Influence of point defects on the elastic properties and phase stability of cubic binary and ternary transition metal (aluminum) nitride thin films

Karimi Aghda, Soheil; Schneider, Jochen M. (Thesis advisor); Anders, Andre (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Book, Dissertation / PhD Thesis

In: Materials chemistry dissertation 41
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023

Abstract

In the first part of this thesis, a correlative experimental and theoretical model is developed to study the influence of ion kinetic energy on the evolution of point defect structure in metastable cubic (V,Al)N. Ion irradiation-induced changes in the structure and mechanical properties of (V,Al)N thin films deposited by reactive high power pulsed magnetron sputtering (HPPMS) are systematically investigated in the ion kinetic energy (E_k) range from 4 to 154 eV. Increasing E_k results in film densification and the evolution from a columnar (111) oriented structure at E_k ≤ 24 eV to a fine-grained structure with (100) preferred orientation for E_k ≥ 104 eV. Furthermore, the compressive intrinsic stress increases by 336 % to −4.8 GPa as E_k is increased from 4 to 104 eV. Higher ion kinetic energy causes stress relaxation to −2.7 GPa at 154 eV. These ion irradiation-induced changes in the thin film stress state are in good agreement with density functional theory (DFT) simulations. Furthermore, the measured elastic moduli of (V,Al)N thin films exhibit no significant dependence on E_k. The apparent independence of the elastic modulus on E_k can be rationalized by considering the concurrent and balancing effects of bombardment-induced formation of Frenkel pairs (causing a decrease in elastic modulus) and evolution of compressive intrinsic stress (causing an increase in elastic modulus). Hence, the evolution of the film stresses and mechanical properties can be understood based on the complex interplay of ion irradiation-induced defect generation and annihilation. In the second part, the developed model was extended towards the isostructural (Ti,Al)N thin films grown by two physical vapor deposition techniques, namely HPPMS and cathodic arc deposition (CAD). Ion irradiation-induced changes in structure, elastic properties, and thermal stability of metastable cubic (Ti,Al)N thin films are systematically investigated by experiments and DFT simulations. While thin films deposited by HPPMS show a random orientation at E_k > 105 eV, an evolution towards (111) orientation is observed in CAD thin films for E_k > 144 eV. The measured ion energy flux at the growing film surface is 3.3 times larger for CAD as compared to HPPMS. Hence, it is inferred that the formation of the strong (111) texture in CAD thin films is caused by the ion flux- and ion energy-induced minimization of strain energy in defective cubic (Ti,Al)N. The ion energy-dependent elastic modulus can be rationalized by considering the ion energy- and orientation-dependent formation of point defects from DFT predictions: The balancing effects of bombardment-induced formation of Frenkel defects and the concurrent evolution of compressive intrinsic stress result in the apparent independence of the elastic modulus from E_k for HPPMS thin films without preferential orientation, similar to the previously investigated (V,Al)N. However, an ion energy-dependent elastic modulus reduction of ~18% for the CAD thin films can be understood by considering the 34% higher Frenkel pair concentration formed at E_k = 182 eV upon irradiation of the experimentally observed (111) oriented (Ti,Al)N in comparison to the (200) configuration at a similar E_k magnitude. Moreover, the effect of Frenkel pair concentration on the thermal stability of metastable (Ti,Al)N is investigated by differential scanning calorimetry for both processing routes: The ion irradiation-induced increase in Frenkel pairs concentration retards the wurtzite formation temperature by up to 206 °C. In the third and last part, the elastic response of binary cubic transition metal nitrides (c-TMNs) with respect to the occupancy of non-metal sublattice is explored. Motivated by the frequently reported deviations from stoichiometry in c-TMNs, the effect of nitrogen vacancy concentration on the elastic properties of TiNx, ZrNx, VNx, NbNx, and MoNx (0.72  x  1.00) is systematically studied by DFT. The predictions are validated experimentally for VNx (0.75  x  0.96). The DFT results clearly indicate a different elastic response of the group IV, V, and VI nitrides with respect to N vacancy concentration. While, TiNx and ZrNx exhibit vacancy-induced reduction in elastic modulus, an elastic modulus enhancement is obtained for VNx and NbNx. These trends could be rationalized by analyzing the bonding characteristic of the binary compounds. The integrated crystal orbital Hamilton population calculations indicate a lower bond strength of Ti-N for TiNx due to presence of N vacancies. However, presence of N vacancies in VNx results in a higher bond strength of V-N, which consequently leads to an overall anomalous increase in elastic modulus of VNx. To validate the elastic modulus behavior with respect to N vacancy concentration experimentally, high crystalline quality, close to single-crystal, VNx thin films are grown on single crystal MgO(001) substrates. Reduction of N content in VNx/MgO(001) from x = 0.96 ± 0.05 to 0.75 ± 0.04 leads to a decrease in the relaxed lattice parameter a0 from 4.128 Å to 4.096 Å, respectively. This reduction in lattice parameter is accompanied by an anomalous 11% increase in elastic modulus. These results are in agreement with the theoretically calculated elasticity data for VNx, which is attributed to vacancy-induced bond strengthening. The results of this thesis pave the way for the design and tailoring of hard protective coatings by considering plasma-surface interactions on the atomistic level. The combination of DFT simulations and growth experiments allowed us to understand the role of plasma condition variation, specifically ion kinetic energy, on the point defect structure and its implications for mechanical properties and thermal stability of nitride thin films.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Materials Chemistry [521110]