Density functional theory studies on Cs activation mechanism between GaN (0001) and Al0.25Ga0.75N (0001) surface

Abstract

In order to study the influence of Al fraction at the AlxGa1−xN photocathode, models of GaN (0001) and Al0.25Ga0.75N (0001) surfaces are built and compared. Plane wave with ultrasoft pseudopotential method based on first-principle density functional theory is used to calculate and compare the Cs activation mechanism for Al0.25Ga0.75N (0001) surface and GaN (0001) surface. In this work, seven possible Cs adsorption sites are chosen for the Al0.25Ga0.75N (0001) surface while five high-symmetry sites are considered in the calculation model of GaN (0001) surface. Adsorption energy, work function, surface dipole moment, and photocurrent curves are obtained. Results show that Cs adsorption lowers the surface work function and benefits to get the most stable adsorption sites. To verify the result of our calculations, an activation experiment is performed on p-type Mg-doped GaN and AlGaN substrate grown by metal organic chemical vapor deposition. The photocurrent curves show that the overall photocurrent value of GaN photocathode is greater than that of Al0.25Ga0.75N photocathode. Spectral response curves of two photocathodes are also investigated and used to explain the difference of photocurrent. This works gives a reference for the further experimental research on the AlxGa1−xN and GaN photocathodes.