Fariba Ferdos

Influence of a thin GaAs cap layer on structural and optical properties of InAs quantum dots

In this letter we investigate the changes in the surface morphology and emission wavelength of InAs quantum dots (QDs) during initial GaAs encapsulation by atomic force microscopy and photoluminescence. The density (2.9×1010 cm−2) and height (7.9±0.4 nm) of the uncapped QDs decrease and saturate at 0.6×1010 cm−2 and 4 nm, respectively, after the deposition of 4 monolayers (MLs) of GaAs. A model for the evolution of surface morphology is proposed. Photoluminescence spectra of the surface dots show a wavelength shift from 1.58 to 1.22 μm when the GaAs capping layer thickness increases from 0 to 8 MLs.

Large ground-to-first-excited-state transition energy separation for InAs quantum dots emitting at 1.3 μm

By capping InAs quantum dots (QDs) with a thin intermediate layer of InAlAs instead of GaAs, the radiative transition wavelengths are redshifted. Surface morphology studies confirm that the redshift is due to a better preserved QD height as compared with capping by GaAs only. In contrast, the energy levels are blueshifted when using AlGaAs instead of GaAs as the barrier material. In both cases, the energy separation between the ground and the first-excited state increases significantly. Combining these approaches, we demonstrate InAs QDs with a record transition energy separation of 108 meV and ground-state emission at 1.3 μm.

Optimisation of MBE growth conditions for InAs quantum dots on (001) GaAs for 1.3 μm luminescence

We present a study of the optimised growth conditions for InAs quantum dots (QDs) grown on GaAs substrates by solid source molecular beam epitaxy (SSMBE). Growth conditions for best luminescence intensity and linewidth were found within narrow windows of substrate temperature (500-520 C) and nominal InAs layer thickness (3.3-3.7 monolayers). The emission wavelength of such InAs QDs capped by GaAs was around 1.24 μm. However, this is red-shifted to 1.3 μm or more by capping the InAs QDs with a thin layer of In x Ga 1 x As. The results show that both In content and thickness of the capping layer can be used to tune the emission wavelength. Atomic force microscopy images show that the surface recovers to two-dimensional when depositing In 0.2 Ga 0.8 As while remaining three-dimensional when depositing In 0.4 Ga 0.6 As.

Gender Equity in Higher Education: Why and How? A Case Study of Gender Issues in a Science Faculty.

At a time when more and more natural science subjects are attracting an increasing number of women (chemistry for example) physics remains a male stronghold. It is not easy to understand this phenomenon or the anomaly that over-representation of males in physics faculties is more likely to occur in countries known for their attempts at equalizing opportunities for women. Sweden, for example, has a parliament in which 40% of its members are women and yet the average percentage of women lecturers in physics faculties is about half of that. In Sweden today women professors of physics (both appointed and promoted) typically represent 10% or less of the total professorial staff. In this paper we report on a qualitative case study of gender equity in a large physics faculty in a Swedish university. In order to locate our study in a more general social and political context we look at Swedish legislation that seeks to equalize opportunities for women in higher education. The rest of the study focuses on a brief review of research in the area of gender issues in higher education and an analysis of interviews with three women in physics: one a professor, one a lecturer and the third a PhD student. The analysis discusses why the current disproportion exists, if it is a good or bad thing for physics and physicists and how one might rectify any perceived problems in terms or gender relations and gender equity.

Influence of initial GaAs and AlAs cap layers on InAs quantum dots grown by molecular beam epitaxy

Capping of InAs quantum dots (QDs) with AlAs or GaAs causes a significant change in the structural properties of the QDs. However, there is a basic difference between these two capping materials. The GaAs capping causes a dramatic reduction of the dot density and height. AlAs capping, on the other hand, results in a partly suppressed height reduction and a higher dot density.

Photoluminescence of an assembly of size-distributed self-assembled InAs quantum dots

We have performed experimental and theoretical studies of the effects of inhomogeneous broadening on the luminescence properties of a self-assembled InAs quantum dot (QD) assembly. From atomic force microscopic (AFM) images the InAs QD assembly is found to have an average lateral size of 20–22 nm and a height of 10–12 nm, and the dot density is in the range of 1–2×1010 cm−2. Using the statistical distribution of the QD size from AFM measurements and the results from the theoretical analysis of the photoluminescene (PL) spectrum, it is found that the distance between QDs is larger than 30 nm (the average distance is about 100 nm), the penetration of the ground-state wave function into the GaAs barrier is negligible, and the calculated PL spectrum agrees well with that measured when the carriers in each QD are assumed to be at a local thermal equilibrium state, resulting in the conclusion that the QDs are physically independent. The width of the PL peak is determined by the inhomogeneous QD size.

Aluminium incorporation for growth optimization of long-wavelength InAs/GaAs quantum dots by molecular beam epitaxy

Summary form only given. We investigate the use of Al in both the lower and the upper embedding layers. We demonstrate that increasing the Al content in the lower InAlGaAs embedding layer increases the QDs density dramatically while introducing a thin InAlAs cap layer can increase the QDs density and also tune the InAs QD ground state emission wavelength to 1.3 /spl mu/m and increase the energy separation between the ground and the first excited state transitions. AFM and PL spectra were used to characterize the QDs.

Influence of thin GaAs and AlAs cap layers on the structural properties of InAs quantum dots grown by molecular beam epitaxy

Self-organised InAs quantum dots (QDs) are used as optical gain material in long wavelength lasers on GaAs. The measured QD height and density are often used as figures of merits, and great efforts have been made to maximise these two parameters to extend the wavelength coverage. In this work, we investigate the influence of initial GaAs and AlAs cap layers on the structural properties of InAs QDs. The study clearly shows that capping of InAs QDs causes a strong modification of not only the QD shape and height but also the QD density.