In this work, we report on the atomic layer deposition (ALD) of HfN x thin films by employing CpHf(NMe 2) 3 as the Hf(IV) precursor and Ar–H 2 plasma in combination with external RF substrate biasing as the co-reactant. Following up on our previous results based on an H 2 plasma and external RF substrate biasing, here we address the effect of ions with a larger mass and higher energy impinging on HfN x film surface during growth. We show that an increase in the average ion energy up to 304 eV leads to a very low electrical resistivity of 4.1 × 10 –4 Ωcm. This resistivity value is achieved for films as thin as ~ 35 nm, and it is an order of magnitude lower than the resistivity reported in literature for HfN x films grown by either CVD or ALD, while being comparable to the resistivity of PVD-grown HfN x films. From the extensive thin film characterization, we conclude that the impinging ions during the film growth lead to the very low electrical resistivity of HfN x films by suppressing the oxygen incorporation and in-grain nano-porosity in the films.