Lattice Dynamical Properties of Semiconductor Superlattices

Fig. (i) Band gap (E_g) variation vs. lattice constant in semiconductors

Fig. (ii) Schematic of the planar "linear chain model" for (BeTe)₄/(ZnSe)₆ SL along SL axis [001] with both Be-Se and Zn-Te interfaces

Fig. (iii) Linear chain model phonon dispersions in bulk ZnTe-BeTe and ZnSe-BeSe for both the real and imaginary part of the wave-vector q₂

Fig. (iv) LO, LA folded phonon dispersion curves for the (ZnSe)₂/(BeTe)₂ SL

Fig. 1 RIM calculation of the Lattice dynamics, density of states, and Debye temperature for -SiC at ambient and 22.5 GPa

A new class of beryllium chalcogenides (BeS, BeSe and BeTe) belonging to group II-VI compound semiconductor family and crystallizing in the four-fold coordinated zinc-blende structure has received considerable attention in recent years. A distinguishing feature of this new class of material system is that the mass and ionic radius of Be atom in beryllium compounds is the smallest among the cations (Mg, Zn, Cd and Hg) and anions (S, Se, and Te) of the IIa-VI compounds. Unlike other IIa-VI compounds which are partially ionic - Be-chalcogenides exhibit a high degree of covalent bonding comparable to III-V compounds. As a result Be-chalcogenides have much higher bonding energy, hardness, and thus exhibit unusual electronic, vibrational, and elastic properties. These unique properties make them potentially useful for various technological applications including laser diodes, high efficiency p-i-n photodetectors, etc. Moreover, a continuous variation of the physical properties can also be achieved in ternary BexZn1-xSe or BeSexTe1-x compounds. These materials are particularly important because they can be grown lattice matched to Si - leading to the possibility of realizing novel low dimensional (quantum-well and superlattices) device. The incorporation of transition metal ions of the iron group into II-VIs compounds has also resulted in dilute magnetic (e.g., BexMn1-xSe) semiconductors commonly used in spintronics applications.

Quite recently, Raman scattering in ZnSe/BeTe superlattices (which share no-common cations or anions in the interfaces) are studied - observing Kliewer-Fuchs-type electrostatic interface phonons (see Fig. 2). We have developed comprehensive theoretical models to understand the lattice dynamical properties of binary, ternary compounds as well as their superlattices. Raman spectra exhibiting interface phonons in ZnSe/BeTe Superlattices (Reshina et al. in Physics of Sol. State 45, 1579 (2003). Theoretical calculations of interface phonons (unpublished) providing excellent corroboration to the Raman scattering data of Reshina et al.