Mechanistic Insights into the Atomic Layer Etching of GaN Using XeF2 and BCl3

Gallium nitride (GaN) thin films can be etched by atomic layer etching (ALE) at a low temperature through sequential exposures of XeF₂ and BCl₃ molecules. However, the reaction mechanisms of XeF₂ and BCl₃ molecules with the surface of the GaN substrates remain to be fully elucidated. Employing density functional theory (DFT), this study systematically investigates the reaction mechanisms and energy barriers of XeF₂ and BCl₃ with the GaN substrate surface. The conventional reaction between XeF₂ and GaN surfaces primarily involves the adsorption of F atoms onto GaN until complete surface fluorination, followed by progressive binding of the N atom with increasing F atoms to form NF₃ molecules. An innovative reaction mechanism for interactions of XeF₂ with GaN surfaces is proposed in this letter. For the N-terminated surface, the new mechanism initiates with complete surface fluorination, followed by the upward migration of the Ga atom, leading to the generation of the vacancy in the subsurface layer. The vacancy facilitates the downward migration of surface N atoms, resulting in the subsequent formation of N₂ in the subsurface. In the case of the Ga-terminated surface, the approach of two adjacent subsurface N atoms, together with the pulling effect of the Ga atom, facilitates the N–N bond formation and N₂ generation. In addition, this work demonstrates that Ga–F coordination lowers the energy barriers for N₂ formation, desorption, and the overall reaction by broadening the desorption pathway of N₂ and pulling the N₂ molecule upward. Furthermore, the effects of Ga and N vacancies on the N₂ formation mechanism in a Ga-terminated surface system are investigated. The DFT results demonstrate that V_Ga promotes N₂ formation and desorption by weakening the constraint on N atoms, while V_N exhibits a dual role in this process. On the one hand, V_N induces adjustments in the electron cloud distribution and geometric positions of adjacent Ga atoms, which enhances their constraint on other N atoms and thereby increases the formation energy of N₂. On the other hand, V_N destabilizes the surface Ga atomic structure, inducing the reconfiguration of the Ga and F atomic layers during N₂ desorption. This reconfiguration widens the desorption channel, reduces constraints on N₂, and thereby lowers the desorption energy of N₂. The DFT calculation results show the thermodynamic feasibility of the N₂-yielding novel mechanism, which has been verified by some existing experimental results. Our analysis provides innovative mechanistic insights into the ALE process of GaN, stimulating further theoretical and experimental studies in this area.