1,721,049 research outputs found
Mg-modified surface kinetics of the GaN growth by molecular beam epitaxy,
No full textThe influence of Mg on the growth kinetics of GaN deposited by plasma-assisted molecular-beam epitaxy has been studied. It has been found that surface diffusion and/or the residence time of both N and Ga were drastically influenced by the presence of Mg on the surface. This leads to an abrupt phase transition from a low-mobility state to a high mobility state for the Ga atoms onto the surface when increasing the Ga/N flux ratio above a critical value. A Ga droplet density has been evaluated by a simple nucleation model, and found to be in agreement with experimental observations
II-VI piezoelectric-barrier heterostructure for infrared light modulation
No full textWe demonstrate a novel all-optical quantum well light modulator device. The device is all-optical in the sense that one “control” laser beam modulates a “read” beam without the use of an external bias. It exploits the large built-in piezoelectric fields present in strained semiconductor layers grown along a polar axis. The new concept here is the use of piezoelectric barriers as opposed to wells. This enables efficient spatial separation of photocarriers created by the “control” beam. We present room-temperature results showing that an optical “control” power of 70 W/cm2 creates in the heart of the structure a space-charge field of about 30 kV/cm, inducing large spectral shifts in the photoluminescence spectra of a CdHgTe quantum well in the mid-IR spectral region
Influence of a compressive strain on the stoichiometry of the (0 0 1)CdTe surface during molecular beam epitaxy
No full textWe present here a reflection high-energy electron diffraction study of the (0 0 1)CdTe growing surface during molecular beam epitaxy. The presence of additional weakly bound Te atoms adsorbed on top of the Te dimers terminating the surface during the growth is demonstrated. The coverage of these weakly bound Te atoms is strongly dependent on the growth conditions (temperature and strain). The resulting surface stoichiometry variations have a significant effect on the CdTe growth rate and on the incorporation mechanisms of Cd and Te atoms
Efficient all-optical light modulation in a piezoelectric heterostructure at room temperature
No full textWe demonstrate a novel piezoelectric-barrier heterostructure designed for efficient all-optical light modulation. The modulation mechanism relies upon drastic photocarrier separation by the piezoelectric field in the barrier layers. We present room temperature results showing that an optical “control” power of 70 W/cm2 creates in the heart of the structure a space-charge field of about 30 kV/cm, which induces large spectral shifts (≈100 nm) in the photoluminescence spectra of a CdHgTe quantum well in the 1.5 μm range
Optical and electronic properties of monomers of eumelanin: A DFT and TD-DFT computational study
Full text linkWe report a systematic investigation on the electronic and optical properties of four monomers which are elementary constituents of some of the protomolecules of eumelanin. Eumelanin is the most important form of melanin which is one of the most universal natural pigments in living organisms. For the isolated monomers we performed all-electrons Density Functional Theory (DFT) and Time Dependent DFT (TDDFT) calculations with a localized Gaussian basis-set. For each mo-nomer we determined a series of molecular properties, namely electron affinities, ionization energies, fundamental energy-gaps, optical absorption spectra, and exciton binding energies. We discuss moreover the possible implications of the above electronic and optical properties of the single monomers with respect to the properties of a recently proposed tetrameric protomolecule of eumelanin
Tunable piezoelectric semiconductor laser controlled by the carrier injection level
No full textWe propose a tunable laser diode based on a piezoelectric heterostructure. The tuning mechanism consists of modulating the gain spectrum during lasing by the quantum-confined Stark effect. The modulating electric field is produced by carrier separation in the active region, and its amplitude depends on the injected carrier density. In a proof-of-principle photopumped experiment, we were able to generate a space-charge field of 20 kV/cm which shifted the lasing wavelength by more than 3 nm in the 800 nm spectral region
Surfactant effect of gallium during molecular beam epitaxy of GaN on AlN(0001)
No full textWe study the adsorption of Ga on (0001) GaN surfaces by reflection high-energy electron diffraction. It is shown that a dynamically stable Ga bilayer can be formed on the GaN surface for appropriate Ga fluxes and substrate temperatures. The influence of the presence of this Ga film on the growth mode of GaN on AlN(0001) by plasma-assisted molecular-beam epitaxy is studied. It is demonstrated that under nearly stoichiometric and N-rich conditions, the GaN layer relaxes elastically during the first stages of epitaxy. At high temperatures the growth follows a Stranski-Krastanov mode, whereas at lower temperatures kinetically formed flat platelets are observed. Under Ga-rich conditions-where a Ga bilayer is rapidly formed due to excess Ga accumulating on the surface-the growth follows a Frank-van der Merwe layer-by-layer mode at any growth temperature and no initial elastic relaxation occurs. Hence, it is concluded that excess Ga acts as a surfactant, effectively suppressing both Stranski-Krastanov islanding and platelet formation. It is further demonstrated that the Stranski-Krastanov transition is in competition with elastic relaxation by platelets, and it is only observed when relaxation by platelets is inefficient. As a result, a growth mode phase diagram is outlined for the growth of GaN on AlN(0001)
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