The effect of low energy helium implantation on the structural, vibrational, and piezoelectric properties of AlN thin films

Abstract Epitaxial AlN thin films were implanted with He at 8 keV along the c-axis growth direction with fluences ranging from 1015 to 1017 atoms/cm2. We find that the He implantation generated uniaxial tensile strain along the c-axis for all fluences that increased with fluence. There was an additional (002) X-ray diffraction peak for the highest He fluences of 5 × 1016 atoms/cm2 and 1017 atoms/cm2 that we interpret in terms of He bubble formation. He implantation leads to significant nitrogen (N) site disorder as evidenced by rapid broadening of the E2 (high) Raman mode with increasing fluence due to a reduction of the phonon mean free path. The piezoelectric coefficient, d33, reduces with increasing fluence, and is ~27% of the unimplanted d33 for the highest fluence. This result can be interpreted as implantation-induced nitrogen site disorder that randomises and reduces the Al–N dipoles. Thus, we show that while He implantation induces uniaxial strain, it decreases d33 due to implantation-induced N site disorder.