Effect of Bilayer Structure on the Long-Term Stability of Nanocrystalline Porous Silicon Ultrasonic Emitter

Abstract

Due to a strong quantum confinement effect, both the thermal conductivity α and heat capacity per unit volume C of nanocrystalline porous silicon (nc-PS) are extremely lowered in comparison to those of single-crystalline silicon (c-Si). This high contrast between the thermal properties of nc-PS and c-Si makes it possible to produce an efficient ultrasound emitter device based on interfacial thermoacoustic transfer with no mechanical surface vibrations. The most important parameter of this device is the thermal effusivity (αC)1/2 of the nc-PS layer, since the theoretical acoustic output is in inversely proportional to (αC)1/2. To stabilize the acoustic output by suppressing possible change in (αC)1/2 during a long-term operation, a bilayer structure is introduced into the nc-PS layer in this work. Due to the increased stability against oxidation, the device operates for a long time without any significant degradation. The observed long-term stability ensures the applicability of this emitter in functional acoustic devices such as sensors, speakers, and actuators.