Fabrication and optical studies of semiconductor quantum well structures

In an effort to investigate modulation doping and 2-dimensional electron gas in wide gap semiconductors and diluted magnetic semiconductors, we carried out systematic studies of n-type modulation doped ZnSe/Zn0.86Cd0.14Se and ZnSe/Zn0.825Cd 0.14Mn0.035Se single quantum well structures. The roles of spacers between doped barriers and undoped wells, as well as doping levels with regard to screening of excitons, were investigated. Low temperature photoluminescence studies were performed under magnetic fields up to 30 tesla. In the presence of a magnetic field, distinct features evolved from the broad luminescence band. These are attributed to interband transitions between electrons occupying Landau levels to photoexcited holes. An analysis of the Landau-level occupation as a function of magnetic field yields the electron sheet density.; Modulation doping was also studied in the context of lasing characteristics, such as doping the barriers in the active region of ZnSe/ZnCdSe quantum well laser structures. With the aid of n-type modulation doping, the optical gain (∝ (fc - fv)) enhanced by the modifications of the Fermi-Dirac distribution functions for electrons and holes in the conduction and valence bands. Resulting threshold conditions were obtained to be 1/2 ∼ 1/3 of those without modulation doping in the active regions. The biexcitonic stimulated emission, ∼10 meV below the main excitonic emission, was also observed in undoped samples, but not in the modulation doped structures due to the instability of excitons caused by the two dimensional electron gas in the well.; We also studied the lasing modes in broad-area, equilateral triangular laser cavities, which take advantage of total internal reflection at the cleaved facets of the cavity for circulating modes. A new approach is proposed to study optical modes in equilateral triangular cavities in an analytical form. The modes were obtained by examining the simplest optical paths inside the cavity, which yields the final solution with the boundary conditions. Experimental results are obtained from cavities fabricated from a superlattice structure of In0.13Ga0.87As/GaAs grown on a (111) GaAs substrate. Through optical pumping, it is shown that the dominant modes inside this triangular cavity are longitudinal circulating modes, which agrees with our theoretical analysis.