Two-dimensional conductive surface oxide on CrN(001) and Cr_1-<i>x</i>Al<i>_x</i>N(001) films

CrN is a Mott-Hubbard insulator but forms a conductive surface oxide with a sheet conductance G_s = 5.9×10^-5 [Ω/sq]^-1 when exposed to oxygen. This is demonstrated by in situ transport measurements on epitaxial CrN(001) layers during a continuously increasing pressure dp/dt = 0.05 Pa/s of a 90% Ar - 10% O₂ mixture, suggesting the formation of a thin n-type doped layer through substitutional replacement of N surface atoms with O. Low-temperature transport measurements indicate incomplete carrier delocalization in the 2D conductive oxide. Alloying with AlN to form cubic Cr_1-xAl_xN(001) films results in similar conductive surface oxides for x < 0.62, an increase in the room temperature resistivity from ρ = 0.070 to 26 Ω-cm for x = 0-0.46, and an increase in the optical band gap from 0.8 to 4.0 eV for x = 0-0.85. Direct oxygen incorporation during reactive deposition in a Ar-N₂-O₂ mixture yields epitaxial CrN_1-xO_x(001) films with a sheet resistance that decreases by over two orders-of-magnitude from 4.4×10³ to 7.5 Ω/sq with an increasing O₂ partial pressure P_O2 = 0-0.060 mTorr, and an insulator-to-metal transition at P_O2 = 0.015 mTorr. The overall results show that oxygen can be used to control transport in CrN, with potential applications in 2D electronics and thermoelectrics.