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Journal of Applied Physics

Issue 1 • Date Jan 1992

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Displaying Results 1 - 25 of 98
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • An infrared dielectric function model for amorphous solids

    Page(s): 1 - 6
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    For the modeling of infrared spectra it is a common approach to use a dielectric function that treats the vibrational modes as damped harmonic oscillators. This model was found to be rather crude for some applications to amorphous solids. A dielectric function model yielding a Gaussian shape of the absorption lines and satisfying Kramers–Kronig relations is suggested. The model function is constructed by a convolution of a Gaussian function with the dielectric function of the damped harmonic oscillator model. An analytical solution of this integral is given. It is demonstrated that this model describes the spectra of thermally grown ultrathin (1.3 nm) silicon oxide films, plasma‐deposited silicon films, plasma‐deposited silicon nitride films, and amorphous aluminum oxide films very well. The physical motivation of the dielectric function model suggested is the randomness of the vibrational frequencies in an amorphous structure. View full abstract»

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  • Analysis of photon scanning tunneling microscope images

    Page(s): 7 - 10
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    Photon scanning tunneling microscope (PSTM) images are made by scanning an optically conducting probe tip in an evanescent field above a sample surface. These images contain a mixture of topographical information, scattering effects, and variations in optical properties across the sample. Proper interpretation of PSTM images is therefore dependent upon knowledge of the field‐surface interactions giving rise to these features. The subject of this work is the calculation of PSTM image features arising from the topography of the sample surface. Image features of a model sinusoidal grating structure are calculated using small perturbation theory. Image amplitudes are shown to depend on the sample orientation and the shape and position of the probe tip. The calculated results are compared with actual PSTM images of a holographically produced sinusoidal grating surface. View full abstract»

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  • Elastic scattering of electrons and positrons by bound phosphorus, indium, and antimony atoms

    Page(s): 11 - 14
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    Elastic differential and total cross sections of electrons and positrons scattered by bound phosphorus, indium, and antimony atoms have been calculated using the method of partial waves. The interaction potential consists of an electrostatic potential, a Buckingham‐type polarization potential, and, in case of electron impact, the Mittleman–Watson exchange potential. The incident energies range from a few eV to 10 keV. A parameterization of the computed partial‐wave total cross sections in terms of the screened Rutherford total cross section is presented. View full abstract»

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  • Ionization coefficients in selected gas mixtures of interest to particle detectors

    Page(s): 15 - 21
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    We have measured and report the total gas‐number‐density normalized effective ionization coefficient, α¯/N, as a function of the density‐reduced electric field, E/N for Ar, CO2, CF4, the binary gas mixtures CF4:Ar (20:80), CO2:Ar(20:80), CO2:CH4(20:80) and for the ternary gas mixtures CO2:CF4:Ar (10:10:80) and H2O:CF4:Ar(2:18:80). We also report the limiting value, (E/N)lim of the above gaseous systems. A comparison of our results on the unitary gases shows good agreement with previous measurements. The results on the ternary mixtures show that the addition of CO2 or H2O to the binary gas mixture CF4:Ar lowers the electron energies and increases α¯/N(E/N). View full abstract»

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  • Automatic quasiphase matching for second‐harmonic generation in a periodically poled LiNbO3 waveguide

    Page(s): 22 - 27
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    A novel technique is proposed and experimentally demonstrated to achieve a quasiphase matching (QPM) condition for second‐harmonic generation (SHG) in a nonlinear optical waveguide with a periodically poled LiNbO3 waveguide. The fundamental wave satisfying the QPM conditions is automatically generated when the laser diode (LD) and the optical waveguide are optically connected. Using the proton‐exchanged LiNbO3 waveguide with a periodically poled grating, which was fabricated by Ti diffusion and the InP/InGaAsP LD with antireflection‐coated facets, it was confirmed that the LD oscillated at the wavelength satisfying the QPM condition, and SHG was observed simultaneously. Using a Ti:sapphire laser, it was also confirmed that the SHG was realized at the third order of QPM condition, as was expected.   View full abstract»

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  • Nonradiative energy‐transfer processes in Cd1-xMnxF2 crystals

