Atomic-scale structure and formation of self-assembled In(Ga)As quantum rings

We present an atomic-scale analysis of the indium distribution of self-assembled (In,Ga)As quantum rings (QRs), which are formed from InAs quantum dots by capping with a thin layer of GaAs and subsequent annealing. We find that the size and shape of QRs as observed by cross-sectional scanning tunneling microscopy (X-STM) deviate substantially from the ring-shaped islands as observed by atomic force microscopy on the surface of uncapped QR structures. We show unambiguously that X-STM images the remaining quantum dot material whereas the AFM images the erupted quantum dot material. The remaining dot material shows an asymmetric indium-rich crater-like shape with a depression rather than an opening at the center and is responsible for the observed electronic properties of QR structures. These quantum craters have an indium concentration of about 55% and a diameter of about 20 nm, which is consistent with the observed electronic radius of QR structures. Based on the structural information from the X-STM measurements, we calculate the magnetization as a function of the applied magnetic field. We conclude that, although the real QR shape differs strongly from an idealized circular-symmetric open ring structure, Aharonov-Bohm-type oscillations in the magnetization can be expected. (c) 2006 Elsevier B.V. All rights reserved.