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2 Feb 2004

Volume 84, Issue 5, pp. 645-830

Issue Cover Spotlight Figure

Appl. Phys. Lett. 84, 810 (2004); http://dx.doi.org/10.1063/1.1644924 (3 pages)

Hendrik F. Hamann, Yves C. Martin, and H. Kumar Wickramasinghe
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Enhanced thermal stability of high-dielectric Gd2O3 films using ZrO2 incorporation

M.-H. Cho, D. W. Moon, S. A. Park, Y. S. Rho, Y. K. Kim, K. Jeong, C. H. Chang, J. H. Gu, J. H. Lee, and S. Y. Choi

Appl. Phys. Lett. 84, 678 (2004); http://dx.doi.org/10.1063/1.1644047 (3 pages) | Cited 2 times

Online Publication Date: 27 January 2004

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The thermal stability of Gd2O3 films, containing added Zr, was investigated using various techniques. The structural characteristics of epitaxial Gd2O3 are maintained on the Si(111) substrate when the Zr is codeposited along with Gd. The incorporation of ZrO2 into Gd2O3 improves the crystallinity of the film and no interfacial layers were observed. In particular, the structural stability with no deformation is greatly enhanced, and silicate formation is drastically suppressed, up to an annealing temperature of 800 °C. The interfacial defects caused by extensive interactions between Gd and Si are also minimized. © 2004 American Institute of Physics.
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68.60.Dv Thermal stability; thermal effects
77.55.-g Dielectric thin films
81.40.Gh Other heat and thermomechanical treatments

Improved technique to determine second-order optical nonlinearity profiles using two different samples

A. Ozcan, M. J. F. Digonnet, and G. S. Kino

Appl. Phys. Lett. 84, 681 (2004); http://dx.doi.org/10.1063/1.1644622 (3 pages) | Cited 5 times

Online Publication Date: 27 January 2004

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An improved inverse Fourier transform Maker fringe (MF) technique is described to retrieve the nonlinearity profile of thin films. It involves measuring the MF curve of two sandwich structures made by combining a pair of different nonlinear samples in two configurations. A simple algorithm is developed to process this data and retrieve the profile of both samples. Compared to our previous technique, which involves a single sandwich structure made of two identical samples, this technique is more flexible, it is insensitive to differences between the samples, and data processing is simpler and much faster. After a sample has been initially characterized by this technique, it can be used as a reference to characterize any other unknown sample using a single MF measurement. Experimental verification is demonstrated with a pair of thermally poled silica wafers. © 2004 American Institute of Physics.
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42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.66.Nk Insulators
42.65.-k Nonlinear optics

Erbium-doped waveguide amplifiers fabricated using focused proton beam writing

K. Liu, E. Y. B. Pun, T. C. Sum, A. A. Bettiol, J. A. van Kan, and F. Watt

Appl. Phys. Lett. 84, 684 (2004); http://dx.doi.org/10.1063/1.1644922 (3 pages) | Cited 29 times

Online Publication Date: 27 January 2004

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Buried channel waveguides were fabricated in Er3+–Yb3+ codoped phosphate glasses using focused proton beam writing. Proton ion doses in the range of 1014–1015 ions/cm2 and 2.0 MeV energy were used. The waveguides were located 38 μm below the substrate surface and are in excellent agreement with the transport and range of ions in matter simulation. The waveguide properties were measured, and the fluorescence spectra and optical gain of the waveguides were characterized. The maximum net gain of the waveguide amplifiers at 1.534 μm wavelength was measured to be ∼1.72 dB/cm with 100 mW pump power at 975 nm wavelength. © 2004 American Institute of Physics.
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42.82.Et Waveguides, couplers, and arrays
42.82.Cr Fabrication techniques; lithography, pattern transfer
42.79.Gn Optical waveguides and couplers
42.65.Yj Optical parametric oscillators and amplifiers

Thermal conductivity of periodic microporous silicon films

David Song and Gang Chen

Appl. Phys. Lett. 84, 687 (2004); http://dx.doi.org/10.1063/1.1642753 (3 pages) | Cited 50 times

Online Publication Date: 27 January 2004

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This letter reports the experimental in-plane thermal conductivity of microfabricated, free-standing, single-crystal silicon thin films with periodically arranged through-film micropores. The experimental data suggest that strong size effects exist even in micro-sized porous silicon structures, particularly at low temperatures. Even at room temperature, all porous membranes show thermal conductivity values lower than expected by porosity and bulk phonon mean free path of silicon, and small-pore membranes have smaller thermal conductivity compared to the large-pore ones despite that they have close porosity values. © 2004 American Institute of Physics.
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81.05.Rm Porous materials; granular materials
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
68.55.-a Thin film structure and morphology

