Be microstructure and optical properties of InAs quantum dots (QDs) grown on a GaAs buffer with a 30 nm thick AlAs insertion layer are investigated and compared with those grown on a plain GaAs buffer by using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The former InAs QDs exhibit larger dot sizes of 20 nm and higher aspect ratios of 0.4, compared to 15 nm and 0.2, respectively, for the latter. Temperature-dependent PL spectra of the larger dots show that the main emission is dominated by band-tail state transitions at low temperatures and ground state transitions at high temperatures. The ground state transition energy in such quantum dots is significantly red-shifted compared to the smaller InAs QDs. Lower thermal activation energy is also observed for the larger QDs with an AlAs layer. All of the phenomena are caused by different In-Ga intermixing behavior occurring during capping, which is discussed in detail. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3561419] All rights reserved.