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It has already been shown that there are defects in semiconductors for which the capture of an electron from the conduction band occurs via a band other than the lowest one. We show here that electron emission from such defects also occurs toward this higher band. This is illustrated in the case of the Te-associated DX center in ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As of various alloy compositions by a comparison between ionization energies measured in a situation of thermal equilibrium and during the process of emission. In that case, the barrier associated with electron capture varies with the alloy composition, since it depends directly on the band structure. The barrier height does not take its origin in the large lattice distortion around the defect. This has been verified by comparing the temperature dependence of the barrier height with a theoretical calculation of the capture by multiphonon emission.

We have studied the defects introduced by electron irradiation in n-doped Si-Ge alloys. Capacitance techniques were used to characterize the defects. Four deep levels are detected, labeled S1 to S4. Only the two main defects, S3 and S4, have been studied, the other ones having low concentration. Their associated energy levels lie at ${\mathit{E}}_{\mathit{c}}$-0.32 and ${\mathit{E}}_{\mathit{c}}$-0.49 eV with capture cross sections equal to 6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ and 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$, respectively. These two defects have a donor character. The study of their introduction rate versus the fluence indicates that they are not primary intrinsic defects but complexes. The variation of the S4 concentration versus the energy of irradiation shows that the threshold energy is 31\ifmmode\pm\else\textpm\fi{}2 eV. Finally, a comparison with electron-irradiation-induced defects in silicon and germanium is made.

We analyze electron capture on a defect that exhibits no electron-phonon interaction. In this case, capture occurs by a cascade mechanism via the defect excited states. In direct-band-gap materials, when the temperature is too high to allow the filling of the \ensuremath{\Gamma} excited states at the bottom of conduction band, capture takes place via the excited states of a higher band, provided that they are deep enough. Experimental evidence for capture via the excited states of the L and X bands is provided for several defects or impurities in InP, GaAs, and related alloys.