Ultrawide-bandgap semiconductors are ushering in the next generation of high-power electronics. The correct crystal orientation can make or break successful epitaxy of such semiconductors. Here, it is found that single-crystalline layers of α-(AlGa)₂O₃ alloys spanning bandgaps of 5.4 to 8.6 eV can be grown by molecular beam epitaxy. The key step is found to be the use of m-plane sapphire crystal. The phase transition of the epitaxial layers from the α- to the narrower bandgap β-phase is catalyzed by the c-plane of the crystal. Because the c-plane is orthogonal to the growth front of the m-plane surface of the crystal, the narrower bandgap pathways are eliminated, revealing a route to much wider bandgap materials with structural purity. The resulting energy bandgaps of the epitaxial layers span a broad range, heralding the successful epitaxial stabilization of the largest bandgap materials family to date.

超宽带隙半导体正在引入下一代大功率电子设备。正确的晶体取向可以成功或破坏这种半导体的外延。在此发现α-（AlGa）₂ O ₃的单晶层。可以通过分子束外延生长跨越5.4至8.6 eV的带隙的合金。发现关键步骤是使用m面蓝宝石晶体。外延层从α-到窄带隙β-相的相变是由晶体的c-平面催化的。由于c平面与晶体的m平面表面的生长前沿正交，因此消除了较窄的带隙通道，从而揭示了具有结构纯度的更宽的带隙材料的路径。所产生的外延层的能带隙范围很宽，预示了迄今为止最大的带隙材料家族的成功外延稳定。