Carbon nanotubes for cold electron sources

Technological advances in the field of microelectronic fabrication techniques have triggered a great interest in vacuum microelectronics. In contrast to solid-state microelectronics, which entails scattering-dominated electron transport in semiconducting solids, vacuum microelectronics relies on the scattering-free, ballistic motion of electrons in vacuum. Since the first international conference on vacuum microelectronics substantial progress in this field has been made. The first technological devices using micrometer-sized electron emitting structures are currently being commercialized. Field-emission flat-panel displays (FED) seem to be an especially promising competitor to LCD displays. Today there is only one mature technology for producing micro-gated field-emission arrays: the Spindt metal-tip process. The drawbacks of this technology are expensive production, critical lifetime in vacuum, and high operating voltage. Carbon nanotubes (CNT) can be regarded as the potential second-generation technology to the Spindt metal micro-tip. In this review we show that the field emission (FE) behavior of CNT can be accurately described by Fowler-Nordheim tunneling and that the field-enhancement factor beta is the most prominent factor. Therefore the FE properties of a CNT thin film can be understood in terms of local field enhancement beta(x, y), which can be determined with scanning anode field emission microscopy (SAFEM). To characterize the FE properties of an ensemble of electron emitters we used a statistical approach (as for thin film emitters), where f(beta)dbeta gives the number of emitters on a unit area with field-enhancement factors within the interval [beta,beta+dbeta]. We show that the field-enhancement distribution function f(beta) gives an almost complete characterization of the FE properties.