The Symmetry of the EL2 Defect in GaAs
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THE SYMMETRY OF THE EL2 DEFECT IN GaAs P. TRAUTMAN AND J.M. BARANOWSKI Institute of Experimental Physics, Warsaw University, Hoa 69, 00-681 Warszawa, Poland ABSTRACT Linear dichroism has been measured in the broad absorption band of the EL2 defect in GaAs under uniaxial stress. In addition, the splittings of the EL2 zero phonon line (ZPL) at 8378 cm- 1 under uniaxial stress applied along [100], [111], and [110] directions have been measured. Splitting of the ZPL under [100] stress is over one order of magnitude smaller than under [111] stress, on the other hand, the linear dichroism in the broad absorption band is roughly equal for these two directions of stress. This is an evidence for the quenching of the coupling to tetragonal strains due to interaction with trigonal modes of the lattice (the Ham effect). Therefore, it is established, that the excited T2 state of EL2 is a localized state subject to dynamical Jahn-Teller coupling to trigonal modes of the lattice. The possibility that the excited T2 state has hydrogenic nature associated with the L minima is ruled out by the present results. The observed splittings of the ZPL together with polarization selection rules clearly indicate the tetrahedral Td symmetry of the EL2 defect ruling out any other point group in particular trigonal C3v. In view of the presented experimental results, their interpretation, and recent theoretical investigations, the isolated arsenic antisite AsGa most successfully accounts for the properties of the neutral charge state of the EL2 defect. INTRODUCTION The EL2 defect in GaAs is currently intensively studied because of its metastability, technological importance, and native origin. Microscopic nature of this defect is still a matter of controversy. From measurements of the splittings of the zero phonon line (ZPL) of EL2 at 8378 cm- 1 under uniaxial stress Kaminska, Skowronski, and Kuszko [1] have deduced that EL2 is a isolated defect of tetrahedral Td symmetry. Combining this result with the technological data on dependence of EL2 concentration on crystal growth conditions, they have concluded that EL2 is the arsenic antisite ASGa. On the basis of electron paramagnetic resonance and deep level transient spectroscopy investigations performed on GaAs samples subjected to electron irradiation and heat treatments, von Bardeleben at al. [2,3] have suggested that EL2 is a complex of AsGa and arsenic interstitial Asi. This suggestion has found strong experimental support in optically detected electron nuclear double resonance (ODENDOR) measurements [4,5] which have indicated that EL2 is a ASGa-Asi axial complex. The validity of the interpretation of the piezospectroscopic experiment of Ref.1 has been questioned by Figielski and Wosinski [6]. They have argued that the observed splitting patterns can be explained assuming that EL2 is a center of orthorhombic I C2 v symmetry. In a recent paper [7], on the basis of independent uniaxial stress experiment, we have confirmed that the ZPL of EL2 is due to the A->T electric dipole transition. Theref
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