Theory of DX Centers in Al x Ga 1-x Alloys
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THEORY OF DX CENTERS IN AIXGal-X ALLOYS D. J. Chadi and S. B. Zhang Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, California 94304
ABSTRACT A theoretical model for DX centers which explains their unusual electronic properties in terms of two distinct bonding configurations for donor impurities in AIXGajxAs alloys is examined. The results of our ab initio self-consistent pseudopotential calculations show that for x > =20%, the normal fourfold coordinated substitutional site becomes unstable with respect to a large lattice distortion. The model explains the large difference between the thermal and optical ionization energies of DX centers. A successful theoretical model for the so called DX centers [1-21 in Al.Gai_•As alloys must explain the following key features of these defect centers: (1) The existence of two states for donors [3-61, the first a delocalized effective-mass type state whose energy follows the conduction band minimum as a function of alloying, the second a very localized state which does not exactly follow any of the conduction band minima [7]; (2) The metastability of the localized with respect to the delocalized state in GaAs and the reverse situation in AIXGal-xAs alloys with x>22% [4]; (3) A repulsive energy barrier for the conversion of each state to the other [1-2]; (4) A large Stokes shift of about 1 eV between the optical and thermal ionization energies of the localized state [1-2,8]; and (5) The conversion of the localized state to the delocalized one at a temperature close to absolute zero through photoexcitation [9]. The theory must be able to give a proper account of other experimental observations such as the pressure, alloy, and impurity concentration dependence of the emission and capture barriers and of the thermal and optical excitation energies. Over the past year we have developed such a theoretical model for the DX center [10] and we have now extended our previous first-principles approach for GaAs to AIxGa_• As alloys [11]. Our theoretical results put our model for DX centers in the large-lattice-relaxation (LLR) category. We believe that such a LLR model is essential in explaining the large differences between the thermal and optical ionization energies of DX centers [1-2,8]. The primary result of our calculations is that donor impurities in Al.Ga 1 As alloys possess two distinct bonding configurations and the
Mat. Res. Soc. Symp. Proc. Vol. 163.
1990 Materials Research Society
766
energy difference between these states changes sign as the Al concentration is increased. For a group IV dopant such as Si, the two structural states are shown in Fig. 1. Independent of x, the normal fourfold coordinated substitutional site in Fig. 1 (a) is found to be stable when the donor is in a neutral or positively charged state. The configuration in Fig. 1 (b) in which the Si atom has moved into an interstitial position (thereby breaking a bond with a nearest-neighbor As atom) is found to occur only when the donor is negatively charged. We have examined [10,11], there
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