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Journal of Applied Physics

Issue 7 • Date Oct 1994

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Displaying Results 1 - 25 of 84
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Heavy‐ion sources for radiation therapy

    Page(s): 3947 - 3969
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    The usefulness of particle beams for radiation therapy has been well and widely recognized. For the cure of cancer patients, many accelerator facilities have already been utilized, and some new facilities are now being put into operation, or are under construction. Considering the medical and biological requirements, light heavy ions with an energy of several hundred MeV/nucleon are regarded as being the most suitable species. A reasonable choice to this end is an accelerator complex, for an example, one comprising an ion source, an injector linac, and a synchrotron. The ion source is of great importance, since its characteristics strongly affect the overall performance of the accelerator system. A pulsed Penning source (PIGIS) has been successfully used at Lawrence Berkeley Laboratory. Recently, at the National Institute of Radiological Sciences a low‐duty pulsed PIGIS for the heavy‐ion medical accelerator in Chiba (HIMAC) has been developed; it has both a long lifetime and a high peak intensity. As other types of ion sources, an electron‐beam ion source (EBIS) and an electron‐cyclotron‐resonance ion source (ECRIS) are being developed at several laboratories. An EBIS is basically a pulsed source, and is being successfully used at Saclay. By using an after‐glow mode, two ECRISs have made remarkable progress at Grenoble and the Grand Accelerateur National d’Ions Lourds; similar tests are proceeding for the Schwer‐Ionen Synchrotron at the Gesellschaft für Schwer‐Ionenforschung, the booster at Centre d’Europeen de Recherche Nucleaire, and the HIMAC. These different types of heavy‐ion sources are discussed from the viewpoint of their application to radiation therapy. View full abstract»

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  • Experimental verification of the smooth‐approximation theory for a space‐charge dominated beam in an axisymmetric periodic focusing channel

    Page(s): 3970 - 3974
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    The predictions of the smooth‐approximation theory for the effective radius of a space‐charge dominated beam in a periodic solenoid focusing channel were checked experimentally over a wide range of focusing conditions. Electron beams with an energy of 5 keV and currents of 50 to 70 mA were transported through a 5‐m‐long periodic channel and beam radii were measured by an axially moveable phosphor screen and a charge‐coupled device camera. The phase advance of betatron oscillation per period without space‐charge, σ0, was varied from σ0=45° to σ0=90°. The tune depression due to space‐charge, σ/σ0, was in the range of 0.2–0.3. The theoretical results for average beam radius R¯ over one period and the ratio R¯/Rmax were found to agree with the experimental data to better than 5% when a correction due to spherical aberration was taken into account. View full abstract»

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  • Crystal growth and the nonlinear optical properties of 4‐nitrophenol sodium salt dihydrate and its deuterated material

    Page(s): 3975 - 3980
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    Large organic ionic crystals on the order of a few centimeters of 4‐nitrophenol sodium(:Na) salt dihydrate (NPNa) and DNPNa, in which the water of crystallization of NPNa was deuterated, were grown for second‐harmonic generation (SHG). The Vickers hardness and thermal conductivities of NPNa were about two times larger than those of conventional organic molecular crystals. The effective nonlinear optical constants of NPNa and DNPNa were estimated to be 5.0 and 5.5 pm/V, respectively. The optical loss coefficients, measured by spectrophotometry, of NPNa and DNPNa were 1.8 and 0.6 dB/cm at 1064 nm, respectively. The substitution of D2O for H2O in NPNa crystal is very effective in reducing the optical loss coefficient. High power of 4.4 mW green SH light was obtained using a 1.5‐mm‐thick DNPNa crystal as the intracavity SHG device of a diode‐pumped Nd:YVO4 laser. View full abstract»

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  • Characterization of proton exchange lithium niobate waveguides

    Page(s): 3981 - 3987
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    Proton exchanged samples of LiNbO3 have been profiled by micro‐Raman spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering channeling, and by x‐ray diffraction (XRD). Following proton exchange (PE) there are two different phases in addition to pure LiNbO3 detected by XRD. After successive annealing steps the outermost phase disappears and an interfacial region forms progressively between PE and LiNbO3. Specific vibrational bands are correlated to electro‐optic and nonlinear optical properties of the system, and the recovery of these properties upon annealing is correlated to chemical bonding changes. View full abstract»

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  • Thermal characteristics of blue‐green II‐VI semiconductor lasers

