Présidents de session
Tutorial: Photocathode physics, a band structure perspective
- Andreas Schroder
Description
Photocathode Physics: A Band Structure Perspective
In this tutorial, after a brief historical introduction, the key components associated with our understanding of photocathode physics will be presented with particular emphasis on the properties of the bulk electronic states from which the photo-excited electrons are emitted. The photoemission physics of both metals and semiconductors will be discussed with attention to two emission mechanisms: (i) transverse momentum conserving direct photoemission from the occupied band states [1] and (ii) momentum and energy resonant Franck-Condon emission involving inelastic scattering processes – electron-(optical)phonon scattering in semiconductors [2] and Umklapp electron scattering in metals [3]. In all cases, the energetic position and dispersion of the emitting bulk band states have a direct influence on the quantum efficiency (QE) and mean transverse energy (MTE) of electron emission. Both the QE and MTE are also strongly dependent upon the temperature Te of the electron distribution in those states, which is generally not equal to the lattice temperature of the photocathode.
Throughout the presentation, direct comparison will be made to experimental measurements of the QE and MTE to demonstrate the connection between the bulk band structure of photocathodes and their photo-excited electron emission properties. In particular, spectral measurements of the photoemission properties of single-crystal metal photocathodes will be shown to be consistent with band-based emission simulations that include the density of states of both the emitting band and recipient vacuum states [4]. Further, the influence of upper conduction bands will be discussed with reference to experimental data obtained from a Cu(111) photocathode [5], while a comparison of the near and below threshold emission properties of Cu(001) and W(111) photocathodes elucidates the effect of Umklapp electron scattering [3]. For semiconductor photocathodes, a Franck-Condon emission mechanism mediated by electron-(optical)phonon scattering usually dominates and the QE and MTE are dependent on both Te and the position of the bottom of the emitting band with respect to the vacuum level (i.e., its electron affinity) – even if the principal emission is from an upper conduction band – in agreement with measurements on cesiated p-type GaAs(001) and p-type GaN(0001) photocathodes [2]. Experimental evidence for additional direct band emission observed from the primary, low effective mass, conduction band of a 300K Fe-doped Ga2O3(010) photocathode, generating a sub-thermal MTE, is also consistent with the expected temperature of the photo-excited electron distribution [6].
[1] W.A. Schroeder and G. Adhikari, New J. Phys. 21, 033040 (2019).
[2] W.A. Schroeder, L.A. Angeloni, I-J. Shan, and L.B. Jones, Phys. Rev. Applied 23, 054065 (2025).
[3] I-J. Shan, L.A. Angeloni, and W.A. Schroeder, https://arxiv.org/abs/2604.12979
[4] G. Adhikari, P. Riley, and W.A. Schroeder, AIP Advances 9, 065305 (2019).
[5] L.A. Angeloni, I-J. Shan, and W.A. Schroeder, AIP Advances 12, 105129 (2022)
[6] L.A. Angeloni, I-J. Shan, J.H. Leach, and W.A. Schroeder, J. Appl. Phys. 139, 095706 (2026).