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InN films were grown on MgO substrates with a β-GaN buffer layer using the gas source molecular beam epitaxy technique. Initially, at typical growth rates from 0.09 to 0.28 ML/sec and at 500 °C substrate temperature, the growth was performed in a layer by layer way as revealed by in situ reflection high-energy electron diffraction (RHEED). In all samples studied, a critical thickness of ∼5 ML in InN pseudomorphic layer was measured with a frame by frame analysis of RHEED patterns recorded on video. After reaching critical thickness, the InN films undergo a relaxation process, going from two-dimensional growth to three-dimensional, as evidenced by the transformation of the RHEED patterns that change from streaky to spotty. Depending on the In cell temperature, either nanocolumnar InN or flat cubic final films are grown, as can be corroborated by scanning electron microscopy. The experimental critical thickness (hc) value of 5 ML is compared to values calculated from different critical thickness models.

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