Thin-film diode structures for advanced energy applications
Creators: name: Donchev, Evgeniy
Publisher: name: Imperial College London
Year: 2015

description: Presented within this thesis is the work towards studies on thin-film diode structures for use in advanced energy applications. The main energy application pursued within this study is the solar rectenna device, considered to be next-generation in energy harvesting due to promises of efficiencies beyond the Shokley-Queisser limit for solar cells. A RECTifying antENNA (RECTENNA) is a device consisting of an antenna, with dimensions scaled to the wavelength of the electromagnetic wave to be absorbed, and the resulting signal rectified by a diode of sufficient cut-off frequency to efficiently operate at the desired harvesting frequency. In studying the rectenna device and its efficiency limits, diode performance requirements have been outlined that are used to identify suitable structures to be used in future prototype manufacturing of energy relevant devices, for example solar rectenna, rectenna for microwave power transmission, antenna-coupled diode photodetectors, hot-electron devices, etc. Silicon based Schottky barrier and Metal-(native oxide)Insulator-Metal diodes have been, for the first time, systematically studied by using conventional manufacturing techniques to produce optimised devices. The result being n-type Si Schottky barrier diodes with various top metals (Ag, Al, Au, Nb, Cr, Mo, Ni and Ti) displaying low ideality factors close to unity (exception made from Al), good low-bias rectification properties, low leakage currents and optimised barrier heights. By analysing and comparing the performance metrics for these diodes, it was established that Cr, Ti, Mo and Ni have the potential to be used as passive devices for energy applications, with Cr displaying the highest cut-off frequency at 39.6 GHz, whereas Mo and Ni having desirable properties that also make them suitable as active components. Native oxide Metal-Insulator-Metal diodes based on Al, Cr, Nb and Ti, and their native oxide derivatives topped with a selection of metals of different workfunctions (Ag, Al, Au, Nb, Cr, Mo, Ni and Ti) all displayed asymmetric current-voltage characteristics. By comparing the rectification properties of all structures, it was established that higher electron affinity insulators (Nb2O5 and TiO2) have better performance metrics than the lower electron affinity insulators (Al2O3 and Cr2O3), with the Nb based structures outperforming the Ti based devices. Three figure of merits were used to characterise the MIM diodes, with the Nb/Nb2O5/Ag device displaying the highest at 35.6 asymmetry, 4.0 nonlinearity and 7.9/V responsivity, making it suitable for lower end energy applications such as IR or Optical sensing. An initial study into Metal-Insulator-Insulator-Metal diodes with insulators grown by reflection high energy electron diffraction assisted pulsed laser deposition has shown a proof of concept for the improved performance of double-insulator over single-insulator tunnel diodes, and identified factors that need to be controlled when considering this novel approach to study the double-insulator devices.