Nanodiamond Electron Emitters: Applications and Fundamental Challenges

Unlike single crystal diamonds, polycrystalline synthetic diamonds are easier to synthesize, and they have various applications, e.g., in tribology, micro- and nano- electromechanical devices. They also appear to be excellent electron field emitters, having turn-on fields on the order of 10 megavolts per meter, which is far below breakdown threshold for any material. Over two years, a Euclid TechLabs/Argonne National Lab team has been working on device applications of n-type nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, electron emitters. Main focus is to use (N)UNCD as highly efficient cathodes for accelerator injectors.

(N)UNCD can be grown directly on any refractory metal, and even on stainless steel with molybdenum buffers. It makes fabrication of cathode plugs for accelerator injectors and conventional direct current guns scalable and facile. By now, efficient (N)UNCD cathode prototypes were demonstrated for copper radiofrequency (RF) injectors in L-band (1.3 GHz) and X-band (9.2 GHz). In collaboration with Brookhaven National Lab, we also conducted first successful cathode tests in a superconducting RF injector (SRF TESLA-type 1.3 GHz) under cryogenic 2-4 K conditions, and obtained a current of a few μA out from the SRF injector.

In the course of device testing and pushing the previously achieved metrics, the ultimate performance of (N)UNCD came into question. To shed more light on fundamental properties of UNCD, we developed experimental and modeling methodologies. We have designed and commissioned a new projection type imaging system. The system can directly image the field emission site distribution on a cathode surface by making use of anode screens in the standard parallel plate configuration. The lateral spatial resolution of the imager is on the order of 1 μm. In uniformly planar (N)UNCD films, we routinely find that the field emitting site distribution is not uniform across the surface. Moreover, we find that the actual electron emitting area depends on the applied electric field. To process extensive imaging datasets, an automated high throughput emission area algorithm was developed and used to calculate emission area per image at a specific electric field point, and determine the actual field emission area dependence on the electric field. By doing so, we have unveiled that when I-E curves, as measured in experiment, are normalized by the field-dependent emission area the resulting j-E curves demonstrate a strong kink, significant deviation from the Fowler-Nordheim law and eventually a saturation type of behavior with a level of saturation of about 100 mA/cm2, which is very similar for all (N)UNCD films studied regardless the substrate. The found j-E saturation type behavior is very similar to the effects reported priory only for semiconductor (Si, SiC, Ge, etc.) field emitters.

All these will be discussed in detail during the presentation.