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 li
ght in the temperature range typical of the persistent photoconductivi
ty (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 investigat
ion of the isothermal photoionization transients is performed to evide
nce features in the curve not directly related to the phenomenology of
the DX center, the free electron density being influenced by the poss
ible occupancy of other impurity levels. The transients show, in parti
cular, an initial nonexponential behavior which is demonstrated as due
to localization of a fraction of the photoexcited electrons into a no
nmetastable 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 i
n the literature. The analysis gives values of the optical ionization
energy and of the Frank-Condon shift varying in the ranges of 0.84-0.9
5 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.