Etching process in molten KOH

The discrepancy of etching characteristics in Ga-face (+c GaN, Ga-polarity) and N-face (-c GaN, N-polarity) has been specifically investigated as illustrated in Fig.

2.3.1. Some reports showed that gallium nitride could be etched in the aqueous sodium hydroxide (NaOH) solution but etching ceased when the formation of an insoluble coating of presumably gallium hydroxide (Ga(OH)3) [14,15]. For further etching, it would need removing of the coating by continual jet action. Various aqueous acid and base solutions have been tested for etching of GaN were list in Table 2.3.1 [16-18]. The undetermined etch rate (nm/min) was because it various from sample to sample and differences in the defect density. According to the research reports in recent years; the common cognition related to gallium nitride etching process was that the most of gallium nitride could be etched rapidly in N-face. The reason for the face-dependent gallium nitride etching process has been studied by Li et al., who utilized the X-ray photoelectron spectroscopy (XPS) to examine the

surface chemistries before and after etching process in aqueous KOH solutions for both Ga- and N-face gallium nitride. The conclusion is that the different etching

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results in Ga- and N-face gallium nitride crystals are due to the different states of surface bonding. Besides, the most important is the etching process only dependent on the polarities, not on the surface morphology, growth condition and which atoms form the surface termination layer. The GaN chemical etching reaction with KOH could be described as the following formula [19]:

2GaN+3H₂O^KOH → Ga₂O₃+2NH₃ (2-11) Here, the molten KOH act as a catalyst and a solvent for the resulting Ga2O3 (Fig.

2.3.2 (d)) as well. The mechanism about etching N-face gallium nitride substrate was illustrated in Fig. 2.3.2. The hydroxide ions (OH^-) were first absorbed on the gallium nitride surface (Fig. 2.3.2 (b)) and finally react with Ga atoms once the OH^- ions with sufficient kinetic energy as shown in the Fig. 2.3.2 (c). The etching could be started at step (c) if the surface was Ga-terminated. The inertness of Ga-face GaN was ascribed to the hydroxide ions would be repelled by the negatively-charged triple dangling bonds of nitrogen near the surface. Thus, if the Ga-face GaN was Ga-terminated, the etching process stops after the first gallium atom layer was removed. In contrast, for the N-face GaN, every nitrogen atom bears a single dangling bond to prevent the hydroxide ions attacking from Ga atoms.

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Table 2.3 Various chemicals etch GaN.[20]

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Fig. 2.3.1 Illustration of different polarity, (a) Ga-face (+c GaN, GaN polarity ), (b) N-face (-c GaN, N-polarity). [22]

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Fig. 2.3.2 Schematic diagrams of the cross section GaN film viewed along [-1-120]

direction for N-polar GaN to explain the mechanism of the polarity selective etching.

(a) Nitrogen terminated layer with one negatively charged dangling bond on each nitrogen atom; (b) absorption of hydroxide ions; formation of oxides; (d) dissolving the oxides. [23]

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2.4 Reference

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