X‐Ray Investigation of the Thermal Expansion Coefficient of Cubic Gallium Nitride on 3C‐SiC (001)/Si (001) Substrates <jats:p> This work investigates the temperature dependence of the lattice constant <jats:italic>a</jats:italic> <jats:sub>exp</jats:sub> of cubic GaN/3C‐SiC/Si (001) epilayers grown at 740°C by plasma‐assisted molecular beam epitaxy is investigated. High resolution X‐ray diffraction is performed to determine the lattice constant, using an Anton–Paar DHS1100 stage to vary the sample temperature from 25°C to 900°C, calibrated against the underlying single‐crystalline silicon substrate. A linear increase in <jats:italic>a</jats:italic> <jats:sub>exp</jats:sub> with rising temperature is observed. The thermal expansion behaviour is modelled using Debye´s phonon dispersion. The fitted lattice parameters are used to calculate the thermal expansion coefficient (TEC). At room temperature the TEC is determined to be <jats:italic>α</jats:italic> <jats:sub>Debye </jats:sub> ≈ 5.25 × 10 <jats:sup>−6</jats:sup>  K <jats:sup>−1</jats:sup> . We further compare the TEC of the cubic GaN epilayer to that of free‐standing hexagonal GaN using the crystallographic relationship of , demonstrating good agreement between both phases. Using literature values for the elastic constants of cubic GaN, the corresponding elastic moduli and Debye temperature Θ <jats:sub>D</jats:sub> are calculated. An average value of Θ <jats:sub>D</jats:sub> of ≈905 ± 25 K is obtained, which is very close to our experimental results. Moreover, tensile strain is found to be present in our sample at room temperature, leading to an increase in the TEC. The impact of strain on the thermal properties of cubic GaN is discussed. </jats:p> 263 2 Wiley