Dennis Gallagher

GaN (0001)-(1×1) surfaces: Composition and electronic properties

We use low energy electron diffraction, Auger electron spectroscopy, and ultraviolet and x-ray photoemission spectroscopy to study the surface structure, stoichiometry, and electronic properties of n- and p-type GaN (0001) grown by metal-organic chemical vapor deposition. Ordered (1×1) surfaces with nearly stoichiometric composition are prepared by nitrogen sputtering and annealing. The band bending is found to be 0.75±0.1 eV up and 0.75±0.1 eV down for n- and p-type samples, respectively. The work function, electron affinity, and Ga 3d core level binding energy are also determined.

Homogeneous precipitation of doped zinc sulfide nanocrystals for photonic applications

A process was developed to prepare nanocrystalline and quantum-confined particles of manganese-doped zinc sulfide. By the reaction of diethylzinc with solubilized hydrogen sulfide, particle sizes of 30–36 A were achieved by control of reactant concentration, and size appeared to vary with the thermodynamic considerations indicative of homogeneous precipitation. Managanese doping required the development of an in situ chemical reaction compatible with the homogeneous precipitation reaction. To that end, ethylmagnesium chloride was reacted with manganese chloride to form the metastable diethylmanganese which acted as the dopant source. Quantum confinement of the particles was accomplished by using methacrylic acid and poly(methyl methacrylate) polymer of low molecular weights. These surfactants were transparent to the ultraviolet wavclcngths of light which allowed luminescent excitation of the material and provided surface passivation which enhanced phosphor brightness. The surfactant adsorption and effect of ultraviolet curing of the surfactant on the luminescent efficiency of the doped nanocrystals was investigated by infrared spectroscopy. These results indicate that the chemisorption of the surfactants to the nanoparticle surface and oxidation followed by crosslinking during curing are responsible for the improvement in luminescent efficiency.

Doped Zinc Sulfide Nanocrystals Precipitated within a Poly(Ethylene Oxide) Matrix-Processing and Optical Characteristics

Zinc sulfide nanocrystals doped with a manganese activator element were precipitated within a poly(ethylene oxide) polymer matrix to study the effects of size and quantum confinement on the luminescent properties of ZnS:Mn. Ultraviolet absorption demonstrates the increased bandgap due to quantum confinement. Photoluminescence measurements show the characteristic Mn 2+ emission from within the ZnS host crystal and photoluminescent excitation measurements show a change in excitation which is indicative of the increased bandgap. These characteristics are shown to change with growth of the particles. The growth rate of the ZnS particles is contrasted with the growth rate of CdS by calculating the molecular diffusivity for diffusion controlled growth in each case