    Page(s): 3988 - 3993
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    Threshold current densities and wavelengths of gain maximum and longitudinal modes have been determined as a function of temperature for various laser structures. The onset of intrapulse heating has been studied and interpreted. In Zn0.92Mg0.08S0.12Se0.88/ZnS0.06Se0.94 /Zn0.8Cd0.2Se lasers, thermal resistances have been measured, using substrate‐up and substrate‐down mounting. From these, continuous‐wave lasing regimes have been determined. View full abstract»

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  • Modeling of the rotation of polarization in polymers using an inhomogeneous birefringence model

    Page(s): 3994 - 4002
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    Polymers exhibiting a weak birefringence mostly posses optical properties which are predicted well by conventional models for birefringence. However, the rotation of polarization behavior was found to disagree with these models. This deviation has been overlooked in usual measurements characterizing birefringence in polymers. In order to accurately predict the rotation of polarization, we propose a model which includes a birefringence inhomogeneity. The model is shown to be related to the birefringence‐induced optical activity. The model predictions are shown to agree well with experimental results using a parallel laser beam. Practical implications with respect to unusually high low‐frequency noise of magneto‐optical disks are discussed briefly. View full abstract»

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  • Dynamic instabilities in the power spectrum of deeply modulated semiconductor lasers

    Page(s): 4003 - 4006
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    Semiconductor lasers under large‐signal direct modulation by a square waveform are found to exhibit a transition from a power spectrum characterized by a fundamental frequency and FM sidebands to a continuous spectrum with a catastrophically broadened linewidth of the order of several GHz. The interesting feature of the phenomenon is that the photon output remains periodic apart from noise‐induced fluctuations, and the broadening of the power spectrum is attributed to the sensitivity of the phase of the optical field to a large difference in the relaxation oscillation frequencies in the on and off states as well as the coupling between motions at the intrinsic resonance frequency of the system and the externally‐imposed modulation frequency. It is shown that under deep modulation by a periodic injection current, the optical phase becomes aperiodic generating a wide range of new frequencies in the power spectrum. It is also demonstrated that by confining the excursions of the injection current to the region of almost‐linear optical response, linewidth broadening may be avoided. Quantitative criteria for determining the boundary of the broadened‐linewidth region are presented for several modulation frequencies. View full abstract»

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  • The effective transverse thermal conductivity of amorphous Si3N4 thin films

    Page(s): 4007 - 4011
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    The effective transverse thermal conductivity of Si3N4 thin films is determined as a function of film thickness. Results indicate that the effective thermal conductivity behavior of Si3N4 thin films is similar to that exhibited by amorphous SiO2 films; that is, there is no significant difference between the thermal conductivity of amorphous Si3N4 and amorphous SiO2 thin films as a function of thickness or temperature. The average effective transverse thermal conductivity decreases drastically as the film thickness is reduced. This strong thickness dependence is ascribed to a thermal resistance that is localized at the amorphous film/Si‐substrate interface. Within the narrow temperature range studied, the interfacial thermal resistance and the intrinsic conductivity of amorphous films increase with temperature; however, the interfacial resistance dominates as the film thickness is reduced. In light of the observed similarities between the Si3N4 results and those previously obtained on SiO2, the reduction in the effective thermal conductivity of amorphous thin films with decreasing thickness is discussed in terms of both interfacial thermal resistance and scattering mechanisms in amorphous solids. View full abstract»

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  • Photoacoustic investigation of thermal and transport properties of amorphous GeSe thin films

    Page(s): 4012 - 4021
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    In this work the possibility of simultaneous determination of thermal and transport properties of thin films using a photoacoustic method was investigated. The properties of amorphous GeSe thin films, which were evaporated on quartz substrata, were examined by measuring their amplitude and phase photoacoustic spectra as a function of the modulated optical laser beam frequency. The measurements were performed in a specially constructed photoacoustic cell which enabled excitation of the sample on one side and detection of the acoustic response on the other. Thermal diffusivity and transport properties (diffusivity coefficient, recombination time, and the surface recombination velocity) of the GeSe thin films were determined by comparing of the experimental results and the calculated theoretical photoacoustic spectra. View full abstract»

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  • Thermal diffusivity measurement of near‐pseudobinary HgCdTe solid and melt, Te‐rich HgCdTe and HgZnTe melts, and pure Te solid and melt by the laser flash technique

