InN (0001) films were grown heteroepitaxially on GaN/Al₂O₃ by radiofrequency plasma assisted molecular beam epitaxy (RF-MBE). The growth conditions and the layers’ thicknesses were systematically varied resulting in different strain levels in the films. High resolution x-ray diffraction (HR-XRD) was employed to study their structural properties.
Films’ lattice parameters were measured using the extended Bond method for the (0004) symmetric and (10-15) asymmetric diffractions. The c versus a relation exhibited a linear behavior which is typical for the case of biaxial strain present in the heteroepitaxial films. The biaxial strain is attributed to the differences between the in-plane lattice parameters and between the thermal expansion coefficients of GaN and InN.
The values of c- and a- lattice parameters varied from 5.706 Å to 5.686 Å and from 3.521 Å to 3.545 Å correspondingly. In the case of films with lower c/a ratio cracks or microcracks were observed, confirming the presence of tensile strain. From the data the value of Poisson’s ratio for InN was determined equal to 0.42+-0.05.

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