Van der Waals multiferroic tunnel junction with giant tunneling electroresistance and magnetoresistance toward bidirectional photoresponse and photoassisted memory

Achieving higher-order multistates with significant tunneling electroresistance (TER) and tunneling magnetoresistance (TMR) in van der Waals (vdW) multiferroic tunnel junction at the nanoscale is essential for multifunctional information storage. Based on first-principles calculations, a strategy is proposed using four-layer vdW heterostructures that ferroelectric (FE) bilayer-$\mathrm{G}{\mathrm{a}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ sandwiched between two half-metal $\mathrm{CoG}{\mathrm{a}}_{2}\mathrm{S}{\mathrm{e}}_{4}$ layers. Reversible transition between quasi-Ohmic and Schottky contacts at the interface can be regulated by FE, stemming from the polarization-field-driven band structure shift and multi-interface electron transfer. Accordingly, the designed symmetric $\mathrm{Cu}/\mathrm{CoG}{\mathrm{a}}_{2}\mathrm{S}{\mathrm{e}}_{4}/\mathrm{bilayer}\ensuremath{-}\mathrm{G}{\mathrm{a}}_{2}\mathrm{S}{\mathrm{e}}_{3}/\mathrm{CoG}{\mathrm{a}}_{2}\mathrm{S}{\mathrm{e}}_{4}/\mathrm{Cu}$ vdW antiferroelectric multiferroic tunnel junction (AFMFTJ) achieves giant TER and TMR ratios up to $5.43\ifmmode\times\else\texttimes\fi{}{10}^{4}%$ and $1.16\ifmmode\times\else\texttimes\fi{}{10}^{3}%$, respectively, accompanied by exceptionally low resistance-area product of $0.1\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\textmu{}{\mathrm{m}}^{2}$. Of note, due to the type-II and degenerate band alignments of bilayer $\mathrm{G}{\mathrm{a}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ controlled by FE-polarized directions, the proposed AFMFTJ exhibits bidirectional photoresponse ($R$) up to 23.3 and \ensuremath{-}23.3 mA/W in FE states, while negligible $R$ in antiferroelectric states. As such, the robust and feeble $R$ are suitable for encoding binary digits as ``1'' and ``0'', respectively, enabling the implementation of photoassisted memory and logic functions. Our findings demonstrate the potential photoelectric spintronic applications of AFMFTJ in nanoscale information storage.