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28 Dec 1992

Volume 61, Issue 26, pp. 3077-3195

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Fabrication of periodically inverted domain structures in LiTaO3 and LiNbO3 using proton exchange

Satoshi Makio, Fumio Nitanda, Kohei Ito, and Masayoshi Sato

Appl. Phys. Lett. 61, 3077 (1992); http://dx.doi.org/10.1063/1.107990 (3 pages) | Cited 24 times

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We report on the fabrication of a first‐order periodically inverted domain structure in LiTaO3 and LiNbO3 for quasi‐phase‐matched devices using proton exchange with one‐directional heating. First‐order periodically inverted domains with a 3.2 μm period and more than 40 μm depth were formed beyond the proton exchanged region. This domain inversion took place during the proton exchange process, far below the Curie temperatures. Taking advantage of these deep domains, we demonstrated second‐harmonic generation without fabricating a channel waveguide.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.82.Cr Fabrication techniques; lithography, pattern transfer

Terahertz optical rectification from a nonlinear organic crystal

X.‐C. Zhang, X. F. Ma, Y. Jin, T.‐M. Lu, E. P. Boden, P. D. Phelps, K. R. Stewart, and C. P. Yakymyshyn

Appl. Phys. Lett. 61, 3080 (1992); http://dx.doi.org/10.1063/1.107968 (3 pages) | Cited 62 times

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We report optical rectification and subsequent generation of subpicosecond submillimeter‐wave radiation from a nonlinear organic crystalline salt. With optical excitation at a wavelength of 820 nm and a 150 fs pulse duration, the magnitude of the rectified field from the organic salt dimethyl amino 4‐N‐methylstilbazolium tosylate is one and two orders of magnitude larger than that from GaAs and LiTaO3 crystals, respectively. This organic crystal presently provides the most intense terahertz radiated field among all of the natural nonexternally biased materials we know.
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42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.66.Qn Polymers; organic compounds
78.20.Jq Electro-optical effects

Correlation between structural and optical properties in proton‐exchanged LiNbO3

Wei‐Yung Hsu, Gabriel Braunstein, Venkatraman Gopalan, Craig S. Willand, and Mool C. Gupta

Appl. Phys. Lett. 61, 3083 (1992); http://dx.doi.org/10.1063/1.107969 (3 pages) | Cited 6 times

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We present a correlation of the structural characterization of proton‐exchanged LiNbO3 provided by ion beam and x‐ray analysis with the optical characterization provided by refractive index and second‐harmonic generation measurements. The process of proton exchange induces disorder in the crystal lattice of LiNbO3. Subsequent thermal annealing removes only partially the lattice disorder and, in particular, the near surface region of the samples generally remains highly distorted. Results of x‐rays studies indicate that the proton exchange induces a strain in the lattice and thermal annealing partially recovers it.  
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
66.30.J- Diffusion of impurities
81.40.-z Treatment of materials and its effects on microstructure, nanostructure, and properties

Correlation between photoinduced absorption and large off‐resonant nonlinear refractive index of polydiacetylene

R. Quintero‐Torres and M. Thakur

Appl. Phys. Lett. 61, 3086 (1992); http://dx.doi.org/10.1063/1.107970 (3 pages) | Cited 3 times

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We have calculated the off‐resonant nonlinear refractive indices of PTS and TCDU polydiacetylenes using photoinduced absorption data and applying the Kramers–Kronig transformation. The results are in good agreement with the recently published experimental measurements. This is the first time that a correlation between photoinduced absorption and the off‐resonant nonlinear refractive index has been established for polydiacetylenes. The calculated values are: n2∼1.3×10−5 cm2/MW for PTS, and ∼2.2×10−6 cm2/MW for TCDU at 1.06‐μm wavelength. The sign of n2 is positive at this wavelength.
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42.70.Jk Polymers and organics
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.65.An Optical susceptibility, hyperpolarizability
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Gain and saturation intensity measurements of a discharge pumped F2 laser at high excitation rates

Masayuki Kakehata, Chuen‐Huei Yang, Yoshiaki Ueno, and Fumihiko Kannari

Appl. Phys. Lett. 61, 3089 (1992); http://dx.doi.org/10.1063/1.108463 (3 pages) | Cited 5 times

