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15 Apr 1992

Volume 71, Issue 8, pp. R1-4094

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Porous silicon formation mechanisms

R. L. Smith and S. D. Collins

J. Appl. Phys. 71, R1 (1992); http://dx.doi.org/10.1063/1.350839 (22 pages) | Cited 233 times

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Recent reports describing photoluminescence in porous silicon have heightened the level of interest in it as a unique electronic material, and have created a need for a more complete understanding of the mechanism of porous silicon formation. The various models describing porous silicon formation are reviewed and the known electrochemical and morphological properties are discussed with the intention of unifying the different models into a comprehensive explanation for the formation of a porous structure in silicon. Because the specific surface dissolution chemistry is critical for a complete understanding of pore formation, some of the more prominent dissolution reactions are also reviewed and their relative importance to pore generation and morphology is discussed. Some aspects of the recently reported quantum effects are also reviewed. Because the mechanism of porous silicon formation involves a wide range of interdisciplinary fields, a considerable number of analogies and examples to related phenomena are also presented throughout the review to aid comprehension.
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61.66.Bi Elemental solids
72.80.Cw Elemental semiconductors
81.20.-n Methods of materials synthesis and materials processing

1H relaxation and diffusion of water and their dependence on suspended γ‐Al2O3 particles

J. O. Kwag, E. K. Jang, and I. Yu

J. Appl. Phys. 71, 3663 (1992); http://dx.doi.org/10.1063/1.350901 (4 pages) | Cited 2 times

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The 1H spin‐lattice relaxation time of water molecules is measured for the aqueous colloidal suspensions of γ‐alumina (Al2O3) particles. We observe that the spin‐lattice relaxation time T1(c) decreases gradually as the concentration c of the alumina particles is increased. We also observe a parallel decrease in the diffusion constant D(c). We attribute the simultaneous decrease of T1(c) and D(c) as c gets larger to the increase in the effective viscosity of the suspension, as explained in the light of the Bloembergen–Purcell–Pound [Phys. Rev. 73, 679 (1948)] theory of spin relaxation in liquids.
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76.60.Es Relaxation effects
82.70.Kj Emulsions and suspensions
66.10.C- Diffusion and thermal diffusion

Variational bound principle for surface wave scattering

Y. Hahn and H. Robinson

J. Appl. Phys. 71, 3667 (1992); http://dx.doi.org/10.1063/1.350902 (13 pages) | Cited 3 times

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The use of a variational bound principle to determine surface wave solutions will be examined. It will be demonstrated that the variational solution for the surface wave velocity in a semi‐infinite substrate is bounded from below and that the value of this bound coincides with the exact solution in the simple case of the substrate only. In the presence of an overlay, the variational solution for the surface wave velocity still maintains the boundedness property, but the solution has changed to reflect the perturbing effect of the overlay. Improved reflection coefficients and velocity shifts are calculated, including the second‐order effects of evanescent waves. The variational bound approach provides a simple but powerful alternative to other conventional methods in which each Floquet component of the solution is treated separately. Bulk wave coupling requires a separate treatment as the bound principle is violated in this case.  
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43.35.Pt Surface waves in solids and liquids
68.35.Gy Mechanical properties; surface strains

Effect of multilayer structure and laser pulse width on the reversible cycling of phase change optical storage media

Kurt A. Rubin, Dunbar P. Birnie, and Martin Chen

J. Appl. Phys. 71, 3680 (1992); http://dx.doi.org/10.1063/1.350877 (8 pages) | Cited 9 times

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The static cycling behavior of chalcogenide phase change optical recording media was investigated. Thin‐film multilayer structure and laser pulse conditions were both varied to determine optimum conditions for good cycling behavior. For all structures, cyclability was always best for short pulses. An aluminum overcoat layer was found to substantially increase the performance. The effect of the temperature excursion experienced during writing on cycling was determined. At the shortest pulses it was demonstrated that the thermal excursions did not depend upon the presence of an aluminum overcoat, suggesting that mechanical strength was important for enhancing cyclability. For longer pulses the aluminum changed the thermal characteristics of the stack. These differences between various samples are compared to try to elucidate modes of failure in optical data storage structures.
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42.79.Vb Optical storage systems, optical disks
42.70.Ln Holographic recording materials; optical storage media
42.88.+h Environmental and radiation effects on optical elements, devices, and systems

