AL AS GAAS LAYERED STRUCTURES FOR SAW SENSORS

To improve the electromechanical coupling of surface acoustic waves (S AWs) on GaAs for sensor and frequency filter applications, the deposit ion of an AlAs layer is considered. Using a transfer-matrix algorithm, the angular dependence of the Rayleigh-type modes on (001)-cut GaAs a nd the velocity dispersion of the Al/GaAs layered system are investiga ted. The dispersion relation and the coupling coefficient for the hete roepitaxial AlAs/GaAs structure are calculated. Results of SAW phase-v elocity measurements carried out by two experimental techniques are pr esented. The SAW mode in the [110] direction of the (001)-cut of AlAs/ GaAs shows anomalous dispersion. For a relative layer thickness kh of about 2.9, with k being the wave number and h the layer thickness, the phase velocity reaches 97% of the velocity of pure AlAs. Above this v alue the Rayleigh mode vanishes. The coupling coefficient has a maximu m value at kh = 1.8, being twice as high as the coefficient of GaAs. I t decreases markedly when kh approaches the cut-off value. Limitations in the experimental determination of the coupling coefficient due to the accuracy of the phase-velocity measurements are discussed.