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Small‐signal gain and saturation intensity of a discharge pumped F2 laser at 157 nm, operated at high pressures (<10 atm) and high excitation rates (7–39 MW/cm3), were measured in an oscillator‐amplifier configuration. The small‐signal net gain reaches 37±4%/cm at an excitation rate of ∼26 MW/cm3 for a 6‐atm laser gas. A saturation intensity, which depends on a nonsaturable absorption coefficient, was estimated.
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42.55.Lt Gas lasers including excimer and metal-vapor lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Nanosecond transient electroluminescence from polymer light‐emitting diodes

D. Braun, D. Moses, C. Zhang, and A. J. Heeger

Appl. Phys. Lett. 61, 3092 (1992); http://dx.doi.org/10.1063/1.107971 (3 pages) | Cited 60 times

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The transient electroluminescence from polymer light‐emitting diodes is reported. When the devices are mounted on a microstrip transmission line, the temporal response is limited by the electrode geometry, with rise and fall times below 50 ns. With low duty‐cycle pulses (0.5%) the electroluminescence intensity remains proportional to the current at values up to 10 A/cm2, two orders of magnitude greater than possible under direct current operation. Since the spectral blue‐shift observed at high current levels (with power dissipation above 1 W/cm2) indicates significant sample heating, still higher levels should be possible with proper thermal management.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.66.Qn Polymers; organic compounds

144 °C operation of 1.3 μm InGaAsP vertical cavity lasers on GaAs substrates

J. J. Dudley, M. Ishikawa, D. I. Babic, B. I. Miller, R. Mirin, W. B. Jiang, J. E. Bowers, and E. L. Hu

Appl. Phys. Lett. 61, 3095 (1992); http://dx.doi.org/10.1063/1.107972 (3 pages) | Cited 22 times

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We report lasing at temperatures as high as 144 °C in long‐wavelength InGaAsP vertical cavity lasers. The devices are optically pumped and employ a novel cavity design using GaAs/AlAs quarter‐wavelength stacks for one mirror. The characteristic temperature T0 of the device increases from 42 K at room temperature to 81 K at temperatures above 80 °C as the gain peak moves into resonance with the longer wavelength cavity mode.
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42.55.Px Semiconductor lasers; laser diodes
42.60.-v Laser optical systems: design and operation

High‐density optical storage based on nanometer‐size arsenic clusters in low‐temperature‐growth GaAs

D. D. Nolte, M. R. Melloch, S. J. Ralph, and J. M. Woodall

Appl. Phys. Lett. 61, 3098 (1992); http://dx.doi.org/10.1063/1.107973 (3 pages) | Cited 20 times

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Nanometer‐size arsenic clusters in low‐temperature‐growth molecular beam epitaxy GaAs provide the basis for a high‐density optical storage medium. The material exhibits a large Franz–Keldysch electro‐optic effect at room temperature. Charge storage on the clusters and the excitonic electro‐optic properties of the material combine to make a low‐power high‐density photorefractive storage medium. The ultrafast lifetimes of the photogenerated carriers produce excellent spatial resolution during the writing of holographic space‐charge gratings. Fringe spacings as small as 0.6 μm can be supported in this material, yielding an optical data density greater than 108 bits/cm2. The saturation intensity is 2 mW/cm2 with a storage time of 2 ms.
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42.79.Vb Optical storage systems, optical disks
42.70.Ln Holographic recording materials; optical storage media
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
72.40.+w Photoconduction and photovoltaic effects

Spreading resistance in proton‐implanted vertical‐cavity surface‐emitting diode lasers

Włodzimierz Nakwaski, Marek Osiński, and Julian Cheng

Appl. Phys. Lett. 61, 3101 (1992); http://dx.doi.org/10.1063/1.107974 (3 pages) | Cited 5 times

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Analytical expressions are derived for calculating electrical spreading resistance in vertical‐cavity surface‐emitting diode lasers. Three contact configurations are considered: annular, circular, and broad area. Calculations performed for proton‐implanted surface‐emitting lasers demonstrate that low values of series resistance can be achieved using the broad‐area contact configuration.
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42.55.Px Semiconductor lasers; laser diodes
85.30.De Semiconductor-device characterization, design, and modeling

