Effect of Fermi Level Motion on ESR and Optical Properties of CuAlS₂

Abstract

Optical absorption and electron spin resonance (ESR) spectra have been studied in as-grown, Cr-doped, and annealed in different atmospheres CuAlS2 crystals. It has been found that the major influence on the optical and ESR properties of CuAlS2 crystals, obtained by chemical vapour transport, arises from the transition ions Fe3+ and Cr2+, which are present in the crystal lattice as residual impurities. Thermal treated crystals exhibit drastic changes in their absorption and ESR spectra depending on the annealing atmosphere, which is explained by the process of the Fermi level motion in the vicinity of the Fe2+/Fe3+ and Cr+/Cr2+ demarcation levels. The results obtained show that the valence states of Fe and Cr ions and their relative amounts in each valence state are controlled by the position of the Fermi level in the band gap of the host crystal, and are stoichiometry on dependent.

Low-temperature electron-paramagnetic-resonance study of extrinsic and intrinsic defects inCuGaSe2

“…8 Even the position of the Fermi level could qualitatively be followed by the observation of differently prepared samples. 27 In the course of these investigations, the position of the Fe 2ϩ /Fe 3ϩ demarcation level in CuInSe 2 was identified close to the middle of the gap. 25 Having these results in mind, the observation of Fe 2ϩ by EPR in CuGaSe 2 is consistent with previous investigations, but appears surprising at the same time.…”

“…13 and 25͒ and CuInS 2 ͑Ref. 16͒ Fe 2ϩ could be identified in vacuum-annealed material, while in as-grown material iron contaminations were observed 5,6,8,12,[26][27][28] as three-valent Fe 3ϩ . The effect may qualitatively be understood from a shift of the Fermi-level energy E F by the annealing procedure causing a selective loss of the high vapor-pressure component of the compound, being sulfur or selenium in the cases considered here.…”

“…Nickel and iron contaminants in other semiconducting chalcopyrites were found to occur with varying valencies. [7][8][9]13,16,27 In the case of nickel both paramagnetic ions Ni ϩ and Ni 3ϩ and in the case of iron Fe 2ϩ and Fe 3ϩ have been identified and especially in CuGaS 2 their relative concentration was realized to monitor stoichiometry deviations. 8 Even the position of the Fermi level could qualitatively be followed by the observation of differently prepared samples.…”

Chemistry of defect induced photoluminescence in chalcopyrites: The case of CuAlS2

“…8 For producing such devices it is desirable to understand the nature of the PL emissions, which are strongly linked with the defect chemistry of CuAlS 2 . So far, to the best of the authors knowledge, although there have been a number of experimental studies, 2,6,[9][10][11][12] there are no theoretical studies on the defect chemistry of CuAlS 2 . NBEPL emissions have been observed in CuAlS 2 powder 6 and epilayers 4 within the 15 K−298 K temperature range.…”

Evaluation of the efficiency potential of intermediate band solar cells based on thin-film chalcopyrite materials

“…Other works have addressed the issue of incorporating transition and rareearth elements in different chalcopyrite compounds for photovoltaic, magnetic, and spintronic applications, both at theoretical [28][29][30] and experimental levels. [31][32][33][34][35][36] The reader is referred to these works and to the references therein for details on issues related to the solubility ranges of impurities, defect formation energies, and their impact on the electronic structure of the host compounds, not treated in further detail here.…”