The quantum efficiency of a vicinal GaAs(100) negative electron affinity (NEA) photocathode is studied and correlated to the surface morphology. Cleaning of a GaAs(100) vicinal surface by atomic hydrogen and by heating are investigated using reflection high-energy electron diffraction (RHEED). After atomic hydrogen cleaning at 500 degrees C, the GaAs surface exhibits a streaky (2x4)-reconstructed RHEED pattern. When the GaAs(100) surface is activated to NEA by the alternate deposition of cesium and oxygen, a quantum efficiency of similar to 9% is measured. The photocathode quantum efficiency correlates with the out-of-phase RHEED intensity measured before activation. After the quantum efficiency decreases with operating time, further atomic hydrogen exposure also produces a (2x4) pattern. Surfaces prepared or revived by atomic hydrogen produce brighter out-of-phase electron diffraction patterns and, when activated to NEA, higher quantum efficiency compared to those that are heat cleaned.