Low temperature chemical vapor deposition of hafnium nitride - Boron nitride nanocomposite films

Nanocomposite HfN_x-BN thin films are deposited by chemical vapor deposition at substrate temperatures of 350 - 800 °C using the single-source precursor hafnium borohydride, Hf(BH₄)₄,in combination with ammonia, NH₃. Below 350 °C, the product is metallic HfB₂ with essentially no incorporation of nitrogen. However, the presence of ammonia decreases the HfB₂ deposition rate considerably; this growth suppression effect is attributed to blocking of reactive surface sites by adsorbed ammonia molecules. At substrate temperatures above 350 °C, film deposition occurs; however, the HfB₂ phase is completely absent. The resulting film stoichiometry is HfB_yN _2.5; although the value of y is difficult to determine precisely, it is about unity. X-ray photoelectron spectroscopy (XPS) analysis detects Hf - N and B - N bonds but no Hf - B bonds; thus the films are nanocomposites that consist of a mixture of hafnium nitride, HfN_x with x > 1 and boron nitride. The deposited films are X-ray amorphous and Raman inactive. Compared to HfB₂ films grown under similar precursor pressure and substrate temperature, the HfN_x-BN films are smoother and have a denser microstructure. The thermal activation energy for growth of HfN_x/BN in the reaction-rate limted regime is ∼0.72 eV (70 kJ/mol), a value 0.3 eV larger than that for the growth of HfB₂ from Hf(BH₄) ₄ alone. This difference in activation energy indicates that growth is governed by a different rate-limiting step; we interpret that the Hf(BH ₄)₄ precursor reacts with ammonia on the growth surface to generate species with Hf - N and B - N bonds, which subsequently lose H ₂ and BH_y to generate the nanocomposite. The HfN _x/BN films have resistivities ∼ 10 ω • cm. Optical transmission and spectroscopic ellipsometry measurements indicate a bandgap of ∼ 2.6 eV.