Reflections on the Use of Density Functional Theory in the Understanding of the Effects of Moderate Amounts of Sulfur Substitutional Impurities on ZnO

A theoretical study on the effects of a moderate amount of sulfur when used as substituent impurity in
place of oxygen in zinc oxide at its crystal form using Density Functional Theory (DFT). S-substituent
amounts in percent go from 0.1% up to 1.0% and we analyze modifications in the crystal properties
such as lattice characteristics, total energy, and gap energy. Lattice parameter c increased slightly as
S-substituent percent increased, lattice parameter a had an opposite behavior because it decreased
as the S-substituent increased and c/a rate had ups and downs but with very slight variation between
consecutive values. Total energy calculations showed an increasing trend at all times and binding
energy showed a decreasing trend at all times as the substituent percent increase but the variation
between consecutive points was small. Gap energy had a decreasing trend with a maximum variation
of 6.57% at 1.0% S-substituent from pristine ZnO. In order to correct the DFT underestimation of
gap energy we applied a correction factor and found a decreasing trend as the substituent percent
increase and observed the highest difference from undoped ZnO was 1.42% at 1.0% S-substituent.
We study the effects on the ZnO structure occurring when moderate S-substituent amounts from
0.1% to 1.0% are used and provide new knowledge to predict if the geometric and electronic structure
changes may be suitable for new applications of ZnO in optoelectronics.