Lattice dynamics and bond polarity of Be-chalcogenides - A new class of II-VI materials

Be-chalcogenides take a special place among II-VI compounds due to their ex pected reduced polarity. We report on an investigation of the phonon proper ties and dielectric constants of BeSe and BeTe by far- and mid-infrared ref lectance and Raman spectroscopy. For comparison with our experimental resul ts we performed first-principles calculations (LDA) to obtain the charge di stribution and the TO-phonon frequencies within the frozen phonon approxima tion. From the spectroscopic data we derive bond polarities, which are dist inctly reduced with respect to classical II-VI compounds (ZnTe, ZnSe). They approach values of III-V compounds, such as GaAs. These results are also r eflected in the calculated charge distributions. The least polar compound B eTe exhibits a very high frequency and narrow optical phonon band due to th e extraordinary mass ratio m(Be) = 9.0 amu versus m(Te) = 127.6 amu. Theref ore, in BeTe, in contrast to almost all other II-VI compounds, the standard optical phonon decay channel into two acoustic phonons with opposite momen ta is not possible. We investigated the consequences for the temperature be haviour of the optical phonon parameters.