p-type behavior of CrN thin films via control of point defects

We report the results of a combined experimental and theoretical study on nonstoichiometric CrN1+δ thinfilms grown by reactive magnetron sputtering on c-plane sapphire and MgO (100) substrates in an Ar/N2 gasmixture using different percentages of N2. There is a transition from n-type to p-type behavior in the layersas a function of nitrogen concentration varying from 48 to 52 at. % in CrN films. The compositional changefollows a similar trend for all substrates, with a N/Cr ratio increasing from approximately 0.7 to 1.06–1.11 byincreasing the percentage of N2 in the gas flow ratio. As a result of the change in stoichiometry, the latticeparameter and the Seebeck coefficient increase together with the increase of N in CrN1+δ ; in particular, theSeebeck value coefficient transitions from –50 μV K–1 for CrN0.97 to +75μV K–1 for CrN1.1. Density functionaltheory calculations show that Cr vacancies can account for the change in the Seebeck coefficient, since they pushthe Fermi level down in the valence band, whereas N interstitial defects in the form of N2 dumbbells are neededto explain the increasing lattice parameter. Calculations including both types of defects, which have a strongtendency to bind together, reveal a slight increase in the lattice parameter and a simultaneous formation of holesin 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 thesemiconducting behavior of CrN with intrinsic defects from n to p type, opening possibilities to integrate thiscompound in energy-harvesting thermoelectric devices.Funding Agencies|Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program [KAW 2020.0196]; Swedish Research Council (VR) [2019-05403, 2016-03365, 2021-03826]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Swedish Energy Agency [46519-1]; Swedish Research Council VR-RFI [2019-00191]; Swedish Foundation for Strategic Research [RIF14-0053]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Research Council [2018-05973]