Effect of Cation Cu/Al Ratio and Surface Proximity on the Electronic Structure and Optical Properties of Cu-Rich Cu_1+<i>x</i>Al_1–<i>x</i>S_2–<i>y</i>: A DFT Study

In this work, we employ density functional theory to investigate the electronic structure and optical properties of both Cu-rich Cu1+xAl1–xS2–y bulk and quantum dot (QD) crystals. The copper deep acceptor, which substitutes Al (CuAl), is dominant under Cu-rich (Al-poor, S-poor) growth conditions, and in QDs, it may form spontaneously. In the presence of copper at interstitial sites, stable acceptor–donor complexes, such as CuAl–2Cui and CuAl–Cui, can be present, which in turn may be a source of p-type conductivity and optical transitions. CuAl reveals a tendency toward the formation of aggregates and Cu acceptor pairing that may decrease empty energy levels. The lowering acceptor states may also be induced by the formation of a large broad band above the valence band maximum that stems from the increasing concentration of host Cu atoms. We find that CuAl, incorporated in the presence of high concentrations of copper in the host structure, gives rise to photoluminescence (PL) transitions at lower energies, in comparison with PL transitions originating from the impurities in a stochiometric crystal. The effect arises as a result of crystal distortion in the vicinity of the dopant: larger distortions occur in higher Cu/Al ratios and under the surface proximity in QDs as a result of stronger energy relaxations.