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Highlights • GaN-based THz-QCL active region was grown by MOCVD on Si substrate. • A GaN/AlGaN QC active layer consisting of two-QW for one-period was grown. • Samples were evaluated by HR-XRD and cross-sectional TEM. • FWHM of the XRD satellite peaks became narrower when growth temperature decreased. Abstract GaN/AlGaN THz quantum-cascade laser (QCL) structures were grown on AlGaN/Si templates by a metal-organic chemical vapor deposition (MOCVD). We aim to fabricate a double metal waveguide (DMW) based GaN THz QCL through removing the Si substrate by wet etching. The DMW is essential for realizing low cavity loss and a very high optical confinement factor (99%). We grew 100 periods of GaN/Al 0.21 Ga 0.79 N two quantum wells (QW) type active region on Al 0.08 Ga 0.92 N/Si templates. Samples were evaluated by cross-sectional transmission-electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD). We found that the full width at half maximum (FWHM) of the XRD satellite peaks became narrower when the growth temperature decreased, and the steepness of the QC layer hetero-interfaces has been improved. [ABSTRACT FROM AUTHOR]