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Journal of Applied Physics

Issue 10 • Date May 2005

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Displaying Results 1 - 25 of 880
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Electrically tunable polymer stabilized liquid-crystal lens

    Page(s): 103101 - 103101-6
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    A tunable focal lens using flat electro-optical liquid-crystal cell with uniform pixel-free electrodes is developed. The lenslike gradient refractive index profile is created in the cell via the spatially distributed polymer network obtained by photopolymerization using a spatially nonuniform laser beam. The conditions of the polymer network generation are optimized to improve the optical quality of the lens and its focusing properties. Low optical loss (scattering) is achieved for a focal length smoothly tunable from infinity to 0.8 m. Obtained results can be applied to develop lenses that have no moving parts and allow the electro-optical zooming. View full abstract»

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  • Chromophore orientational mobility and index grating rise time in azo-dye-doped photorefractive polymer composites

    Page(s): 103102 - 103102-6
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    The influence of chromophore solubility enhancements on photorefractive grating rise time and device lifetime is investigated. Three azo chromophores differing primarily in compatibility with a polyvinylcarbazole host polymer were synthesized. Aromatic substitutions to the chromophore increased the device lifetime from several days to years although electric-field-induced poling experiments indicated that chromophore orientational mobility is severely hindered, resulting in photorefractive grating rise times approaching several hours. The incorporation of a flexible butyl chain to the aromatic substituted chromophores significantly enhanced the orientational mobility. These chromophores could be loaded as high as 60 wt % with no degradation in transparency for one year following fabrication. View full abstract»

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  • Terahertz wave detection performance of photoconductive antennas: Role of antenna structure and gate pulse intensity

    Page(s): 103103 - 103103-6
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    We studied the receiver performance of two photoconductive antennas (bow tie and dipole antennas) fabricated on the same low-temperature-grown GaAs substrate to clarify the effect of the antenna structure and gate pulse intensity on terahertz wave detection. We observed the gate pulse intensity dependence of the temporal profiles of the terahertz waves or terahertz spectra. For both antennas, the sensitivity in the low-frequency regime (≪0.5 THz) was enhanced compared to that in the high-frequency regime for large gate pulse intensities. This is because the carrier trap time increased due to the saturation of the GaAs defect levels. We also observed that the peak-to-peak amplitude of the terahertz wave detected by one antenna was not always larger than that detected by the other antenna, and the peak-to-peak amplitude of the bow tie antenna was larger (smaller) than that of the dipole antenna when the gate pulse intensity was high (low). This was explained by the gate pulse intensity dependence of the frequency-dependent detection sensitivity and also by the resonance frequency of the antenna structure. View full abstract»

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  • Optical second-harmonic imaging of PbxCd1-xTe ternary alloys

    Page(s): 103104 - 103104-6
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    We employ femtosecond laser pulses (80 fs, 1.59 eV, and 80 MHz) to study the optical second-harmonic (SH) response of PbxCd1-xTe ternary alloys (x about 0.2) grown by the vertical Bridgman method. The alloy segregates into a Pb-rich and a Cd-rich phase, the latter dominating the SH response of the ternary alloy by at least two orders of magnitude. Several sample regions show a regular layer-by-layer accommodation of the Pb-rich and Cd-rich phases as seen by a periodic alternation of the alloy’s SH response on a ∼10-μm length scale. Furthermore, we employ polarization-resolved SH imaging as well as SH imaging at different azimuthal angles to obtain spatially resolved mappings of the sample, which are sensitive to the composition as well as the growth orientation of the PbxCd1-xTe material system. We observe an azimuthal phase shift of approximately 30° between coherent macroscopic regions (several mm2) in the Cd-rich phase of the ternary alloy. We interpret these regions as large area crystalline grains of (111) and (411) crystal orientations and approximately equal composition. Hence, SH imaging is shown to spatially resolve regions of different growth directions within the PbxCd1-xTe sample. View full abstract»

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  • Investigation of the spectra of phosphorescent organic light-emitting devices in relation to emission zone

    Page(s): 103105 - 103105-4
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    The dependence of the electroluminescent spectra on the emission zone of blue electrophosphorscent light emitting diodes (PHOLEDs) was investigated. The light emission of a PHOLED was tuned from blue to greenish blue by adjusting the position of the emission zone in the PHOLED. Experimental results agreed well with the numerical simulation based on the effect of the wide-angle optical interference by the metal cathode. The comparison of the numerical results and the electroluminescent spectra of the PHOLED was then extended to serve as the basis of another method to determine the location of the emission zones of PHOLEDs. View full abstract»

