Comprehensive study of photoinduced changes kinetics in ternary Ge-Sb-Se chalcogenide thin films

Abstract

In this work, the kinetics of photoinduced changes in sputtered ternary Ge₂₉Sb₈Se₆₃ chalcogenide thin films with different thicknesses is studied. The optical bandgap energy of virgin thin films is 1.87±0.02 eV and the refractive index at 1 550 nm is 2.55±0.01 as determined by spectroscopic ellipsometry using Cody-Lorentz oscillator model. An annealing treatment caused bleaching of thin films resulting in optical bandgap energy increase to 1.96±0.02 eV accompanied with refractive index decrease down to 2.54±0.01. Subsequently, the photoinduced shift of the absorption edge was determined by the analysis of transmission data obtained by fibre-coupled high-resolution spectrometer. The irradiation of virgin thin films by near-bandgap light coming from continuous-wave diode-pumped solid-state laser leads to a fast photodarkening (PD) followed by slow photobleaching (PB) effect. The PB effect persists in virgin films and the maximum magnitude of this effect was found in film with the thickness of ~ 350 nm. Rise of the optical bandgap energy was ~ 0.04±0.02 eV using optical intensity of 125.0±5.0 mW ∙ cm⁻². On the other hand, in annealed thin films, only PD occurs under the same conditions indicating that the PB component of the photoinduced change disappears when the film is annealed. Maximum decrease in optical bandgap energy due to the PD effect in annealed films was about ~ 0.05±0.02 eV found in film with the thickness of ~ 650 nm. An influence of the thickness and laser optical intensity onto the kinetics of photoinduced changes is discussed.