Metal-insulator transitions in epitaxial rocksalt-structure Cr 1 − x / 2 N 1 − x O x (001)

Substitutional replacement of N with O in epitaxial CrN(001) layers is achieved by reactive sputter deposition in a mixed ${\mathrm{N}}_{2}$ and $\mathrm{Ar}+{\mathrm{O}}_{2}$ atmosphere. O incorporation facilitates Cr vacancies, yielding a rocksalt-structure solid solution ${\mathrm{Cr}}_{1\ensuremath{-}x/2}{\mathrm{N}}_{1\ensuremath{-}x}{\mathrm{O}}_{x}$ with a single compositional parameter $x$ and a measured lattice constant that decreases from $a=0.4175$ to 0.4116 nm for $x=0$ to 0.59. First-principles calculations predict $da/dx=+0.0200$, \ensuremath{-}0.0018, and \ensuremath{-}0.0087 nm for $\mathrm{Cr}{\mathrm{N}}_{1\ensuremath{-}x}{\mathrm{O}}_{x}, {\mathrm{Cr}}_{1\ensuremath{-}x/3}{\mathrm{N}}_{1\ensuremath{-}x}{\mathrm{O}}_{x}$, and ${\mathrm{Cr}}_{1\ensuremath{-}x/2}{\mathrm{N}}_{1\ensuremath{-}x}{\mathrm{O}}_{x}$, respectively, and confirm, in combination with ion-beam compositional analyses and x-ray diffraction results, a vacancy concentration of $x/2$ per cation site. The room-temperature resistivity decreases by over two orders of magnitudes from $\ensuremath{\rho}=1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ to $7.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{m}$ for $x=0--0.26$. This is accompanied by a transition from a negative to a positive temperature coefficient of resistivity, an increase in the Hall mobility from ${\ensuremath{\mu}}_{\mathrm{H}}=0.37$ to $1.7\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{Vs}$, an increase in the carrier concentration $n=1.1\ifmmode\times\else\texttimes\fi{}{10}^{20}$ to $4.9\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, and a decrease in the calculated band gap from 0.54 to 0 eV. However, $\ensuremath{\rho}$ increases again to $6.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{m}$ for $x=0.34$ and to $g10\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{m}$ for $x\ensuremath{\ge}0.59$, with a corresponding drop in ${\ensuremath{\mu}}_{\mathrm{H}}$ and the opening of a gap. The maximum in the measured carrier density and conductivity at $x=0.26$ is attributed to a maximum in the density of states near the Fermi level, in perfect agreement with calculations using supercells with randomly distributed O substitutions and Cr vacancies that predict a maximum at $x=0.25\ifmmode\pm\else\textpm\fi{}0.05$. The measurements indicate insulator-to-metal and subsequent metal-to-insulator transitions at $x=0.04\ifmmode\pm\else\textpm\fi{}0.03$ and $0.30\ifmmode\pm\else\textpm\fi{}0.03$, respectively. The transition to metallic conduction is attributed to substitutional O on anion sites acting as donors, while the measured $n$ for $x=0.01--0.26$ quantitatively indicates charge compensation by cation vacancies that act as acceptors. The insulating properties at large $x$ are likely caused by increased electron correlation effects.