Full polarization reversal at room temperature in unsubstituted AlN

Room temperature ferroelectricity in unsubstituted AlN films is studied to examine the role of cation substitution into wurtzite materials. AlN and (Al0.7Sc0.3)N films deposited on (111) 0.5 wt. % Nb-doped SrTiO3 have a (0001)-orientation with different in-plane lattice alignments with respect to those of the substrate, depending on the composition and the deposition temperature. The AlN films deposited at 450 degrees C showed complete ferroelectric switching above 140 degrees C but local polarization switching at room temperature because a dielectric breakdown occurred before complete switching, while full polarization reversal was observed at all measurement temperatures for (Al0.7Sc0.3)N. Low-temperature deposition, such as at 250 degrees C, significantly enhanced the dielectric breakdown field and also increased leakage current. As a result, sufficient polarization switching at room temperature was observed in the AlN film deposited at 250 degrees C. Positive-up/negative-down pulse measurements showed remanent polarization of 150 mu C/cm(2) and a coercive field of 8.3 MV/cm, in agreement with the theoretical value and temperature dependence observed for the AlN film deposited at 450 degrees C. The observed coercive field value lies on the line composed of the previously reported data in Sc concentration dependence. This tendency implies that the reduction in the coercive field is primarily attributable to the alteration of crystal lattice anisotropy caused by Sc.

Automated summary

Room temperature ferroelectricity was studied in unsubstituted AlN films to investigate the effect of cation substitution. AlN and (Al0.7Sc0.3)N films deposited on (111) 0.5 wt.% Nb-doped SrTiO3 showed different in-plane lattice alignments depending on the composition and deposition temperature. The AlN films deposited at 450 degrees C exhibited incomplete ferroelectric switching due to dielectric breakdown, while the (Al0.7Sc0.3)N films showed full polarization reversal. Low-temperature deposition at 250 degrees C enhanced the dielectric breakdown field and allowed for sufficient polarization switching at room temperature. The reduction in coercive field was primarily attributed to the alteration of crystal lattice anisotropy caused by Sc.