  We present investigations of the optical properties of stacked InGaN quantum
dot layers and demonstrate their advantage over single quantum dot layer
structures. Measurements were performed on structures containing a single layer
with quantum dots or threefold stacked quantum dot layers, respectively. A
superlinear increase of the quantum dot related photoluminescence is detected
with increasing number of quantum dot layers while other relevant GaN related
spectral features are much less intensive when compared to the
photoluminescence of a single quantum dot layer. The quantum dot character of
the active material is verified by microphotoluminescence experiments at
different temperatures. For the possible integration within optical devices in
the future the threshold power density was investigated as well as the modal
gain by using the variable stripe length method.
