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Applied Physics Letters

Issue 17 • Date Oct 1989

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Displaying Results 1 - 25 of 37
  • Excimer and CO2 laser interactions with foamed polystyrene

    Page(s): 1701 - 1703
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    We report the results of a study of excimer (308 nm) and CO2 laser etching of foamed polystyrenes. The relative role of photochemical and photothermal effects in material removal can be seen by comparing vaporization energies λ obtained under different material removal regimes. The most efficient material removal occurs with 308 nm pulses at a high repetition rate. Under these conditions λ=10–20 J cm-3 corresponding to 0.3–0.6 eV per monomer. This should be compared to 2–5 eV per monomer with photochemical ablation. View full abstract»

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  • Theory and experiment of the parasitic‐free frequency response measurement technique using facet‐pumped optical modulation in semiconductor diode lasers

    Page(s): 1704 - 1706
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    The letter describes the theory and experiment of the newly developed parasitic‐free frequency response measurement technique using facet‐pumped optical modulation on diode lasers. It was found that the modulation damping rate depends on the relative position of the pump laser wavelength with respect to the Fabry–Perot resonance wavelength of the laser under study. This effect is fully explained in terms of the carrier‐induced index change. For multilongitudinal mode lasers, it is shown experimentally and theoretically that the damping rate and frequency response measured by this technique give the intrinsic frequency response corresponding to direct current modulation only if the cavity lengths of pump and probed lasers are equal, and then only if the pump laser wavelength coincides with the Fabry–Perot transmission maximum or minimum of the probed laser. View full abstract»

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  • Image formation in a superresolution phase conjugate scanning microscope

    Page(s): 1707 - 1709
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    Analytical and experimental results of a new type of optical scanning microscope, which uses a phase conjugate mirror and pinholes to achieve superresolution, are presented. This phase conjugate scanning microscope has a higher Rayleigh resolution limit than conventional, single‐pass, and double‐pass scanning microscopes. View full abstract»

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  • Double waveguide in LiNbO3 by ion implantation

    Page(s): 1710 - 1712
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    Ion implantation has been employed to produce superposed planar waveguide structures in LiNbO3. This was achieved by constructing two optical barriers at different depths beneath the crystal surface using 1.1 and 2.2 MeV He+. The composite refractive index profile was compared with those of the two individual barriers, by means of a dark mode analysis technique based on a calculation of the reflectivity function at the coupling prism. The profiles were found to be directly additive except for a noticeable annealing effect of the first implanted barrier, and a range enhancement of the second barrier. These effects were confirmed by varying the order and temperature of the implantations. The implications of these results to possible device construction have been discussed. View full abstract»

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  • Interference filters using indium phosphide‐based epitaxial layers grown by metalorganic vapor phase epitaxy

    Page(s): 1713 - 1714
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    Bandpass interference filters with center wavelengths between 1.3 and 1.7 μm have been made with InGaAsP/InP epitaxial layers grown by metalorganic vapor phase epitaxy. The transmission spectra measured exhibit a full width half maximum of 7 nm with peak rejections up to 20 dB and are in good agreement with the results obtained from theoretical modeling of the 81‐layer single half‐wave Fabry–Perot structure used. This indicates very good control of the composition and thickness of the layers grown, which is confirmed by transmission electron microscopy of the structure. View full abstract»

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  • Reflection high‐energy electron diffraction intensity oscillation induced by electric current during Si epitaxial growth on Si (001) 2×1 surfaces

    Page(s): 1715 - 1717
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    We have observed the oscillations of specular beam intensity of reflection high‐energy electron diffraction (RHEED) during Si molecular beam epitaxy on Si (001) 2×1 surfaces. The oscillation modes were measured as a function of incident beam direction, substrate temperature, or especially, dc current direction for resistive heating of the substrate. For the first time in [110] incident beam direction and at a substrate temperature above 450 °C, the phase of specular beam oscillations with the biatomic‐layer growth mode can be chosen by the direction of the heating current. View full abstract»

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  • Tunneling acoustic microscope

    Page(s): 1718 - 1720
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    A new type of microscope, which is based upon both a scanning tunneling microscope (STM) and a technique for detecting acoustic waves, is described. An acoustic wave generated in a STM’s sample, by vibration of its tip, is detected by a piezoelectric transducer coupled to the sample. The amplitude of the acoustic wave corresponds to the strength of the force interaction between the tip and the sample, and is sensitive to tip‐sample spacing. We have been successful in keeping the spacing constant by using a new feedback loop that holds this amplitude constant without tunneling. This method enhances the features of the STM without reducing its functions and enables simultaneous use of both force interactions and tunneling current to investigate the properties of samples. Topographies taken by the new feedback system and tunneling current images are shown. View full abstract»

