Strain-engineered step-flow growth of highly conductive n-AlGaN with Al content over 80% at low temperature by MOCVD

The n-doping of Al-rich AlGaN with Al content over 80% remains a crucial challenge for the advancement of far-ultraviolet C (far-UVC) optoelectronic devices. Although low-temperature growth can improve electrical performance by suppressing defect compensation, it will deteriorate the surface morphology and impede the subsequent epitaxy. Particularly for n-AlGaN grown on high-temperature-annealed AlN/sapphire (HTA-AlN) templates, which usually exhibit strong compressive stress, the significant compressive stress accumulated within n-AlGaN will further hamper the formation of a smooth surface. To address this issue, we propose a strain engineering strategy for n-AlGaN growth by metalorganic chemical vapor deposition (MOCVD) at a relatively low temperature of 1075 °C, facilitating step-flow growth while maintaining high electrical conductivity. Nearly stress-free AlN templates were obtained through TMAl pretreatment of the HTA-AlN templates, reducing the in-plane compressive strain of the epitaxial n-AlGaN layer from 0.74% to 0.44%. This strain relaxation drives a distinct morphological evolution of the epitaxial n-AlGaN from three-dimensional hillocks to step-terrace structures, significantly reducing the surface roughness. The strain-modified Burton–Cabrera–Frank model indicates that relieving compressive strain can reduce the maximum surface supersaturation, facilitating step-flow growth of epitaxial n-AlGaN at the low-temperature window. Ultimately, an epitaxial n-Al0.8Ga0.2N with a step-flow surface exhibits a conductivity of 34.1 S/cm, and its application in far-UVC light-emitting diodes demonstrates its effectiveness. This strain engineering strategy provides a promising pathway for the low-temperature growth of Al-rich n-AlGaN by MOCVD on HTA-AlN templates with strong compressive stress, advancing their application in far-UVC optoelectronic devices.