The influence of InAs coverage on the formation of self-assembled quantum rings (QRs) is investigated in this report. With decreasing InAs coverage from 2.6 to 2.2 monolayers (MLs), the QR density would decrease from 5 x 10(9) cm(-2) to invisible according to the atomic force microscope (AFM) measurements. The results are attributed to a much smaller 2.2 ML InAs quantum dot (QD) height and therefore much smaller QR structures would be obtained with the almost fully capping GaAs cap layer. Compared with QD sample with the same 2.6 ML InAs coverage, it is found that the QD density is about one order of magnitude larger than that of QRs. The phenomenon is attributed to the QD size non-uniformity such that instead of partial capping, the 2-nm-GaAs cap layer has fully covered the smaller QDs. Also observed is the blue shift of photoluminescence (PL) peak wavelengths of the QR structures with decreasing InAs coverage, which is attributed to the increase of Ga composition and the decrease of QR height during the intermixing of GaAs capping layer and the InAs QDs. (c) 2007 Elsevier B.V. All rights reserved.