Nonlinear Bragg reflection waveguide

C. Wächter, F. Lederer, L. Leine, U. Trutschel, and M. Mann

J. Appl. Phys. 71, 3688 (1992); http://dx.doi.org/10.1063/1.350878 (5 pages) | Cited 9 times

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The ability of Bragg reflection waveguides to support TE‐polarized nonlinear guided waves is studied. The dispersion relation for these waves is derived analytically. The numerical studies address the dependence of their propagation constants upon the interface intensity, the guided field power, and their evolution as they propagate down the guide. The characteristic features of the nonlinear guided waves are identified as power thresholds, instability on positively‐sloped branches of the dispersion curve and symmetry breaking with the formation of spatial solitons.
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42.82.Et Waveguides, couplers, and arrays
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

Effects of the structure and composition of lead glasses on the thermal lensing of pulsed laser radiation

B. Taheri, A. Munoz F., W. D. St. John, J. P. Wicksted, Richard C. Powell, D. H. Blackburn, and D. C. Cranmer

J. Appl. Phys. 71, 3693 (1992); http://dx.doi.org/10.1063/1.350879 (8 pages) | Cited 9 times

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The thermal lensing characteristics of several silicate, germanate, phosphate, and borate glasses were studied using a laser with a 7 ns pulse at 457 nm in a tight focus geometry. A geometric model was developed to describe the quadratic radial profile of the refractive index resulting from the laser‐induced temperature profile. This model was utilized to interpret the effects of some of the relevant experimental parameters on the fluence transmission experiments. The influence of material properties such as different types of network former and modifier ions on the nonlinear optical properties of these materials were also studied. It was found that: (i) the greatest influence of the network modifier ions was due to their effect on the absorption coefficient of the glasses; (ii) in lead glasses, the thermo‐optic coefficients dn/dT of the germanates and silicates with random network structures were greater than those of the borate and phosphate glasses with ring and chain structures; and (iii) the main contribution to the thermo‐optic coefficient comes from the thermally induced changes in the electronic polarizability of the glass components. In these glasses, the oxygen polarizability provides the dominant contribution and is affected by the variations in the polarizing power (charge to radius‐squared ratio) of the network former ions.
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42.65.Tg Optical solitons; nonlinear guided waves
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
61.43.Fs Glasses
61.43.-j Disordered solids
61.44.Br Quasicrystals

Heat flow in substrates induced by a scanning laser beam

Yong‐Feng Lu

J. Appl. Phys. 71, 3701 (1992); http://dx.doi.org/10.1063/1.350880 (12 pages) | Cited 9 times

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A general model is derived for computing the heat flow induced by a scanning continuous‐wave laser beam with a Gaussian intensity distribution in a semi‐infinite substrate. Temperature‐dependent thermal conductivity, thermal diffusivity, and surface reflectivity are incorporated in the model. The model is then applied to different substrate materials such as silicon, GaAs, and Mn–Zn ferrite. The numerical results show that the heat flow intensity in the substrate depends on the incident laser power, substrate temperature, scan speed, and beam radius. This study is expected to be useful in investigating the distributions of the laser‐induced thermal stresses and lattice damages.
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44.30.+v Heat flow in porous media
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
65.90.+i Other topics in thermal properties of condensed matter (restricted to new topics in section 65)

Effects of magnetic field orientation on a liquid‐metal free surface in a sliding electrical contact

J. S. Walker, D. M. Audet, G. Talmage, S. H. Brown, and N. A. Sondergaard

J. Appl. Phys. 71, 3713 (1992); http://dx.doi.org/10.1063/1.350881 (8 pages) | Cited 1 time

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This paper treats a free surface between a liquid metal and an inert gas in the presence of a magnetic field with arbitrary orientation relative to the free surface. The free surface intersects a perfectly conducting surface at rest and an insulated surface rotating about an axis which is perpendicular to both surfaces and which is far from the liquid‐metal region. This problem models free surfaces in liquid‐metal sliding electric contacts for motors and generators. There is a primary azimuthal liquid‐metal velocity which is driven by the rotation of the insulated surface, and there is a secondary flow which involves radial and axial velocities and which is driven by the centrifugal force due to the primary velocity. The free‐surface positions, pressures, and velocities are presented as functions of the magnetic‐field orientation and strength.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
84.32.Dd Connectors, relays, and switches
47.90.+a Other topics in fluid dynamics (restricted to new topics in section 47)

