Acoustic Wave Effect on Bound Excitons and Size-quantized Excitons

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Followings are results we obtained from the present study.1) When the AC voltage of 8 MHz is applied to the CdS crystal, the decay time of the PL intensity gradually increases from 1.2 nsec and reaches the maximum at 1200 V/cm and then decreases down to 1.05 nsec at 1600 V/cm. The increase of the decay time in the low electric fields is caused by the fact that the overlap of the wave functions between the electron and hole decreases for piezo electric fields induced by the acoustic wave. The decrease in the high electric fields is caused by broadening of the potential well in the impurity center for the bound exciton. Moreover, the growth in the acoustic phonon wing of the IィイD21ィエD2 PL line, the shift of the absorption edge to the low energy side and the disappearance of the AィイD21ィエD2 phonon Raman line have been observed, and these phenomena are explained by the inhomogeneous electric fields generated around the impurity center by the acoustic wave.2) When an acoustic wave is applied to the ZnSe quantum well, there appears a short decay component in the time response of the PL intensity due to the exciton in the ZnSe quantum well. The PL intensity in the 4 eV light excitation decreases gradually with increasing the acoustic wave intensity and decreases suddenly at a given intensity of the acoustic wave. Such a phenomenon does not occur in the excitation of 3.5 eV light. We interpreted as follows. The free exciton is ionized by the electric field resulting in the decrease in the PL intensity. On the other hand, the free exciton is localized by the acoustic wave and causes the decrease of the free exciton lifetime. As a result, the lifetime of the PL decreases.