Transmission electron microscopy (TEM) images of 3C-SiC thin films on Si (111) grown by solid-source molecular beam epitaxy (MBE) often reveal interfacial voids just underneath the film because of the Si outdiffusion from the substrate into the layer. The same phenomenon can be seen in the 2H-AlN thin film/Si (111) heterosystem grown by plasma-assisted MBE. We demonstrate in both cases the influence of growth parameters on the created voids. In the SiC/Si system we show an important influence of the growth temperature. At 850 degreesC a well-known triangular void is formed, whereas at 1050 degreesC we found an unusual hexagonal void shape. In this case not only low surface energy {111} facets form the void shape, but also facets with higher surface energy. We discuss this new appearance as a void shape which is near the equilibrium void shape in a cubic crystal. In the AlN/Si heterosystem the initial covering of the substrate has an influence on the amount of the Si outdiffusion and therefore on the size of the voids. In samples with an initial nitrogen cover the Si content in the AIN layer is higher (similar to 10(21) cm(3)) and the voids are more larger compared to samples with an initial Al cover (Si content similar to 10(18) cm(-3)).