RAMAN AND INFRARED-SPECTROSCOPY OF OPTICAL PHONONS IN II-VI ALLOYS, EPILAYERS AND SUPERLATTICES

The multimode behavior of the zone center optical phonons in bulk crys tals and molecular beam epitaxy (MBE) grown epilayers of the tetrahedr ally coordinated II-VI semiconductor alloys was investigated using Ram an and infrared spectroscopy, Depending on the inter-atomic force cons tants and the relative nuclear masses of the constituents, the zone ce nter optical modes of the ternary A(1-x)B(x)C exhibit either a two-mod e behavior with distinct AC-like and BC-like TO and LO modes across th e entire composition range, or an intermediate mode behavior when the masses of A and B are more closely matched. When B is much lighter tha n A, the BC-like TO and LO modes emerge from the localized vibrational mode of B in AC when x approximate to 0 and the AC-like TO and LO mod es converge into the gap mode of A in BC as x --> 1. A quaternary A(1- x-y)B(x)C(y)D may similarly exhibit a distinct three-mode or intermedi ate-mode behavior depending on the relative masses of the constituents , The two-mode behavior in bulk Zn1-xMgxTe and the three-mode behavior in bulk Cd1-x-yZnxMgyTe were studied with Raman scattering. Using inf rared transmission at oblique incidence, i.e. the Berreman technique, the zone center TO and LO modes of MBE-grown zinc blende MnTe are obse rved as transmission minima. The observation of confined optical phono ns in ultra-thin CdTe/ZnTe superlattices by exploiting the Berreman te chnique and Raman scattering permits their bulk dispersions to be deli neated.