Fabrication of crystal plane oriented trenches in gallium nitride using SF₆ + Ar dry etching and wet etching post-treatment

During the last few years and with the commercialization of the gallium nitride based high electron mobility transistor, research effort on gallium nitride has been strongly increasing. Besides activities regarding lateral devices like the gallium nitride high electron mobility transistor, progress in the growth of native gallium nitride substrates encourages the development of vertical devices. In particular, for power electronics above 600 V, vertical architecture shows superior performance compared to lateral devices. This makes the vertical approach interesting for the use in traction inverters in the rising market of e-mobility. A key aspect in the fabrication of most vertical devices is the formation and optimization of trenches in the semiconductor. In this work, the fabrication of 1.5–$2μm$ deep, crystal plane oriented trenches in gallium nitride with lateral dimension as small as $1μm$ is demonstrated. The trenches were produced by means of plasma etching based on sulfur hexafluoride and argon as well as a subsequent wet etching step in tetramethylammonium hydroxide and potassium hydroxide. By accurately aligning the trenches along the $[1¯010]$- and $[12¯10]$-directions, the authors were able to evaluate the wet etching behavior of the respective crystal planes and achieved smooth vertical sidewalls.