Charge storage and retention in electret dielectric layers for energy harvesting applications

By providing a permanent electric polarization, electrets have a wide range of applications in different fields like sensors and actuators, energy harvesting or biomedicine. Stable electrets with silicon-based compatible technology are particularly pursued. In this work, different dielectric layer stacks, including SiO₂, Si₃N₄, AL₂O₃ and AlN, are evaluated in terms of charge storage and retention. The corona charging characteristics are analyzed and local micro-breakdowns of the dielectric layers are observed for critical electric fields in the range of 6-7 MV/cm. A hexamethyldisilazane (HMDS) surface treatment is confirmed to provide an effective protection for electret charge retention. Unfortunately for potential applications involving liquids, the charge is found to vanish after electret surface soaking with either deionized water or ethanol. In principle, deeper charge storage in the electret dielectric layers could be achieved by using an alternative method based on charge injection by means of a direct contact. The results of first attempts with such alternative technique are also reported.