    Page(s): 28 - 36
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    The absorption and fluorescence spectra and fluorescence decay kinetics of Cd1-x MnxF2 crystals with x=0.018, 0.063, 0.084, 0.26 were measured as a function of temperature under argon‐ion and nitrogen laser excitation. In addition, laser‐induced grating measurements were made as a function of write beam crossing angle for various temperatures between 15 and 50 K. It was found that a characteristic intrashell luminescence comes from two Mn subsystems, one composed of single Mn2+ ions and the second of Mn ion aggregates. Based upon the four‐wave‐mixing experiment, a relatively strong energy migration was found in each of these subsystems, but with a weak, if any, communication between them. The diffusion coefficients for both subsystems increase as the temperature is raised, and above 50 K strong diffusion erases the light‐induced gratings beyond the possibility of detection. The long‐lived components of the four‐wave‐mixing transient signals are associated with the other point defects, either created or photoexcited by the strong laser beams. View full abstract»

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  • Fluence limiting via photorefractive two‐beam coupling

    Page(s): 37 - 44
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    The optimum performance of a photorefractive two‐beam coupling fluence limiter is presented. A two‐level, single‐dopant‐species model is used to determine the minimum transmitted fluence for different crystal dopant densities and incident fluences. The effects on limiter performance for different beam parameters such as the modulation ratio and crossing angle and for different crystal parameters such as the mobility, electro‐optic coefficient, and dielectric constant are determined. View full abstract»

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  • High‐efficiency second‐Stokes‐order Raman conversion of KrF laser radiation in hydrogen

    Page(s): 45 - 48
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    Generation of the second Stokes order with a photon efficiency of 68% has been obtained from a single‐pass hydrogen Raman oscillator. The third‐order Stokes generated by four‐wave mixing is effectively suppressed by phase mismatching. The 1.6 times diffraction‐limited pump‐beam quality used in the present experiment was found to be essential to the high‐efficiency second‐Stokes‐order generation. View full abstract»

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  • Ion‐implanted optical waveguides in KTaO3

    Page(s): 49 - 52
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    Optical waveguides have been formed by helium‐ion implantation in KTaO3. The implantation forms a confinement barrier near the projected range of the ion by decreasing the refractive index as much as 16% for ion doses of 4×1016 ions/cm2. This is the highest refractive‐index change yet reported for ion‐implanted crystalline planar waveguides. Guiding modes (with moderately low loss) are produced without the need for annealing out of color centers. During annealing studies, the index change reduces during an anneal stage near 400 °C, but waveguiding is maintained even after anneals to 900 °C. Loss measurements indicate a planar‐waveguide loss of ≪1 dB/cm after a 400 °C anneal. View full abstract»

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  • Photothermal deflection measurements for monitoring heat transfer during modulated laser heating of solids

    Page(s): 53 - 63
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    Photothermal deflection (PTD) techniques have been used to monitor various laser‐heating processes, including melt, vaporization, and ablation of solids. To interpret the complex signal response resulting from transient phase changes at a surface, the temporal profile of the PTD signal response must be considered. In doing so, the case of the linear heating of a target without phase change is first studied here. Numerical and experimental work is presented to show the effect on the shape, magnitude, and phase of a PTD signal due to changes in (1) the thermophysical properties of the target material and deflecting medium, (2) the dimensions and boundary conditions of the target, (3) the distance of the probe beam from the surface of the target, and (4) the modulation frequency of the heating source. Copper and lead target materials heated in air are used in the experimental work. The PTD signals show qualitative agreement with the temperature gradient normal to the surface calculated using a numerical finite‐difference two‐dimensional thermal‐diffusion model. The results also show that an unusual phenomenon occurs when heating with a laser or other finite‐sized heating source. When the thermal diffusivity of the target and deflecting medium are different and the probe beam is close to the surface, a local maximum is observed in the time‐response profile of the PTD signal during the heating cycle. The maximum occurs as a result of asymmetric changes in the temperature field over time. The shape of the PTD signal, therefore, can provide information about the laser‐heating process at a surface in real time. View full abstract»

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  • Gain measurements at 780.8 nm in a copper hollow anode cathode laser

