Overcoming diffusion-related limitations in semiconductor defect imaging with phonon-plasmon-coupled mode Raman scattering.

Carrier diffusion is of paramount importance in many semiconductor devices, such as solar cells, photodetectors, and power electronics. Structural defects prevent such devices from reaching their full performance potential. Although a large carrier diffusion length indicates high material quality, it also implies increased carrier depletion by an individual extended defect (for instance, a dislocation) and obscures the spatial resolution of neighboring defects using optical techniques. For commonly utilized photoluminescence (PL) imaging, the spatial resolution is dictated by the diffusion length rather than by the laser spot size, no matter the spot is at or below the diffraction limit. Here, we show how Raman imaging of the LO phonon-plasmon-coupled mode can be used to recover the intrinsic spatial resolution of the optical system, and we demonstrate the effectiveness of the technique by imaging defects in GaAs with diffraction-limited optics, achieving a 10-fold improvement in resolution. Furthermore, by combining Raman and PL imaging, we can independently and simultaneously determine the spatial dependence of the electron density, hole density, radiative recombination rate, and non-radiative recombination rate near a dislocation-like defect, which has not been possible using other techniques.

Spatial resolution versus data acquisition efficiency in mapping an inhomogeneous system with species diffusion.

Traditionally, spatially-resolved photoluminescence (PL) has been performed using a point-by-point scan mode with both excitation and detection occurring at the same spatial location. But with the availability of high quality detector arrays like CCDs, an imaging mode has become popular for performing spatially-resolved PL. By illuminating the entire area of interest and collecting the data simultaneously from all spatial locations, the measurement efficiency can be greatly improved. However, this new approach has proceeded under the implicit assumption of comparable spatial resolution. We show here that when carrier diffusion is present, the spatial resolution can actually differ substantially between the two modes, with the less efficient scan mode being far superior. We apply both techniques in investigation of defects in a GaAs epilayer - where isolated singlet and doublet dislocations can be identified. A superposition principle is developed for solving the diffusion equation to extract the intrinsic carrier diffusion length, which can be applied to a system with arbitrarily distributed defects. The understanding derived from this work is significant for a broad range of problems in physics and beyond (for instance biology) - whenever the dynamics of generation, diffusion, and annihilation of species can be probed with either measurement mode.

Pharmacokinetics, cortisol release, and hemodynamics after intravenous and subcutaneous injection of human corticotropin-releasing factor in humans.

OBJECTIVE: Two clinical trials investigated the pharmacokinetics of human corticotropin-releasing factor (hCRF), resulting cortisol release, and associated hemodynamic changes. METHODS: In a 3 x 3 Latin square design, subjects were randomized to receive a single dose of 5 microg x kg(-1) hCRF as a 10-minute intravenous infusion, a 180-minute infusion, and a subcutaneous injection in separate study sessions 7 days apart. Twelve additional subjects obtained a subcutaneous dose of either 300, 600, or 1200 microg hCRF on 3 consecutive days. Noncompartmental and compartmental pharmacokinetic analysis was performed. Hemodynamic response was characterized with use of pharmacodynamic models. RESULTS: The volume of distribution at steady state was 9.81 +/- 3.0 and 15.61 +/- 2.9, and the clearance was 256 +/- 40 mL x min(-1) and 345 +/- 90 mL x min(-1) for the 10-minute and 180-minute intravenous infusion, respectively (P < .05). Corresponding elimination half-life was 45 +/- 7 minutes and 37 +/- 10 minutes. Two-compartment and 1-compartment models adequately described the 10-minute and 180-minute infusions, respectively. The bioavailability of hCRF after subcutaneous administration was 67% +/- 17%. Apparent clearance remained unchanged for different subcutaneous doses. Peak plasma cortisol concentrations were similar after subcutaneous and intravenous administration of hCRF. Repetitive administration of hCRF did not result in accumulation but produced a reduced plasma cortisol response. A sigmoidal model related plasma hCRF concentrations to increase in heart rate (maximum, 39 beats x min(-1)). The relationship between the modest decrease in diastolic blood pressure and plasma hCRF concentrations was linear. CONCLUSION: The pharmacokinetics of intravenously administered hCRF were nonlinear, but apparent clearance was constant for various subcutaneous doses. An excellent bioavailability and preserved bioactivity make the subcutaneous route an attractive choice. Repetitive administration of hCRF probably caused tolerance of the cortisol response.