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  • 1
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    Observation of the role of subcritical nuclei in crystallization of a glassy solid (2009)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2009-12-08
    Description: Phase transformation generally begins with nucleation, in which a small aggregate of atoms organizes into a different structural symmetry. The thermodynamic driving forces and kinetic rates have been predicted by classical nucleation theory, but observation of nanometer-scale nuclei has not been possible, except on exposed surfaces. We used a statistical technique called fluctuation transmission electron microscopy to detect nuclei embedded in a glassy solid, and we used a laser pump-probe technique to determine the role of these nuclei in crystallization. This study provides a convincing proof of the time- and temperature-dependent development of nuclei, information that will play a critical role in the development of advanced materials for phase-change memories.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Lee, Bong-Sub -- Burr, Geoffrey W -- Shelby, Robert M -- Raoux, Simone -- Rettner, Charles T -- Bogle, Stephanie N -- Darmawikarta, Kristof -- Bishop, Stephen G -- Abelson, John R -- New York, N.Y. -- Science. 2009 Nov 13;326(5955):980-4. doi: 10.1126/science.1177483.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign (UIUC), Urbana, IL 61801, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/19965508" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 2
    Electronic Resource
    Electronic Resource
    Subsurface hydrogenated amorphous silicon to μc-hydrogenated silicon transformation during magnetron sputter deposition determined by spectroscopic ellipsometry (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 1769-1771 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We use in situ spectroscopic ellipsometry to analyze the microstructure of the μc-Si:H/a-Si:H interface deposited by reactive magnetron sputtering of a Si target in (Ar+H2). We increase the hydrogen pressure to promote μc-Si:H formation and observe several effects. Initially, H penetrates ∼45 A(ring) into the a-Si:H substrate and increases its hydrogen content. Then ∼55 A(ring) of hydrogen-rich a-Si:H deposits. Finally, μc-Si:H nucleates on top of this ∼100 A(ring) thick, high H-content a-Si:H interface layer. As μc-Si:H grows, the thickness of the amorphous interface layer decreases by ∼40 A(ring); the void fraction in the μc-Si:H layer is always ≤15 vol %, ruling out the possibility that the a-Si:H is etched away. These results suggest that a-Si:H can be transformed into μc-Si:H in a subsurface region under appropriate conditions. © 1994 American Institue of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.113003
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  • 3
    Electronic Resource
    Electronic Resource
    Si–C–H bonding in amorphous Si1−xCx:H film/substrate interfaces determined by real time infrared absorption during reactive magnetron sputter deposition (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 1659-1663 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We determine the evolution of Si–H and C–H bonding during the growth of hydrogenated amorphous silicon carbide films by reactive magnetron sputtering of a Si target in (Ar+H2+CH4). Si–H and C–H modes are observed by infrared reflectance spectroscopy. An optical cavity substrate is used to enhance the sensitivity. We identify Si–H stretching modes at 2110 and 2145 cm−1 due to Si–H clusters in microvoids and Si–H back-bonded to carbon, respectively. C–H stretching modes are identified at 2870, 2900, and 2950 cm−1. These indicate dominant sp3 bonding configuration for C. During initial growth, a transition layer rich in H and C is observed. Steady state growth is not achieved until (approximately-greater-than)250 A(ring) on SiO2 substrates, and ∼70 A(ring) on a-Si:H substrates. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360260
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  • 4
    Electronic Resource
    Electronic Resource
    Surface reaction probability in hydrogenated amorphous silicon growth (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 3123-3129 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We deposit hydrogenated amorphous silicon (a-Si:H) on a novel "macroscopic'' trench substrate using both remote hollow cathode (HC) silane discharges and reactive magnetron sputter (RMS) deposition sources. Both methods produce state of the art optoelectronic quality a-Si:H. We analyze the surface coverage profiles in terms of the surface reaction probability β, using a Monte Carlo simulation to correct for particle reflection and loss. We also measure the deposited film quality as a function of position in the trench. For low power silane HC deposition, we find β=0.28±0.05, whereas for the RMS case β=0.97±0.05. In contrast to the prevailing thinking in the a-Si:H field, this result demonstrates that β is not universally correlated with film quality. We discuss the role of energetic particle bombardment in RMS that permits high quality films to be deposited despite the high precursor reactivity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357494
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  • 5
    Electronic Resource
    Electronic Resource