    Page(s): 64 - 69
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    The results of a parametric study of a copper hollow anode cathode laser operating on the He‐Cu+ infrared lines are reported. The discharge voltage, the small‐signal unsaturated gain on the 780.8‐nm transition, and the ground‐state copper atom concentration each exhibit a universal dependence on the ratio J/p of the discharge current density to the total pressure of the helium‐argon buffer gas mixture. The laser output power dependencies (780.8 and 782.5 nm) on J/p, while not resulting in universal curves, have common threshold values of J/p. The threshold value of J/p for the 782.5‐nm line is approximately twice that for the 780.8‐nm line. View full abstract»

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  • Diffraction of light by a focused ultrasonic wave

    Page(s): 70 - 75
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    The measurement of the acoustic pressure of a planar ultrasonic wave by light diffraction is well established. The ability to do similar measurements in the case of spherical waves has been doubted. However, we show that the range of validity can be extended to the focal region of a spherical concave piezoelectric transducer. Light is passed through the focal plane of a spherical concave transducer and is diffracted as a result of the variation in the index of refraction. The peak pressure can be calculated from the diffraction intensity by making the following simplification. We assume that in the focal plane the ultrasound can be approximated by a profiled planar wave, which in turn can be modeled by a wave of constant amplitude and effective width. The experimental results for moderate‐pressure amplitudes in water compare favorably with the calculations using the Khokhlov–Zabolotskaya–Kuznetsov equation, which incorporates both nonlinearity and diffraction effects of the acoustic field. View full abstract»

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  • Optimal heating and cooling strategies for heat exchanger design

    Page(s): 76 - 79
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    The optimal heating or cooling strategy that minimizes entropy production is derived for a simple class of common heat transfer processes that are constrained to proceed in a fixed, finite time. The empirical wisdom embodied in conventional single‐pass counterflow heat exchanger design is examined in light of this solution. For judiciously selected system parameters, the counterflow heat exchanger can yield the optimal solution. View full abstract»

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  • Droplet diameter, flux, and total current measurements in an electrohydrodynamic spray

    Page(s): 80 - 84
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    The measured diameter and flux distributions and total current of ethanol droplets generated electrohydrodynamically from a single hypodermic needle at two different spray charge densities are presented and discussed. The resultant droplet diameter and droplet flux distributions reveal a progressive production of an increasing number of smaller diameter droplets with increasing spray charge density. These are consistent with observations made of the filament breakup‐to‐droplet process in this region. The droplet flux distributions are used to construct droplet charge flux distributions by assuming various models of the droplet charging process. The resultant charge fluxes are logarithmically related to lateral position, regardless of the droplet charging model assumed. The integrals of the various charge flux profiles are compared to the measured total current. This approach reveals the best agreement between measured and predicted total droplet currents when an equilibrium end‐state process for droplet charging is assumed. View full abstract»

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  • Laser‐plasma x‐ray generation using an injection‐mode‐locked XeCl excimer laser

    Page(s): 85 - 93
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    A discharge‐excited XeCl excimer laser equipped with unstable resonator optics was injection locked to a low‐power picosecond pulse train resulting in an efficient high‐power output consisting of a train of ∼150‐ps pulses separated by ∼2 ns and lasting for the duration of the excimer gain. The laser had a total energy of approximately 250 mJ and when focused to an irradiance of ∼5×1013 W cm-2 on metal targets produced keV x rays with a conversion efficiency of ≊2.5%.   View full abstract»

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  • Breakdown characteristics in nonplanar geometries and hollow cathode pseudospark switches

    Page(s): 94 - 100
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    Breakdown voltages of gases in parallel‐plate geometries are well represented by Paschen’s law, whose scaling parameter is pd (gas pressure×electrode separation). In nonplanar geometries, Paschen’s law is not directly applicable due to the ambiguity in the distance between the electrodes and distortion of the electric field. A Monte Carlo computer model is used to investigate breakdown characteristics in nonplanar geometries and hollow cathode pseudospark switches in particular. The model tracks the trajectories of both electrons and ions, including ionizing collisions in the gas phase by electrons and ions, and secondary electron emission by ions on surfaces. It is found that under typical operating conditions in helium (0.1 to a few Torr, voltages of tens of kV, effective electrode separation of a few mm), approximately two‐thirds of ionizing collisions are attributable to ion impact, of which half are due to ion impact in the gas phase. View full abstract»

