PHYSICAL REVIEW B VOLUME 48, NUMBER 20 15 NOVEMBER 1993-II Optical properties of Mg-based II-VI ternaries and quaternaries: Cd1 Mg Te and Cdr „Mg Mn„Te Eunsoon Oh, C. Parks, I. Miotkowski, M. Dean Sciacca, A. 3. Mayur, and A. K. Ramdas Department of Physics, Purdue University, West Lafayette, Indiana $7907 199-6 (Received ll 3une 1993) We have investigated the photoluminescence, Raman, and infrared spectra of Cd~ Mg Te and Cdi ~ „Mg Mn„Te bulk crystals with the zinc-blende structure. From the band-gap (Es) photo- luminescence peak we deduce Eg= 1. 595+ 1. 607x + 1. 592y eV at 10 K. The Raman spectra of the quaternary alloys exhibit a classic three-mode behavior with MgTe-like, MnTe-like, and CdTe-like LO-TO pairs of zone-center optical phonons, in decreasing order of frequency. Their composition dependence follows the modified random-element isodisplacement model. For very small x and/or y, the infrared absorption spectra recorded with a Fourier-transform spectrometer clearly reveal the local modes of Mn + and Mg +. Mn + exhibits one and Mg + three local modes corresponding to their isotopic abundances; the latter have frequencies proportional to 1/QMM 2+. I. INTRODUCTION The Mg-based II-VI ternaries and quaternaries such as Cdi ~Mg Te (Ref. 1) and Zni ~Mg Sei „S„, in which a fraction of the group II elements are replaced by Mg, have interesting physical properties. Besides being of fun- damental interest, their importance for potentially useful optoelectronic devices has recently attracted attention. For example, the present intense activities focused on the realization of II-VI-based diode lasers involve II-VI ternaries and quaternaries with Mg as a constituent; the large band gaps of ternaries and quaternaries such as Zn-Mg-Se and Zn-Mn-S-Se have been exploited both for quantum confinement as well as for achieving wave- guide (cladding) structures. In this context, the study of their basic physical properties such as band gap, lattice constant, and lattice dynamics has acquired special im- portance. Since Mg is much lighter than Hg, Cd, or Zn, the typical group II elements in the II-VI's, the incor- poration of Mg in place of the heavier group II element results in an increase of the band gap as well as in lattice vibrational modes with frequencies significantly higher than those of the host II-VI semiconductor. The relative masses of the constituent atoms in al- loys and their interatomic force constants characterize the composition dependence of their zone-center opti- cal phonon frequencies. In ternary alloys, their varia- tion with composition can be categorized by one-mode, two-mode, and intermediate behavior. For example, in Cdq „Mn&Te, due to the significant mass difference be- tween Cd and Mn, the optical phonons exhibit a two- mode behavior, whereas in Znq „Mn„Te, where masses of Zn and Mn are more closely matched, they show an in- termediate behavior. The zone-center optical phonons in Cdq Mg Te exhibit two-mode behavior, i.e. , the local mode of Mg2+ in CdTe (x — &0) evolves into Mg Te-like LO and TO phonons with increasing x, and finally becomes the LO-TO modes of the the tetrahedrally coordinated (zinc-blende) Mg Te; on the other hand, the CdTe LO and TO phonons converge into the Cd +-gap mode in MgTe with x ~1. ' The physical properties of quaternary compounds are often of interest in the context of an additional control of material parameters such as band gap, lattice pa- rameter, and valence band offsets in heterostructures. Lattice vibrations in some III-V quaternaries have al- ready been reported in the literature. We reported recently the vibrational modes of a II-VI-based qua- ternary, viz. , Cdi „Zn Mn„Te (Ref. 8), and showed that there are three longitudinal and transverse optical phonons, whose frequency variation with composition is similar to the intermediate behavior, exhibited, for ex- ample, by the ternary Zn~ „Mn„Te. In contrast, for Cdl „Mg Mn„Te one expects an unambiguous three- mode (CdTe-like, MnTe-like, and MgTe-like modes) be- havior, due to the distinct masses of the three kinds of cations. The Mg + local mode in Cdi „Mn„Te (x — +0) should split into MgTe-like LO and TO phonons in the quaternary Cd& — yMg Mny Te, and finally become the MgTe-LO and TO phonons in MgTe (x — +I). Similarly, the Mn +-impurity mode in Cdi Mg Te (y — +0) should evolve into the MnTe-LO and TO phonons in the zinc- blende MnTe (y -+1), whereas the Cd2+-gap mode in Mg Mn„Te (x + y — +1) should become the CdTe-LO and TO phonons in CdTe. In this paper, we present the results of a study on Raman scattering, band-gap photoluminescence, and in- frared absorption spectroscopy in Cdi Mg Te and Cd& — yMg& Mny Te, motivated by the above consider- ations. II. EXPERIMENTAL The II-VI quaternaries and ternaries studied in the present investigation were grown by the modified Bridg- man method. While CdTe, MnTe, and MgTe exhibit zinc-blende, NiAs, and wurtzite structure, respectively, 0163-1829/93/48(20)/15040(7)/$06. 00 1S 040 1993 The American Physical Society