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  • AlxGa1-xN polarity determination by x‐ray diffraction

    Page(s): 1721 - 1723
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    The crystallographic polarities of (0001) oriented AlxGa1-xN epitaxial layers grown on basal plane sapphire were determined using the anomalous dispersion effect of x‐ray diffraction. Integrated intensities of three (00l) reflections were measured using the Lγ1 and Lβ2 excitation lines from a tungsten x‐ray source. Calculations that included the effects of anomalous dispersion were compared with the measurements to identify the polarity. Each epitaxial layer had a single polarity rather than a mixture of oppositely polarized grains. However, both A‐ and B‐oriented layers were observed. The method also provides a nondestructive estimate of the layer thickness. View full abstract»

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  • Enhanced corrosion resistance in noble metal containing amorphous chromium alloys

    Page(s): 1724 - 1726
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    The addition of noble metals to Cr‐based amorphous alloys can dramatically improve their corrosion resistance. The results of x‐ray photoemission spectroscopy and cathodic polarization measurements show that the noble metals concentrate on the surface of the alloys and affect both the cathodic and anodic reaction kinetics. The amorphous Cr alloys require far less noble metal content to provide corrosion resistance in HCl compared to the corresponding crystalline Cr alloys. This allows the development of amorphous Cr alloys that are extremely stable in both reducing and oxidizing acids at high temperatures. View full abstract»

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  • Nanometer‐scale hole formation on graphite using a scanning tunneling microscope

    Page(s): 1727 - 1729
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    We present a reproducible technique for forming holes on a graphite surface with a scanning tunneling microscope. The holes have an average diameter of 40 Å (20 Å minimum) with an average resolvable spacing of 60 Å. Holes are produced by applying a short voltage pulse (3–8 V, 10–100 μs) across the tunneling gap which removes one or more layers of graphite in a small region directly below the tip. Arrays of hundreds of holes have been formed with yields as high as 99.6%. The writing process has a higher success rate in air or in the presence of water vapor. This suggests that the physical mechanism is a chemical process. View full abstract»

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  • Elastic constants of sodalite Na4Al3Si3O12Cl

    Page(s): 1730 - 1731
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    The elastic stiffness constants of sodalite Na4Al3Si3O12Cl (cubic, a=8.882 Å;Z=2), a framework‐type aluminosilicate with cubo‐octahedral cages, have been determined by an ultrasonic method. The measured values are C11,88.52(71); C12,38.70(50); and C44,36.46(33) GPa. Because of its structural flexibility, the bulk modulus KS(55.30 GPa) and shear modulus μ (31.30 GPa) values in sodalite are considerably smaller than those in densely packed cubic silicate structures such as spinel and garnet. View full abstract»

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  • Electron capture by DX centers in AlGaAs and related compounds

    Page(s): 1732 - 1734
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    n‐type dopants in Alx Ga1-x As, GaAs1-x Px, and related compounds create deep donors that control the free‐electron concentration for x≫0.2. The electron capture by the deep donors shows a very nonexponential dependence with filling time. In this letter the electron capture kinetics is modeled, and the influence of the donor ionization factor, sample degeneracy, and size effects, are introduced. Capture barrier energy determination is discussed, and a comparison with experimental capture data is made. View full abstract»

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  • Resistance of very small area ohmic contacts on GaAs

    Page(s): 1735 - 1737
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    Very small area, alloyed ohmic contacts on n+‐GaAs etched mesas have been fabricated and their resistance measured. The nominal contact size ranges from 10 down to 0.1 μm on a side. The data show an extremely large increase of the resistance spread as the contact size is reduced. Grain size effects in the alloyed contacts and dead zones due to ion etching are discussed as possible causes for the observations. View full abstract»

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  • Interfacial work functions and extrinsic silicon infrared photocathodes

    Page(s): 1738 - 1740
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    It is shown that n+ and/or p+ contacts on p‐i‐n diodes can function as solid‐state photoemitters at temperatures ≲20 K. Infrared radiation can excite electrons or holes over small n‐i or p‐i interfacial barriers and into the intrinsic region when the diode is forward biased. Photoelectric thresholds in the far infrared corresponding to 37 and 61 μm cutoffs have been observed for silicon devices using a Fourier transform spectrometer. Suggestions are made to tailor the cutoff wavelengths using different concentrations of various impurities near the metal‐insulator transition. Epitaxially grown multilayered (superlattice) detectors are proposed. View full abstract»

