UC Davis

Comprehensive investigations of the materials properties and device applications made from molecular beam epitaxy (MBE) prepared ZnSe-GaAs epilayers have been performed. The properties of ZnSe-GaAs (100) interfaces were studied in order to enable the fabrication of high quality ZnSe-GaAs heterovalent structures (HS). The atomic structure of the ZnSe/GaAs interface with different surface terminations of GaAs was examined. ZnSe deposited on Ga-terminated GaAs was found to have a superior optical and microstructural quality. It is a highly coherent interface consisting of a mixture of both GaAs and ZnSe atomic constituents. To prepare GaAs on ZnSe interfaces, a low-temperature migration enhanced epitaxy (LT-MEE) growth technique was developed to grow GaAs layers under the conditions compatible with ZnSe. Both Ga and As-initialized LT-MEE GaAs/ZnSe interfaces were investigated. A defective transition layer was observed along the As-initialized GaAs/ZnSe interface, which may be attributed to the formation of the Zn3As2 compound. The correlation between the observed optical and structural properties of ZnSe-GaAs interfaces and growth conditions is discussed in detail.The second component of this thesis was to study ZnSe/GaAs/ZnSe quantum well (QW) structures for potential light emitting devices. Unfortunately, ZnSe/GaAs based QWs formed with abrupt interfaces cannot produce ideal photoluminescence (PL) performance due to inherent defect formation at heterovalent interfaces. However, Ga-initialized MEE GaAs growth and high temperature annealing was found to be able to facilitate a compositionally graded ZnSe-GaAs interface. The annealed QW structures exhibited strong, novel PL emission with broad peaks from 500-800 nm at room temperature. Transmission electron microscopy (TEM) results revealed that the annealed QW structure has an intermixed ZnSe-GaAs layer in the QW region with all four elements. Detailed investigations have been conducted to understand the luminescence mechanism. Evidence suggested that intermixed ZnSe-GaAs was responsible for the broad PL emission. Finally, novel ohmic contacts to n-ZnSe were demonstrated using both in-situ deposited single crystal Al films by MBE, and ex-situ deposited Cu films. For Al contacts, electron backscatter diffraction (EBSD) confirmed the single crystalline structure of the Al films. The (110)-oriented Al layer was rotated 45 relative to the substrate to match the ZnSe (100) lattice constant. Leaky Schottky behavior in lightly doped ZnSe samples suggested that Al-ZnSe formed a low-barrier height, Schottky limit contact. For Cu contact, X-ray photoelectron spectroscopy (XPS) detected the traces of Cu2Se bonding environment at Cu/ZnSe interface, which contributed to the ohmic contact formation. Both as-grown contacts exhibited nearly ideal ohmic electrical characteristics without any additional treatment. The contact resistances are in a range of 10-3 Ω cm2 for even lightly doped ZnSe. The novel metallization method could greatly simplify the ZnSe-based device fabrication complexity and lower the cost.