Surface activation of n-type AlGaN with cesium and oxygen to enhance thermionic emission

Abstract

The aim of this study is to enhance the characteristics of thermionic emission of AlGaN surface through surface control employing cesium (Cs) and oxygen. Cs-deposited AlGaN has significant applications in thermionic energy converters. However, as the emitter temperature increases, the thermal desorption of Cs from AlGaN surface increases, resulting in a decrease in the thermionic emission current. Therefore, focusing on the high affinity between Cs and oxygen, we investigated the possibility of suppressing thermal desorption by depositing Cs and oxygen on AlGaN surface. The thermionic emission current measured when Cs and oxygen were alternately deposited on AlGaN surface was 1.9 × 10−3 A cm−2 at 500 °C. The thermionic emission current was significantly higher than that obtained with Cs-only deposition (2.0 × 10−5 A cm−2). In addition, we attempted to reproduce the effect of dynamic surface changes on thermionic emission employing a new thermionic emission model (modified Richardson–Dushman model) that considers the correlation between a specific surface reconstruction phase and its thermionic emission component. The results suggest that the adsorbed component of Cs-deposited AlGaN exhibits three Cs adsorption sites with different desorption energies, while the adsorbed component of Cs/O2 co-deposited AlGaN exhibits at least four Cs adsorption sites with different desorption energies. It is suggested that the increase in adsorption components with higher desorption energies, caused by the deposition of oxygen, may have reduced the thermal desorption and improved Cs coverage and stability.