Abstract: In this study, we epitaxially grew a multilayer structure of gallium nitride (GaN) photocathode film material on a sapphire substrate and conducted a surface photovoltage test. The effects of doping type, thickness, and doping method on the surface photovoltage of the gallium nitride material were compared and analyzed, and the mechanism of surface photovoltage generation of the multi-layered gallium nitride material was determined. A surface photovoltage test was performed on uniformly doped and delta-doped gallium nitride photocathode thin film materials using sub-band-gap laser. Experimental data shows that better growth quality was achieved using δ-doping than that achieved using uniform doping; however, δ-doping increased the density of defect states in the (E_v+0.65)–(E_v+1.07) eV energy levels.