One-Dimensional Thickness Scaling Study of Phase Change Material $(\hbox{Ge}_{2}\hbox{Sb}_{2}\hbox{Te}_{5})$ Using a Pseudo 3-Terminal Device

To address the scalability of phase change memory (PCM), we study a 1-D thickness scaling effect on threshold switching voltage
$(V_{\rm th})$
,
$V_{\rm th}$
drift, high resistance state (RESET) resistance
$(R_{\rm RESET})$
drift, and crystallization temperature
$(T_{\rm crys})$
. We use a pseudo three-terminal device to accurately correlate the amorphous region thickness to the observed characteristics. The pseudo 3-terminal device is a fully functional PCM cell and enables 1-D thickness scaling study down to 6 nm without the need for ultrafine lithography.
$V_{\rm th}$
scales down to 0.65–0.5 V (at 25
$^{ \circ}\hbox{C}$
–75
$^{\circ}\hbox{C}$
) for 6-nm-thick
$\hbox{Ge}_{2}\hbox{Sb}_{2}\hbox{Te}_{5}$
(GST), showing that stable read operation is possible in scaled PCM devices. The
$V_{\rm th}$
drift measurement suggests that
$V_{\rm th}$
drift can be attributed to threshold switching field
$(E_{\rm th})$
drift, whereas
$V_{{\rm th}0}$
, i.e.,
$V_{\rm th}$
at zero thickness, stays almost constant.
$R_{\rm RESET}$
drift shows no dependence on the amorphous GST thickness.
$T_{\rm crys}$
is
$ \sim\!\hbox{175} \ ^{\circ}\hbox{C}$
for the device with 6-nm-thick GST, compared with
$\sim\!\hbox{145} \ ^{\circ}\hbox{C}$
of thick GST. From the 1-D scaling study, no significant hurdles against scaling are found down to 6 nm. Further study of scaling effect on endurance and development of scalable selection device is needed to assess the ultimate scalability of PCM.