Thermionic energy conversion is one of the promising technologies for transforming thermal energy into electrical energy. A stable cathode electrode with high conductivity and low work function is required to realize this application. It is already well known that hydrogen-terminated diamond exhibits negative electron affinity, which is a great advantage of an electron emission electrode. In this study, we focus on phosphorus doping, which gives n-type donors with 0.57 eV in single crystal diamond to control the conductivity of diamond, in an attempt to develop a nano-crystalline diamond (NCD) electrode for thermionic emission by heavily phosphorus doping. Phosphorus concentration and structural characterization were performed by secondary ion mass spectroscopy and Raman spectroscopy, respectively. The thermionic emission properties were characterized in vacuum as a function of cathode temperature from 300 to 600 degrees C. The work function of heavily phosphorus doped NCD electrode was estimated based on the Richardson Dushman equation, and the effect of heavily phosphorus doping on thermionic emission was briefly discussed

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In this work, we have developed an innovative epitaxial growth process named the “Migration Enhanced Embedded Epitaxial” (ME3) growth process. It was found that at elevated growth temperatures, the epitaxial growth at the bottom of the trenches is greatly enhanced compared to growth on the sidewalls. This is attributed to the large surface diffusion length of reactant species mainly due to the higher growth temperature. In addition, it was found that this high temperature ME3 growth process is not influenced by the crystal-orientation. Similar growth behavior was observed for stripe-trench structures aligned either along the [11-20] or [1-100] directions. No difference was observed in the electrical performance of the pn diodes fabricated on either oriented stripe geometry. The ME3 process can also be used as an alternative to ion-implantation technology for selective doping process.

The migration enhanced embedded epitaxy (ME3) mechanism and 2D dopant distribution of the embedded trench region is investigated with the aim to realize the all-epitaxial, normally-off junction field effect transistor (JFET). We found that the embedded growth consists of two main components. First one is the direct supply without gas scattering and the other one is the surface migration supply via the trench opening edge, which dominate the ME3 process. An inhomogeneous 2D distribution of Aluminum (Al) concentration was revealed for the first time in the 4H-SiC embedded trench regions by the combined analysis of secondary ion mass spectrometry (SIMS) and scanning spreading resistance microscopy (SSRM) results. The maximum variation of Al concentration in the trench is estimated to be about 4-times, which suggests that the Al concentration is highest for the (0001) plane and lowest for the trench corner (1-10x) plane. Al concentration in the (1-100) plane, which determines the JFET p-gate doping level is 1.5-times lower than (0001) plane for trench region fabricated on Si-face wafers.

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