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0003-2670

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The photoluminescence (PL) of porous Si that has been hydrogenated in boiling water has been investigated. The PL intensity is observed to increase with a concurrent shift of the spectral peak to shorter wavelengths. These effects can be explained in terms of size effects in the microstructures of porous Si. The spectral changes after annealing and after rehydrogenation are similar to the behavior of a-Si:H. We conclude that hydrogen plays an important role in the luminescence of porous Si.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Single-crystal GexSi1−x/Si superlattices have been successfully fabricated using remote plasma-enhanced chemical vapor deposition, a novel low-temperature thin-film growth technique. Reflection high-energy electron diffraction, cross-sectional transmission electron microscopy (XTEM), plan view TEM, x-ray-diffraction, and secondary-ion mass spectroscopy techniques have been applied to study the crystallographic properties of the superlattice structures. Arrays of dislocation lines, which are either parallel or perpendicular to each other, have been observed in the superlattices for those cases in which the total layer thickness exceeds the critical layer thickness. The location, orientation, and Burgers vectors of the misfit dislocation lines have been analyzed. Possible mechanisms of the generation of the misfit dislocations are also discussed.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Photoreflectance spectroscopy has been used to study the surface electric-field strength and the surface potential of delta-doped GaAs. Franz–Keldysh oscillations in the reflectance spectra were clearly observed and the oscillation periods were used to calculate the internal electric fields of the delta-doped samples. Based on the measured results and the self-consistent calculation, a surface potential of 0.6 eV is obtained.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The p-d hybridization of the valence bands of copper indium disulfide has been studied by photoreflectance. A simple model has been used to measure the separation E between the p and d levels, the interaction strength M between these levels, the d-electron contributions x0 to the E0 energy level, and x1 to the E0 + Δ0 energy level. Our results confirm that the d-electron contributions decrease when the temperature is increased. Our measurements of x0 and x1 agree with the calculation of Yoodee et al. [Phys. Rev. B 30, 5904 (1984)].

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The temperature dependence of the energy gaps for sulfur-annealed copper indium disulfide has been studied by photoreflectance in the temperature range of 10–300 K. The sulfur-annealed sample has been found to have larger transition energies, smaller positive temperature coefficients of energy gaps, and larger spin-orbit splitting energy than the as-grown sample. This can be explained by the reduction of d-level contributions in the upper valence band probably caused by the variation of lattice distance due to native defects.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Si-δ-doped GaAs (N2D ≈ 1011 cm−2) samples grown by molecular-beam epitaxy are investigated by using photoreflectance spectroscopy. The oscillations observed above the GaAs fundamental band gap are attributed to the Franz–Keldysh effect in the region between the δ-doped layer and the crystal surface. This ascription is confirmed by detailed studies through varying the cap thickness (250–2500 A(ring)), temperature (10–450 K), and laser pump power (0.05–7 mW/cm2). The surface potential deduced from the Franz–Keldysh oscillations is found to be temperature and laser pump power dependent, which is explained by taking the surface photovoltaic effect into account. The surface Fermi level has been measured by this method and is found to have the value 0.73±0.02 V.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

High oxygen pressure annealed PrBa2Cu3O7−y (y∼0) cuprates were prepared in order to study the effect of oxygen stoichiometric parameter y on the unusual Pr/Cu magnetic properties and/or recently reported superconductivity. The oxygen-rich orthorhombic 123-chain phase is highly unstable under high-oxygen pressure synthesis and decomposes completely in 10 bar pressure. For a smaller 2 bar prepared sample a relatively clean phase was preserved with an oxygen parameter y=0.05, as compared with y=0.11 from a conventional 1 bar flowing oxygen method. No superconductivity can be detected for all high-oxygen pressure prepared samples. Instead, Mott-insulator behavior with anomalous high Pr ordering TN(Pr)=19 K was observed for PrBa2Cu3O6.95. Comparison with other Pr/Ba intersubstituted Pr1−xBa2−xCu3O7−y cuprates is discussed. © 2001 American Institute of Physics.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

