Tuning the electromechanical properties and polarization of Aluminium Nitride by ion beam-induced point defects

We report the evolution of uniaxial strain, resulting in an expansion of the c-axis in the wurtzite structure by up to 1 %, without significant degradation of the crystal structure of 30 keV Zr+ implanted epitaxial AlN films, grown on Si substrates. Raman and X-ray absorption spectroscopies demonstrated that the dominant defects are Zr-Al, V-Al and V-N. The uniaxial strain can be attributed to a weakening of the Metal-N pi bond along the c-axis. Monte Carlo simulations further predict the formation of a cation-rich region within the Zr implantation range, along with a buried anion-rich layer for all investigated fluences. The anion-rich layer undergoes a polarity inversion, which was experimentally confirmed by high-resolution high-angle annular dark field scanning transmission electron microscopy. Those microstructural changes influence the macroscopic electromechanical properties of AlN. The effective piezoelectric coefficient, d(33), reduces from (7.0 +/- 0.5) pm/V to (5.2 +/- 0.5) pm/V at a fluence of 10(15) at./cm(2) Zr+. At higher fluences AlN undergoes a strain- and compositionally-induced polarity change, and the piezoelectric coefficient decreases due to crystalline damage. These results provide a pathway to optimise the performance of AlN by ion implantation for applications such as energy harvesting, light-emitting diodes and acoustic wave devices. In addition, the capability to engineer a buried polarisation inversion after AlN growth may enable novel device design. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The evolution of uniaxial strain in Zr+ implanted epitaxial AlN films on Si substrates leads to an expansion of the c-axis in the crystal structure, with the main defects being Zr-Al, V-Al, and V-N. Monte Carlo simulations predict the formation of cation-rich and anion-rich regions within the implantation range, with an anion-rich layer undergoing a polarity inversion. The effective piezoelectric coefficient decreases as the fluence of Zr+ increases, due to strain- and compositionally-induced polarity changes and crystalline damage.