Dependence of texture tilt and excitation efficiency of shear waves for ZnO films on working gas pressure in a DC magnetron system

Dependence of the texture tilt and excitation efficiency of shear waves on the working gas pressure in an interval of 0.14 – 0.74 mTorr that corresponds to the transition from collisionless to almost diffusion deposition is studied for the ZnO films with a thickness of about 0.45 – 1.2 $\mu$m that are synthesized in a planar dc magnetron system. It is shown that an increase in the pressure from about 0.14 – 0.24 to 0.74 mTorr causes a decrease in the tilt angle of the column texture from $\sim$25$^{\circ}$ – 27$^{\circ}$ to $\sim$7$^{\circ}$ and a decrease in the efficiency of acoustic excitation. Films that are synthesized at pressures of $\sim$0.14 – 0.24 mTorr close to the transition from the Townsend to glow discharge exhibit the highest excitation efficiency of shear waves. For such films, the insertion loss reaches a minimum level at thicknesses of 0.45 – 0.75 $\mu$m and the number of echo pulses amounts to 20 – 40, so that the reflected sound can be observed with a delay of up to 80 $\mu$s at a length of an acoustic guiding crystal of 10 mm.