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  • 1
    Electronic Resource
    Electronic Resource
    Temperature dependent vibrational lifetimes in supercritical fluids near the critical point (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 5971-5979 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational relaxation measurements on the CO asymmetric stretching mode (∼1980 cm−1) of tungsten hexacarbonyl (W(CO)6) as a function of temperature at constant density in several supercritical solvents in the vicinity of the critical point are presented. In supercritical ethane, at the critical density, there is a region above the critical temperature (Tc) in which the lifetime increases with increasing temperature. When the temperature is raised sufficiently (∼Tc+70 °C), the lifetime decreases with further increase in temperature. A recent hydrodynamic/thermodynamic theory of vibrational relaxation in supercritical fluids reproduces this behavior semiquantitatively. The temperature dependent data for fixed densities somewhat above and below the critical density is in better agreement with the theory. In fluoroform solvent at the critical density, the vibrational lifetime also initially increases with increasing temperature. However, in supercritical CO2 at the critical density, the temperature dependent vibrational lifetime decreases approximately linearly with temperature beginning almost immediately above Tc. The theory does not reproduce this behavior. A comparison between the absolute lifetimes in the three solvents and the temperature trends is made. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477222
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  • 2
    Electronic Resource
    Electronic Resource
    Temperature dependence of vibrational lifetimes at the critical density in supercritical mixtures (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 9741-9748 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Experimental measurements are reported for the temperature dependence of the vibrational lifetime, T1, of the asymmetric CO stretching mode of tungsten hexacarbonyl in supercritical ethane at constant density from just above the critical temperature to substantially higher temperatures. T1 is found initially to increase with temperature along an isochore (reaching a maximum at about 70° above the critical point of ethane), and then subsequently to decrease. Using a recent classical theory of vibrational relaxation, we attempt to rationalize the T1 data. This behavior can be semiquantitatively reproduced by the theory if quantum corrections to the classical rate expressions are assumed to be temperature independent in the limit when the transition energy is much greater than thermal energy. In this case, the theory indicates that the initial increase in T1 with temperature arises because of a competition between properties of the solvent which are changing rapidly as the temperature is raised above the critical temperature. At sufficiently high temperature, properties of the solvent vary slowly with temperature, and the explicit temperature dependence of the vibrational relaxation dominates, producing a decrease in T1 with increasing temperature. The predictions of the theory are also examined when other postulated forms of the quantum correction factors are used, and the implications of these results for theoretical approaches to vibrational relaxation are discussed. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475270
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  • 3
    Electronic Resource
    Electronic Resource
    Vibrational lifetimes and vibrational line positions in polyatomic supercritical fluids near the critical point (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3747-3757 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Picosecond infrared pump–probe experiments are used to measure the vibrational lifetime of the asymmetric (T1u) CO stretching mode of W(CO)6 in supercritical CO2, C2H6, and CHF3 as a function of solvent density and temperature. As the density is increased at constant temperature from low, gaslike densities, the lifetimes become shorter. However, in all three solvents, it is found that within a few degrees of the critical temperature (Tr≡T/Tc(approximate)1.01), the lifetimes are essentially constant over a wide range of densities around the critical value (ρc). When the density is increased well past ρc, the lifetimes shorten further. At higher temperature (Tr=1.06) this region of constant vibrational lifetime is absent. Infrared absorption spectra of W(CO)6 and Rh(CO)2acac in supercritical CO2, C2H6, and CHF3 acquired for the same isotherms show that the vibrational spectral peak shifts follow similar trends with density. The peak positions shift to lower energy as the density is increased. Near the critical point, the peak positions are density independent, and then redshift further at densities well above ρc. It is shown that critical fluctuations play a dominant role in the observed effects. Theoretical calculations ascribe the density independence of the observables to the cancellation of various rapidly changing quantities near the critical point. The theory's calculation of density independence implicitly involves averages over all local densities and does not involve any form of solute–solvent clustering. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474732
