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Intracenter Transitions of Iron-Group Ions in II–VI Semiconductor Matrices

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Intracenter Transitions of Iron-Group Ions in II–VI Semiconductor Matrices Physics of the Solid State, Vol. 44, No. 11, 2002, pp. 2013–2030. Translated from Fizika Tverdogo Tela, Vol. 44, No. 11, 2002, pp. 1921–1939. Original Russian Text Copyright © 2002 by Agekyan. REVIEWS Intracenter Transitions of Iron-Group Ions in II–VI Semiconductor Matrices V. F. Agekyan Institute of Physics, St. Petersburg State University, St. Petersburg, Petrodvorets, 198504 Russia e-mail: [email protected] Received December 27, 2001 Abstract—A review is made of studies of intracenter optical transitions in 3d shells of iron-group divalent (magnetic) ions. Attention is focused on the emission spectra of Mn2+ ions in CdTe, ZnS, and ZnSe crystals. An analysis is made of the structure of intracenter absorption and luminescence and of the effect that the elemental matrix composition, magnetic-ion concentration, temperature, hydrostatic pressure, and structural phase tran- sitions exert on the intracenter transitions. The mutual influence of two electronic excitation relaxation mecha- nisms, interband and intracenter, is considered. The specific features of the intracenter emission of magnetic ions embedded in two-dimensional systems and nanocrystals associated with a variation in sp–d exchange interaction and other factors are discussed. Data on the decay kinetics over the intracenter luminescence band profile are presented as a function of temperature, magnetic ion concentration, and excitation conditions. The saturation of the luminescence and the variation of its kinetic properties under strong optical excitation, which are caused by excitation migration and the cooperative effect, as well as the manifestation of a nonlinearity in intracenter absorption, are studied. © 2002 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION metal, its 3d shell can undergo charge transfer, thus imparting new properties to excitons bound to Cu2+, Crystals and glasses containing iron-group atoms Ni2+, Co2+, and Fe2+ in crystals of the wide-band-gap exhibit intracenter transitions in the unfilled 3d shells of II–VI compounds [2, 3]. DMSs and related quantum these atoms that manifest themselves strongly in structures exhibit specific properties associated with absorption and luminescence. The intracenter emission the large magnetic moment of the unfilled 3d shell, of these ions, particularly of the divalent manganese, 2+ such as giant Zeeman splitting of the electronic levels, has a potential use, because ZnS:Mn is the most effi- giant Faraday rotation, and the magnetic polaron effect. cient electroluminescent phosphor known to date [1]. DMSs reveal two types of electronic excitations and of The main subject of the present review is optically their relaxation, more specifically, the conventional excited intracenter luminescence (IL). While IL has semiconductor interband and the intracenter mecha- been studied on a broad range of objects, most reports 2+ nisms, which can affect each other substantially. The have dealt only with crystals containing Mn . Of most intracenter absorption and emission of light in the 3d interest is the investigation of the II–VI crystals, in 2+ shell of a magnetic ion become manifest in crystals which the relative concentration of the Mn cations with a broad enough band gap. The most popular DMSs can range from a few hundredths of a percent to tens of 2+ a percent. Considered over such a broad range of con- with Mn ions are model systems for studying mag- centrations, the manganese may act both as a paramag- netic and magnetooptical properties and intracenter netic impurity and an activator and as a solid-solution transitions. The application potential of DMSs contain- component capable of affecting the fundamental prop- ing various magnetic impurities lies in the area of pho- erties of a crystal. A large number of studies of the opti- toinduced magnetism, laser gain media, laser shutters, cal properties of II–VI solid solutions with a magnetic electroluminescent devices, Faraday cells, and magne- component and of the related quantum structures have tosensitive low-dimensional quantum structures. recently been reported. These compounds make up a Recently, electroluminescence has been studied in thin class of dilute magnetic semiconductors (DMS) some- epitaxial films and composites containing a polymer times referred to as semimagnetic semiconductors. and manganese-activated crystallites [4, 5]. Investiga- Typically, DMSs exhibit properties basically inherent tion of the properties of intracenter photoluminescence in crystalline solid solutions, such as a dependence of promotes our understanding of the electroluminescence the fundamental parameters on the solid-solution com- processes occurring in bulk crystals and quantum struc- position, inhomogeneous broadening, electron local- tures and shows the path to their optimization. There is ization associated with local compositional fluctua- a