2‐picosecond, GaAs photodiode optoelectronic circuit for optical correlation applications

K. D. Li, A. S. Hou, E. Özbay, B. A. Auld, and D. M. Bloom

Appl. Phys. Lett. 61, 3104 (1992); http://dx.doi.org/10.1063/1.107975 (3 pages) | Cited 6 times

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An ultrafast GaAs Schottky photodiode is monolithically integrated with a microwave detector. By using this new optoelectronic circuit in place of a nonlinear crystal in an optical correlation setup, the high‐speed photodiode can measure laser pulse durations without using expensive sampling oscilloscopes. Key advantages are that this circuit works over a broad wavelength range and at low peak optical powers. The correlated temporal response of the circuit is measured to be 1.9 ps full width at half maximum. Due to its wavelength flexibility, cross correlation with different lasers may be performed with this single device.
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85.60.Dw Photodiodes; phototransistors; photoresistors
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Probing of surface acoustic wave fields by a novel scanning tunneling microscopy technique: Effects of topography

E. Chilla, W. Rohrbeck, H.‐J. Fröhlich, R. Koch, and K. H. Rieder

Appl. Phys. Lett. 61, 3107 (1992); http://dx.doi.org/10.1063/1.108478 (3 pages) | Cited 18 times

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A new method for the investigation of SAW (surface acoustic wave) fields is presented using a modified version of a scanning tunneling microscope. By adding a high frequency sinusoidal signal to the dc tip voltage the tunneling current contains a component at the difference of the frequencies of this signal and the SAW. There is a remarkable variation of phase for areas much smaller than the SAW wavelength, due to different contributions of the normal and horizontal components of the SAW.
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43.35.Pt Surface waves in solids and liquids
07.79.Cz Scanning tunneling microscopes
61.05.-a Techniques for structure determination
68.35.B- Structure of clean surfaces (and surface reconstruction)

New method of generating cylindrical imploding shocks using a flyer disk

K. Fujiwara, T. Hiroe, and H. Matsuo

Appl. Phys. Lett. 61, 3110 (1992); http://dx.doi.org/10.1063/1.107976 (3 pages)

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A pancake‐like explosive layer loaded over a circular disk (flyer) was detonated from the periphery. The flyer was locally accelerated downward causing a deformation front to propagate toward the center as it was swept by the detonation front. This acted as a cylindrical piston and a cylindrical imploding shock was generated. Shocks were observed by framing camera photographs. The motion of the flyer was also investigated analytically assuming a simple theoretical model.
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47.40.-x Compressible flows; shock waves
52.50.Lp Plasma production and heating by shock waves and compression

Effect of oxygen on the nucleation and growth of diamond thin films

S. I. Shah and M. M. Waite

Appl. Phys. Lett. 61, 3113 (1992); http://dx.doi.org/10.1063/1.107977 (3 pages) | Cited 11 times

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We have used x‐ray photoelectron spectroscopy to study the effects of oxygen on the nucleation and growth of diamond films during microwave plasma assisted chemical vapor deposition. The high film growth rate in the presence of oxygen in the feed gas mixture was found to be related to the accelerated saturation of carbon onto the Si substrate surface. This reduces the incubation period and promotes a much faster diamond nucleation and growth than with oxygen‐less plasmas. In the presence of oxygen, diamond has been detected as early as 1 min into the deposition, whereas without oxygen it was not observed until after 40–50 min.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
68.55.Nq Composition and phase identification

Substrate surface step effects on microstructure of epitaxial films

J. Guo, H. L. M. Chang, and D. J. Lam

Appl. Phys. Lett. 61, 3116 (1992); http://dx.doi.org/10.1063/1.107978 (2 pages) | Cited 15 times