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  • 4
    Electronic Resource
    Electronic Resource
    Zinc-implantation-disordered (InGa)As/GaAs strained-layer superlattice diodes (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 1131-1134 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have examined the properties of (InGa)As/GaAs strained-layer superlattices (SLSs) that have been disordered by implantation of 5×1015/cm2, 250 keV 64Zn+ followed by controlled atmosphere annealing at 680 °C for 30 min. Ion channeling techniques indicate that the Zn-disordered regions of the SLS contain extensive crystalline damage after annealing. Simulations of the disordering process using an analytic ion range code predict that the electrical junction resulting from the implantation process is located outside the disordered region of the SLS in both the vertical and the lateral directions. Junction electroluminescence intensity for given drive current densities from the Zn-disordered SLS devices is comparable to that from reference Be-implantation-doped (SLS retained) devices and greatly exceeds that from heavily dislocated grown-junction mesa diodes in the homogeneous alloy of the average SLS composition; this result is consistent with the results of the simulations. This study demonstrates that implantation disordering can be as useful for strained-layer systems as for less severely mismatched heterojunction systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337356
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  • 5
    Electronic Resource
    Electronic Resource
    Characterization of ion-implantation doping of strained-layer superlattices. I. Structural properties (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 3631-3640 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have characterized the effects of Be+ implantation and controlled-atmosphere annealing on the structure of Ga(AsP)/GaP strained-layer superlattices (SLSs). Damage and strain distributions within the implanted layers were examined by cantilever-beam bending measurements, double-crystal x-ray rocking curves, and a variety of ion-channeling techniques. Implantation-induced displacement damage produces additional stress in the SLS, in this case reaching 4.5×109 dyn/cm2, a value comparable to that of the built-in stresses in these SLSs. The depth distribution of ion damage as measured by ion channeling agrees well with the predictions of the trim code, although substantial recovery occurs during the room-temperature implant. Rocking curve analysis indicates that the interlayer strain in the SLS is retained despite the ion damage, and that the ion damage can be modelled as an independent additional source of strain in the as-implanted structure. The linear expansion of the layers due to point defect generation for the 1×1015 Be/cm2 implant is determined to be approximately 0.3% by all three techniques. After controlled-atmosphere annealing at the nominal SLS growth temperature, both the x-ray and ion-channeling measurements indicate removal of the implant damage with the as-grown strain retained and no resolvable intermixing of the layers in the SLS. These results demonstrate that ion-implantation technologies developed for bulk semiconductors can successfully be applied to strained-layer superlattice systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337570
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  • 6
    Electronic Resource
    Electronic Resource
    Characterization of ion-implantation doping of strained-layer superlattices. II. Optical and electrical properties (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 3641-3650 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have investigated the properties of Ga(AsP)/GaP strained-layer superlattices (SLSs) that have been doped by implantation of 1×1015/cm2, 75 keV Be+ followed by controlled-atmosphere annealing at 825 °C for 10 min. Our results indicate that doping of these strained-layer superlattices without disordering is a viable process. Liquid-helium temperature photoluminescence suggests a binding energy for the implanted acceptors of 50 meV, consistent with that of beryllium in GaP-based alloys. The implantation-doped regions exhibit room-temperature electrical activation of 15% and hole mobilities of 20 cm2/V s, consistent with the values expected for type-converted GaP-based alloys. SLS diodes fabricated by this process exhibit excellent rectification properties, with a forward turn-on voltage of approximately 1.8 V and low values of room-temperature reverse leakage current densities. Diodes formed from SLSs with original n-type doping of 1×1017/cm3 have typical reverse leakage current densities of 1×10−7 A/cm2 at −10 V, despite the depletion region penetrating approximately ten interfaces of the SLS at this bias. Deep-level transient spectroscopy demonstrates the existence of defect centers, whose densities and signatures are similar to those found in ion-implanted GaP. The