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Applied Physics Letters

Issue 24 • Date Jun 2004

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Displaying Results 1 - 25 of 73
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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  • Effects of high temperature forming gas anneal on the characteristics of metal-oxide-semiconductor field-effect transistor with HfO2 gate stack

    Page(s): 4839 - 4841
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    The effects of high temperature forming gas (N2:H2=96:4) anneal (600 °C) prior to metallization have been evaluated in terms of the improvement in the carrier mobility of HfO2/nitride layer gate stack metal-oxide-semiconductor field-effect transistors with TaN gate electrode. The high-temperature forming gas anneal has been found to be effective in improving the interface quality by lowering both interface state density and interface charges. The improvements resulting in decreased Coulombic scattering centers can be attributed to hydrogen atoms that were concentrated near the interface. © 2004 American Institute of Physics. View full abstract»

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  • Electromechanically induced transition from nonohmic to ohmic behavior at contact interfaces

    Page(s): 4842 - 4844
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    A straightforward method for removing thin insulating films from contact interfaces of conductive surfaces that enables changing the contact behavior from nonohmic to ohmic was derived in this study. The efficacy of this method is demonstrated by experiments performed with a special microdevice consisting of polysilicon. It is shown that the native oxide film can be removed from asperity nanocontacts at the contact interface by electromechanical means without damaging the microdevice. Measurements of electrical contact resistance versus applied current are used to characterize the contact interface. The present approach for removing oxide surface films that are responsible for the nonohmic behavior at microdevice contact interfaces is straightforward and can easily be applied to other types of miniaturized devices to enhance their reliability and performance. An additional benefit of this method is the release of adhered microdevices by nondestructive means. © 2004 American Institute of Physics. View full abstract»

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  • Crystalline silicon oxycarbide: Is there a native oxide for silicon carbide?

    Page(s): 4845 - 4847
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    Using variable cell ab initio molecular dynamics, we have investigated hypothetical crystalline phases of silicon oxycarbide (Si1-xCxO2). We found that silicon oxide remains energetically stable with carbon incorporation, and the resulting oxycarbide material has a moderately large bulk modulus. Our results also indicated that there are at least two possible, and competing, crystalline phases for the Si2CO6. We discuss the possibility of those phases forming near the SiC/SiO2 interfaces. © 2004 American Institute of Physics. View full abstract»

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  • Multiple relaxation mechanisms in SrTiO3/SrRuO3 heterostructures

    Page(s): 4848 - 4850
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    We have studied stress relaxation mechanisms in epitaxial (001) SrTiO3 films grown on (001) LaAlO3 substrates with SrRuO3 buffer layers. A theoretical analysis has been undertaken to understand the variation of the lattice parameters of SrTiO3 epitaxial films, taking into account stress relaxation due to the formation of an orthorhombic polydomain structure in the SrRuO3 buffer layer as well as the formation of misfit dislocations at the LaAlO3/SrRuO3 and the SrTiO3/SrRuO3 interfaces. There exists a critical SrRuO3 buffer layer thickness, above which the SrRuO3 buffer layer can “screen” the effect of the LaAlO3 substrate. It is shown that the internal stress level in films can be controlled using buffer layers that exhibit a structural phase transformation. © 2004 American Institute of Physics. View full abstract»

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  • Thickness-related features observed in GaN epitaxial layers

    Page(s): 4851 - 4853
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    Electrical properties of gallium nitride (GaN) epitaxially grown on sapphire show significant dependence on layer thickness. In this letter we show some of the main features observed by spectral photoconductivity (PC), electron beam induced current, and current–voltage characteristics. We focus our attention on the blueshift of the PC peak corresponding to the energy gap, which we associated to the strain acting in the GaN epilayers. The good energetic resolution of photoconductivity spectra allows for a direct study of the energy gap dependence on thickness. © 2004 American Institute of Physics. View full abstract»

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  • Giant optical rectification effect in nanocarbon films

