Bandgap bowing in crystalline (ZnO)1−x(GaN)x thin films; influence of composition and structural properties

Thin films of (ZnO)1−x(GaN)x with an optical bandgap between 3.3 and ∼2.4 eV at room temperature for 0 ≤ x ≤ 0.2 have been grown by magnetron sputtering on (0001)-sapphire substrates. A strong dependence of the bandgap shrinkage on x for low values (x ≤ 0.07) not previously reported enables bandgap-tuned films of high crystal quality with limited distortion of the ZnO matrix suitable for optoelectronic applications. X-ray diffraction, transmission electron microscopy and Rutherford backscattering spectrometry all show a single phase and highly crystalline films. The a- and c-lattice constants are found to be 3.252 and 5.224 for x = 0.15, i.e. larger than that predicted by Vegard's law. The films are highly c-axis oriented and exhibit a good heteroepitaxial relationship with the sapphire substrate, growing predominantly with aligned domains. A distinct shift in optical absorption is attributed to a shift in the bandgap, where compositions of x = 0.07 to x = 0.2 all result in bandgaps around or below 2.5 eV, while a lower GaN content increases the optical bandgap towards that of ZnO. This strong dependence of the bandgap shrinkage on x for low values (x ≤ 0.07) enables bandgap-tuned films of high crystal quality with limited distortion of the ZnO matrix.