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Journal of Applied Physics

Issue 12 • Date Jun 2001

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Displaying Results 1 - 25 of 110
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • High-transmittance surface textures formed by plasma etching of metallophthalocyanine films

    Page(s): 7711 - 7715
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    The effect of rf-induced plasma etching on thermally evaporated metallophthalocyanine films is investigated. Etching by a gas mixture of nitrogen and CF4 results in a transparent microstructuring residue with a grain size in the 100 nm range. The residue-covered surface increases visible transmittance up to 3%–4% over the glass substrate. The high-transmittance effect, which is nearly insensitive to wavelength, is characterized by modeling a gradient refractive-index profile bounded on discrete interfaces with surrounding media. © 2001 American Institute of Physics. View full abstract»

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  • Low-loss optical planar waveguides in Li2B4O7 crystal formed by He+ implantation

    Page(s): 7716 - 7721
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    Formation of planar waveguide in lithium tetraborate (Li2B4O7) crystal by 2 MeV helium (He+) implantation is reported. Investigation of the induced defects is performed by using a modified m-lines setup. Results indicate that the optical properties of the guiding core are not affected by the implantation process. Moreover the produced defects are shown to be isotropic and localized at the optical barrier position without any extension towards the surface. Optical losses measured by using a new approach are found to be less than 2 dB cm-1. Finally the influence of postannealing procedure applied to the sample showed a stability of the optical properties of the guide with 10% improvement of optical losses. © 2001 American Institute of Physics. View full abstract»

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  • Periodic light coupler gratings in amorphous thin film solar cells

    Page(s): 7722 - 7726
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    Efficient light trapping structures for amorphous hydrogenated silicon (a-Si:H) solar cells have been realized using periodically structured aluminum doped zinc oxide (ZnO:Al) with periods between 390 and 980 nm as a transparent front contact. Atomic force microscopy, optical reflection, and diffraction efficiency measurements were applied to characterize solar cells deposited on such gratings. A simple formula for the threshold wavelength of total internal reflection is derived. Periodic light coupler gratings reduce the reflectance to a value below 10% in the wavelength range of 400–800 nm which is comparable to cells with an optimized statistical texture. Diffraction efficiency measurements and theoretical considerations indicate that a combination of transmission and reflection gratings contribute to the observed reduction of the reflectance. © 2001 American Institute of Physics. View full abstract»

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  • Topographic cross talk in reflection mode near-field optical microscopy on patterned structures

    Page(s): 7727 - 7729
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    In scanning near-field optical microscopy the sample topography may have a strong effect on the optical image signal. This cross talk has been investigated in subwavelength-periodically patterned thin-film structures using a reflection-mode near-field optical microscope. A comparison between measured and simulated line scans shows that far-field light waves emitted from the tip aperture play a major role in the imaging process.© 2001 American Institute of Physics. View full abstract»

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  • Photoinduced in-plane switching of a photochromic nematic liquid crystal

    Page(s): 7730 - 7734
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    Photoinduced fast in-plane switching of the optic axis of a photochromic nematic liquid crystal is found in a sandwich cell with substrates promoting a twofold degenerate anchoring. The switching process is governed by the modification of the anchoring conditions associated with the photoisomerization of the photochromic nematic liquid crystal. Photoinduced in-plane reorientation of the sample optic axis of about 80° has been found. Together with the model of photoinduced in-plane switching, some implementations of this effect are briefly discussed. © 2001 American Institute of Physics. View full abstract»

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  • High duty cycle operation of quantum cascade lasers based on graded superlattice active regions

    Page(s): 7735 - 7738
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    High duty cycle operation of quantum cascade superlattice lasers with graded superlattice active regions is investigated with the goal of achieving high average optical power. The optical output power increases with pulse width and decreases with heat sink temperature. This behavior is explained on the basis of the laser core temperature oscillations during the pulsed, high duty cycle operation. Between 175 and 325 K heat sink temperature, optimum duty cycles vary from 10% to 1% and average power levels vary from 50 to 1 mW for various lasers used in this study. © 2001 American Institute of Physics. View full abstract»

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  • Optical characterization of a laser dye in a solid state host

