# PIEZO- AND PHOTO-MODULATION STUDIES OF SEMICONDUCTORS: I. DILUTED MAGNETIC SEMICONDUCTORS. II. GALLIUM-ARSENIDE/ALUMINUM-GALLIUM - ARSENIDE, ZINC-SELENIDE, AND CADMIUM - MANGANESE - TELLURIDE HETEROSTRUCTURES

#### Abstract

The reflectivity and transmission spectra of the diluted magnetic semiconductors (DMS) were investigated using piezo- and photo-modulation techniques. The spectra for Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te and Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te show a characteristic exciton A of the zinc blende DMS. The energy E $\sb{A}$, linear in x, is given by 1.595 + 1.592x (eV) for Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te and 2.376 + 0.820x (eV) for Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te at liquid helium temperature, yielding E $\sb{A}$ = 3.187(3.196) eV for the "hypothetical" zinc blende MnTe. For the wurtizite DMS's, such as Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se, signatures characteristic of the crystal field split valence band--the A and the B exciton--are observed for electric vector ${\rm (\vec{E})}$ perpendicular and parallel to the c -axis, respectively. For x $\leq$ 0.35, Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se has the zinc blende structure; correspondingly only one exciton feature appears in the spectrum. For x $>$ 0.35, two exciton features are observed consistent with the wurtzite structure of Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se for higher x. A signature with a sign opposite to that of free exciton is observed at $\sim$ 2.2 eV for Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te and Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se for x $>$ 0.4 and for all x in Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te and Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se. The feature, identified with a Mn$\sp{2+}$ internal transition, shows no x dependence. In addition, we have observed signatures associated with imperfections in the crystals. In the magnetoreflectivity measurements, the Mn$\sp{2+}$ transition fails to show any observable Zeeman shift or splitting when examined in magnetic fields up to 15.58 T. In contrast, the free exciton exhibits huge Zeeman splittings, a consequence of the large Mn$\sp{2+}$-band electron exchange interaction. These observations favor the assignment of $\sp6 A\sb1(\sp6 S)$ $\to$ $\sp4 T\sb1(\sp4 G)$ to the 2.2 eV Mn$\sp{2+}$ transition where the levels are associated with the crystal-field-split 3 d $\sp5$ manifold of Mn$\sp{2+}$. We have also applied the piezo- and photo-modulation techniques to investigate the electronic transitions associated with the MBE grown GaAs/Al$\sb{\rm x}$Ga$\sb{\rm 1-x}$As quantum well structures and pseudomorphic ZnSe epilayers and the LPE grown Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te epilayers. Our results obtained at temperatures down to that of liquid helium with single-, double-, and multiple-quantum wells reveal electronic transitions in the wells, the barriers and the buffer layer with exceptional clarity. The effects of coupling in the double and multiple quantum wells are clearly identified. The results of the 0.1 $\mu$m ZnSe epilayer show "heavy hole"--"light hole" splitting in the heterostructure.

Ph.D.

#### Subject Area

Condensation

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