The f k e f k h term defines the probability to find an electron and a hole with same k . Such a form is expected for a probability of two uncorrelated events to occur simultaneously at a desired k value. Therefore, f k e f k h is the spontaneous-emission source originating from uncorrelated electron–hole plasma. The possibility to have truly correlated electron–hole pairs is defined by a two-particle exciton correlation Δ Δ N λ λ ; the corresponding probability is directly proportional to the correlation. Nevertheless, both the presence of electron–hole plasma and excitons can equivalently induce the spontaneous emission. A further discussion of the relative weight and nature of plasma vs. exciton sources is presented in connection with the SLEs.