Optical properties of 3d transition metals in II VI compounds

Measurements are made of linear electroabsorption spectra in the range of the 4A2 4T1(4F) and 4A2 4T1(4P) absorption bands of substitutional Co2+ in CdS and CdSe crystals for electric fields parallel and perpendicular to the hexagonal c-axis, at 77 and 300 K. The observed dependences of these spectra on field and polarization direction can be completely explained on the basis of the theoretical treatment given in the first part of this work. A moment analysis is performed for CdS: Co2+, and this leads to values for the linear Stark splittings and field-induced changes in transition probability. The axial crystalfield splittings of the T, excited levels of CdS: Co2+ are also determined, and their similarity to the external field splittings for F ‖ c is emphasized.



Es werden Messungen der linearen Elektroabsorptionsspektren im Bereich der 4A2 + 4T1(4F)- und 4A2 + 4T1(4P)-Absorptionsbanden von Co2+- Substitutionsstiirstellen in CdS und CdSe fur elektrische Felder parallel und senkrecht zur hexagonalen c-Achse bei 77 und 300 K durchgefuhrt. Die beobachteten Abhangigkeiten von der Feld- und Polarisationsrichtung konnen vollstandig auf der Gruridlage der im ersten Teil der Arbeit gegebenen theoretischen Behandlung erklart werden. Fur CdS: Co2+ werden die Spektren rnit Hilfe der Monientenmethode analysiert. Diese Analyse liefert Werte fur die linearen Stark-Aufspnltungen und fur die feldinduzierten Anderungen der Ubergangswahrscheinlichkeiten. Die Aufspaltnngen der angeregten T1-Niveaus von CdS: Co2+ durch das axiale Kristallfeld werden ebenfalls bestimmt; auf ihre Ahnlichkeit mit den Aufspaltungen durch ein auseres Feld parallel zur c-Achse wird hingewiesen.

Linear electroabsorption due to substitutional deep impurities in wurtzite-type crystals: II. Experimental study of Co2+ in CdS and CdSe†

A theoretical study is made of the first-order effects of an external electric field on optical transitions between strongly localized electronic states of isolated substitutional impurities in a wurtzite-type crystal taking into account the near-tetrahedral symmetry of these centers The field effect on optical absorption bands due to A T type transitions is treated on the basis of the method of moments which is extended to include mixing with the A ground level The field-induced changes of the zeroth and first moment (corresponding to changes in transition probability and to the Stark splitting, respectively) are calculated as functions of field and polarization direction The polarization dependence of the zero-field moments which is due to the axial part of the impurity potential is also discussed On the basis of a rigid-shift approximation a brief semi-phenomenological treatment is given of the field-induced changes in band shape



Die Effekte erster Ordnung eines auseren elektrischen Feldes auf optische Ubergange zwischen stark lokalisierten Elektronenzustanden von isolierten Substitutionsstorstellen in Wurtzitkristallen werden theoretisch untersucht, wobei die angenaherte Tetracder- symmetrie dieser Zentren berucksichtigt wird Der Feldeinflus auf optische A T-Absorptionsbanden wird auf der Basis der Momentenmethode behandelt, die auf den Fall der Mischung mit dem A-Grundzustand verallgemeinert wird Die feldinduzierten Anderungen des nullten und ersten Moments (die Anderungen der Ubergangswahrscheinlichkeit bzw der Stark-Aufspaltung entsprechen), werden als Funktion der Feld-und Polarisationsrichtung berechnet Die Polarisationsabhangigkeit der Nullfeld-Momente, die durch den axialen Anteil des Storstellenpotentials bedingt ist, wird ebenfalls diskutiert Auf der Grundlage der Naherung der starren Verschiebung erfolgt eine halbphanomenologische Behandlung der feldinduzierten hderungen der Bandenform

Linear electroabsorption due to substitutional deep impurities in wurtzite-type crystals: 1. Theoretical analysis for A T absorption bands†

The Stark effect of localized electronic centres is reviewed by using a few selected examples of impurity centres or simple defects. Important information on the symmetry of centres and on the position of energy levels is obtained through the electric-field-induced absorption changes. Typical results are summarized for the Stark effect on the zerophonon lines and on the broad absorption band of point defects in ionic crystals and of impurities in semiconductors.

The stark effect of localized electronic centres

A study is made of the linear (first-harmonic) electro-absorption spectra due to substitutional Ti2+ centres in CdS crystals between 0.5 and 1.5 eV for electric fields parallel to thec-axis. It is found that the signals due to the3A2→3T1(3F) absorption band show the polarization dependence and spectral shape characteristic of the linear Stark effect. In contrast to this, the signal due to the3A2→3T1(3P) transition exhibits an anomalous behaviour, which is attributed to the fact that the excited levels of this centre are very probably in resonance with the conduction band.

J. Gardavsky

Papers

Linear Stark effect in absorption spectrum of CdS:Ni2+