(Color online) (a) and (b) Sample surface under a metallographical microscope with 50× and 500× object lens, respectively. Dense hexagonal pits with different sizes and depths are observed. (c) and (d) AFM pictures of sample surface inside and outside the pit area, respectively. The sample was annealed in H2 atmosphere at 1030 °C for 5 min.
(Color online) AFM pictures of sample surface annealed at different temperatures in H2 atmosphere. From (a) to (d), the annealing temperature is 970, 1000, 1030, and 1050 (C∘C, respectively, all for 5 minutes. An increase of dot density toward higher temperature is evident.
(Color online) AFM pictures of sample surface annealed for different times in H2 atmosphere. From (a) to (d), the annealing time is 3, 4, 5, and 6 min, respectively, and the temperature is kept at 1030 °C. Dramatic variation of dot morphology as a function of time is observed.
(Color online) Three-dimensional AFM picture of sample surface near a macropit. Nanodots are clearly seen on the normal face, while their precursors, like upright peaks, are mainly distributed near lateral walls. Such a morphology feature reveals the etching mechanism and the dot formation process.
(Color online) (a) Height (red circle line) and base length (blue square line) of peaks as a function of the distance from the origin marked in Fig. 3 . The results are averaged within a particular area centered at each distance. (b) Decomposition rate (red circles) measured from the number of macropits as a function of temperature in log scale. The fitting by linear function (blue dashed line) gives the activation energy of GaN decomposition.
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