In this article, photo luminescence measurements are used to investigate the vertical correlation effect on stacked layers of GaN self-assembled quantum dots in AlN matrix. The samples have been grown by plasma-assisted molecular beam epitaxy on the Si-face and C-face of Sic substrates, leading to AlN or GaN layers with metal (Al, Ga) or N polarity, respectively. For both polarities, photoluminescence reveals that the energy and the width of the quantum dot emission line strongly depend on the number of quantum dots planes, consistent with the progressive built-up of correlation. It is concluded that the evolution of both emission line position and broadening is mostly determined by the average internal electric field.