The transverse momentum of photoelectrons released from a negative electron affinity GaAs cathode is small compared with other thermonic or field emission electron sources. A low photoelectron transverse momentum in vacuum promises highly focused, low-energy beams useful for numerous applications. A simplified theory for electron emission from GaAs predicts much lower electron transverse momentum than those previously measured experimentally. To address this theory–experiment mismatch, Monte Carlo based calculations were compared with experimental data. We checked the possibility of there being electron scattering in the Cs,O layer; however, none of the scattering checked (isotropically distributed, cosine distributed, and Rutherford scattering) properly fit the experimental results. The assumption of conservation of the parallel component of the crystal momentum k during the emission is mainly believed to be responsible for the calculation-theory disagreement. The best simulation-experiment fit was obtained through a relaxation of the conditions imposed on the transverse momentum inside and outside of the semiconductor when an ideal interface is considered. We obtained the best results by assuming that the effective mass of the electron inside GaAs is equal to the effective mass of the electron in vacuum. Two independent experiments confirmed that in both cases, this same-mass approximation gives the best fit. The physical meaning of this is not clear, but it seems to be related to the amorphous nature of the Cs,O layer. We conclude that the way to get lower electron transverse energy spread cathodes is to study alternative activation methods and new materials with smaller effective electron masses.
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Vergara et al. (1996) studied this question.