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  • 1
    Electronic Resource
    Electronic Resource
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Pharmacology 44 (2004), S. 399-421 
    ISSN: 0362-1642
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Medicine , Chemistry and Pharmacology
    Notes: New cells are continuously generated from immature proliferating cells throughout adulthood in many organs, thereby contributing to the integrity of the tissue under physiological conditions and to repair following injury. In contrast, repair mechanisms in the adult central nervous system (CNS) have long been thought to be very limited. However, recent findings have clearly demonstrated that in restricted areas of the mammalian brain, new functional neurons are constantly generated from neural stem cells throughout life. Moreover, stem cells with the potential to give rise to new neurons reside in many different regions of the adult CNS. These findings raise the possibility that endogenous neural stem cells can be mobilized to replace dying neurons in neurodegenerative diseases. Indeed, recent reports have provided evidence that, in some injury models, limited neuronal replacement occurs in the CNS. Here, we summarize our current understanding of the mechanisms controlling adult neurogenesis and discuss their implications for the development of new strategies for the treatment of neurodegenerative diseases.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 7389-7391 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Metastable pseudomorphic Ge0.08Si0.92 layers grown by chemical-vapor deposition on Si(100) substrate were implanted at room temperature with 90 keV As ions to a dose of 1×1013 cm−2. The samples were subsequently annealed for short 40 s durations in a lamp furnace with a nitrogen ambient, or for long 30 min periods in a vacuum tube furnace. For samples annealed for a 30-min-long duration at 700 °C, the dopant activation can only reach 50% without introducing significant strain relaxation, whereas samples annealed for short 40 s periods (at 850 °C) can achieve more than 90% activation without a loss of strain. We conclude that it is advantageous to anneal a low-dose As-implanted pseudomorphic and metastable GeSi epilayers briefly at an elevated temperature, rather than to anneal it for a 30-min-long period at a lesser temperature, when high activation without a strain loss is desired. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The finding of neurogenesis in the adult brain led to the discovery of adult neural stem cells. TLX was initially identified as an orphan nuclear receptor expressed in vertebrate forebrains and is highly expressed in the adult brain. The brains of TLX-null mice have been reported to have no ...
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 8341-8348 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Metastable pseudomorphic epi-GexSi1−x (x=8%, 16%) films of a common thickness of 145±10 nm were deposited on Si(100) substrates by chemical vapor deposition and then implanted at room temperature with 90 keV arsenic ions to a dose of 1.5×1015/cm2. This implantation amorphizes the top ∼125 nm of the epi-GeSi layers. Implanted as well as nonimplanted GeSi samples were subsequently annealed by (1): short (10–40 s) lamp annealing in nitrogen ambient at 600–800 °C; or (2): long (30 min) furnace annealing in vacuum (∼5×10−7 Torr) at 500–800 °C. Silicon samples were also implanted and annealed as references. The amorphized epi-GeSi recrystallizes via solid-phase epitaxy when annealed at or above 500 °C. The initial pseudomorphic strain of the epi-GeSi is thereby lost for both short and long annealing. High densities of dislocations (1010–1011/cm2) are typically present in the regrown GeSi layers, but not in the regrown Si samples. Just after the completion of solid-phase epitaxial regrowth, ∼80%–100% of the implanted arsenic ions become electrically active; further annealing decreases the activation. We conclude and generalize that metastably strained GeSi layers amorphized by a high dose of implanted dopants will not recover their original crystallinity and strain after solid-phase epitaxial regrowth, regardless of annealing procedure, although the implanted dopants are electrically activated in the process. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 5160-5166 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Several 265-nm-thick metastable pseudomorphic Ge0.12Si0.88 films grown on a Si(100) substrate by molecular-beam epitaxy were implanted at room temperature with 100 keV phosphorus ions to a dose of 1.5×1015/cm2. The implantation amorphizes the top portion (∼190 nm) of the GeSi layer and leaves the rest of the film single crystalline. Implanted and nonimplanted samples were subsequently annealed simultaneously in vacuum for 30 min from 400 to 800 °C. The implanted samples undergo layer-by-layer solid-phase-epitaxial regrowth during annealing at or above 500 °C. The regrown GeSi layer is relaxed with a high density of threading dislocations (∼1010–1011/cm2). The nonamorphized portion of the layer remains fully strained when annealed between 400 and 600 °C. At or above 700 °C misfit dislocations are observed at the Si/Ge0.12Si0.88 interface. After 800 °C annealing the strain in the whole epilayer is fully relaxed. The strain relaxation is facilitated by the implantation. The presence of phosphorus in GeSi raises its regrowth velocity by about an order of magnitude over that of Ge0.12Si0.88 amorphized by irradiation of Si. The implanted phosphorus reaches ∼100% activation after the completion of solid-phase-epitaxial regrowth. The room-temperature sheet electron mobility in GeSi is ∼20% below that of a Si sample implanted and annealed under the same conditions. It is concluded that metastable Ge0.12Si0.88 on Si(100) amorphized at room temperature by P implantation and recrystallized by solid-phase epitaxy can- not recover its crystalline perfection and its pseudomorphic strain upon steady-state furnace annealing. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 6039-6045 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The damage and strain induced by irradiation of both relaxed and pseudomorphic GexSi1−x films on Si(100) with 100 keV 28Si ions at room temperature have been studied by MeV 4He channeling spectrometry and x-ray double-crystal diffractometry. The ion energy was chosen to confine the major damage to the films. The results are compared with experiments for room temprature Si irradiation of Si(100) and Ge(100). The maximum relative damage created in low-Ge content films studied here (x=10%, 13%, 15%, 20%, and 22%) is considerably higher than the values obtained by interpolating between the results for relative damage in Si-irradiated single crystal Si and Ge. This, together with other facts, indicates that a relatively small fraction of Ge in Si has a significant stabilizing effect on the retained damage generated by room-temperature irradiation with Si ions. The damage induced by irradiation produces positive perpendicular strain in GexSi1−x, which superimposes on the intrinsic positive perpendicular strain of the pseudomorphic or partially relaxed films. In all of the cases studied here, the induced maximum perpendicular strain and the maximum relative damage initially increase slowly with the dose, but start to rise at an accelerated rate above a threshold value of ∼0.15% and 15%, respectively, until the samples are amorphized. The pre-existing pseudomorphic strain in the GexSi1−x film does not significantly influence the maximum relative damage created by Si ion irradiation for all doses and x values. The relationship between the induced maximum perpendicular strain and the maximum relative damage differs from that found in bulk Si(100) and Ge(100).
