Thermionic emission and conversion properties of n-type AlGaN thin film cathodes grown on 6H–SiC substrates

We observed the thermionic emission (TE) and conversion (TC) characteristics of Si-doped AlGaN thin film cathodes grown on n-type 6H-SiC substrates with Cs adsorption on the surfaces. The threshold temperatures of TE decreased as the AlN mole fraction was increased, with threshold temperatures of around 205 °C for Si–doped Al0.5Ga0.5N. TE was detected even when the applied external voltage was 0 V at 600 °C, which means that the cathodes had TC characteristics at a temperature dramatically lower than that of conventional systems with metal-based cathodes. In addition, we observed a notable enhancement of the emission current when the cathodes were irradiated with white light having energy lower than that of the bandgap of the AlGaN films at low temperatures. We suggest that the enhancement was caused by an internal photoelectron effect due to the absorption of the light in the n–type 6H–SiC substrate.