Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userMechanical strain in capped and uncapped self-assembled Ge/Si quantum dots
112
0
0.0
(
0
)
Added by
Paulo Freitas Gomes
Publication date
2015
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
In this study we numerically calculate the spatial profile of mechanical strain on self-assembled germanium (Ge) quantum dots (QDs) grown on a silicon (Si) substrate. Although the topic has been exhaustively studied, interesting features have not been explained or even mentioned in the literature yet. We studied the effect of the cap layer considering two cases: capped QDs (where a Si cap is present above the Ge QDs) and uncapped QDs (where no Si is present above the Ge QDs). We observed that Ge in the capped QDs is more strained compared with the the uncapped QDs. This expected effect is attributed to the additional tension from the Si cap layer. However, the situation is opposite for the Si substrate, it is more strained in the uncapped QD because the Ge layer is less strained in this case. We also calculated the band-edge alignment for the electrons and holes.
rate research
Read More
We report on the influence of hyperfine interaction on the optical orientation of singly charged excitons X+ and X- in self-assembled InAs/GaAs quantum dots. All measurements were carried out on individual quantum dots studied by micro-photoluminescence at low temperature. We show that the hyperfine interaction leads to an effective partial spin relaxation, under 50kHz modulated excitation polarization, which becomes however strongly inhibited under steady optical pumping conditions because of dynamical nuclear polarization. This optically created magnetic-like nuclear field can become very strong (up to ~4 T) when it is generated in the direction opposite to a longitudinally applied field, and exhibits then a bistability regime. This effect is very well described by a theoretical model derived in a perturbative approach, which reveals the key role played by the energy cost of an electron spin flip in the total magnetic field. Eventually, we emphasize the similarities and differences between X+ and X- trions with respect to the hyperfine interaction, which turn out to be in perfect agreement with the theoretical description.
We investigate the electronic structure of the InAs/InP quantum dots using an atomistic pseudopotential method and compare them to those of the InAs/GaAs QDs. We show that even though the InAs/InP and InAs/GaAs dots have the same dot material, their electronic structure differ significantly in certain aspects, especially for holes: (i) The hole levels have a much larger energy spacing in the InAs/InP dots than in the InAs/GaAs dots of corresponding size. (ii) Furthermore, in contrast with the InAs/GaAs dots, where the sizeable hole $p$, $d$ intra-shell level splitting smashes the energy level shell structure, the InAs/InP QDs have a well defined energy level shell structure with small $p$, $d$ level splitting, for holes. (iii) The fundamental exciton energies of the InAs/InP dots are calculated to be around 0.8 eV ($sim$ 1.55 $mu$m), about 200 meV lower than those of typical InAs/GaAs QDs, mainly due to the smaller lattice mismatch in the InAs/InP dots. (iii) The widths of the exciton $P$ shell and $D$ shell are much narrower in the InAs/InP dots than in the InAs/GaAs dots. (iv) The InAs/GaAs and InAs/InP dots have a reversed light polarization anisotropy along the [100] and [1$bar{1}$0] directions.
Epitaxial self-assembled quantum dots (SAQDs) represent an important step in the advancement of semiconductor fabrication at the nanoscale that will allow breakthroughs in electronics and optoelectronics. In these applications, order is a key factor. Here, the role of crystal anisotropy in promoting order during early stages of SAQD formation is studied through a linear analysis of a commonly used surface evolution model. Elastic anisotropy is used a specific example. It is found that there are two relevant and predictable correlation lengths. One of them is related to crystal anisotropy and is crucial for determining SAQD order. Furthermore, if a wetting potential is included in the model, it is found that SAQD order is enhanced when the deposited film is allowed to evolve at heights near the critical surface height for three-dimensional film growth.
We demonstrate a method to grow height controlled, dislocation-free InGaAs quantum posts (QPs) on GaAs by molecular beam epitaxy (MBE) which is confirmed by structural investigations. The optical properties are compared to realistic 8-band k.p calculations of the electronic structure which fully account for strain and the structural properties of the QP. Using QPs embedded in n-i-p junctions we find wide range tunability of the interband spectrum and giant static dipole moments.
We study spin dynamics of excitons confined in self-assembled CdSe quantum dots by means of optical orientation in magnetic field. At zero field the exciton emission from QDs populated via LO phonon-assisted absorption shows a circular polarization of 14%. The polarization degree of the excitonic emission increases dramatically when a magnetic field is applied. Using a simple model, we extract the exciton spin relaxation times of 100 ps and 2.2 ns in the absence and presence of magnetic field, respectively. With increasing temperature the polarization of the QD emission gradually decreases. Remarkably, the activation energy which describes this decay is independent of the external magnetic field, and, therefore, of the degeneracy of the exciton levels in QDs. This observation implies that the temperature-induced enhancement of the exciton spin relaxation is insensitive to the energy level degeneracy and can be attributed to the same excited state distribution.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
