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.