Yonatan Calahorra - Video Lecture

Yonatan Calahorra - Video Lecture

IVS-IPSTA 2020 Online Conference December 13, 2020

Nanomaterials, Thin films, and Surface Science
Morning Session

Session chair:
David Zitoun

Bar-Ilan University

Enhanced Piezoelectricity and Electromechanical Efficiency in Nanoporous GaN

Yonatan Calahorra

Department of Materials Science and Metallurgy & BPI Institute
University of Cambridge


Piezoelectricity, first formulated by the Curie brothers, is the linear interconversion of mechanical stress and electrical polarization. It is a material property of non-centrosymmetric crystal structures. This electromechanical relation is beneficial for applications such as mechanical sensors and actuators. An interesting class of materials is piezoelectric semiconductors. Particularly, polarization effects in III-Ns are inherent along the direction, and are responsible for high electron mobility transistors, and conversely are a limiting factor in GaN based light emitting diodes. Nanostructuring of materials, may bring different physical properties into play, e.g. through increased surface area, or relaxation of bulk constraints. In this talk I will show that nanoscale porosification of a doped GaN layer results in measurable piezoelectricity, 2-3 times stronger than nominally undoped GaN, and the effect of porosity on efficiency. Recently, researchers in the Cambridge Centre for GaN have developed an electrochemical process to etch pores in epitaxially grown GaN, and used this process to implement Bragg reflectors, improving optical properties of GaN structures. In a recent work, we demonstrated that the electromechanical properties of porous GaN are enhanced, and that for stress-based applications the energy conversion figure of merit is comparable to that of ceramics. PFM measurements show a clear transition from the initial conductive layer, to a piezoelectric layer. The piezoelectric strain coefficient measured, d33 ~ 8 pm/V, is about 3 times that of bulk GaN. A capacitor structure formed by depositing a top contact on the porous layer exhibited energy harvesting performance, by generating a current pulse across a resistive load following sinusoidal applied pressure. We will also discuss finite element simulations analysing the electromechanical efficiency, and the significant effect of porosity. Overall, these results establish porosification as a handle to induce and control piezoelectricity in GaN, and encourage further studies, e.g. in integration of active piezoelectric layers in GaN devices.