Theoretical study for heterojunction surface of NEA GaN photocathode dispensed with Cs activation

Abstract

For the disadvantages of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, new-type NEA GaN photocathodes with heterojunction surface dispensed with Cs activation are investigated based on first-principle study with density functional theory. Through the growth of an ultrathin [Formula: see text]-type GaN cap layer on [Formula: see text]-type GaN emission layer, a [Formula: see text]–[Formula: see text] heterojunction is formed on the surface. According to the calculation results, it is found that Si atoms tend to replace Ga atoms to result in an [Formula: see text]-type doped cap layer which contributes to the decreasing of work function. After the growth of [Formula: see text]-type GaN cap layer, the atom structure near the [Formula: see text]-type emission layer is changed while that away from the surface has no obvious variations. By analyzing the E-Mulliken charge distribution of emission surface with and without cap layer, it is found that the positive charge of Ga and Mg atoms in the emission layer decrease caused by the cap layer, while the negative charge of N atom increases. The conduction band moves downwards after the growth of cap layer. Si atom produces donor levels around the valence band maximum. The absorption coefficient of GaN emission layer decreases and the reflectivity increases caused by [Formula: see text]-type GaN cap layer.