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  • Optically pumped terahertz laser based on intersubband transitions in a GaN/AlGaN double quantum well

    Page(s): 103106 - 103106-5
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    A design for a GaN/AlGaN optically pumped terahertz laser emitting at 34 μm (ΔE∼36 meV) is presented. This laser uses a simple three-level scheme where the depopulation of the lower laser level is achieved via resonant longitudinal-optical-phonon emission. The quasibound energies and associated wave functions are calculated with the intrinsic electric field induced by the piezoelectric and the spontaneous polarizations. The structures based on a double quantum well were simulated and the output characteristics extracted using a fully self-consistent rate equation model with all relevant scattering processes included. Both electron-longitudinal-optical phonon and electron-acoustic-phonon interactions were taken into account. The carrier distribution in subbands was assumed to be Fermi–Dirac-like, with electron temperature equal to the lattice temperature, but with different Fermi levels for each subband. A population inversion of 12% for a pumping flux Φ=1027 cm-2 s-1 at room temperature was calculated for the optimized structure. By comparing the calculated modal gain and estimated waveguide and mirror losses the feasibility of laser action up to room temperature is predicted. View full abstract»

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  • Magneto-optic Kerr effect: Incorporating the nonlinearities of the analyzer into static photometric ellipsometry analysis

    Page(s): 103107 - 103107-6
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    A static photometric ellipsometer allows the measurement of the magneto-optic Kerr effect at a very high signal-to-noise ratio. To gain information about the state of magnetization a mathematical description of the optical components is required. This involves the use of trigonometric functions, e.g., within the Müller calculus. As these are transcendental functions the inversion of the formulae defining the final state of polarization is impossible. The commonly applied linearization is only valid for thin samples with small Kerr angles. For large Kerr angles the nonlinearity of the analyzer setup becomes important. The influence of these nonlinearities on static ellipsometry is studied analytically and numerically. A method is developed to reveal the magnetic properties with high accuracy despite of the nonlinearities due to the optical setup. The applicability of the method is demonstrated for the experimental data obtained from an ion-irradiated iron film on silicon, revealing the magnetic behavior of the investigated sample. The encountered effects are of fundamental importance for the full vectorial magnetization analysis when large Kerr signals are obtained in experiments. View full abstract»

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  • Generation of continuous-wave terahertz radiation using a two-mode titanium sapphire laser containing an intracavity Fabry–Perot etalon

    Page(s): 103108 - 103108-4
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    Continuous-wave terahertz (THz) radiation was generated by photomixing two modes of a titanium sapphire laser. The laser was induced to oscillate on two modes by placing a Fabry–Perot etalon in the laser resonator. The frequency of terahertz radiation, which was equal to the difference frequency of the two modes, was varied by adjusting the free spectral range (FSR) of the etalon. Photomixing was performed by logarithmic spiral antennas fabricated on low-temperature-grown GaAs; and the emitted THz radiation was characterized. The THz power, measured by a Golay cell, was 1 μW at 0.3 THz and 0.7 μW at 0.5 THz. The THz frequency, as determined by a Fourier transform interferometer, was seen to correspond to the etalon FSR. The current-voltage characteristics of photomixers were also determined, and photocurrent modulation was observed by the autocorrelation of the laser beam. View full abstract»

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  • Electron-phonon relaxation rates and optical gain in a quantum cascade laser in a magnetic field

    Page(s): 103109 - 103109-5
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    We present a model for calculating the optical gain in a midinfrared GaAs/AlGaAs quantum cascade laser in a magnetic field, based on solving the set of rate equations that describe the carrier density in each level, accounting for the optical- and acoustic-phonon scattering processes. The confinement caused by the magnetic field strongly modifies the lifetimes of electrons in the excited state and results in pronounced oscillations of the optical gain as a function of the field. Numerical results are presented for the structure designed to emit at λ∼11.4 μm, with the magnetic field varying in the range of 10–60 T. The effects of band nonparabolicity are also included. View full abstract»

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  • Real-time photodisplacement imaging using parallel excitation and parallel heterodyne interferometry

    Page(s): 103110 - 103110-7
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    A parallel photodisplacement technique that achieves real-time imaging of subsurface structures is presented. In this technique, a linear region of photothermal displacement is excited by a line-focused intensity-modulated laser beam and detected with a parallel heterodyne interferometer using a charge-coupled device linear image sensor as a detector. Because of integration and sampling effects of the sensor, the interference light is spatiotemporally multiplexed. To extract the spatially resolved photodisplacement component from the sensor signal, a scheme of phase-shifting light integration combined with a Fourier analysis technique is developed for parallel interferometry. The frequencies of several control signals, including the heterodyne beat signal, modulation signal, and sensor gate signal, are optimized so as to eliminate undesirable components, allowing only the displacement component to be extracted. Two-dimensional subsurface lattice defects in silicon are clearly imaged at a remarkable speed of only 0.26 s for an area of 256×256 pixels. Thus, the proposed technique allows for real-time imaging more than 10 000 times faster than conventional photoacoustic microscopy. View full abstract»

