Thickness-Dependent Flat-Band Voltage of Ferroelectric Al0.7Sc0.3n Films on SiO2/Si Substrate

Abstract

Ferroelectric films have a high potential for random access memory (RAM) thanks to the switching speed with low power consumption. Along with the advent of ferroelectric HfO2 films, various materials have been reported to show high remnant polarization (P r) with thickness scalability. Although its high P r can be utilized for data storage, and charge trapping in addition to the depolarization field to compensate the P r has been the issue for ferroelectric/paraelectric stacked structure. It was recently reported that Al1-x Sc x N (ASN) films have a high P r of above 100 µC/cm2 and are expected for high-density RAM applications [1]. However, the charge trapping behavior at AlScN/paraelectric interface is not characterized yet. In this work, we fabricated Al0.7Sc0.3N(1~50nm)/SiO2(27nm)/nSi MOS capacitors to measure the surface potential change induced by the thickness of the ASN films.ASN films were sputter deposited on a thermally oxidized 27-nm-thick SiO2. The thickness of the ASN films was changed from 1 to 50 nm. TiN films were in-situ deposited atop, and patterned to be the gate electrodes. Finally, annealing at 420oC in forming gas was conducted. The ASN films used in this study have a coercive field of 5.3 MV/cm and are already up-polarized [2]. Therefore, no ferroelectric switching is involved during the capacitance-voltage (C-V) measurements.Fig. 1 shows the obtained C-V curves of the MOS capacitors, where we can see a strong thickness-dependent shift in the C-V curves. No apparent hysteresis nor SiO2/Si interface degradation was observed. Fig. 2 shows the summary of the extracted V fb on equivalent oxide thickness (EOT). In the thin layer region, V fb shifts to a positive direction and saturates with a shift by 0.27 V. Further increase in the thickness shifts back to the negative direction, and again gradually to the positive direction. Although the physical origin of the complicated V fb shift behavior is not clear, we anticipate that the thickness-dependent stain to induce an inverse piezoelectric effect with charge trapping is the origin of the behavior.In conclusion, we measured the V fb shift of ASN films on the SiO2/Si substrate. The thickness-dependent V fb shift showed complicated behavior with a thickness that can be attributed to the piezoelectric property of ASN films.