Study of GaN/AlGaN photocathode with variable aluminum AlxGa1−xN material in emission layer

To introduce the built-in electric field in photoelectron emission layer, the GaN/AlGaN photocathode with variable aluminum (Al) AlxGa1-xN material in emission layer was grown. This sample was treated with surface cleaning and Cs/O activation to achieve negative electron affinity (NEA) status. The vacuum degree changes with heating temperature in the ultra-high vacuum (UHV) chamber indicated that almost all the carbon and oxygen residua were removed from GaN surface by chemical cleaning alone. The introduction of O after Cs deposition on GaN surface made a comparatively high photocurrent increase, which was observed for GaN photocathode for the first time and could be well explained with the Ga-O-Cs dipole model proposed before. High quantum efficiencies (QEs) were achieved in the wavelength from 240 nm to 275 nm for this sample, and the lower QEs at longer wavelengths can be explained with the small optical absorption coefficient of AlxGa1-xN material.