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Sapphire (0001) basal plane is a commonly used substrate for film depositions. Due to the threefold rotational symmetry of the (0001) substrate surface, epitaxial films deposited are expected to form at most three variants with relative orientations of 120° and 240°. However, epitaxial TiO2 (tetragonal) and VO2 (monoclinic) films deposited on sapphire (0001) substrates by the metalorganic chemical vapor deposition technique were found to have six variants of relative orientations of 60°, 120°, 180°, 240°, and 300° based on the x‐ray diffraction studies. Furthermore, epitaxial MgO (cubic) films deposited on sapphire (0001) substrates by the molecular beam epitaxy technique were found to have two variants which are mirror images about the (1210) plane from the high resolution electron microscopy. We show that these unconventional film microstructures found experimentally are caused by the substrate surface steps.
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68.55.-a Thin film structure and morphology

Chiral thin solid films: Method of deposition and applications

R. M. A. Azzam

Appl. Phys. Lett. 61, 3118 (1992); http://dx.doi.org/10.1063/1.107979 (3 pages) | Cited 22 times

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Chiral thin solid films (CTSFs) can be deposited on a solid substrate in vacuum by letting a vapor stream of film material impinge on the substrate at oblique incidence and rotating the substrate during deposition. The direction of substrate rotation determines the handedness of the resulting helical structure which resembles that of a cholesteric or twisted‐nematic liquid crystal layer. CTSFs are useful in making new optical rotators and beam splitters that separate the orthogonal circular polarization components of light. A quadrant‐detector ellipsometer that uses chiral and achiral obliquely deposited thin films is described for measuring the state of polarization of light.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
42.79.Fm Reflectors, beam splitters, and deflectors
42.79.Ci Filters, zone plates, and polarizers

Electromigration induced transgranular slit failures in near bamboo Al and Al‐2% Cu thin‐film interconnects

John E. Sanchez, Oliver Kraft, and Eduard Arzt

Appl. Phys. Lett. 61, 3121 (1992); http://dx.doi.org/10.1063/1.107980 (3 pages) | Cited 46 times

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We report the morphology of electromigration induced transgranular slit‐like voids in Al and Al‐2% Cu thin‐film interconnects with near bamboo microstructures. The grain structure surrounding fatal voids was determined from images produced by scanning electron and focused ion beam microscopy. Fatal voids were seen to traverse grains near to, but apart from, bamboo boundaries or grain triple points, and often appear to emanate from small wedge‐shaped voids at the interconnect edge. The small volume slit voids are consistent with an observed decrease in fatal void volume with reduced linewidth/grain size ratio. However, the transgranular void pathway represents a new failure mechanism responsible for reduced reliability in the narrow interconnects which are more relevant for advanced integrated circuit technologies. The conditions for stress‐induced transgranular slit voiding are briefly discussed.
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66.30.Qa Electromigration
61.72.Bb Theories and models of crystal defects
73.61.At Metal and metallic alloys
85.40.Ls Metallization, contacts, interconnects; device isolation

Single‐crystal diamond plate liftoff achieved by ion implantation and subsequent annealing

N. R. Parikh, J. D. Hunn, E. McGucken, M. L. Swanson, C. W. White, R. A. Rudder, D. P. Malta, J. B. Posthill, and R. J. Markunas

Appl. Phys. Lett. 61, 3124 (1992); http://dx.doi.org/10.1063/1.107981 (3 pages) | Cited 55 times

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We describe a new method for removing thin, large area sheets of diamond from bulk or homoepitaxial diamond crystals. This method consists of an ion implantation step, followed by a selective etching procedure. High energy (4–5 MeV) implantation of carbon or oxygen ions creates a well‐defined layer of damaged diamond that is buried at a controlled depth below the surface. For C implantations, this layer is graphitized by annealing in vacuum, and then etched in either an acid solution, or by heating at 550–600 °C in oxygen. This process successfully lifts off the diamond plate above the graphite layer. For O implantations of a suitable dose (3×1017 cm−2 or greater), the liftoff is achieved by annealing in vacuum or flowing oxygen. In this case, the O required for etching of the graphitic layer is also supplied internally by the implantation. This liftoff method, combined with well‐established homoepitaxial growth processes, has considerable potential for the fabrication of large area single crystalline diamond sheets.  
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61.80.Jh Ion radiation effects
81.65.-b Surface treatments

C60 encapsulation of the Si(111)‐(7×7) surface

Hawoong Hong, W. E. McMahon, P. Zschack, D.‐S. Lin, R. D. Aburano, Haydn Chen, and T.‐C. Chiang

