With the increasing demand for new devices for application to short-range telecommunications, the development of GaAs-based active materials emitting in the telecom wave-length regions around 1.3 and 1.55 μm has become mcre and more important. The realization of long-wavelength InGaAs/GaAs quantum dot (QD) lasers is one of the most promising ways to reach this goal. Since the electronic state of QDs depends on the size, shape and composition of the nanostructure, accurate studies of the growth conditions and, therefore, of the structural properties of the samples are crucial to better understand the changes in the optoelectronic properties and to achieve a careful control of the various parameters, necessary to obtain devices emitting at 1.3 µm. Laser structures grown by MOCVD show an interesting effect: when tuning the photoluminescence (PL) emission wavelengths at 1.3 μm, the electroluminescence (EL) emission occurs at a shorter wavelength, in structures grown on n-type substrates, whereas this is not found in those grown on p-type substrates'.In this work, we investigated and compared the structural and optical features of QDs samples grown by MOCVD on different substrates, namely on semi-insulating (SI) GaAs (sample A), on p-type (Zn doped) GaAs (sample B), and on n-type (Si doped) GaAs (sample C), by means of conventional and high resolution transmission electron microscopy (CTEM and HRTEM), PL and AFM. The samples reported here are representative of a series of samples already investigated (above all by AFM) and they are test samples, differing from the full laser device only for the terminated growth after …