Telluride materials, including the canonical GST composition, Ge₂Sb₂Te₅, exhibit unique behaviour (nanosecond crystallization times, large opto-electronic property contrasts between glassy and crystalline phases) which enable them to function as non-volatile memory materials, in which digital information is stored as the atomic-structural state rather than as stored electronic charge, as in conventional Si CMOS (flash) memory. Intermediate memory states, i.e. partially crystalline and glassy, can also be programmed, thereby allowing more than 1 bit to be stored in each memory cell. However, such multilevel operation is compromised by 'resistance drift' in the glassy state, whereby the electrical resistance increases with time after a RESET voltage pulse producing the glassy state. The origin of resistance drift remains obscure.
In this talk, I will outline computer-simulational work that we have undertaken to study the nature, and behaviour, of structural defects in models of glassy GST and other phase-change materials, which give rise to shallow- and deep-lying localized electronic states in the bandgap of the materials. In particular, I will describe results obtained from simulations of charge-trapping by such defects, as well as the effects of applied high electric fields. The implications for resistance drift will also be discussed.