Influence of etching condition on surface morphology of AlN and GaN layers

The surface morphology of GaN and AlN thin films after etching was examined with a scanning electron microscope to seek the optimum condition for estimation of the density and distribution of threading dislocations (TDs). The correspondence of the pits and TDs was also confirmed by transmission electron microscopy. By etching GaN in HCl + N-2, a pit is formed one by one on the end of each TD. The etching in a relatively high concentration of HCl for a short time is found to be preferable for specimens with a high density of TDs. The Burgers vector can be identified from the morphology of the pit. No pits are formed on the surface of AlN by etching in (HCl + N2). By etching with molten NaOH, distinct pits are formed on whole TDs in GaN and on (a + c)-type TDs in AlN. The preferable conditions of etching are 400 degreesC or lower for 6-9 min for GaN, and 320 degreesC for 10 sec for AlN. By a treatment of reactive ion etching with Cl-2, whiskers remain on AlN. The whiskers contain a TD at the center.