Spectroscopic evidence for different laser gain mechanisms in optically pumped ZnCdS/ZnS quantum well structures

The role of biexcitons has been studied in two optically pumped quantum well (QW) structures of Zn1-xCdxS/ZnS, one with 18% Cd concentration and the other with 3% in the wells. For the x = 18% QW structure, high excitation photoluminescence and stimulated emission indicate that the laser gain mechanism involves biexcitons. For the x = 3% QWs, even though biexcitons are clearly observed in the spontaneous emission, they are not responsible for laser gain in this structure. Instead exciton-exciton scattering may be the more likely mechanism responsible for laser gain close to threshold, while at higher densities an estimate of the carrier density indicates an electron-hole plasma as the likely source of optical gain. The different mechanisms in the two cases can, very likely, be attributed to one of differing degrees of localization both within the QW and at alloy fluctuations. (C) 1998 American Institute of Physics. [S0021-8979(98)00922-0].