Carrier-induced refractive index change and optical absorption in wurtzite InN and GaN: Fullband approach
arXiv:0911.2312 · doi:10.1103/PhysRevB.81.155206
Abstract
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. For high doping levels the intraband absorption becomes significant in InN. 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~cm in GaN, and 10~cm 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~cm at 1.55~m 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~m is observed to be similar to the n-doped case up to a carrier density of 10~cm. However, in the photoexcited case this is now accompanied by a strong absorption in this wavelength region due to intravalence band transition. Our findings suggest that the alloy composition of InGaN 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.
Revised with an appendix, two additional figures, and more discussions; 10 pages, 14 figures; published version