    Page(s): 4854 - 4856
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    We present observations of the optical rectification effect in the nanocarbon film, which is excited by nanosecond pulses of a Nd:YAG laser in the absence of an external electric field. Effective second order susceptibility of the film material is found to be 10-6 CGSE, which is higher than that of conventional noncentrosymmetric crystals. The measured ratio of the dc voltage to the laser power is 500 and 650 mV/MW at the wavelengths of 1064 and 532 nm, respectively. This makes the nanocarbon materials a promising alternative to conventional semiconductor-based terahertz radiation sources. © 2004 American Institute of Physics. View full abstract»

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  • Enhanced stability of the second-order optical nonlinearity in poled glasses

    Page(s): 4857 - 4859
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    Glasses in a wide range of chemical composition were thermally poled and the stability of the second-order optical nonlinearity induced was studied through isothermal annealing experiments. Enhanced stability was found in aluminosilicate and aluminoborosilicate with respect to silica (at least a five order of magnitude increase in lifetime). Such dramatic enhancement was explained by the strong decrease of the alkali ion mobility due to specific changes in glass composition. © 2004 American Institute of Physics. View full abstract»

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  • Optical in-well pumping of a vertical-external-cavity surface-emitting laser

    Page(s): 4860 - 4862
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    A scheme is demonstrated for optical pumping of a vertical-external-cavity surface-emitting laser. The scheme is based on absorption of the pump light within the wells of the multiple-quantum-well gain structure rather than the conventional approach of absorption of a shorter wavelength in the barrier regions. The operation of a laser around 850 nm pumped by an 808 nm source demonstrates the potential of this technique for allowing operation at a significantly shorter range of wavelengths for these devices in general and specific application of high-brightness pump lasers for devices in this spectral region. A further advantage is the smaller quantum defect which results in reduced heating of the gain medium. These advantages are achieved while maintaining a slope efficiency of up to 18%, which is comparable to results obtained with a traditional pumping scheme with a similar gain medium. © 2004 American Institute of Physics. View full abstract»

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  • Varying the effective buoyancy of cells using magnetic force

    Page(s): 4863 - 4865
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    We introduce a magnetic force buoyancy variation (MFBV) technique that employs intense inhomogeneous magnetic fields to vary the effective buoyancy of cells and other diamagnetic systems in solution. Nonswimming Paramecia have been suspended, forced to sediment and driven to rise in solution using MFBV. Details of their response to MFBV have been used to determine the magnetic susceptibility of a single Paramecium. The use of MFBV as a means by which to suspend cell cultures indefinitely is also described. © 2004 American Institute of Physics. View full abstract»

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  • Critical point transitions of wurtzite AlN in the vacuum–ultraviolet spectral range

    Page(s): 4866 - 4868
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    The optical reflection spectra have been measured on a high-quality wurtzite aluminum nitride (AlN) single crystal with synchrotron radiation in the range of 6–16 eV at different temperatures. The energy positions of the dominant structures due to the critical point (CP) transitions have been extracted by employing Adachi’s dielectric function model. By the aid of the band structure of AlN, we have assigned up to ten CP transitions in the reflection spectra. The crystal-field splitting at the center of the Brilliouin zone is observed to be 110 meV. We have further revealed the temperature dependencies of these interband transitions. © 2004 American Institute of Physics. View full abstract»

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  • Carbon nanotube filaments in household light bulbs

    Page(s): 4869 - 4871
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    Household light bulbs made from macroscopic single-walled and double-walled carbon nanotube filaments were fabricated and tested. The nanotube bulbs are found to possess several interesting features when compared to a conventional tungsten filament in safelight (36 V, 40 W), such as lower threshold voltage for light emission and higher brightness at high voltages. Electrically induced excited peaks at 407, 417, 655 nm were identified to be an intrinsic property of nanotubes and these peaks are observed to become stronger in the light emission spectra at high temperatures which cannot be explained easily with the concept of blackbody emission. © 2004 American Institute of Physics. View full abstract»

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  • Martensitic transformation from α-Ti to β-Ti on rapid heating

