Intrinsic emittance properties of an Fe-doped Beta-Ga2O3(010) photocathode: Ultracold electron emission at 300K and the polaron self-energy
Abstract
Measurements of the spectral emission properties of an iron-doped Beta-Ga2O3(010) photocathode at 300K reveal the presence of ultracold electron emission with a 6meV mean transverse energy (MTE) in the 3.5-4.4eV photon energy range (282-354nm). This extreme sub-thermal photoemission signal is consistent with direct emission of electrons photoexcited from the Fe dopant states into the low effective mass and positive electron affinity primary conduction band, and it is superimposed on a stronger signal with a larger MTE associated with an (optical)phonon-mediated momentum resonant Franck-Condon (FC) emission process from a thermally populated and negative electron affinity upper conduction band. For photon energies above 4.5eV, a transition from a long to a short transport regime is forced by an absorption depth reduction to below 100nm and both MTE signals exhibit spectral trends consistent with phonon-mediated FC emission if the polaron formation self-energy is included in the initial photoexcited electron thermalization.