Hot Electron Photoemission from Tunable Electron Affinity Semiconductor Cathodes

A voltage-tunable negative electron affinity (NEA) semiconductor photocathode offers one key advantage over current materials such as cesiated NEA photocathodes: stability under ambient conditions. A semiconductor/insulator/graphene heterostructure can inject electrons into the conduction band of the insulator, where an electric field "heats" them up so that the effective emission barrier seen by the "hot" electrons is negative, enabling a voltage-tunable NEA surface. Here, we have experimentally demonstrated a peak emission current density of 2.253 × 10^-3 A/cm² and a peak external quantum efficiency (EQE) of ∼1.53% from a p-Si/amorphous-Al₂O₃/graphene-based hot electron laser-assisted cathode (HELAC). We have developed a full-band Monte Carlo Boltzmann Transport Equation (MCBTE) solver to study the hot electron transport behavior in three different crystalline insulators: SiO₂, Al₂O₃, and MgO. Through MCBTE and semiconductor device simulations, we have predicted a peak emission current density of ∼10³ A/cm², far above our experimental value, indicating that the optimal performance of state-of-the-art HELACs has not yet been realized. This theoretical framework provides an understanding of the key performance limitations of the device and can be used to guide the optimal design (e.g., through the selection of new materials) of voltage-tunable NEA semiconductor photocathodes.