  The low-lying structure of $^{10}_\Lambda$Be and $^{11}_\Lambda$Be are
investigated within the framework of the antisymmetrized molecular dynamics. We
focus on the modifications of the excitation spectra and dynamical changes of
the 2$\alpha$ cluster structure caused by a $\Lambda$ particle as an impurity
in these hypernuclei. It is found that the excitation energies of
well-pronounced cluster states are largely shifted up by the addition of a
$\Lambda$ particle. Furthermore, we also find that the 2$\alpha$ cluster
structure is significantly changed in the excited states, whereas it is almost
unchanged in the ground states.
