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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]

• ELO-AlN was performed by varying off-cut angle of sapphire using MOVPE. • Dislocations were reduced by increasing off-cut angle of sapphire substrate. • Increasing off-cut angle, low dislocation region of ELO became smaller. • ELO-AlN layer exhibited low dislocation density of 2.1 × 106cm−2. Using a metal–organic vapor phase epitaxy, the epitaxial lateral overgrowth (ELO) of AlN was performed by varying a vicinal off-cut angle from 0.15 to 2.0° of sapphire substrates oriented to the m -axis, i.e., the a -axis of AlN. During overgrowth, step bunching resulted in the efficient conversion of dislocations to elongated voids. The dislocations were terminated at the voids formed over the groove regions and the low and high etch pit density (EPD) regions were observed. The dislocation was reduced by increasing the off-cut angle of the sapphire substrates up to 0.8°; however, low-EPD region became small in the ELO. Thus, we have attempted to achieve a longer growth of the ELO-AlN layer on the sapphire substrate with the off-cut angle of 0.15°. As a result, the EPD decreased with the increasing thickness. The ELO-AlN layer with the thickness of 15.9 µm exhibited the EPD of 2.1 × 106 cm−2 in the low dislocation region. [ABSTRACT FROM AUTHOR]