Resonant defect states of the SnO2:Ta transparent conductive oxide revealed by excitation wavelength-dependent Raman spectroscopy and hybrid functional DFT calculations

Abstract

Excitation wavelength-dependent Raman spectroscopy, optical spectroscopy, and density functional theory (DFT) calculations with hybrid functionals were used to analyse the electronic structure of defects in SnO₂:Ta (1.25 at% Ta) transparent conductive oxide thin films. Based on the Raman excitation profiles of the characteristic D₁ and D₂ defect modes of two tin vacancy V_Sn-type defects and one oxygen interstitial O_i-type defect, we derived the corresponding defect-induced electronic transitions of the involved defect states. DFT calculations revealed additional density-of-states for the three point defects at the top of the valence band (VB) in comparison to defect-free SnO₂ and SnO₂:Ta. The largest distortion of the VB electronic structure was caused by the V_Sn-type defect with the farthest possible distance from the Ta dopant in the studied 96-atom supercell, and the smallest distortion was caused by the O_i-type defect. Accordingly, the amount of VB splitting showed a reverse order to the electronic transition energies. From the projected defect-density-of-states, we found a delocalized nature of the V_Sn-type defects and a localized nature of the O_i-type defect, accounting for the different degrees of distortion of the SnO₂:Ta electronic structure. Based on these complementary experimental and theoretical results, the electronic structure of point defects in the SnO₂:Ta transparent conductive oxide was elucidated in detail. Thus, the proposed approach has great potential to resolve the ongoing controversy about point defects in SnO₂.