Interfacial Barrier of Heterojunction Photocathodes

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Abstract
The theory of N, large band gap; P, small band gap (NLPS) heterojunctions is examined in the case, where EAp‐EAn>Egn, and it is shown that the height of the interfacial barrier (from P to N) can be no larger than Egn as measured from the Fermi level. This means that the interfacial barrier of N‐type Cs2O deposited on III–V P‐type semiconductors can be no greater than ∼2.0 eV. This barrier will be reduced in practice if interfacial states stabilize the Fermi level in the N‐type material by providing a depository for electronic charge concentrated right at the surface. These states can be caused by the first layer of Cs deposited on a III–V semiconductor in the fabrication process for a III–V, Cs2O photocathode. In that case, the height of the interfacial barrier can be expected to be Iads‐EAn, where Iads is the ionization energy of Cs alone deposited on the III–V semiconductor and EAn is the electron affinity of Cs2O. Good correlation is shown with experiment for GaSb, N‐type GaAs, and Ge.