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  • 1
    Electronic Resource
    Electronic Resource
    Synthesis and biological properties of .alpha.-mono and .alpha.-difluoromethyl derivatives of tryptophan and 5-hydroxytryptophan (1988)
    s.l. : American Chemical Society
    Journal of medicinal chemistry 31 (1988), S. 30-36 
    Details
    ISSN: 1520-4804
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/jm00396a007
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  • 2
    Electronic Resource
    Electronic Resource
    The structure and spin dynamics of lanthanide-bearing silicate glasses : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6825-6827 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The structure of 3Na2O⋅R2O3⋅6SiO2 (R=Nd, Yb and Lu) glasses has been studied by time-of-flight neutron diffraction. The average Si-O and O-O nearest-neighbor coordination is found to be similar to those of the SiO4 tetrahedral units in vitreous SiO2 and other silicate glasses. The rare-earth (RE) ions coordinate with no more than seven nearest-neighbor oxygen atoms. The dynamic and static response of these glasses and their isochemical crystalline analogs has been investigated by inelastic neutron scattering and magnetic susceptibility measurements. The magnetic excitation spectra were analyzed by a ligand-field model using a method of descending symmetry. The correlation between short-range atomic order in a network glass and magnetic interactions of the RE ions with the local environment is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.356797
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  • 3
    Electronic Resource
    Electronic Resource
    Epitaxial overgrowth of 13C diamond films on diamond substrates predamaged by ion implantation (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 1720-1725 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Homoepitaxial chemical vapor deposited (CVD) 13C diamond films were grown on 〈100〉 diamond substrates predamaged by implantation with 620 keV Xe ions. The structural quality of the overgrown films was analyzed by plain-view and cross-sectional micro-Raman spectroscopy. Implantation doses below 2×1014 cm−2, for which no damage detectable by Raman spectroscopy was observed in the substrate, had no effect on the quality of the overgrown films. For doses around 4×1014 cm−2, a pronounced predamage of the diamond substrate was found which had a strong degrading effect on the quality of the overgrown layer resulting in a drastic increase of the width of the optic zone-center phonon and in the appearance of Raman scattering from sp2-bonded carbon. Higher implantation doses up to 1×1015 cm−2 resulted in a complete etch removal of the predamaged graphitic surface layer during the initial phase of CVD growth, which thus had no effect on the quality of the film produced by the subsequent overgrowth. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.364029
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  • 4
    Electronic Resource
    Electronic Resource
    Influence of interdiffusion processes on optical and structural properties of pseudomorphic In0.35Ga0.65As/GaAs multiple quantum well structures (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 6818-6825 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Interdiffusion has been investigated in molecular-beam epitaxially (MBE)-grown, highly strained In0.35Ga0.65As/GaAs multiple quantum well (MQW) structures. Thermal intermixing and impurity-free interdiffusion (IFID) was induced via rapid thermal annealing (RTA) at temperatures between 700 and 950 °C using GaAs proximity caps and electron-beam evaporated SiO2 cap layers, respectively. Both reduced photoluminescence (PL) linewidths and increased PL intensities were observed following interdiffusion-induced band-gap shifts ranging from 6 to 220 meV. PL microscopy (PLM) investigations were utilized to study the onset of strain relaxation due to dislocation generation. Two types of line defects were found in the proximity-cap annealed samples, depending on the annealing temperature and the number of QWs: misfit dislocations with the dislocation lines parallel to 〈110〉 directions and 〈100〉-oriented line defects. No dislocations were observed in the SiO2-cap annealed samples over the entire temperature range investigated here. Resonant Raman scattering measurements of the 1LO/2LO phonon intensity ratio were used for a semiquantitative assessment of the total defect densities, including point defects (PDs). Whereas increasing PD densities and the formation of line defects were observed in the proximity-capped samples as the annealing temperature was increased, no deterioration of the structural quality due to an increased PD density was observed in the case of the SiO2-cap annealed samples. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361503
