In this presentation we review the results of our recent studies aimed at understanding the electronic structure and its effects on electrical and optical properties of InN and In-rich In_1-xGa_xN alloys. We have investigated MBE-grown films, unirradiated as well as irradiared with high energy particles. Measurements of the fundamental absorption edge give the value of 0.64 eV for the room temperature energy gap of InN. The low energy gap and a strong kp interaction leads to a small electron effective mass and a distinctly nonparabolic conduction band. In samples with high electron concentrations, a large increase in the absorption edge energy is observed due to the Burstein-Moss shift. In addition to having a narrow gap, InN also has an extremely high electron affinity of 5.8 eV, which places the conduction band edge of this material 0.9 eV below the average energy of dangling bond defects represented by the Fermi level stabilization energy, E_FS. This unusual band alignment explains the extreme proclivity for n-type conduction and the large surface electron accumulation densities in InN and In-rich alloys. As grown, undoped InN is always n-type with electron concentrations ranging from mid 10¹⁷ cm^-3 to as high as 10²¹ cm^-3. A similar range of electron concentrations can be achieved by irradiation with 2 MeV He⁺ ions. At a sufficiently high dose, the electron concentration saturates. In agreement with the amphoteric defect model the saturation concentration n_s, corresponds to Fermi energy pinned at E_FS. In In_1-xGa_xN the conduction band shifts towards the E_FS resulting in reduced n_s with increasing x. For x>0.66 E_FS moves below the conduction band edge and acceptor like defects are preferentially formed, leading to a decrease of electron concentration in irradiated samples. Photoluminescence results show that samples with x<0.6 are two orders of magnitude less sensitive to irradiation damage than standard III-V compounds.

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