Ambiguity introduced by the strong local electric field induced by the polarization charge density and the background doping strongly disturbs the optical properties of nitrides. The detailed model of the absorption and emission spectra in the vicinity of the fundamental gap is therefore of fundamental importance in studying such systems. Systematic studies were conducted to determine an influence of change of the well/barrier width ratio on the physical properties of AlN/GaN superlattice system, such as the electric field, polarization charges and polarization dipoles. To overcome the Kohn-Sham band gap (BG) underestimation of experimentally measured values, we have applied two different approaches i.e. empirical LDA+U correction method [1] and a method proposed recently by Ferreira et al., called LDA-1/2 which approximately includes the self-energy of excitations in semi-conductors, providing BG energies, effective masses, and band structures in very good agreement with experimental bulk properties [2]. The results obtained within this two approaches were compared. It was shown that the electric fields depend critically on the well-barrier thickness ratio. The overlap of wavefunctions of electrons and holes is calculated, showing considerable separation in space which significantly reduces the oscillator strength of the optical transitions, and consequently, the optical efficiency of nitride based light emitting diodes (LED) and laser diodes (LD). We have determined the dependence of the absorption spectra edge on the type of structure and also on the carrier concentration. Blue shift of the emission peak has been observed when comparing structures with higher dopants concentration.

[1] A. I. Liechtenstein, V. I. Anisimov and J. Zaane, Phys. Rev. B 52, R5467 (1995). 

[2] L.G. Ferreira, M. Marques, and L.K. Teles 78(12), 125116 (2008).

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