PHYSICAL REVIEW B 87, 014104 (2013) Insights into the atomic and electronic structure triggered by ordered nitrogen vacancies in CrN Zaoli Zhang, 1 Hong Li, 1 Rostislav Daniel, 2 Christian Mitterer, 2 and Gerhard Dehm 1,3 1 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben, Austria 2 Department of Physical Metallurgy and Materials Testing, Montanuniversit¨ at Leoben, Austria 3 Department of Materials Physics, Montanuniversit¨ at Leoben, Leoben, Austria (Received 29 July 2012; revised manuscript received 21 November 2012; published 8 January 2013) We report on the atomic and electronic structure of ordered nitrogen vacancies in CrN by using spherical aberration-corrected high-resolution transmission electron microscopy, electron energy-loss spectra, and ab initio calculations. The ordered nitrogen vacancies are identified to be distributed on {111} atomic planes. The vacancy concentrations were evaluated by quantitative high-resolution transmission electron microscopy. A direct consequence of the ordered nitrogen vacancies is a lattice shrinking leading to a reduced lattice constant, displaying a distorted CrN. The experimental measured lattice constant is quantitatively compared to the ab initio calculations. A relationship between the lattice constant and nitrogen vacancy concentration is theo- retically and experimentally established, and quantitatively compared. The presence of the ordered N vacancies further induces the electronic changes as reflected in a very small core-level shift as well as a shift of the volume plasmon energy. Moreover, the change of the ionicity in CrN with nitrogen vacancy concentration is revealed. A direct relation between the covalent-ionic level of the bonding and the nitrogen vacancy concentration is shown. DOI: 10.1103/PhysRevB.87.014104 PACS number(s): 61.72.jd, 61.05.−a, 68.37.Og, 79.20.Uv I. INTRODUCTION Transition metal nitrides are known as hard metallic compounds that are characterized by their extreme hard- ness, thermal stability, and resistance to corrosion. As a consequence, they have found numerous applications in the cutting- and machining-tool industry. 1 Typical materials are TiN, TiAlN, and CrN. 1–3 Among them, CrN has—besides its application as wear-resistant coating—recently gained considerable interest 4–13 as it has unique antiferromagnetic configurations, and has been used as a prototype mate- rial for strong magnetrostructural interactions. 4 CrN has an orthorhombic crystal structure and antiferromagnetic prop- erties below a N´ eel temperature (T N ) of ∼286 K, and transforms into a cubic paramagnetic insulator above the T N . The studies relevant to CrN films can be simply classified into structural studies 2–4,6,13 and electrical, optical, and me- chanical property measurements. 5,8,10 Furthermore, structural calculations using density functional theory (DFT) 7 have also been performed. However, controversial results were obtained regarding whether CrN possess a metallic or semiconductor electronic structure. 5,8,10,14,15 The discrepancy is essentially closely related to the different crystalline quality of the samples, such as various defects, which were used by different authors. The defects and their critical densities in the film can alter the electronic structure of CrN. 11 The frequently encountered defects are nitrogen (N) vacancies, which are known to act as carriers, that is, donors, and play an important role for the resulting electronic properties of CrN. 11 It is noted that the stoichiometry or structural defects, such as vacancies, interstitial atoms, and dislocations, influence the properties of transition metal nitrides. 1,16,17 The knowledge about the influence of N vacancies on phase stability and mechanical properties of CrN coatings is still in its infancy. Thus, systematic data based on a comprehensive characteriza- tion of N vacancy effects in CrN is definitely needed. It was reported that these N vacancies can affect the chemical bonding and electronic structures as revealed by x-ray photoemission spectroscopy in TiVN and TiN x , 16,18,19 by electron energy-loss spectroscopy (EELS) in TiN x , 20,21 and VN x . 22,23 Theoretical calculations uncover that N vacancies can strongly affect the mechanical properties 17 and thermally induce atomic changes in TiN. 24 So far, experimentally evaluating the atomic and electronic structure changes induced by N vacancies in hard coating mate- rials is relatively scarce, although it is obviously of importance for understanding the various properties of transition metal nitrides. It seems conceivable that the study of simultaneous atomic and electronic structures enabled by a combination of modern spherical aberration-corrected (C S -corrected) high- resolution transmission electron microscopy (HRTEM), scan- ning transmission electron microscopy (STEM), and EELS, can elucidate the effects caused by N vacancies. It is known that a substoichiometric CrN could occur during CrN synthesis, and N vacancies and other defects are consequently likely to be introduced. Despite this fact, however, characterizing the atomic and electronic structures of N vacancies at the atomic scale is still not yet available. Particularly for CrN, a fundamental knowledge is required to advance the perfor- mance of this material. Here we report on the atomic and electronic structures triggered by ordered N vacancies in CrN films using modern TEM techniques combined with ab initio calculations. II. METHODS The CrN coatings used in this study were grown using a Cr interlayer on 300 and 500 μm thick Si(100) wafers by direct current unbalanced magnetron sputtering from a hot pressed Cr target (Ø 145 mm) in an industrial-scale Oerlikon Balzers rapid coating system (RCS) in static mode at a constant total pressure of 1 Pa. A target power of 6 kW and a temperature of 350 ◦ C were used to deposit the films in a pure Ar atmosphere (Cr) and in an Ar + N 2 gas mixture with a nitrogen partial pressure of 0.25 Pa (CrN). 014104-1 1098-0121/2013/87(1)/014104(9) ©2013 American Physical Society