Quantum simulation of thermionic emission from diamond films

Recent advances in wide-band gap thermionic materials have brought to question the applicability of well accepted theories for thermionic emission from metallic surfaces. The authors developed a nonequilibrium Green's function (NEGF) self-consistent model based on quantum mechanics to investigate thermionic emission from nitrogen-incorporated diamond cathodes. The model allows us to relax several assumptions typical of Richardson's equation. The NEGF method is a self-consistent Schrodinger-Poisson formalism where the transport is calculated from an effective mass description and Fermi-Dirac statistics. The predictions were validated against experimental measurements from nitrogen-incorporated diamond cathodes. The model captures key emission characteristics such as the onset temperature of emission and the emission from low or negative electron-affinity materials. The results indicate that Richardson's equation overestimates emission for many cases, especially in low electron affinity materials. In addition, the model allowed them to estimate the heat flux at the cathode using the spectral emission as opposed to the mean velocity approximation, which under-predicts the cooling potential. Finally, they developed a relation between Richardson's constant, work function, and electron affinity to aid in identifying a range of Richardson's parameters applicable to experimental characterization of materials. (C) 2013 American Vacuum Society. [http://dx.doi.org/10.1116/1.4792522]