Reducing the formation enthalpy is believed to be the key issue in solving the problem HSP990 in vitro of Mg incorporation. The formation enthalpy is governed by two important factors, as given by [11]: (2) Here, ΔE = E Mg  - E host, where E Mg and E host are the total energies of the supercell with and without Mg substitution; Δμ = μ Al/Ga – μ Mg, where μ i (i = Al, Ga, Mg) is the chemical potential. ΔE is mainly induced by the strain caused by the atom size mismatch. Consequently,

larger atom size mismatch results in large ΔE, thus resulting in larger ΔH f as mentioned above. The strain induced by the C-dopant into the Si matrix becomes smaller on the surface than in the bulk [12]. The question of whether the surface also plays a similar role in the Mg check details incorporation in Al x Ga1 – x N arises. To address this issue, we further investigated the formation Selleck JQ-EZ-05 enthalpies of MgAl and MgGa on Al x Ga1 – x N surface, and the results are shown in Figure 1b. In contrast to the bulk case, both of the formation enthalpies in the surface are negative, suggesting favorable Mg substitution. The value of MgAl becomes lower than that of MgGa and decreases as the Al content increases. These interesting reversed tendencies provide us a possible way to promote the Mg incorporation in Al-rich Al x Ga1 – x N by utilizing the surface effect. An epitaxy growth, e.g., MOVPE and molecular

beam epitaxy systems, is conducted under an inherently non-equilibrium process with the surface transforming into a bulk [12]. Therefore, enhancing the Mg incorporation oxyclozanide by using the surface effect should be practically feasible. Two Mg-doped Al x Ga1 – x N (x = 0.33, 0.54) films were grown by MOVPE using the conventional method (the inset of Figure 1c) to validate the prediction of the surface effect on Mg incorporation. As shown in Figure 1c, the Mg concentration

(C Mg) on the surface is about one order higher than that of in the bulk for both samples. Although C Mg rapidly falls beneath the topmost surface (about 10 nm), C Mg is still several orders higher than the theoretical prediction by Equation 1. This phenomenon can be understood in terms of the competition between the Mg incorporation enhancement on the growing surface due to the surface effect and the Mg segregation as the epitaxy continues. Simply, when the surface with the enhanced Mg solubility is covered by newly added layers during further growth, most of these Mg segregates to the new surface to regain equilibrium because the surface transforms into a bulk. Meanwhile, considerable part of these Mg is frozen in because of solidification. As a result, the C Mg in the bulk is lower than that of in the final epilayer surface but is much higher than the equilibrium value of the ideal bulk. Considering this competition, Mg incorporation can be modified by balancing the surface time and solidification time. As shown in Equation 2, the factor Δμ also affects Mg incorporation.