variety of reviews and papers that primarily describe tions, etc. If the isoelectronic impurity is a transition the band electronic states of DMSs and the effect of an 1063-7834/02/4411-2013 $22.00 © 2002 MAIK “Nauka/Interperiodica” 2014 AGEKYAN internal field of magnetic ions on them (electron–ion replaced by Dq) is commonly used to measure the crys- ∆ exchange interaction), as well as the interaction of mag- tal field. It is due to the comparatively small value of t netic ions with one another (ion–ion exchange interac- that the tetrahedral complexes in a II–VI matrix are tion) (see, e.g., [6–10]). The present review deals with classed among high-spin, weak-field complexes. In this the optical transitions occurring in the 3d shell of the case, the nearest neighbors (anions) forming the tetra- iron-group divalent ions, mainly of Mn2+, in II–VI crys- hedron do not affect the level diagram of a single man- tals and related nanostructures. Main attention is ganese atom noticeably. The tetrahedral type of the focused on the interaction between various mecha- anion environment also accounts for the small change nisms of electronic-excitation relaxation, on the effect that the fields acting on the Mn2+ ion undergo in the of external factors, elemental composition of the transition of a II–VI crystal from the wurtzite to zinc- matrix, and confinement, as well as on the kinetic prop- blende structure; indeed, the ligand field has the Td erties of the IL. symmetry and there is no inversion center. The level diagram for the Mn2+ ion in a tetrahedral field is pre- sented in Fig. 1, with the energy reckoned from the 2. ENERGY LEVELS AND OPTICAL 6 TRANSITIONS IN THE 3d SHELLS ground state (the spin sextet A1 with spin S = 5/2 and OF IRON-GROUP IONS IN THE II–VI CRYSTAL orbital number L = 0), which is only weakly sensitive to MATRICES the field. The lowest excited states correspond to S = 3/2 and L = 1, 2, 3, 4, the 4G term with L = 4 lying at the The properties of the 3d shell of an iron-group ion in lowest energy. The 4G term splits into four levels in a a crystal and the methods to be used to calculate its tetrahedral field. The T level is the lowest, because its electronic levels are governed by a number of factors. 1 Among these are the electron–electron spin interaction energy decreases considerably with increasing field, V , the spin–orbit interaction V , hybridization of the d whereas the A and E levels respond very weakly to a ss so field variation. Energy level calculations for iron-group states with s and p states of the band electrons, and the ions in crystal matrices (the ligand field) can be found crystal field Vc. The crystal field can be written in the in a number of monographs and papers (see, e.g., form [11, 12]). V s = V o ++V 1 V 2, (1) While the positions of the 3d-shell levels relative to where V is the spherically symmetric part of the field, the band-state extrema are difficult to derive from opti- o cal spectra, the absence of clearly pronounced transi- V1 provides the main contribution to the lowering of the 6 tions from the A1 level to the conduction band in all spherical field symmetry, and V2 contains small terms of an even lower symmetry. In the systems of interest to Mn-doped crystals of the II–VI compounds indicates us here, the strongest is the electron–electron interac- that this level lies below the top of the valence band tion, which corresponds to the case of a weak crystal derived from the chalcogenide p states. Photoelectron field. If the 3d shell is filled to one half or less, the emission spectra of Cd1 – xMnxTe were studied both weak-field complexes retain, in accordance with experimentally and theoretically in [13–18]. Two meth- Hund’s rule, the maximum possible total free-ion spin ods of photoelectron spectroscopy, integrated and moment (5/2 for Mn2+). In high-symmetry crystal angle-resolved, yielded contradictory results for the fields, the electron–electron interaction is described by extent of hybridization of various states. These contra- three Racah parameters, A, B, and C, with the first of dictions were finally removed, and it has been shown them yielding only the general shift of the whole that the observation of a strong photoemission peak 3d-level system. The ligand crystal field acting on the located a few electronvolts below the valence-band top cation in a zinc-blende-type lattice, to which II–VI and corresponding to the 3d-shell contribution to the cubic crystals belong, is tetrahedral. In this field, the d electron density of states is not at odds with a consider- able sp–d hybridization. The energy level correspond- orbitals can be divided in two groups, with d 2 2 , d 2 ()x – y z ing to the removal of one 3d electron from Mn2+ lies belonging to the T2 term and dxy, dxz, dyz, to term E. The 3.5 eV below the valence-band top, whereas that due to tetrahedral and octahedral fields acting on the magnetic the addition of a sixth electron to the 3d shell is located ion located at the cube center are created by the ligands 3.0–3.5 eV above the valence-band top.
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