Low-Frequency Noise Spectroscopy for Navigating Geometrically Varying Strain Effects in HfO

Strain engineering has been widely employed to control and enhance the ferroelectric properties of hafnium oxide (HfO₂)-based thin films. While previous studies focused on the influence of the strain in simple metal-ferroelectric-metal structures, the integration of strain-induced ferroelectricity into field-effect transistors (FETs) requires consideration of geometrical factors, such as the interfaces between the channel and source/drain contacts, as well as device dimension. Here, we demonstrate strain effects in HfO₂-based ferroelectric FETs (FeFETs) with poly-Si channels via low-frequency noise (LFN) spectroscopy. LFN analysis reveals that the strain during the post-metal annealing introduces damage to channel interface with its severity depending on the device geometry. This strain-dependent behavior results in a unique noise characteristic, which we refer to as the reverse scaling effect, where noise increases with longer channel lengths-contrary to the conventional trend in typical FETs, where noise decreases with increasing channel length. Furthermore, we observe that while increased strain enhances ferroelectricity, it also degrades the electrical performance of poly-Si FeFETs, primarily through damage to the channel interfaces. These findings underscore the critical role of strain engineering in FeFETs and provide important guidelines for balancing strain effects to achieve optimal ferroelectricity and reliability in future device designs.