We compare the photoluminescence (PL) properties of silicon (Si) single quantum well structures, consisting of an either amorphous or crystalline Si layer of 3 nm thickness, embedded between silicon–nitride layers. These structures are grown by plasma‐enhanced chemical vapor deposition on Si substrates. After crystallization of the originally amorphous Si layers and passivation by hydrogen, strongly polarized PL is observed in the entire visible spectrum. In contrast, a reference structure without the 3 nm silicon layer, but otherwise identical, shows PL in the red and infrared only, but also strongly polarized. Thus, the PL in the blue and green part of the spectrum comes from recombination via Si quantum well states, while the red and infrared PL is due to states at the interface between crystalline Si and silicon–nitride. © 1996 American Institute of Physics.