Crystallographic and topographical evolutions of a cylinder patterned sapphire substrate etched with a sulfuric acid and phosphoric acid mixture: An SEM and AFM study

Crystallographic and topographical evolutions of a cylinder patterned sapphire substrate (PSS) under ordinary wet etching conditions (H₂SO₄:H₃PO₄ = 5:1 volume ratio, 230 °C) were systematically studied using complementary SEM and AFM characterization techniques. The strongly time-dependent crystallographic and topographical evolutions of cylinder protrusions were revealed. Cylinders changed to triangular frustums with co-existing top c-planes, to triangular pyramids with multiple high Miller index planes, then to hexagonal pyramids, and finally to triangular pyramids with a single family of planes. The Miller indices of five major crystallographic planes were determined: CY₂ {1 1 0 15}, CY₃ {1 1 0 6}, CY_4b {1 1 0 5}, CY₅ {4 5 1 38}, and CY₆ {1 1 0 12}, with the first three being reported for the first time to the best of our knowledge. The etching rates of the crystallographic planes follow the order: CY₃ > CY₅ > CY₆ > CY₂ > c plane. It was clearly demonstrated that cylinder protrusions, featuring steep sidewalls, circular symmetry, and top c-planes, are suitable for studying the full spectrum of crystallographic and topographical evolution characteristics of the PSS, as well as for fabricating optimized PSSs with a specific slant angle for enhancing the performance of light emitting diodes in the future.