Abstract
The understanding of the correlation between structural and photoluminescence (PL) properties of self-assembled semiconductor quantum dots (QDs), particularly InGaAs QDs grown on (001) GaAs substrates, is crucial for both fundamental research and optoelectronic device applications. So far structural and PL properties have been probed from two different epitaxial layers, namely top-capped and buried layers respectively. Here, we report for the first time both structural and PL measurements from an uncapped layer of InGaAs QDs to correlate directly composition, strain and shape of QDs with the optical properties. Synchrotron X-ray scattering measurements show migration of In atom from the apex of QDs giving systematic reduction of height and enlargement of QDs base in the capping process. The optical transitions show systematic reduction in the energy of ground state and the first excited state transition lines with increase in capping but the energy of the second excited state line remain unchanged. We also found that the excitons are confined at the base region of these elliptically shaped QDs showing an interesting volume-dependent confinement energy scaling of 0.3 instead of 0.67 expected for spherical dots. The presented method will help us tuning the growth of QDs to achieve desired optical properties.
Highlights
It is becoming increasingly clear that this assumption is not valid as In-Ga intermixing during growth of these two layers on GaAs (001) substrate, the buried-layer just above the buffer layer and the top-layer just below capping layer can be quite different
The atomic force microscopy (AFM) images obtained from uncapped quantum dots (QDs) sample enabled us to reconfirm the results obtained from synchrotron X-ray scattering measurements
Grazing incidence diffraction (GID) X-ray measurements[14,21,22,23,24,25,26,27,28] are ideal non-destructive techniques to extract the structural information of the nanometer-thin layer of QDs averaged over a large area depending on the footprint of the X-ray beam over a sample
Summary
The results from AFM measurements presented here clearly show that the InGaAs QDs are ellipsoidal in shape with a preferred elongation in the direction perpendicular to the miscut stair-steps (refer Fig. 1c) We performed both radial and angular momentum scans in GID around in-plane 400 and 200 Bragg diffraction peaks of GaAs, by keeping incidence angle of the X-ray beam below or around the critical angle of GaAs to make the technique sensitive to QDs layer near the surface (refer method section for details). The variation of Indium content as a function of in-plane lattice parameter aII has been calculated using equation (1) given in method section and the obtained profile reveals compositional variation for differently capped QDs. It has been found that the concentration x within an average InxGa1−xAs QD varies between 0.85 and 0.1 as shown in the plot shown in Fig. 1g for the uncapped sample. The average height of QDs was found to be around 2.2 ± 0.4 nm and we shall
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