Modeling radio‐frequency discharges: Effects of collisions upon ion and neutral particle energy distributions

P. W. May, D. Field, and D. F. Klemperer

J. Appl. Phys. 71, 3721 (1992); http://dx.doi.org/10.1063/1.350882 (10 pages) | Cited 39 times

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Using a recently published description of the potentials in a radio‐frequency discharge [Y. P. Song, D. Field, and D. F. Klemperer, J. Phys. D 23, 673 (1990)] Monte Carlo trajectories of ions and neutral particles for an Ar plasma have been computed, including collisions at the realistic pressures of tens of mTorr used for reactive ion etching. The model for collisions involves both charge exchange and momentum‐transfer scattering. Ion and neutral energy and angular distributions have been calculated for particles impacting on the powered electrode and the calculations agree with an experimental ion energy distribution for Ar+. The presence of a large proportion of energetic neutrals is predicted, and it is suggested that these may have an important role to play in etch mechanisms.
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52.65.-y Plasma simulation
52.80.Pi High-frequency and RF discharges
52.40.Hf Plasma-material interactions; boundary layer effects

Experiments on a current‐toggled plasma‐opening switch

C. W. Mendel, M. E. Savage, D. M. Zagar, W. W. Simpson, T. W. Grasser, and J. P. Quintenz

J. Appl. Phys. 71, 3731 (1992); http://dx.doi.org/10.1063/1.350883 (16 pages) | Cited 8 times

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Plasma‐opening switches have been used to improve pulsed‐power wave shapes for over a decade. These switches have used the inertia of the plasma to hold the switch closed. This results in conflicting requirements when long hold‐off time and fast opening are required, and also results in variation in opening current due to variation in initial plasma fill. The current‐toggled plasma‐opening switch attempts to overcome these problems by using external magnetic fields rather than inertia to control the plasma conductor. Data will be presented showing several features of the operation of this switch. These data will be compared to models used to design the switch. The comparisons indicate that the mass can be measured approximately from fast coil data and that the slow coil flux does set the opening level of the current. They also indicate that the opening current is somewhat dependent upon plasma mass, and that the design of the field coils that provide the control fields must be done more carefully to provide a switch that opens satisfactorily.
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52.75.Kq Plasma switches (e.g., spark gaps)
84.32.Dd Connectors, relays, and switches
52.35.Tc Shock waves and discontinuities

Damage coefficient associated with free exciton lifetime in GaAs irradiated with neutrons and electrons

M. Parenteau, C. Carlone, and S. M. Khanna

J. Appl. Phys. 71, 3747 (1992); http://dx.doi.org/10.1063/1.350884 (7 pages) | Cited 6 times

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The lifetime τ of the free exciton at T=17 K in irradiated GaAs has been determined by the reflectance of layers grown by metalorganic chemical vapor deposition and of semi‐insulating samples grown by the liquid‐encapsulated Czochralski method. The samples had been irradiated with 1 MeV and thermal neutrons, and with 7 MeV and white electrons up to 7 MeV. The linear increase of τ −1 as a function of the fluence is explained by a simple kinetics model, which applies especially well to the epitaxial layers. The damage coefficient kτ associated with this lifetime has been measured. For the defects created by all types of radiation, the radius associated with the cross section for the capture of excitons is three to ten times the radius of the free exciton. The exciton transition energy is found to decrease as its reflectance structure is broadened.
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71.35.-y Excitons and related phenomena
61.80.Fe Electron and positron radiation effects
61.80.Hg Neutron radiation effects

Raman scattering of coupled longitudinal optical phonon‐plasmon modes in dry etched n+‐GaAs

P. D. Wang, M. A. Foad, C. M. Sotomayor‐Torres, S. Thoms, M. Watt, R. Cheung, C. D. W. Wilkinson, and S. P. Beaumont

J. Appl. Phys. 71, 3754 (1992); http://dx.doi.org/10.1063/1.350885 (6 pages) | Cited 26 times

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We have carried out extensive Raman scattering investigations of the damage caused by the dry etching in GaAs. The heavily doped n+‐GaAs (2–3×1018 cm−3) allows the study of the coupled longitudinal optical (LO) phonon‐plasmon mode as a probe to assess the dry etch‐induced damage. Three etching techniques were used including conventional radio frequency (rf) reactive ion etching (RIE), ion beam etching (IBE), and electron cyclotron resonance radio frequency reactive ion etching (ECR‐RIE). It is demonstrated that the etched damage is confined to a few tens of nanometers after 20 nm of material is etched away. ECR‐RIE etching produces the smallest damage. It is found that in RIE etching, as etching proceeds, the depletion depth saturates while for purely physical etching (IBE) the depletion depth increases continuously, at least under the conditions used.
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78.30.Fs III-V and II-VI semiconductors
81.65.-b Surface treatments
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
52.40.Hf Plasma-material interactions; boundary layer effects

