Oscillations in the threshold photoemission spectra of GaN(0001) with submonolayer Cs coverages

It is found that Cs adsorption on the $n$-type GaN(0001) surface generates an unusual change in the electronic properties of the surface and the near-surface space-charge layer, which leads to the appearance of photoelectron emission upon excitation in the transparent region of GaN. It is established that the photoemission is due to the formation of quasimetallic states induced by Cs adsorption in the band-bending region near the surface. The behavior of the photoemission threshold upon excitation by $s$-polarized light is studied as a function of the Cs coverage. It is found that the minimum value of the threshold corresponds to $\sim1.4\,$eV at a concentration of Cs atoms of $\sim4.5\cdot10^{14}\,$atom/cm$^2$ in the submonolayer coverage. A new effect is revealed, namely, the appearance of oscillations in the spectral curves of threshold photoemission. A model is proposed for photocurrent oscillations that takes into account the formation of quasimetallic states in the near-surface layer of GaN band bending and the occurrence of interference in the GaN slab upon light irradiation in the transparent region.