  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.
