Investigation Of Electronic Structures And Optical Properties Of â…¢A And Cu Elements Doped Wurtzite CdS

Due to having excellent photoelectric properties and potencial applications, II-VIsemiconductor material CdS has attracted considerable interest and attention.Undoped CdS generally shows the n-type conductivity and its conductivity isextremely low. The experimental results show that properly doping IIIA (B, Al, Ga, In)can be improved its conductivity and help to obtain ideal n-type CdS semiconductormaterial. With Cu can be realized in a p-type doped CdS. Employing the local densityapproximation based on the density functional theory, we have studied systematicallythe electronic strcture and optical properties of B, Al, Ga, In and Cu doped wurtziteCdS.First, we have calculated systematically the electronic structures and opticalproperties of undoped and IIIA-doped wurtzite CdS. When one impurity atom (B, Al,Ga, or In) is introduced into the supercell of CdS, the Fermi level moves upward intothe conduction band from the band gap of undoped CdS. The relatively largecontribution of the partial DOS of the cations to the total DOS near the Fermi level isobserved in each of the doped cases, especially the s states of these cations. Thecalculated energy differences between the Fermi level and the maximum of thevalence band for the group-IIIA doped CdS are bigger than the band gap of theundoped one, being consistent with the absorption edge blueshift for the CdS filmswith group-IIIA element doping. Relative weak absorptions in the visible range of theelectromagnetic spectrum are observed for group-IIIA doped CdS, especially for thepolarization vectors perpendicular to the c axis. Our calculations provide reasonableexplanations to the experimental observations of the greatly decrease of resistivity ofCdS while retaining high transparency with the doping of group-IIIA element. It alsoillustrates that Al, Ga, or In doped CdS could be promising window materials for solarcells.Secondly, we have also investigated the electronic structures and opticalproperties of different concentrations Cu-doped wurtzite CdS. Compared to undopedCdS, the band structure and DOS of the CdS:Cu illustrate that the Fermi level movesdownward into the valence band from the band gap of undoped CdS. And the top of valence band appear extra carriers-holes, explaining introduced the acceptor levelsnear the Fermi level. The relatively large DOS of CdS:Cu integration calculations ofabove Fermi level and below valence band, and the relatively large contribution of thepartial DOS of Cu to the Total DOS near the Fermi level show that the introduction ofCu make the hole concentration of CdS increase. As the Cu doping concentrationincreasing, the DOS of between Fermi level and the top of valence band also increase,indicating that the larger the percentage of Cu in CdS is, then the more obvious thep-type characteristic of CdS:Cu is. Furthermore, we found that the bands of near thetop of valence band are mainly formed by highly localized sulfur p states and copper dstates, that is to say, the holes mass of CdS:Cu become bigger than undoped CdS,indicating that it is not conductive to improve the conductivity of CdS with the dopingCu. Our calculations manifest that doping Cu is very likely to attain p-type CdS, butmay be not a ideal p-type doping element in terms of improving its conductivity.