Effects of nitrogen on oxygen precipitation in silicon

Q. Sun, K. H. Yao, H. C. Gatos, and J. Lagowski

J. Appl. Phys. 71, 3760 (1992); http://dx.doi.org/10.1063/1.350886 (6 pages) | Cited 43 times

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Defect interactions among oxygen, carbon, and nitrogen during a three‐step high‐low‐high intrinsic gettering process (1100 °C+750 °C+1000 °C) were studied using differential analysis of Fourier transform infrared (FTIR) absorption spectra of Czochralski‐Si crystals intentionally doped with carbon and nitrogen. Two phenomena related to nitrogen were observed: (1) Nitrogen‐enhanced oxide nucleation in the temperature range from 750 to 1100 °C, thus having a negative effect on the denuding step; and (2) defect interaction associated with carbon during oxygen precipitation was significantly affected by the co‐existence of nitrogen. For the latter effect the first‐step annealing at 1100 °C was crucial. The results presented are consistently interpreted assuming that nitrogen participates in creation of nucleation sites for heterogeneous oxygen precipitation, and also assuming that nitrogen aggregates at the strain regions surrounding precipitates and thus retards Si self‐interstitial migration. This explanation was supported by observed differences in oxygen‐precipitate‐related FTIR absorption and by the behavior of stress‐related photoluminescence D lines.
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61.72.Yx Interaction between different crystal defects; gettering effect
78.30.-j Infrared and Raman spectra
78.40.Fy Semiconductors

Interfacial reactions between nickel–chromium alloys and aluminum

Zhengquan Tan and S. M. Heald

J. Appl. Phys. 71, 3766 (1992); http://dx.doi.org/10.1063/1.350887 (7 pages)

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We have studied the interface structure and reactions of Ni–Cr alloy and Al thin film bilayers using glancing‐angle x‐ray reflectivity and extended x‐ray‐absorption fine structure techniques. The Al/Ni–Cr bilayers were prepared by evaporation in ultrahigh vacuum with a clean interface (Clean) or an interface exposed to 600 Langmuir oxygen (O‐exposed). No interfacial reaction is observed at room temperature in either the Clean or the O‐exposed samples. Upon annealing for 10 minutes, initial reaction occurs at about 250 °C in the Clean sample and 310 °C in the O‐exposed sample. The oxygen impurity at the interface retards the initial reaction and also modifies the diffusion behavior. In both Clean and O‐exposed systems, the reaction starts between Ni and Al forming NiAl3. The reaction results in a Cr‐rich region near the interface which transforms to the bcc structure. Only at higher temperatures do Cr and Al react to form CrAl7. The role of various diffusion channels and oxygen impurity at the interface in relationship to the reaction pattern will be discussed.
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61.05.cf X-ray scattering (including small-angle scattering)
61.05.cj X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.
68.35.Fx Diffusion; interface formation
68.60.Dv Thermal stability; thermal effects

Ion‐beam induced structural transformations in hydrogenated microcrystalline silicon

S. N. Yedave, S. B. Ogale, R. O. Dusane, S. V. Rajarshi, V. G. Bhide, and S. T. Kshirsagar

J. Appl. Phys. 71, 3773 (1992); http://dx.doi.org/10.1063/1.350888 (7 pages) | Cited 1 time

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Ion‐implantation induced structural transformations are investigated in hydrogenated microcrystalline silicon (μc‐Si:H). Ion‐beam processing of the μc‐Si:H samples was done using 160 keV Ar+ ions at different dose values, in the range of 1013–1016 ions/cm2. Ion‐beam induced transformation from the microcrystalline‐to‐amorphous phase was examined with the help of x‐ray diffraction (XRD) and laser‐Raman spectroscopic techniques. The conductivity changes in the samples were also monitored as a function of ion dose. It is observed that at a specific threshold ion dose value, the conductivity decreases by more than an order of magnitude. There are concomitant changes in XRD and laser Raman features. Furthermore the results clearly indicate grain growth under ion‐beam irradiation at high dose values.
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61.80.Jh Ion radiation effects
68.55.-a Thin film structure and morphology
61.43.Fs Glasses
61.43.-j Disordered solids
61.44.Br Quasicrystals

