Local Crystallization of ${\rm LaB}_{6}$ Yielding Compact, Strong Thermionic Electron Emission Source

Local microheating of amorphous LaB₆ film could control the degree of crystallization as determined by spatially resolved Raman spectroscopy and transmission electron microscopy. With full crystallization of the LaB₆, we achieved micrometer-sized thermionic electron emission source with the maximum current density of 1.2 A/cm². The advantage of fabricating a micrometer-sized emitter with high current density enables versatile applications such as compact X-ray or vacuum type terahertz radiation sources. A new structure of micrometer-sized, lateral-type vacuum channel transistor is proposed based on the simulation. The calculated electron travelling time from emitter-to-collector was 8.3 ps at the traveling distance of ~10 μm, meaning that the maximum operation frequency is 120 GHz.

I. Introduction

Lanthanium hexaboride is an inorganic material that has a low work function and nature of low volatility to be one of the highest electron emitter [1] . Therefore, its principal use is in thermionic cathodes, including electron microscopes, microwave tubes, and X-ray tubes. To achieve desired performance, should have high crystallinity. Therefore, the most prepared has the bulky form although it is highly single crystalline. To reduce such bulky dimension, polycrystalline has been studied as an alternative thermionic emitter, which is obtained by sputter deposition of amorphous on tungsten (W) wire followed by successive crystallization through electrical joule heating [2]. Recent chemical vapor deposition of single-crystalline nanowire on a silicon substrate might be a useful technique for miniaturization of thermionic emitters [3]. However, all these attempts have not yet been successful because of limitations in integrating the materials into compact heating source formed on semiconductor-process compatible substrates. On the other hand, recent report on the vacuum-channel transistor based on field emission exhibits potential to fast electronics (0.46 THz). But, the device exhibits nonsaturating behavior with increasing drain bias [4]. This hinders the stable operation of vacuum transistor. Thermionic electron source can have saturation behavior even when formed in the micrometer-scale transistor. Thermionic electrons have an additional advantage of being easily focused than that by field emission because of lower initial kinetic energy, which is strongly required for high-performance terahertz (THz) or X-ray sources.