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Based on the full band electronic structure calculations, first we considerthe effect of n-type doping on the optical absorption and the refractive indexin wurtzite InN and GaN. We identify quite different dielectric response ineither case; while InN shows a significant shift in the absorption edge due ton-type doping, this is masked for GaN due to efficient cancellation of theBurstein-Moss effect by the band gap renormalization. For high doping levelsthe intraband absorption becomes significant in InN. Furthermore, we observethat the free-carrier plasma contribution to refractive index change becomesmore important than both band filling and the band gap renormalization forelectron densities above 10$^{19}$~cm$^{-3}$ in GaN, and 10$^{20}$~cm$^{-3}$ inInN. As a result of the two different characteristics mentioned above, theoverall change in the refractive index due to n-type doping is much higher inInN compared to GaN, which in the former exceeds 4\% for a doping of10$^{19}$~cm$^{-3}$ at 1.55~$\mu$m wavelength. Finally, we consider intrinsicInN under strong photoexcitation which introduces equal density of electron andholes thermalized to their respective band edges. The change in the refractiveindex at 1.55~$\mu$m is observed to be similar to the n-doped case up to acarrier density of 10$^{20}$~cm$^{-3}$. However, in the photoexcited case thisis now accompanied by a strong absorption in this wavelength region due to$\Gamma^v_5 \to \Gamma^v_6$ intravalence band transition. Our findings suggestthat the alloy composition of In$_x$Ga$_{1-x}$N can be optimized in theindium-rich region so as to benefit from high carrier-induced refractive indexchange while operating in the transparency region to minimize the losses. Thesecan have direct implications for InN-containing optical phase modulators andlasers.

Carrier-induced refractive index change and optical absorption in wurtzite InN and GaN: Fullband approach