Appl. Phys. Lett. 61, 3127 (1992); http://dx.doi.org/10.1063/1.107982 (3 pages) | Cited 22 times

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The structure of a Si(111)‐(7×7) surface capped by a 200 Å film of C60 was studied by grazing‐incidence x‐ray diffraction. The Si(111)‐(7×7) reconstruction prepared in vacuum, including the loosely bonded ‘‘adatoms’’ on the surface, is preserved under the C60 overlayer. This result illustrates that C60 can be used as an inert cap for surfaces and suggests potentially interesting applications in surface science research and electronic device engineering.  
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68.35.B- Structure of clean surfaces (and surface reconstruction)
81.65.-b Surface treatments

Interband absorption in α‐Sn/Ge short‐period superlattices

Janos Olajos, Werner Wegscheider, and Gerhard Abstreiter

Appl. Phys. Lett. 61, 3130 (1992); http://dx.doi.org/10.1063/1.107983 (3 pages) | Cited 1 time

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Band‐to‐band infrared absorption has been studied in a series of Sn1Gem (m=11, 15, and 21) strained layer superlattices for the first time. The absorption coefficient shows a quadratic increase with two onsets, characteristic for indirect transitions. From a numerical fit to the absorption spectra, band gap energies are determined in the energy range between 0.55 and 0.75 eV, in accordance with theoretical calculations. The temperature dependence of the band gaps show the usual behavior, at low temperatures quadratic and at higher temperatures linear decrease with increasing temperature.
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78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
78.30.Fs III-V and II-VI semiconductors

Photoluminescence of ZnSe/CdSe short‐period superlattices grown by self‐limiting monolayer epitaxy

P. Juza, H. Zajicek, H. Sitter, M. Helm, W. Faschinger, and K. Lischka

Appl. Phys. Lett. 61, 3133 (1992); http://dx.doi.org/10.1063/1.107984 (3 pages) | Cited 7 times

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A series of symmetric ZnSen/CdSen superlattices (n=2–6, where n is the number of monolayers) was prepared by a novel growth method, the self‐limiting monolayer epitaxy. For n≤3, the low temperature photoluminescence spectra of these superlattices are dominated by near‐band‐gap emission with full widths at half maximum (FWHM) of 17 meV. For longer periods the photoluminescence intensity drops drastically and the FWHM increases by a factor of 2 which is attributed to the formation of misfit dislocations. The energetic positions of the photoluminescence peaks are in good agreement with transition energy calculations based on a Kronig–Penney‐type model.
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68.55.-a Thin film structure and morphology
78.55.Et II-VI semiconductors

Ion‐induced fluorination in electron cyclotron resonance etching of silicon studied by x‐ray photoelectron spectroscopy

David Vender, Marco Haverlag, and Gottlieb S. Oehrlein

Appl. Phys. Lett. 61, 3136 (1992); http://dx.doi.org/10.1063/1.107985 (3 pages) | Cited 9 times

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X‐ray photoelectron spectroscopy has been used to investigate the effect of varying substrate bias when silicon is etched in a CF4 electron cyclotron resonance plasma. After etching, the silicon surface is found to be covered by a two layer structure consisting of damaged, fluorinated silicon and a fluorocarbon overlayer. The thickness of the fluorinated silicon layer depends on the self‐bias voltage. Reduction of the ion energy leads to a thinner fluorinated layer as well as a lower etch rate, suggesting that damage and fluorination of the crystal lattice are important in the ion enhanced etching of silicon in fluorine containing plasmas.
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81.65.-b Surface treatments
79.60.-i Photoemission and photoelectron spectra
61.80.Jh Ion radiation effects

Temperature‐dependent transition from two‐dimensional to three‐dimensional growth in highly strained InxGa1−xAs/GaAs (0.36≤x≤1) single quantum wells

S. M. Wang, T. G. Andersson, and M. J. Ekenstedt

Appl. Phys. Lett. 61, 3139 (1992); http://dx.doi.org/10.1063/1.107986 (3 pages) | Cited 22 times

