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

Abstract 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 ZrAl, VAl and VN. The uniaxial strain can be attributed to a weakening of the Metal-N π 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, d33, reduces from (7.0 ± 0.5) pm/V to (5.2 ± 0.5) pm/V at a fluence of 1015 at./cm2 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.