    Page(s): 7739 - 7744
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    The optical properties of C18H16N2O2B2F4H2O, an organic laser dye embedded in solid plastic host, have been characterized for a number of laser wavelengths in the visible region. The index of refraction of the dye in plastic host is measured by the conventional method of minimum deviation at these wavelengths. The inverse adding doubling method based on the diffusion approximation and radiative transport theory have been employed to determine the absorption, scattering, and scattering anisotropy coefficients of the dye in plastic host from the measurements of total diffuse transmittance, total diffuse reflectance, and collimated transmittance using an integrating sphere. The total attenuation coefficients obtained by this method have been compared with those determined by the collimated transmission and from the total attenuation measurement. © 2001 American Institute of Physics. View full abstract»

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  • Optical recording using a photochromic ferroelectric liquid crystal

    Page(s): 7745 - 7749
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    The direction of molecular switching, i.e., the sign of the electro-optic response, in ferroelectric liquid crystals depends on the polarity of the applied electric field as well as on the sign of the spontaneous polarization. Light-induced switching of the molecules by ultraviolet light was found in a photochromic ferroelectric liquid crystal at fixed field polarity. A high contrast image was recorded whose contrast could easily be inverted by reversing the field polarity. Moreover, in the presence of bistable boundary conditions, the optically recorded image can be stored after removal of the electric field. The effect of light-assisted switching in the photochromic ferroelectric liquid crystal seems to be promising for applications such as optical recording, all-optical switches, and image storage. © 2001 American Institute of Physics. View full abstract»

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  • Absolute fluorine atom concentrations in fluorocarbon plasmas determined from CF2 loss kinetics

    Page(s): 7750 - 7755
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    We present a simple technique for determining the absolute fluorine atom concentration in pure CF4 capacitively coupled radio-frequency discharges. It is based on the measurement, by laser-induced fluorescence, of the loss rate kloss of the CF2 radical in the afterglow of a pulsed plasma. We first demonstrate that in our conditions, CF2 is lost only by gas phase recombination with F atoms (with a known rate constant krec) and by recombination at the reactor walls at a rate kwall, independent of the rf power injected. Hence, the total CF2 loss rate, kloss=kwall+krec[F], varies linearly with [F] when the rf power is increased. By recording kloss and the relative variation of the F atom concentration (by optical emission actinometry) as a function of rf power, kwall and [F] can be determined. These measurements of [F] complement previous quantitative measurements of CF and CF2 radicals [Booth etal, J. Appl. Phys. 85, 3097 (1999); and Cunge and Booth, J. Appl. Phys. 85, 3952 (1999)] made in the same reactor for the same plasma conditions. © 2001 American Institute of Physics. View full abstract»

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  • Two-dimensional modeling of a microcell plasma in a mixture of Ne/Xe driven by a capacitively coupled high-frequency source

    Page(s): 7756 - 7763
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    The basic characteristics of a micro-cell plasma in a gas mixture sustained by a high-frequency voltage source with a ring-shaped electrode are described in this article. The key to maintaining a microcell plasma is to reduce wall loss and increase plasma production. The advantage of a gas mixture of Ne/Xe is the increase of the plasma production rate in a low electric field compared to that in pure gas. As a result, a microcell plasma can be sustained by a lower applied voltage by using a gas mixture of Ne/Xe under the same power condition as compared with that in pure Xe. © 2001 American Institute of Physics. View full abstract»

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  • Correlation between cathode properties, burning voltage, and plasma parameters of vacuum arcs

    Page(s): 7764 - 7771
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    Burning voltages of vacuum arcs were measured for 54 cathode materials and compared with literature data. As anticipated, a correlation between the arc burning voltage and the plasma temperature was found. However, more importantly, a correlation between the cohesive energy of the cathode material and the arc burning voltage could be demonstrated. This link between a cathode material property, the cohesive energy, and a discharge property, the arc burning voltage, is essential for the operation of the vacuum arc discharge because is determines the plasma temperature. Energy balance considerations show that this “cohesive energy rule” is responsible for several other secondary relationships, such as the correlation between the mean ion charge state and the boiling temperature of the cathode. © 2001 American Institute of Physics. View full abstract»