GaN quantum dot density control by rf-plasma molecular beam epitaxy

J. Brown, F. Wu, P. M. Petroff, and J. S. Speck

Appl. Phys. Lett. 84, 690 (2004); http://dx.doi.org/10.1063/1.1645333 (3 pages) | Cited 32 times

Online Publication Date: 27 January 2004

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We report on the growth of GaN quantum dots and the control of their density in the Stranski–Krastanov mode on AlN (0001) by rf-plasma molecular beam epitaxy at 750 °C. After depositing the equivalent of 2–3 ML GaN coverage, as limited by N fluence under Ga-droplet growth conditions, excess Ga was desorbed and Stranski–Krastanov islands formed under vacuum. We present the dependence of island density as a function of GaN coverage (for two growth rates: 0.10 and 0.23 ML/s), as estimated from atomic force microscopy and cross-sectional transmission electron microscopy. With a GaN growth rate of 0.23 ML/s, the island density was found to vary from less than 3.0×108–9.2×1010 cm−2 as the GaN coverage was varied from 2.2 (critical thickness) to 3.0 ML. For a GaN growth rate of 0.10 ML/s, the island density varied from 2.0×1010 to 7.0×1010 cm−2 over a GaN coverage range of 2.0–3.0 ML. For each growth rate, the GaN islands were found to be of nearly uniform size, independent of the quantum dot density. © 2004 American Institute of Physics.
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68.65.Hb Quantum dots (patterned in quantum wells)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.07.Ta Quantum dots
68.43.Mn Adsorption kinetics
68.37.Ps Atomic force microscopy (AFM)
68.37.Lp Transmission electron microscopy (TEM)
52.77.Dq Plasma-based ion implantation and deposition

Optical properties of Cd1−xMgxTe (x = 0.00, 0.23, 0.31, and 0.43) alloy films

Y. S. Ihn, T. J. Kim, Y. D. Kim, D. E. Aspnes, and J. Kossut

Appl. Phys. Lett. 84, 693 (2004); http://dx.doi.org/10.1063/1.1639506 (3 pages) | Cited 4 times

Online Publication Date: 27 January 2004

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Pseudodielectric functions 〈ε〉 of Cd1−xMgxTe ternary alloy films of compositions x = 0.00, 0.23, 0.31, and 0.43 have been determined from 1.5 to 6.0 eV by spectroscopic ellipsometry. We obtain approximations to the bulk dielectric functions ε by performing wet-chemical etching to remove overlayers and using parametric modeling to remove interference oscillations below the fundamental band gap. The values of the E0, E0+Δ0, E1, E1+Δ1, E2, and E0′ critical point energies and their x dependences at room temperature were determined from numerically calculated second energy derivatives of these data. © 2004 American Institute of Physics.
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77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)

Air-bridged lateral growth of crack-free Al0.24Ga0.76N on highly relaxed porous GaN

R. S. Qhalid Fareed, V. Adivarahan, C. Q. Chen, S. Rai, E. Kuokstis, J. W. Yang, M. Asif Khan, J. Caissie, and R. J. Molnar

Appl. Phys. Lett. 84, 696 (2004); http://dx.doi.org/10.1063/1.1644621 (3 pages) | Cited 19 times

Online Publication Date: 27 January 2004

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We report on the strain reduction in AlGaN layers grown on porous GaN (P-GaN) by metalorganic chemical vapor deposition (MOCVD). The P-GaN was obtained by ultraviolet radiation-enhanced electroless wet chemical etching of thick hydride vapor phase epitaxial grown GaN (HVPE-GaN) templates over (001) sapphire substrates. By optimizing the growth conditions, lateral growth of AlGaN was enhanced resulting in air-bridge formation between the P-GaN and the AlGaN layers. X-ray diffraction studies showed significant strain relaxation in AlGaN layers primarily due to the strain sharing between P-GaN and the AlGaN layers. This allowed us to grow crack-free good optical quality layers with thickness exceeding the critical limits for AlGaN deposition on the conventional MOCVD GaN or HVPE-GaN. The obtained results demonstrate the potential of this approach for the development of efficient ultraviolet light emitters. © 2004 American Institute of Physics.
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68.60.Bs Mechanical and acoustical properties
81.05.Ea III-V semiconductors

Power-law load dependence of atomic friction

C. Fusco and A. Fasolino

Appl. Phys. Lett. 84, 699 (2004); http://dx.doi.org/10.1063/1.1644617 (3 pages) | Cited 3 times

