Isoelectronic substitutions in II-VI semiconductors: Photoluminescence, absorption, and Raman spectroscopy

Maeng-Je Seong, Purdue University

Abstract

Photoluminescence and absorption measurements on ZnTe specimens with significantly higher oxygen concentration disclosed spectral features with four no-phonon lines, all displaced to lower energies with respect to that of isolated oxygen. Photoluminescence spectra of the new no-phonon lines show a temperature dependence characteristic of exchange coupling between an electron and a hole. The new features, including phonon replicas, appear to be due to excitons bound to oxygen pairs, with different OO separations. Photoluminescence (PL) and absorption spectra of ZnTe doped with Ca, Sr, and Ba, substituting the cation, Zn, exhibit features characteristic of excitons bound in a short range potential generated by isoelectronic impurities. The energy of the excitonic signature, Eg( x), in the wavelength modulated reflectivity spectrum of Zn 1−xMgxTe shows a monotonic increase with x in contrast to a large downward bowing of Eg( x) in ZnSxTe1− x and ZnSexTe1−x. Raman spectra of these ternaries display a remarkable resonance enhancement of the zone center LO phonon and its overtones when the scattered photon energy approaches the free excitonic transition energy. The binding energy of excitons bound to oxygen impurity present in ZnSexTe 1−x and ZnSxTe 1−x is proportional to the difference between the electronegativity of oxygen and the concentration-weighted average of those of Se (or S) and Te, both scaling with x. Due to the strong localization of the excitons bound to isoelectronic oxygen impurities, its no-phonon line (NPL) exhibits a significantly faster alloy broadening compared with that associated with excitons bound to shallow acceptors. Electronic Raman transitions due to the spin flip of the Co2+ 3d electrons in Zn1− xCoxTe and Cd1− xCoxTe (x ≤ 0.01) are observed at ħωPM = g(Co2+)μBH as both Stokes and anti-Stokes shifts. The intensity of the Raman-EPR shows strong resonant enhancement when the scattered photon energy coincides with the excitonic energy. Under resonant conditions, the Raman spectra also reveal “ZnTe-like” (or “Me-like”) and “CoTe-like” longitudinal optical (LO) phonons in combination with the spin-flip transitions.

Degree

Ph.D.

Advisors

Ramdas, Purdue University.

Subject Area

Condensation

COinS