| Abstract Scope |
Semiconductor nanostructures with type-II band alignment result in spatially separated electron and hole wavefunctions, resulting in reduced oscillator strength and corresponding reduction in optical absorption, spontaneous radiative recombination rate, and increased radiative carrier lifetime. The reduced optical response, however, provides a tradeoff with carrier lifetime that may be useful in applications where materials with longer radiative lifetime are desired in order to match coupled generation-recombination or transport processes. Furthermore, the optical characteristics of type-II structures may be tuned by varying structural dimensions rather than limited by intrinsic material properties. ZnTe/ZnSe quantum structures provide such a type-II band alignment, along with favorable energy transitions for efficient solar energy conversion. In this work, the epitaxial growth and photoluminescence properties of ZnTe quantum dots grown on ZnSe will be presented. Materials are grown by solid source molecular beam epitaxy on GaAs (001) substrates. Strained layer growth of ZnTe on ZnSe results in the spontaneous formation of 3-D islands in the Stranski-Krastanow growth mode as observed by in-situ reflection high energy electron diffraction. Low temperature photoluminescence (PL) spectra demonstrate optical transitions for ZnTe/ZnSe in the range of 1.7-2.8eV. Emission near 2.8eV corresponds to bandedge emission of the ZnSe, while emission below 2.1eV is attributed to a defect band observed in ZnSe control samples without ZnTe. Emission peaks are observed near 2.6eV and 2.7eV corresponding to Te substitutional impurities in ZnSe. Broadband emission in the range of 2.1-2.4eV is attributed to ZnTe/ZnSe quantum dots, is both below the ZnSe and ZnTe bandedge, and is evidence that this emission corresponds to the type-II band lineup in the material. Temperature dependent PL indicates that the emission peak energy for ZnSe follows the Varshni relation. The emission from the ZnTe/ZnSe quantum dots demonstrates a sharp transition from 2.24eV to 2.1 eV near 100K, implying a thermal activation process that is attributed to carrier transfer between quantum dots of varying size. The temperature dependent integrated PL intensity of the ZnTe/ZnSe quantum dots follows an Arrhenius relation with activation energy of 54meV. Further details of the ZnTe/ZnSe quantum dot structural properties, electronic structure, and temperature dependent carrier transfer and emission properties observed in PL studies will be presented. |