    Page(s): 4022 - 4028
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    The thermal diffusivities of mercury cadmium telluride, (Hg1-xCdx)1-yTey, and mercury zinc telluride, (Hg1-xZnx)1-yTey, with 0.55≤y≤1.0 and 0.0125≤x≤0.054 65, and of pure Te were measured from 350 to 850 °C by the laser flash technique. The diffusivity of near‐pseudobinary Hg1-xCdxTe solids decreased more rapidly with temperature approaching the melting point than did pseudobinary solids previously reported: The solid diffusivity for x=0.028 17 and y=0.55 was 0.83 mm2/s at 371 °C, decreasing to 0.22 mm2/s at 614 °C. The diffusivity of Te‐rich (Hg1-xCdx)1-yTey melt increased with x and with temperature: The melt diffusivity for x=0.039 36 and y=0.782 was 0.91 mm2/s at 485 °C, increasing to 4.93 mm2/s at 851 °C. For Te‐rich (Hg1-xZnx)1-yTey melt with x=0.0125 and y=0.7944 there appeared to be a minimum diffusivity of about 2.6 mm2/s near 690 °C. The thermal diffusivity of pure Te solid was 0.97 mm2/s at 300 °C and decreases to 0.64 mm2/s at 439 °C. The melt diffusivity of pure Te was 1.52 mm2/s at 486 °C, increased to 3.48 mm2/s at 584 °C. Experimental data presented can be used to calculate the thermal conductivity needed for designing systems capable of growing a better quality single crystal of these materials. View full abstract»

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  • Simple model of foam drainage

    Page(s): 4029 - 4035
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    A simple model of foam drainage is introduced in which the Plateau borders and quadruple junctions are identified with pools that discharge through channels to pools underneath. The flow is driven by gravity and there are friction losses in the exhausting channels. The equation of Bernoulli combined with the Hagen–Poiseuille equation is applied to describe the flow. The area of the cross section of the exhausting channels can be taken as a constant or may vary during drainage. The predictions of the model are compared with standard drainage curves and with the results of a recently reported experiment in which additional liquid is supplied at the top of the froth. View full abstract»

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  • Electric fields in high‐frequency parallel‐plate helium discharges

    Page(s): 4036 - 4044
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    Spatially and temporally resolved electric fields in the sheath region of a 13.56‐MHz, parallel‐plate helium discharge have been measured as a function of voltage, pressure, phase, bias, and electrode drive configuration. The electric field was determined from laser‐induced fluorescence measurements of the Stark structure of the n=11 singlet Rydberg manifold. Two‐dimensional maps of the electric field indicate that the field is uniform across a large fraction of the electrode diameter. Measured values of the sheath electric field are compared with the results of a hybrid Boltzmann‐equation–average‐equation simulation. View full abstract»

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  • Characterization of nanocrystallites in porous p‐type 6H‐SiC

    Page(s): 4045 - 4049
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    We report the formation of porous p‐type 6H‐SiC. The existence of uniformly dispersed pores was confirmed by transmission electron microscopy, with interpore spacings in the range of 1–10 nm. The porous film as a whole is a single crystal. Luminescence peaks above the normal band gap of 6H‐SiC have been observed in the porous layer, but were not distinguished in the bulk SiC substrate. Quantum confinement is discussed as a possible mechanism for the luminescence effects. View full abstract»

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  • Spatially resolved measurement of lattice damage in alpha‐particle‐irradiated type IIa natural diamond by confocal photoluminescence microscopy

    Page(s): 4050 - 4053
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    Confocal fluorescence microscopy has been used to measure the three‐dimensional distribution of the H3 color center produced in type IIa natural diamonds by 5 MeV He+ irradiation at a total fluence of 8×1015 cm-2. The peak of the H3 emission occurs 16 μm below the surface of the irradiated side of the diamond, which is in fair agreement with the peak of the vacancy distribution predicted by a Monte Carlo calculation (transport of ions in matter or trim). The H3 distribution is broader in the direction normal to the surface (10 μm full width at half maximum) than the trim calculation. This is attributed to diffusion of vacancies caused by self‐annealing during irradiation. View full abstract»

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  • A scanning tunneling microscope study of a palladium sphere in hydrogen gas: Expansion and surface topology

    Page(s): 4054 - 4060
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    A scanning tunneling microscope monitored the time dependence of both the expansion and surface morphology changes of 1–2 mm palladium spheres exposed to 0.5 atm hydrogen gas. The surfaces were initially smooth except for 2–10 nm high steps separating flat terraces 50–150 nm wide. Spheres exposed to hydrogen expanded, during which time the surface buckled forming 30–50‐nm‐deep and 1000‐nm‐wide features and the step structures vanished. There were two distinct regimes in the expansion: 25%–30% of the total expansion occurred in the first regime, which only lasted about 5% of the time required to reach 90% of the total expansion; and in the second regime the sphere asymptotically approached the final size. Some samples took ∼500 h to expand, but other samples with only slightly different surface morphology expanded in as little as 8 h. We developed a quantitative model describing the time dependence of the observed expansion in the asymptotic regime by assuming the eventual formation of β‐Pd hydride and the elastic properties of the material. The model also suggested the cause of the surface buckling. View full abstract»

