COVALENCY ENGINEERING THROUGH ALLOYING WITH BERYLLIUM CHALCOGENIDES IN WIDE BAND-GAP II-VI CRYSTALS

Defects are at the origin of a device problem in ''classical'' IIB-VI wide-bandgap compounds, e,g., the elusive ''longevity'' of [(Zn,Cd)-VI ]-based blue lasers. Based on a new review of literature data, this wo rk follows our first proposals of novel Be-chalcogenides potentialitie s (1994, 1995) to obviate it. We identify the crystal shear modulus C- s = (C-11-C-12)/2 as a structural signature of the epitaxial layer lat tice and perform an improved semi-empirical calculation of Cs vs a def inite Harrison's covalency parameter deduced from his LCAO model. As a result, this leads to three observations: (i) As noticed in our previ ous papers, from the Phillips-Van Vechten ionicity scale the Be-VI com pounds have a unique strong covalency degree in the II-VIs, even highe r than in GaAs; (ii) the Phillips and Harrison covalency parameters ar e shown to compare reasonably well if proper scaling is used; (iii) (Z n,Cd)-chalcogenides are characterized by a strong ionicity which induc es low C-s-values, leading to a number of defects. As we proposed befo re, this work confirms the possibility of achieving an elastic rigidit y reinforcement of the lattice in (Zn,Cd)-VI materials through alloy B e-substitution, paving the way to what me call a ''covalency engineeri ng'' in the II-VI family. We discovered previously the possibility of Be-based alloys that are Si lattice-matched. We briefly describe the S i substrate innovative utilizations for heteroepitaxy of electronic an d blue or UV devices-stressing salient potential applications of the c ovalency engineering in II-VI blue lasers for acceptable longevities.