Reduction of secondary defects in MeV ion‐implanted silicon by means of ion beam defect engineering

Zhong‐lie Wang, Bo‐xu Zhang, Qing‐tai Zhao, Qi Li, J. R. Liefting, R. J. Schreutelkamp, and F. W. Saris

J. Appl. Phys. 71, 3780 (1992); http://dx.doi.org/10.1063/1.350889 (5 pages) | Cited 14 times

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A new concept of ion beam defect engineering is proposed. Reduction of secondary defects in 1 MeV As ion‐implanted Si(100) has been investigated by Rutherford backscattering/channeling and high resolution electron microscopy. It is found that the additional irradiation of 1.6 MeV Si ions prior to two‐step thermal annealing leads to a noticeable reduction of secondary defects. Good recrystallization of a buried amorphous layer is also obtained by irradiation of 1.6 MeV, 2×1015 Si/cm2 into the implanted Si sample held at elevated temperatures.
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61.72.uf Ge and Si
61.80.Jh Ion radiation effects
07.79.Cz Scanning tunneling microscopes
61.05.-a Techniques for structure determination

Effect of annealing on carbon concentration in edge‐defined film‐fed grown polycrystalline silicon

B. Pivac, M. Amiotti, A. Borghesi, A. Sassella, and J. Kalejs

J. Appl. Phys. 71, 3785 (1992); http://dx.doi.org/10.1063/1.350890 (3 pages) | Cited 7 times

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Carbon complex formation during annealing in the temperature range from 450 to 1150 °C is studied in a carbon‐rich polycrystalline edge‐defined film‐fed grown (EFG) silicon sheet. The result is compared with that obtained in Czochralski (CZ) single‐crystal silicon. Infrared (IR) spectroscopy reveals that carbon removal from substitutional sites above 600 °C is greatly inhibited in EFG silicon with respect to that seen in the CZ material. A broad IR peak attributed to C‐O complexes appears only after annealing at highest temperatures, while there is no evidence for appearance of the sharp band at 794 cm−1 usually assigned to SiC precipitation. The suppression of carbon complex formation in EFG material is attributed to decreased availability of silicon self‐interstitials necessary to promote removal of carbon from substitutional sites and to enhance its diffusivity.
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61.72.Bb Theories and models of crystal defects
61.72.S- Impurities in crystals
78.30.Hv Other nonmetallic inorganics

Structural properties of ion‐beam‐synthesized β‐FeSi2 in Si(111)

D. Gerthsen, K. Radermacher, Ch. Dieker, and S. Mantl

J. Appl. Phys. 71, 3788 (1992); http://dx.doi.org/10.1063/1.350891 (7 pages) | Cited 33 times

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A continuous buried β‐FeSi2 layer was obtained by implantation of 200 keV Fe+ ions into Si(111) wafers at elevated temperature. During the subsequent rapid thermal annealing at 1150 °C for 10‐s, a continuous buried layer of the metallic α‐FeSi2 phase is formed. During the second annealing step at 800 °C, the α phase is completely transformed into the semiconducting β phase. The epitaxial relationship between the β‐FeSi2 and the silicon substrate was investigated by transmission electron microscopy. It was found that the β‐FeSi2(010) plane grows parallel to the Si(111) substrate. Two different azimuthal orientations were observed. For the first azimuthal orientation, the β‐FeSi2[001] direction is oriented almost parallel to one of the three Si〈110〉 directions lying in the interface. In the second azimuthal orientation, the β‐FeSi2[100] direction lies parallel to one of the Si〈110〉 directions in the interface. The lattice parameter mismatch and the growth mechanism must be considered to be the main reasons for the epitaxial relationship of the Si(111)/β‐FeSi2/Si(111) heterostructures studied in this investigation. The orientation of the β‐phase is likely to be predetermined by the orientation of the α‐phase which is formed during the first annealing step. Different orientation relationships were observed for β‐FeSi2 prepared by solid phase epitaxy and ion beam synthesis without high‐temperature annealing.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.72.uf Ge and Si
07.79.Cz Scanning tunneling microscopes
61.05.-a Techniques for structure determination

