X‐ray, photo‐ESR and optical spectroscopy studies of ZnS_xSe_1−x solid solutions doped with Co

Abstract

PACS 71.55. Gs, 78.70.En, Experimental studies of X-ray photoelectron and Co Lα X-ray emission spectra of the ZnS:Co semiconductor were carried out. It was established that Co ions are in a Co 2+ configuration and that the Co 3d impurity states are localized above the top of the valence band by 1.0 ± 0.2 eV. It was found that the covalency reduction factor for Co 2+ defect changes through the alloy from 0.62 in ZnSe lattice to 0.59 in ZnS.1 Introduction Interest in the study of the electronic structure of wide-gap II-VI semimagnetic semiconductors (ZnS, ZnSe, CdTe, etc.) increased substantially in recent years, motivated both by the construction of ZnSe-based semiconductor lasers operating in the blue-green region of the spectrum and due to ferromagnetism with high Curie temperature. Doping with 3d elements changes the optical properties of these compounds and gives rise to magnetic effects depending on the energetic position of the 3d ion states and their interaction with the host bands. Therefore, it is very important to know the positions of the 3d impurity levels. The positions of the 3d impurity levels relative to the valence band or the conduction band in the II-VI semiconductors have been established by optical absorption measurements [1]. For Mn-doped II-VI semiconductors (ZnS:Mn, ZnSe:Mn, CdTe:Mn), the energy positions of Mn 3d impurity states have been deduced both by means of photoelectron spectroscopy [2,3,4,5] and using a combination of X-ray emission Mn L α spectra and the Mn 2p binding energies determined from X-ray photoelectron spectroscopy experiments [6,7]. It was found that Mn 3d impurity states are localized at 2-3 eV under the valence-band edge [4,5,7]. According to optical data, Co 3d impurity levels should be localized in the forbidden band [1]. Kowalski et al. [8] have carried out measurements of 3p-3d resonant photoemission spectra for a Zn 0.9 Co 0.1 S semiconductor and found that the Co 3d contribution to the valence band has a wide structure with two maxima: at the edge of the valence band and about 4 eV below the edge. In our work [11] we have presented X-ray photoelectron core-level and valence-band spectra of a ZnS:Co semiconductor. We have found that cobalt ions are in a 3d 7 ground-state configuration and that the degree of covalency of a Co-S bond in ZnS:Co is somewhat lower than for CoS.In this work we have carried out a direct observation of the Co 3d impurity levels in ZnS:Co using Xray emission spectroscopy. The combination of the X-ray emission and X-ray photoelectron spectroscopies allowed us to determine the position of the Co 3d states relative to the valence band edges.