Dominant Effect of Near-Interface Native Point Defects on ZnO Schottky Barriers

The authors used depth-resolved cathodoluminescence spectroscopy and current-voltage measurements to probe metal-ZnO diodes as a function of native defect concentration, oxygen plasma processing, and metallization. The results show that resident native defects in ZnO single crystals and native defects created by the metallization process dominate metal-ZnO Schottky barrier heights and ideality factors. Results for ZnO(0001) faces processed with room temperature remote oxygen plasmas to remove surface adsorbates and reduce subsurface native defects demonstrate the pivotal importance of crystal growth quality and metal-ZnO reactivity in forming near-interface states that control Schottky barrier properties

Remote Hydrogen Plasma Doping of Single Crystal ZnO

We demonstrate that remote plasma hydrogenation can increase electron concentrations in ZnO single crystals by more than an order of magnitude. We investigated the effects of this treatment on Hall concentration and mobility as well as on the bound exciton emission peak I4 for a variety of ZnO single crystals–bulk air annealed, Li doped, and epitaxially grown on sapphire. Hydrogen increases I4 intensity in conducting samples annealed at 500 and 600 °C and partially restores emission in the I4 range for Li-diffused ZnO. Hydrogenation increases carrier concentration significantly for the semi-insulating Li doped and epitaxial thin film samples. These results indicate a strong link between the incorporation of hydrogen, increased donor-bound exciton PL emission, and increased n-type conductivity

Optical Polarizability of Zigzag Single-Walled Carbon Nanotubes Fullerene-Capped at One End and Covalently Bonded with Benzene Rings at the Other End

We report on the results of numerical simulations for the linear optical polarizability of single-walled zigzag (9,0) carbon nanotubes with modified ends. The nanotubes of a variable length are fullerene-capped at one end and covalently bonded to a hydrophobic cluster of nine benzene rings at the other end. We investigate electronic and optical properties of such structures within a framework of the Su-Schrieffer-Heeger model. We demonstrated that the localized states in this system exhibit nonlinear characteristics of excited states. The nanotubes have a strongly oscillating dependence of their optical polarizability on the energy of incident light. Spectral features of the optical polarizability drop in intensity and shift towards higher energies with a decrease in the length of a nanotube or upon fullerene-uncapping. The length dependence is similar for the nanotubes without benzene rings, capped either at one or both ends. Potential applications are suggested for hydrophobic pollutant control in liquid-purification systems

Role of Near-Surface States in Ohmic-Schottky Conversion of Au Contacts to ZnO

A conversion from ohmic to rectifying behavior is observed for Au contacts on atomically ordered polar ZnO surfaces following remote, room-temperature oxygen plasma treatment. This transition is accompanied by reduction of the “green” deep level cathodoluminescence emission, suppression of the hydrogen donor-bound exciton photoluminescence and a ∼ 0.75 eV increase in n-type band bending observed via x-ray photoemission. These results demonstrate that the contact type conversion involves more than one mechanism, specifically, removal of the adsorbate-induced accumulation layer plus lowered tunneling due to reduction of near-surface donor density and defect-assisted hopping transport