Laser patterning of diamond films

J. Narayan and X. Chen

J. Appl. Phys. 71, 3795 (1992); http://dx.doi.org/10.1063/1.350892 (7 pages) | Cited 11 times

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We report here selective deposition and fine‐scale patterning of hot filament deposited diamond films by the use of pulsed laser irradiation on silicon and copper substrates. The substrates were abraded with diamond and alumina powders before hot‐filament chemical vapor deposition. A drastic enhancement in diamond nucleation (using hot‐filament chemical vapor deposition) was observed on specimens treated with diamond powder, whereas enhancement on specimens pretreated with alumina powder was relatively insignificant. We have found that the seeding of diamond crystals was substantially reduced by pulsed laser annealing/melting which removes the plastic damage as well as the seed crystals introduced by diamond powder pretreatment. The selective deposition or fine‐scale patterning of diamond films was achieved either by a shadow masking or by scanning a focused laser beam to generate desired patterns. The nucleation can also be enhanced by laser deposition of thin films, such as diamond‐like carbon and tungsten carbide (WC), and selective deposition and patterning achieved by controlled removal or deposition of the above films.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
79.20.Ds Laser-beam impact phenomena
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

In situ observation of the initial growth stages in GaAs/InAs by coaxial impact collision ion scattering spectroscopy: Transition from two‐dimensional‐like to three‐dimensional island growth

T. Saitoh, A. Hashimoto, and M. Tamura

J. Appl. Phys. 71, 3802 (1992); http://dx.doi.org/10.1063/1.350893 (4 pages) | Cited 1 time

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The initial growth stages of the highly lattice‐mismatched GaAs/InAs system was studied by coaxial impact collision ion scattering spectroscopy (CAICISS). The GaAs coverage on the InAs substrate during GaAs molecular beam epitaxial growth was monitored in situ by low incident angle CAICISS. The scattering intensity as a function of the deposited amount of GaAs can be divided into three characteristic regions. First, the scattering intensity from In decreases proportionally with the amount of deposited GaAs molecules. However, the intensity decrease stops abruptly before the surface is completely covered with a GaAs layer, and remains constant. Then, the intensity gradually decreases. This result shows that there exist three kinds of growth stages in the process of GaAs deposition on an InAs substrate. The mechanism of the growth mode transition, corresponding to the three kinds of growth stages is discussed from the viewpoint of a strain energy change on the surface.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.Fx Diffusion; interface formation

Adsorption of benzene on porous amorphous hydrogenated carbon films

Y. Yin, R. E. Collins, and B. A. Pailthorpe

J. Appl. Phys. 71, 3806 (1992); http://dx.doi.org/10.1063/1.350894 (6 pages) | Cited 4 times

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The annealing effect on porosity of amorphous hydrogenerated carbon thin films was studied using a quartz crystal microbalance. As‐prepared films show a mesoporous structure and a low adsorption capacity which is proportional to film thickness. Porosity of the as‐prepared film is estimated at about 0.15. Baking in vacuum changes the film to be highly microporous and significantly increases the porosity of the films to over 0.40, depending annealing history. Outgassing during the annealing process plays an important role in the formation of porosity. Heat treatment first opens micropores in the slightly mesoporous film. Further heating increases the number and size of micropores, and also produces mesopores. With prolonged heating at high temperatures, further outgassing occurs, resulting in collapsing of the films.
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68.55.-a Thin film structure and morphology
68.03.Fg Evaporation and condensation of liquids
68.43.Mn Adsorption kinetics
61.43.Fs Glasses
61.43.-j Disordered solids
61.44.Br Quasicrystals

Growth of epitaxial C54 TiSi2 on Si(111) substrate by in situ annealing in ultrahigh vacuum

Kun Ho Kim, Jeoung Ju Lee, Dong Ju Seo, Chi Kyu Choi, Sung Rak Hong, Jeoung Dae Koh, Sung Chul Kim, Jeong Yong Lee, and Marc A. Nicolet

