Interfacial Barrier of Heterojunction Photocathodes

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.