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  • Observation of Fe-related defects in neutron irradiated semi-insulating InP

    Page(s): 7772 - 7776
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    Optical absorption and positron lifetime measurements have been performed on Fe-doped semi-insulating InP single crystals irradiated with thermal neutrons in a wide dose range from 0.1 to 2.7×1017n cm-2. Two lifetimes were found: τ1=210 ps is constant in all the irradiation range; and τ2=340 ps reaches an intensity of almost 40% at the higher fluence used. When comparing these results with those obtained on unintentionally doped InP, a large increase of the longest lifetime is observed, from 300 ps in the nondoped InP to 340 ps in the semi-insulating InP. The increase of the second lifetime in InP:Fe means that the positron traps are less attractive to positrons. These positron traps have been associated to a complex defect generated by the main neutron-originated defect, the indium vacancy, and the clusters or interstitial atoms of Fe. The optical absorption spectra show a background absorption related to Fe precipitates in as-grown InP:Fe. This background absorption disappears after neutron irradiation, suggesting the destruction of Fe precipitates by the energetic particles generated in the transmutation process of 115In. © 2001 American Institute of Physics. View full abstract»

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  • Dewetting and nanopattern formation of thin Pt films on SiO2 induced by ion beam irradiation

    Page(s): 7777 - 7783
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    Dewetting and nanopattern formation of 3–10 nm Pt thin films upon ion irradiation is studied using scanning electron microscopy (SEM). Lateral feature size and the fraction of exposed surface area are extracted from SEM images and analyzed as functions of ion dose. The dewetting phenomenon has little temperature dependence for 3 nm Pt films irradiated by 800 keV Kr+ at temperatures ranging from 80 to 823 K. At 893 K, the films dewet without irradiation, and no pattern formation is observed even after irradiation. The thickness of the Pt films, in the range 3–10 nm, influences the pattern formation, with the lateral feature size increasing approximately linearly with film thickness. The effect of different ion species and energies on the dewetting process is also investigated using 800 keV Kr+ and Ar+ irradiation and 19.5 keV He+, Ar+, Kr+, and Xe+ irradiation. The lateral feature size and exposed surface fraction scale with energy deposition density (J/cm2) for all conditions except 19.5 keV Xe+ irradiation. © 2001 American Institute of Physics. View full abstract»

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  • Continuous monitoring of the zinc-phosphate acid-base cement setting reaction by proton nuclear magnetic relaxation

    Page(s): 7784 - 7790
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    Proton nuclear magnetic relaxation is a well-established technique for continuous and non destructive monitoring of hydration of conventional Portland building cements. Here, we demonstrate the feasibility of nuclear magnetic resonance (NMR) monitoring of the setting reaction of zinc-phosphate acid-base dental cements, which harden in minutes as compared to days, as in the case of Portland cements. We compare the setting of cement powder (mainly, zinc oxide) prepared with clinically used aluminum-modified orthophosphoric acid solution with the setting of a model system where cement powder is mixed with pure orthophosphoric acid solution. In contrast to previously published NMR studies of setting Portland cements, where a decrease of spin-lattice relaxation time is attributed to enhanced relaxation at the growing internal surface, spin-lattice relaxation time T1 increases during the set of clinically used zinc-phosphate cement. Comparison of these results with a detailed study of diffusion, viscosity, and magnetic-field dispersion of T1 in pure and aluminum-modified orthophosphoric acid demonstrates that the increase of T1 in the setting cement is connected with the increase of molecular mobility in the residual phosphoric acid solution. Although not taken into account so far, such effects may also significantly influence the relaxation times in setting Portland cements, particularly when admixtures with an effect on water viscosity are used. © 2001 American Institute of Physics. View full abstract»

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  • Elastic constants of yttria (Y2O3) monocrystals to high temperatures