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  • A study of Cu50Fe50 produced by mechanical alloying and its thermal treatment

    Page(s): 4061 - 4067
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    A specimen of Cu50Fe50 equiatomic composition was mechanically alloyed (MA) by ball milling starting from the pure elements, which are immiscible according to the equilibrium phase diagram. Structural analysis by x‐ray and neutron diffraction has shown that the mechanical process initially reduces the crystallite size of both elements as a function of the milling time. The diffraction data show that the bcc iron phase is subsequently consumed, due to progressive incorporation of the iron atoms into the fcc copper matrix. The Mössbauer spectra of a specimen MA for 16 h has a broad magnetic profile typical of a Fe‐Cu extended solid solution, with some evidence of two local environments of the iron atoms and a small admixture of the γ‐Fe. The annealing of these MA treated specimens effects a decomposition of the extended solid solution into FCC copper and both α‐ and γ‐iron allotropes. This decomposition process is discussed in relation to spinodal decomposition and to nucleation‐and‐growth mechanisms. View full abstract»

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  • Vacancy‐controlled model of degradation in InGaAs/AlGaAs/GaAs heterostructure lasers

    Page(s): 4068 - 4071
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    A model is proposed for dark line defect (DLD) growth in semiconductor lasers by dislocation climb. The model assumes that climb occurs by the migration of randomly distributed vacancies toward existing dislocations. The effects of a strained layer on the number of vacancies and their stability are investigated. It is concluded that vacancies can act as strain relievers, thereby reducing their energy of formation. In strained‐layer lasers this has the effect of reducing the driving force for DLD growth by climb but increasing the pool of vacancies available for the process to occur. These findings are offered as an explanation for the anomalous behavior of DLDs in strained‐layer lasers. View full abstract»

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  • Relaxation of radiation damage in silicon planar detectors

    Page(s): 4072 - 4076
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    The behavior of radiation‐induced carbon‐related defects in high‐resistivity silicon detectors has been investigated. The defects were introduced by α‐particle irradiation and investigated by deep‐level transient spectroscopy. An unusual defect behavior consists in low‐temperature annealing, including self‐annealing at room temperature, of the interstitial carbon Ci with a simultaneous increase of the Ci‐Oi‐complex concentration. The kinetic parameters of the process have been determined from the increase of the Ci‐center concentration versus time. Two annealing velocities have been observed, which arise from different heat treatments during the detector fabrication process. View full abstract»

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  • Equation of state of aluminum nitride and its shock response

    Page(s): 4077 - 4085
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    Equation of state of aluminum nitride (AlN) is determined from the ultrasonic wave velocity measurements of longitudinal and shear modes to 0.7 GPa. The equation of state obtained from the ultrasonic data is used in conjunction with the shock Hugoniot data on AlN to estimate its strength under plane shock wave compression. Further, a better understanding of the existing shock Hugoniot data above its transition stress from wurtzite to rock salt structure is realized through a comparison with the static high‐pressure investigations pertaining to this phase transition in AlN. View full abstract»

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  • Failure of three‐dimensional random composites

    Page(s): 4086 - 4094
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    Using an optimized bond breaking algorithm, we simulated the failure of three‐dimensional networks composed of two randomly interspersed components. We studied the strength and the number of bonds broken prior to the failure instability (the damage). When one phase is of low concentration, damage mostly occurs when the dilute phase is very weak and fails prematurely, although a limited ‘‘matrix damage’’ mechanism does exist. In contrast, when the composite is a 50/50 mixture and the two phases are macroscopically interpenetrating (both phases percolate), the ability to absorb damage is greatly enhanced. In the interpenetrating phase regime the strength of the composite is, in some cases, enhanced beyond that of the analogous particle reinforced composite. These features suggest that interpenetrating phase composites can have improved properties in comparison to conventional ‘‘particulate’’ composites. Analysis of the composite is neatly summarized in ‘‘damage maps’’ which give a quick indication of the regions in which the ability to absorb damage is enhanced. Using lattices of up to linear dimension L=40 we also study the finite‐size‐scaling laws for the average strength and damage of the networks. View full abstract»

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  • Structure of LaFe9Si4 intermetallic compound

