Femtosecond pump-probe study of negative electron affinity GaAs/AlGaAs photocathodes

Negative electron affinity (NEA) photocathodes have attracted a lot of interest over the last two decades due to their high quantum efficiency and low dark emission, which are desirable for night vision and other low-light applications. Recently, gradient-doping technique has shown promise to significantly improve the quantum yield of GaAs/AlGaAs heterojunction photocathodes by assisting electron diffusion toward the surface. In the present work, femtosecond pumpprobe transient reflectivity measurement has been used to study the ultrafast carrier dynamics in NEA GaAs/AlGaAs photocathodes. The research focuses on the comparison between a traditional, uniform-doped structure (1.7 μm p-GaAs (1×1019 cm-3) / 0.7 μm p-Al0.57Ga0.43As (3×1018 cm-3) / si-GaAs substrate) and a gradient-doped structure (0.1 μm pGaAs (1×1018 cm-3) / 1.2 μm p-Al0.63Ga0.37As (doping level gradually changes from 1×1018 cm-3 to 1×1019 cm-3) / 0.5 μm p-GaAlAs (1×1019 cm-3) / si-GaAs substrate). Our result indicates that gradient doping not only leads to more efficient electron transportation but also results in better electron accumulation (i.e. higher concentration and longer lifetime) near device surface, a feature well-suited for photocathodes. Moreover, we have shown that pump-probe transient reflectivity measurement is able to offer a direct picture of electron diffusion inside NEA photocathodes, which can be of significant importance to device development.