Lattice dynamics of the mixed semiconductors (Be,Zn)Se from first-principles calculations

Abstract

Vibration properties of Zn1−xBexSe, a mixed II-VI semiconductor characterized by a high contrast in elastic properties of its pure constituents, ZnSe and BeSe, are simulated by first-principles calculations of electronic structure, lattice relaxation and frozen phonons. The calculations within the local density approximation has been done with the Siesta method, using norm-conserving pseudopotentials and localized basis functions; the benchmark calculations for pure endsystems were moreover done also by all-electron WIEN2k code. An immediate motivation for the study was to analyze, at the microscopic level, the appearance of anomalous phonon modes early detected in Raman spectra in the intermediate region (20 to 80%) of ZnBe concentration. This was early discussed on the basis of a percolation phenomenon, i.e., the result of the formation of wall-to-wall -Be-Se-chains throughout the crystal. The presence of such chains was explicitly allowed in our simulation and indeed brought about a softening and splitting off of particular modes, in accordance with experimental observation, due to a relative elongation of Be-Se bonds along the chain as compared to those involving isolated Be atoms. The variation of force constants with interatomic distances shows common trends in relative independence on the short-range order.

Linearized force constants method for lattice dynamics in mixed semiconductors

“…1 shows diagonalized force constants for (Zn,Be)Se alloys (those previously shown in Ref. [5]) along with new results for (Ga,In)As. We address the reader to Ref.…”

“…We address the reader to Ref. [5] for a discussion on a more ionic character of Zn-Se bonds in contrast to more covalent character of the Be-Se bonds which follows from these results; obviously there is no such strong distinction in the degree of covalence between Ga-As and In-As bonds. What is striking for both mixed systems is a markedly linear variation of the force constant values with the bond length.…”

“…The supercells used were a prototype "chain+impurities" Be 6 Zn 26 Se 32 one described in Ref. [5], "Be-clustering" supercells of the same size containing 1. . .4 Be atoms grouping around the same Se atom in ZnSe; the same supercells constructed for (Ga,In)As system, and a number of smaller (32-atom) supercells for both alloys, which were subject to discussion in Ref.…”

Impurity Modes and Effect of Clustering in Diluted Semiconductor Alloys

“…It should be noted (and was already discussed in Ref. 4) that the absolute values frequencies are shifted upwards in our calculation, with respect to their experiment values, due to a slight overbinding caused by the local density approximation to the exchange-correlation. However, the frequency difference between the single-impurity and double-impurity modes is fairly reproduced.…”

“…Shorter bonds imply larger force constants and hence higher vibration frequency (again, see Ref. 4 for a more detailed discussion). It is noteworthy that an addition of further Be atoms as neighbors to the central Se brings in some additional structure, but leaves in place the initial major splitting into two big groups of peaks, those characterizing a "Be-poor" environment (at ∼480 cm −1 ) and "Be-rich" environment (at ∼440 cm −1 ).…”