AVS2010 Session TF-ThM: Modeling and Analysis of Thin Films

Multinary nitride materials are widely used as coatings for wear protection of cutting tools, e.g. TiAlN, as well as in thin film electronic applications. One important aspect of the success of TiAlN is a composition dependent thermodynamically driven age-hardening process through spinodal decomposition into TiN and cubic AlN at cutting tool operational temperatures. To understand this phenomena and to be able to tailor optimal compositions for different applications, we perform a thorough theoretical thermodynamics investigation based on first principles calculations.

By mapping the complete quantum mechanical complexity of the system onto a generalizes Ising Hamiltonian for the configuration of Ti and Al atoms, we are able both to predict thermodynamics using accurate Monte Carlo simulations and achieve a deeper understanding of the interactions governing the system.

One physical property that is difficult to find in nitrides and which disfavors the usage of the materials class in spintronics, is room temperature ferromagnetism. TiCrN is one of the rare nitride systems where strong ferromagnetism is seen. We use our methodological framework to analyze and explain why the magnetic interactions that favor anti-ferromagnetism in pure CrN is changed in the TiCrN solid solution, as well as the intricate dependence of the Curie temperature on the CrN content.

We use Density Functional Theory (DFT) calculations in the generalized gradient approximation (GGA) to predict the properties of a number of novel V-M-N thin films in the B1 (NaCl) structure. The new compounds are obtained by alloying VN with Nb, Mo and W, in concentrations of 50 %. We evaluate the elastic moduli and constants of these ternaries and perform a detailed analysis of their electronic structure. These results are compared with the corresponding properties of typical binaries such as TiN and VN. Our calculations show that the new ternaries have hardness comparable with TiN and VN, and significantly, a resolute ductile behavior. This unique combination of hardness/ductility, which is in contrast to the hardness/brittleness relationship typically found in hard coatings, equates to significantly increased toughness, as confirmed by the stress-strain relationship we obtain for all these compounds. The electronic structure results presented herein reveal a layered charge density, consisting in alternating high and low electron density regions, similar to that recently reported for Ti-Mo/W-N thin films [1]. To fully understand the mechanism responsible for this interleaved arrangement of electrons, we carry out crystal orbital overlap population (COOP) and electron localization function (ELF) calculations, and succeed to energetically resolve the bonding/antibonding contributions, of the first and second neighbors, to the chemical bonds in these compounds. Based on the results of this analysis, we find that the electronic mechanism responsible for the observed toughness enhancement in these compounds is rooted in the increased metal-metal (second neighbors) interaction of d-t_2g orbitals.

[1] D.G. Sangiovanni, V. Chirita and L. Hultman, Phys. Rev. B 81, 104107 (2010)

Correlated atomic positions of Si-atoms bonded to 2-fold coordinated O-atoms are determined by Si 3d-dervied T_2g states on these atoms [1]. These states constrain dihedral angles contributing to medium range order with correlation lengths of ~0.4 and 0.45 nm, and, a coherence length of ~1 nm, each obtained from the first sharp diffraction peak in X-ray diffraction [2]. O K edge soft X-ray SiO₂ spectra reveal conduction band edge states with singly degenerate A₁ symmetry, and stronger doubly and triply degenerate E and T₂ d-state beginning at energies ~3 eV higher. This difference is smaller in c-Si, ~1 eV. Analysis of O K and L_2,3 spectra are based on a charge transfer multiplet many electron theory [3]. Energy differences are the same for band edge features obtained from transmission/reflectivity studies in the visible/VUV where the d-state character was not previously recognized [4]. The correspondence derives from O 1s core hole localization, and a coherent process in which 1s core holes are filled by electrons from valence band O 2p p states, accounting for the one-to-one correspondence between sequenced features in the O K edge, and in the Si L_2,3 spectrum of SiO₂. L_2,3 spectra, studied by electron energy loss spectroscopy (EELS), have not detected the weaker Si A₁ features in the 100 to 104 eV regime of SiO₂, or 98 to 100 eV regime of c-Si. The O K edge X-ray spectra of transition metal elemental and complex oxides are qualitatively different with either doubly or triply degenerate d-state derived spectral features at the band edge. This results in significant quantitative differences in (i) the high-spin excited d-states of O-atom vacancy occupied d² states, and (ii) the negative ion electronically active d-state traps populated by charge injection. O-vacancy states in SiO₂ are close to mid-gap, and do not contribute to gate stack tunneling processes. This explains the order of magnitude higher interfacial trap densities, d_it, and trap-assisted tunneling in HfO₂ gate stacks. It also accounts for significant differences in radiation hardness. XAS spectra are presented for the first time for remote plasma deposited GeO₂. These deposited films display significantly different O K edge spectra than those obtained by oxidation of Ge; more importantly A₁, and E and T_2g features stronger and narrower than those of SiO₂. Electrical data for deposited GeO₂ dielectrics on Si and Ge substrates are compared for the first time with SiO₂ test devices on the same substrates.

[1] Whitten J, et al., J. Vac. Sci. Technol. B 20, 1710 (2002).

[2] Lucovsky G, et al. physica status solidi (a) 207, 631 (2010).

[3] de Grott F, Kotani A. Core level spectroscopy of solids (Boca Raton, CRC, 2008).

[4] Laughlin RB, Phys. Rev. B 22, 3021 (1980).