Chemical etching of freestanding N-polar GaN in control of the surface morphology

Surface treatment methods were investigated on freestanding N-polar GaN surfaces with chemical etching methods in H3PO4 and NaOH solutions at room temperature. Two kinds of step-flow structures could be obtained on the surfaces. Merely etching with diluted H3PO4 solutions resulted in straight step-flows towards the off-cut direction of the wafer, while coaxial hexagonal step-flows were formed after a three-step etching scheme with H3PO4, NaOH and H3PO4 in turn. During these procedures, etching with H3PO4 solutions exhibits the ability to sharpen atomic step edges, form flat terraces and remove hexagonal pyramids. On the contrary, NaOH etching will roughen well-defined terraced surfaces and produce hexagonal pyramids.