Online Publication Date: 27 January 2004

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We present a theoretical study of the dynamics of a tip scanning a graphite surface as a function of the applied load. From the analysis of the lateral forces, we extract the friction force and the corrugation of the effective tip-surface interaction potential. We find both the friction force and potential amplitude to have a power-law dependence on applied load with exponent ∼ 1.6. We interpret these results as characteristic of sharp undeformable tips in contrast to the case of macroscopic and elastic microscopic contacts. © 2004 American Institute of Physics.
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81.40.Pq Friction, lubrication, and wear
62.20.Qp Friction, tribology, and hardness

Cr2O3 surface layer and exchange bias in an acicular CrO2 particle

R. K. Zheng, Hui Liu, Y. Wang, and X. X. Zhang

Appl. Phys. Lett. 84, 702 (2004); http://dx.doi.org/10.1063/1.1644919 (3 pages) | Cited 22 times

Online Publication Date: 27 January 2004

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It has been found by electron diffraction and high-resolution transmission electron microscopy that each commercial CrO2 particle is enclosed by an epitaxial Cr2O3 layer ∼2.5 nm thick. The epitaxial relationships of Cr2O3 layers on CrO2 particles have been identified. Magnetic hysteresis loops measured at different temperatures (5–300 K), after the sample was cooled from 350 K with a 50 kOe field, were found to be perfectly symmetrical, i.e., no exchange bias was observed, which is unexpected for ferromagnetic/antiferromagnetic core/shell structured particles. To gain a deeper understanding of the phenomenon, the “as-received” commercial CrO2 particles were annealed at 600 °C in air for 1 h to thicken the Cr2O3 shells. Exchange bias was clearly observed in the annealed particles with thicker Cr2O3 shells. At 5 K, the exchange bias field reached 220 Oe. These experimental observations can be explained well within the framework of the random field model. © 2004 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
75.50.Dd Nonmetallic ferromagnetic materials
68.37.Lp Transmission electron microscopy (TEM)

Boron alloying in GaN

Laurian Escalanti and Gus L. W. Hart

Appl. Phys. Lett. 84, 705 (2004); http://dx.doi.org/10.1063/1.1644910 (3 pages) | Cited 5 times

Online Publication Date: 27 January 2004

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Using first-principles calculations in the local density approximation, we studied effects of adding up to 6% boron to zinc-blende GaN. We found that the band gap increases monotonically with boron incorporation, in agreement with experiment. A composition-independent band-gap bowing parameter of 4.30 eV was determined, and proved to be large compared to bowing for other mixed cation systems. The formation enthalpy of mixing, ΔH, was determined for BxGa1−xN, BxGa1−xAs, and GaAs1−xNx. A comparison of enthalpies indicates that the production of BxGa1−xN films with boron concentrations of at least 5% may be possible. © 2004 American Institute of Physics.
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65.40.G- Other thermodynamical quantities
68.43.Bc Ab initio calculations of adsorbate structure and reactions
64.75.-g Phase equilibria
71.20.Nr Semiconductor compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
82.60.Cx Enthalpies of combustion, reaction, and formation

Strain relaxation by directionally aligned precipitate nanoparticles in the growth of single-crystalline Gd-doped ceria thin films

D. X. Huang, C. L. Chen, L. Chen, and A. J. Jacobson

Appl. Phys. Lett. 84, 708 (2004); http://dx.doi.org/10.1063/1.1644035 (3 pages) | Cited 9 times

Online Publication Date: 27 January 2004

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Transmission electron microscopy has been used to investigate the microstructure and epitaxial behavior of gadolinium-doped ceria (Ce0.8Gd0.2O2−δ) thin films on single crystal (001) LaAlO3. The results show that the films have single-crystal cubic structure and a sharp interface with an interface relationship of (001)film∥(001)sub and [100]film∥[110]sub. Accompanying the high film crystallinity, a directionally aligned, precipitated nanoparticle structure has been observed. The precipitated particles have an average size of ∼4 nm, a Ga-rich composition of Ce0.7Gd0.3O2−δ, a rhombic shape with mainly {111} facets, and are uniformly distributed over the entire film area. The nanoparticles contribute a uniform tensile strain to the film that effectively compensates the compressive film strain induced by the substrate, and also leads to a uniform relaxation of the residual film strain by generating misfit dislocations at the film/particle interfaces. The high film crystallinity is believed to result from this uniform film strain relaxation mechanism. © 2004 American Institute of Physics.
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68.60.Bs Mechanical and acoustical properties
68.55.-a Thin film structure and morphology
61.46.-w Structure of nanoscale materials

