Gallium nitride (GaN) is an attractive piezoelectric material as same as aluminum nitride (AlN). These wurtzite nitrides have a good output voltage coefficient rather than oxide piezoelectric materials such as lead zirconium titanate Pb(Zr, Ti)O3. They have been regarded as attractive candidates for featuring a sensor, an energy harvester and a bulk acoustic wave resonator. However, it has not been progressed the investigation of GaN as piezoelectric materials, compared with AlN. Indeed, high quality GaN films can be prepared by MOCVD, but they have not been prepared by RF-sputtering, a common method of MEMS. The lattice mismatch is large between GaN and silicon (Si) which is standard substrate material. Then, it is not easy to grow GaN on Si directly.
In this study, we investigated some metal layers as a buffer layer. (002) oriented hafnium (Hf) was most effective. The FWHM of rocking curve for GaN 002 diffraction was about 1°, which is sufficient narrow compared with those of general (002) oriented AlN MEMS films. This would be caused by that the a-axis of hexagonal Hf (3.196 Å) is close to that of GaN (3.19 Å). This film showed a good piezoelectric coefficient, which was comparative to that of single crystal GaN.
Some calculation papers predict that the scandium (Sc) doping increase the piezoelectric coefficient of GaN. However, it has not been demonstrated experimentally. We prepared the Sc-doped GaN films using Hf buffer layer. While the crystallinity of films directly-grown on Si was very low, those of doped films on Hf layer was good as same as above non-doped films. The piezoelectric coefficient significantly increased by Sc-doping. The maximum value was 4-times of non-doped GaN. This significantly increase indicates experimentally the possibility of GaN as a piezoelectric material as same as AlN.