In the fabrication of next-generation power electronic devices based on the III-V materials i.e. gallium nitride (GaN), AlGaN, etc., atomic layer etching (ALE) with the cyclic process of ion irradiation and Cl adsorption steps has been attracted for the reduction of plasma induced damages and the precise definition of etch depth. For controlling the GaN ALE, we investigated the surface layer of GaN at each Ar ion and Cl radical reaction step using a beam experiments with in situ X-ray photoelectron spectroscopy (XPS).[1,2]Samples were GaN films on a sapphire substrate by Hydride Vapor Phase Epitaxy (HVPE) method. Native oxide on GaN surface was removed by wet cleaning (5% HF) and Ar ion sputtering before the beam experiments. The wet cleaned surface was exposed to Cl radicals generated in Cl2 gas plasma. Then, Ar ions were irradiated with the accelerating voltage of 100 V or 200 V.The one cycle consisted of these Cl radical exposure and Ar ion irradiation. To stabilize the GaN surface, five cyclic processes were carried out. The GaN surface at each step was analyzed by angle-resolved XPS with the take-off-angles (TOA) of 20, 30, 40, 60, and 90 degrees with respect to the wafer surface. The depth profiles were analyzed by the maximum entropy method.In the GaN cycle process by Cl adsorption and Ar ion irradiation, the reaction in the Cl adsorption step is saturated at the order of 1019 cm−2. The reaction in the Ar ion irradiation step was saturated by Ar ion irradiation of the order of 1016 cm−2, and the formation of Ga dangling bonds occurred with the etching of Ga chloride and the desorption of N atoms. Increasing the Ar ion energy increased the amount of Ga dangling bonds formation and the subsequent increase of Cl adsorbed amount by the radical irradiation. At the same time, higher the Ar ion energy, higher the required doses of Cl and Ar ions to saturate the reaction in each step.The Cl adsorption behavior was depended on the depth profile of the Ga dangling bonds and varied with Ar ion energy. At 47.6 eV, which is below the energy of GaN ALE window, the Ga chloride could not be desorbed, but ions in this energy break Ga-N bonds and desorb N atoms. At 86.8 eV, which is the energy band of the ALE window of GaN, Ga chloride and N atoms can be desorbed, and it was confirmed that the Cl adsorption behavior depends on the ion energy. From these results, it is considered that the Cl adsorption depth and etching depth continue to change even in the energy band of the ALE window.Consequently, to achieve a higher Synergy and precise etching of GaN, tuninng of ion energy beside the ALE window should be necessary.AcknowledgementThe author thanks Drs. Masaki Hasegawa, Takayoshi Tsutsumi, Kenji Ishikawa, Masaru Hori, Hiroki Kondo, Shouhei Nakamura, Atsushi Tanide, Souichi Nadahara, Osamu Oda, Jia He for their discussions and providing the data.[1] T. Takeuchi et al., J. Phys. D: Appl. Phys. 46, 102001 (2013).[2] Y. Zhang et al., J. Vac. Sci. Technol. A 35, 060606 (2017).
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