Tracking visible pulsed laser annealing of Hf0.5Zr0.5O2 heterostructures with <i>in situ</i> transmission electron microscopy

Abstract

Laser annealing offers a promising route to investigate and control phase formation in hafnium-zirconium oxide (HZO) thin films under rapid thermal conditions. Due to the wide bandgap of this material, previous reports have studied the crystallization of HZO using ultraviolet or infrared light. In contrast, we monitor its crystallization in a Si3N4/TiN/Hf0.5Zr0.5O2 thin film heterostructure upon irradiation with visible nanosecond laser pulses. This geometry mimics the structure of complementary metal–oxide–semiconductor devices and harnesses the absorption of TiN in the visible regime to generate the heat necessary for the transformation. Through a series of local in situ measurements using a modified transmission electron microscope, we quantify the relationship between the HZO film thickness, critical laser energy density, and the ferroelectric HZO phase fraction, finding a sharp threshold behavior in the laser pulse energy necessary to crystallize HZO. The optimal condition of irradiating an 8-nm HZO film with a single laser pulse with an energy density of 177 mJ/cm2 is found to produce 86% of the ferroelectric orthorhombic phase. Heat transfer dynamics within the heterostructure during laser annealing is revealed by finite-element simulations, where the partial melting of the silicon nitride substrate is found to play an important role limiting the temperature to 1900 °C. This finding and the observed laser pulse energy threshold behavior support a kinetic crystallization pathway involving the tetragonal phase. More generally, these findings provide insight into laser-driven phase engineering in ferroelectric heterostructures and establish a basis for future studies aimed at functional electronic applications.