Photoionization cross-section of the DX center in Te-doped AlxGa1-xSb

The photoionization of DX centers in Te-doped AlxGa1-xSb layers grown by molecular beam epitaxy is investigated by measuring the increase of the Hall free electron density after illumination by monochromatic light in the temperature range typical of the persistent photoconductivity (PPC) effect. The investigated samples have AlSb molar fractions in the 0.3 less than or equal to x less than or equal to 0.5 range and n-type doping in the 10(17)-10(18) cm(-3) range. An accurate investigation of the isothermal photoionization transients is performed to evidence features in the curve not directly related to the phenomenology of the DX center, the free electron density being influenced by the possible occupancy of other impurity levels. The transients show, in particular, an initial nonexponential behavior which is demonstrated as due to localization of a fraction of the photoexcited electrons into a nonmetastable impurity state which is responsible for the semiconductor-to-metal transition observed under the PPC regime. When this effect is accounted for, the dependence of the photoionization cross section of the DX center on the photon energy was obtained from the analysis of the linear part of the transients and analyzed through a model given in the literature. The analysis gives values of the optical ionization energy and of the Frank-Condon shift varying in the ranges of 0.84-0.95 and 0.70-0.74 eV, respectively, depending on the alloy composition. This confirms a large lattice relaxation for the DX center related to the Te-impurity in AlxGa1-xSb. (C) 1998 American Institute of Physics.