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  • Theoretical and experimental study of a high‐current vacuum arc in a uniform axial magnetic field

    Page(s): 101 - 107
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    Measurements of the electron temperature, floating potential, plasma density, and average plasma pressure in a magnetically confined high‐current vacuum arc are described. A 40‐mm‐diam, 30‐mm‐long arc was initiated between OFCu copper electrodes in a uniform axial magnetic field by triggering the gap. The crest values of the arc current ranged from 3 to 20 kA. The electron temperature by Langmuir probe measurement was Te= 2.5–3.0 eV with a uniform space distribution and was independent of the arc current. The space distribution of the ion density was parabolic in the radial direction and with a zero derivative in the axial direction. The average plasma pressure by paramagnetic measurement increased with the square of the arc current. The experiment showed that the arc column could be considered as an infinitely long column in which the plasma parameters have zero axial derivatives. Theoretical analysis has been carried out under the following assumptions. An infinitely long cylindrical and fully ionized steady‐state plasma in which all quantities varied in the r direction only was considered. The anisotropy factor σ for electrical conductivities was taken into account in the theoretical analysis, where σ and σ were electrical conductivities parallel to the magnetic field and perpendicular to it, respectively. The plasma parameters determined from the experiments and theoretical analysis agreed closely with each other when the factor σ was equal to ∼2, which is the theoretical upper limit in a high magnetic field. Constant temperature characteristics independent of the arc current have been found to be clearly seen by using line radiation as the dominant power loss in the power balance equation. View full abstract»

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  • Numerical study of single‐vapor‐bubble dynamics in insulating liquids initiated by electrical current pulses

    Page(s): 108 - 112
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    In this paper we present a study of the dynamics (growth, collapse, and rebound) of a single bubble (typical radius 1–10 μm) as a function of various parameters: injected energy, hydrostatic pressure (for a wide range 0.1–12 MPa), and several liquid hydrocarbons of different physical properties. It appears that between two rebounds the dominant role is played by liquid inertia. However, rapid damping occurs, and we examine here the influence of both liquid viscosity and acoustic radiation on this mechanism, discarding heat and mass transfers. Two simple models are proposed: the first one, based on a Herring modified model, and the second one, based on the Rayleigh–Plesset equation. The discrepancy with experimental results suggests that further insight must be achieved, taking into account the neglected transfers between vapor and liquid. View full abstract»

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  • Numerical simulation of plasma sheath expansion, with applications to plasma‐source ion implantation

    Page(s): 113 - 117
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    In plasma‐source ion implantation a target is pulse biased to a high negative voltage, forming an expanding plasma sheath. A numerical simulation model for the evolution of the sheath has been developed and compared successfully with experimental results. The model is one dimensional (planar, cylindrical, or spherical). The time‐dependent, self‐consistent potential profile is calculated from Poisson’s equation coupled with collisionless fluid equations for the ions and a Boltzmann assumption for the electrons. In addition to the density and potential profile, the simulation yields the ion current to the surface and the energy spectrum of the ions hitting the surface. View full abstract»

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  • Temperature dependence of incorporation processes during heavy boron doping in silicon molecular beam epitaxy

    Page(s): 118 - 125
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    Boron doped layers were grown by silicon molecular beam epitaxy to establish incorporation processes at temperatures between 900 and 450 °C. For temperatures exceeding 650 °C a surface accumulated phase of boron was formed when doping levels exceeded solid solubility limits. The properties of this surface phase were used to determine solubility limits for boron in silicon. Above 750 °C, the measured equilibrium solubility limit was in the 1019‐cm-3 range in good agreement with previously published annealing data and showing a gradual decrease with decreasing temperature. Below 650 °C, the processes leading to the formation of the surface phase were kinetically limited, manifested by a sharp increase in boron solubility limit, with completely activated levels above 1 × 1020 cm-3 realized. At intermediate growth temperatures the degree of dopant activation was found to be dependent on growth rate. The stability of fully activated highly‐doped boron layers, grown at low temperatures, to ex situ annealing is also discussed. View full abstract»

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  • Elevated‐temperature 3‐MeV Si and 150‐keV Ge implants in InP:Fe

