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

Abstract

We report the results of a combined experimental and theoretical study on nonstoichiometric $\mathrm{Cr}{\mathrm{N}}_{1+\ensuremath{\delta}}$ thin films grown by reactive magnetron sputtering on $c$-plane sapphire and MgO (100) substrates in an $\mathrm{Ar}/{\mathrm{N}}_{2}$ gas mixture using different percentages of ${\mathrm{N}}_{2}$. There is a transition from $n$-type to $p$-type behavior in the layers as a function of nitrogen concentration varying from 48 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--1.11 by increasing the percentage of ${\mathrm{N}}_{2}$ 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 $\mathrm{Cr}{\mathrm{N}}_{1+\ensuremath{\delta}}$; in particular, the Seebeck value coefficient transitions from $\text{--}50\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\text{--}1}$ for ${\mathrm{CrN}}_{0.97}$ to $+75\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\text{--}1}$ for ${\mathrm{CrN}}_{1.1}$. Density functional theory calculations show that Cr vacancies can account for the change in the Seebeck coefficient, since they push the Fermi level down in the valence band, whereas N interstitial defects in the form of ${\mathrm{N}}_{2}$ 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 ${\mathrm{N}}_{2}$ 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.