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  • Multiple-wavelength-transmission filters based on Si-SiO2 one-dimensional photonic crystals

    Page(s): 103111 - 103111-4
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    The Si/SiO2 one-dimensional photonic crystals of heterostructural multilayers with two periods, ΛA and ΛB, have great potential for multiple-wavelength-transmission filters. These structures were prepared by inserting N pairs of ΛB (as the defect region) in the middle of two sets of two pairs of ΛA, so that the structure becomes air→[(2∙ΛA)→(N∙ΛB)→(2∙ΛA)]→substrate. N means the number of ΛB pairs in the defect region. The complex refractive indices of Si and SiO2 are assumed to be 3.7+i0 and 1.5+i0 in the transfer matrix calculation. The number of transmission channels or defect branches m is given by 2N, that is, m=2N. For large N(≫10), the photonic band gap exists in a normalized frequency range ω of 0.0846–0.3838, which corresponds to the wavelength range of 0.84–6.67 μm. The defect branches are placed on a branch band between two symmetric flat bands. For a filling factor η=0.406, a matching condition of optical length in two alternating layers, the branches at the center of the branch band are divided into a uniform frequency interval. In particular, we claim that the transmission-defect branches can be precisely tuned by controlling the incident angles without external applied bias. View full abstract»

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  • Investigation of dye-doped red emitting organic electroluminescent devices with metal-mirror microcavity structure

    Page(s): 103112 - 103112-5
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    Organic electroluminescent (EL) devices with planar microcavity structure, indium-tin-oxide/Ag/N,N-diphenyl-N , N ′-bis(3-methylphenyl)-1,1-biphenyl-4 , 4-diamine/tris(8-hydroxyquinoline)-aluminum (AlQ):4-(dicyanomethylene)-2-methyl-6-(p-dimethyl aminostyryl)-4H-pyran/AlQ/LiF/Al, were fabricated. The Ag and Al layers acted as not only hole-injection layer and cathode, respectively, but reflective mirrors, resulting in strong microcavity effects, such as spectral narrowing and directional emission. The effects of device parameters on the EL performance were studied in detail and were discussed in terms of conventional microcavity theory. On-axis light magnification with a coefficient (EL enhancement ratio between cavity and noncavity devices) of ∼5 was observed, which was consistent with the theoretical calculation. At the same time, optimized microcavity device with bright pure red emission showed maximum luminance of 5140 cd/m2, peak at 624 nm, Commission International de I’Eclairage coordinates of x=0.663 and y=0.336, and high EL efficiency of 1.71 cd/A were obtained. View full abstract»

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  • Multiguide directional coupler using switchable liquid-crystalline optical channels

    Page(s): 103113 - 103113-4
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    A multiguide directional coupler with a gratinglike indium-tin-oxide electrode on a planar nematic liquid-crystal film is studied. The linearly polarized beam can be coupled into adjacent channels with the periodic modulation of the refractive index by applying a periodic electric field. The intensity distribution of coupling in the transverse direction varies with the distance of beam propagation. The coupling effects, which depend on the polarization of the incident beam and the temperature of the liquid crystals, are also discussed. View full abstract»

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  • Probing interactions between colloidal particles with oscillating optical tweezers

    Page(s): 103114 - 103114-11
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    The potential of optical traps for the study of weak interactions in colloidal systems is well recognized. In this paper, a theoretical model is developed for two oscillating interacting hard spheres in a fluid in the low Reynolds number limit. The amplitude of oscillation of the particle and the phase lag between the motion of the particles and the optical traps are a function of both the hydrodynamic coupling between the particles and the interaction potential. The effect of the particle-particle interaction is most pronounced for particles oscillated asynchronously along their line of centers. The experimental realization of this scheme is demonstrated for the model system of polystyrene latices in water and the theoretical predictions are compared with the experimental results. The extension of this idea to oil-in-water emulsions is discussed. View full abstract»

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  • Absolute and relative density measurements in a N2/Ar dielectric barrier discharge by diode-laser absorption spectroscopy and resolved plasma emission