    Page(s): 126 - 132
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    Variable‐fluence 3‐MeV Si+ and 150‐keV Ge+ implants were performed into InP:Fe at 200 °C. Lattice damage in the material is greatly reduced over comparable room‐temperature (RT) implantations and is rather insensitive to fluence for Si+ implantation in the range of 8 × 1014–5 × 1015 cm-2, and no amorphization occurs. For 8 × 1014‐cm-2 Si+ implantation at 200 °C, the dopant activation is 82% and carrier mobility is 1200 cm2/V s after 875 °C/10‐s annealing, whereas for the RT implantation the corresponding values are 48% and 765 cm2/V s, respectively. The reasons for the improved mobility in the elevated‐temperature implants were investigated using Rutherford‐backscattering spectrometry. At a dose of 8 × 1014 cm-2, the aligned yield after annealing is close to that of a virgin sample, indicating a low concentration of residual damage in the 200 °C implant, whereas the lattice remained highly defective in the RT implanted sample. Elevated‐temperature implantation of Si+ and Pi+ ions was also investigated. Coimplantation did yield an improvement in activation for an implanted fluence of 2 × 1015 cm-2 Si+, but resulted in an inferior lattice quality which degraded the carrier mobility compared to a Si+ (only) implant. For a 1 × 1014‐cm-2 Ge+ implant, the maximum dopant activation is 50% (donor) and the material did not turn p type even after 925 °C annealing. View full abstract»

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  • Monte Carlo simulation of ion‐channeling spectra from partially damaged crystals

    Page(s): 133 - 139
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    We have elaborated a new Monte Carlo program to simulate ion‐channeling spectra from partially damaged crystals. This program provides the transverse‐flux distribution together with the fractions of channeled and dechanneled ions, their average energies, and energy fluctuations at any depth by calculating channeled‐ion trajectories under a multistring potential. Thus we can determine the depth profiles of defects and dopant atoms, including preferential lattice positions as well as atomic configurations at interfaces. The present program is applied to deriving the depth profiles of defects induced by 100‐, 150‐, and 200‐keV Ar+ irradiation upon GaP(111) crystals. Overall agreement is obtained between the present results and those determined from an analytic manner and calculated from a modified trim program, which simulates collision cascades in solids. View full abstract»

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  • Cell thickness and surface pretilt angle measurements of a planar liquid‐crystal cell with obliquely incident light

    Page(s): 140 - 144
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    Two simple and fast optical methods are presented for the cell thickness and the surface pretilt angle measurements of a planar‐oriented liquid‐crystal (LC) cell, in which the LC director always lies in a plane of a Cartesian coordinate system. The methods employ an obliquely incident light propagating in the same plane containing the liquid‐crystal molecules. Simple analytic formulas for the angular‐dependent optical transmission are obtained. The phase retardation between the ordinary and extraordinary waves is used to measure the cell thickness with the use of a compensator. Independent data can be obtained for the cell thickness measurement by varying the angle of incidence of the incident light. The phase retardation‐extremum method is used to measure the pretilt angle. Approximate formulas relating the pretilt angle and the angle of incidence corresponding to the transmission minimum are also obtained for the pretilt angle measurement for vertically and parallel‐oriented LC cells with a small pretilt angle. For the parallel cell, the approximate formula agrees with that obtained by Birecki and Kahn [The Physics and Chemistry of Liquid Crystal Devices, edited by G. J. Sprokel (Plenum, New York, 1980), p. 115]. The methods are applicable to a planar LC cell, including the planar parallel nematic LC, homeotropic nematic LC cell, obliquely oriented nematic LC cell, and the planar‐oriented ferroelectric smectic LC cell. View full abstract»

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  • Studies of coherent and diffuse x‐ray scattering by porous silicon

    Page(s): 145 - 149
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    The microstructure of porous silicon layers has been studied by means of x‐ray diffraction. Using a double‐crystal diffractometer, the observed diffraction patterns give directly the mismatch between the lattice parameters of the porous layers and of the substrate, and the curvature of various porous silicon samples obtained in different conditions. From measurements with the same experimental set‐up, but with a larger scan range, broad diffuse bumps produced by the pore structure have been observed. This new feature allows us to obtain structural informations on the porous silicon in a nondestructive way. In particular, we have observed the anisotropic pattern showing a preferential elongation of the pores perpendicular to the (100) surface. 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