Room-temperature photoluminescence of ZnO/MgO multiple quantum wells deposited by reactive magnetron sputtering

Wurtzite ZnO/MgO superlattices were successfully grown on Si (001) substrates at 750 degrees C using radio-frequency reactive magnetron sputtering method. X-ray reflection and diffraction, electronic probe and photoluminescence analysis were used to characterize the multiple quantum wells (MQWs). The results showed the periodic layer thickness of the MQWs to be 1.85 to 22.3 nm. The blueshift induced by quantum confinement was observed. Least square fitting method was used to deduce the zero phonon energy of the exciton from the room-temperature photoluminescence. It was found that the MgO barrier layers has a much larger offset than ZnMgO. The fluctuation of periodic layer thickness of the MQWs was suggested to be a possible reason causing the photoluminescence spectrum broadening.

Effect of an indium-doped barrier on enhanced near-ultraviolet emission from InGaN/AlGaN: In multiple quantum wells grown on Si(111)

Low indium content InGaN/AlGaN multiple quantum wells (MQWs) have been grown on Si(111) substrate by metal-organic chemical vapour deposition (MOCVD). A new method of using an isoelectronic indium-doped AlGaN barrier has been found to be very effective in improving the crystalline quality and interfacial abruptness of InGaN quantum well layers. We grew five periods of In0.06Ga0.94N/Al0.20Ga0.80N:In MQWs with In-doped barrier layers and obtained strong near-ultraviolet (UV) emission (similar to 400 nm) at room temperature. An In-doped AlGaN barrier improves the room-temperature PL intensity of InGaN/AlGaN MQWs, making it a candidate barrier for a near-UV source on Si substrate.

Non-exponential photoluminescence decay dynamics of localized carriers in disordered InGaN/GaN quantum wells: the role of localization length

In this article, we report a combined experimental and theoretical study on the luminescence dynamics of localized carriers in disordered InGaN/GaN quantum wells. The luminescence intensity of localized carriers is found to exhibit an unusual non-exponential decay. Adopting a new model taking the radiative recombination and phonon-assisted hopping transition between different localized states into account, which was recently developed by Rubel et al., the non-exponential decay behavior of the carriers can be quantitatively interpreted. Combining with precise structure characterization, the theoretical simulations show that the localization length of localized carriers is a key parameter governing their luminescence decay dynamics. (c) 2006 Optical Society of America.

Rapid photoluminescence quenching in GaInNAs quantum wells at low temperature

Temperature dependence of optical properties of GaInNAs/GaAs quantum wells (QWs) has been studied by photoluminescence (PL) and time-resolved PL. A rapid PL quenching is observed even at very low temperature and is of the excitation power dependence. These results strongly suggest that the non-radiative recombination process plays a very important role at low temperature. In the TRPL measurement the shape of the PL decay curve shows significant difference under different excitation powers. It is attributed to the different involvement of non-radiative recombination in the overall recombination process. The TRPL data are well fitted with the rate equation involving both the radiative and non-radiative recombination. (c) 2006 Elsevier B.V. All rights reserved.

Nonlinear Rashba model and spin relaxation in quantum wells

We find that the Rashba spin splitting is intrinsically a nonlinear function of the momentum, and the linear Rashba model may overestimate it significantly, especially in narrow-gap semiconductors. A nonlinear Rashba model is proposed, which is in good agreement with the numerical results from the eight-band k center dot p theory. Using this model, we find pronounced suppression of the D'yakonov-Perel' spin relaxation rate at large electron densities, and a nonmonotonic dependence of the resonance peak position of the electron spin lifetime on the electron density in [111]-oriented quantum wells, both in qualitative disagreement with the predictions of the linear Rashba model.

Design and fabrication of 1.06 mu m resonant-cavity enhanced reflective modulator with GaInAs/GaAs quantum wells

A resonant-cavity enhanced reflective optical modulator is designed and frabricated, with three groups of three highly strained InGaAS/GaAs quantum wells in the cavity, for the low voltage and high contrast ratio operation. The quantum wells are positioned in antinodes of the optical standing wave. The modulator is grown in a single growth step in an molecular beam epitaxy system, using GaAs/AIAs distributed Bragg reflectors as both the top and bottom mirrors. Results show that the reflection device has a modulation extinction of 3 dB at -4.5 V bias.

Effect of nonradiative recombination on carrier dynamics in GaInNAs/GaAs quantum wells

The nonradiative recombination effect on carrier dynamics in GaInNAs/GaAs quantum wells is studied by time-resolved photoluminescence (TRPL) and polarization-dependent TRPL at various excitation intensities. It is found that both recombination dynamics and spin relaxation dynamics strongly depend on the excitation intensity. Under moderate excitation intensities the PL decay curves exhibit unusual non-exponential behaviour. This result is well stimulated by a rate equation involving both the radiative and non-radiative recombinations via the introduction of a new parameter of the effective concentration of nonradiative recombination centres in the rate equation. In the spin dynamics study, the spin relaxation also shows strong excitation power dependence. Under the high excitation power an increase of spin polarization degree with time is observed. This new finding provides a useful hint that the spin process can be controlled by excitation power in GaInNAs systems.

Enhancement of photoluminescence intensity of GaInNAs/GaAs quantum wells by two-step rapid thermal annealing

We investigate the effect of rapid thermal annealing on InGaNAs/GaAs quantum wells. At optimized annealing temperatures and times, the greatest enhancement of the photoluminescence intensity is obtained by a special two-step annealing process. To identify the mechanism affecting the material quality during the rapid thermal annealing, differential temperature analysis is applied, and temperature- and power-dependent photoluminescence is carried out on the samples annealed under different conditions. Our experiment reveals that some composition redistribution or other related ordering process may occur in the quantum-well layer during annealing. Annealing at a lower temperature for a long time primarily can remove defects and dislocations while annealing at a higher temperature for a short time primarily homogenizes the composition in the quantum wells.

