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We study the surface charge accumulation on InN thin films that strongly effects mobility of charge carriers. The films are formed by MBE in the temperature range (400–470 °C) yielding films with different morphology, crystallinity, and optical properties. The band-gap values determined by optical absorption and PL studies are found to depend on the Hall carrier concentration as per the Moss-Burstein relation. The magnitude of the near surface band bending is measured on InN thin films by knowing the surface and bulk Fermi level position with respect to the valence band maximum using X-ray photoelectron spectroscopy and optical absorption measurements, respectively. The studies show that the surface charge occurs due to the In adlayer on the film causing band-bending is up to 1 eV for low band gap single crystalline films and nearly a flat band for the highly degenerate polycrystalline films. The absence of the band bending is seen to occur when the bulk carrier concentration related to the crystalline quality of the InN films, tends to equate with the surface charge density.

The issue of ambiguous values of the band gap (0.6 to 2 eV) of InN thin film in literature has been addressed by a careful experiment. We have grown wurtzite InN films by PA-MBE simultaneously on differently modified c-plane sapphire substrates and characterized by complementary structural and chemical probes. Our studies discount Mie resonances caused by metallic In segregation at grain boundaries as the reason for low band gap values (≈ 0.6 eV) and also the formation of Indium oxides and oxynitrides as the cause for high band gap value (≈ 2.0 eV). It is observed that polycrystallinity arising from azimuthal miss-orientation of c-oriented wurtzite InN crystals increases the carrier concentration and the band gap values. We have reviewed the band gap, carrier concentration, and effective mass of InN in literature and our own measurements, which show that the Moss-Burstein relation with a non-parabolic conduction band accounts for the observed variation of band gap with carrier concentration.

We address the carrier concentration, strain, and bandgap issue of InN films grown on c-sapphire at different N-flux by molecular beam epitaxy using x-ray diffraction and x-ray photoelectron spectroscopy. We demonstrate that the strain in InN films arises due to point defects like nitrogen interstitials and nitrogen antisites. We report minimal biaxial strain due to relaxed growth morphology and a minimal hydrostatic strain arising due to interstitial nitrogen atoms being partially compensated by nitrogen antisites. We find that the variation in absorption edge can be attributed to defect induced carrier concentration and that nitrogen interstitials and nitrogen antisites act as donors that yield the respective absorption edge and Moss-Burstein shift. Our studies are a step towards the ability to form low carrier concentration strain-relaxed films and to determine the intrinsic band gap value for this technologically important material.

The issue of variable bandgap values for InN films grown on c-sapphire has been addressed in this work. α-InN films have been deposited in nitrogen rich condition at different substrate temperatures on bare Al2O3 (0001) by plasma assisted molecular beam epitaxy (PA-MBE). The results of several complementary characterization techniques show that single crystalline wurtzite InN is formed, but their orientation depends on the substrate temperature. The bandgap measured on these samples (1.78 eV) is explained by Moss-Burstein shift on these degenerately n-doped samples. Our results discount effects of crystal orientation, presence of oxygen at grain boundaries and In/N stoichiometry as factors that influence the measured band-gap values.