The AlGaN/GaN epitaxial layers were grown by metalorganic chemical vapor deposition on both Ga-face and N-face free-standing GaN substrates, which were fabricated by halide vapor phase epitaxy method. Time-resolved photoluminescence technique was applied to study the optical properties of the grown AlGaN/GaN heterostructures. A strong emission at similar to 3.41 eV has been observed in the Ga-polar AlGaN/GaN structure, which is confirmed to associate to the two-dimensional electron gas. The temporal behavior of the two-dimensional electron gas emission is studied, which correlates well with the recombination processes in an asymmetric triangular potential well formed by an AlGaN/GaN structure grown in [0 0 0 1] direction. (C) 2019 Elsevier B.V. All rights reserved.