(Sc,Al,Ga)N materials for energy harvesting

Dr Michelle Moram (Semiconductor Physics and Materials Science and Metallurgy) and Prof Dave Ritchie (Semiconductor Physics)

In-plane stress produce out-of-plane crystal distortions

Piezoelectric energy harvesting is used to convert mechanical vibrations into electrical energy. This is done using a piezoelectric crystal, in which cyclic stresses (applied in the appropriate direction) are used to generate an a.c. voltage. For this to work effectively, the material should have both a high piezoelectric coefficient and a low dielectric constant. Unfortunately, it has so far proven very difficult to find a material that can fulfil both criteria, resulting in very limited device output powers. However, our recent predictions suggest that performance may be improved dramatically using new (Sc,Al,Ga)N materials, in which both high piezoelectric coefficients and low dielectric constants may be achieved. To test the potential of this unexplored material system, we are currently growing thin films of (Sc,Al,Ga)N materials by MBE, for use in improved proof-of-principle energy harvesting devices.  Figure shows in-plane stresses produce out-of-plane crystal distortions, which can be used to generate a voltage.

Project Completed end of January 2013

During the project 100 films for Sc-based nitrides were grown and characterized with the support of members of the Semiconductor Physics group (Dr Christine Nicoll and Dr Harvey Beere) and Dr Moram’s PhD students in the Materials Department.  These results enabled the confirmation of the theoretical predictions regarding the key structural properties of these new nitrides and created epitaxial films of Sc-based nitrides which were under compressive stress, counteracting the tensile stress which presents a significant problem for comparable AlN-based devices.  A number of publications are in preparation related to this work.

These results have stimulated further research on Sc-based nitrides, with funding through a ERC Starting Grant (€1.5 million) awarded recently to Dr Moram entitled Scandium-based nitrides for optoelectronic, polaritonic and magnetoelectric applications.  Dr Moram has now moved to Materials Department at Imperial College, and this grant will support further work on MBE-grown Sc–based nitrides in the Semiconductor Physics group at the Cavendish over the next 5 years.