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Damage and strain produced in a 370-nm-thick strained epitaxial Ge0.10Si0.90 film on Si(100) by irradiation with 320 keV 28Si+ ions at fixed temperatures ranging from 40 to 150 °C and for doses from 1 to 30×1014/cm2 have been measured by MeV 4He channeling spectrometry, transmission electron microscopy, and high-resolution x-ray diffractometry. The ion energy was chosen so that the maximum damage created by irradiation occurs very near the GeSi-Si interface. For all temperatures, the retained damage and the perpendicular strain induced by the irradiation are significantly greater in the GeSi epilayer than in the Si substrate. For all doses the retained damage and the induced perpendicular strain become small above 100 °C. Both rise nonlinearly with increasing ion dose. They are related to each other differently in GeSi than in bulk Si or Ge irradiated at room temperature. Postirradiation furnace annealing can remove a large portion of the induced damage and strain for nonamorphized samples. Amorphized samples regrow by solid-phase epitaxy after annealing at 550 °C for 30 min; the regrown GeSi is, however, highly defective and elastically relaxed. A consequence of this defectiveness is that irradiation-induced amorphization in metastable GeSi is undesirable for applications where good crystalline quality is required. Ion implantation above room temperature can prevent amorphization. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 592-594 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Metastable pseudomorphic Ge0.12Si0.88 films were grown by molecular beam epitaxy on Si(100) substrates and then implanted with 100 keV 31P at room temperature for a dose of 5×1013/cm2. Samples were subsequently annealed by rapid thermal annealing (RTA) in nitrogen and by steady-state furnace annealing in vacuum. Both damage and strain introduced by implantation can be completely removed, within instrumental sensitivity, by RTA at 700 °C for 10–40 s. Vacuum annealing for 30 min at 500–550 °C removes most of the damage and strain induced by the implantation but the activation of the P is poor. At 700 °C, the activation is nearly 100%, but the crystallinity worsens and the pseudomorphic strain begins to relax. We conclude that for a lightly implanted metastable and pseudomorphic GeSi epilayer on Si, steady-state vacuum annealing cannot achieve good dopant activation without introducing significant strain relaxation to the heterostructure, while RTA can. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 1700-1703 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Dual-energy carbon implantation (1×1016/cm2 at 150 and at 220 keV) was performed on 260-nm-thick undoped metastable pseudomorphic Si(100)/ Ge0.08Si0.92 with a 450-nm-thick SiO2 capping layer, at either room temperature or at 100 °C. After removal of the SiO2 the samples were measured using backscattering/channeling spectrometry and double-crystal x-ray diffractometry. A 150-nm-thick amorphous layer was observed in the room temperature implanted samples. This layer was found to have regrown epitaxially after sequential annealing at 550 °C for 2 h plus at 700 °C for 30 min. Following this anneal, tensile strain, believed to result from a large fraction of substitutional carbon in the regrown layer, was observed. Compressive strain, that presumably arises from the damaged but nonamorphized portion of the GeSi layer, was also observed. This strain was not significantly affected by the annealing treatment. For the samples implanted at 100 °C, in which case no amorphous layer was produced, only compressive strain was observed. For samples implanted at both room temperature and 100 °C, the channelled backscattering yield from the Si substrate was the same as that of the virgin sample. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 63 (1993), S. 1405-1407 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An epitaxial Ge layer is grown by solid-phase epitaxy on an underlying Ge0.82Si0.18 seeding layer with a Ge-SiO2 matrix positioned between them. To this end, a (100)Si substrate with a Ge0.82Si0.18 epilayer is first oxidized in a wet ambient at 700 °C for 30 min to transform an upper fraction of the epilayer to amorphous Ge0.82Si0.18O2. A second annealing step (700 °C/16 h) in a 95% N2+5% H2 ambient (forming gas) reduces the GeO2 to Ge which grows epitaxially by solid-phase reaction on the remaining Ge0.82Si0.18 layer. A self-induced intermediate layer of epitaxial Ge with SiO2 inclusions restricts the propagation of dislocations, resulting in a crystalline perfection of the overlying Ge epilayer superior to that of the Ge0.82Si0.18 template.
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