Optical properties of InN with stoichoimetry violation and indium clustering

We demonstrate that nonstoichiometry is one of the main reason of strong deviation of the InN optical gap in the 0.7 – 2 eV range, with N/In 1 corresponding to the lower and higher energies, respectively. The phenomenon is discussed in terms of atomic orbital energies, which are strongly different for indium and nitrogen, therefore both excess atom incorporation and elimination could change the optical gap. We estimate such trends using the approximation of the empirical nearest-neighbor tight binding theory. It is also demonstrated that resonant absorption in In-enriched regions is an additional factor lowering an effective absorption edge. InN is an artificial material, whose thin films are formed by modern high-purity technologies. No bulk crystals of reasonable optical quality exist so far. In spite of recent breakthroughs in its study, debates on the basic properties of InN continue. Among these, is the exceptional variation of its experimental gap energy over a huge spectral range from ~0.7 up to 2 eV. The theoretical band-gap values, being strongly dependent on the model used, are also varied in the wide range. In particular, 2 eV [1], 1.55 eV [2] and ~0.7 eV [3] had been reported. At present the 0.7 eV value is commonly accepted, although the narrowgap InN possesses some unusual properties, such as disagreement with the common cation/anion rule [4], extraordinary position of the universal hydrogen acceptor/donor crossover level [5], superconductivity [6], high carrier mobility in layers with a huge defect density >10