The elastic behaviour of finite-stage quasiperiodic Fibonacci a-Si:H/a-Si1-xCx:H multilayers

Abstract

The Brillouin light scattering technique has been exploited in order to study the elastic properties of finite-stage a-Si:H/a-Si1-xCx:H semiconductor quasiperiodic Fibonacci multilayers, deposited by plasma enhanced chemical vapour deposition on crystalline silicon, fused quartz and corning glass substrates. Detection of the Rayleigh surface acoustic mode enabled us to study the evolution of the phase velocity of this mode as the number of Fibonacci stages is increased from eight to twelve and to evaluate the effective elastic constant c44. The values obtained in these finite-stage Fibonacci structures are consistent with those calculated, by means of the effective modulus model, for an infinite quasiperiodic structure. In contrast to the anomalous elastic behaviour previously found in metallic superlattices, no elastic anomaly has been observed in the a-Si:H/a-Si1-xCx:H amorphous semiconductor multilayers, in agreement with recent theoretical models, which predict the presence of elastic anomalies in cases of symmetry difference and lattice mismatch between adjacent crystalline layers. Moreover, a slight decrease of the Rayleigh mode velocity has been observed on changing the substrate from c-Si to corning glass or fused quartz.