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  • 1
    Electronic Resource
    Electronic Resource
    Modeling uphill diffusion of Mg implants in GaAs using suprem-iv (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 2615-2623 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Transient, uphill diffusion of implanted Mg in GaAs during a 900 °C anneal is simulated using suprem-iv. The diffusion is believed to occur via the substitutional-interstitial-diffusion (SID) mechanism, with excess interstitials and vacancies produced by the implantation process causing this abnormal diffusion behavior. The SID mechanism is shown to be equivalent, in terms of the governing equations, to the interstitial-dopant pair diffusion model used in suprem-iv. This allows one to use suprem-iv, a silicon process simulator that includes dopant–point-defect interactions, to model uphill diffusion once the appropriate diffusivity and defect parameters are included. The profiles of excess interstitials and vacancies produced by the implantation process are obtained from Boltzmann transport equation calculations. The transient uphill diffusion phenomenon can be well simulated using the diffusion model in suprem-iv, with the dopant diffusing from the region of excess interstitials toward the surface and the region of excess vacancies. Once the defect concentrations return to their steady-state levels, either by diffusion, recombination, or capture by sinks, the normal concentration-dependent diffusion into the substrate occurs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351354
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  • 2
    Electronic Resource
    Electronic Resource
    Hole-dependent diffusion of implanted Mg in GaAs (1991)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 2800-2802 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnesium implants in GaAs exhibit two types of diffusion during annealing: uphill diffusion in the peak of the implant and concentration-dependent diffusion into the bulk. The uphill diffusion predominates at short times and low temperatures, while the concentration-dependent diffusion is dominant at long times and high temperatures. By studying implants that were annealed at temperatures where no uphill diffusion occurs, diffused profiles could be modeled and an expression for the Mg diffusivity obtained. The activation energy for this process is 1.77 eV. Results from Fermi level experiments show that the diffusivity is hole dependent rather than concentration dependent. The hole-dependent exponent is unity for Mg implanted into semi-insulating substrates, but may change to two at high hole concentrations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.104740
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  • 3
    Electronic Resource
    Electronic Resource
    Diffusion modeling of zinc implanted into GaAs (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 1670-1676 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The diffusion of implanted zinc in GaAs is studied and modeled for annealing temperatures of 625 through 850 °C. Secondary ion mass spectrometry data for the annealed profiles are presented. The substitutional interstitial diffusion (SID) mechanism is used to explain how the deviation of the local gallium interstitial concentration from its equilibrium value regulates the Zn diffusion. We are able to simulate both the box shaped profiles resulting from high temperature anneals and the kink-and-tail profiles resulting from lower temperature anneals. The simulation results have allowed us to determine Arrhenius relations for: the intrinsic diffusion coefficient for implanted Zn, DZnint=0.6075 exp(−3.21 eV/kBT) cm2 s−1; the equilibrium Ga interstitial concentration, CIGa*=7.98×1030 exp(−3.47 eV/kBT) cm−3; and the Ga interstitial diffusion coefficient, DIGa=0.4384 exp(−2.14 eV/kBT) cm2 s−1. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.364024
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  • 4
    Electronic Resource
    Electronic Resource
    Damage-induced uphill diffusion of implanted Mg and Be in GaAs (1990)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 56 (1990), S. 554-556 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The redistribution of Be and Mg implants upon post-implant annealing is studied in order to evaluate the influence of implant damage on the diffusion process. Rapid uphill diffusion is observed in the peak of Mg implants in GaAs, whereas Be implants show only uniform, concentration-dependent diffusion. This behavior is explained by the substitutional-interstitial-diffusion mechanism and computer simulations of damage-produced point defects. In the region of uphill diffusion, the dopants diffuse from areas of excess interstitials toward areas of excess vacancies. A critical concentration of point defects is necessary to initiate uphill diffusion. Uphill diffusion can be induced in Be implants by co-implanting with a heavier element such as Ar.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.102743
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  • 5
    Electronic Resource
    Electronic Resource
    Diffusion of implanted beryllium in n- and p-type GaAs (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 1990-1992 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The diffusion of ion-implanted Be in GaAs is studied by comparing the diffusion of implanted Be in undoped GaAs and in GaAs uniformly doped with Zn or Si. The Si-doped sample exhibits much less Be diffusion compared to the undoped case, while the Zn-doped sample shows much more Be diffusion. The diffusion in the doped substrate cases can be simulated with a constant Be diffusivity, as opposed to a concentration-dependent diffusivity in the undoped case. The results are consistent with the substitutional-interstitial diffusion mechanism, which predicts a diffusivity that is dependent on the net hole concentration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.102142
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  • 6
    Electronic Resource
    Electronic Resource
