Metastable atomic-ordered configurations for Al_1/2Ga_1/2N predicted by Monte-Carlo method based on first-principles...

Metastability of Al_n/12Ga_1-n/12N (n= 2-10: integer) with the 1-2 monolayer (ML) in-plane configuration towards thec[0001] direction has been demonstrated recently. To theoretically explain the existence of these metastable structures, relatively large calculation cells are needed. However, previous calculations were limited to the use of small calculation cell sizes to estimate the local potential depth (Δσ) of ordered Al_1/2Ga_1/2N models. In this work, we were able to evaluate large calculation cells based on the interaction energies between proximate Al atoms (δE_Al-Al) in AlGaN alloys. To do this,δE_Al-Alvalues were estimated by first-principles calculations (FPCs) using a (5a₁× 5a₂× 5c) cell. Next, a survey of the possible ordered configurations using various large calculation cell models was performed using the estimatedδE_Al-Alvalues and the Monte-Carlo method. Then, various Δσvalues were estimated by FPCs and compared with the configurations previously reported by other research groups. We found that the ordered configuration obtained from the (4a₁× 2a₂× 1c) calculation cell (C₄₂) has the lowest Δσof -9.3 meV/cation and exhibited an in-plane configuration at thec(0001) plane having (-Al-Al-Ga-Ga-) and (-Al-Ga-) sequence arrangements observed along them11-00planes. Hence, we found consistencies between the morphology obtained from experiment and the shape of the primitive cell based on our numerical calculations.