  Dependence of the spectral functions on temperature and rare-earth
substitution was examined in detail for Kondo semiconductor alloys Sm1-xEuxB6
and Yb1-xLuxB12 by bulk-sensitive photoemission. It is found that the 4f
lattice coherence and intrinsic (small) energy gap are robust for SmB6 against
the Eu substitution up to x = 0.15 while both collapse by Lu substitution
already at x = 0.125 for YbB12. Our results suggest that the mechanism of the
intrinsic gap formation is different between SmB6 and YbB12 although they were
so far categorized in the same kind of Kondo semiconductors.