implanted photodiodes exhibit a wavelength-dependent photoresponse characteristic of grown-junction SLS photodetectors in the same chemical system. Examination of the spatial response of the photodiodes to a tightly focussed (FWHM=2.45 μm) laser beam at a wavelength of 488 nm indicates that the photoresponse from the device is uniform to within 10% for regions away from the edges of the implanted regions. Modelling of the wavelength-dependent and the spatially dependent photoresponse allows an estimate of minority-carrier diffusion lengths for electrons and holes of 1.0 μm parallel to the SLS layers and 0.1 μm perpendicular to the SLS layers. The excellent electrical and optical properties of the implanted and annealed SLS materials implies additional device applications for these novel materials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337796
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  • 7
    Electronic Resource
    Electronic Resource
    Characterization of thin AlGaAs/InGaAs/GaAs quantum-well structures bonded directly to SiO2/Si and glass substrates (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 459-462 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Strained GaAs/InGaAs/AlGaAs quantum-well structures grown on GaAs have been removed from their original substrates by a lift-off process and bonded directly to glass or SiO2-coated Si substrates. Both undoped and modulation-doped structures have been characterized before and after transfer by Hall measurements, variable temperature x-ray diffraction, and photoluminescence. The bonded structures retain the high quality of the as-grown layers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.343850
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  • 8
    Electronic Resource
    Electronic Resource
    Useful design relationships for the engineering of thermodynamically stable strained-layer structures (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 65 (1989), S. 4769-4773 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent studies have provided sufficient knowledge about the dominant failure mechanisms for lattice-mismatched strained-layer heterostuctures to permit the design of thermodynamically stable strained-layer systems for device applications. We have developed procedures that summarize this knowledge for the working device designer, and apply these relationships to the design of ion-implanted, strained-layer, quantum-well lasers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.343231
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  • 9
    Electronic Resource
    Electronic Resource
    Selective porous silicon formation in buried p+ layers (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 62 (1987), S. 4182-4186 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report a systematic microstructural study of enhanced lateral porous silicon formation in the buried p+ layers of n/p+/p− and p−/p+/p− structures. We find, surprisingly, extremely selective porous silicon formation due to the thin p+ layer in both structures, despite the absence of a p-n junction in the p−/p+/p− structure. The interface between the isolated island and the buried porous silicon layer was always located at the depth where the net p-type dopant concentration was 1–8×1015/cm3. The observed microstructure can largely be understood in terms of a recent model for porous Si formation in uniformly doped Si, proposed by Beale et al. [J. Cryst. Growth 73, 622 (1985)]. However, we also observe, for the first time, important effects unique to a nonuniform dopant concentration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.339086
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  • 10
    Electronic Resource
    Electronic Resource
    Validity of the Ruff–MKW boiling point method: Vapor pressures, diffusion coefficients in argon and helium, and viscosity coefficients for gaseous cadmium and zinc (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2447-2456 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Ruff–MKW boiling point method is used to determine equilibrium vapor pressures greater than 660 Pa (5 Torr). Samples are vaporized from a Ruff cell, which has a capillary exit, in the presence of an inert gas. Viscosity coefficients and gaseous interdiffusion coefficients may be determined also. This is a second study of the method using Cd(l) and Zn(l) as samples. For the first study with CsCl(l), see J. Chem. Phys. 81, 915 (1984). Vapor pressure data are in good agreement with previous data and gave a third-law ΔsubH0(298) for Cd(s) of 111.95±0.42 kJ/mol and for Zn(s) of 130.65±0.48 kJ/mol. Analyses of the diffusion coefficients gave atomic diameters of 4.06×10−10 m for Cd and 3.46×10−10 m for Zn; these values are somewhat larger than previously measured values. In these experiments when the equilibrium vapor pressures were greater than 13 000 Pa (100 Torr), the need to consider heat transfer from the furnace to the vaporizing sample was noted, i.e., sample cooling occured due to rapid vaporization. Validity of the MKW analysis was found.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449291
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