Effect of Chemical Treatment of the Surface on Optical Properties of ZnSeAl Substrates

The influence of the surface morphology of single-crystal ZnSe substrates on the formation of their luminescence and absorption spectra was studied. It was established that the treatment of substrates in the chrome etchant Cr2O3:HCl  2:3 leads to the formation of a mirror surface, and in a solution of the composition H2SO4:H2O2  3:1 – of matte. Analysis of the AFM-topograms of the mirror surface shows that the root-mean-square roughness does not exceed 0.6 nm, and such a surface can be considered an atomically smooth. AFM image of a matte surface is a set of smoothed pyramidal grains with lateral dimensions of 30-300 nm, and their maximum height reaches ~ 200 nm. Nanoclusters with an average lateral size of ~ 90 nm and a maximum height of ~ 27 nm are also observed on the surface of these substrates. In the luminescence spectra of samples with a surface nanostructure, in contrast to substrates with a mirror surface, the edge emission band and a wide U-band appear in the range of photon energies larger than the bandgap Eg of zinc selenide. In this case, the U-band is formed by the tops of small pyramidal grains, and its large half-width and lack of structure is due to the dispersion of the lateral sizes of these grains. It was found that the optical absorption edge of substrates with a mirror surface is described by the «crystalline» approximation of the Urbach rule, and the energy position of the focal point Ef is consistent with Eg ≈ 2.81 eV of zinc selenide at 0 K. The behavior of the optical absorption edge of samples with a surface nanostructure obeys the «glassy» approximation of the Urbach rule, and its displacement with increasing temperature to the low-energy region occurs without a change in the slope. This is a consequence of static disordering, in contrast to samples with a mirror surface, for which dynamic disorder due to electron-phonon interaction dominates.