Excitonic transitions in ZnSe epilayers grown on GaAs

ZnSe epilayers grown on GaAs substrates frequently show free- and bound-excitonic transitions in low-temperature photoluminescence. In particular, the excitonic transition at ∼2.7968 eV ($I_2^1$) and ∼2.7948 eV ($I_2^2$) are commonly observed. The higher-energy transition corresponds closely in energy to a typical donor-bound exciton transition energy in ZnSe. However, there has been much uncertainty as to origin of the lower-energy transition. We show that both of these transitions correspond to a bound-excitonic transition at the same donor which has been split due to the residual tensile strain present in the ZnSe heteroepilayers. This strain also causes similar splittings of the free-exciton transitions. We show that our model is consistent with the experimental fact that the precise energy position of the free- and bound-exciton transitions is a function of the epilayer thickness and the growth temperature. The temperature dependence of the photoluminescence in the free- and bound-exciton region is also shown to be consistent with our model of thermal population of the strain-split states.