ZnSe films with thickness between 800 and 7500 Å were grown on GaAs(100) by molecular beam epitaxy (MBE), and characterized by photoluminescence (PL), photoreflectance (PR), transmission electron microscopy (TEM), and high resolution x-ray diffraction. A first set of films was prepared with ZnSe directly grown on the GaAs substrate. Another set was prepared using an arsenic capped GaAs buffer layer grown on the GaAs substrate in a separated MBE system. PL studies at 18 K showed that the ZnSe films have more defects for samples grown directly on the GaAs substrate. The behavior of stacking faults and dislocations as a function of film thickness were investigated by TEM, and by the variation of the intensity of PL signals related to these defects. For both sets of samples the intensity of these signals decreased with increasing film thickness, but the decrease is steeper for films grown on GaAs buffer layers. A signal in PL spectra at ∼ 2.7 eV was observed only for the samples grown directly on GaAs substrates, it was associated with donor–acceptor transitions involving GaZn and VZn. The room temperature PR spectra showed, besides the GaAs and ZnSe band-gap signals, oscillations associated with the Franz–Keldysh effect due to internal electric fields. The strength of these fields was obtained by employing the asymptotic Franz–Keldysh model. A signal 22 meV below the GaAs band-gap energy was observed only in the PR spectra of the samples grown directly on GaAs substrates. This signal was associated with Zn interdiffused into the GaAs, and is correlated to the PL signal observed at 2.7 eV for the same set of samples. © 1998 American Institute of Physics.