Analysis of spatial distribution of remnant polarization in inverted staggered a-IGZO/HZO ferroelectric thin-film transistor using capacitance-voltage characteristics

초록

A HfZrOx (HZO)-based ferroelectric field-effect transistor (FeFET) is considered a promising candidate for use in high-density, low-power nonvolatile memory. In general, the remnant polarization (P-r) is the origin of the memory property, and a larger P-r is desired. However, coercive voltage (V-C) tends to increase with increasing P-r, and a large memory window indicates that the electric field is increased in HZO when polarization switching occurs, resulting in electric field-induced defect generation. Considering that defect generation is one of the primary causes of endurance failure, a quantitative analysis of the defect states is essential. In addition, the application of HZO as a gate insulator in FeFET requires an understanding of the electric field distribution inside the device, thus requiring a comprehensive analysis of the device operation perspective. In this paper, a quantitative analysis is performed encompassing both changes in defect states and structure effects depending on the ferroelectric polarization state of a ferroelectric thin-film transistor (FeTFT). FeTFTs were designed and fabricated, and their switching characteristics and subgap density of states (DOS) distribution were investigated through positive-up-negative-down (PUND) measurements and gate capacitance-gate voltage characteristics (C-g-V-g) at various frequencies. We confirmed that the distribution of subgap DOS increases significantly in the large V-g sweep range (-4.5 V similar to 3.5 V) compared to the small V-g sweep range (-2 V similar to 2 V), which implies that the ferroelectric polarization switching affects the subgap DOS of a-IGZO and/or interface states, even just once. In a previous study, it was found that TiN/HZO/a-IGZO/Mo (MFSM, source/drain contact region) primarily contributes to the memory operation of FeTFTs, while the TiN/HZO/a-IGZO (MFS, channel region) exhibits one-time memory operation with irreversible polarization switching. Based on this, TCAD simulations were conducted, incorporating the switching effects of MFSM, MFS, and the extracted subgap DOS. This allowed for the verification of consistency with the measured C-g-V-g curve of FeFETs, enabling a quantitative analysis of changes in subgap DOS and P-r distribution in different regions of the device according to the applied V-g.

키워드