Controlled growth of epitaxial wurtzite BeMgZnO alloy films and two microscopic origins of Be-Mg mutual stabilizing mechanism

A narrow growth window for high quality BeMgZnO quaternary alloy films was discovered. The key for this growth window is to keep the Be/Mg atomic ratio low. Here we examine this growth window in more detail and specify the optimum Be/Mg ratio, which is between 0.2 and 0.4. Within this region, the doping level and band-gap can be tuned in a wide range with constant crystal quality, which is certainly favorable to ZnO-based bandgap engineering. More importantly, we discuss the microscopic origins of this growth window to unveil the Be-Mg mutual stabilization mechanism. Our semi-quantitative analysis demonstrates that lattice mismatch compensation between Mg2+ and Be2+ and the strong fourfold coordination preference of Be2+ are two major origins. We have clarified that the upper limit of the growth window is determined by the lattice mismatch and the lower limit is determined by the coordination number (CN) preference, which use the new criterion parameter delta to take both ionic and covalent bond into account under the frame of Pauling's Rules. We believe that Mg can alleviate the serious lattice mismatch due to Be incorporation and Be can reversely relieve the ionic CN6 anxiety due to Mg incorporation. That is the way they stabilize each other and come to a balance. Our research provides a better insight into the stability of BeMgZnO alloys and a very meaningful way to design and engineer this quaternary alloys. (C) 2015 Elsevier B.V. All rights reserved.