Aarnoud L. Roest

Coulomb blockade of electron transport in a ZnO quantum-dot solid

The temperature dependence of electron transport was studied in an assembly of ZnO quantum dots. The number of electrons per quantum dot was controlled by the electrochemical potential. For assemblies permeated with organic electrolyte solutions, the electron conductance measured in the linear response regime shows an activation energy of 80–120 meV. Our analysis indicates that this is due to Coulomb blockade of electron transport.

Electron-conducting quantum-dot solids with ionic charge compensation.

Electron-conducting quantum-dot solids can be prepared on the basis of assemblies of colloidal insulating nanocrystals if electrons can be injected in the delocalized conduction orbitals. We discuss the energetics of electron injection in such an artificial solid consisting of weakly coupled quantum dots. We show that quantum confinement and electron electron repulsion determine the charging characteristics. The electron electron repulsion energy can be screened by three-dimensional charge compensation from trapped holes or positive inert ions inserted in the assembly. We present experimental results on the electron storage and long-range transport in assemblies of ZnO nanocrystals in which the electron charge is compensated by positive ions. The electron electron repulsion energy in assemblies permeated with an aqueous electrolyte solution is strongly screened. In contrast, the repulsion energy is about 100 meV in aprotic solvents; the repulsion energy strongly influences electron storage and the characteristics of long-range electron transport.

Ferroelectric Thin-Film Capacitors and Piezoelectric Switches for Mobile Communication Applications

Thin-film ferroelectric capacitors have been integrated with resistors and active functions such as ESD protection into small, miniaturized modules, which enable a board space saving of up to 80%. With the optimum materials and processes, integrated capacitors with capacitance densities of up to 100 nF/mm2 for stacked capacitors combined with breakdown voltages of 90 V have been achieved. The integration of these high-density capacitors with extremely high breakdown voltage is a major accomplishment in the world of passive components and has not yet been reported for any other passive integration technology. Furthermore, thin-film tunable capacitors based on barium strontium titanate with high tuning range and high quality factor at 1 GHz have been demonstrated. Finally, piezoelectric thin films for piezoelectric switches with high switching speed have been realized.