Refractive index tuning with Burstein-Moss effect in Indium Nitrite under photoexcitation
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The band filling effect due to free carriers introduces a shift in the absorption edge, which in turn modifies the refractive index of the medium through the Kramers-Kronig relation. This is known as the Burstein-Moss effect. Based on the full band pseudopotential electronic structure calculations, we demonstrate that Burstein-Moss effect will be crucial in the design of InN based lasers. The primary reason is the small effective mass and the strong nonparabolicity of the conduction band of InN where the shift in the absorption edge is more than 0.5 eV for an electron density of the order of 1019 cm−3. On the other hand, for the case of valence band, the shift in the absorption edge is approximately 0.04 eV. However due to high density of states at the edge of the valence band, also this shift becomes crucial since it opens intraband transitions in the medium. In the case of laser structures, the Burstein-Moss effect in both conduction and valence bands needs to be considered. Furthermore, we take into account the band gap renormalization due to high free carrier concentration. For the case of semiconductor laser structures, which can be also considered as an n-p junction, we predict about 2% change in the refractive index for a wavelength 1.55 µm at an electron-hole density of 1019 cm−3. When we compare photoexcited (i.e., n = p) InN with n-type doped InN, in the former case the intraband transitions in the valence band which is a result of Γv 5 → Γv 6 transition, partially cancels the Burstein-Moss effect. Our findings can also have direct implications for InN based optical modulators.