Theoretical and experimental investigation of low and negative electron affinity cold cathodes based on rare-earth monosulfides.

摘要

Recently, there has been renewed interest in cold cathode emitters for applications to a variety of electronic devices, including microwave vacuum transistors and tubes, pressure sensors, thin panel displays, high temperature and radiation tolerant sensors, among others. Introduction of such emitters would permit an unprecedented compactness and weight reduction in device and equipment design. Cold cathodes exhibit a narrower energy distribution than conventional thermionic cathodes and are therefore expected to offer several advantages such as reduced noise, improved electron beam focusing, imaging, and beam discharge lag in camera tubes. To keep the emitter temperature from increasing, emitters must be built on thin epitaxial films (using vertical layering architecture) so as to keep extremely short heat paths and thus provide excellent heat sinking.; In this thesis, we investigate Metal(or n++-InP)/CdS/LaS cold cathode based on the negative electron affinity (NEA) between CdS and LaS (= 60 meV). Various effects like self heating, current self-quenching of anode current due to space-charge effects, and the interplay between current crowding and space-charge effects have been studied as a part of the doctoral work.; Rare-earth sulfides (like LaS, NdS etc.) form stable alternatives to reach NEA at various III--V semiconductor surfaces and do not suffer from all the limitations of conventionally used caesiated surfaces. LaS has a low work-function (at room temperature) of 1.14 eV, a high melting point of 2200°C, and a good lattice matching with InP and CdS. Bulk LaS is successfully grown and XRD scans show the rocksalt phase with a lattice constant a0 = 5.857(2)A . To realise the device, thin-films of LaS have been grown by RF Magnetron sputtering on various substrates like Si, glass and InP. The parameters leading to samples with stoichiometric amounts of La and S atoms are identified. Thin-film samples are characterized by X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM), and X-Ray Diffraction (XRD).