    Page(s): 7791 - 7796
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    The single-crystal elastic moduli of yttria have been measured by Brillouin spectroscopy up to 1200 °C. The room temperature values obtained are C11=223.7±0.6 GPa, C44=74.6±0.7 GPa, and C12=112.4±1.1 GPa. The resulting bulk and (Voigt–Reuss–Hill) shear moduli are K=149.5±1.0 GPa and GVRH=66.3±0.8 GPa, respectively. These agree much more closely with experimental values reported for polycrystalline samples than do previous single-crystal measurements. Linear least squares regressions to the variation of bulk and shear moduli with temperature result in derivatives of dK/dT=-17±2 MPaC and dGVRH/dT=-8±2 MPaC. Elastic anisotropy was found to remain essentially constant over the temperature range studied. © 2001 American Institute of Physics. View full abstract»

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  • Effects of dielectric materials on electromigration failure

    Page(s): 7797 - 7808
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    The effects of dielectric layers on electromigration failure were studied in situ using a high-voltage scanning electron microscope and at the wafer level using conventional accelerated testing. Several different passivation layers were deposited on wafers with A1 interconnect test structures. Prior to the deposition of the final dielectric, the wafers were processed identically and, whenever possible, simultaneously. Interconnects encapsulated with compliant polymer and very thin (0.1 μm) SiO2 layers demonstrated substantial lifetime extensions over those with more rigid (1 μm thick) SiO2 layers. Unpassivated lines behaved dramatically differently and failed much sooner than those covered with only 0.1 μm of SiO2. As expected, increasing the passivation thickness from 0.5 to 4 μm increased the electromigration lifetime for SiO2 covered specimens. The fabrication of silicon dioxide dielectrics using electron-cyclotron-resonance chemical-vapor deposition (CVD) and silicon nitride dielectrics via plasma-enhanced CVD damaged the interconnects. This damage nearly completely removed the barrier to void nucleation during electromigration. © 2001 American Institute of Physics. View full abstract»

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  • Diffusivity measurements of silicon in silicon dioxide layers using isotopically pure material

    Page(s): 7809 - 7813
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    We report measurement of the silicon diffusion coefficient in silicon dioxide films using isotopically enriched 28Si silicon dioxide layers that enable relatively low 30Si concentration measurements to be performed using secondary ion mass spectrometry. Two types of experiments are made. 30Si atoms are introduced in excess in a stoichiometric isotopically pure silicon dioxide layer either by ion implantation or by a predeposition technique. These experiments are representative of any physical situation in which excess silicon atoms are introduced into silicon dioxide layers during silicon processing. The estimated diffusivity values are significantly higher than previously reported values for Si diffusion within a stoichiometric oxide and closer to reported values for excess Si diffusion within an oxide. The activation energy of the diffusivity is found to be 4.74 eV. © 2001 American Institute of Physics. View full abstract»

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  • Surface energy of amorphous carbon films containing iron

    Page(s): 7814 - 7819
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    Iron containing diamond-like amorphous carbon (a-C:Fe) films were deposited by filtered cathodic vacuum arc technique. The influences of Fe content and substrate bias on the surface energy of the films were investigated. The surface energy of a-C:Fe films was determined by the contact angle measurement. Atomic force microscopy, Raman spectroscopy, and x-ray induced photoelectron spectroscopy were employed to analyze the origin of the variation of surface energy with various Fe content and substrate bias. It is found that the contact angle for water increases significantly after incorporating Fe into the films and the films become hydrophobic. The roughness of these films has no effect on the contact angle. The surface energy is reduced from 42.8 to 25 dyne/cm after incorporating Fe into the a-C film (10% Fe in the target), which is due to the reduction of both dispersive and polar component. The reduction in dispersive component is ascribed to the decrease of atomic density of the a-C:Fe films due to the increase in sp2 bonded carbon. When sp2 content increases to some extent, the atomic density remains constant and hence dispersive component does not change. The absorption of oxygen on the surface plays an important role in the reduction of the polar component for the a-C:Fe films. It is proposed that such network as (CnO–Fe)–O–(Fe–O–Cn) may be formed and responsible for the reduction of polar component. © 2001 American Institute of Physics. View full abstract»

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  • Control of strain in GaN by a combination of H2 and N2 carrier gases

