Interface Reactions during Annealing-Induced Ferroelectric Phase Formation in Hf_0.5Zr_0.5O₂ Films on HfS₂ and MoS₂

Interface reactions during postdeposition annealing (PDA) of Hf_0.5Zr_0.5O₂ (HZO) films grown on HfS₂ and MoS₂ crystals by atomic layer deposition are investigated for potential application in future top-gate ferroelectric transistors. Both the HZO/HfS₂ and HZO/MoS₂ samples experience blistering of the HZO films when annealed for ferroelectric phase formation at temperatures greater than 500 and 600 °C, respectively, for 1 min in an N₂ atmosphere. The different critical PDA temperatures are associated with different onset temperatures for the thermal decomposition of HfS₂ and MoS₂ crystals, which occur because of reactions with oxygen inadvertently introduced into the N₂ annealing atmosphere. In both samples, the S- and O-related byproduct gases delaminate the HZO film, forming blisters and ruptures with increasing PDA temperatures. Because of the formation of solid HfO_x films on HfS₂ via the diffusion of ambient oxygen through the HZO film and randomly located blisters, the HZO/HfS₂ sample develops deep and irregularly shaped encroachment defects with an uneven HfS₂ surface. By contrast, owing to the desorption of the volatile oxide phase (e.g., MoO₃) produced during the oxidation reaction, the MoS₂ crystal maintains a flat surface through its layer-by-layer decomposition as the PDA temperature increases beyond 600 °C.