Comparison of first and second annealing GaN photocathode

We anneal GaN samples in ultra high vacuum system for two times at the same temperature of 710 °C, and activate the sample after each heating by Cs/O. The vacuum level, residual gas, QE, and photocurrents are compared. We find, for the 1^st annealing vacuum level line has a shape of “W”, but the 2^nd looks like “V”. The residual gases include H₂, H₂O, N₂, and CO₂ mainly. Nothing else has been detected significantly. For the 1^st annealing, there are two stages the residual gases come out quickly, but nothing comes out until the maximum temperature during the 2^nd annealing. Before activation, the photo-current after 2^nd heating is higher than the 1^st, which shows the stability of GaN. No obvious difference of QE has been found between the two times annealing, and more researches will be done on this topic.