    Si(001):B gas-source molecular-beam epitaxy: Boron surface segregation and its effect on film growth kinetics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 82 (1997), S. 2288-2297 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: B-doped Si(001) films, with concentrations CB up to 1.7×1022 cm−3, were grown by gas-source molecular-beam epitaxy from Si2H6 and B2H6 at Ts=500–800 °C. D2 temperature-programed desorption (TPD) spectra were then used to determine B coverages θB as a function of CB and Ts. In these measurements, as-deposited films were flash heated to desorb surface hydrogen, cooled, and exposed to atomic deuterium until saturation coverage. Strong B surface segregation was observed with surface-to-bulk B concentration ratios ranging up to 1200. TPD spectra exhibited β2 and β1 peaks associated with dideuteride and monodeuteride desorption as well as lower-temperature B-induced peaks β2* and β1*. Increasing θB increased the area under β2* and β1* at the expense of β2 and β1 and decreased the total D coverage θD. The TPD results were used to determine the B segregation enthalpy, −0.53 eV, and to explain and model the effects of high B coverages on Si(001) growth kinetics. Film deposition rates R increase by ≥50% with increasing CB〉˜1×1019 cm−3 at Ts≤550 °C, due primarily to increased H desorption rates from B-backbonded Si adatoms, and decrease by corresponding amounts at Ts≥600 °C due to decreased adsorption site densities. At Ts≥700 °C, high B coverages also induce {113} facetting. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.366036
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  • 6
    Electronic Resource
    Electronic Resource
    Hydrogen-surface reactions during the growth of hydrogenated amorphous silicon by reactive magnetron sputtering: A real time kinetic study by in situ infrared absorption (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 6247-6256 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This article experimentally identifies the hydrogen incorporation and release processes which control the final hydrogen content of hydrogenated amorphous silicon films (a-Si:H). We deposit films using reactive magnetron sputtering of a silicon target in an Ar and H2 atmosphere. Hydrogen incorporation or loss is measured using real time infrared reflectance spectroscopy. An optical cavity substrate increases the sensitivity, allowing us to observe Si–H bonding in layers ≥5 A(ring) thick via the stretching mode absorption (1800–2300 cm−1). We observe a narrow component at ∼2100 cm−1 corresponding to all SiHx bonds on the physical surface; the line width allows us to distinguish this contribution from the broader bulk modes. Various combinations of growth flux (isotope labeling, hydrogen partial pressure between 0.1 and 2.0 mTorr) and substrate material (on SiO2, a-Si, or a-Si:D) at substrate temperatures between 120 and 350 °C are used to distinguish various mechanisms. From the deposition of a-Si:H films on SiO2, we quantify the H surface coverage at the end of the nucleation stage (10 A(ring) of growth) to be 1.2±0.3×1015 cm−2, essentially independent of growth conditions. From the evolution of Si–H bonding during the initial growth (≤25 A(ring)), we infer a reduction of the surface area (smoothening) during nucleation and coalescence. During a-Si:H growth on unhydrogenated a-Si, we observe H implantation up to a depth of 40 A(ring), and derive the total flux of arriving H as a function of hydrogen partial pressure. For the exposure of a-Si:H to atomic deuterium and of a-Si:D to atomic H, we observe a loss/gain of surface H due to abstraction or exchange reactions. When a-Si:D or a-Si films are deposited on a-Si:H films, we observe H loss from the bulk of the film due to fast particle bombardment at the growing surface. Based on the experimental evidence, we discuss the dependence of various H incorporation and release processes on the incident H flux, substrate temperature, and deposition rate. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.359156
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  • 7
    Electronic Resource
    Electronic Resource
    The improved stability of hydrogenated amorphous silicon films grown by reactive magnetron sputtering at high substrate temperature (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 3704-3706 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report the electronic properties, stability, and microstructure of a-Si:H films grown at high substrate temperature (320–440 °C) by dc reactive magnetron sputtering. The initial defect state density, determined by the constant photocurrent method, varies from 2–5×1015 cm−3 with H content changing from 15–10 at. % as Ts increases from 320–375 °C. For 100 h of white light exposure at 1 W/cm2, the midgap state density reached an apparent saturation at 2–3×1016 cm−3 over this temperature range. By contrast, films grown at 230–300 °C saturate at 9×1016 cm−3.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.356042
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  • 8
    Electronic Resource
    Electronic Resource
