Correlating Photoluminescence and Structural Properties of Uncapped and GaAs-Capped Epitaxial InGaAs Quantum Dots.

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.

Density dependent composition of InAs quantum dots extracted from grazing incidence x-ray diffraction measurements.

Epitaxial InAs quantum dots grown on GaAs substrate are being used in several applications ranging from quantum communications to solar cells. The growth mechanism of these dots also helps us to explore fundamental aspects of self-organized processes. Here we show that composition and strain profile of the quantum dots can be tuned by controlling in-plane density of the dots over the substrate with the help of substrate-temperature profile. The compositional profile extracted from grazing incidence x-ray measurements show substantial amount of inter-diffusion of Ga and In within the QD as a function of height in the low-density region giving rise to higher variation of lattice parameters. The QDs grown with high in-plane density show much less spread in lattice parameter giving almost flat density of In over the entire height of an average QD and much narrower photoluminescence (PL) line. The results have been verified with three different amounts of In deposition giving systematic variation of the In composition as a function of average quantum dot height and average energy of PL emission.