988 resultados para quantum well electrode


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The growth techniques which have enabled the realization of InGaN-based multi-quantum-well (MQW) structures with high internal quantum efficiencies (IQE) on 150mm (6-in.) silicon substrates are reviewed. InGaN/GaN MQWs are deposited onto GaN templates on large-area (111) silicon substrates, using AlGaN strain-mediating interlayers to inhibit thermal-induced cracking and wafer-bowing, and using a SiN x interlayer to reduce threading dislocation densities in the active region of the MQW structure. MQWs with high IQE approaching 60% have been demonstrated. Atomic resolution electron microscopy and EELS analysis have been used to study the nature of the important interface between the Si(111) substrate and the AlN nucleation layer. We demonstrate an amorphous SiN x interlayer at the interface about 2nm wide, which does not, however, prevent good epitaxy of the AlN on the Si(111) substrate. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Ultrashort superradiant pulse generation from a 1580 nm AlGaInAs multiple quantum-well (MQW) semiconductor structure has been experimentally demonstrated for the first time. Superradiance is confirmed by analyzing the evolution of the optical temporal waveforms and spectra. Superradiant trends and regimes are studied as a function of driving condition. An optical pulse train is obtained at 1580 nm wavelength, with pulse durations as short as 390 fs and pulse peak powers of 7.2 W.

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The structural and optical properties of trench defects, which are poorly understood yet commonly occurring defects observed on the surfaces of InGaN multiple quantum wells (MQW), are reported. These defects comprise near-circular trenches which enclose areas of MQW which give rise to a red shift in peak photoluminescence emission and a change in cathodoluminescence intensity with respect to the surrounding material. Atomic force microscopy shows that the height of trench-enclosed areas differs from that of the surrounding quantum well structure, and that trenches are unrelated to the commonly observed V-defects in InGaN films, despite being occasionally intersected by them. Cross-sectional electron microscopy analysis of trenches with raised centres suggests that the red shift in the observed cathodoluminescence peak emission may be due to the quantum wells being thicker in the trench-enclosed regions than in the surrounding quantum well area. The mechanism of trench formation and its implication for the control of the emission properties of light-emitting diodes is discussed. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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We have investigated the growth, structural properties and photoluminescence of novel GaAs/AlGaAs radial heterostructure nanowires, fabricated by metalorganic chemical vapour deposition. The effect of growth temperature on nanowire morphology is discussed. Strong photoluminescence is observed from GaAs nanowires with AlGaAs shells. Core/multishell nanowires, of GaAs cores clad in several alternating layers of thick AlGaAs barrier shells and thin GaAs quantum well shells, exhibit a blue-shifted photoluminescence peak believed to arise from quantum confinement effects. A novel two-temperature growth procedure for obtaining GaAs cores is introduced, and other nanowire heterostructures are addressed. © 2006 IEEE.

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A spin-injection/-detection device has been fabricated based on the multiple quantum well light emitting diode (LED) structure. It is found that only a broad electroluminescence (EL) peak of a full width at half maximum of 8.6 nm appears at the wavelength of 801 nm in EL spectra with a circular luminescence polarization degree of 18%, despite PL spectra always show three well resolved peaks. The kinetic energy gained by injected electrons and holes in their drift along opposite directions broadens the EL peak, and makes three EL peaks converge together. The same process also destroys the injected spin polarization of electrons mainly dominated by the Bir-Aronov-Pikus spin relaxing mechanism.

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Using effective-mass Hamiltonian model of semiconductors quantum well structures, we investigate the electronic structures of the Gamma-conduction and L-conduction subbands of GeSn/GeSiSn strained quantum well structure with an arbitrary composition. Our theoretical model suggests that the band structure could be widely modified to be type I, negative-gap or indirect-gap type II quantum well by changing the mole fraction of alpha-Sn and Si in the well and barrier layers, respectively. The optical gain spectrum in the type I quantum well system is calculated, taking into account the electrons leakage from the Gamma-valley to L-valley of the conduction band. We found that by increasing the mole fraction of alpha-Sn in the barrier layer and not in the well layer, an increase in the tensile strain effect can significantly enhance the transition probability, and a decrease in Si composition in the barrier layer, which lowers the band edge of Gamma-conduction subbands, also comes to a larger optical gain.

