Hafnium diboride (HfB2) is a promising candidate as a seed layer for GaN growth on Si substrates due to its excellent lattice and thermal coefficients matching with both materials. This work investigates the epitaxial growth of AlB2-type non-stoichiometric HfB2 (HfB(2+delta )with -0.1 < delta < 0.6) thin films on Si(111) using magnetron co-sputtering. We demonstrated that the process temperature significantly affected the surface roughness (R-RMS similar to 0.5-4 nm), film composition, and the nucleation of secondary impurity phases. Films deposited between 700 and 900 degrees C exhibit epitaxial growth on the Si substrate with a well-defined relationship of ( 0001 ) (HfB 2) & Vert; ( 111 ) (Si )and [ 11 2 <overline> 0 ] (HfB 2) & Vert; [ 1 1 <overline> 0 ] (Si). Detailed x-ray diffraction and scanning transmission electron microscopy analyses reveal that impurity phases detected at high temperatures are primarily carbon-rich phases, identified as HfCx or HfCxBy. Interestingly, this secondary phase's crystal orientation follows the orientation of its surroundings. The different findings in terms of contamination (C and O) and deposition temperature offer valuable insights for further growth optimizing of high-quality epitaxial HfB2 thin films on Si(111) for future GaN-on-Si integration.