Large g factors of higher-lying excitons detected with reflectance difference spectroscopy in GaAs-based quantum wells

Exciton g factors in GaAs-based quantum wells (QWs) were evaluated by reflectance difference spectroscopy (RDS) under a weak magnetic field. The well-width dependence of the n=1 heavy-hole exciton (1H1E) g factor agrees well with the reported results, demonstrating RDS as a sensitive tool for detection of g factor. By comparison, the n=1 light-hole exciton g factor increases with the well width, and shows a larger value than that of 1H1E. In a 20-nm-wide Al0.02Ga0.98As/AlAs multiple QW sample, the g factors of up to ten excitons are obtained, and the higher-lying exciton g factors are found to be one order larger than that of the 1H1E exciton.

Influence of longitudinal electric field on the hot-phonon effect in quantum wells

Using the Huang-Zhu model [K. Huang and B.-F. Zhu, Phys. Rev. B 38, 13377 (1988)] for the optical phonons and associated carrier-phonon interactions in semiconductor superlattices, the effects of longitudinal electric field on the energy-loss rates (ELRs) of hot carriers as well as on the hot-phonon effect (HPE) in GaAs/AlAs quantum wells (QWs) are studied systematically. Contributions of various bulklike and interface phonons to the hot-carrier relaxation are compared in detail, and comprehensively analyzed in relation to the intrasubband and intersubband scatterings for quantum cascade lasers. Due to the broken parity of the electron (hole) states in the electric field, the bulklike modes with antisymmetric potentials are allowed in the intrasubband relaxation processes, as well as the modes with symmetric potentials. As the interface phonon scattering is strong only in narrow wells, in which the electric field affects the electron (hole) states little, the ELRs of hot carriers through the interface phonon scattering are not sensitive to the electric field. The HPE on the hot-carrier relaxation process in the medium and wide wells is reduced by the electric field. The influence of the electric field on the hot-phonon effect in quantum cascade lasers is negligible. When the HPE is ignored, the ELRs of hot electrons in wide QWs are decreased noticeably by the electric field, but slightly increased by the field when considering the HPE. In contrast with the electrons, the ELRs of hot holes in wide wells are increased by the field, irrespective of the HPE. (c) 2006 American Institute of Physics.

High-quality multiple quantum wells selectively grown on taper-patterned substrates by ultra-low-pressure MOCVD

High quality InGaAsP/InGaAsP multiple quantum wells ( MQWs) have been selectively grown by ultra-low-pressure (22 mbar) metal-organic chemical vapor deposition. A large bandgap energy shift of 46 nm and photoluminescence with FWHM less than 30 meV were obtained with a rather small mask width variation (15-30 mu m). In order to study the uniformity of the MQWs grown in the selective area, novel tapered masks were employed, and the transition effect W the tapered region was also studied. The energy detuning of the tapered region was observed to be saturated at larger ratios of the mask width to the tapered region length.

High-quality multiple quantum wells selectively grown with tapered masks by ultra-low-pressure MOCVD

An InGaAsP/InGaAsP multiple quantum wells (MQWs) selectively grown by ultra-low-pressure (22 mbar) metal-organic chemical vapor deposition was investigated in this article. A 46 nm photoluminescence peak wavelength shift was obtained with a small mask width variation (15-30 mu m). High-quality crystal layers with a photoluminescence (PL) ftill-width-at-half-maximum (FWHM) of less than 30 meV were achieved. Using novel tapered masks, the transition-effect of the tapered region was also studied. The energy detuning of the tapered region was observed to be saturated with the larger ratio of the mask width divided to the tapered region length. (C) 2005 Elsevier B.V. All rights reserved.

Strong in-plane optical anisotropy of asymmetric (001) quantum wells

It is well known that asymmetry in the (001) direction can induce in-plane optical anisotropy (IPOA) in (001) quantum wells (QWs). In this letter, asymmetry is introduced in (001) GaAs/AlGaAs QWs by inserting 1 ML (monolayer) of InAs or AlAs at interfaces. Strong IPOA, which is comparable to that in the InGaAs/InP QWs with no common atom, is observed in the asymmetric GaAs/AlGaAs QW by reflectance difference spectroscopy. (C) 2006 American Institute of Physics.

Influence of growth parameters of frequency-radio plasma nitrogen source on extending emission wavelengths from 1.31 mu m to 1.55 mu m GaInNAs/GaAs quantum wells grown by molecular-beam epitaxy

High (42.5%) indium content GaInNAs/GaAs quantum wells with room temperature emission wavelength from 1.3 mu m to 1.5 mu m range were successfully grown by Radio Frequency Plasma Nitrogen source assisted Molecular Beam Epitaxy. The growth parameters of plasma power and N-2 How rate were optimized systematically to improve the material quality. Photoluminescence and transmission electron microscopy measurements showed that the optical and crystal quality of the 1.54 mu m GaInNAs/GaAs QWs was kept as comparable as that in 1.31 mu m.

Rashba spin splitting in biased semiconductor quantum wells

Rashba spin splitting (RSS) in biased semiconductor quantum wells is investigated theoretically based on eight-band k center dot p theory. We find that at large wave vectors, RSS is both nonmonotonic and anisotropic as a function of in-plane wave vector, in contrast to the widely used isotropic linear model. We derive an analytical expression for RSS, which can qualitatively reproduce such nonmonotonic behavior at large wave vectors. We also investigate numerically the dependence of RSS on the various band parameters and find that RSS increases with decreasing band gap and subband index, increasing valence band offset, external electric field, and well width. All these dependences can be qualitatively described by our analytical model.