ABSTRACT
Germanium monosulfide with an anisotropic puckered crystalline structure has recently attracted much attention due to its unique optical and electronic properties; however, exciton-phonon interactions were only superficially elucidated. We study the resonant Raman scattering and the photoluminescence of the optically active Γ-exciton in layered GeS flakes and evaluate the exciton and phonon responses on variations in the excitation energy, laser-light and emission polarizations, temperature, and laser power. A double-resonance mechanism allows for observing Raman forbidden (dark) first- and second-order longitudinal-optical phonon modes whose symmetries and energies are moreover calculated by density functional perturbation theory. For (quasi)-resonant exciton excitation, the selection rules become relaxed so that a fourth-order Fröhlich intraband process is mediated by the scattering of the electron with a longitudinal-optical and an acoustic phonon. Our results demonstrate considerable coupling between phonons and photogenerated carriers in GeS flakes and the high efficiency of multiorder scattering in optical processes.
ABSTRACT
In this paper, the photoluminescence spectra of excitons in ZnO/ZnMgO/ZnO double asymmetric quantum wells grown on a-plane Al2O3 substrates with internal electric-field bands structures were studied. In these structures, the electron and the hole in the exciton are spatially separated between neighbouring quantum wells, by a ZnMgO barrier with different thickness. The existence of an internal electric field generates a linear potential and, as a result, lowers the energy of quantum states in the well. For the wide wells, the electrons are spatially separated from the holes and can create indirect exciton. To help the understanding of the photoluminescence spectra, for single particle states the 8 k·p for wurtzite structure is employed. Using these states, the exciton in the self-consistent model with 2D hydrogenic 1s-like wave function is calculated.
ABSTRACT
In this study, we propose designs of an interband cascade laser (ICL) active region able to emit in the application-relevant mid infrared (MIR) spectral range and to be grown on an InP substrate. This is a long-sought solution as it promises a combination of ICL advantages with mature and cost-effective epitaxial technology of fabricating materials and devices with high structural and optical quality, when compared to standard approaches of growing ICLs on GaSb or InAs substrates. Therefore, we theoretically investigate a family of type II, "W"-shaped quantum wells made of InGaAs/InAs/GaAsSb with different barriers, for a range of compositions assuring the strain levels acceptable from the growth point of view. The calculated band structure within the 8-band k·p approximation showed that the inclusion of a thin InAs layer into such a type II system brings a useful additional tuning knob to tailor the electronic confined states, optical transitions' energy and their intensity. Eventually, it allows achieving the emission wavelengths from below 3 to at least 4.6 µm, while still keeping reasonably high gain when compared to the state-of-the-art ICLs. We demonstrate a good tunability of both the emission wavelength and the optical transitions' oscillator strength, which are competitive with other approaches in the MIR. This is an original solution which has not been demonstrated so far experimentally. Such InP-based interband cascade lasers are of crucial application importance, particularly for the optical gas sensing.
ABSTRACT
We investigate a hybrid system containing an In0.53Ga0.47As quantum well (QW), separated by a thin 2 nm In0.53Ga0.23Al0.24As barrier from 1.55 µm emitting InAs quantum dots (QDs), grown by molecular beam epitaxy on an InP substrate. Photoreflectance and photoluminescence (PL) spectroscopies are used to identify optical transitions in the system, with support of 8-band kp modelling. The main part of the work constitute the measurements and analysis of thermal quenching of PL for a set of samples with different QW widths (3-6 nm). Basing on Arrhenius plots, carrier escape channels from the dots are identified, pointing at the importance of carrier escape into the QW. A simple two level rate equations model is proposed and solved, exhibiting qualitative agreement with experimental observations. We show that for a narrow QW the escape process is less efficient than carrier supply via the QW due to the narrow barrier, resulting in improved emission intensity at room temperature. It proves that with carefully designed energy level structure, a hybrid QW/QD system can be used as an active region in telecom lasers with improved efficiencies.
