Discrete AlN mole fraction of n/12 (n = 4-8) in Ga-rich zones functioning as electron pathways created in nonflat AlGaN layers grown on high-miscut sapphire substrates

Ga-rich zones created along macrosteps in n-AlGaN plausibly function as electron pathways of AlGaN-based deep-ultraviolet (DUV) LEDs fabricated on AlN templates using 1.0°-miscut c(0001) sapphire substrates toward the m[1-100] axis. This work was performed to clarify AlN mole fractions (x_Al) of Ga-rich zones. x_Al ≃ (7/12, 6/12, and 5/12) was observed in Ga-rich zones in AlαGa₁₋αN (α ≃ 0.63, 0.55, and 0.43, respectively) by the method proposed in our previous article in which we showed that Ga-rich zones of Al_8/12Ga_4/12N were created in Al_0.7Ga_0.3N. x_Al in the Ga-rich zones obtained from an energy-dispersive x-ray signal by scanning transmission electron microscopy calibrated by Rutherford backscattering well agreed with x_Al obtained by cross-sectional cathodoluminescence (CL) spectroscopy using scanning electron microscopy. A weak CL shoulder peak corresponding to Al_4/12Ga_8/12N was also observed for Al_0.43Ga_0.57N. In addition, x_Al ≃ n/12 (n = 6-9) in Al-rich zones appeared in the rest of the Ga-rich zones. Furthermore, nanobeam electron diffraction patterns of the Ga-rich zones indicated a high possibility of a regular configuration of Ga and Al atoms on the c(0001) plane in our samples. Consequently, x_Al values in nonflat AlGaN layers with macrosteps were often determined to be near n/12 (n: integer). Thus, Ga-rich zones (x_Al = n/12: n = 4-8) formed in our nonflat AlGaN layers, which originated from the macrosteps along [11-20] edgelines normal to the m[1-100] axis, are suggested to be metastable. The creation of discrete x_Al in Ga-rich zones should contribute to the stable production of DUV-LEDs using high-miscut sapphire substrates.