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 $g5.2\\phantom{\\rule{0.16em}{0ex}}\\mathrm{eV}$. The large band gap allows low frequency hysteresis measurement (200 Hz) with modest leakage contributions. At B concentrations of $xg0.15$, $c$-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.