Comparison between extended microtunnels along different crystal orientations in GaN

Triangular prism-like extended microtunnels (EMTs) can be easily formed in specially designed epitaxial lateral overgrowth GaN thick films by selective wet chemical etching in molten potassium hydroxide (KOH). In this study, extended microtunnels along the 〈1 100〉 and the 〈11 20〉 directions were fabricated and compared. If tunnels along the 〈1 100〉 direction, the {11 22} planes would be the etch stop plane. While the tunnels along 〈11 20〉 direction, the etch stop plane would become to {10 11} planes. The activation energies of wet chemical etch for the {11 22} planes and the {10 11} planes were determined to be 23.6 kcal/mol (1.02 eV) and 22.8 kcal/mol (0.99 eV) using Arrhenius plot. On the other hand, the etching depths of the tunnels along the 〈1 100〉 direction were more than twice the depths of the tunnels along the 〈11 20〉 direction during the same etching time. The highest etch rate of the tunnels along the axial direction can reach 1000 μm/hr in 〈1 100〉 direction, which is believed to be the highest etch rate of GaN ever reported. Additional crystal facet of m-plane in GaN was also observed in the sample with extended microtunnels along the 〈11 20〉 direction.