Resonant Raman scattering has been used to study amorphous carbon and polycrystalline diamond films. The incident photon energies were varied over the range 2.2-4.8 eV. In hydrogenated amorphous carbon films containing both sp high 3 and sp high 2 bonded carbon, a high-frequency shift is interpreted in terms of scattering from pi-bonded carbon clusters which is resonantly enhanced for photon energies approaching the pi-pi resonance of sp high 2 bonded carbon. In polycrystalline diamond films excitation with photon energies equal or bigger than 3.0 eV enhances the Raman signal from sp high 3 bonded diamond phase relative to the scattering by sp high 2 bonded carbon and with respect to the underlying broadband luminescence. The Raman band arising from scattering by sp high 2 bonded carbon shows a high-frequency shift with increasing photon energy for energies equal or bigger than 3.0 eV. Possible models for the structure of this sp high 2 bonded carbon phase are discussed on the basis of the present Raman data
The optical and electrical properties of infrared photodiodes diodes based on InAs/(GaIn)Sb superlattices grown by molecular beam epitaxy were investigated. The diodes, with a cut-off wavelength around 8 µm show a current responsivity of 2 A/W. By proper adjustment of the p-doping level above the n-background concentration the depletion width exceeds a critical size of about 60 nm, leading to the suppression of band-to-band tunneling currents. Above that critical width the dynamic impedance R0A at 77 K reaches values above 1 kV cm2 leading to a Johnson-noise-limited detectivity in excess of 1X10(12) cm/Hz/W
We report the direct observation of hot carriers generated by Auger recombination via photoluminescence spectroscopy on tailored (AlGaIn)N multiple quantum well (QW) structures containing alternating green and ultra-violet (UV) emitting (GaIn)N QWs. Optically pumping solely the green QWs using a blue emitting high power laser diode, carrier densities similar to electrical light-emitting diode (LED) operation were achieved, circumventing possible leakage and injection effects. This way, luminescence from the UV QWs could be observed for excitation where the emission from the green QWs showed significant droop, giving direct evidence for Auger generated hot electrons and holes being injected into the UV QWs. An examination of the quantitative relation between the intensity of the UV luminescence and the amount of charge carriers lost due to drooping of the QWs supports the conclusion that Auger processes contribute significantly to the droop phenomenon in (AlGaIn)N based light-emitting diodes. Due to their high lifetimes and efficiencies along with rapidly declining prices, light-emitting diodes (LEDs) based on (AlGaIn)N multiple quantum well (MQW) structures are on their way to replace incandescent as well as fluorescent lighting. Despite great progress in recent years, resulting in peak power conversion efficiencies of up to 81%, 1 one obstacle still to overcome is the decrease in efficiency towards high operating current densities, a phenomenon commonly known as droop. 2,3 The current dependency of the internal quantum efficiency (IQE) can be modeled in good quantitative agreement with experimental data using an ABC rate equation model [4][5][6]
Resonant Raman spectroscopy has been used to study amorphous hydrogenated carbon films. For films containing both sp2 and sp3 bonded carbon a well-defined high-frequency shift of the main Raman peak is observed with increasing exciting photon energy. This shift is interpreted in terms of scattering from π-bonded carbon clusters which is resonantly enhanced for incident photon energies approaching the π–π* resonance.
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