Fabrication of Ultrathin Ferroelectric Al_0.7Sc_0.3N Films under Complementary-Metal-Oxide-Semiconductor Compatible Conditions by using HfN_0.4 Electrode

Aluminum scandium nitride (AlScN) has emerged as a promising candidate for next-generation ferroelectric memories, offering a much higher remanent charge density than other materials with a stable ferroelectric phase. However, the inherently high coercive field requires a substantial decrease in film thickness to lower the operating voltage. Significant leakage currents present a severe challenge during the thickness scaling, especially when maintaining compatibility with complementary-metal-oxide-semiconductor (CMOS) fabrication standards. This study adopts a HfN_0.4 bottom electrode, which minimizes lattice mismatch with Al_0.7Sc_0.3N (ASN), forming a coherent bottom interface that effectively reduces leakage currents even at thickness < 5 nm. CMOS-compatible HfN_0.4/ASN/TiN stack, deposited without vacuum break between each layer, demonstrates exceptional scalability, confirming the ferroelectricity of ASN films at thicknesses down to 3 nm. The coercive voltage is decreased to 4.35 V, significantly advancing low-voltage AlScN devices that align with CMOS standards.