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 H
2
, H
2
O, N
2
, and CO
2
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.