    Page(s): 4095 - 4098
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    The crystal structure of LaFe9Si4 intermetallic compound equilibrated at 1173 K has been determined by the x‐ray powder diffraction and refined by the Rietveld technique. The crystal structure belongs to the tetragonal symmetry with space group I4/mcm, which can be derived from the cubic NaZn13‐type structure. Each unit cell contains four formula units of LaFe9Si4. The lattice parameters are a=7.932(1) Å and c=11.677(2) Å. The calculated density is Dx=6.86(4) g/cm3. In the structure there are five kinds of equivalent positions, i.e., 4a, 16l(1), 16k, 16l(2), and 4d, which are occupied by 4La, 16Fe(1), 16Fe(2), 16Si, and 4Fe(3) atoms, respectively.   View full abstract»

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  • Investigation of nucleation and growth processes of diamond films by atomic force microscopy

    Page(s): 4099 - 4106
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    The nucleation and growth of plasma‐enhanced chemical‐vapor deposited polycrystalline diamond films were studied using atomic force microscopy (AFM). AFM images were obtained for (i) nucleated diamond films produced from depositions that were terminated during the initial stages of growth, (ii) the silicon substrate‐diamond film interface side of diamond films (1–4 μm thick) removed from the original surface of the substrate, and (iii) the cross‐sectional fracture surface of the film, including the Si/diamond interface. Pronounced tip effects were observed for early‐stage diamond nucleation attributed to tip convolution in the AFM images. AFM images of the film’s cross section and interface, however, were not highly affected by tip convolution, and the images indicate that the surface of the silicon substrate is initially covered by a small grained polycrystalline‐like film and the formation of this precursor film is followed by nucleation of the diamond film on top of this layer. X‐ray photoelectron spectroscopy spectra indicate that some silicon carbide is present in the precursor layer. View full abstract»

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  • Fundamentals of two‐step etching techniques for ideal silicon‐hydrogen termination of silicon(111)

    Page(s): 4107 - 4112
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    White deposits are seen to form in highly basic etching solutions which contain NH4F such as 40% NH4F or pH‐enhanced buffered HF (BHF) (a mixture of BHF and NH4OH). Surface infrared spectroscopy, x‐ray photoelectron spectroscopy, and Auger electron spectroscopy are used to characterize a hydrogen terminated Si(111) surface and the chemistry of these white deposits; such analyses show significant amounts of nitrogen, nonbonded to silicon, and fluorine in the deposits. The morphology of the flat surface is examined with optical microscopy, scanning tunneling microscopy, and atomic force microscopy. A two‐step etching, a 7 min dipping in 1% HF followed by a short dipping (∼3 s) in pH‐enhanced 1% HF solution (a mixture of 1% HF and NH4OH, pH=9.25), is proposed to obtain an atomically smooth and impurity‐free surface, based on studies with all spectroscopies and microscopies used. The second etching step converts a microscopically rough surface with all defects on Si(111) into an atomically smooth, ideally monohydride‐terminated surface within 3 s. Similar results are obtained with a 3 s dipping in pH‐enhanced BHF solution (a mixture of BHF and NH4OH, pH=9.25); however, studies with the spectroscopies and microscopies used show that longer treatments with this solution result in the formation of white surface deposits. Possible ways of inhibiting the formation of white surface deposits from cleaning solutions are found to be the use of purer chemicals and shorter last‐step cleaning times. For example, an increase of the last‐step cleaning duration from 10 to 20 min is observed to increase the substrate area covered by the white deposits from about 1% to 5% for purer 40% NH4F, and from about 20% to 30% for less pure 40% NH4F solutions. View full abstract»

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  • Scanning tunneling microscope contrast of perylene‐3,4,9,10‐tetracarboxylic‐dianhydride on graphite and its application to the study of epitaxy

    Page(s): 4113 - 4120
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    Epitaxial films of perylene‐3,4,9,10‐tetracarboxylic‐dianhydride (PTCDA) on graphite (0001) were investigated by scanning tunneling microscopy. Molecular image contrast of PTCDA was found to depend strongly upon the molecular orientation and the position on graphite. In particular, the periodic discrepancy between PTCDA and graphite lattice points results in a modulation of contrast, which can be used to determine the epitaxial relation of PTCDA relative to the substrate accurately. By analyzing this modulation of contrast, we determined two kinds of epitaxial orientation of PTCDA. These orientations have no exact commensurate relation with graphite, but every lattice point of PTCDA lies on a lattice line parallel to the a axis (or b axis) of graphite. This specific feature contributes to decreasing the interfacial energy. The contrast mechanism of adsorbed molecules is also discussed.   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