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  • 27.6% efficiency (1 sun, air mass 1.5) monolithic Al0.37Ga0.63As/GaAs two‐junction cascade solar cell with prismatic cover glass

    Page(s): 1741 - 1743
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    A 27.6% efficiency measured under 1 sun, air mass 1.5 illumination has been achieved in a two‐terminal, monolithic two‐junction cascade solar cell consisting of an Al0.37Ga0.63As (Eg=1.93 eV) upper cell and a GaAs lower cell. The component cells were electrically connected together using a metal‐interconnect contact fabricated during post‐growth processing. Also, a prismatic cover glass was bonded to the front surface of the cascade structure to minimize the obscuration effect introduced by the grid lines and metal‐interconnect contacts. As the cascade cell was operated under air mass 0 illumination, an efficiency of 23.0% was obtained. Both results represent the highest 1 sun efficiencies ever reported. The implications of achieving this high efficiency in a two‐junction cascade solar cell for terrestrial and space applications are also described. View full abstract»

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  • Differential photoconductive sampling with a resolution independent of carrier lifetime

    Page(s): 1744 - 1746
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    We present a novel approach to photoconductive sampling with a resolution which is virtually independent of the carrier lifetime and is governed solely by the circuit‐limited gap charging time. Using photoconductors with a carrier lifetime of 150 ps, accurate measurement of a 65‐ps‐wide electrical signal is demonstrated and the actual resolution is believed to be 10 ps. View full abstract»

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  • Dynamic behavior of negative charge trapping in thin silicon oxide

    Page(s): 1747 - 1749
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    The trapping of negative charges in thin oxide under bipolar and unipolar dynamic stressing is compared. Bipolar stressing causes significantly less trapping than unipolar stressing. The difference is attributed to the enhanced electron detrapping during the bipolar stress. This detrapping mechanism is strongly dependent on the amount of charge injected through the oxide and, for a fixed amount of injected charge, is a weak function of the stressing current density. View full abstract»

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  • Ultrahigh doping of GaAs by carbon during metalorganic molecular beam epitaxy

    Page(s): 1750 - 1752
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    Recent advances in heterostructure bipolar transistor technology have created a need for p‐type doping at levels ≥1020 cm-3. Furthermore, such levels may eliminate the need for alloying during ohmic contact formation. We have achieved p‐type doping levels as high as 5×1020 cm-3 using an unconventional dopant, C, derived from the gaseous source chemical, trimethylgallium (TMG), during metalorganic molecular beam epitaxial (MOMBE) growth of GaAs. We have controllably achieved doping levels between 1019 and 5×1020 cm-3 by diluting the TMG flow with another metalorganic, triethylgallium (TEG). By utilizing the so‐called δ‐doping or atomic planar doping method we have also been able to grow C‐doped spikes with hole concentrations as high as 7×1019 cm-3, with a full width at half maximum of ∼50 Å at 300 K. This doping level is the highest yet reported for planar doping, and the narrow width indicates that the C atoms are restricted to one or two atomic planes. By switching out the TMG, and switching in the TEG to continue the growth of C‐free GaAs we have grown sandwich‐type structures with C levels of 1020 cm-3, which fall off within 210 Å to C levels of ≪1017 cm-3. High‐temperature annealing of such structures reveals a C diffusion coefficient of ≤10-16 cm2 s-1 at 950 °C, in agreement with other reports. This is at least three orders of magnitude less than for the other conventional p‐type dopants, Be and Zn. Finally, we report the presence of strain in the highly - - C‐doped layers, detected by x‐ray diffraction. The lattice constant obtained corresponds roughly to that calculated by assuming a Vegard’s law mixture of GaAs and 0.7% GaC. This distortion of the GaAs lattice has not been previously measured. View full abstract»

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  • Growth of Al on GaAs(001): Observation of interfacial submonolayer structure

    Page(s): 1753 - 1755
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    Submonolayer structure has been observed using reflection high‐energy electron diffraction during room‐temperature deposition of Al onto (2×4) reconstructed GaAs(001) surfaces prepared by molecular beam epitaxy. This structure with a (4×1) symmetry occurs after ∼0.25 monolayer of Al deposition. It is growth‐rate independent, reproducible, and stable. This result shows that there is a strong, directionally dependent adatom‐adatom interaction at submonolayer coverages. View full abstract»

