INVESTIGATION OF THE EFFECT OF QUANTUM-WELL WIDTH ON THE BINDING-ENERGY OF EXCITONS TO NEUTRAL DONORS

Observations of bound exciton states in some bulk semiconductor materi als has shown the validity of Haynes' rule, namely that the binding en ergy of the exciton to a donor is a multiple 1/zeta (say) of the bindi ng energy of the electron to the donor (i.e., the neutral donor bindin g energy E(D)), although in other bulk materials more general linear d ependencies are required. Quantum well structures (QWS) typically exhi bit such donor bound exciton complexes. There are several points of di fference however, E(D) is a function of donor position, the donor dist ribution is unknown and interface roughness could also influence the d onor bound exciton emission. Hence in order to determine whether a gen eralised form of Haynes' rule can be applied to excitons bound to dono rs in quantum wells of varying width requires a careful combination of theory with experiment. The binding energies of the donors at various positions in each well region must be calculated with due allowance m ade for the effects of interface roughness. A presumed distribution of the donor concentration is then made and, on the assumed validity of Haynes' rule, the lineshape of the bound exciton emission calculated. Comparison with the observed emission spectra will then give insight i nto whether Haynes' rule is satisfied as a function of position and we ll width in QWS. This analysis has been carried out for a series of si ngle quantum wells, all of different widths and all grown in the same sample by the technique of molecular beam epitaxy. A careful analysis of all the data showed that Haynes' rule is not applicable-i.e., the o bserved exciton energies are not a constant multiple of the donor bind ing energy for QWS of different well width.