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 (106), 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 (104 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.

We present an innovative dielectric layer composed of a ternary oxide composite, including high-k materials Al2O3, ZrO2, and HfO2. These composite films were deposited using the Chemical Beam Vapor Deposition (CBVD) technique with a combinatorial approach. We investigated their structural, nanostructural, morphological, and electrical properties for three different Al:Zr:Hf atomic concentrations (∼27:61:12), (∼44:42:13), and (∼67:20:12), which are normalized as AZH (2.2:5:1), AZH (3.3:3.2:1), and AZH (5.5:1.6:1), respectively. X-ray diffraction (XRD) and Raman spectroscopy revealed cubic phase Al2O3 and monoclinic phases of ZrO2 as well as HfO2 in all AZH films. X-ray photoelectron spectroscopy (XPS) showed +3, +4, and +4 oxidation states for Al, Zr, and Hf, respectively. Transmission electron microscopy (TEM) images demonstrated a uniform, dense morphology with no signs of interdiffusion. Among the films, the AZH (5.5:1.6:1) sample exhibited the lowest leakage current density (∼4 × 10−11 A/mm2 at 10 V) and the highest capacitance density (∼15 fF/μm2 at 100 Hz). These results suggest that the proposed dielectric layer has significant potential for advanced applications requiring high capacitance and low leakage.

Combinatorial approach has been widely recognized as a powerful strategy to develop new-higher performance materials and shed the light on the stoichiometry-dependent properties of known systems. Herein, we take advantage of the unique features of chemical beam vapor deposition to fabricate compositionally graded Na_1+xTaO_3±δ thin films with −0.6 < x < 0.5. Such a varied composition was enabled by the ability of the employed technique to deliver and combine an extensive range of precursors flows over the same deposition area. The film growth occurred in a complex process, where precursor absolute flows, flow ratios, and substrate temperature played a role. The deviation of the measured Na/Ta ratios from those predicted by flow simulations suggests that a chemical-reaction limited regime underlies the growth mechanism and highlights the importance of the Ta precursor in assisting the decomposition of the Na one. The crystallinity was observed to be strongly dependent on its stoichiometry. High under-stoichiometries (e.g., Na_0.5TaO_3−δ) compared to NaTaO₃ were detrimental for the formation of a perovskite framework, owing to the excessive amount of sodium vacancies and oxygen vacancies. Conversely, a well-crystallized orthorhombic perovskite structure peculiar of NaTaO₃ was observed from mildly under-stoichiometric (e.g., Na_0.9TaO_3−δ) to highly over-stoichiometric (e.g., Na_1.5TaO_3+δ) compositions.