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Abstract:
In gasphase-grown p-type ZnTe crystals, the substitutional Cr impurity is detected in the
neutral (Cr2+) and both ionized states Cr3+ and Cr+. While these three oxidation states
are now identified by EPR, two of them emerge in the optical spectra as well. Optical
irradiation of the samples at low temperatures influences the relative concentrations of
these species. By monitoring the respective EPR or optical signals, thresholds for photoionization
and photo-neutralization processes are derived. In addition to the strong
5E(D)→5T2(D) optical absorption of Cr2+(d4), the samples display a luminescence band near 4500 cm-1, structured by Jahn—Teller interaction. For the first time, a no-phonon
line at 4989 cm-1 is resolved, accompanied by a TA(L) satellite and some local-mode structure. In EPR, the angular dependence of the ∆M = ±2 transition characteristic for Cr2+(d4) ions on lattice sites indicates the 5E(D) ground state subject to a static Jahn-Teller effect in the three (100) directions. Whereas an internal optical transition of Cr3+(d3) could not be identified, in EPR, an isotropie signal at g=3.3 discloses a strongly Jahn—Teller-disturbed 4T1(F) ground state. On irradiation with band-tail photons, the ionized acceptor state Cr+(d5) becomes detectable by EPR. Furthermore, an emission with
an onset near 10,300 cm-1 is recorded for the first time. Its excitation and optical stimulation spectra are studied in wide spectral ranges; the exponential decay with ζ≅1 ms supports an assignment to a spin-forbidden transition. The acceptor ionization energy is
determined as 10,500 cm-1 (1.3 eV).
