Activation process of GaAs NEA photocathode and its spectral sensitivity

NEA GaAs photocathodes with high quantum efficiency (QE) and high response speed are expected as a electron source for THz frequency vacuum devices. To achieve high QE and high response speed, further understanding of the electron emission mechanism is necessary. This study focused on the formation process of NEA surface on a GaAs photocathode. The results suggests that NEA surface allows the electron emission from both the Γ and L valleys in the conduction band. On the contrary, in the Cs or O excess state, vacuum level is between Γ and L, and electrons can emit only from L valley, and cannot be allowed to emit from the Γ valley. Contribution of Γ and L valleys to the emission current was confirmed, of which balance depended on surface conditions and excitation wavelength.