On the negative Hubbard correlation energy of the DX center in In-doped CdMnTe

Abstract

Persistent photoeffects have been investigated in indium doped Cd1−xMnxTe of manganese content x=0.1, by means of photocapacitance and photoconductivity transient measurements run at 77 K. The transients are superpositions of two exponents with short and long time constants. The two exponents have been attributed to the two-stage ionization of two energy states of the DX centers present in the material. A detailed analysis of photoionization kinetics leads to the conclusion that the DX center which is responsible for the observed persistent photoeffects possesses negative effective Hubbard correlation energy. Thus the “fast” component of the phototransients corresponds to the ionization of the two-electron ground state of the DX center to an intermediate one-electron state. The “slow” component is a result of the photoionization of the electron from this state into the conduction band. The value of the optical ionization energy for the first transition is equal to E20=0.85 eV. Photoionization of an electron from the intermediate state to the conduction band requires energy E10=0.77 eV. It was also found that both states are localized and strongly coupled to the lattice as large numbers of phonons take part in the photoionization in both cases.