    Page(s): 103301 - 103301-6
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    The population density of metastable Ar 4s[12 in a 70% Ar/30% N2, short-pulsed (≪15-ns full width at half maximum) dielectric barrier discharge was determined from the time-resolved diode-laser absorption as a function of pulse repetition rate for a gas pressure of 100 Torr. The relative population density change of N2+ B 2Σu+ and N2 C 3Πu was also determined from N2+ first negative and N2 second positive plasma emission for the same pulse repetition rates and pressure. The net power deposited per pulse was obtained from measurements of the voltage and current wave forms. The fractional energy dissipated in metastable Ar production was estimated from the power and absorption number density measurements to be roughly 20% for pulse repetition rates ≤10 kHz, decreasing to ∼10% at 30 kHz for 3.5-kV applied voltage and 100 Torr. View full abstract»

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  • Measurement of CH density in a pulsed-dc hydrocarbon-gas- mixture discharge

    Page(s): 103302 - 103302-6
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    Optical and electrical measurements are performed in methane/diluent mixtures in a 250-ns pulsed-dc discharge using a fast-rise-time pulser. In particular, emission and laser-induced-fluorescence studies of CH produced by direct-electron-impact dissociation reveal the postdissociation kinetics of CH as well as a means of estimating the CH density. In a 20-Torr 10%/20%/70% mixture of argon/methane/nitrogen, the instantaneous CH density is determined to be 2.1×1013 cm-3 at the end of the steady-state voltage and current condition. Kinetics analysis reveals that neutral chemistry with methane is the largest loss process for the CH fragments. Analysis of N2(C-B) rotational temperatures indicates that less than 30% of the input electrical energy is deposited as heat in the gas, with the balance going toward dissociation, excitation of internal molecular states, and ionization. View full abstract»

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  • Effects of nanosecond-duration laser prepulses on solid targets

    Page(s): 103303 - 103303-4
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    A critical issue in high-intensity laser-solid interactions is the effect of the laser prepulse on the target, but the experimental details of these lower-intensity interactions are often difficult to measure due to the subsequent high-intensity pulse. We have performed target experiments using a 0.5-ns duration, 800-nm wavelength laser pulse, specifically designed to mimic the typical amplified spontaneous emission (ASE) prepulse from a high-power Ti:Sapphire laser. Using this “artificial” ASE prepulse, we find that the threshold for relevant changes to typical solid targets occurs at a fluence of ∼0.1 J/cm2, or ∼108 W/cm2, well below the plasma formation threshold. Notably, the results are not consistent with simple surface vaporization, and suggest that the ASE prepulse causes multiatom clusters to be ejected from the target surface. In a full high-intensity experiment, this ablated material would then strongly interact with the subsequent primary laser pulse. View full abstract»

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  • The impact of frequency mixing on sheath properties: Ion energy distribution and Vdc/Vrf interaction

    Page(s): 103304 - 103304-4
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    A dual frequency rf sheath is analyzed using a simple rf sheath model to study the interaction between the two driving rf currents and their effect on sheath parameters. A symmetric rf discharge with defined electron density and dc sheath potential is modeled using a sharp boundary sheath approximation. Three results of this study are reported: (1) reproduction of trends in ion energy distribution functions predicted and measured in previous studies, (2) a frequency-mixing-dependent relationship between the dc sheath potential and applied rf potential, and (3) an additional asymmetry in the ion energy distribution function generated by the intermodulation components resulting from the nonlinear sheath. View full abstract»

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  • Atomic hydrogen densities in capacitively coupled very high-frequency plasmas in H2: Effect of excitation frequency

    Page(s): 103305 - 103305-6
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    Absolute hydrogen atom densities in pure hydrogen capacitive discharges were measured as a function of excitation frequency (13.56, 27.12, and 40.68 MHz), nominal electrical power, and gas pressure (between 0.1 and 1 Torr). Quantitative measurements were made using two-photon absorption laser-induced fluorescence (TALIF), put on an absolute scale by comparison with the TALIF signal from a known density of krypton gas, as proposed by Niemi, Schultz von Gathen, and Döbele [J. Phys. D 34, 2330 (2001)]. The H atom density increases with gas pressure and electrical power, and at a given power and pressure it increases significantly with excitation frequency. The latter can be attributed in part to increased electron density. However, time-resolved TALIF measurements in the afterglow showed that the H atom surface loss probabilities are not constant, becoming somewhat smaller when the sheath voltage is lowered, as is the case when the excitation frequency is increased, contributing to the increase in H density. View full abstract»

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  • Charge-state-resolved ion energy distributions of aluminum vacuum arcs in the absence and presence of a magnetic field

