This paper presents an approach to enhance Hf0.5Zr0.5O2 (HZO) ferroelectric orthorhombic phase (O-phase) formation via in situ NH3 plasma treatment. High-resolution non-disruptive hard x-ray photoelectron spectroscopy confirmed that O-phase formation can be enhanced by suppressed interfacial diffusion between HZO and the top TiN electrode. Additional N-bonding facilitated by NH3 treatment was shown to suppress the interaction between TiN and HZO, thereby reducing the formation of oxygen vacancies within HZO. It was shown to improve the reliability and ferroelectric performance (examined by the leakage current and positive-up-negative-down measurements) of HZO devices. After cyclic operations, NH3-treated ferroelectric FETs (FeFETs) exhibited stable transfer characteristics and memory windows, whereas untreated devices presented unstable behaviors. Our results demonstrate the efficacy of the proposed in situ NH3-treatment scheme in enhancing the stability of HZO-based FeFETs.