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  • 5
    Electronic Resource
    Electronic Resource
    Silicon incorporation in GaAs: From delta-doping to monolayer insertion (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 2431-2434 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Raman spectroscopy was used to study the incorporation of Si into doping layers in GaAs, grown by molecular beam epitaxy at a temperature of 400 °C, for Si concentrations ranging from the δ-doping level to a ML coverage. The strength of the scattering by local vibrational modes of substitutional Si was almost constant for Si areal concentration [Si]A in the range 5×1012〈[Si]A〈5×1013 cm−2 but then decreased, dropping below the detection limit for [Si]A(approximately-greater-than)3×1014 cm−2. At these concentrations a new vibrational band emerged at a frequency close to 470 cm−1 and developed into the optic zone center phonon of a coherently strained epitaxial layer of Si embedded in GaAs when a coverage of ≈1.5 ML (9.3×1014 cm−2) was reached. These findings strongly indicate that the observed saturation and the eventual decrease of the concentration of substitutional silicon is caused by an increasing incorporation of deposited Si into two-dimensional islands of covalently bonded Si. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360165
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  • 6
    Electronic Resource
    Electronic Resource
    The bonding of CAs acceptors in InxGa1−xAs grown by chemical beam epitaxy using carbon tetrabromide as the source of carbon (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 6754-6760 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: InxGa1−xAs layers (0≤x≤0.37) doped with carbon (〉1020 cm−3) were grown on semi-insulating GaAs substrates by chemical beam epitaxy using carbon tetrabromide (CBr4) as the dopant source. Hall measurements imply that all of the carbon was present as CAs for values of x up to 0.15. The C acceptors were passivated by exposing samples to a radio frequency hydrogen plasma for periods of up to 6 h. The nearest-neighbor bonding configurations of CAs were investigated by studying the nondegenerate antisymmetric hydrogen stretch mode (A−1 symmetry) and the symmetric XH mode (A+1 symmetry) of the H–CAs pairs using IR absorption and Raman scattering, respectively. Observed modes at 2635 and 450 cm−1 had been assigned to passivated Ga4CAs clusters. New modes at 2550 and 430 cm−1 increased in strength with increasing values of x and are assigned to passivated InGa3CAs clusters. These results were compared with ab initio local density functional theory. Modes due to AlInGaCAs clusters were detected in samples containing grown in Al and In. These results demonstrate that for InGaAs, CBr4 is an efficient C doping source since both In–CAs bonds as well as Ga–CAs bonds are formed, whereas there is no evidence for the formation of In–CAs bonds in samples doped with C derived from trimethylgallium or solid sources. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363803
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  • 7
    Electronic Resource
    Electronic Resource
    Influence of In on Si local vibrational modes in InxGa1−xAs (0≤x≤0.12) (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 7797-7804 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Local vibrational modes (LVM) of Si in substitutional sites have been observed by resonant Raman spectroscopy in highly doped (≥8×1018 cm−3) InxGa1−xAs layers, either relaxed or under strain, on [100] GaAs substrates. The peak frequency ωLVM of the Si on Ga site (SiGa) LVM in unstrained samples shifts to lower values with increasing In content. For x≤0.10 this shift is clearly higher than expected from a linear interpolation between the measured values in the binaries. The comparison between the SiGa peak frequency measured in both a full strained layer and a relaxed layer with similar composition provides a rough determination of the deformation potentials for the SiGaLVM in these layers: q/ω2LVM=−2.7±1 and p/ω2LVM=−2.5±1. As the In content becomes higher the width of the SiGa peak increases much more than that of the GaAs-like longitudinal optical-phonon peak, revealing the splitting due to the loss of local symmetry introduced by the In. New calibration factors for the Si-defect concentrations have been deduced, which allow estimation of the solubility limit for the Si incorporation in substitutional positions, which ranges from 2.3×1019 to 2.6×1019 cm−3 for the layers at the growth conditions used. The analysis of the integrated intensity of the LVM Raman peaks indicates that the degree of electrical compensation is clearly reduced for increasing In up to x≤0.05, due to both an increase of the solubility limit for Si in these layers and a saturation or slight reduction of the SiAs-related defect concentrations. This conclusion is also supported by Hall and plasmon measurements. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358521
