Electron emission suppression from hydrogen-terminated n-type diamond

By total photoelectron yield spectroscopy (TPYS), we have found a thermal instability of a negative electron affinity in hydrogen-terminated n-type diamond films, which has never been observed for intrinsic and p-type diamond films. Experimentally, we have succeeded in detecting surface defect states and surface valence band extended states on the same n-type sample after soft annealing from 100 to 300 °C, which allowed us to evaluate the surface Fermi level and surface band bending using phosphorus doping parameters. Our results show that “quasi-positive” electron affinity prevents electron emission from an n-type diamond film even though its surface has a negative electron affinity.     

Using negative electron affinity diamond emitters to mitigate space charge in vacuum thermionic energy conversion devices

A negative electron affinity (NEA) diamond surface is employed as an emitter electrode in a vacuum thermionic energy conversion device in order to mitigate the negative space charge effect. The motive diagram of an NEA device operating at the virtual saturation point is compared to a similar device with a conventional emitter material operating in the space charge limited regime in order to understand how NEA mitigates space charge. Output current characteristics are calculated for various NEA values, and the results are compared to an ideal (no space charge) model. Increasing the value of the NEA causes the output current characteristic to approach that of the ideal model. Motive diagrams for various values of NEA are calculated and used to explain this phenomenon. It is shown that an NEA device can achieve a maximum output power density equal to the maximum output power density of a similar ideal device.