Brillouin scattering from porous p-type 6H silicon carbide

Abstract

Brillouin scattering was used to study acoustic modes in porous silicon carbide (PSC). Six samples, with porosities varying from 30% to 58%, were studied. Initially the samples were coated with a 40 nm aluminum layer and two Brillouin peaks were found with frequency shifts that increased with increasing angle of incidence but that could not be classified as Rayleigh surface modes. Two of the samples were studied while in a near vacuum environment but it was found that this did not affect the resultant frequency shifts. 50% HCl was used to remove the aluminum coatings and each sample was found to have a Rayleigh surface mode, with velocities ranging from 2.8x10³ m/s to 5.2x10³ m/s. The velocities were compared to, and agree closely with, those theoretically determined using the Mori-Tanaka acoustic effective medium model for spherical and prolate spheroid pore shapes. Four of the samples also had a mode that could be due to a pseudo-surface mode, with velocities ranging from 4.6x10³ m/s to 8.4x10³ m/s. Each sample was also found to have a transverse and longitudinal bulk mode, with velocities ranging from 3.1 x 10³ m/s and 5.3 x 10³ m/s and from 5.5x10³ m/s to 9.7x10³ m/s respectively. The velocities were compared to, and agree closely with, those determined by using the Mori-Tanaka model for prolate spheroid pore shapes. Due to the optical transparency of the sample, the slow and fast transverse bulk modes and the longitudinal bulk mode of the sample's silicon carbide (SiC) substrate were also detected. The average velocity values for these peaks were determined to be 7.2x 10³ m/s, 7.5x 10³ m/s, and 12.8x 10³ m/s respectively. Using the data that was collected a more accurate value for the shape factor could possibly be determined. As well, further analysis would allow for the determination of the elastic constants for the PSC layers.