The DX Center: Evidence for Charge Capture Via an Excited Intermediate State
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TlIF T)X CENTER: EVIDENCE FOR CHlARGE CAPTURE VIA AN EXCITEI) INTERMEDIATE STATE Thomas N. Theis and Patricia M. Mooney IBM Research, T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598
ABSTRACT We review three important experimental results which suggest that electron capture and emission by the DX center in A1IGa 1 .5 As proceeds via an excited intermediate state: the very different dependencies of the thermal capture and emission rates on alloy composition, the exponential dependence of the thermal capture rate on the quasi-equilibrium Fermi energy, and the thermal activation of the hot electron capture rate. None of these results is readily explained by a conventional lattice relaxation model, in which an electron is captured directly from the lowest lying band edge, but each can be simply explained if the dominant channel for multiphonon capture is via a transition state which lies well above the band edge. This picture is consistent with recent pseudopotential calculations which predict that the lattice relaxed state (the DX state) is stabilized by capture of more than one electron, since such a model naturally admits the possibility of an intermediate one-electron state.
INTRODUCTION Calculating the spectrum of electronic states of a substitutional donor is an old, but still unfinished problem in semiconductor physics. The hydrogenic states, bound by the long range Coulomb potential, are well predicted by effective mass theory. llighly localized or deep states, bound by the short range ion core potential, are predicted by tight binding theory.[I] The energy of such deep states relative to the spectrum of hydrogenic states depends sensitively on the balance between the long and short range potentials, and calculational techniques suitable for the hydrogenic spectrum are unsuitable for localized states, and vice versa. Whether the ground state of the spectrum is hydrogenic or deep, the wave functions of deep and hydrogenic states will exhibit large spatial overlap. Excited states should therefore be relatively short-lived, decaying radiatively to the ground state. In contrast to this view, substitutional donors in some compound semiconductors are now believed to exhibit bistability. The most studied example is the DX center[2] in GaAs and A 5G(ia 1_ As, which captures electrons either to hydrogenic states,[3-5] or to a deep state[2] which we shall refer to as the DX state. Depending on the donor species, alloy composition, local alloy environment, and hydrostatic pressure, the ground state may be either hydrogenic or deep. The hydrogenic states are separated from the deep state by an energetic barrier, so that transitions between them are unobservably slow at low temperatures. This behavior implies a strong suppression of energetically allowed radiative transitions which is attributed to a large lattice relaxation associated with electron capture to the deep (I)X) levcl.[2] A configuration coordinate diagram, appropriate for the I)X center in direct gap A1 5(ia 1 5xAs is shown in figure 1.
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