Wurtzite to zinc-blende phase transition in gallium nitride thin films

Joo Han Kim and Paul H. Holloway

Appl. Phys. Lett. 84, 711 (2004); http://dx.doi.org/10.1063/1.1640801 (3 pages) | Cited 17 times

Online Publication Date: 27 January 2004

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The phase transition in gallium nitride (GaN) thin films at room temperature from the thermodynamically stable wurtzite to the metastable zinc-blende structure is studied. The GaN films were grown at room temperature on (100)-oriented silicon substrates by radio-frequency planar magnetron sputtering of a GaN target in pure nitrogen atmosphere. As the nitrogen gas pressure during the film growth is decreased from 30 to 10 mTorr, the crystalline phase of GaN films changed from a hexagonal wurtzite to a cubic zinc-blende structure with (111) crystallographic planes oriented preferentially parallel to the film surface. The mechanisms responsible for the formation of the nonequilibrium cubic zinc-blende GaN phase at room temperature are suggested, in which the impact of the hyperthermal species impinging on the surface of growing films plays a key role in stabilizing the zinc-blende phase of GaN. © 2004 American Institute of Physics.
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68.55.-a Thin film structure and morphology
81.15.Cd Deposition by sputtering
64.70.K- Solid-solid transitions

Molecular dynamics study of dislocation formation in a [001] face-centered-cubic epitaxial island under tensile stress

P. Liu, Y. W. Zhang, B. Fox, and C. Lu

Appl. Phys. Lett. 84, 714 (2004); http://dx.doi.org/10.1063/1.1644343 (3 pages) | Cited 1 time

Online Publication Date: 27 January 2004

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Dislocation formation in homoepitaxial pyramidal [001] Cu islands under tensile stress is studied using molecular dynamics simulations. It is found that 90° Shockley partial dislocations are dominant in the island strain relaxation. For a low-aspect-ratio island, the dislocations are nucleated from the island surface and propagate downwards to form misfit dislocations. For a high-aspect-ratio island, a pair of the dislocations on the same slip plane are simultaneously nucleated respectively from the two island edges, propagate inwards, and react to form a 90° Shockley misfit partial dislocation. These dislocations can form sequentially and cooperatively. © 2004 American Institute of Physics.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Bb Theories and models of crystal defects

Intense blue–white luminescence from carbon-doped silicon-rich silicon oxide

Se-Young Seo, Kwan-Sik Cho, and Jung H. Shin

Appl. Phys. Lett. 84, 717 (2004); http://dx.doi.org/10.1063/1.1645989 (3 pages) | Cited 31 times

Online Publication Date: 27 January 2004

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The effect of carbon doping on the enhancement of visible luminescence from silicon-rich silicon oxide (SRSO), which consists of Si nanoclusters embedded inside a SiO2 matrix, is investigated. C-doped SRSO films were fabricated by electron cyclotron resonance-plasma enhanced chemical vapor deposition method using SiH4, O2, and CH4 source gases followed by a high-temperature anneal. Intense blue-white visible luminescence, visible to the naked eye under daylight conditions, was observed from the film with a nearly equal amount of C and excess Si (∼16 at. %) after an anneal at 950 °C. Furthermore luminescence could be tuned from 1.8 to 2.5 eV by controlling the C to excess Si ratio, the C content, and the anneal temperature. Taken together with the infrared absorption spectra, these results indicate that the luminescence is attributed to exciton recombination in C-incorporated Si nanoclusters. © 2004 American Institute of Physics.
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78.55.Ap Elemental semiconductors
61.46.-w Structure of nanoscale materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.S- Impurities in crystals
78.30.Am Elemental semiconductors and insulators
71.35.-y Excitons and related phenomena

Isolated nickel impurities in diamond: A microscopic model for the electrically active centers

R. Larico, L. V. C. Assali, W. V. M. Machado, and J. F. Justo

Appl. Phys. Lett. 84, 720 (2004); http://dx.doi.org/10.1063/1.1645327 (3 pages) | Cited 9 times

Online Publication Date: 27 January 2004

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We present a theoretical investigation on the structural and electronic properties of isolated nickel impurities in diamond. The atomic structures, symmetries, formation and transition energies, and hyperfine parameters of isolated interstitial and substitutional Ni were computed using ab initio total energy methods. Based on our results, we ultimately propose a consistent microscopic model which explains several experimentally identified nickel-related active centers in diamond. © 2004 American Institute of Physics.
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61.72.up Other materials
71.15.Nc Total energy and cohesive energy calculations
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Mq Elemental semiconductors
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling
71.55.Cn Elemental semiconductors
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