The Effect of Helium and Scandium Ion Implantation on the Structural, Vibrational and Piezoelectric Properties of Aluminium Nitride Thin Films

Aluminium nitride (AlN) is one of the leading piezoelectric materials for commercial bulk acoustic wave devices due to its useful properties such as moderate piezoelectricity, high acoustic velocities and low thermal drift. In this thesis, possibilities of strain generation or alloy formation in AlN thin films are investigated with the aim to further increase the piezoelectric response. To this end, helium and scandium ions are implanted in the AlN films at different concentrations. The changes in the vibrational, structural and piezoelectric properties of the AlN thin films from the implantation are investigated using experimental techniques such as Raman spectroscopy, X-ray diffraction (XRD) and piezoelectric force microscopy (PFM).    

This study demonstrates that the ion beam implantation technique is well-suited to generate uniaxial strain along the c-axis, without modifying the strain in the a-axis basal-plane. We measure a strain  increase of 1% for He and Sc at the highest fluence of 1 x 1017 ions/cm2. Interestingly, for both implantations, we observe a secondary phase created at fluences above 1 x 1016 ions/cm2 and above, which could mark an amorphization threshold. A possible interpretation is that for the He implantation, the secondary phase is from the formation of He bubbles in the lattice. For the case of Sc implantation this phase could be from formation of nano-crystalline Sc in the first few nanometres of  the film or ScAlN alloying effects.    

We did not find evidence of an increase in the piezoelectric response for the He implantation. XRD scans revealed the implantation of He into the films generated uniaxial strain along the c-axis, with a simultaneous reduction in piezoelectric response as measured by PFM. The reduction in piezoelectricity is attributed to introduction of disorder  in the lattice as confirmed by Raman spectroscopy. For the Sc implantation, a reduction in the measured amplitude from PFM for fluences of 1 x 1015 ions/cm2 and 5 x 1015 ions/cm2 is also attributed to disorder introduced by the implantation. We find that at a fluence of 1 x 1016 ions/cm2, the film has similar electromechanical responses as  un-implanted which makes an interesting case to investigate its piezoelectric properties at higher fluences and after annealing.