    Diffusion of implanted beryllium in gallium arsenide as a function of anneal temperature and dose (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 996-998 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The diffusion of implanted Be in GaAs was studied by annealing samples of GaAs implanted with low and high doses of Be. The high-dose (1×1014 cm−2) samples show an increase in diffusion with increasing anneal temperature from 700 to 900 °C. However, the low-dose (2×1013 cm−2) samples show a decrease in diffusion as the temperature is increased. The temperature dependence of the low-dose case can be reversed by coimplantation of 1×1014 cm−2 boron. This behavior is explained in terms of the substitutional-interstitial diffusion mechanism and the relative concentrations of Be interstitials and Be substitutionals for the different cases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.101700
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  • 7
    Electronic Resource
    Electronic Resource
    Eliminating dopant diffusion after ion implantation by surface etching (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 3302-3304 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Implantation damage is believed to play a significant role in dopant diffusion. Described here is an attempt to modify the point defect damage profile of a 40 keV, 29Si implant in GaAs by chemically etching away the top 100 A(ring) of the sample after implantation. In these samples, no Si diffusion was observed after annealing, while significant Si redistribution did occur in a similar sample which received no post-implant etch. trim simulations predict an excess Ga vacancy surface layer and excess Ga interstitials deeper in the sample. It is thought that by removing the vacancy-rich surface layer, the etch alters the implant damage profile and thus the diffusion behavior of the Si. Surface effects from etching as related to the Si diffusion are shown to be consistent with a vacancy-assisted diffusion mechanism. Evidence that this model may be applicable to B implants in Si is also shown.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111319
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  • 8
    Electronic Resource
    Electronic Resource
    Diffusion of implanted Be in AlxGa1−xAs as a function of Al concentration and anneal temperature (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 355-357 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Diffusion of ion implanted Be in AlxGa1−xAs was studied as a function of Al concentration and annealing temperature and was compared to diffusion in GaAs. The behavior of Be in AlxGa1−xAs is similar to that in GaAs, even showing the anomalous behavior of increasing redistribution with decreasing temperature. The diffusivity of Be appears to increase with Al content which may be due to increasing bonding strength of matrix atoms with the addition of Al, preventing the easy transferral of Be from interstitial to substitutional sites. The oversaturation of Be interstitials may also explain the persistence of anomalous diffusion behavior in AlxGa1−xAs with respect to anneal temperature. This is discussed in terms of the substitutional-interstitial diffusion mechanism, the relative amount of interstitial and substitutional Be, and the relative difficulty of moving from an interstitial to a substitutional site. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.114211
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  • 9
    Electronic Resource
    Electronic Resource
    Effect of encapsulant material on the diffusion of beryllium in molecular beam epitaxy gallium arsenide (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 1939-1941 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments were done that illustrate the role of the wafer surface in the transient diffusion of Be in GaAs. Samples were doped during molecular beam epitaxial growth and annealed at 900 °C for 15 min and 2 h under two different caps. In some of the annealed samples, the dopant was initially located near the surface. Other samples had the dopant initially located in a buried layer. Both types of samples were analyzed by secondary ion mass spectrometry measurement. The variations in the diffusion behavior for these different experimental conditions can all be qualitatively explained by a model which accounts for three important effects: the transient evolution of point defect populations; the injection of Ga vacancies by an oxide cap; and the efficiency of the surface in restoring point defect equilibria. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.115632
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  • 10
    Electronic Resource
    Electronic Resource
    Nullspace shuttles (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 124 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In seismic tomography the problem is generally underdetermined. The solution to the tomographic problem depends on the specific optimization condition used and is inherently distorted due lo noise in the data and approximations in the theory. Smoothing is often applied to reduce inversion artefacts with short correlation lengths. However, a posteriori smoothing generally affects the data fit. For more sophisticated, non-linear filters this effect can be severe. We present a technique to conserve data fit for filters of arbitrary complexity. The difference between the ‘optimal’ solution and a filtered version is projected onto the nullspace of the model space in order to preserve the data fit. Thus, we only allow changes to the image that do not conflict with the data. We demonstrate the benefits of such conservative filters using several different non-linear filters to reduce noise, smooth the image, and highlight edges.The method is exact in small-scale experiments, where we can use the method of singular value decomposition: eigenvectors with large eigenvalues are used to project the difference between the original model and the filtered version onto the nullspace. With large-scale tomographic problems, calculation of all of the large eigenvectors is unrealistic. We show how to use the iterative method of conjugate gradients to apply conservative filters to large-scale tomographic problems with minimum computational effort.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb07027.x
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