Gas adsorption on activated AlGaN photocathode: A first-principle study on charge redistribution, work function variation and emission performance attenuation

Abstract

AlGaN photocathode has important applications in photoelectronic imaging and vacuum electronic devices. However, The short lifetime of the photocathode results in higher costs of usage. This can be attributed to the decrease in vacuum degree and the adsorption of residual gases. In this study, first-principle study and transmission matrix are utilized to investigate five gases (H2O, CO, CO2, H2, CH4) adsorbed on Cs-activated AlGaN (100) surface. Results suggest that all these five gases are prior to deposit on the surface at low coverage of 0.25 ML with adsorption energy of -2 eV. CO has the highest adsorption possibility with its adsorption energy lower than 0 eV. Significant charge transfer occurs at Cs-gas interface, particularly near the oxygen atoms of oxygen-containing molecules. CO2 and CO exhibit a noticeable increase in work function. The increase in the work function of the H2O adsorption is primarily due to its longer dipole length rather than dipole charge by analysing the surface dipole moment. CH4 and H2 have negligible impact on surface properties. Upon gas adsorption, the electrons on the AlGaN surface flow back to the adatoms, causing the CBM shift towards higher energy and upwards band bending. Combined with the surface electron affinity, the iteration of band structures further explains the increase of work function. The open energy of free electron escaping into the vacuum after CO2 adsorption increases to the level of inactivated surface of 5.55 eV from PBE-D2. This study reveals the mechanism of performance decay of the photocathode and is expected to guide the optimization of packaging technology.