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We have fabricated and characterized GaN-based vertical cavity surface emitting lasers (VCSELs) with a unique active region structure, in which three sets of InGaN asymmetric coupled quantum wells are placed in a half-wavelength (0.5 lambda) length. Lasing action was achieved under optical pumping at room temperature with a threshold pumping energy density of about 6.5 mJ/cm(2). The laser emitted a blue light at 449.5 nm with a narrow linewidth below 0.1 nm and had a high spontaneous emission factor of about 3.0x10(-2). The results indicate that this active region structure is useful in reducing the process difficulties and improving the threshold characteristics of GaN-based VCSELs.

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We present the monolithic integration of a sampled-grating distributed Bragg reflector (SC-DBR) laser with a quantum-well electroabsorption modulator (QW-EAM) by combining ultra-low-pressure (55 mbar) selective-area-growth (SAG) metal-organic chemical vapour deposition (MOCVD) and quantum-well intermixing (QWI) for the first time. The QW-EAM and the gain section can be grown simultaneously by using SAG MOCVD technology. Meanwhile, the QWI technology offers an abrupt band-gap change between two functional sections, which reduces internal absorption loss. The experimental results show that the threshold current I-th = 62 mA, and output power reaches 3.6 mW. The wavelength tuning range covers 30 nm, and all the corresponding side mode suppression ratios are over 30 dB. The extinction ratios at available wavelength channels can reach more than 14 dB with bias of -5 V.

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Magnetotransport measurements have been carried out on In0.53Ga0.17As/In0.52Al0.48 As quantum wells in a temperature range between 1.5 and 77 K. We have observed a large positive magnetoresistance in the low magnetic field range, but saturating in high magnetic fields. The magnetoresistance results from two occupied subbands in the two-dimensional electron gas. With the intersubband scattering considered, we obtained the subband mobility by analyzing the positive magnetoresistance. It is found that the second subband mobility is larger than that of the first due to the existence of the intersubband scattering.

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A two-color time-resolved Kerr rotation spectroscopy system was built, with a femtosecond Ti:sapphire laser and a photonic crystal fiber, to study coherent spin transfer processes in an InGaAs/GaAs quantum well sample. The femtosecond Ti:sapphire laser plays two roles: besides providing a pump beam with a tunable wavelength, it also excites the photonic crystal fiber to generate supercontinuum light ranging from 500 nm to 1600 nm, from which a probe beam with a desirable wavelength is selected with a suitable interference filter. With such a system, we studied spin transfer processes between two semiconductors of different gaps in an InGaAs/GaAs quantum well sample. We found that electron spins generated in the GaAs barrier were transferred coherently into the InGaAs quantum well. A model based on rate equations and Bloch-Torrey equations is used to describe the coherent spin transfer processes quantitatively. With this model, we obtain an effective electron spin accumulation time of 21 ps in the InGaAs quantum well.

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The confined longitudinal-optical phonon-assisted tunneling through a parabolic quantum well with double barriers in a magnetic field perpendicular to the interfaces is studied theoretically based on a dielectric continuum model. The numerical results show that the applied magnetic field sharpens and heightens the phonon-assisted tunneling peaks in agreement with experimental observation. Furthermore, the phonon-assisted magnetotunneling peaks shift towards the higher biases as the magnetic field increases. In contrast to the results for a rectangular quantum well, the ratio of peak to valley of the phonon-assisted tunneling is larger for the wider well case. It also indicates that the phonon-assisted tunneling current peaks can be easily observed for a wider parabolic quantum well. (C) 2008 Published by Elsevier B.V.

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Hexagonal nanopillars with a single InGaAs/GaAs quantum well (QW) were fabricated on a GaAs (111) B substrate by selective-area metal-organic vapor phase epitaxy. The standard deviations in diameter and height of the nanopillars are about 2% and 5%, respectively. Zincblende structure and rotation twins were identified in both the GaAs and the InGaAs layers by electron diffraction. The excitation-power-density-dependent micro-photoluminescence (mu-PL) of the nanopillars was measured at 4.2, 50, 100 and 150 K. It was shown that, with increasing excitation power density, the mu-PL peak's positions shift to a higher energy, and their intensity and width increase, which were rationalized using a model that includes the effects of piezoelectricity, photon-screening and band-filling. It was also revealed that the rotation twins significantly reduce the diffusion length of the carriers in the nanopillars, compared to that in the regular semiconductors.