The structural and optical properties of improved quality thick InN films grown by plasma assisted radio frequency molecular beam epitaxy (rf-MBE) on GaN/Al₂O₃ templates were investigated. The InN epilayers, up to 1.1 μm in thickness, were grown either by a single-step mode at 300 ^oC or by a two-step mode, with nucleation at the same temperature and overgrowth at 400 ^oC. The films were characterized by High Resolution Transmission Electron Microscopy (HRTEM), temperature dependent photoluminescence (PL) and Raman spectroscopy. At low temperatures the main PL emission was at 0.8 eV and 0.767 eV for samples with single-step and two-step growth mode, respectively. The improved properties of the two-step grown InN were outlined by the characterisation results. Such films exhibited lower threading dislocation density values, higher by a factor of two PL intensity spectra and sharper Raman peaks compared to single-step films. Both types of InN films exhibit tensile residual strain due to thermal origin and/or structural defects present in the epilayer. The amount of the residual strain deduced by the Raman spectra correlates well with that calculated from electron diffraction analysis through the variation of InN lattice constants. The difference in the main emission peak of the PL spectra between the single-step and the two-step samples by 33 meV provides further evidence of the different residual strain present in InN epilayers.

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