Surface polarity dependence of thermionic emission and conversion characteristics of n-type GaN cathodes

We observed the polarity-dependent thermionic emission (TE) and conversion characteristics of n-type GaN-based cathodes with Cs adsorbed on their surfaces. TE current from the surface of an n-GaN sample with N-polarity was 0.18 mA at an applied anode voltage of 30 V at 500 °C. This TE current was markedly higher than that of a sample with Ga-polarity, which had a corresponding TE current of 0.063 mA. We consider the N-polarity with spontaneous polarization to be the cause of the increase in electron density at the Cs/n-GaN interface. TE current was also detected from both samples with Ga- and N-polarity even when the applied anode voltage was 0 V or lower, indicating the presence of thermionic conversion characteristics. From the viewpoint of a thermionic converter, the electromotive force for TE was 0.12 V higher when using the N-polarity n-GaN cathode compared with the Ga-polarity cathode. The short-circuit currents at 500 °C were 4.8 and 0.97 μA for the sample with N-polarity and the one with Ga-polarity, respectively.