Progress in Understanding the Optical Properties of EL 2
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PROGRESS IN UNDERSTANDING THE OPTICAL PROPERTIES OF EL2 G. A. BARAFF A.T.&T. Bell Laboratories, Murray Hill, N.J., 07974
ABSTRACT The material covered in the talk given under this title has already been published [1] or has been accepted for publication [2]. For this reason, the material to be presented below will consist only of a summary with references.
SUMMARY Although there is still not universal agreement on the question of whether the EL2 defect consists of an isolated AsGa antisite or whether, in addition, an Asi arsenic intersitial atom is an essential part of its structure, [3] the experimentally observed optical absorption gives no evidence of anything except the isolated antisite. The main features of the optical absorption spectrum can be broadly understood in terms of a combination of three processes, namely, transfer of an electron from the occupied midgap level to the conduction bands, transfer of an electron from the valence band to the midgap level when it is not occupied, and an internal transition in which an electron in the occupied midgap level is excited to a long lived resonance which, for most practical purposes, behaves as a truly localized state. In particular, this resonance has T 2 symmetry, is three-fold degenerate, and its occupation by an electron causes a symmetry lowering Jahn-Teller relaxation. Occupation of this resonant state is the trigger that, with finite but small probability, causes the defect to transfer to its metastable configuration. [4] There are, however, problems with the detailed understanding of the internal optical transition. These have to do with aspects relating to lattice relaxation. In particular, although the broad shape of the absorption peak certainly seems to be indicative of lattice relaxation, and although there is what appears to be a zero-phonon line (ZPL) at its lower limit, the intensity of the ZPL and the energy of the observed multi-phonon replicas are not compatible with the width of the line. Furthermore, under hydrostatic pressure, the peak of the absorption and the ZPL move in opposite directions. [5] Although this is not impossible to rationalize, it is not what one would expect. Most difficult to explain is the fact that under hydrostatic pressure, the ZPL is observed to ride up over the onset of the broad absorption. [5] This by itself indicates that there is a component of the broad peak that has nothing to do with lattice relaxation, since the ZPL must lie at the very lower limit of any lattice relaxed absorption. Motivated by these problems, we have undertaken a more detailed look at the information that can be obtained from these hydrostatic pressure experiments [5,6] and from the experiments in which uniaxial pressure was used to split the ZPL.[3a] We have found [2] that the observed lattice relaxation must be assigned to two different symmetries of lattice relaxation, with the Jahn-Teller relaxation contributing approximately 25% of the total and a breathing relaxation possibly accounting for the rest. The estimate of the breathing
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