Photoemission Physics of Wide-Bandgap Semiconductors: An Empirical Development in Photocathode Theory

For photoinjector applications, a photocathode with a low intrinsic emittance (i.e. low emitted beam divergence) and high quantum yield (i.e. number of emitted electrons per absorbed photon) is desired and, implicitly, physical robustness and reasonable cost are also sought after qualities. These properties are material dependent and one class of material that has the potential to satisfy all these conditions is wide-bandgap semiconductors like those used in high power electronics applications. As a result, some of these materials, specifically diamond, gallium oxide (Ga2O3), and gallium nitride (GaN), were evaluated to determine their feasibility as photocathodes. To that end, a combined theoretic and experimental approach is employed. Specifically, materials were first identified and screened by computationally evaluating their electronic properties using density function theory (DFT) in accordance with predictions made by the (at the time) state-of-the-art photoemission theory. Promising candidates were then procured and subsequently investigated through the use of an ultrafast tunable-UV laser system and DC electron gun. The resulting experimental data, which represents the real photoemission physics that occurred, was then used as an empirical basis to further develop the relevant theories. None of the investigated materials met the low intrinsic emittance criteria. However, the results obtained through the experimentation have led to the realization of significant new physics in the field of solid-state photoemission devices. In particular, phonon assisted, momentum resonant emission processes (“Franck-Condon Emission”) are a dominant and, potentially, unavoidable aspect of photoemission in many otherwise promising semiconductor photocathodes. Since such processes generally increase intrinsic emittance and since semiconductor photocathodes are ubiquitous in modern photoinjectors, this result has the potential to be of critical importance for the field.