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In this study, hysteresis-free double-layer gate-all-around stacked poly-Si nanosheet channel ferroelectric HfxZr1-xO2 negative capacitance field-effect transistors (DL GAA NS FE-HZO NC-FETs) with an internal metal gate (IMG) and NH3 plasma nitridation were experimentally investigated and comprehensively discussed for the first time. The results revealed that NH3 plasma nitridation at both the ZrO2/TiN and TiN/HZO interfaces can effectively enhance the HZO ferroelectricity and quality by suppressing the generation of oxygen vacancies (${V}_{o}{)}$. Furthermore, the SiON interfacial layer quality can be enhanced by passivating bulk defects within the SiON interfacial layer through NH3 plasma nitridation at the ZrO2/TiN interface. When NH3 plasma nitridation is performed at both the ZrO2/TiN and TiN/HZO interfaces, the devices exhibit excellent electrical characteristics: 1) an extremely low subthreshold swing (SS) of 45.77 mV/decade; 2) an ultrasteep average SS (ASS) of 61.39 mV/decade; 3) a relatively high ON/OFF current ratio (${I}_{ \mathrm{\scriptscriptstyle ON}} / {I}_{ \mathrm{\scriptscriptstyle OFF}}{)}$ of $ > 5\,\,\times \,\,10^{{7}}$ ; and 4) a quite high effective breakdown voltage (${V}_{\text {EBD}}$) of 6.7 V at ${V}_{D}$ = 0.1 V. Thus, these devices are promising candidates for low-power-consumption monolithic 3-D integrated circuit (IC) applications. [ABSTRACT FROM AUTHOR]