Local electronic structures of the native defects in modulation-doped AlAs/GaAs superlattices

The authors investigate the character of the native defects (antisites, interstitials, vacancies and complexes of these or with impurities) in undoped and n- or p-type modulation-doped AlAs/GaAs superlattices by means of the developed recursion method. The presented electronic structures, and local and partial density of states give a detailed identification of these defects in various types of AlAs/GaAs superlattice. They have calculated the energies of highly localised electronic states and compared the results with the existing theoretical and experimental data. The stable states of the EL2 defect (As_Ga-I_As pair) are studied under the assumption that it has the same atomic configuration as that in the bulk GaAs. With respect to the energy distribution, the electronic occupancy of various defects and of their atomic orbitals are determined; these show the occupied charge states directly. On the basis of these calculations, they find that the ionicity of a normal group III or V atom in AlAs/GaAs superlattices will always be positive when it becomes an interstitial and will be changed with a different type of doping, when it becomes an antisite. By considering the Coulombic nature of a Frenkel pair, some specific complexes between the native defects themselves or with the impurity Si are suggested. Finally, they describe the self-diffusion and the impurity diffusion mechanisms which occur in AlAs/GaAs superlattices with and without doping. Results are compared and contrasted with those of the bulk GaAs materials.