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Transition from two‐dimensional to three‐dimensional growth mode has been investigated by photoluminescence in highly strained InxGa1−xAs/GaAs (0.36≤x≤1) single quantum wells. The structures were grown by molecular beam epitaxy from 410 to 590 °C. The critical layer thickness based on this transition decreased as the growth temperature increased. This behavior was well described by the single‐kink Matthews model [J. Vac. Sci. Technol. 12, 126 (1975)] including the simplest expression of the Peierls–Nabarro friction stress [J. Appl. Phys. 41, 3800 (1970)].
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.-a Thin film structure and morphology
78.55.Cr III-V semiconductors

Distribution of Ge in O+ implanted silicon

M. E. Twigg, M. E. Zvanut, R. Stahlbush, D. J. Godbey, and W. C. Jenkins

Appl. Phys. Lett. 61, 3142 (1992); http://dx.doi.org/10.1063/1.107987 (3 pages) | Cited 2 times

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We have studied 50 nm Si0.8Ge0.2 buried layers and 800 nm Si0.9Ge0.1 layers, grown by molecular beam epitaxy, which were implanted with 200 keV oxygen O+ ions to a dose of 1.8×1018/cm2. This implantation procedure is similar to that used in the formation of SIMOX (separation by implantation of oxygen), although it was not followed by the usual high temperature anneal. Because the initial implantation profile extends as far as 600 nm below the Si surface, the 50 nm Si0.8Ge0.2 layers were embedded in Si at depths of 600, 500, 400, and 300 nm from the Si surface. The effects of Ge incorporation could then be studied as a function of distance from the surface and position within the implantation profile. Using a scanning transmission electron microscope equipped with energy dispersive x‐ray spectroscopy, we examined cross‐sectional specimens for the distribution of the Ge in and around the implanted oxide layer. We found that the Ge tended to be excluded from the oxide in all the samples with the 50 nm buried layer. For the case where the oxide was implanted into the midst of an 800 nm layer, the Ge remained in the oxide.
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61.72.uf Ge and Si
66.30.J- Diffusion of impurities

Single‐electron effects in a point contact using side‐gating in delta‐doped layers

Kazuo Nakazato, Trevor J. Thornton, Julian White, and Haroon Ahmed

Appl. Phys. Lett. 61, 3145 (1992); http://dx.doi.org/10.1063/1.107988 (3 pages) | Cited 23 times

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Side‐gated point‐contact structures in delta (δ)‐doped layers have been used to form single‐electron tunnel junctions with variable resistance. Clear Coulomb‐blockade effects have been observed in the current‐voltage characteristics. The measured characteristics are described in terms of a series of single‐electron transistors formed by microsegments within the point contact. The effective tunnel capacitance and side‐gate capacitance are estimated to be 10 and 1 aF, respectively, which are both one order of magnitude smaller than the reported capacitance of tunnel junctions made from Al or GaAs/AlGaAs heterostructures.
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72.15.Eb Electrical and thermal conduction in crystalline metals and alloys
72.20.Ht High-field and nonlinear effects
72.80.Ey III-V and II-VI semiconductors
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)

Damage saturation during high‐energy ion implantation of Si1−xGex

O. W. Holland and T. E. Haynes

Appl. Phys. Lett. 61, 3148 (1992); http://dx.doi.org/10.1063/1.107989 (3 pages) | Cited 4 times

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Saturation of radiation damage during MeV Si+‐ion implantation of unstrained Si1−xGex (x=0.15,0.50) alloy layers, as well as bulk Ge, was investigated. First observed in self‐ion irradiated Si, damage saturation is distinguished by a low concentration of lattice defects in the near‐surface region ahead of the ions’ end‐of‐range which remains constant over an extended range of implantation dose. A previously proposed model accounted for saturation by assuming that damage nucleates homogeneously during ion irradiation. Different mechanisms for nucleation of damage are discussed and substrate conditions are specified under which each dominates. A characteristic temperature is defined for each substrate above which damage nucleates primarily by a homogeneous mechanism. Results are presented which show the damage saturation occurs above this characteristic temperature but not below, thus establishing homogeneous nucleation as a necessary condition for the occurrence of damage saturation, as previously suggested.
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61.80.Jh Ion radiation effects
61.72.Yx Interaction between different crystal defects; gettering effect
61.72.Bb Theories and models of crystal defects
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