    Page(s): 4872 - 4874
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    Instead of conventional quenching martensitic transformation from β-Ti to α-Ti, unusual martensitic transformation from α-Ti to β-Ti induced by rapid heating has been achieved simply by using an electric current pulse. A large amount of the high-temperature martensitic phase remained in a Ti–6Al–4V alloy. We verified that such transformation is structurally and kinetically achievable, which agrees well with the phenomenological crystallographic theory of martensitic transformation. © 2004 American Institute of Physics. View full abstract»

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  • Quasiperiodic photonic crystal microcavity lasers

    Page(s): 4875 - 4877
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    We fabricated a 12-fold symmetric quasiperiodic photonic crystal (QPC) point defect laser, and obtained the lasing action at room temperature. When photopumping a seven-hole-missing defect or a uniform QPC part, the lasing occurred by different modes. The finite-difference time-domain analysis showed that the defect mode is a whispering gallery mode, which is confined by the photonic gap or the boundary reflection. It also indicated that the defect-free mode is an extended but partly localized mode, which cannot be seen in photonic crystals. © 2004 American Institute of Physics. View full abstract»

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  • Determination of dielectric constant of a thin and low-dielectric film in the millimeter wave region

    Page(s): 4878 - 4880
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    This letter presents a method for measuring free-space transmittance to study the dielectric property of a thin and low-dielectric film in the millimeter wave region. We found that multireflection keeps the height of the repeated peaks of the transmittance almost constant and that their widths quickly narrow with increase in the incident angle. We further show how the dielectric constant is set for a 20-μm-thick silicon dioxide film on a 700-μm-thick silicon substrate near the electromagnetic waves of frequency 65 GHz. © 2004 American Institute of Physics. View full abstract»

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  • Controllable excimer-laser fabrication of conical nano-tips on silicon thin films

    Page(s): 4881 - 4883
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    We have found conditions for the reproducible, direct laser fabrication of sharp conical tips with heights of about 1 μm and apical radii of curvature of several tens of nanometers. An individual cone is formed when single-crystalline silicon on a silica substrate is irradiated with a single pulse from a KrF excimer laser, homogenized and shaped to a circular spot several microns in diameter. Atomic force microscopy and field-emission scanning electron microscopy were used to characterize these structures. A simple mechanism of formation based on movement of melted material is proposed. Our results suggest that this technique could produce even smaller structures by optimizing the laser processing geometry. © 2004 American Institute of Physics. View full abstract»

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  • Leakage mechanism in GaN and AlGaN Schottky interfaces

    Page(s): 4884 - 4886
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    Based on detailed temperature-dependent current–voltage (I–V–T) measurements the mechanism of leakage currents through GaN and AlGaN Schottky interfaces is discussed. The experiments were compared to calculations based on thin surface barrier model in which the effects of surface defects were taken into account. Our simulation method reproduced the experimental I–V–T characteristics of the GaN and AlGaN Schottky diodes, and gave excellent fitting results to the reported Schottky I–V curves in GaN for both forward and reverse biases at different temperatures. The present results indicate that the barrier thinning caused by unintentional surface-defect donors enhances the tunneling transport processes, leading to large leakage currents through GaN and AlGaN Schottky interfaces. © 2004 American Institute of Physics. View full abstract»

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  • Vacancy defects in O-doped GaN grown by molecular-beam epitaxy: The role of growth polarity and stoichiometry

    Page(s): 4887 - 4889
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    Positron annihilation spectroscopy is used to study vacancy defects in GaN grown by molecular-beam epitaxy due to different polar directions and varying stoichiometry conditions during oxygen doping. We show that Ga-polar material is free of compensating Ga vacancies up to [O]=1018 cm-3 in Ga stable growth, but high concentrations of VGa are formed in N-stable conditions. We also show that vacancy clusters are formed in N-polar material grown in Ga stable conditions, which may be related to the higher reactivity of the N-polar surface. These clusters have no apparent influence on the electrical properties of the material. We thus infer that their charge state is neutral. © 2004 American Institute of Physics. View full abstract»

