  To probe the influence of structural disorder on low-temperature behavior of
magnetic penetration depth in electron-doped high-T_C superconductors, a
comparative study of high-quality Pr_{1.85}Ce_{0.15}CuO_4 (PCCO) and
Sm_{1.85}Ce_{0.15}CuO_4 (SCCO) thin films is presented. The obtained results
confirm a d-wave pairing mechanism in both samples, substantially modified by
impurity scattering (which is more noticeable in less homogeneous SCCO films)
at the lowest temperatures. The value of the extracted impurity scattering rate
correlates with the quality of our samples and is found to be much higher in
less homogeneous films with lower T_C.
