HfO₂ Thin Films by Chemical Beam Vapor Deposition for Large Resistive Switching Memristors

We present chemical beam vapor deposition (CBVD) as a valuable technique for the fabrication of good quality HfO2-based memristors. This deposition technique gives the opportunity to rapidly screen material properties in combinatorial mode and to reproduce the optimized conditions homogenously on large substrates. Cu/HfO2/Pt memory devices with three different oxide thicknesses were fabricated and electrically characterized. A bipolar resistive switching and forming free behavior was seen in all the tested devices. Lower switching voltages than similar devices fabricated by employing different deposition techniques were observed. The conduction mechanism in the low resistance state can be ascribed to filamentary copper, while a trap-controlled space charge limited current conduction was observed in the high resistance state. The comparative evaluation of devices with different oxide thicknesses allows to infer that devices with thicker HfO2 film (25 nm) are more performing in terms of ROFF/RON ratio ( $10{^{{6}}}$ ), and reproducible resistive switching over more than 100 cycles in both low and high resistance states. Thinner oxide devices (20 nm and 16 nm), despite similar long retention time ( $10{^{{4}}}$ s), and lower SET/RESET voltages show instead a smaller memory window and a switching instability. These results, compared also with other reported in literature for similar memristive structures realized with other deposition techniques, show that CBVD can be considered as a promising technique for realizing HfO2-based non-volatile memory devices with good performance.