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  • Imaging Joule heating in a conjugated-polymer light-emitting diode using a scanning thermal microscope

    Page(s): 4890 - 4892
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    We have used a scanning thermal microscope to image Joule heating in a conjugated-polymer light-emitting diode (LED). Our LEDs had an active diameter of 100 μm, which was defined using an insulating layer of silicon nitride patterned onto the LED anode. At an average power input of 0.2 mW into the LED, we find that the center of the cathode is some 0.2 K warmer than its periphery. The observed temperature distribution across the pixel is slightly asymmetric, an effect which may be correlated with spatial inhomogeneity in the local current density across the device. We present a finite element analysis thermal model which is able to accurately describe the observed temperature distribution across the LED cathode. © 2004 American Institute of Physics. View full abstract»

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  • Light amplification by dye-doped holographic polymer dispersed liquid crystals

    Page(s): 4893 - 4895
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    In this letter we report a “one-step” fabrication technique of an optical active organic photonic band gap (PBG) structure allowing the onset of the amplification spontaneous emission effect and narrowing of the corresponding band. This result has been achieved through the addition of the dye rhodamine 6G to a standard holographic polymer dispersed liquid crystal pre-polymer mixture. The effects of the dye addition on the optical properties of the polymerized PBG structure have been analyzed. The spectra emitted by this optically active periodic structure have been studied as a function of the excitation energy. © 2004 American Institute of Physics. View full abstract»

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  • The effect of an additional infrared laser on the carrier collection efficiency of InAs quantum dots

    Page(s): 4896 - 4898
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    We report a micro-photoluminescence study on the influence of single and multi-quantum dots (QDs) on the exposure by a low-energy laser, in addition to the principal exciting laser. At low temperatures, the presence of the low-energy laser effectively quenches the single QD luminescence. This can be explained in terms of an induced screening of a built-in electric field, which plays an important role as a carrier capture mechanism. The influence of the low-energy laser is successively decreasing when the capture efficiency is increased either by elevated crystal temperature or by increased QD densities, full consistent with the proposed model. © 2004 American Institute of Physics. View full abstract»

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  • Analysis of optical emission from high-aluminum AlGaN quantum-well structures

    Page(s): 4899 - 4901
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    The letter presents theoretical analysis of spontaneous emission in AlGaN wurtzite quantum wells. It is found that the combined effects of strain, internal electric field, and many-body Coulomb interactions lead to a significant dependence of optical properties on quantum-well configuration. In particular, the effects of the internal electric field are mitigated for certain Al concentration in the quantum well. Calculations of the emitted photon energy show good agreement with experimental measurements. Presented results are of interest for applications such as ultraviolet light-emitting diodes and lasers. © 2004 American Institute of Physics. View full abstract»

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  • High power polymer dye laser with improved stability

    Page(s): 4902 - 4904
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    A polymer dye laser was developed suitable for high-power and high-repetition-rate pump lasers. A maximum output power of the dye laser of 1.5 W was achieved. The output power of the polymer gain medium is demonstrated to be absolutely stable for 4 h within 3% accuracy in a cyclic operation mode with a duty cycle of 83%. The enhanced stability of the gain medium stems from a partial recovery of photodegradation due to the dissociation of intermediate chemically active products which are detected with electron spin resonance and are shown to be produced much faster at elevated temperature. © 2004 American Institute of Physics. View full abstract»

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  • All-pass transmission or flattop reflection filters using a single photonic crystal slab

    Page(s): 4905 - 4907
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    We show that a single photonic crystal slab can function either as optical all-pass transmission or flattop reflection filter for normally incident light. Both filter functions are synthesized by designing the spectral properties of guided resonance in the slab. The structure is extremely compact along the vertical direction. We expect this device to be useful for optical communication systems. © 2004 American Institute of Physics. View full abstract»

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Applied Physics Letters, published by the American Institute of Physics, features concise, up-to-date reports on significant new findings in applied physics.

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Nghi Q. Lam
Argonne National Laboratory