Ferroelectricity of hafnium oxide and its variants: thin film growth and memory device application

Abstract

Ferroelectric materials have emerged as pivotal components in the landscape of modern electronics, offering unprecedented potential for non-volatile memory and novel transistor designs. Among these materials, hafnium dioxide (HfO2)-based thin film exhibits remarkable promise, largely attributable to its compatibility with conventional semiconductor processes. This doctoral thesis describes the multifaceted realm of ferroelectric materials, especially HfO2, and their applications in two paramount domains as follows: non-volatile memory devices utilizing graphene and lowering the dielectric constant. In order to resolve the problem of low on/off ratio (Ion/off) of the ferroelectric graphene field- effect transistor (FeGFET) structure, a transconductance measurement scheme is proposed. The effect of an ultra-thin Al2O3 insertion layer is also discussed regarding the ferroelectricity and dielectric constant of a Hf0.5Zr0.5O2 layer, as one way to overcome the problem of high-k property of hafnia ferroelectric systems. First, we propose a new approach for read-out process in ferroelectric random-access memory (FeRAM) and prove its viability and reliability. This method utilizes a graphene layer as the channel material in a bottom-gated field-effect transistor structure. We found that the transconductance of graphene channel changes its sign based on the spontaneous polarization (SP) direction of the underlying ferroelectric layer. This phenomenon enables the unambiguous detection of the memory state in FeRAM, determined by the SP direction in the ferroelectric layer, through transconductance measurements. We also fabricated an array of FeRAM cells organized in a cross-point structure, with an HfO2-based ferroelectric thin film. Notably, it allows for the self-selective measurement of memory state in a cross-point structure without the need for additional selector. As another topic, we propose a method to decrease a dielectric constant of the Hf0.5Zr0.5O2 (HZO) ferroelectric film while preserving its ferroelectric properties. We achieved this by introducing an ultra- thin Al2O3 layer within the HZO film, creating an HZO/Al2O3/HZO trilayer structure during the atomic layer deposition process. This trilayer structure is combined with TiN electrodes deposited in an Ar-rich environment using the magnetron sputtering process. We observed that it possesses an effective dielectric constant of approximately 18, without compromising its ferroelectric nature. For this, we exploited the role of Al2O3 which is expected to provide in-plain tensile stress and inhibits grain size growth. This effective dielectric constant is around 37% smaller than the dielectric constant of our HZO layer and even lower than the theoretically calculated value which considers the series connection of each layer in the trilayer system. By combining the strengths of conventional ferroelectric memory (low power, fast switching of polarization direction), proposed read-out process (non-destructive read-out, selector free), and relatively low-k ferroelectric HZO layer (small RC delay), it is expected to find a solution fabrication of 3D stackable non-volatile random access memory array with ultra-low power, high density, high- speed long lifetime. Such type of memory device can be used as solid state drive fast as RAM.