From the above results, the emission mechanism for the
Al,Eu-codoped sample was deduced and is shown in Fig.
6. There are two traps, a hole trap and an electron trap. The
hole trap is an Al center or Eu-related center with trap
depth of ca. 2 eV and the electron trap is probably Eu
31 21
(from sn Eu to Eu transition), for which the trap depth
is ca. 0.5 eV.