Properties of Si/Cs/O nanocluster thin films with negative electron affinity
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ABSTRACT Thin films of Si/Cs/O nanoclusters have been synthesized by the technique of supersaturated thermal vaporization of Si and Cs in an oxygen background gas. These films, which were deposited onto conducting or semiconducting substrates, exhibit negative electron affinity (NEA) as evidenced by ultraviolet photoemission spectroscopy (UPS). Photo and field electron emission properties of these nanocluster films were investigated with photo-electron emission microscopy (PEEM), field electron emission microscopy (FEEM), and current-voltage measurements. Flat cathodes covered with thin films of Si/Cs/O nanoclusters exhibited high current outputs and low turn-on fields. The films' NEA is unaffected by air exposure and is stable to high temperature annealing (550 TC). A field emission display unit with a simple diode structure containing a flat cathode coated with a patterned thin film of Si/Cs/O nanoclusters has also been built to demonstrate the potential application of this material in cold cathode electron emitting devices, particularly field emission flat panel displays. INTRODUCTION Theoretical and experimental studies of the phenomenon of negative electron affinity (NEA), in which the vacuum energy level lies below that of the conduction band minimum, have been reported.'' 8 In most cases, only certain well prepared and highly oriented surfaces of some semiconductors or insulators [most notably: diamond (100) and (111), Si (001), and GaAs (100) surfaces] exhibit NEA. The preparation of these surfaces usually requires etching and high temperature annealing in an ultrahigh vacuum (UHV) chamber. The clean surfaces are then, in the case of diamond, terminated with hydrogen or coated with well ordered layers of cesium or cesium oxide. Diamond has also been coated with metals such as Ni, Cu, Co, or Ti. In all cases, surface dipole formation is thought to induce a strong NEA. However, most of these NEA surfaces tend to be unstable when exposed to air or under prolonged electron emission. Many do not survive the thermal conditions associated with device manufacturing and packaging. Furthermore, due to the large band gaps associated with insulators, electron supply to insulating materials such as diamond for electron emission is not a trivial issue. Recently, there have been reports on NEA surfaces of AIN and AlGal-xN epitaxially grown on c(6H)-SiC(000l) substrates. 5' 6 There was no effort to control the surface termination for these samples mentioned by the authors in obtaining NEA, but lattice matching for epitaxial growth limits the number of useful subtrates these NEA films can be grown on. Additionally, NEA from e-beam deposited BN has been reported.4 However, there has been no report of a NEA Si-based material that is capable of producing strong electron emission, stable with respect to environment and temperature, and yet does not require a stringent surface preparation or epitaxial growth. Such a Si-based material would be an excellent candidate for applications in cold/photo-cathode emitters, and ready for i
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