J. Appl. Phys. 71, 3812 (1992); http://dx.doi.org/10.1063/1.350895 (4 pages) | Cited 15 times

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The growth of Ti and the formation of epitaxial Ti silicide on Si(111)‐7×7 were investigated by using reflection high‐energy electron diffraction (RHEED) and high‐resolution transmission electron microscopy (HRTEM). The growth mode of Ti is Stransky–Krastanov type when the substrate temperature is room temperature (RT). On the other hand, it is Volmer–Weber type when the substrate temperature is ∼550 °C. The HRTEM lattice image and transmission electron diffraction pattern show that C54 TiSi2 is grown epitaxially on a Si substrate when 160 ML of Ti is deposited on a Si(111)‐7×7 surface at RT followed by in situ annealing at 750 °C for 10 min in ultrahigh vacuum (UHV). The TiSi2/Si interface is somewhat incoherent, but the developed TiSi2 crystallites are single crystal with matching face relationships of TiSi2(111)∥Si(111), TiSi2(311)∥Si(111), and TiSi2(022)∥Si(111). A thin single‐crystal Si overlayer with [111] direction is grown on the TiSi2 surface when TiSi2/Si(111) is annealed at ∼900 °C in UHV, which is confirmed by observing the Si(111)‐7×7 RHEED pattern.
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68.55.-a Thin film structure and morphology
61.05.jh Low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED)
07.79.Cz Scanning tunneling microscopes
61.05.-a Techniques for structure determination

Analysis of the strain profile in thin Au/Ni multilayers by x‐ray diffraction

J. Chaudhuri, V. Gondhalekar, and A. F. Jankowski

J. Appl. Phys. 71, 3816 (1992); http://dx.doi.org/10.1063/1.350896 (5 pages) | Cited 10 times

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The strain relaxation in Au/Ni multilayers was analyzed in detail using a dynamical theory of x‐ray diffraction. The depth profile of strain in the modulation direction was determined by an iterative fitting of the calculated rocking curve with the experimental one. The repeat periods of Au/Ni multilayers used in this study range from 0.82 to 9.0 nm. The analysis indicates that the theoretical x‐ray patterns are extremely sensitive to the amount of strain at the interface.
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68.60.Bs Mechanical and acoustical properties
61.05.cc Theories of x-ray diffraction and scattering

Impact ionization thresholds in GexSi1−x alloys and strained layers

I. K. Czajkowski, J. Allam, A. R. Adams, and M. A. Gell

J. Appl. Phys. 71, 3821 (1992); http://dx.doi.org/10.1063/1.350897 (6 pages) | Cited 3 times

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Impact ionization rates in GexSi1−x alloys and strained layers are expected to be strongly influenced by the threshold energies for ionization. We have calculated the threshold energies along the principal crystal axes from empirical pseudopotential band structures for Ge, Si, and GexSi1−x alloys (x=0.25, 0.5, 0.75). Both relaxed (cubic) and strained [tetragonal, grown on (001) cubic Si] alloys were examined. The ionization process with the lowest threshold energy in Si is different from that in Ge, for both electron‐ and hole‐initiated ionization. For the cubic alloys, the lowest thresholds are ‘‘Si‐like’’ for compositions up to approximately x=0.65. The effect of strain on the GexSi1−x alloys is to increase the ratio of the lowest hole threshold to the lowest electron threshold for all compositions, suggesting the possibility that GexSi1−x strained layer avalanche photodiodes may exhibit superior noise properties.
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72.20.Ht High-field and nonlinear effects
72.80.Jc Other crystalline inorganic semiconductors
85.60.Dw Photodiodes; phototransistors; photoresistors

Intersubband relaxation dynamics in ternary/binary quantum wells: Role of the electron‐optical phonon interaction

R. P. Joshi

J. Appl. Phys. 71, 3827 (1992); http://dx.doi.org/10.1063/1.350872 (9 pages) | Cited 1 time

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Ensemble Monte Carlo calculations of the intersubband dynamics in binary‐ternary double‐heterostructure systems are presented. The presence of a ternary alloy has been explicitly incorporated to account for complexities arising from the multimode nature of phonons in real heterostructures. Electronic scattering rates are derived as a function of energy and quantum‐well width for both confined and interface modes on the basis of a continuum model. Results of Monte Carlo simulations yield an intersubband time constant that is in reasonable agreement with the experimental value, but only when details of the phonon modes and their dispersion, spreading of the electronic wave functions due to poor confinement, and the phonon amplification effects are comprehensively included.
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72.20.Dp General theory, scattering mechanisms
72.80.Ey III-V and II-VI semiconductors
78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
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