Optical Studies of Monolayer Structures and Interfaces in Epitaxial II-Vi Semiconductor Systems

In this thesis we apply a series of optical techniques to investigate fundamental aspects of two groups of epitaxially grown heterostructures involving II-VI semiconductor compounds. The first part of the thesis is devoted to "digital alloys", i.e., to multilayer systems fabricated by epitaxial deposition of sequences of atomic monolayers of different semiconductor materials. For specificity, our study of digital alloys concentrates on systems of alternating monolayers of ZnSe and CdSe. Our choice is motivated by the relevance of this wide-gap semiconductor material to blue and blue-green optoelectronic devices. Using photoluminescence spectroscopy, we demonstrate that quantum wells made from such digital systems are effective in achieving optical confinement. We further show that electronic properties of such quantum wells can be quantitatively analyzed by the finite element method of numerical computation. We then extend our measurement of digital alloys to photoluminescence excitation and optical absorption. The latter studies provide important input on the longitudinal optical phonons in digital-alloy media, and their coupling to excitons. In the second part of the thesis we investigate two families of super-lattices combining magnetic and non -magnetic semiconductors: MnSe/ZnTe and MnSe/ZnSe by luminescence spectroscopy. Our studies of MnSe/ZnTe superlattices reveal that the band alignment at the MnSe/ZnTe interface is of Type-II, i.e., that conduction band electrons and valence band holes are confined in the different layers. We then extend out studies to magnetic field dependence of photoluminescence in both MnSe/ZnTe and MnSe/ZnSe systems. These measurements reveal that significant out-diffusion of Mn must take place at the interface between MnSe and the non-magnetic compound resulting in the formation of a diluted magnetic semiconductor alloy near the boundary. This leads to profound magnetic shifts of the photoluminescence energy, and an especially dramatic increase of the emitted photoluminescence intensity, as the magnetic field is increased. A model is proposed, based on the relative variation of free and bound exciton levels in an applied magnetic field, which provides a satisfactory explanation of the observed effects.