    Hydrogen release kinetics during reactive magnetron sputter deposition of a-Si:H: An isotope labeling study (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 1856-1870 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The release of molecular hydrogen from the growing surface of hydrogenated amorphous silicon films is determined using an isotope labelling technique. The results demonstrate that surface-bonded H atoms are readily abstracted by atomic hydrogen arriving from the gas phase. The films are deposited by dc reactive magnetron sputtering of a silicon target in an argon-hydrogen atmosphere. To achieve isotope labeling, we first deposit a deuterated amorphous silicon film, then commence growth of hydrogenated amorphous silicon and measure the transient release of HD and D2 from the growing surface using mass spectrometry. Release occurs when the supply of reactive hydrogen in the growth flux exceeds the incorporation rate into the film, and is observed under all experimental conditions. The net rate of H incorporation is known from ex situ measurements of film growth rate and hydrogen content. We combine the H release and incorporation data in a mass balance argument to determine the H-surface kinetics. Under conditions which produce electronically useful films, (i) 0.5–1.0 hydrogen atoms react with the growing surface per incorporated silicon atom, (ii) the near surface of the growing film contains 1–3×1015/cm2 of excess hydrogen, (iii) the dominant hydrogen release mechanism is by direct abstraction to form H2 molecules, and (iv) the kinetics of H release and incorporation can be described by constant rate coefficients. These data are supported by studies of H interactions with single-crystal silicon and amorphous carbon surfaces.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357706
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  • 9
    Electronic Resource
    Electronic Resource
    Iron redistribution and compensation mechanisms in semi-insulating Si-implanted InP (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 65 (1989), S. 1009-1017 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Si and Fe depth distributions have been measured in Si-implanted semi-insulating (SI) InP as a function of implant temperature and post-implant annealing technique (either furnace annealing or rapid thermal annealing). Depth profiles obtained by secondary ion mass spectrometry and damage measurements obtained by Rutherford backscattering demonstrate that Fe redistributes into regions of residual damage during thermal processing. The active electrical carrier profiles measured by electrochemical profiling show differences between atomic and electrical carrier profiles which depend on whether the substrate is semi-insulating or undoped, and on the implant temperature. These differences are interpreted in terms of three different compensating mechanisms: (a) the amphoteric nature of Si impurities in InP, so that Si can be self-compensating, (b) carrier compensation caused by the redistribution of Fe, and (c) implantation-related damage effects and/or stoichiometry imbalance induced by the Si implant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.343086
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  • 10
    Electronic Resource
    Electronic Resource
    Deposition of microcrystalline silicon: Direct evidence for hydrogen-induced surface mobility of Si adspecies (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 1463-1469 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Hydrogenated microcrystalline silicon thin films can be deposited at low substrate temperatures using plasma enhanced–or hot wire–chemical vapor deposition using silane, reactive magnetron sputtering of silicon, or related techniques. Microcrystalline silicon is deposited when a large quantity of molecular hydrogen is added to the process gas such that a large flux of atomic hydrogen impinges on the film growth surface; otherwise, the films are amorphous. Three different microscopic mechanisms have been hypothesized to explain the formation of the microcrystalline phase. In essence, the hypotheses are that atomic hydrogen: (i) enhances the surface diffusion of Si adspecies, which in turn raises the probability of crystalline phase formation, (ii) promotes a subsurface transformation of amorphous into microcrystalline Si, or (iii) preferentially etches amorphous regions such that only microcrystalline Si survives to produce film growth. In this work, we critically test mechanism (i) as follows. We deposit films using dc reactive magnetron sputtering of a Si target in an argon–hydrogen plasma, which yields very poor adspecies mobility at low rates of hydrogen injection. We then increase the hydrogen injection and measure the increase in adspecies motion via the enhanced rate at which the surface smoothens for film growth on substrates with a calibrated roughness of ∼80 Å. The dynamic surface roughness and the structural phase are determined by real-time spectroscopic ellipsometry. The combination of high atomic hydrogen flux and high surface hydrogen coverage uniquely correlates with microcrystalline phase formation. Higher substrate temperatures do not increase adspecies mobility, and actually decrease it when the rate of thermal desorption becomes sufficient to decrease the surface hydrogen coverage. These results also suggest that the original identity of the Si-bearing growth species is relatively unimportant, because the atomic hydrogen flux appears to produce mobile adspecies via surface reactions. We have previously shown that subsurface transformations, mechanism (ii), can also occur. However, we find no evidence for competitive etching, mechanism (iii), under our experimental conditions. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1334639
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