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

摘要

Based on the full band electronic structure calculations, first we consider the effect of n-type doping on the optical absorption and the refractive index in wurtzite InN and GaN. We identify quite different dielectric response in either case; while InN shows a significant shift in the absorption edge due to n-type doping, this is masked for GaN due to efficient cancellation of the Burstein-Moss effect by the band gap renormalization. Moreover, for high doping levels the intraband absorption becomes significant in InN. For energies below 1 eV, the corresponding shifts in the real parts of the dielectric function for InN and GaN are in opposite directions. Furthermore, we observe that the free-carrier plasma contribution to refractive index change becomes more important than both band filling and the band gap renormalization for electron densities above 10~(19) cm~(-3) in GaN, and 10~(20) cm~(-3) in InN. As a result of the two different characteristics mentioned above, the overall change in the refractive index due to n-type doping is much higher in InN compared to GaN, which in the former exceeds 4% for a doping of 10~(19) cm~(-3) at 1.55 μm wavelength. Finally, we consider intrinsic InN under strong photoexcitation which introduces equal density of electron and holes thermalized to their respective band edges. The change in the refractive index at 1.55 μm is observed to be similar to the n-doped case up to a carrier density of 10~(20) cm~(-3). However, in the photoexcited case this is now accompanied by a strong absorption in this wavelength region due to Γ_5~v→Γ_6~v intravalence band transition. Our findings suggest that the alloy composition of In_xGa_(1-x)N can be optimized in the indium-rich region so as to benefit from high carrier-induced refractive index change while operating in the transparency region to minimize the losses. These can have direct implications for InN-containing optical phase modulators and lasers.