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  • 8
    Electronic Resource
    Electronic Resource
    A Raman spectroscopic study of the Si, Be, and C incorporation in InxGa1−xAs relaxed layers (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 4690-4695 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The incorporation of high concentrations ((approximately-greater-than)1019 cm−3) of Si, Be, and C in InxGa1−xAs relaxed layers has been studied as a function of In content (x≤0.16) by Raman spectroscopy of local vibrational modes (LVM). The frequencies of the Raman peaks resulting from the convolution of several split modes shift to lower values as the In content is increased, the carbon acceptor (CAs) local mode frequency showing the strongest dependency on x. Features attributed exclusively to the influence of In on second-neighbor sites are identified only in the Si-doped samples. The transverse and longitudinal modes expected from the splitting of a LVM of CAs with one In first neighbor are not observed in layers with x up to 0.085. A new calibration for the BeGa, CAs, and Si-related LVM leads to the conclusion that In increases the total electrical activation of Si and favors its incorporation on group-III sublattice sites. In contrast, no influence of In on the Be or C doping activation has been detected. The analysis of the CAs LVM spectra supports the view that in InxGa1−xAs CAs is preferably surrounded by Ga instead of In atoms for x≤0.085. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.359816
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  • 9
    Electronic Resource
    Electronic Resource
    The transition from dilute aluminum δ structures to an AlAs monolayer in GaAs and a comparison with Si δ doping (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 7627-7629 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Superlattices of Al δ layers embedded in GaAs have been grown by molecular beam epitaxy at 400 °C on (001) GaAs. Infrared absorption measurements revealed a shift of the Al localized vibrational mode (LVM) from 361.4 to 358.3 cm−1 as the doping was increased from 0.25 monolayers (ML) to 1.4 ML per δ plane, while Raman scattering demonstrated a clear transition from the TO-like LVM to a longitudinal LO-like mode. X-ray measurements showed that the Al atoms were confined to a thickness of 6 A(ring) for the highest areal concentrations. The LVM behavior is compared with that of Si δ layers where the absorption tends to zero as the areal concentration increases up to 0.5 ML. It is concluded that SiGa atoms move off their lattice sites and/or change their charge states as the coverage approaches 0.5 ML. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357931
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  • 10
    Electronic Resource
    Electronic Resource
    A local vibrational mode investigation of p-type Si-doped GaAs (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 7839-7849 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Infrared absorption (IR) and Raman scattering measurements have been made of the localized vibration modes (LVM) due to defects incorporating silicon impurities in p-type Si-doped GaAs grown by liquid phase epitaxy (LPE) on (001) planes and by molecular beam epitaxy (MBE) on (111)A and (311)A planes. Analysis of a closely compensated LPE sample indicated that an existing calibration factor for the SiAs LVM (399 cm−1) relating the integrated absorption coefficient (IA) to the concentration [SiAs] should be increased by 40%, so that IA=1 cm−2 corresponds to [SiAs]=7×1016 cm−3. The SiAs LVM appeared as a Fano dip in the hole absorption continuum at ∼395 cm−1 in the highly doped p-type material, some 4 cm−1 lower in frequency than its normal position in compensated GaAs. Electron irradiation of samples led to the progressive removal of the Fano dip and a shift with the emergence of the expected SiAs LVM absorption line at 399 cm−1. In MBE samples the irradiation also generated SiGa donors, but the site switching was not detected in LPE material. By contrast, Raman spectra of as-grown p-type samples exhibited a symmetrical peak at 395 cm−1, which also shifted towards 399 cm−1 as the free carriers were removed. MBE (111)A GaAs:Si compensated by SnGa donors revealed the SiAs LVM at its normal position. After hydrogenation of MBE and LPE samples, only stretch modes due to H-SiAs were observed. Passivated MBE GaAs (111)A codoped with Si and Be showed stretch modes due to both shallow acceptors. It was thereby concluded that only one type of acceptor (SiAs) was present in p-type Si-doped GaAs, contrary to previous proposals. There was no evidence for the presence of SiAs pairs or larger clusters. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357892
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