Energy gap, excitonic, and ‘‘internal’’Mn2+optical transition in Mn-based II-VI diluted magnetic semiconductors

The piezomodulated and the photomodulated reflectivity spectra of Mn-based diluted magnetic semiconductors (${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$Te,${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$ Te,${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$Se,${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$Se) reveal sharp signatures associated with the excitonic transitions near the band gap. The dependence of the excitonic energy on the manganese concentration (x), on temperature (T), and, in the wurtzite structure, on the polarization with respect to the optic axis, are determined from the modulation experiments. The excitonic energy in the ‘‘hypothetical’’ tetrahedrally coordinated MnTe and MnSe, viewed as the end members of the alloy systems, are deduced from the x dependence of the excitonic transition. A signature associated with ${\mathrm{Mn}}^{2+}$ is observed in piezomodulated ‘‘pseudoreflection,’’ when the energy gap is sufficiently large; its energy is insensitive to x. The absence of the Zeeman effect for the ${\mathrm{Mn}}^{2+}$ signature, in contrast to the large Zeeman effect of the excitonic feature, demonstrates that the former is an optical transition associated with the 3$d^5$ shell of ${\mathrm{Mn}}^{2+}$.