Crystal-orientation-dependent flatband voltage of non-polar GaN MOS interfaces investigated using trench sidewall capacitors

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We systematically characterized gallium nitride (GaN) metal-oxide semiconductor interfaces formed on trench sidewalls, paying particular attention to eliminating the parasitic capacitance caused by the trench bottom. The flatband voltage (Vfb) was found to be clearly higher on the m-face trench than on the a-face trench, and an interface state density of ∼1 × 1011 cm−2 eV−1 near the conduction band edge was achieved, irrespective of the trench direction. For trench sidewalls treated with a tetramethyl ammonium hydroxide (TMAH) solution, variation in Vfb among the trench direction was suppressed, reflecting that microscopic m-faces were generated after the TMAH treatment, irrespective of the direction of the trench. Therefore, the origin of the Vfb variation was attributed to the GaN crystal face dependence of fixed charge density at SiO2/GaN interfaces.