The explanation of InN bandgap discrepancy based on experiments and first-principle calculations

Indium nitride (InN) films with different free electron concentration and optical bandgap were grown either directly on sapphire substrate or on pre-covered gallium nitride (GaN) buffer through metal-organic chemical vapor deposition (MOCVD) method. Based on first-principle calculations, we confirm that the widening of InN optical bandgap reported before is caused by high density of free electrons. To find the contributor of the free electrons, the characteristic energetic levels of O_N, V_N and Si_In are investigated. We find that they are all high enough to uplift the optical bandgap from about 0.78 eV to 1.9 eV, which almost can't be enlarged further when it reaches 2.09 eV.