We have observed and characterized a delayed secondaryhyphen;electron emission in dense layers of MgO, 50ndash;650 Aring; thick, which persists for as long as 10minus;2s after interruption of the primary electron beam. The total delayed charge as well as the initial value of the delayed current are proportional to the secondary current under bombardment. The delayed current is proportional to the sample thickness for nonpenetrating primary electrons (100 eV), but decreases as the primary energy increases. During electron bombardment, these layers acquire a field of about 5times;106V/cm across them. The field permits the extraction into vacuum of electrons thermally released from bulk traps. This is the origin of the delayed emission. Part of the electron current injected at the metal contact is also extracted, yielding a fieldhyphen;enhanced component of secondary emission. The current through the MgO is mostly carried by electrons; the holes are relatively immobile. Impact ionization is not involved in these phenomena when the film thickness lies in the above range.
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