A series of transport experiments has been carried out in Alcator-C-Mod. [Phys Plasmas 1, 1511 (1994)]. Data from both Ohmic and ICRF (ion cyclotron range of frequencies) heated plasmas can be fitted with an L-mode (low mode) scaling law. The Ohmic τE's show no scaling with density in any regime and can reach values of 2–3 times neo-Alcator. Impurity confinement has been studied with the laser blow-off technique with τI showing nearly linear scaling with plasma current. Ohmic and ICRF H modes are obtained over a wide range of discharge parameters, extending the range in the international database for nB, by almost a factor of 10. The power threshold for ELM-free (edge localized mode) discharges is in rough agreement with the scaling P/S=0.044nB. Energy diffusivities of Ohmic and ICRF heated plasmas have been measured from local analysis of plasma profiles and power fluxes. The same analysis produces a value for plasma resistivity which lies between the Spitzer and neoclassical calculations. Analysis of plasma transients have yielded values for particle diffusivity and convection velocity. © 1995 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We present the scanning electron-filling modulation reflectance (SEFR) of charged In0.5Ga0.5As self-assembled quantum dots. The SEFR is performed by applying a small constant modulation voltage and scanning the dc bias through the quantum dot energy levels. The spectral intensity of the SEFR can be related to the electron density confined in each quantum state, which shows a good agreement with the apparent carrier concentration deduced from the capacitance–voltage measurements. In this study, we find that the transition energy of quantum-dot excited state is about 10 meV smaller than the values obtained from the photoluminescence and electron-filling modulation reflectance. This phenomenon is explained by the Coulomb interactions between the optical excited electron–hole pair and the electrons occupied in the quantum dots. © 2002 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We studied the subband energy levels in AlGaAs/GaAs multiple quantum wells grown at different temperatures (200–600 °C) by using the piezoreflectance and photoreflectance in room temperature. Under subsequent 30 s rapid-thermal annealing at different temperatures (600–1000 °C), we observed a large energy blueshift in samples with growth temperature below 400 °C. This blueshift energy may be attributed to the modification of quantum wells caused by gallium vacancy enhanced Al–Ga interdiffusion. The energy blueshifts were analyzed by solving Fick's second law for Al diffusion in quantum wells, obtaining an effective activation energy of 0.75 eV. We discuss our results using the nonequilibrium diffusion equation and comparing them with other experiments. © 2000 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A natural monolayer {111} superlattice (the CuPt ordered structure) is formed spontaneously during organometallic vapor phase epitaxial (OMVPE) growth of Ga0.52In0.48P. The extent of this ordering process is found to be a strong function of the input partial pressure of the phosphorus precursor during growth due to the effect of this parameter on the surface reconstruction and step structure. Thus, heterostructures can be produced by simply changing the flow rate of the P precursor during growth. It is found, by examination of transmission electron microscope (TEM) and atomic force microscope (AFM) images, and the photoluminescence (PL) and PL excitation (PLE) spectra, that order/disorder (O/D) (really less ordered on more ordered) heterostructures formed by decreasing the partial pressure of the P precursor during the OMVPE growth cycle at a temperature of 620 °C are graded over several thousands of Å when PH3 is the precursor. The ordered structure from the lower layer persists into the upper layer. Similarly, D/O structures produced by increasing the PH3 flow rate yield PL spectra also indicative of a graded composition at the heterostructure. The grading is not reduced by a 1 h interruption in the growth cycle at the interface. Similar heterostructures produced at 670 °C using tertiarybutylphosphine (TBP) as the P precursor show a totally different behavior. Abrupt D/O and O/D heterostructures can be produced by abruptly changing the TBP flow rate during the growth cycle. PL and PLE studies show distinct peaks closely corresponding to those observed for the corresponding single layers. TEM dark field images also indicate that the interfaces in both for D/O and O/D heterostructures are abrupt. The cause of the difference in behavior for TBP and PH3 is not clear. It may be related to the difference in temperature. © 1997 American Institute of Physics.

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