Prediction and identification of point defect fingerprints in the X-ray photoelectron spectra of TiN_x

We investigate the effect of selected N and Ti point defects in B1 TiN on N 1s and Ti 2p_3/2 binding energies (BE) by experiments and ab initio calculations. X-ray photoelectron spectroscopy (XPS) measurements of Ti-deficient TiN films reveal additional N 1s spectral components at lower binding energies. Ab initio calculations predict that these components are caused by either Ti vacancies, which induce a N 1s BE shift of -0.53 eV in its first N neighbors, and/or N tetrahedral interstitials, which have their N 1s BE shifted by -1.18 eV and also shift BE of their first N neighbors by -0.53 eV. However, the ab initio calculations also reveal that the tetrahedral N interstitial is unstable at room temperature. We, therefore, unambiguously attribute the detected signal to Ti vacancies. Furthermore, the vacancy concentration in Ti-deficient TiN was quantified with XPS supported by ab initio calculations. The largest BE shifts of -1.53, -1.80 and -2.28 eV for Ti 2p_3/2 electrons are predicted for the Ti tetrahedral, split (101)-aligned and split (111)-aligned interstitial atoms, respectively, and we, therefore, propose XPS could detect them. Other defects such as N vacancy or N split (101)-aligned interstitial introduce smaller N 1s and Ti 2p_3/2 BE shifts and are unlikely to be detectable experimentally. Our work highlights the potential of ab initio-guided XPS measurements in detecting and quantifying point defects in B1 TiN.