Identification of acoustic waves in ZnO materials by Brillouin light scattering for SAW device applications

摘要

Brillouin light scattering (BLS) was conducted on melt-grown ZnO bulk crystals and ZnO thin films grown by pulsed laser deposition. The bulk ZnO crystals presented both longitudinal and transverse bulk acoustic waves. Theoretical calculations agreed well with there being one piezoelectric longitudinal branch and two transverse branches. BLS measurements conducted on ZnO thin films also revealed Rayleigh surface acoustic waves (R-SAW) guided by only the surface of the layer and Sezawa modes, guided by the film thickness. Measurements were conducted for three incidence angles in order to investigate different SAW wave numbers. Higher frequency features were identified as being related to a new class of guided longitudinal (LG) SA W modes which are not usually detected for ZnO thin films. The LG-SA W modes were observed for two incidence angles (Ө=45° and 55°) corresponding to frequencies of 17.88 and 20.75 GHz, respectively. BLS measurements enable us to estimate the LG-SA W velocity as 6500 m/s. This value is three times higher than that of the currently used R-SA W. Theoretical simulations were coherent with the presence of LG modes in the ZnO layers. Such LG-SA W modes are promising for the development of novel, higher-speed SAW devices operating in the GHz-band and which could be readily incorporated in Si-based integrated circuitry.