    Page(s): 103306 - 103306-6
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    The charge-state-resolved ion energy distributions (IEDs) of aluminum vacuum arc plasma species were measured and analyzed for different geometric and magnetic field configurations. The IEDs were fitted by shifted Maxwellian distributions. Plasma expansion in the absence of a magnetic field showed higher ion energies for higher charge states. The introduction of a magnetic field (independent of geometric configuration) resulted in a broader distribution and increased average ion energies. The energy gain was approximately proportional to the charge state, which may be due to the presence of electric fields in the magnetized plasma. The evolution of ion energy distributions is relevant to thin-film growth, and it is shown that the IEDs can be modified by suitable magnetic field configurations. View full abstract»

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  • Electrohydrodynamic force and aerodynamic flow acceleration in surface dielectric barrier discharge

    Page(s): 103307 - 103307-10
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    Surface discharges created in dielectric barrier discharge (DBD) configurations have been proposed as actuators for flow control in aerodynamic applications. We focus on DBDs operating in a glow regime, i.e., where the discharge is sustained by ion-induced secondary electron emission from the surface and volume ionization. After a brief discussion of the force per unit volume acting on the flow and due to the momentum transfer from charged particles to neutral molecules, we present calculations of this force based on a two-dimensional fluid model of the surface discharge. We show that this force is of the same nature as the electric wind in a corona discharge. However, the force in a DBD is localized in the cathode sheath region of the discharge expanding along the dielectric surface. While its intensity is much larger than the analogous force in a direct-current corona discharge, it is active during less than one hundred nanoseconds for each discharge pulse and the time-averaged forces in the two cases are comparable, at least for the conditions we have chosen for this study. View full abstract»

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  • A diagnostic method for real-time measurements of the density of nitrogen atoms in the postglow of an ArN2 discharge using a catalytic probe

    Page(s): 103308 - 103308-7
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    We determined the density of neutral nitrogen atoms in an ArN2 postglow using a fiber-optics catalytic probe. The probe, which had a catalyst made of nitrided iron, was calibrated with a NO titration. The recombination coefficient for the heterogeneous recombination of N atoms on the nitrided iron surface was determined by comparing the probe signal with the NO titration. Within the limits of experimental error the coefficient was found to be independent of the catalyst temperature between 400 and 650 K and had a value of 0.21±0.04. Real-time measurements of the N-atom density were performed at a nitrogen flow of 600 SCCM (standard cubic centimeter per minute) for several discharge powers between 80 and 300 W, and for argon flow rates between 200 and 3000 SCCM. With increasing discharge power the N-atom density increased monotonously; with increasing Ar flow the N-atom density at first increased, reaching a broad maximum at about 1.8×1021 m-3 for an Ar flow of 2000 SCCM, after which the N-atom density decreased with any further increase of Ar flow. View full abstract»

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  • Experimental studies of anode sheath phenomena in a Hall thruster discharge

    Page(s): 103309 - 103309-9
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    Both electron-repelling and electron-attracting anode sheaths in a Hall thruster were characterized by measuring the plasma potential with biased and emissive probes [L. Dorf, Y. Raitses, V. Semenov, and N. J. Fisch, Appl. Phys. Lett. 84, 1070 (2004)]. In the present work, two-dimensional structures of the plasma potential, electron temperature, and plasma density in the near-anode region of a Hall thruster with clean and dielectrically coated anodes are identified. Possible mechanisms of anode sheath formation in a Hall thruster are analyzed. The path for current closure to the anode appears to be the determining factor in the anode sheath formation process. The main conclusion of this work is that the anode sheath formation in Hall thrusters differs essentially from that in the other gas discharge devices, such as a glow discharge or a hollow anode, because the Hall thruster utilizes long electron residence times to ionize rather than high neutral pressures. View full abstract»

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  • Microfabricated strained substrates for Ge epitaxial growth

    Page(s): 103501 - 103501-5
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    The manipulation of strain in micromachined silicon structures presents an opportunity in the control of surface processes in epitaxial growth. With appropriate fabrication techniques, the magnitude, crystallographic direction, and symmetry of the strain at a Si surface can be precisely controlled with this strategy. Synchrotron x-ray microdiffraction techniques allow simultaneous independent measurements of the strain and bending in these structures and serve to calibrate the fabrication process. Bending is the dominant source of strain in a microfabricated Si bridge loaded at its ends by silicon nitride thin films that we have used as a strained substrate in studies of Ge epitaxial growth. The total strain difference between the top and bottom of the bent bridge exceeds 10-3 in present structures and can potentially be increased in optimized devices. These micromachined substrates complement other methods for producing strained silicon and silicon–germanium structures for improved electrical device performance and for fundamental studies of epitaxial growth. 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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P. James Viccaro
Argonne National Laboratory