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  • Assessment of GaAs heteroepitaxial films grown on silicon‐on‐sapphire upgraded by double solid phase epitaxy

    Page(s): 1756 - 1758
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    Characterization of heteroepitaxial GaAs grown on silicon‐on‐sapphire that had been microstructurally upgraded by the double solid phase epitaxy process reveals that microtwining and surface roughness are substantially reduced. Moreover, the thermally induced elastic strain in the GaAs film is found to be reduced by over a factor of 4 from the strain measured in GaAs‐on‐Si (100) grown in the same manner. The low‐temperature photoluminescence characteristics are similar to GaAs homoepitaxial films grown by molecular beam epitaxy. View full abstract»

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  • Dependence of optimal growth temperature on carbon content in hydrogenated amorphous silicon carbide alloys

    Page(s): 1759 - 1761
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    The temperature (TE) above which optimal growth takes place during plasma‐enhanced chemical vapor deposition of undoped hydrogenated amorphous silicon carbide alloys from methane/silane gas mixtures is shown to increase with the relative gas‐phase mole fraction of methane XCH4. The increase in TE from ≊220 °C for XCH4=0.0 to ≊350 °C for XCH4=0.90 at 2 W rf power is attributed to a corresponding decrease in the rate of hydrogen diffusion with carbon incorporation. View full abstract»

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  • New model for dopant redistribution at interfaces

    Page(s): 1762 - 1764
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    A new model for dopant redistribution at an interface is derived and verified by data from segregation and outdiffusion experiments. In the present model, a third phase, the interface layer itself (interphase), is considered in addition to the adjacent bulk phases for the first time. The dynamics of the three‐phase system is described in terms of rate equations. In case that all three phases are identical, i.e., constitute one single homogeneous phase, this model effectively reduces to an ordinary diffusion equation for a bulk phase. The immediate advantage of this formulation is that the coupling between the redistribution at the interface and the diffusion in adjacent bulk phases can be described in an unambiguous and consistent way derived from first principles. In the limit of equilibrium segregation, the interface dynamics and bulk diffusion decouple, and the model reduces to diffusion equations in bulk phases supplied with Dirichlet boundary conditions at the interface given by analytic solutions of the equilibrium interface dynamics. View full abstract»

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  • Anisotropic and inhomogeneous strain relaxation in pseudomorphic In0.23Ga0.77As/GaAs quantum wells

    Page(s): 1765 - 1767
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    The structural properties of pseudomorphic In0.23Ga0.77As/GaAs single quantum wells are investigated with x‐ray double‐crystal diffractometry. Anisotropic tilting of lattice planes along dislocations and anisotropic reflectivity of fully relaxed domains coexisting with strained domains are reported for the first time. Due to the anisotropic strain relaxation the crystal symmetry changes from tetragonal in the fully strained case to monoclinic in a partially relaxed quantum well. View full abstract»

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  • Zn diffusion enhancement of interdiffusion in a GaInAsP‐InP heterostructure

    Page(s): 1768 - 1770
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    The influence of concurrent Zn diffusion on the interdiffusion in a Gax In1-x Asy P1-y ‐InP heterostructure (x=0.28, y=0.61) was investigated using Auger electron spectroscopy and secondary‐ion mass spectrometry. The measured profiles showed that the Zn diffusion (600 °C, 1–4 h) predominantly enhanced the cation (In‐Ga) interdiffusion. The result could not be interpreted by the Zn‐vacancy complex model. Under conditions of a group V overpressure, our results suggest that cation interstitials may control both the rate of Zn diffusion and the mixing of the group III sublattices in the InP‐based alloy system. View full abstract»

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  • Quantum‐switched heterojunction bipolar transistor

    Page(s): 1771 - 1773
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    We propose and demonstrate a negative differential resistance transistor—the quantum‐switched heterojunction bipolar transistor (QSHBT). It has the highest current peak‐to‐valley ratio ever reported at room temperature (15 in an InGaAs/InP QSHBT). More important, the switching and peak‐to‐valley ratio can be controlled by a base current injected electronically or optically. For example, a peak current as high as 72 mA or 2.9 kA/cm2 can be controlled by either a few microamperes of base current or a few microwatts of optical signal. A gain of peak current of 8650 at room temperature is obtained. The present device is grown by chemical beam epitaxy. 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