Brillouin scattering measurements of off-axis acoustic energy distribution in semiconducting CdS

The acoustic domain formation in semiconducting CdS crystals with with their c axis parallel to the applied electric field is discussed. For the first time the Brillouin scattering technique is used not only for on-axis phonons but for the whole spatial distribution, and reveals a continuous variation of the tilt angle between the off-axis domain and the c axis during the acoustic build-up. The results are interpreted in terms of the small signal linear theory of sound amplification in off-axis directions. The agreement between experiments and calculated prediction is very good if one takes into account the high value of the electric field inside the domain and the associated decrease in conductivity by carrier trapping.