Intriguing morphological evolution during chemical vapor deposition of HfS₂ using HfCl₄ and S on sapphire substrate

For chemical vapor deposition (CVD) of HfS₂, HfCl₄ is commonly used as the Hf precursor, and a sapphire (Al₂O₃) substrate is preferred to improve film crystallinity. During the HfS₂ CVD process using HfCl₄ and S precursors on the sapphire substrate, we observed an anomalous morphological evolution due to the chemical interaction between the Al₂O₃ substrate and HfCl₄, particularly when the substrate was exposed to HfCl₄ vapor too early, prior to the introduction of S. The excessive HfCl₄-induced local chemical etching produced a serrated surface on the substrate, while pyramid-shaped HfO₂ crystals (a by-product of chemical etching) and CVD-HfS₂ flakes formed on the unetched and etched substrate surface regions, respectively. A mechanism for the observed morphological evolution is proposed based on thermodynamic and kinetic considerations, which emphasizes the importance of precursor supply timing to minimize the potential process-related defects.