Plasma-assisted atomic layer deposition of HfNx: Tailoring the film properties by the plasma gas composition

Abstract

The authors synthesized HfNx (x ≥ 1) thin films by plasma-assisted atomic layer deposition at stage temperatures of 350–450 °C by using the high-thermal-stability CpHf(NMe2)3 monomer as Hf precursor and either H2 plasma or N2 plasma as coreactant. Most notably, the selection of the plasma gas composition enabled us to tune the film properties: films fabricated using N2 plasma led to the formation of highly resistive and amorphous Hf3N4 films (6 × 102 Ω cm), while the use of the strongly reducing H2 plasma generated conductive (resistivity of 6 × 10−1 Ω cm) films with the signature of the δ-HfN fcc crystal structure. Via x-ray photoelectron spectroscopy, the authors observed that the use of the H2 plasma facilitates the reduction of the oxidation state of Hf from Hf4+ to Hf3+. This result was corroborated by the simultaneous increase in the free carrier absorption observed in the infrared range via spectroscopic ellipsometry. The δ-HfNx films fabricated via the present route are promising as highly reflective back contacts for thin films solar cells, Cu diffusion barriers, and as a gate metal for metal–oxide–semiconductor capacitors, provided that the resistivity values can be further decreased by suppressing the formation of the resistive Hf2ON2 impurity phase and grain-boundary scattering of the charge carriers.