Ferroelectricity in boron-substituted aluminum nitride thin films

Abstract

This manuscript demonstrates ferroelectricity in B-substituted AlN thin films and a complementary set of first-principles calculations to understand their structure-property relationships. ${\mathrm{Al}}_{1--x}{\mathrm{B}}_{x}\mathrm{N}$ films are grown by dual-cathode reactive magnetron sputtering on $(110)\mathrm{W}/(001){\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates at 300\ifmmode^\circ\else\textdegree\fi{}C at compositions spanning $x=0$ to $x=0.20$. X-ray diffraction studies indicate a decrease in both the $c$ and $a$ lattice parameters with increasing B concentration, resulting in a decrease in unit cell volume and a constant $c$/$a$ axial ratio of 1.60 over this composition range. Films with $0.02\ensuremath{\le}x\ensuremath{\le}0.15$ display ferroelectric switching with remanent polarizations exceeding $125\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--2}$ while maintaining band gap energies of $>5.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The large band gap allows low frequency hysteresis measurement (200 Hz) with modest leakage contributions. At B concentrations of $x>0.15$, $c$-axis orientation deteriorates and ferroelectric behavior is degraded. Density-functional theory calculations corroborate the structural observations and provide predictions for the wurtzite $u$ parameter, polarization reversal magnitudes, and composition-dependent coercive fields.