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P-type behavior of CrN thin films by control of point defects

We report the results of a combined experimental and theoretical study on nonstoichiometric CrN1+d thin films grown by reactive magnetron sputtering on c-plane sapphire, MgO (100) and LaAlO3 (100) substrates in a Ar/N2 gas mixture using different percentage of N2. There is a transition from n-type to p-type behavior in the layers as a function of nitrogen concentration varying from 48 at. % to 52 at. % in CrN films. The compositional change follows a similar trend for all substrates, with a N/Cr ratio increasing from approximately 0.7 to 1.06-1.10 by increasing percentage of N2 in the gas flow ratio. As a result of the change in stoichiometry, the lattice parameter and the Seebeck coefficient increase together with the increase of N in CrN1+d; in particular, the Seebeck value coefficient transitions from -50 uV.K-1 for CrN0.97 to +75 uV.K-1 for CrN1.1. Density functional theory calculations show that Cr vacancies can account for the change in Seebeck coefficient, since they push the Fermi level down in the valence band, whereas N interstitial defects in the form of N2 dumbbells are needed to explain the increasing lattice parameter. Calculations including both types of defects, which have a strong tendency to bind together, reveal a slight increase in the lattice parameter and a simultaneous formation of holes in the valence band. To explain the experimental trends, we argue that both Cr vacancies and N2 dumbbells, possibly in combined configurations, are present in the films. We demonstrate the possibility of controlling the semiconducting behavior of CrN with intrinsic defects from n- to p-type, opening possibilities to integrate this compound in energy-harvesting thermoelectric devices.