Impact of the Crystal Phase of ZrO₂ on Charge Trapping Memtransistor as Synaptic Device for Neural Network Application

In this work, we investigated the effects of the crystal phase of ZrO
₂
on charge trapping memtransistors (CTMTs) as synaptic devices for neural network applications. The ZrO
₂
deposited through thermal (t-ZrO
₂
) atomic layer deposition (ALD) and plasma (p-ZrO
₂
) ALD were analyzed using an X-ray diffractometer, which indicated that the t-ZrO
₂
consisted of pure cubic phase, whereas p-ZrO
₂
consisted of both cubic and tetragonal phases. Through X-ray photoelectron spectroscopy analysis, we then constructed the energy band diagram of the gate stacks. The ΔEC of tand p-ZrO
₂
with respect to tunneling and blocking Al
₂
O
₃
were 1.84 and 1.19 eV respectively. Because of the relatively large ΔEC of t-ZrO
₂
, the window of the flat band voltage (VFB) shift extracted from charge trapping capacitors was enlarged by 591.9 mV more than the one using p-ZrO
₂
as the charge trapping layer. Retention was also improved by 10.4% after 105 s in the t-ZrO
₂
case. Finally, we fabricated the CTMTs with the gate stack of the t-ZrO
₂
case and demonstrated their characteristics as synaptic devices. With the optimization of pulse schemes, we reduced the nonlinear factors of depression (αd) and potentiation (αp) from -6.72 and 6.47 to 0.03 and 0.01 respectively, enlarged the ON/OFF ratio from 15.6 to 70.4 and increased the recognition accuracy from 27.6% to 86.5% simultaneously.