2, Saha Institute of Nuclear Physics, Kolkata, , India
Self-assembled semiconductor quantum dots (QDs), such as Ge QD on Si and InAs QD on GaAs exhibit size and composition dependent optical and electrical properties. The prime challenge remains to determine the growth parameters to obtain predictable composition and strain profiles within a QD and size-distribution of QDs so that optoelectronic properties of these quantum structures can be tuned with fundamental physics calculations. It has been pointed out that the confinement length of the carriers depends upon the composition profile within a QD and not just on their size. Structure-spectroscopy correlation allowed us to address these problems with accuracy.
We have studied the growth process of Ge Inverted Quantum Hut (IQH) structures, embedded in a silicon lattice, using anomalous x-ray scattering techniques . The results showed that the deposited Ge layer relaxes strain by uniform intermixing with the previously deposited lower Si layer to form a SiGe alloy wet layer and the IQH structure forms just below the wet layer with its tip pointing towards the substrate. The interfacial strain in such IQH structures is greatly reduced, leading to a Type - I band-alignment at the hetero-interface, which is different from that obtained for conventional QDs , and hence these are significant for application in optoelectronics. We have also studied the in-plane density dependent properties of InAs QDs grown on GaAs substrate. We showed that composition and strain profile of the QDs can be tuned by controlling in-plane density of the dots over the substrate with the help of substrate-temperature profile .
Anomalous grazing incidence x-ray diffraction (GIXD) study of the Ge IQH structures on Si substrate grown by Molecular Beam Epitaxy technique will be presented. GIXD measurements were performed at Petra III, DESY, Germany by tuning the incident X-ray beam at two different energies corresponding to Ge K-edge (11103eV) and away from it (11043eV), in order to improve the sensitivity of Ge. Measurements were performed to probe (400) and (800) in-plane diffraction peaks at both the energies and their intensities were compared. The study allows us to directly determine the composition variation in such structures and also mapping of strain in the different regions constituting them. Important morphological details of the IQH structure have been revealed by the analysis of x-ray reflectivity data by Born-approximation. The results are supported by cross-sectional transmission electron microscopy images of the IQH structures.
 M. Sharma, M. K. Sanyal, B. Satpati, O. H. Seeck and S. K. Ray, Physical Review B, 89, 205304 (2014).
 M. Sharma, M. K. Sanyal, A. Katiyar and S. K. Ray, Applied Physics A, 119, 55 (2015).
 M. Sharma, M. K. Sanyal, I. Farrer, D. Ritchie, A. B. Dey, A. Bhattacharyya, O. H. Seeck, J. S. Szymanska, M. Felle, A. J. Bennett and A. J. Shields, Scientific Reports, 5, 15732 (2015).