    Page(s): 7820 - 7824
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    We study the effect of a combination of N2 and H2 carrier gases on the residual strain and crystalline properties of GaN, and we propose its application to the improvement of crystalline quality of GaN/Al0.17Ga0.83N multiple quantum well (MQW) structures. GaN was grown with H2 or N2 carrier gas (H2– or N2–GaN) on an AlN low-temperature-deposited buffer layer. A (0001) sapphire substrate was used. N2–GaN was grown on H2–GaN. The total thickness was set to be 1.5 μm, and the ratio of N2–GaN thickness to the total thickness, x, ranged from 0 to 1. With increasing x, the tensile stress in GaN increased. Photoluminescence intensity at room temperature was much enhanced. Moreover, the crystalline quality of GaN/Al0.17Ga0.83N MQW was much higher when the MQW was grown with N2 on H2–GaN than when it was grown with H2 on H2–GaN. The- se results were due to the achievement of control of strain in GaN using a combination of N2–GaN and H2–GaN. © 2001 American Institute of Physics. View full abstract»

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  • Topographic study of sputter-deposited film with different process parameters

    Page(s): 7825 - 7832
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    In this study, molecular dynamics simulation is employed to investigate the surface topography of thin films produced by the sputtering process for different parameters such as substrate temperature, incident energy, and incident angle. Interface width is used to quantify the quality of the deposited film. The Morse potential is used to model the atomic interaction between atoms. From the results of this study, it is found that for lower substrate temperature, lower incident energy, and larger incident angle, the growing film structure tends toward a three-dimensional columnar structure, and a rougher film is produced. Conversely, for higher substrate temperature, higher incident energy, and smaller incident angle, the growing film structure tends toward a two-dimensional (Frank–van der Merwe) quasi-layer-by-layer structure, and a smoother film is produced. Finally, average surface kinetic energy is found to be an important factor in determining the surface properties produced in the process. Generally, the produced film is smoother when the average surface kinetic energy is larger. © 2001 American Institute of Physics. View full abstract»

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  • Microstructure of laterally overgrown GaN layers

    Page(s): 7833 - 7840
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    Transmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed light on the optical properties of such samples. © 2001 American Institute of Physics. View full abstract»

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  • Interfacial reactions in epitaxial Al/TiN(111) model diffusion barriers: Formation of an impervious self-limited wurtzite-structure AIN(0001) blocking layer

    Page(s): 7841 - 7845
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    Single-crystal TiN(111) layers, 45 nm thick, were grown on MgO(111) by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges at Ts=700 °C. Epitaxial Al(111) overlayers, 160 nm thick, were then deposited at Ts=100 °C in Ar without breaking vacuum. Interfacial reactions and changes in bilayer microstructure due to annealing at 620 and 650 °C were investigated using x-ray diffraction and transmission electron microscopy (TEM). The interfacial regions of samples annealed at 620 °C consist of continuous ≃7-nm-thick epitaxial wurtzite-structure AlN(0001) layers containing a high density of stacking faults, with ≃22 nm thick tetragonal Al3Ti(112) overlayers. Surprisingly, samples annealed at the higher temperature are more stable against Al3Ti formation. TEM analyses of bilayers annealed at 650 °C (10 °C below the Al melting point!) reveal only the self-limited growth of an ≃3-nm-thick interfacial layer of perfect smooth epitaxial wurtzite-structure AlN(0001) which serves as an extremely effective deterrent for preventing further interlayer reactions. © 2001 American Institute of Physics. View full abstract»

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  • Metalorganic chemical vapor deposition of GaN on Si(111): Stress control and application to field-effect transistors

    Page(s): 7846 - 7851
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    Two schemes of nucleation and growth of gallium nitride on Si(111) substrates are investigated and the structural and electrical properties of the resulting films are reported. Gallium nitride films grown using a 10–500 nm-thick AlN buffer layer deposited at high temperature (∼1050 °C) are found to be under 260–530 MPa of tensile stress and exhibit cracking, the origin of which is discussed. The threading dislocation density in these films increases with increasing AlN thickness, covering a range of 1.1 to ≫5.8×109cm-2. Films grown using a thick, AlN-to-GaN graded buffer layer are found to be under compressive stress and are completely crack free. Heterojunction field effect transistors fabricated on such films result in well-defined saturation and pinch-off behavior with a saturated current of ∼525 mA/mm and a transconductance of ∼100 mS/mm in dc operation. © 2001 American Institute of Physics. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory