Direct Epitaxial Growth of Polar (1-x)HfO2-(x)ZrO2 Ultrathin Films. on Silicon

Ultrathin Hf1_xZr 02 films have attracted tremendous interest since they show ferroelectric behavior at the nanoscale, where other ferroelectrics fail to stabilize the polar state. Their promise to revolutionize the electronics landscape comes from the well-known Si compatibility of Hf02 and Zr02, which (in amorphous form) are already used as gate oxides in MOSFETs. However, the recently discovered crystalline ferroelectric phases of hafnia-based films have been grown on Si only in polycrystalline form. Better ferroelectric properties and improved quality of the interfaces have been achieved in epitaxially grown films, but these are only obtained on non -Si and buffered Si(100) substrates. Here, we report direct epitaxy of polar Hfl_Zrx02 phases on Si, enabled via in situ scavenging of the native a-Si0,, layer by Zr (Hf), using pulsed laser deposition under ballistic deposition conditions. We investigate the effect of substrate orientation and film composition to provide fundamental insights into the conditions that lead to the preferential stabilization of polar phases, namely, the rhombohedral (r-) and the orthorhombic monoclinic (m-), on Si.