The minimization of plasma-induced damage (PID) in plasma etching is important for the precise and smooth removal of a depth of approximately 7 nm of GaN films to fabricate gate-recess GaN-based normally-off power electronic devices. We have systematically studied the photoluminescence (PL) properties and surface morphologies of GaN films exposed to Cl2 plasma at 400 °C, focusing on their dependences on etch time and ion energy. It is noticeable that PL degradation saturated at etch times of more than 2 min, while surface roughness increased continuously with etch time. Variations of surface roughness with bias voltage were negligible. PID was successfully suppressed by reducing bias voltage, leading to the decrease in incident ion energy on the surface, and thus the near-band-edge emission (NBE) intensity as a PL property was increased to 98.8% of the initial value.
Deep ultraviolet (UV) photons emitted from Cl2 plasmas become a critical cause of degradation in both photoluminescence (PL) properties and surface stoichiometry as a result of plasma-induced damage on GaN films in Cl2 plasma etching at high temperatures. The damages were formed thermally by photon-irradiations of plasma UV emissions with wavelengths of ∼258–306 nm from Cl2 plasma at temperatures greater than 500 °C. The damage were observed with a depth of approximately 3.2 nm. The PL property degraded by the UV emission-induced damage at an early period of plasma etching and reached a constant value.
Surface chemical reactions on the GaN surface with Cl radicals are thermally enhanced in the high-temperature Cl2 plasma etching of GaN, resulting in the formation of etch pits and thereby, a roughened surface. Simultaneous irradiation of ultraviolet (UV) photons in Cl2 plasma emissions with wavelengths of 258 and 306 nm reduces the surface chemical reactions because of the photodissociation of both Ga and N chlorides, which leads to a suppression of the increase in surface roughness. Compared with Si-related materials, we point out that photon-induced reactions should be taken into account during the plasma processing of wide-bandgap semiconductors.
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