Enhanced ferroelectricity by strain gradient in few-layer HfO2thin films

Abstract

Due to the great potential in reducing the size of ferroelectric memory cells and the good compatibility with traditional silicon process, the ferroelectricity of few-layer HfO2 thin films has received huge attention, but its microscopic mechanism is still unclear. Based on first-principles calculations, the ferroelectricity of few-layer HfO2 thin films in the presence of both strain and strain gradient has been investigated systematically. It is shown that the orthorhombic ferroelectric phase exists stably with the strain between −7% and 3%, and the ferroelectric polarization decreases monotonically with the strain increasing, which is consistent with previous studies. In particular, the ferroelectric polarization increases monotonically with the strain gradient increasing within the strain less than −2.5%, which is due to the fact that the strain gradient further separates the positive and negative centers. Our findings provide possible theoretical explanations for recent experimental results and technical guidance for the design of ultra-thin ferroelectric devices.