Optical spectroscopy of 3d7and 3d8impurity configurations in a wide-gap semiconductor (ZnO:Co,Ni,Cu)

Abstract

Four emission bands have been studied at low temperatures with differently doped ZnO crystals. In the ${\mathrm{Co}}^{2+}$(${\mathrm{d}}^7$) ion, two transitions to the ${}_{}{}^4{}_{}{}^{}\mathrm{A}_2^{}{}_{}{}^{}$(F) ground state give rise to new luminescence bands, viz., starting from the ${}_{}{}^4{}_{}{}^{}\mathrm{T}_2^{}{}_{}{}^{}$(F) term (around 3600 ${\mathrm{cm}}^{\mathrm{-}1}$) and from the mixed ${}_{}{}^4{}_{}{}^{}\mathrm{T}_1^{}{}_{}{}^{}$(P)${,}^2$ ${\mathrm{T}}_1$(G), ${}_{}{}^2{}_{}{}^{})$ levels (around 15 100 ${\mathrm{cm}}^{\mathrm{-}1}$). Fine-structure and polarization properties of these transitions agree with conclusions on $C_{3v}$ and spin-orbit splittings drawn from previous absorption data. An emission of ZnO:Ni near 6000 ${\mathrm{cm}}^{\mathrm{-}1}$ is identified as the ${}_{}{}^4{}_{}{}^{}\mathrm{T}_2^{}{}_{}{}^{}$(F)${\to }^4$ ${\mathrm{A}}_2$(F) transition of the ${\mathrm{Ni}}^{3+}$(${\mathrm{d}}^7$) ion in a trigonal environment. While the samples display the ${\mathrm{Ni}}^{2+}$(${\mathrm{d}}^8$) transmission spectrum, a charge-transfer process leads to a donor-type conversion of the center entailing the transient ${\mathrm{Ni}}^{3+}$ state. A similar mechanism is proposed to explain the emission of ZnO:Cu in the 5600–6900-${\mathrm{cm}}^{\mathrm{-}1}$ range. Its fine-structure and polarization behavior can be understood in terms of $T_2$ ${\mathrm{-}}^3$ ${\mathrm{T}}_2$(F) →${\mathrm{A}}_1$ ${\mathrm{-}}^3$ ${\mathrm{T}}_1$(F) transitions of a ${\mathrm{Cu}}^{3+}$(${\mathrm{d}}^8$) ion. Its occurrence is established in a model of one-electron configurations also covering the other known Cu transitions. The excitation spectrum presented is explained by a Tanabe-Sugano type of reasoning for the $d^8$ excited states which are reached in the $d^9$→$d^8$ conversion.