic molecular materials, i.e., N,N′-bis-(1-naphthyl)-N,N′-diphenyl1-1,1-biphenyl1-4,4′-diamine; para-sexiphenyl; pentacene, and two high work function electrode materials, i.e., gold and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate). Although both electrode surfaces have a similar work function ( ∼ 5 eV), the hole injection barrier and the interfacial dipole barrier are found to be significantly smaller for all the interfaces formed on the polymer as compared to the metal. This important and very general result is linked to one of the basic mechanisms that control molecular level alignment at interfaces with metals, i.e., the reduction of the electronic surface dipole contribution to the metal work function by adsorbed molecules. © 2003 American Institute of Physics.
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79.60.Jv Interfaces; heterostructures; nanostructures
73.30.+y Surface double layers, Schottky barriers, and work functions
73.20.Hb Impurity and defect levels; energy states of adsorbed species
79.60.Fr Polymers; organic compounds
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Determination of spin distributions in ion-beam magnetic textured iron films by magnetic orientation Mössbauer spectroscopy

G. A. Müller, R. Gupta, K.-P. Lieb, and P. Schaaf

Appl. Phys. Lett. 82, 73 (2003); http://dx.doi.org/10.1063/1.1533836 (3 pages) | Cited 13 times

Online Publication Date: 30 December 2002

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A promising approach by which to gain information about the spin distributions in different sample layers using depth-sensitive angle-dependent conversion electron Mössbauer spectroscopy is presented in combination with magneto-optical Kerr effect, Rutherford backscattering spectrometry and x-ray diffraction investigations. Three 77 nm thick Fe films containing a 15 nm 57Fe marker layer at different depths were deposited on Si(100) substrates and irradiated under well defined mechanical stress with 200 keV Xe ions. The distribution of the spins in the sample plane was analyzed by Mössbauer spectroscopy and by tilting the sample toward the incident γ beam and observing the variation in angle-dependent relative intensity of the sextet lines. For the sample with the sensitive marker layer in the middle of the ferromagnetic film, all spins were found to be oriented exactly along the mechanical stress axis. This agrees with the angular properties of the macroscopic hysteresis curves measured by the magneto-optic Kerr effect. © 2003 American Institute of Physics.
Show PACS
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.50.Bb Fe and its alloys
75.70.Ak Magnetic properties of monolayers and thin films
76.80.+y Mössbauer effect; other γ-ray spectroscopy
61.80.Jh Ion radiation effects
78.20.Ls Magneto-optical effects
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.66.Bz Metals and metallic alloys
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