We have observed magnetic-field-induced charged excitonic transitions from one-side-modulation-doped Al x Ga 1−x As/ GaAs asymmetric double quantum wells ͑ADQWs͒ where two low-energy conduction subbands e1 and e2 are coupled strongly, whereas the heavy-hole ground state hh1 is localized mostly in a quantum well.In the presence of a magnetic field applied parallel to the growth axis, the photoluminescence spectrum of the e1-hh1 transition develops into a Landau fan for filling factors of Ͼ 2. However, at Ͻ 2, the lowest Landaulevel ͑LL͒ transition reveals the charged-exciton behavior with an abrupt changeover in the field dependence of the transition energy, whereas the intensities of other LL transitions in the same Landau fan diminish. When the density of free electrons in the ADQWs is about 1.4ϫ 10 11 cm −2 , a new charged excitonic e2-hh1 transition also appears in the + polarization at Ͻ 2.The many-body interaction of a two-dimensional electron gas ͑2DEG͒ in modulation-doped quantum wells ͑QWs͒ has been the subject of extensive investigations in the past years. 1-9 The photoluminescence ͑PL͒ spectroscopy is a useful tool in studying the many-body effect on interband optical transitions in QWs. In particular, a number of PL investigations 10-13 have observed that at a filling factor ͑=n e h / eB͒ of 2 ͑here n e , h, and B denote the electron density, the Planck constant, and the magnetic field, respectively͒, there is an abrupt change in the slope of the energy of the lowest Landau-level ͑LL͒ transition with line narrowing, indicating the formation of 2DEG-hole complexes. The transformation from a LL behavior to an excitonic one for Ͻ 2 has been explained by two different interpretations.According to an interpretation in terms of the screening effect, 4 the transition of electron-hole ͑e-h͒ plasma into exciton, the so-called exciton-Mott transition, occurs when the density of plasma is decreased below a critical value. A similar transition can occur in QWs containing 2DEG and a photogenerated hole when the density of electrons is decreased. At a high electron density of n e ജ 1/a B 2 ͑here, a B is the Bohr radius of exciton in the QW͒, the e-h Coulomb interaction is screened effectively by free carriers, resulting in the prohibition of exciton formation. The critical electron density for the transition of electron-plasma into exciton increases in the presence of a magnetic field. Since the magnetic field shrinks the wave function of an electronic bound state, the screening effect on the e-h Coulomb interaction is reduced, allowing the formation of many-electrons-hole complexes such as X − the negatively charged exciton. 14,15 According to this mechanism, there is no difference between symmetric and asymmetric QWs in the changeover from a LL behavior to an excitonic behavior.On the other hand, a rather recent theory explains the abrupt changeover from LL behaviors to excitonic behaviors in both energy and linewidth as a consequence of hidden symmetry 11 ͑HS͒ inherent in a symmetric 2DEG system in strong magneti...
The reduction mechanism of threading dislocation at the interface of InGaN/low-temperature GaN (LT-GaN) layers was investigated by atomic force microscopy, transmission electron microscopy and secondary ion mass spectroscopy measurements. Introducing the LT-GaN intermediate layer onto the InGaN active layer not only prevented indium evaporation during the growth of the p-GaN layer but also suppressed the propagation of threading dislocations from InGaN to p-GaN. The propagation of threading dislocations is reduced by the formation of two-dimensional lateral islands, and further defect generation is prevented by the formation of In x Ga 1Àx N alloy due to the relaxation of lattice mismatch between active InGaN and p-GaN.
The Au/Ni/Al/Ti/Ta ohmic contact (7.5 Â 10 --7 Wcm 2 ) on the AlGaN/GaN heterostructure was demonstrated through 700 C annealing for 1 min. The nitride phases were identified as the TaN/TiN for Au/Ni/Al/Ti/Ta and TiN for Au/Ni/Al/Ti. The strong formation of interfacial nitrides appears to be responsible for the good ohmic contact behavior in the Au/Ni/Al/Ti/Ta metal scheme. The contact resistance was found to depend on the thickness of the nitride phases.
The performance limitations have been studied for various optical bistable symmetric seif-electooptic effect devices (S-SEEDs) using extremely shallow quantum wells (ESQWs). We consider the four ESQW SEEDs; anti-reflection (AR)-coated ESQW S-SEED, back-to-back AR-coated ESQW S-SEED, asymmetric Fabry-Perot (AFP)-ESQW S-SEED, and back-to-back AFP-ESQW S-SEED. As the optical input power increases, the ohmic heating dominantly reduces the on-state reflectivity, while the exciton saturation dominantly increases the off-state reflectivity, both of which seriously degrade the device performances limiting the maximum input power for fast switching. For the quantitative estimation of the limitation, we simulate and analyze the bit-rates of the optically cascaded set-reset latch systems employing the various ESQW S-SEED. The optimized number of ESQWs for each structure is also obtained.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.