Untersuchung von Umladungsprozessen der Chromionen in ZnSe-Einkristallen

Abstract

An investigation is made of the recharging processes of ions of chromium inserted by substitution on zinc sites in ZnSe single crystals. Opposed are corresponding spectra of the photoconductivity against spectra of excitation and quenching of the Cr+ ‐EPR signal. Supplementary statements are obtained from optical and electrical measurements. With the exception of the optical ionization of the Cr+ ‐centres three typical mechanisms of recharging are confirmed by the measurements. The energy of the unoccupied Cr|Zn|′ ‐term is determined to 2.45 eV above the top of valence band. For the occupied Cr|Zn|′ ‐term it is found, that the corresponding energy exceeds not a maximum distance of 0.9 eV below the conduction band edge.

Identification of near-infrared luminescence in ZnSe

“…Under intensive additional irradiation including the charge-transfer process (2), however, an absorption can be detected which is probably inverse to the NIR PL (graph C of figure 8). This photoinduced transition decays exponentially with a time constant of approximately 320 s, which is comparable with the decay time observed in photo-EPR experiments of ZnSe:Cr + [13]. The absorption is only detectable after creation of a sufficiently high concentration of Cr + centres.…”

“…In PLE spectra, broad bands just below the band gap are usually caused by charge-transfer reactions. From the comparison with photo-EPR experiments [13,14], the main PLE band in both spectra can be connected to the acceptor process Cr 2+ 5 T 2 (D) + 16 900 cm −1 → Cr + 6 A 1 (S) + e + V B (2) where e + V B denotes a hole in the valence band. The radiative transitions within Cr 2+ are obviously much more efficiently pumped by an intermediate creation of a transient Cr + state (reaction (2)) than by internal Cr 2+ transitions.…”

“…Second, the decay time of the luminescence (τ = 1.8 ms [9]) is comparable with that of the Mn 2+ emission in different host crystals. Third, the excitation (PLE) spectrum corresponds to the charge-transfer band of Cr 2+ → Cr + transitions known from photo-EPR experiments [13,14], as indicated by a coinciding threshold value (about 16 900 cm −1 ) and maximum (around 19 000 cm −1 ). Fourth, an optically detected magnetic resonance (ODMR) attributed to Cr + was found [10] possessing the same spectral excitation characteristics as the PLE spectrum.…”